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Page 1: Pablo Giménez Gavarrell
Page 2: Pablo Giménez Gavarrell

Pablo Giménez Gavarrell

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Departamento de Ingeniería Energética

Escuela Superior de Ingeniería

Universidad de Sevilla

Thermal Energy Storage for High

Temperature Applications

Autor:

Pablo Giménez Gavarrell

Director:

Sonia Fereres Rapoport

Investigador Senior Abengoa Research

Sevilla, 2017

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Tesis doctoral: Thermal Energy Storage for High Temperature Applications

Autor: Pablo Giménez Gavarrell

Tutor: José Julio Guerra Macho

Programa de doctorado en Ingeniería Energética, Química y Ambiental (2011)

Línea de investigación: Eficiencia Energética e Integración de Energías Renovables

en la Edificación y en la Industria

El tribunal nombrado para juzgar el Proyecto arriba indicado, compuesto por los

siguientes miembros:

Presidente:

Dr. Servando Álvarez Domínguez (Universidad de Sevilla)

Vocales:

Dr. Ignacio González Loscertales (Universidad de Málaga)

Dr. Luis Allan Pérez Maqueda (Consejo Superior de Inv. Científicas)

Dr. Yulong Ding (Universidad de Birmingham)

Secretario:

Dra. Luisa F. Cabeza Fabra (Universidad de Lleida)

Acuerdan otorgarle la calificación de:

Sevilla, 2017

El Secretario del Tribunal

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"La página escrita nunca recuerda todo lo que se ha intentado, sino lo poco que

se ha conseguido."

Antonio Machado

A mis padres, familia y Lorena

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Acknowledgements

First of all, I am sincerely thankful to my director Sonia Fereres for demonstrating

immense faith in my work and for being the only one who has believed in this

research from the beginning of this thesis. She has always tried to motivate me and

without her support I could not have completed the thesis. I would like to thank

Abengoa Research for economically funding this PhD thesis, Y-Flow for helping in

the development of the idea of PCM-borosilicate capsules, my supervisor Prof.

José Guerra and the Energetic Engineering department for accepting me as PhD

student as well as Prof. D. Shin for accepting me in his lab at UTA.

We were 21 students when we started this adventure more than 4 years ago.

Unfortunately, only few of them finished it and nobody knows how many will be

able to continue in this exciting world of research. Despite the fact that nothing was

what we were promised while pursuing this endeavor, I have personally enjoyed it

and I would not change the valuable experience I gained. I like to further thank all

my colleagues from Abengoa, especially who started with me: Ramos, my favorite

cellist Bea, Sol, Irene and Elyas, and some still working on it like Javi and Jacobo.

I would also like to thank Eva, Vince, Kike and Maria for sharing not only work

place but also our life in Seville. Everything changes when you have the best

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housemates. From my visit to the States, I would like to thank my friend Tu Price

who, together with Vince, made my stay in America awesome.

Last but not least, I especially dedicated my thesis to my parents, Paqui and Juan

Ramón, my brothers JuanRa and Franc and my family, Ambrosio, Elena, Adrian,

Amparin, Adri, Amparo, Roberto, Chritine, María, Morgane, David and Sylvan.

Lastly, I want to thank Lorena, for being with me these lovely last years in Seville.

Pau Giménez Gavarrell

Sevilla, 2017

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Resumen

La producción de energía eléctrica a gran escala a partir de energía solar ha

recibido en las últimas décadas un gran impulso debido a las diferentes ventajas

que ofrece, no sólo medioambientales sino también en temas geopolíticos como son

su independencia de los combustibles fósiles y su alta disponibilidad geográfica.

Sin embargo, antes de que la energía solar sea capaz de penetrar significativamente

en el mix de energía que abastece un país, todavía quedan algunos retos por

resolver.

El desafío principal de la energía solar es su intermitencia intrínseca, lo que a

menudo dificulta la coincidencia entre la disponibilidad de la fuente de energía con

la necesidad de satisfacer la demanda eléctrica. Por tanto, el almacenamiento de

energía es la clave para conseguir desacoplar la producción de energía eléctrica de

la radiación solar.

En este aspecto, la tecnología solar térmica está un paso por delante otras

tecnologías como la fotovoltaica. Tal vez debido a la histórica utilización del calor

para diferentes propósitos, el almacenamiento de energía térmica está bastante

desarrollado y plantea hoy en día pequeños problemas tecnológicos en

comparación con otras aún incipientes -pero muy prometedoras- tecnologías de

almacenamiento como las baterías. Gracias a esta ventaja, junto con las mayores

eficiencias en la producción a gran escala en comparación con la energía

fotovoltaica, la energía termosolar promete desempeñar un papel relevante en un

futuro próximo. Aunque menor que otras tecnologías, la elevada inversión

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necesaria para la implementación de sistemas de almacenamiento térmico es una

gran desventaja. Por tanto, el desarrollo de nuevos sistemas de almacenamiento

térmico más densos energéticamente y sobre todo de menor coste específico,

además de la mejora de los sistemas existentes, es crucial para la implantación de la

energía solar como vector energético a gran escala.

Esta tesis se plantea, dentro de este contexto, con el objetivo principal de

explorar diferentes estrategias para incrementar la densidad energética de los

sistemas de almacenamiento térmico que se utilizan actualmente en plantas

solares térmicas de concentración, específicamente las de torre, por su mayor

potencial debido a su mayor temperatura de operación y por tanto mayor

eficiencia en la transformación de la energía térmica en eléctrica. Las dos

tecnologías de almacenamiento térmico que se utilizan actualmente a nivel

comercial en este tipo de plantas son: los acumuladores de vapor y los tanques

de sal fundida. Ambos sistemas utilizan almacenamiento de calor sensible: el

primero en agua líquida saturada a alta temperatura y presión, y el segundo

sistema almacena energía mediante el incremento de la temperatura de sales

fundidas.

Como alternativas a las actuales tecnologías en esta tesis se ha investigado el

uso de a) materiales de cambio de fase (PCM) que utilizan el calor latente de

fusión como mecanismo de almacenamiento térmico complementario a los

acumuladores de vapor, y b) la modificación de las propiedades termofísicas de

las sales fundidas a través de la adición de nanopartículas con el objetivo de

incrementar su densidad energética del sistema basado en tanques de sales.

El uso de materiales de cambio de fase como sistema de almacenamiento ha

requerido la realización de una selección de materiales basada en valores de

temperatura de fusión y calores latentes de la literatura en el rango de temperatura

de interés (~300 ºC) validando dichos valores experimentalmente mediante el uso

de calorimetría diferencial barrido. El intercambio de calor entre el material de

almacenamiento y el fluido caloportador por medio de un sistema de lecho fijo de

bolas ha requerido el diseño, desarrollo y prueba del sistema PCM-capsula. Se ha

identificado el borosilicato como material encapsulante de PCM utilizándolo para

el desarrollo de diferentes pruebas de concepto.

Las cápsulas de PCM, una vez fabricadas, se han probado individualmente en una

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instalación experimental, donde el objetivo principal era el ciclado térmico con un

flujo convectivo de aire a temperaturas entre 200 y 400ºC con el fin de fundir y

congelar el material. Se ha implementado un modelo numérico, intentando

aproximarse en la medida de lo posible a la instalación experimental, para ayudar a

entender la influencia de parámetros como la naturaleza del material de cambio de

fase (sal - metal), material de cápsula, espesor, etc., en los tiempos de inicio y fin del

proceso de cambio de fase así como perfiles de temperatura dentro de la cápsula. El

modelo es capaz de capturar la física principal que tiene lugar a pesar de su

simplicidad y se utiliza para ayudar a entender los resultados experimentales. Las

simulaciones se han comparado con los experimentos cualitativa y

cuantitativamente identificando algunas fuentes de incertidumbre que podrían

explicar el desajuste entre ellos. En conclusión, el uso de PCM como

almacenamiento térmico tiene sentido desde un punto de vista de eficiencia

energética. Con este análisis se demuestra que una cápsula aislada funciona

debidamente, intercambiando calor con al aire caliente circundante. El siguiente

paso sería probar un lecho fijo de cápsulas en condiciones no sólo de alta

temperatura sino también de alta presión. Sin embargo, hasta el momento los

costes asociados al contenedor del lecho fijo (tanque de vapor) superan los

beneficios de estas cápsulas de PCM, llevando a pensar en diseños de

intercambiadores de calor alternativos.

Finalmente, se ha investigado la adición de nanopartículas para mejorar la

capacidad de almacenamiento de los sistemas basados en sales fundidas. El

objetivo inicial era reproducir el aumento de calor específico reportado en la

literatura por varios grupos de investigación. Desafortunadamente, las diferentes

concentraciones de nanopartículas, tamaños, tipos y diferentes composiciones de

fluido base ensayadas no mostraron ninguna mejora sustancial ni estadísticamente

significativa respecto al fluido base. Se introdujeron ligeras modificaciones del

proceso de síntesis, descubriendo que la etapa de evaporación de la disolución

parece separar la sal en diferentes regiones con composiciones ligeramente

diferentes, lo que podría explicar ligeras modificaciones en el calor específico y el

calor latente.

Durante esta investigación se han cuestionado algunas suposiciones que se daban

por validas en el proceso de síntesis por los diferentes grupos de investigación. El

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protocolo de síntesis basado en la disolución de la sal en agua, sonicación y

vaporación, el más ampliamente utilizado, no parece apropiado ya que es posible

encontrar muestras con Cp superior o inferior a la media incluso sin

nanopartículas. También se ha cuestionado la capacidad de los nanofluidos de sal

fundida para mantener las nanopartículas homogéneamente distribuidas en

suspensión. Las imágenes de microscopía electrónica de barrido no mostraron la

presencia de nanoestructuras "especiales" o "anómalas" en la sal.

También se ha investigado el calor latente de los nanofluidos de la sal fundida. Se

ha observado que el calor latente disminuye en una cantidad mayor que la prevista

teóricamente, a diferencia de las limitadas investigaciones existentes con sales

fundidas, pero alineadas con un gran número de estudios de materiales de cambio

de fase con nanopartículas de menor temperatura. Por último, hemos planteado la

hipótesis de la existencia de una capa líquida en la interfase partícula-fluido como

responsable de esta mayor reducción en el calor latente del cálculo del espesor de la

capa en cada una de las composiciones del líquido base analizada.

Aunque la adición de nanopartículas a fluidos caloportadores podría resultar muy

interesante para mejorar las propiedades termofísicas de los mismos, actualmente

la tecnología está muy lejos de poder ser implementada industrialmente en una

planta termosolar. Con este análisis detallado se han identificado muchos de los

problemas por resolver: la dificultad en realizar medidas a muy altas temperaturas

y con nanopartículas en suspensión, la falta de consenso en la mejora de

propiedades a obtener, la falta de un método de síntesis que no influya en los

resultados y, sobre todo, donde las partículas se mantengan uniformemente

distribuidas durante los ciclos de trabajo y a las temperaturas requeridas. Hasta el

momento, ninguno de los autores que han evaluado estas sales con nanopartículas

han reportado detalles de la estabilidad de sus dispersiones en estado líquido, paso

previo esencial a cualquier trabajo futuro.

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Abstract

Large scale electricity production from solar energy has been gaining attention in

recent years due to its advantages to solve environmental problems as well as

issues such as fossil fuel dependence. However, there are some challenges to be

solved before solar power becomes a feasible alternative representing a significant

portion of the energy consumption of a country.

The main challenge of solar energy is its intrinsic intermittence, which makes it

difficult to match the energy resource availability with the electricity demand.

Therefore, energy storage is the key element to decouple electricity production

from solar radiation.

In this aspect, solar thermal technology is one step ahead other technologies such as

photovoltaic. Perhaps due to the historical use of heat for different purposes, the

storage of thermal energy is a more cost-effective solution compared to other

storage technologies such as batteries. Thanks to this advantage, together with

higher efficiencies in electricity production compared to photovoltaics, solar

thermal energy is playing and will continue to play an important role in the near

future. On the other hand, the high investment cost for the implementation of

thermal storage systems is its main disadvantage. Therefore, the development of

newer, higher energy density and, especially, lower specific cost storage

technologies, in addition to improving the existing systems, is crucial for the

deployment of the solar thermal energy.

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In this context, the main objective of this thesis is to explore different strategies to

increase the energy density of the thermal storage systems currently used in

concentrating solar power plants, specifically those of tower plants. Tower plants

have a greater potential due to their higher operating temperatures and greater

efficiency in the transformation of thermal energy into electricity. The two thermal

storage technologies currently used commercially in this type of plants are: vapor

accumulators and molten salt tanks. Both systems use sensible heat storage: the

first, in saturated liquid water at high temperature and pressure, and the second

system stores energy by increasing the temperature of molten salts.

As alternative to current technologies, this thesis investigates the use of a) the latent

heat of fusion of phase change materials (PCM) as a thermal storage mechanism

complementary to vapor accumulators, and b) the modification of the

thermophysical properties of the molten salts through the addition of nanoparticles

in order to increase the energy density of the molten salt tanks.

The use of phase change materials as thermal storage requires a screening of

materials based on melting temperature and latent heat values from the literature

in the temperature range of interest (~ 300 °C), validating experimentally these

values using a differential scanning calorimeter. The heat exchange between the

storage material and the heat transfer fluid by using a packed bed required the

design, development and testing of a PCM-capsule system. Borosilicate has been

identified as encapsulating material and it has been used to develop different proof

of concepts.

The PCM-capsules have been manufactured and tested in an experimental facility,

where the main objective was to assess the functionality of the capsules by

performing thermal cycling with a convective flow of air at temperatures between

200 and 400 ° C in order to melt and freeze the material. A finite difference 1-D

numerical model was developed trying to reproduce the phenomena taking place

in the experimental installation. It has been used to understand the influence of

parameters such as the nature of the phase change material (salt - metal), the

capsule material, its thickness, etc… on the start and end times of the phase change

process and on the temperature gradients within the capsule. The model is able to

capture the main physics of the process despite its simplicity and it is used to help

understand the experimental results. The simulations have been compared with

the experiments qualitatively and quantitatively, identifying some sources of

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uncertainty that could explain the mismatch between them.

In conclusion, the use of PCM as thermal storage makes sense from an energy

efficiency point of view. This analysis demonstrates that a single capsule functions

properly, exchanging heat with the surrounding hot air as the PCM melts and

solidifies while maintaining its physical integrity. This demonstrates the volume

expansion of the PCM during the phase transition is managed adequately in the

capsules. The next step would be to test a fixed bed of capsules under conditions

not only of high temperature but also of high pressure. However, the current costs

associated with the fixed-bed container (steam tank) outweigh the benefits of these

PCM capsules, leading to evaluate alternative heat-exchanger designs.

Finally, the addition of nanoparticles has been investigated in order to improve the

storage capacity of the systems based on molten salts. The initial objective was to

reproduce the specific heat increase reported in the literature by several research

groups. Unfortunately, the different concentrations of nanoparticles, sizes, types

and different base fluid compositions tested did not show any substantial or

statistically significant improvement over the base fluid. Slight modifications of the

synthesis process were introduced, finding that the evaporation stage of the

solution appears to separate the salt in different regions with slightly different

compositions, which could explain slight modifications in specific heat and latent

heat.

During this research, some common assumptions regarding the synthesis process

have been questioned. The widely used synthesis protocol based on the dissolution

of the salt in water, sonication and vaporization of the solvent water, does not seem

appropriate since it is possible to find samples with higher or lower Cp even

without adding nanoparticles to the base salt. The ability of the molten salt

nanofluids to keep homogeneously dispersed nanoparticles in suspension has also

been questioned. Scanning electron microscopy images did not show the presence

of "special" or "anomalous" nanostructures in the salt.

The latent heat of the molten salt nanofluids has also been investigated. It has been

observed that latent heat decreases by a greater amount than theoretically

anticipated, unlike the limited existing research with molten salts, but in line with a

large number of studies of phase change materials with lower temperature

nanoparticles. Finally, we have hypothesized the existence of a liquid layer at the

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particle-fluid interface as responsible for this greater reduction in the latent heat.

Although the addition of nanoparticles to heat transfer fluids could be very

interesting to improve their thermophysical properties, the technology is far from

being industrially viable in a solar thermal plant. This detailed analysis has

identified many of the problems to be solved: the difficulty in carrying out

measurements at very high temperatures and with suspended nanoparticles, the

lack of consensus on the possible property improvement, the lack of a synthesis

method that does not influence the results and, above all, the particles’ ability to

remain uniformly dispersed during the work cycles at the required temperatures.

So far, none of the authors who have evaluated these salts with nanoparticles have

reported details of the stability of their dispersions in the liquid state, an essential

prior step to any future work.

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Contents

Acknowledgements ix

Resumen xi

Abstract xv

Contents xix

List of Figures xxiii

List of Tables xxxv

Nomenclature xli

1 Introduction 1 1.1 Concentrating Solar Thermal Power Systems 2 1.2 Thermal Energy Storage (TES) 3 1.3 Mechanisms to store thermal energy 4 1.4 Classification of TES 5

1.4.1 TES in commercial central receiver solar power plants 6 1.4.2 Steam accumulators 8 1.4.3 Two molten salt tanks direct system 9

1.5 Main objectives of the thesis and structure 9

2 Phase Change Materials 13 2.1. Introduction and main objectives 13

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2.1.1. Objectives 16 2.2. PCM screening, characterization and selection 16

2.2.1. Metallic PCM candidates 19 2.2.2. Inorganic salts PCM candidates 27

3 Macro Encapsulation of PCM 53 3.1. Introduction and Main Objectives 53 3.2. Background on High Temperature Encapsulation 56 3.3. Shell Material Selection 66 3.4. Capsule Design 70 3.5. Capsule Manufacturing 71 3.6. Capsule Testing 80

3.6.1. Set-up Design 80 3.6.2. Experimental Procedure 85 3.6.3. Experimental Results and Discussion 88 3.6.4. Melting Results 99 3.6.5. Freezing Results 102

3.7. Conclusions 104

4 Single Capsule Model 107 4.1. PCM-capsule heat transfer model 108 4.2. Grid and time-step convergence 116 4.3. Model Validation 118 4.4. Material properties 121 4.5. Boundary condition 125 4.6. Results and Discussion 127

4.6.1. Effect of the new boundary condition on the phase change times127 4.6.2. Effect of capsule size on phase change 130 4.6.3. Effect of capsule shell thickness on phase change 132 4.6.4. Effect of shell material on phase change times: borosilicate vs. steel 134 4.6.5. Effect of the latent heat on the phase change times 139 4.6.6. Effect of PCM characteristics on phase change times 141 4.6.7. Effect of experimental conditions on the phase change times 144

4.7. Comparison to experimental results 147 4.7.1. Convection correlation for experimental set-up 147

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4.7.2. Model vs. Experiments 150 4.8. Double PCM solution 164 4.9. Discussion and conclusion of encapsulated PCM as TES system 166

5 Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage 169 5.1. Introduction 170

5.1.1. Background on Nano-enhanced HTF-TES materials 170 5.1.2. Impact of a Cp enhancement on a CSTP plant cost 182 5.1.3. Aim and objectives 185

5.2. Materials and Methods 186 5.2.1. Synthesis of nanofluids 186 5.2.2. DSC Measurements 188

5.3. Specific heat of nitrate base nanofluids 194 5.3.1. Results 194 5.3.2. Discussion 207 5.3.3. Results new synthesis method 212 5.3.4. Discussion new synthesis method 215 5.3.5. Conclusions 226

5.4. Latent heat of nitrate base nanofluids 227 5.4.1. Results 232 5.4.2. Discussion 235 5.4.3. Conclusions 240

5.5. Stability of the molten salt nanofluid 241 5.5.1. Motivation 241 5.5.2. Materials and methods 242 5.5.3. Results and discussion 243 5.5.4. Conclusions 250

5.6. Summary and conclusions 251

6 General Conclusions and Future Challenges 253

References 259

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List of Figures

Figure 1-1: Schematic representation of the study presented ....................................... 11

Figure 2-1: Classification of performance enhancement techniques. .......................... 15

Figure 2-2: Schematic representation of the study presented with the contents of

this chapter marked in yellow. ............................................................................................. 16

Figure 2-3: Heat of fusion vs. melting point for metals, fluorides, chlorides, nitrates

and other salts (data extracted from 11). .............................................................................. 18

Figure 2-4: Schematic representation of a typical tube and housing thermal storage

unit .............................................................................................................................................. 20

Figure 2-5: MgZn alloying with Aluminum crucibles. ................................................... 23

Figure 2-6: Lead (Pb) and Tin (Sn) phase change properties: onset melting

temperature (Pb and Sn, left), and lead melting and freezing latent heat (right) ..... 24

Figure 2-7: Specific heat of Pb and Sn ................................................................................. 25

Figure 2-8: KNO3-KCl-KBr phase diagram, composition (1) and (2) .......................... 28

Figure 2-9: NaNO3-NaCl-Na2CO3 phase diagram, composition (3) and (4) .............. 29

Figure 2-10: NaNO3-NaBr-NaCl phase diagram, composition (5) and (6) ................. 29

Figure 2-11: Initial mixture of components (left), manual milling (center) and

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melting in a beaker on a hot plate (right) ........................................................................... 32

Figure 2-12: KNO3 heat flow DSC curves during three subsequent melting/freezing

loops at 10 ºC·min-1 showing the solid-solid transition peaks (a)heating, (d)cooling

and the solid-liquid transition peaks for (b)melting (c)crystallization. Water is

released during the 1st heating ramp, leading to a reduction in the area integrated in

peak (a). ...................................................................................................................................... 35

Figure 2-13: KNO3-KCl-KBr (1) melting curves at 5 ºC·min-1 ........................................ 37

Figure 2-14: KNO3-KCl-KBr (1) freezing curves at 5ºC·min-1......................................... 38

Figure 2-15: Heat flow curves for three subsequent loops of KNO3-KCl-KBr (1);

freezing curves at 1) 20ºC·min-1, 2) 20ºC·min-1, and 3) 2ºC·min-1. Top curves

represent cooling (heat is released during crystallization) and bottom curves

represent heating (heat is absorbed during melting). ..................................................... 39

Figure 2-16: KNO3-KBr-KCl (2) DSC curves heating/cooling at 20ºC·min-1: (a)/(d)

solid-solid transition and (b)/(c) solid-liquid transition. ................................................. 40

Figure 2-17: KNO3 – KCl (6 mol%) heat flow curve, solid-liquid transition. ............. 43

Figure 2-18: Heat flow curve of mixtures (3) and (4) and NaNO3 while heating at 20

ºCmin-1 ........................................................................................................................................ 46

Figure 2-19: Durferrit heat flow curves: phase change evaluation (left) and

variability between samples (right) ..................................................................................... 47

Figure 2-20: Latent heat and melting temperature of several PCM tested. ................ 51

Figure 3-1: Schematic representation of a PCM-capsule ................................................ 54

Figure 3-2: KNO3 encapsulated particles53,54 ...................................................................... 57

Figure 3-3: Encapsulated PCM: Encapsulated NaCl-MgCl2 (left)65; Encapsulated

MgCl2 (center)66; Encapsulated Ternary carbonate eutectic (lithium, sodium and

potassium carbonates) (right)67 ............................................................................................. 60

Figure 3-4: Vshell/Vpcm ratio vs Rpcm/Rcapsule for spherical and cylindrical capsules ...... 61

Figure 3-5: High temperature PCM successfully encapsulated .................................... 64

Figure 3-6: Shell/solid PCM volume ratio vs. capsule external diameter. .................. 65

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Figure 3-7: Shell materials used to encapsulate high temperature PCM. (*Present

work) .......................................................................................................................................... 67

Figure 3-8: SWOT (Strengths-Weaknesses-Opportunities-Threats) analysis

performed to evaluate potential of borosilicate as a PCM shell material. .................. 69

Figure 3-9: Shaping a spherical capsule performed at the University of Zaragoza –

glass blowing service. ............................................................................................................. 71

Figure 3-10: Pre-shaped capsules (left) and local stresses in the sphere (right) ........ 72

Figure 3-11: Annealing treatment of the pre-shaped capsules performed at the

University of Zaragoza glass blowing service. ................................................................. 72

Figure 3-12: Crucibles created for the capsule filling and filled capsule performed at

the University of Zaragoza glass blowing service. .......................................................... 73

Figure 3-13: Capsule closure procedure performed at the University of Zaragoza

glass blowing service. ............................................................................................................. 73

Figure 3-14: Pressure inside the capsules for different PCM and different capsule

filling percentages (in solid state) at room temperature (worst-case scenario, no

vacuum inside). The dots represent the expected pressure inside the capsules

manufactured ........................................................................................................................... 76

Figure 3-15: NaNO3 capsules after the last heat treatment (1-2-3-4) ............................ 78

Figure 3-16: Durferrit capsules after the last heat treatment, capsules (5-6-7) ........... 78

Figure 3-17: Capsules after the last heat treatment: lead capsules (11-12-13) (left

image); tin (14) and KNO3 (10) capsules (right image) ................................................... 79

Figure 3-18: Intermediated relative pressure before the blow heater vs. volumetric

flow rate (liters per minute, LPM) ....................................................................................... 81

Figure 3-19: Experimental set-up schematic. All dimensions are in mm. .................. 81

Figure 3-20: Measured temperature profile in the experimental set-up: average

temperature thermocouple 1 to 3, 4 to 6, and capsule estimated temperature. ........ 82

Figure 3-21: Schematic representation of the experimental set-up installation ......... 83

Figure 3-22: Initial experimental set-up installation performed at the laboratories of

Yflow Sistemas y Desarrollos, S.L. under the collaboration with Prof. I. G.

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Loscertales from the University of Malaga. ....................................................................... 83

Figure 3-23: Schematic of the expected thermal response of the set-up ...................... 84

Figure 3-24: Temperature vs. Voltage applied for different experiments performed

with the set-up at different volumetric flow rates (LPM, liters per minute). The

melting temperatures of two PCM (Durferrit, NaNO3) are shown as reference. ..... 85

Figure 3-25: Capsule temperature measured and approximated equation adjusted

to be used in the capsule model for two experiments ..................................................... 86

Figure 3-26: Total optical transmittance of borosilicate.71 ............................................... 87

Figure 3-27: NaNO3 capsule freezing in free flow: Image A (top left, 400 s) shows a

completely liquid salt-borosilicate capsule, while image B (top right, 410 s) shows

the beginning of the salt crystallization process, as marked by a change in slope in

the IR camera temperature traces (bottom right). The different temperature traces

correspond to the locations numbered 1-5 in the IR image (bottom left). .................. 89

Figure 3-28: Sequential images of a NaNO3 capsule during the solidification process

as recorded with a visual camera (top) and an IR camera (bottom). ........................... 90

Figure 3-29: NaNO3 capsule freezing with test duct: IR camera temperature trace

and video snapshots during the liquid-solid phase change highlighted in the red

area. ............................................................................................................................................. 91

Figure 3-30: NaNO3 capsule freezing with test duct ....................................................... 92

Figure 3-31: Capsule temperature trace and sequential images from IR camera (top

row) and visual camera (bottom row) of a NaNO3 capsule during the melting

process under free flow heating, showing: A melting starts at the upper border, B

melting extends over the complete capsule external surface, C, D, E during the

melting process, F completely liquid capsule after melting ends. ................................ 93

Figure 3-32: Visual and IR images at different stages of the melting process of a

NaNO3 capsule inside a borosilicate duct. ......................................................................... 94

Figure 3-33: NaNO3 capsule melting with test duct recorded with visual and IR

camera. Slope changes in the IR temperature traces correspond to melting. ............ 95

Figure 3-34: Tin (Sn) capsule. Change in reflectivity when melting. Completely

solid Sn (left), during the solid-liquid phase change (middle), and completely liquid

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Sn (right). ................................................................................................................................... 96

Figure 3-35: Determination of the phase change process for melting and freezing

experiments on the Pb-Capsule number 11 based on the IR camera’s temperature

curves. ........................................................................................................................................ 97

Figure 3-36: Comparison of IR camera temperature traces from Pb and NaNO3

PCM capsules during sequential melting and solidification in free flow experiment.

Shadowed area represents the standard deviation due to capsule surface location.

..................................................................................................................................................... 98

Figure 3-37: Thermocouple temperature (red), IR camera temperature (blue) and

derivative of the IR temperature (green).......................................................................... 101

Figure 3-38: Recorded images of the melting experiment number 6. At t~60 seconds

the melting process seems to begin, confirmed by the IR temperature. ................... 102

Figure 3-39: Example of a Durferrit capsule melting experiment. ............................. 102

Figure 3-40: Example of a Durferrit capsule freezing experiment ............................. 103

Figure 3-41: NaNO3 capsule freezing experiment (Number 15). ............................... 103

Figure 4-1: Schematic of the problem to be solved ........................................................ 111

Figure 4-2: Melting starting time and melting duration time as a function of time

steps. ......................................................................................................................................... 117

Figure 4-3: Start melting and melting duration time as a function of mesh size. ... 118

Figure 4-4: Temperature at a various radial locations as a function of time: Zhao et

al.44 (left) and present work (right). Ni-Zn capsule ........................................................ 119

Figure 4-5: Location of the interface as a function of time: Zhao et al.44 (left) and this

work (right). Ni-Zn capsule ................................................................................................ 120

Figure 4-6: Temperature at a various radial locations as a function of time: Zhao et

al.44 (left) and This work (right). Steel NaCl-MgCl2 capsule. ...................................... 121

Figure 4-7: Location of the interface as a function of time: Zhao et al.44 (left) and this

work (right). Steel-(NaCl-MgCl2) capsule. ....................................................................... 121

Figure 4-8: Borosilicate thermo-physical properties71 (Black dots extrapolated at

higher temperature) .............................................................................................................. 123

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Figure 4-9: Schematic of air temperature boundary condition: previous model vs.

experiment. SM: start melting, EM: end melting............................................................ 125

Figure 4-10: Example of the new boundary condition introduced in the

mathematical model and temperature evolution for different capsule radial

positions ................................................................................................................................... 127

Figure 4-11: Effect of the new boundary condition applied to the capsule in the

present work compared to Zhao’s boundary condition on the temperature profiles

at the center of the capsule (R=0) and at the shell-PCM interface (R=R2) for a fixed

hconv=150 Wm-2K-1. .................................................................................................................. 129

Figure 4-12: Effect of the new boundary condition applied to the capsule in the

present work compared to Zhao’s boundary condition on the temporal evolution of

the melt fraction for two different convective heat transfer coefficients (50 and 150

Wm-2K-1). .................................................................................................................................. 130

Figure 4-13: Dimensionless temperature at the center of the capsule (R=0) and at the

shell-PCM interface (R=R2) for three different capsule radii. ...................................... 131

Figure 4-14: Location of the solid-liquid interface for three different capsule radii

(left) and melt fraction for three different capsule radii (right) ................................... 132

Figure 4-15: Effect of the capsule thickness on the capsule temperature: at the center

(R=0) and at the shell-PCM interface (R=R2). .................................................................. 133

Figure 4-16: Location of the solid-liquid interface vs. time for different capsule

thickness (left); Melt fraction vs. time for different capsule thickness (right) .......... 133

Figure 4-17: Dimensionless temperature for different radial positions: center (R=0),

shell-PCM interface (R=R2) and capsule surface (R=R1) for NaNO3-capsule with

different shell materials. Convective heat transfer coefficient around the capsule 150

W m-2 K-1. ................................................................................................................................. 135

Figure 4-18: Melt fraction vs time (left) and Location of the solid liquid interface

(right) for different shell materials and different convective heat transfer coefficients

around a NaNO3 capsule ..................................................................................................... 137

Figure 4-19: Dimensionless temperature for different radial positions: center (R=0),

shell-PCM interface (R=R2) and capsule surface (R=R1) for a Sn-capsule with

different shell materials. Convective heat transfer coefficient around the capsule 150

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Wm-2K-1. ................................................................................................................................... 138

Figure 4-20: Location of the solid liquid interface vs time (left) and Melt fraction vs

time (right) in the Sn capsule for different shell materials and different convective

heat transfer coefficients around the capsule. ................................................................. 139

Figure 4-21: Melting time duration vs. latent heat times the latent heat of NaNO3

(left); Effect of the latent heat on melt fraction for three different latent heats tested

(right) ........................................................................................................................................ 140

Figure 4-22: Dimensionless temperature of the capsule at the center (R=0), PCM-

shell interface (R=R2) and the capsule surface (R=R1) for two different PCM

(NaNO3 and Sn) encapsulated with borosilicate. Convective heat transfer

coefficient 50 Wm-2K-1 ........................................................................................................... 142

Figure 4-23: Location of the solid-liquid interface vs. time (left) and melt fraction vs.

time (right) for two different PCM (NaNO3 and Sn) encapsulated with borosilicate.

Convective heat transfer coefficient 50Wm-2K-1. ............................................................. 143

Figure 4-24: Start melting and melting duration time for borosilicate capsules with

Sn and NaNO3 as PCM for different convective heat transfer coefficients. ............. 144

Figure 4-25: Melting start time (left) and Melting duration (right) vs. heat transfer

coefficient for different temperature steps, fixed dimensionless melting

temperature θm= 0.5 and time constant τ=45.7 s. ............................................................ 145

Figure 4-26: Melting start time (left) and Melting duration time (right) for different

convective heat transfer coefficients ‘h’ and dimensionless melting temperature θm

for a fixed T=100ºC .............................................................................................................. 146

Figure 4-27: Convective heat transfer coefficient vs. Reynolds number (at 300ºC) 149

Figure 4-28: Experimental infrared melting and freezing curves for a metallic (Pb,

blue) and Salt (NaNO3, brown) borosilicate capsule ..................................................... 153

Figure 4-29: Melting (left) and freezing (right) comparison: experimental infrared

temperature history curves (blue) for a metallic borosilicate capsule compared to

the model results (brown) ................................................................................................... 154

Figure 4-30: Melting (left) and freezing (right) comparison: experimental infrared

temperature history curves (orange) for a NaNO3 borosilicate capsule compared to

the model results (blue and green) .................................................................................... 154

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Figure 4-31: Experimental freezing start time vs. dimensionless phase change

temperature for freezing experiments .............................................................................. 156

Figure 4-32: Model and experimental results comparison: Freezing start time vs

dimensionless phase change temperature for freezing experiments ........................ 157

Figure 4-33: Experimental melting start time vs. dimensionless melting

temperature. ............................................................................................................................ 159

Figure 4-34: Model and experimental results comparison: Melting start time vs.

dimensionless melting temperature. ................................................................................. 160

Figure 4-35: Experimental phase change start time vs. dimensionless phase change

temperature. ............................................................................................................................ 161

Figure 4-36: Model and experimental results comparison: Melting duration time vs.

temperature step applied for different dimensionless melting temperatures and

convective heat transfer coefficients. ................................................................................. 162

Figure 4-37: Double PCM TES solution ............................................................................ 165

Figure 5-1. Main research groups analyzing molten salt nanofluids (main author

marked in red) ........................................................................................................................ 172

Figure 5-2. Experimental studies (39) on molten salt nanofluids measuring the

specific heat capacity of the liquid salt.............................................................................. 174

Figure 5-3. Specific heat enhancement vs. Nanoparticle concentration. ................... 178

Figure 5-4. a) Fractal-like fluid nanostructures formed by nanoparticles in a

conventional nanofluid. b) Fractal-like fluid nanostructures formed by separated

base molten salts in a molten salt nanofluid.132 ............................................................... 180

Figure 5-5. Schematic representation of two possible predicted thermal behavior of

adsorbed layer in nanofluid: a) extended solid-liquid phase transition (left); b)

nanoporous substrate studies and experimental observation (right).133 ................... 181

Figure 5-6. Break-Down TES cost of a 50MWe -TES 15h central receiver CSP

Plant.135 ..................................................................................................................................... 183

Figure 5-7. Break-Down cost of a 50MWe -TES 15h central receiver CSTP Plant.135

.................................................................................................................................................... 184

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Figure 5-8. Comparison between a conventional molten salt Tower (50MW, 15h

TES) CSP plant cost vs. the potential use of a nano-HTF TES with an increase of

25% of the base salt specific heat. ....................................................................................... 185

Figure 5-9. Schematic representation of the synthesis method ................................... 187

Figure 5-10. Neat and nanofluid solar salt water solution after sonication, and flasks

during solvent evaporation ................................................................................................. 187

Figure 5-11. Powder-form nanofluid (1% SiO2 10nm) after synthesis, before and

after scratching ready for testing ....................................................................................... 188

Figure 5-12. Schematic showing onset and peak temperatures, width at half peak

height, and latent heat as determined by the DSC tests. .............................................. 189

Figure 5-13. Specific heat of sapphire: theoretical (blue), measured (red) and

corrected (green) for the temperature range 100 - 250 ºC ............................................. 191

Figure 5-14. Position of nitrate salt inside the aluminum crucibles after testing in the

DSC: salts creep up the crucible walls away from the base center. ........................... 192

Figure 5-15 Standard 40μl aluminum crucible after testing KNO3 ............................ 193

Figure 5-16. Specific heat of neat and nanofluid solar salt in solid and liquid phase.

................................................................................................................................................... 194

Figure 5-17. Average specific heat of neat solar salt vs. solar salt nanofluid (10 and

30 nm SiO2 nanoparticles at a concentration of 1 wt %) ............................................... 195

Figure 5-18. Freezing (left) and melting (right) average latent heat of the neat salt

and nanofluids synthesized. Measurements performed at 20 ºC/min and 5 ºC/min

respectively. ............................................................................................................................ 197

Figure 5-19. Effect of 1% of SiO2 (10 nm diameter) nanoparticles on the specific heat

(average 260 – 400ºC for mixtures and 350 – 400ºC for pure components) for

different base fluid mixture compositions. ...................................................................... 198

Figure 5-20. Neat and nanofluid (1% SiO2 10 nm in diameter) specific heat heat

(average over 260 – 400ºC for mixtures and 350 – 400ºC for pure components) vs.

base fluid composition (NaNO3 wt %) in a NaNO3-KNO3 mixture. ......................... 198

Figure 5-21. Specific heat capacity vs. nanoparticle concentration by mass (10 nm

diameter SiO2 nanoparticles) for two different base fluids Na-KNO3 (60-40 wt. %,

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solar salt) and Na-KNO3 (30-70 wt. %) (at 400ºC) .......................................................... 200

Figure 5-22. Na-KNO3 (60-40 wt. %, solar salt) neat and nanofluid, specific heat for

different SiO2 (20 - 60 nm in diameter) nanoparticles (0 – 0.5 – 1 – 3.21 – 5.35 wt. %)

at 400 ºC. .................................................................................................................................. 201

Figure 5-23. Effect of 1 wt. % of nanoparticles (CuO and Al2O3) on the specficic heat

of the eutectic NaNO3-KNO3 (45-55 wt. %) (average over 260 – 400ºC) .................... 202

Figure 5-24. SEM (left) and SEM-EDS analysis (right) of the neat NaNO3-KNO3 (30-

70 wt. %) ................................................................................................................................... 203

Figure 5-25. SEM (left) and SEM-EDS analysis (right) of the nanofluid 1% SiO2 (10

nm) NaNO3-KNO3 (30-70 wt. %)........................................................................................ 204

Figure 5-26. SEM images of the nanofluid 1% SiO2 (10 nm) NaNO3-KNO3 (30-70 wt.

%) ............................................................................................................................................... 204

Figure 5-27. SEM (left) and SEM-EDS analysis (right) of the nanofluid NaNO3-

KNO3 (30-70 wt. %) with 5% SiO2 (10 nm) ....................................................................... 205

Figure 5-28. SEM images of the nanofluid NaNO3-KNO3 (30-70 wt. %) with 5% SiO2

(10 nm) ..................................................................................................................................... 205

Figure 5-29. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic

NaNO3-KNO3 (45-55 wt. %) with 1% CuO ...................................................................... 206

Figure 5-30. Vial (left) vs. larger surface area evaporation receptacles such as glass

petri-dish (center) and steel pan (right, from Schuller et al. (2012)139). Larger surface

area leads to a shorter evaporation times. ........................................................................ 212

Figure 5-31. Different areas tested on a petri-dish. Solar salt nanofluid with 1wt. %

of SiO2 (10 nm) ........................................................................................................................ 213

Figure 5-32. Solar salt nanofluid (1 wt. %, 10 nm SiO2 nanoparticles) specific heat

results vs. Temperature synthesized using petri-dish (PD) evaporation. Neat solar

salt is mixed and evaporated in a vial. .............................................................................. 214

Figure 5-33. Latent heat of Type A and B nanofluid vs. Neat solar salt .................... 217

Figure 5-34. Freezing onset temperature difference between Type A and B

nanofluid. ................................................................................................................................ 219

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Figure 5-35. Specific heat and latent heat results of the base salt and nanofluid (1%

of SiO2 10nm) synthesized using petri-dish and mixing completely the entire batch

of salt before testing instead of selectively choosing samples. .................................... 221

Figure 5-36. Specific heat and latent heat results of the neat solar salt (vial) and the

neat solar salt synthesized with petri dish selecting the highest and lowest Cp

value. ........................................................................................................................................ 222

Figure 5-37. Heat flow curves vs. temperature for the sample with the maximum

and minimum Cp synthesized with Petri dish for a representative test from the

data shown in Figure 5-36. .................................................................................................. 223

Figure 5-38. Reported effect of nanoparticles on the latent heat of fusion of organic

PCM .......................................................................................................................................... 228

Figure 5-39. Summary of the effect of nanoparticle on the latent heat of high

temperature inorganic PCM ............................................................................................... 229

Figure 5-40. Phase diagram NaNO3-KNO3 from Factsage43 highlighting the

compositions tested: a hypoeutectic at 34 mol% NaNO3, the eutectic at 49 mol%

NaNO3, a hypereutectic at 64 mol % NaNO3, and the pure components KNO3 and

NaNO3. ..................................................................................................................................... 230

Figure 5-41. Example of DSC cooling and heating curve example, showing

temperature vs. time in blue plotted on the right axis and heat flow vs. time in

green plotted on the left axis. .............................................................................................. 231

Figure 5-42. Heat flow curves vs. temperature for the different compositions tested.

................................................................................................................................................... 232

Figure 5-43. Latent heat of fusion results for different NaNO3-KNO3 mixtures ..... 233

Figure 5-44. Latent heat vs. nanoparticle concentration for different NaNO3-KNO3

mixtures ................................................................................................................................... 234

Figure 5-45. Latent heat vs. NaNO3 content in a Na-KNO3 mixture for different

nanoparticle concentrations ................................................................................................ 234

Figure 5-46. Normalized latent heat vs. nanoparticle concentration for different

NaNO3-KNO3 mixtures. Theory corresponds to a simple mixing rule calculation.

................................................................................................................................................... 235

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Figure 5-47. Neat salt and nanofluids (dissolved in water) after sonication. ........... 243

Figure 5-48. The nanofluid powder after solvent water evaporation is ground in a

mortar and placed in smaller vials for the thermal cycling tests ................................ 244

Figure 5-49. Solid-state nanofluid before melting, from left to right: neat salt

NaNO3-KNO3 eutectic, SiO2 nanofluid, Al2O3 nanofluid, CuO nanofluid, and CNT

nanofluid with gum arabic (GA) dispersions. ................................................................ 244

Figure 5-50. Molten state nanofluid during the first melting process on the hot plate

at 350ºC from left to right: neat salt NaNO3-KNO3 eutectic, SiO2 nanofluid, Al2O3

nanofluid, CuO nanofluid, and CNT with gum arabic nanofluid. ............................ 245

Figure 5-51. Difference between SiO2, Al2O3, and CuO nanofluids after 6 melting

and freezing cycles. ............................................................................................................... 245

Figure 5-52. Molten SiO2-nanofluid on a hot plate at 350ºC after 2, 3, 5 and 6 (left to

right) thermal cycles. ............................................................................................................. 246

Figure 5-53. Molten Al2O3-nanofluid (left) and in the hot plate at 350ºC. CuO

nanofluid (right) in molten state and solidified after several freeze/thaw cycles

showing nanoparticle stratification ................................................................................... 247

Figure 5-54. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic

NaNO3-KNO3 (45-55 wt. %) with 1% CuO after the stability test .............................. 248

Figure 5-55. SEM analysis of the nanofluid eutectic NaNO3-KNO3 (45-55 wt. %)

with 1% CuO after the stability test ................................................................................... 248

Figure 5-56. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic

NaNO3-KNO3 (45-55 wt. %) with 1 wt. % of SiO2 after the stability test ................... 249

Figure 5-57. SEM analysis of the nanofluid eutectic NaNO3-KNO3 (45-55 wt. %)

with 1 wt. % of SiO2 after the stability test ....................................................................... 249

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List of Tables

Table 1-1. List of commercial CSP central receiver plants with TES operating, under

construction (UC) or under development (UD).13 ............................................................. 6

Table 2-1: Desirable PCM properties.7,17,24 .......................................................................... 17

Table 2-2: Metals price29 ......................................................................................................... 21

Table 2-3: Estimated specific cost of different metal alloys and cost per thermal

storage unit ............................................................................................................................... 22

Table 2-4: Average (cycle 2 to 8) onset melting temperature and melting latent heat

for lead and tin (first melting discarded). .......................................................................... 24

Table 2-5: MgZn measured onset, peak, and endset temperature (melting and

freezing) at 10ºC·min-1 during ten subsequent heating and cooling ramps. .............. 26

Table 2-6: Melting temperature and latent heat of Mg-Zn alloys, literature values

vs. measurements. ................................................................................................................... 26

Table 2-7: Nitrate based compositions to be tested and predicted melting

temperature. *Estimated based on FactSage43 , **literature value 41,44,45 ...................... 27

Table 2-8: Purity grade and supplier of the chemical components used in this study,

and estimated market price ................................................................................................... 31

Table 2-9: KNO3 measured latent heat, onset, peak, and endset temperatures

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(melting and freezing) at 10ºC·min-1 during three subsequent heating and cooling

ramps. ......................................................................................................................................... 34

Table 2-10: Heating rate effect on phase transition properties: KNO3 measured

latent heat, onset, peak, and endset temperature (melting and freezing) at 10

ºC·min-1 and 20 ºC·min-1 .......................................................................................................... 36

Table 2-11: KNO3-KCl-KBr(1) measured latent heat, onset, peak and endset

temperature (melting and freezing) at 5 ºCmin-1 (average values of three samples,

5th thermal cycle) ...................................................................................................................... 37

Table 2-12: KNO3-KBr-KCl (2) solid-solid transition measured values ...................... 41

Table 2-13: KNO3-KBr-KCl (2) solid-liquid transition measured values .................... 41

Table 2-14: KNO3 – KCl (6 mol %) solid-liquid transition measured values ............. 42

Table 2-15: Summary of solid-liquid transition properties for KNO3 vs three

different mixtures at a heating rate of 20ºC/ min (except for mixture (1) measured at

10 ºC/min). All compositions are in wt. % ......................................................................... 44

Table 2-16: DSC analysis of NaNO3 (melting and freezing results). Average values

of three different samples. ..................................................................................................... 45

Table 2-17: Melting DSC analysis: comparison between pure NaNO3 vs mixtures

(3), (4), (5), (6) (20 ºC·min-1). Average values of three different samples. .................... 45

Table 2-18: Freezing DSC analysis: comparison between pure NaNO3 vs mixtures

(3), (4), (5), (6) (20 ºC·min-1) .................................................................................................... 46

Table 2-19: Average results for four Durferrit samples .................................................. 48

Table 2-20: Specific cost analysis of the different mixtures calculated with the

experimental latent heat measurement. (*FactSage, **literature) ................................. 49

Table 3-1: High temperature PCM candidates34 ............................................................... 58

Table 3-2: Summary of experimental studies on high temperature encapsulated

PCM. Spherical capsules are marked in blue, cylindrical capsules in black. ............. 61

Table 3-3: Solid and liquid PCM density and calculated amount of PCM added to

each borosilicate capsule type. (*measured) ...................................................................... 74

Table 3-4: List of spherical PCM capsules manufactured included in this study ..... 77

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Table 3-5: Variability of the capsule thickness .................................................................. 79

Table 3-6: Melting experiments summary (NaNO3 Tm 302ºC) .................................... 100

Table 3-7: Freezing experiments summary.*SF: start Freezing ................................... 104

Table 4-1: Comparison Zhao et al. vs. present work Zn-Ni capsule (50 mm in

diameter) ................................................................................................................................. 119

Table 4-2: Comparison Zhao et al. vs. present work Steel-NaCl-MgCl2 capsule .... 120

Table 4-3: Thermo-physical properties used in the model. (*Measured) ................. 122

Table 4-4 Properties of the HTF (air) used in the experiments to estimate the

convective heat transfer coefficient used in the model.85.............................................. 124

Table 4-5: Melting start time and melting duration for three different capsule sizes.

................................................................................................................................................... 131

Table 4-6: Shell material properties ................................................................................... 134

Table 4-7: Melting start time and melting time duration for different convective

heat transfer coefficients and different shell materials. NaNO3 as PCM. ................. 136

Table 4-8: Melting start time and melting time duration for different convective

heat transfer coefficients and different shell materials. Tin (Sn) as PCM. ................ 138

Table 4-9: Melting start time and melting time duration for three different latent

heats tested. ............................................................................................................................. 140

Table 4-10: Nusselt number and convective heat transfer coefficient using different

correlations .............................................................................................................................. 149

Table 4-11: Estimated experimental convective heat transfer coefficient that will

make the simulations fit the experimental times. .......................................................... 163

Table 4-12: Material properties ........................................................................................... 165

Table 5-1. Number of publications on the specific heat capacity of high temperature

nanofluids. .............................................................................................................................. 174

Table 5-2. Review on molten salt nanofluids including the base fluid, nanoparticle

concentration and specific heat enhancement. Nitrate, when not specified, refers to

solar salt (NaNO3-KNO3 60-40 wt. %). Studies marked in blue correspond to

references that do not belong to Texas A&M University. ............................................ 175

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Table 5-3. Hypothesis test between the base solar salt and solar salt nanofluid (1 wt.

% 30 nm SiO2 nanoparticles) ............................................................................................... 196

Table 5-4. Onset and peak melting and freezing temperatures .................................. 197

Table 5-5 Specific heat of neat salt and nanofluid (1 wt. % SiO2 10 nm

nanoparticles) modifying the base fluid (average over 260 – 400ºC for mixtures and

350 – 400ºC for pure components) ..................................................................................... 199

Table 5-6. Properties of common nanoparticle materials (bulk properties).90.......... 207

Table 5-7. Solar salt specific heat measurement in molten state by different authors.

(two step nanofluid synthesis procedure) ....................................................................... 208

Table 5-8. Chieruzzi et al. (2013)110 results for solar salt modified with different

nanoparticle types and concentrations. ............................................................................ 210

Table 5-9. Differences in the solar salt nanofluid synthesis procedure among

researchers. Enhancement at 1wt. % of nanoparticles concentration if not specified.

.................................................................................................................................................... 211

Table 5-10. New synthesis method nanofluids - Cp results ......................................... 214

Table 5-11. Type A nanofluid SS Vial 1%SiO2 (10 nm) vs neat solar salt .................. 215

Table 5-12. Type A vs Type B nanofluid SS Vial 1%SiO2 (10 nm) .............................. 215

Table 5-13. Possible explanation regarding the specific heat enhancement ............. 217

Table 5-14. New synthesis method nanofluids – Solid-liquid phase change results

.................................................................................................................................................... 218

Table 5-15. Hypothesis testing for the onset freezing temperatures for neat solar salt

vs. type A and type B nanofluid ......................................................................................... 219

Table 5-16. Onset melting and freezing temperatures values corresponding to the

tests shown in Figure 5-35 ................................................................................................... 221

Table 5-17.Maximum and minimum Cp neat solar salt (PD) sample compared to

the average values of the neat SS (vial) ............................................................................. 222

Table 5-18. Specific heat results: present work vs. Andreu-Cabedo et al. (2014). Neat

solar salt vs. solar salt nanofluid (1 wt. % of SiO2 nanoparticles). *Type A

nanofluid. ................................................................................................................................ 224

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Table 5-19. Solubility of nitrate salts in water.141 ............................................................. 225

Table 5-20. NaNO3-KNO3 compositions tested .............................................................. 230

Table 5-21. Slope of the linear fit of the normalized latent heat vs. the nanoparticle

concentration. ......................................................................................................................... 236

Table 5-22. Estimated thickness (∆) of the hypothetical interfacial liquid layer that

could explain the larger reduction on the latent heat observed in nitrate base

nanofluids. .............................................................................................................................. 239

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Nomenclature

Abbreviations

CSP Concentrated solar power

DSC Differential Scanning Calorimeter

DSG Direct Steam Generation

HTF Heat transfer fluid

PCM Phase change materials

SEM Scanning Electron Microscope/Microscopy

TES Thermal energy storage

Symbol

Unit

Meaning

Latin symbols

BR - blockage ratio

Cp J/(kg K) Specific isobaric heat capacity

D m Diameter

e m thickness

h W/(m2 K) Convective heat transfer coefficient

k W/(m K) Thermal conductivity

Kg/s Mass flow

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Nu - Nusselt numbers

P Bar Pressure

Pr - Prandtl

R m Radius

Re - Reynolds

T ºC/K Temperature

V m3 Volume

w - Weight fraction

Greek symbols

α m²/s Thermal diffusivity

η - Thermal efficiency

ρ kg/m³ Density

∆ nm Width of the interfacial liquid layer

∆T ºC Temperature difference

θ - Dimensionless temperature

μ kg/(s·m) Dynamic viscosity

Indices

bf Base fluid

C cold

H hot

i Interfacial liquid layer

nf Nanofluid

nm Nanomaterial

np Nanoparticle

p particle

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1

1 INTRODUCTION

he increase in the global energy demand in the last decades, linked to the

rapid growth of the population has resulted in a human activity strongly

dependent on fossil fuels. Nowadays, eighty percent of the present

worldwide energy use is based on fossil fuels according to the World Energy

Council.1

This situation of unprecedented energy consumption has led to concerns about the

security of the energy supply in the midterm. As a response to this energy context,

the big challenge for the society is to achieve sustainable development supported

by the use of clean and renewable energy. The production of clean energy using

renewable energy sources is crucial and it is a necessary component to address the

climate change issue.

From this perspective, solar energy stands as the most abundant permanent energy

resource on earth. The potential of the solar energy becomes even clearer when the

world’s total annual primary energy consumption is compared with total annual

solar radiation falling on the earth, being the latter more than 7500 times higher.1 Its

endless nature makes it a very interesting energy source in the development of

renewable energy.

T

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Introduction

2

1.1 Concentrating Solar Thermal Power Systems

Concentrating solar thermal technologies are based on the concept of concentrating

the radiation from the sun using reflective mirrors in order to achieve a high

temperature heat source. This heat source is used to generate electricity through

thermal power cycles.

The different concentration technologies can be classified depending on how the

solar radiation is focused: line-focus (linear Fresnel and parabolic trough) and

point-focus technologies (dish-Stirling and CSP central receiver plant). The main

differences between these two technologies are the sun tracking system and

concentration factor, defined as the ratio between the collector aperture area and

the receiver aperture area. Whereas line-focus systems track the sun along a single

axis achieving a low concentration factor (i. e. approximately 30 for Linear Fresnel),

point focusing systems track the sun along two axis and present considerable

higher concentrating factors. The concentration factor is directly related to higher

working operation temperatures which in turn define the maximum possible

thermodynamic cycle efficiency by Carnot’s theorem:

Equation 1-1

Where TC and TH are the sink and source temperatures respectively. This leads to a

higher efficiency in converting heat into mechanical motion and, hence, to

electricity. This means that point-focus systems can convert into electricity a larger

fraction of the energy that falls on the receiver than linear systems.

According to the International Energy Agency2,3 the central receiver CSP

technology is the one with the highest outlook for improvements linked to the

increase in the maximum operation temperature. Although at the present time line-

focus systems are a more mature technology, tower technology could be the

winning concept in a near future when some challenges such as the receiver

technology and the stability of the fluid flowing through it, called heat transfer

fluid (HTF), are overcome. The new development relies on the use of advanced

power cycles and new heat transfer fluids capable of operating at higher

temperatures.

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3

Thermal Energy Storage for High Temperature Applications

There are, however, other more important challenges to be solved before solar

power becomes a feasible alternative capable of, for example, supplying the base

load of a whole country. The main challenge of solar energy nowadays is its

intrinsic intermittency, which makes it difficult to match supply and demand at

any time. The variability of the heat source, unlike traditional systems such as

conventional power plants, comes from the fluctuation of the solar radiation due to

the day and night cycle and intermittencies in cloudy periods. Therefore, a lot of

efforts are being put on trying to decouple power output and solar radiation. There

is only one known way to achieve this: through energy storage.

In this aspect, concentrating solar thermal technology is one step ahead of other

technologies such as photovoltaics: maybe due to the historical predominance of

heat utilization for different purposes, heat storage is a rather well-known issue

and poses nowadays minor technological problems when compared to the still

incipient –but very promising- electricity storage in batteries. Thanks to this

advantage, together with the higher conversion efficiencies compared to

photovoltaics, concentrated solar thermal power is bound to play a major role in

the solar race.

1.2 Thermal Energy Storage (TES)

A thermal energy storage system consists of an insulated container and a storage

medium which absorbs (releases) heat from (to) the heat transfer fluid. Due to the

fact that a CSP plant uses thermal energy as a primary energy form, thermal energy

storage can be direct and more easily integrated than other systems. The energy can

be stored through different mechanisms, described in the following section.

The design and operation of thermal energy storage in a solar thermal power plant

requires specific strategies for the accumulation of energy and production of

electricity depending on the season, location, climatology, number of sun light

hours, and thermal energy storage type and capacity. Nevertheless, it allows a

more manageable electricity production, reduces the transient periods in the plant

operation, and the operation time can be extended beyond the hours of sunlight.

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Introduction

4

1.3 Mechanisms to store thermal energy

There are three main types of thermal energy storage (TES) according to the storage

mechanism involved: sensible heat storage, latent heat storage and thermo-

chemical storage.

Sensible heat storage: this type of storage relies on a TES medium’s

temperature change to store energy. The energy is stored/ recovered by

heating up/cooling down the TES medium. The amount of thermal energy

stored in the system depends on the temperature change, the mass, and

specific heat of the energy storage material.

Latent heat storage: this system utilizes the energy change involved in a

phase change transformation to store and release heat. The large enthalpy

change in these transformations provides a high storage density. This type

of TES system usually operates over a much narrower temperature range

than those of the sensible heat storage systems because the phase change

transformation occurs isothermally. Among the different phase change

transformations, latent heat storage by solid–liquid transformations

provides high energy storage density with a relatively low change in

volume.

Thermo-chemical storage: this system utilizes the solar energy to drive

reversible chemical reactions which store energy in chemical bonds. When

a chemical TES system is discharged, the chemical bonds are broken and

the thermal energy can be extracted as needed.

So far, the few thermal storage systems introduced in some solar plants have

followed the path initiated by Solar Two4 in the nineties: they store thermal energy

in form of sensible heat, i.e. as a temperature increase of the storage material.

Lately, the search for improvements in the field of thermal energy storage is

shifting interest towards new solutions, namely to latent heat thermal energy

storage, based on the solidification and melting of the storage material and not on

its temperature change. In comparison with sensible heat technology, latent heat

features a much higher energy density, which in turn means smaller volume of

material and lower heat losses. It also works at a nearly constant temperature,

something that can be especially interesting for the evaporation of steam in a

Rankine cycle. The development of the latent heat technology represents the

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5

Thermal Energy Storage for High Temperature Applications

cutting edge in the field of thermal energy storage and has the potential to increase

greatly the competitiveness of solar power. For this reason, there is a growing

amount of studies on the possible materials, how to overcome the challenges that

arise when using phase change materials (PCM) for latent heat thermal storage.5–12

1.4 Classification of TES

According to Gil et al. (2010)8 high thermal storage systems can be classified into

active or passive systems:

The active storage system is characterized by forced convection heat transfer

into the storage material. The storage medium itself circulates through a heat

exchanger. Active systems can be subdivided into direct systems using the heat

transfer fluid as the storage medium, which reduced the number of heat

exchangers, and indirect systems where a second medium is used for storing

the heat.

Passive storage systems are generally dual medium storage systems. The

storage medium itself does not circulate, being the HTF which passes through

the storage only for charging and discharging. The HTF carries energy received

from the energy source to the storage medium, which is mainly solid systems

(concrete, PCM…) during charging, and receives energy from the storage

when discharging. The main disadvantages of passive storage systems are on

one hand, that the HTF decreases in temperature during the discharge as the

storage material cools down. On the other hand, the low heat transfer rates

between the storage material and HTF.

Ideally, one would like to use the same medium as HTF, TES material, and

working fluid in the power block, to avoid any irreversibilities associated with the

exchange of heat and to simplify the installation. However, in practice this can only

be achieved with steam and the quality and duration of the heat storage with steam

to this date is poor.

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Introduction

6

1.4.1 TES in commercial central receiver solar power plants

A central receiver solar power plant uses reflective mirrors called heliostats to focus

the radiation from the sun into a small area on top of the tower called the receiver.

A fluid (heat transfer fluid) passes through the receiver absorbing heat. This heat is

used as a heat source in a Rankine Cycle where pressurized water is converted into

superheated steam feeding a steam turbine to generate electricity. This plant

configuration leads to a more compact design where the working fluid piping and

power block are located in a small area.

In the last and present decade some commercial CSP central receiver plants with

thermal storage have started its operation. Data regarding the actual CSP plant

projects that are currently operational or under construction worldwide can be

readily found at the National Renewable Laboratory (NREL) website.13 It is data

compiled by the SolarPACES (Solar Power and Chemical Energy Systems)

organization. It gives a clear picture of the CSP technology maturity, installed

capacity, project location and development trends. Some of the first commercial

plants are presented in Medrano et al.9 describing in more detail the different

technologies and materials used. Table 1-1 summarizes recent commercial solar

power tower plants with TES projects in operation, under construction or under

development.

Table 1-1. List of commercial CSP central receiver plants with TES operating, under

construction (UC) or under development (UD).13

Name Power TES: technology

(hours)

Temperatures Status (Start

Year)

Planta Solar 10

(PS10) (Spain)

11MW Steam

Rankine

(45bar)

50-minute at 50%

load. Steam

accumulator

Water/Steam

(HTF)

250 – 300ºC

(receiver

outlet)

2007

Planta Solar 20

(PS20) (Spain)

20MW Steam

Rankine

(45bar)

50-minute at 50%

load.

Water/Steam

(HTF)

2009

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7

Thermal Energy Storage for High Temperature Applications

Gemasolar

Thermosolar

Plant (Spain)

19.9MW Steam

Rankine Molten Salts (MS)

(15h)

290-565ºC

(receiver inlet

outlet)

2011

Khi Solar One

(South Africa)

50MW Steam

Rankine 2h Steam

accumulator

Water/Steam

(HTF)

2014

Crescent Dunes

Solar Energy

Project (USA)

110MW Steam

Rankine

(115bar)

MS (10h) 2015

Rice Solar

Energy Project

(RSEP) (USA)

150MW Steam

Rankine

(115bar)

MS 287-565ºC

(receiver inlet

outlet)

2016 (UD)

NOOR

III(Morocco)

150MW MS (8h)

2017 (UC)

Qinghai

Delingha Solar

Thermal

Generation

Project (China)

270MW Steam

Rankine MS (3.5h)

2017 (UD)

Atacama1

(Chile)

110MW Steam

Rankine

MS (17.5h) 300-550ºC 2018 (UC)

Redstone Solar

Thermal Power

Plant (South

Africa)

100MW Steam

Rankine MS (12h)

288-566ºC

(receiver inlet

outlet)

2018 (UD)

Copiapó (Chile) 260MW Steam

Rankine

MS (14h) 2019 (UD)

Supcon Solar

Project (China)

50MW Steam

Rankine MS (2.5h)

(UC)

As can be seen from Table 1-1, there are two main TES commercial technologies for

this type of plants: saturated steam storage and molten salts (MS) storage. These

two TES technologies would fall within the active storage systems using the same

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Introduction

8

fluid as heat transfer fluid and thermal storage material. The main difference

between these two technologies is the number of hours of storage due to the

optimization of each technology leads to different storage capacities.14

1.4.2 Steam accumulators

During the normal operation of the plant, part of the high temperature steam

produced in the solar receiver is derived to the insulated stainless steel tanks, called

steam accumulators, instead of being piped to the steam turbine to generate

electricity. This energy is stored in the form of pressurized saturated liquid water at

high temperature.

In special conditions such as transitory cloudy periods without sunshine in which

the vapor from the receiver is insufficient to maintain the turbine operation, steam

is produced by lowering the pressure of the saturated liquid during discharge.

Steam accumulators operate in many industrial processes and also since the fifties

in some conventional thermal power plants. The energy density of this technology

can reach 20-30 kWh/m3.15 PS10 (Seville, Spain), the first commercial solar tower

power plant in the world with a nominal power of 11 MW, is one of the

commercial solar power plants using this technology. The storage provides 50

minutes of storage capacity at 50% load (50 bars and 285 °C) to handle cloud

transients.

The main advantages of the plants incorporating this technology include: a lower

investment and operation and maintenance costs and greater simplicity of the

overall plant configuration, allowing the installation to operate at higher

temperatures compared to facilities using a different heat transfer fluid than water.

Other advantage is the rapid availability of the storage energy.

Since the heat transfer fluid is the same water used in the power block, the

intermediate heat exchanger between the heat transfer fluid and the steam

generation can be eliminated. This results in a higher steam temperatures and

consequently higher power cycle efficiencies. On the other hand instabilities of the

two phase flow inside the receiver together with the increase of the pipe installation

cost because of the high pressure required are the main drawbacks.8

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9

Thermal Energy Storage for High Temperature Applications

1.4.3 Two molten salt tanks direct system

In this thermal storage system molten salts are used as a heat transfer fluid as well

as the storage medium, absorbing the solar thermal energy in the receiver and

storing the fluid in two tanks at different thermal level, one at high temperature

and the other at low temperature, both above the freezing temperature of the salt.

The fluid, known as solar salt, is a well-known off-eutectic mixture of sodium

nitrate (60 wt. %) and potassium nitrate (40 wt. %).

The storage material from the low temperature tank is pumped through the solar

receiver, where it heats up to a higher temperature (565ºC), flowing to the high

temperature tank. The fluid is kept in the hot tank until extra heat is required to

operate the power block. At this point the system is discharged by pumping the

fluid through a heat exchanger where it generates steam for electricity production

while it decreases its temperature. The cycle ends with the fluid in the cold tank as

the original starting point.

The main limitations of this storage system are the high investment costs of the

thermal storage material/HTF, the storage tanks and the heat exchangers.

Furthermore, working with molten salts with relatively high freezing temperature

has the additional risk of solidification of the storage fluid. This increases the

maintenance and operation costs due to the requirement for electric heat tracing on

all salt equipment. On the other hand, the reason why molten salts is the most

widely used thermal storage material in CSP is the combination of heat transfer

and thermo-physical properties together with its very low vapor pressure, which

reduces the thickness of the storage tank and also the piping mechanical stress.

1.5 Main objectives of the thesis and structure

The main objective of the thesis is to improve existing commercial TES solutions for

CSP central receiver plants by increasing their storage density. Two different

research areas can be distinguished: the first part focus on improving the steam

accumulator technology and the second, the two molten salt tanks technology.

The current solution of direct storage in steam accumulators is not cost competitive

for larger storage capacities due to the low volumetric energy density. Research in

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Introduction

10

order to increase the storage density of this solution is directed towards latent

storage materials. Phase change materials (PCM) are studied because of its

potential in the system energy density increase for DSG applications. The thermal

storage mechanism based on phase change transformation is used to store the

evaporation enthalpy of water/steam. The main advantage is a temperature profile

in the storage system matches the temperature profile of the water/steam and

therefore higher efficiencies can be obtained in these systems.

Figure 1-1 shows a schematic representation of the approach followed in this study.

Chapter 2 to 4 aim at developing both materials and systems for thermal energy

storage at high temperature based on PCM. The use of PCM consisting of

encapsulating the PCM to exchange heat through capsules in a packed bed system

is investigated. The idea is to provide a suitable design and performance analysis of

the storage units that will form the storage system. A material screening for PCM

candidates is performed in Chapter 2 combining thermo-physical and economic

criteria. Several potential materials are characterized through a differential

scanning calorimeter. Different encapsulation attempts for high temperature PCM

are reviewed in Chapter 3, addressing the most important challenges in the

development and testing of the thermal storage components. For PCM

encapsulation, a new macro-encapsulation procedure is designed, developed, and

tested. In Chapter 4 a single capsule thermal model is used to analyze the impact of

different parameters on the phase change process of the PCM-capsule. This model

provides valuable information to guide the design of PCM capsules. Capsules are

fabricated and a laboratory set-up is used to test single capsules. The encapsulation

procedure, numerical and experimental validation of the proposed solutions are

presented and discussed.

The second part of this thesis is focus on improving the commercial solution based

on two molten salt tanks, specifically on improving the specific heat of the molten

salt by adding nanoparticles (Chapter 5). We have also investigated the effect of

nanoparticles on the phase change characteristics of the salts. The use of

nanoparticles could be used to improve both systems: nanoparticle addition on

nitrate salts to modify the phase change properties for PCM applications and to

enhance the specific heat for sensible heat applications.

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11

Thermal Energy Storage for High Temperature Applications

Figure 1-1: Schematic representation of the study presented

TES in Commercial CSP

Tower Plants

Steam Acumulator

PCM

Screen & characterize PCM

(Chapter 2)

Packed bed

Macro - encapsulation (Chapter 3-4)

Screen Shell Materials

Fabricate capsules

Model Single Capsule

Testing

Tube & housing

Double PCM (Chapter 4)

Molten salt tanks

Nano enhanced HTF-TES

(Chapter 5)

Specific heat

Latent heat

Stability

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Introduction

12

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13

2 PHASE CHANGE MATERIALS

oday’s thermal energy storage commercial solutions in large scale CSP plants

are based on sensible heat materials such as inorganic molten salts and steam

accumulators. The molten salt storage can provide sufficient thermal energy,

not only to avoid the natural intermittencies of the solar resource but also to

produce power throughout the entire night (~ 15 hours), while steam accumulators,

using sensible heat storage in pressurized saturated liquid water are used for peak

power, although this is an inefficient and not economically attractive solution for

large storage capacities and high pressure. On the other hand, higher energy

density systems utilizing latent heat storage are still being developed.

2.1. Introduction and main objectives

Many latent heat TES have been tested at laboratory scale and in demonstrator

prototypes, but they are not currently cost competitive with sensible heat storage.

The most interesting property of these materials is their ability to absorb and

release heat at almost constant temperature.7 This temperature corresponds to the

phase transition (i.e. melting and solidification) of a phase change material (PCM).

T

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Phase Change Materials

14

This approach is of particular interest in Direct Steam Generation (DSG) solar

thermal systems, which currently do not have a cost-effective storage solution.

Since the major part of the energy is required in the evaporator ~55%16 this

technology might allow a more efficient heat exchange between the storage system

and the water/vapor by maintaining the temperature difference along the heat

exchanger, thus reducing the heat exchanger pinch point.

Abhat (1983)17 described the three challenging elements involving the design of a

latent heat thermal energy storage system:

1. The phase change material: a heat storage substance that undergoes a

solid-to-liquid phase transition within the desired operating temperature

range and where the bulk of the heat added is stored as the latent heat of

fusion.

2. The containment for the thermal storage substance: this could be

micro/macro-encapsulation of the PCM, for example.

3. A heat exchanging surface for transferring heat from the heat source to the

heat storage substance and from the latter to the heat sink. This heat

exchanger is inherently related to the heat transfer fluid (HTF) of choice

(for DSG this HTF is pressurized steam at high temperature, while for

molten salts tower plants the salts are both the TES material and HTF, and

in parabolic trough plants synthetic oil is typically the HTF).

The preferred latent heat TES candidates for DSG are inorganic salts due to their

relatively low cost, high latent heat, and appropriate melting temperature for this

application (between 200-400ºC). However, the main limitation of these PCM is

their low thermal conductivity (on the order of 1 W·(m K)-1)18, which governs the

heat exchange rate between the storage material and the working fluid. The low

thermal conductivity causes:

Long response times in the thermal storage discharge process

High temperature gradients

Limits the heat flux absorbed/ released from the PCM

This low thermal conductivity has resulted in researchers developing different

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15

Thermal Energy Storage for High Temperature Applications

techniques to improve this property. The goal of all these methods is to achieve a

higher thermal conductivity, preferably in the range of 5-20W·m-1·K-1. These

different approaches to improve the thermal performance of PCM TES systems are

presented schematically in Figure 2-1. Fabricating PCM-composite materials with

increased thermal conductivity can be achieved through 1) highly conductive

matrices (carbon foams19, metal foams20, meshes21, wool, sponges) where the

composite is created by infiltrating the molten PCM through the porous matrix;

and 2) dispersing high conductivity fillers (particles, flakes, fibers22 in the PCM,

commonly mixed in solid state at room temperature, although in some cases the

dispersion can be done in molten state or in water suspension/solution.

Alternatively, the thermal conductivity of PCM can be enhanced by adding

traditional extended surfaces to improve the heat transfer. The extended heat

transfer surface techniques include the insertion of fins fabricated with high

thermal conductivity materials (metallic fins on the heat exchanger tubes in contact

with the PCM23 or encapsulating the PCM. Heat transfer effectiveness in this last

solution increases as capsule size decreases, minimizing the influence of a PCM

with low thermal conductivity. PCM encapsulation will be reviewed and analyzed

in detail in Chapter 3. On the other hand, metallic PCM, which present other

drawbacks and generally higher specific cost, have been also studied due to their

higher thermal conductivities.

Figure 2-1: Classification of performance enhancement techniques.

Composite materials with increased

thermal conductivity

Matrix infiltrated with

PCMs

Dispersed filler-PCM

Extended heat transfer surface

Fins

Capsules

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Phase Change Materials

16

2.1.1. Objectives

A material screening for potential PCM candidates is performed in this chapter

(Chapter 2) combining thermo-physical and economic criteria. Several potential

materials are characterized through calorimetry comparing with the literature

values. Figure 2-2 shows a schematic representation of the approach followed in

this study.

Figure 2-2: Schematic representation of the study presented with the contents of

this chapter marked in yellow.

2.2. PCM screening, characterization and selection

An extended literature review has been performed for the PCM selection. The main

criteria are materials with a melting temperature in the range 300-400 ºC and a high

latent heat of fusion. However, there is a long list of appropriate physical, thermal,

chemical and economic properties, often competing with each other. Table 2-1

shows the desirable characteristics that the storage materials must possess to enable

a more cost-effective system.

PCM TES heat

exchanger system

Packed bed

Macro-encapsulation

Screen & characterize

PCMs

Screen Shell Materials

Model Single Capsule

Fabricate capsules

Test single

capsule

Compare Experiments

& Model

Evaluate performance & challenges

Tube & housing

Metal PCM

Double PCM

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17

Thermal Energy Storage for High Temperature Applications

Table 2-1: Desirable PCM properties.7,17,24

Thermal

properties

Physical

properties

Kinetic

properties

Chemical

properties

Economics

(i) Suitable phase-

transition

temperature.

(ii) High latent

heat of transition.

(iii) High thermal

conductivity.

(iv) Thermal

stability.

(i) High density.

(ii) Low density

variation

during phase

change.

(iii) Low vapor

pressure at the

operating

temperature.

(i) Little or no

super-cooling

during

freezing.

(ii) Sufficient

crystallization

rate.

(i) Long-term

chemical

stability.

(ii) Compatibility

between storage

medium and

containment

material.

(iii) No toxicity.

(iv) No fire

hazard.

(i) Abundant.

(ii) Available.

(iii) Cost

effective.

(iv) Minimal

environmental

and health

impact.

Each of the properties in Table 2-1 comes from the inherent drawbacks that appear

when PCM are used in thermal storage systems. For example, heat transfer

properties are important when the PCM starts solidifying on the heat transfer

surface impeding the heat transfer process; the density change between solid and

liquid PCM is an important issue that must be addressed for encapsulated PCM.

Even if some candidates meet most of the requirements, they can be discarded if

the appropriate balance among the different requirements is not achieved. Finding

an ideal PCM that is able to satisfy all these different requirements is very

challenging.

Storage materials at high temperature are mainly inorganic salts or metals, pure or

often eutectic mixtures, which means that these compositions present minimum

melting temperature melting and freezing congruently like a pure substance.

A first list of possible PCM candidates for a specific TES application can be chosen

based on having a melting point in the rage of interest in order to be able to store

the energy coming from a source or process, and a high latent heat of fusion. Many

extended lists of PCM candidates with their melting temperature and latent heat of

fusion can be found in different literature reviews.5,8,25–27 Zalba et al. (2003) 5

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Phase Change Materials

18

presented inorganic substances with potential use as PCM in the temperature

range 100 - 900 ºC. Apart from the melting point and latent heat, Kenisarin (2010)

summarized other thermo-physical properties such as specific heat, conductivity,

safety and compatibility with container materials such as stainless steel. 11

Figure 2-3: Heat of fusion vs. melting point for metals, fluorides, chlorides, nitrates

and other salts (data extracted from 11).

Figure 2-3 summarizes the different PCM candidates (metals, fluorides, chlorides,

hydroxides and nitrates) arranged considering the heat of fusion versus their

melting point. The temperature range used in different CSP plant thermodynamic

cycles has been superposed. For conventional (subcritical) Rankine cycles, the PCM

melting temperature should be in the 250-450ºC range with the highest possible

0

100

200

300

400

500

600

700

800

900

1000

200 300 400 500 600 700 800 900

Heat

of

fusi

on

[k

J·k

g-1

]

Melting Point [ºC]

Alloy Carbonate Chloride Fluoride Hydroxide Nitrate

Rankine

Subcritical

Cycle

Rankine

Supercritical

Cycle

Brayton Cycle

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19

Thermal Energy Storage for High Temperature Applications

heat of fusion and best combination of properties as shown in Table 2-1. However,

for CSP plants with more advanced power cycles such as supercritical-H2O or

Brayton-air cycles, PCM with higher melting temperatures should be utilized.

The different PCM candidates for high temperature applications have been divided

into salts and metals, since each group presents their own challenges and

limitations.

2.2.1. Metallic PCM candidates

Pure metals and metal alloys have been proposed as possible candidates for TES

using their phase change enthalpy between solid and liquid state. The main

components of these alloys are zinc, magnesium, aluminum, copper, silicon and

calcium. The melting point (Tm) in the 250-500ºC temperature range and high latent

heat makes these materials competitive with inorganic salts for the same TES

application. The most interesting property of metallic PCM, which gives them an

advantage over salts, is their high thermal conductivity. This implies higher

charging and discharging rates in the thermal energy storage tank heat exchangers.

As an example, the alloy Al-12wt.%Si with a Tm = 576 ºC and heat of fusion = 560

kJ/kg presents a thermal conductivity of 160 W/(m·K), which is 100 times higher

compared to inorganic salt PCM. The effect of this large difference in thermal

conductivity can be understood based on the comparison performed in Hoshi et

al.18 where the use of lead (Pb) versus potassium nitrate (KNO3) as a phase change

material in a shell and tubes TES configuration (similar to Figure 2-4) was

evaluated (dimensional parameters: ri=0.01 m, ro=0.1 m, ∆T = 10 ºC). The charge of

100MJ·m-3 of thermal energy as a latent heat is 8.25 times faster in the case of lead

compared to the inorganic salt. However, the differences become much higher in

the discharge process. The discharging speed for potassium nitrate is considerably

slower than for lead. The salt system is unable to discharge the same amount of

energy as the metal system even in times that are an order of magnitude higher,

since the discharging time is substantially longer for equal volumetric energy

density. These large discrepancies are caused mainly by the differences in the

thermal conductivity (k) of the two media (kPb is 30 to 60 times higher than kKNO3).

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Phase Change Materials

20

Figure 2-4: Schematic representation of a typical tube and housing thermal storage

unit

On the other hand, the main drawback of using these metallic materials is their

high cost and limited availability in general. Nevertheless, metallic PCM can

become competitive since a simple design and a reduction in the heat transfer area

(less number of pipes embedded in the bulk material) can be achieved for charging

and discharging due to their high thermal conductivity.

In the temperature range of 300-400 ºC one of the interesting PCM candidates is the

Mg-Zn binary system. Rodríguez-Aseguinolaza et al.28 studied this PCM,

specifically Mg-Zn (49 - 51 wt. %). Its thermal diffusivity, heat capacity, and energy

density were measured and compared to nitrate salts. This alloy shows a thermal

diffusivity two orders of magnitude higher than salts, more than two orders of

magnitude higher thermal conductivity, similar latent heat and volumetric energy

density (155 kJ·kg-1, 442 MJ·m-3) to sodium nitrate’s (172 kJkg-1, 389 MJ·m-3 ). The

price of these metals is around ~2 €/kg29,30 compared to ~0.7 €/kg31 for sodium

nitrate. This means that, for these two PCM’s to be comparable, the difference in

price gives the maximum allowable cost of any heat transfer enhancement

PCM

Heat Transfer

Fluid

x

ro

ri

ξ

HTF

solid

liquid

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21

Thermal Energy Storage for High Temperature Applications

technique for sodium nitrate to approximate its heat transfer properties to those of

the alloy.

In Adinberg & Epstein32 Sn-Zn alloys were proposed in order to stabilize the steam

temperature in direct steam solar power plants. The results of this numerical study

showed that the steam sent to a turbine after the thermal buffer may have stable

temperature near 330ºC during the entire charge-discharge cycle. The same idea

was applied in Bellard (2012)33 in concentrated solar power plants using alloy Al-

12wt%Si as PCM in the solar receiver (with a higher melting temperature). This

alloy has been also studied in Yagi et al.34 and Wang et al.35 using a stainless steel

capsule and a clay-ceramic container respectively. Table 2-2 shows the market

prices for some metals in two different dates (at the present time and when this

study was performed) and a thermo-economic evaluation is presented in Table 2-3.

As this data shows, the competitiveness of a metallic PCM is highly dependent on

the fluctuations of the market.

Table 2-2: Metals price29

Price [€/ton]

(accessed 1/11/2012)

Price [€/ton]

(accessed 27/05/2016)

Copper 6017 4208

Zinc 1422 1703

Nickel 12570 7499

Aluminum 1403 1379

Lead 1636 1517

Tin 15761 14551

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Phase Change Materials

22

Table 2-3: Estimated specific cost of different metal alloys and cost per thermal

storage unit

Phase Change Materials Melting

Temperature

Heat of

Fusion Price

Price/Stored

Energy

Composition (wt. %) [ºC] [kJ/kg] [€/kg] [€/kWh]

Sn 227 60 15.07 904.2

Pb 327 23 1.50 234.78

Zn(52)–48Mg 340 180 2.10 41.94

Zn(53.7)–46.3Mg 340 185 2.08 40.42

Zn(96)–4Al 381 138 1.54 40.15

Mg(55)–28Ca–17Zn 400 146 1.75 43.07

Zn 419 112 1.76 56.57

Al(59)–35Mg–6Zn 443 310 1.93 22.46

Mg(60)–25Cu–15Zn 452 254 3.38 47.97

Mg(52)–25Cu–23Ca 453 184 2.94 57.48

Al(65.35)–34.65Mg 497 285 1.93 24.36

Al(60.8)–33.2Cu–6.0Mg 506 365 3.12 30.80

Al(64.1)–5.2Si–28Cu–2.2Mg 507 374 2.88 27.70

Al(54)–22Cu–18Mg–6Zn 520 305 2.75 32.45

Al(68.5)–5.0Si–26.5Cu 525 364 2.77 27.44

Al(64.3)–34.0Cu–1.7Sb 545 331 3.32 36.09

Al(66.92)–33.08Cu 548 372 3.05 29.48

Al(83.14)–11.7Si–5.16Mg 555 485 1.65 12.28

Al(87.76)–12.24Si 557 498 1.60 11.55

Al(65)–30Cu–5Si 571 422 2.94 25.05

Al(46.3)–4.6Si–49.1Cu 571 406 3.81 33.83

Al(86.4)–9.4Si–4.2Sb 575 471 2.15 16.40

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23

Thermal Energy Storage for High Temperature Applications

In the temperature range of interest three metallic PCM have been selected and

characterized: Mg-Zn alloy due to its thermo-economic properties and two pure

metals: lead, also used as PCM in 34,36, and tin. Most of the other alloys from Table

2-3 have higher melting temperatures which may be more convenient for PCM

cascade configurations in the superheated steam region.

A Differential Scanning Calorimeter (DSC1 from Mettler-Toledo) is used to

thermally characterize the PCM candidates. The calibration procedure is performed

by using the melting temperatures and latent heat of standard certified reference

materials (In, Zn), at a heating rate of 10ºC·min-1, resulting within the limits

specified by the equipment manufacturer. The interactions between the crucible

material and the sample tested must be evaluated before the measurements are

performed. Figure 2-5 shows how the solubility of the aluminum crucibles on the

MgZn sample damages the crucible and can damage the DSC sensor. Sapphire

crucibles have been used instead to avoid any possible alloying between the PCM

and crucible.

Figure 2-5: MgZn alloying with Aluminum crucibles.

The preparation of the Mg-52wt%Zn alloy was slightly more cumbersome than

initially anticipated. It was achieved by melting the magnesium at 665ºC before

adding zinc, both of them in the form of solid bricks. The mixture was then melted

and crystallized twice (30 minutes from 300 to 500ºC and 60 minutes from 300 to

400ºC) in a stainless steel A316 crucible cover by Ar-1%SF6 atmosphere.

The melting temperature, the latent heat and specific heat has been measured

through different freezing /melting cycles. The thermal cycle heats up from 230 ºC

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Phase Change Materials

24

to 400 ºC for lead, from 100 ºC to 300 ºC for tin, and 60 ºC to 460 ºC for MgZn alloy,

and cools down to the initial temperature at a heating rate of 10 ºCmin-1. Eight

thermal cycles have been programmed. Nitrogen is used (50 ml·min-1) as over gas.

Figure 2-6: Lead (Pb) and Tin (Sn) phase change properties: onset melting

temperature (Pb and Sn, left), and lead melting and freezing latent heat (right)

Figure 2-6 shows the onset melting temperature (on the left) for tin and lead, and

the melting and freezing latent heat for lead (on the right). The onset melting

temperature is stable over eight thermal cycles. Only the first melting presents

significant differences for both metals (Table 2-4). It might be caused by a larger

energy required to break down the crystalline lattice the first time. This difference

could be also produced by an inadequate contact between the sample and the

crucible before the first melting.

Table 2-4: Average (cycle 2 to 8) onset melting temperature and melting latent heat

for lead and tin (first melting discarded).

Onset Melting T [ºC] Melting Latent Heat [J·g-1]

Average 2-8 melting First melting Average 2-8 melting First melting

Pb 315.5 +/-0.1 317.8 20.9+/-0.1 21.6

Sn 179.2 +/-0.6 182.5 44.4+/-0.7 46.1

150

170

190

210

230

250

270

290

310

330

350

0 1 2 3 4 5 6 7 8 9

On

set

Mel

tin

g T

emp

erat

ure

[ºC

]

Thermal Cycle

Pb Sn

20.5

20.7

20.9

21.1

21.3

21.5

21.7

21.9

0 1 2 3 4 5 6 7 8 9

Lat

ent

Hea

t [J

/g]

Thermal cycle

Pb Melting LH

Pb Freezing LH

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25

Thermal Energy Storage for High Temperature Applications

The expected melting temperature and latent heat of these metals is 231.9 ºC and 59

J/g for tin and 327.5 ºC and 23 J/g for lead. Our measurements indicate a low purity

grade of the metals tested leading to a lower melting point and melting enthalpy.

Only the latent heat of lead is near its theoretical value. The differences between the

freezing and melting latent heat (+2% for freezing) can be neglected.

The specific heat of lead and tin has been also measured for different thermal

cycles (Figure 2-7) using ASTM E1269 protocol37. The measured values in molten

state are slightly higher than the theoretical values in solid state at room

temperature 0.13 J·(gK)-1 for lead and 0.21 J·(gK)-1 for tin. Even though the relative

differences (14% and 31%) for tin and lead respectively are not negligible, the

absolute differences are within the accuracy limits of the DSC.

Figure 2-7: Specific heat of Pb and Sn

The characterization of MgZn alloy has been also performed (table 2-5). The

measured latent heat of this alloy is 152.5 +/- 4.6 J·g-1 which is consistent considering

recent literature values (Table 2-6). The onset melting temperature is 342 ºC for the

first melting changing to 336.6 +/- 0.2 ºC for subsequent melting ramps (average

cycles 3 to 10). The onset freezing temperature is stable (331.1+/-0.1 ºC) from the

0.17 0.17

0.24 0.27

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

160 180 200 220 240 260 280 300 320 340 360 380 400

Sp

ecif

ic h

eat

[J/

(g·K

)]

Temperature [ºC]

Pb 2nd Melt

Pb 3rd Melt

Pb 4th melt

Sn 2nd Melt

Sn 3rd Melt

Sn 4th Melt

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Phase Change Materials

26

first freezing.

Table 2-5: MgZn measured onset, peak, and endset temperature (melting and

freezing) at 10ºC·min-1 during ten subsequent heating and cooling ramps.

Onset

Temperature [ºC]

Peak

Temperature [ºC]

Endset

Temperature [ºC]

Melting 336.6 +/- 0.2

(342 1st melting)

339.2 +/- 0.1

(344.8 1st melting)

343.2 +/- 1.1

(351.6 1st melting)

Freezing 331.1 +/- 0.1 327.1 +/- 0.3 320.9 +/- 0.6

Table 2-6: Melting temperature and latent heat of Mg-Zn alloys, literature values

vs. measurements.

Melting/solidification

Temperature [ºC]

Melting

Latent

Heat [J·g-1]

Present work (Mg-52wt%Zn) 336.6/331.1 152.5

Blanco et al (2014)28 (Mg-52wt%Zn) 342/337 155

Gasanaliev & Gamataeva (2000)38

(Mg-53.7wt%Zn) 340/- 185

Farkas & Birchenall (1985)39 (Mg-52wt%Zn) 340/- 180

Birchenall & Riechman (1980)40 (Mg/Mg2Zn) 343/- 138

As a summary, among the different metallic PCM considered and thermo-

economically compared, three different metals have been characterized with DSC

measuring the phase change properties and specific heat. The pure metals selected

have shown significan impurities content based on its lower latent heat and

melting temperature. On the other hand, the MgZn alloy has shown agreement

with recent literature values reported for the same alloy in terms of latent heat.

However, the similarities in the melting tempearture can only be observed for the

first melting cycle, reducing the melting temperature by ~6ºC for the third and

subsequent melting cycles. The onset freezing temperture is stable upon cycles.

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Thermal Energy Storage for High Temperature Applications

2.2.2. Inorganic salts PCM candidates

Different nitrate based mixtures have been pre-selected as possible encapsulated

PCM candidates with a melting temperature in the range 290-340ºC. Other authors

such as Gomez (2011) and Tamme et al. have also analyzed single to quaternary

salt mixtures.41,42 The nitrate based compositions, with an expected melting

temperature in the desired temperature range, evaluated in this study are

described in Table 2-7. Hydroxide and chloride based mixtures were also initially

considered but discarded due to their corrosiveness and their high hygroscopic

nature which makes them extremely complicated to synthesize and to measure

their properties.24

Table 2-7: Nitrate based compositions to be tested and predicted melting

temperature. *Estimated based on FactSage43 , **literature value 41,44,45

Phase Change Materials Composition (wt. %)

Melting

Point

[ºC]

Heat of

Fusion

[kJ·kg-1]

NaNO3 306* 172**

KNO3 335* 95**

Durferrit (Hitec XL) 120** -

NaNO3, NaCl (4.5) 284** 171**

KNO3, KCl (4.5) 320** 74**

(1) KNO3 (91.43)-KCl (7.32)-KBr (1.25) (wt. %) 306.7* -

(2) KNO3 (80.7)-KBr (11.9)-KCl (7.4) (wt. %) 342.0** 140**

(3) NaNO3 (86.67)-NaCl (8.27)-Na2CO3 (5.06) (wt. %) 358.9* -

(4) NaNO3 (93.11)-NaCl (4.67)-Na2CO3 (2.22) (wt. %) 294.6* -

(5) NaNO3 (90.01)-NaCl (2.64)-NaBr (7.35) (wt. %) 290.5* -

(6) NaNO3 (89.21)-NaCl (1.35)-NaBr (9.44) (wt. %) 290.0* -

Sodium and potassium nitrate are the main components in the different mixtures

evaluated. All mixtures are thermally characterized to confirm the reported data.

These pure components (sodium and potassium nitrate) are measured as a

reference in order to analyze whether the combination with other components

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Phase Change Materials

28

produce significant differences in their phase change properties. DurferritTM

(another commercial name for HitecXLTM) has been also considered as a PCM. This

mixture is composed by NaNO3, KNO3 and Ca(NO3)2 with a very low melting

point, around ~120 ºC.45 It will be used as a proof of concept for the initial

manufactured PCM-capsule because it makes testing at lower temperatures

possible. The initial tests with this salt mixture are used to demonstrate the ability

of the capsule to withstand initial thermal shock and cycling at lower temperatures

before testing other higher temperature PCM. The mixtures (1, 3-6) have been

selected based on their phase change diagrams43 (Figure 2-8 to Figure 2-10), while

looking for mixtures with eutectic points around 300 ºC. The new compositions

also provide some a priori economic benefits such as lowering the price of the

mixtures using carbonate or chloride. The mixture (2) has been evaluated based on

Gomez (2011)41.

Figure 2-8: KNO3-KCl-KBr phase diagram, composition (1) and (2)

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29

Thermal Energy Storage for High Temperature Applications

Figure 2-9: NaNO3-NaCl-Na2CO3 phase diagram, composition (3) and (4)

Figure 2-10: NaNO3-NaBr-NaCl phase diagram, composition (5) and (6)

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Phase Change Materials

30

Zhao et al. 44 also consider the melting temperature and latent heat of some of the

nitrate mixtures in Table 2-7. Different trends can be observed in the values

reported: the addition of a small amount of NaCl to NaNO3 seems to lower the

melting point but the latent heat remains unchanged. On the other hand, the

addition of a similar amount of KCl to KNO3 reduces in the same order of

magnitude the melting point (-15 ºC), although a significant change in the latent

heat is observed. In contrast, the addition of a third component (KBr) to the KNO3-

KCl mixture seems to increase significantly both, the melting point (+22 ºC) and the

latent heat (+66 J·g-1) compared to the binary composition, and +7 ºC and +45 J·g-1

compared to the pure KNO3. These observations indicate that further research is

needed to clarify the discrepancies on these novel compositions on the

melting/solidification characteristics.

Thermo-physical properties are only one criterion to take into account when

selecting an appropriate PCM. Economic considerations also play a major role (see

Table 2-1). The purity, supplier and market price of the different components are

summarized in Table 2-8. The market price of different chemical components has

been obtained based on quotes for large purchase volume (hundreds of tons). A

quick assessment of this information gives an idea of the economic impact on the

final price of the mixture.

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31

Thermal Energy Storage for High Temperature Applications

Table 2-8: Purity grade and supplier of the chemical components used in this study,

and estimated market price

Component Purity Supplier

Market price

(quotes for tons)

[€/kg] 31

NaNO3 Technical

grade

Thermosolar - Crystals, SQM

Northamerica Corporation,

Atlanta, GA

0.77

KNO3 Technical

Grade

Thermosolar - Crystals, SQM

Northamerica Corporation,

Atlanta, GA

1.08

Na2CO3 99.5% Panreac Química S.L.U.,

Barcelona, Spain 0.23

NaBr 99% Panreac Química S.L.U.,

Barcelona, Spain 2.03

NaCl 99% Panreac Química S.L.U.,

Barcelona, Spain 0.23

KBr 99.5% Panreac Química S.L.U.,

Barcelona, Spain 1.89

KCl 99.5% Panreac Química S.L.U.,

Barcelona, Spain 0.5

2.2.2.1. Synthesis Method

The amount of salt prepared for the thermal analysis for each composition is 3g.

The different compositions have been prepared using the method described in

Gimenez & Fereres (2015)46: firstly, prills from each component are milled and

mixed in a mortar. The resulting powder is melted into a glass beaker on a hot plate

at 400 ºC. Once the content is in liquid state, it is homogenized by stirring

manually. The resulting liquid is poured over the mortar for fast cooling. The

resulting frozen thin layer of salt is easily milled again and the aforementioned

process is repeated three times (milling, mixing, melting) to guarantee

homogenization (Figure 2-10).

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Phase Change Materials

32

Figure 2-11: Initial mixture of components (left), manual milling (center) and

melting in a beaker on a hot plate (right)

2.2.2.2. Thermal characterization (Differential Scanning Calorimeter)

A Differential Scanning Calorimeter (DSC1 from Mettler-Toledo) is used to

thermally characterize the PCM candidates by measuring the melting temperature

and latent heat. Aluminum crucibles (40 µl) are filled with salt powder (~15 mg)

and the salt is pre-melted on a hot plate before sealing the crucibles to eliminate the

presence of moisture and to ensure good salt contact with the bottom surface of the

crucible.

The samples are introduced in the DSC equipment, heated to 420 ºC and cooled to

80 ºC at 10 ºC·min-1 five times. Other heating rates 2, 5 and 20 ºC·min-1 have been

also investigated. Nitrogen (50 ml·min-1) is used as inert gas in the thermal

program. Parameters such as the onset and endset temperatures are estimated as

the intersection point between the baseline connecting the points before and after

the transition and the tangent at the point of largest positive and negative slope on

the heat flow DSC curve respectively. The peak temperature is the temperature at

which heat flow during phase changes reaches the maximum. The latent heat of

fusion is determined by numerical integration of the area under the peaks.

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Thermal Energy Storage for High Temperature Applications

2.2.2.3. Potassium nitrate mixtures

KNO3 is the main component of mixtures (1) and (2). The phase change diagram

from FactSage Thermochemical Database 43 predicts a solid-solid transformation at

122 ºC and a liquid-solid phase change at 335 °C. Both transitions are measured and

presented in Table 2-9 and Figure 2-12, showing the results for three subsequent

melting/freezing cycles performed at a heating/cooling heating rate of 10 ºC·min-1.

Subsequent melting cycles are needed in many cases because salts are highly

hygroscopic and sample moisture can have a significant effect on the measured

latent heat. For instance, water absorption can modify the sample mass, leading to

a reduction in the measured value of the latent heat of the PCM. Additionally, the

latent heat of evaporation of water could be mistaken for a particular transition in

the 120 ºC range in certain salts, as is the case with KNO3.

The results in Table 2-9 show that the main phase transition properties vary mostly

after the first melting process takes place and they remain constant over

subsequent cycles. The energy absorbed by KNO3 in the first solid-solid transition

is different to the same transition after the melting temperature of KNO3 has been

reached, once moisture is assumed to be completely removed. The energy absorbed

changes from approximately 51 J·g-1 to 25 J·g-1, indicating that one should perform a

pre-melt for accurate results (or discard the initial melting DSC curve). Pre-melts

are otherwise typically recommended to improve the contact between the sample

and the crucible base. If water absorption is suspected, the samples should be

measured in a crucible with a small perforation on the cover, allowing water to be

evaporated during the sample heating without creating a pressure buildup. This

has been known to cause the abrupt rupture of crucibles during salt testing with

damaging consequences for the DSC equipment. Another solution could be to pre-

melt the salt inside the crucibles before sealing them hermetically.

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Phase Change Materials

34

Table 2-9: KNO3 measured latent heat, onset, peak, and endset temperatures

(melting and freezing) at 10ºC·min-1 during three subsequent heating and cooling

ramps.

KNO3 Enthalpy

[J·g-1]

Onset

[ºC]

Peak

[ºC]

Endset

[ºC]

S-S

transformation

1st melt 51.52 130.24 136.74 143.93

2nd melt 25.09 128.80 131.23 134.7

3rd melt 24.45 129.11 131.25 134.42

1st crystallization 24.8 119.33 117.13 113.73

2nd crystallization 22.18 119.62 116.97 113.72

3rd crystallization 22.06 118.84 117.13 114.02

S-L

transformation

1st melt 96.78 332.74 333.16 341.97

2nd melt 95.93 331.32 332.25 340.25

3rd melt 95.4 331.3 332.10 339.94

1st crystallization 97.81 329.85 329.61 320.26

2nd crystallization 97.05 329.77 329.51 321.34

3rd crystallization 96.37 329.84 329.41 321.88

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35

Thermal Energy Storage for High Temperature Applications

Figure 2-12: KNO3 heat flow DSC curves during three subsequent melting/freezing

loops at 10 ºC·min-1 showing the solid-solid transition peaks (a)heating, (d)cooling

and the solid-liquid transition peaks for (b)melting (c)crystallization. Water is

released during the 1st heating ramp, leading to a reduction in the area integrated in

peak (a).

Another parameter to take into account is the velocity of the dynamic segments

(heating/cooling rates). The experiments shown in Figure 2-12 were also performed

at a higher heating rate of 20ºC·min-1. The results are shown in Table 2-10. There is

essentially no difference between the phase transitions at 10 and 20 ºC·min-1 because

there are two clear and narrow peaks for each transition. The melting and freezing

onset temperature and latent heat remain unchanged, while a higher heating rate

extends artificially the phase transition into a wider temperature range.

1st

2nd

3rd

(a) (b)

(c) (d)

HEATING

COOLING

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Phase Change Materials

36

Table 2-10: Heating rate effect on phase transition properties: KNO3 measured

latent heat, onset, peak, and endset temperature (melting and freezing) at 10

ºC·min-1 and 20 ºC·min-1

Heating rate

[ºC/min]

Enthalpy

[J·g-1]

Onset

[ºC]

Peak

[ºC]

Endset

[ºC]

S-S transformation

10 25.09 128.80 131.23 134.7

20 24.65 129.68 130.65 136.43

% Difference -2% 1% 0% +1%

Melting

10 96.78 332.74 333.16 341.97

20 96.4 332.89 334.19 346.43

% Difference 0% 0% 0% 1%

Freezing

10 97.81 329.85 329.61 320.26

20 97.47 329.91 329.31 316.70

% Difference 0% 0% 0% -1%

However, the analysis of the mixture (1) KNO3-KCl-KBr (Table 2-11) shows the

presence of a sharp peak together with a smaller one with wider temperature

amplitude. The limits of integration are taken between 312ºC and 347ºC for melting

and 300°C to 340°C for freezing using a straight based line between the flat

segments before and after the phase transition. Figure 2-13 and Figure 2-14 show

the selected integration limits. The shape of the curves suggests that either a) the

mixture is not an eutectic composition and the solid-liquid phase transition extends

over a temperature range or that b) there are two peaks overlapping in the phase

change region that are not entirely visible when sampling at that specific

heating/cooling rate. The temperatures (onset, peak and endset) reported in Table

2-11 correspond to the sharpest peak while the latent heat corresponds to the total

integrated area between the base line and the heat flow curve. The standard

deviation found among three different samples is less than 0.5 J/g for latent heat

measurements and below 0.5ºC in all the transition temperatures.

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Thermal Energy Storage for High Temperature Applications

Table 2-11: KNO3-KCl-KBr(1) measured latent heat, onset, peak and endset

temperature (melting and freezing) at 5 ºCmin-1 (average values of three samples,

5th thermal cycle)

(1) KNO3-KCl-KBr Melting Freezing

Latent heat [J·g-1] -73.48+/-0.45 72.73+/-0.32

Onset [ºC] 319.45+/-0.09 319.14+/-0.06

Peak [ºC] 321.36+/-0.15 319.04+/-0.15

Endset [ºC] 324.29+/-0.31 314.96+/-0.45

Figure 2-13: KNO3-KCl-KBr (1) melting curves at 5 ºC·min-1

Page 80: Pablo Giménez Gavarrell

Phase Change Materials

38

Figure 2-14: KNO3-KCl-KBr (1) freezing curves at 5ºC·min-1

Comparing the melting process (only latent heat and onset temperature) between

pure KNO3 (Table 2-9) with the composition tested (1) (Table 2-11) to analyze the

effect of adding a small amount of KCl and KBr, the latent heat for the pure

component is absorbed and released in a Tendset-Tonset ~13.5 ºC temperature range.

The latent heat of the mixture (1) is ~24% lower than the pure component and part

of the energy is absorbed in a narrow temperature range (~5 ºC, peak) while the

rest is absorbed in the range 319-345 ºC. Note that the measured enthalpy of fusion

is 73 J/g, in contrast to 140 J/g reported by Zhao et al.44. This highlights the

importance of testing and understanding the phase transitions of each of these

PCM materials, as the composition in (1) is not a eutectic mixture and,

consequently, does not have a clear, narrow melting/freezing peak.

Testing at lower heating rates can sometimes separate phase transition peaks, but

they are often avoided as they increase testing time. Figure 2-15 shows how the

double hump shown in Figure 2-13 and Figure 2-14 can be separated into two

independent transitions by decreasing the heating rate to 2 ºC/min. These

transitions are very clear in the cooling (upper curve) segment.

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39

Thermal Energy Storage for High Temperature Applications

Figure 2-15: Heat flow curves for three subsequent loops of KNO3-KCl-KBr (1);

freezing curves at 1) 20ºC·min-1, 2) 20ºC·min-1, and 3) 2ºC·min-1. Top curves

represent cooling (heat is released during crystallization) and bottom curves

represent heating (heat is absorbed during melting).

Summarizing, the tested mixture (1) shows values an onset temperature and latent

heat very similar to KNO3-KCl mixture reported in Table 2-7. However, based on

the heat flow curves, this composition does not behave as a eutectic. The phase

change expands from 319 ºC to 345 ºC with a latent heat ~73 J·g-1 and with a

pronounced peak around 321 ºC that represents between 34-44% of the latent heat.

Mixture number (2) (KNO3(80.7)-KBr(11.9)-KCl(7.4) (wt%)) has been analyzed.

DSC curves are shown in Figure 2-16. The melting is not congruent, as mixture (1),

and the freezing presents an extended phase change, as it is not a eutectic

composition. There is a solid-solid transition with onset at approximately 128 ºC

and a solid-liquid transition beginning around 320 ºC (solidus) and finishing

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Phase Change Materials

40

around 400 ºC (liquidus). The measured values and comparison with recent

literature41 are shown in Table 2-12 and Table 2-13. Because it is not a eutectic

composition, melting occurs over a temperature range starting at the solidus

temperature, extending over the “mushy region” where solid and liquid phases

coexist, until the liquidus temperature, where the mixture is completely liquid. If

the integration intervals are not adequately adjusted, it is possible to obtain

different values for the latent heat of this solid-liquid transition that could explain

differences with the literature. It is unclear why this system has been previously

proposed as a potential PCM since it has a phase change process takes place over

such a large temperature range (~80 ºC).

Figure 2-16: KNO3-KBr-KCl (2) DSC curves heating/cooling at 20ºC·min-1: (a)/(d)

solid-solid transition and (b)/(c) solid-liquid transition.

2nd CYCLE

(a)

(c)

1st CYCLE

HEATING

COOLING

(b)

(d)

Page 83: Pablo Giménez Gavarrell

41

Thermal Energy Storage for High Temperature Applications

Table 2-12: KNO3-KBr-KCl (2) solid-solid transition measured values

solid-solid

transformation Enthalpy [J·g-1] Onset T [ºC]

Heating

Cycle 1 30.31 128.51

Cycle 2 20.01 127.61

Gomez2011 17.92 129.47

Cooling Cycle 1 18.92 107.1

Cycle 2 19.1 107.1

Table 2-13: KNO3-KBr-KCl (2) solid-liquid transition measured values

solid-liquid transformation

Enthalpy 1

(narrow

peak) [J·g-1]

Enthalpy 2

(extended

phase change)

[J·g-1]

Total

Enthalpy

[J·g-1]

Onset T

[ºC]

Heating /

melting

Cycle 1 12.53 63.28 75.81 322

Cycle 2 23.18 55.41 78.59 322

Gomez 2011 - - 75.89 326.6

Cooling /

solidification

Cycle 1 19.85 63.34 83.19 320.8

Cycle 2 19.45 61.71 81.16 320.7

Gomez 2011 - - 77.3 324.4

The shape of the curves, temperatures and latent heats are similar to mixture (1)

indicating that we are moving in a small area along the ternary phase change

diagram (Figure 2-8). The differences between these two compositions

(KNO3(91.43)-KCl(7.32)-KBr(1.25)(wt%))(1) and KNO3(80.7)-KBr(11.9)-

Page 84: Pablo Giménez Gavarrell

Phase Change Materials

42

KCl(7.4)(wt%)(2)) is an increase of the KBr content and a consequent reduction on

the KNO3 content, fixing the percentage of KCl. Although mixture (1) contains

higher amount of KNO3, its latent heat (72.7 - 73.5 J·g-1) is lower than mixture

(2)(78.6 - 83.2 J·g-1).

KNO3-6mol%KCl (4.5wt. %) from Table 2-7 was included in the testing plan to

further analyze these differences. This composition has been tested at 2 ºC·min-1

and 20 ºC·min-1 in order confirm whether it behaves as a eutectic mixture and,

consequently be of interest as a PCM or not. Moreover, it is important to clarify if

the presence of KBr is responsible for the extended solid-liquid phase change in

mixtures (1) and (2). Table 2-14 contains the results of the analysis and Figure 2-17

shows the heat flow curves.

Table 2-14: KNO3 – KCl (6 mol %) solid-liquid transition measured values

solid-liquid transformation Enthalpy

[J·g-1]

Onset T

[ºC]

Peak T

[ºC]

Endset T

[ºC]

Heating /

melting

Cycle 2

2 ºC/min 81.1 321.0 322.4 324.5

Cycle 3

20 ºC/min 77.54 321.9 323.2 325.6

Zhao et al. 44 74 320 - -

Cooling /

solidification

Cycle 2

2 ºC/min 80.46 322.9 321.6 319.1

Cycle 3

20 ºC/min 77.96 321.0 321.4 317.2

Page 85: Pablo Giménez Gavarrell

43

Thermal Energy Storage for High Temperature Applications

Figure 2-17: KNO3 – KCl (6 mol%) heat flow curve, solid-liquid transition.

A comparison of the potassium nitrate mixtures with the pure salt results analyzed

above is shown in Table 2-15 to summarize the findings. Adding a small amount of

KCl to KNO3 reduces the melting point by 11 ºC but also reduces the latent heat

from 96.4 J·g-1 to 77.5 J·g-1. Considering that the theoretical value of the latent heat

of KCl is over three times that of KNO3 47, a reduced latent heat is an undesired and

unexpected result. Adding KBr to the KNO3-KCl mixture does not seem to modify

this outcome and essentially results in the same melting point. Moreover, the

KNO3-KCl mixture behaves as a eutectic even though, based on its binary phase

diagram from Factsage, is an off-eutectic. Since the ternary mixtures (1) and (2)

clearly have extended phase transitions, it seems that the addition of KBr to the

binary mixture KNO3-KCl might be responsible of the extended phase change

observed.

-10

-5

0

5

10

15

Hea

t F

low

(W

/g)

305 310 315 320 325 330 335 340Temperature (°C)

KNO3-KCl_TA_s1_L2-2ºCmin– – – – KNO3-KCl_TA_s1_L3-20ºCmin––––– ·

Exo Up

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Phase Change Materials

44

Table 2-15: Summary of solid-liquid transition properties for KNO3 vs three

different mixtures at a heating rate of 20ºC/ min (except for mixture (1) measured at

10 ºC/min). All compositions are in wt. %

Potassium based mixtures (wt. %) Onset T [ºC] Latent heat [J·g-1]

KNO3 332.9 96.4

KNO3-KCl (4.5) 321.9 77.5

(1) KNO3 (91.43) – KCl (7.32) – KBr (1.25) 319.5 73.5

(2) KNO3 (80.7) – KBr(11.9) – KCl (7.4) 322 78.6

2.2.2.4. Sodium nitrate mixtures

Apart from potassium nitrate mixtures, other mixtures such as (3) to (6) have been

evaluated, with NaNO3 as the main component. The reference melting point of

NaNO3 is 306 ºC47. Additionally, this salt is reported to have a solid-phase

transition around 275 ºC48,49.

Table 2-16 shows the melting and freezing results for the pure component. There

are no significant differences between melting and freezing latent heats. The

enthalpy of the second order transition in heating and cooling is also similar. In

freezing experiments 1.5 ºC of supercooling is observed (the freezing onset

temperature is lower than the peak temperature). The width of the phase change

transition for melting and freezing are similar (~7.4 ºC). The second order transition

in cooling is about ~273 ºC, similar to the literature value.

Table 2-17 and Table 2-18 compares the average values of the DSC for 4 different

samples, for each NaNO3 based mixtures, 5 thermal cycles per sample, averaging

cycles 2 to 5, in melting and freezing respectively.

Page 87: Pablo Giménez Gavarrell

45

Thermal Energy Storage for High Temperature Applications

Table 2-16: DSC analysis of NaNO3 (melting and freezing results). Average values

of three different samples.

NaNO3 2nd Order

transition

Melting Freezing 2nd Order

transition

Latent heat [J·g-1] -17.15+/-0.22 -176.75+/-1.95 175.81+/-2.04 15.02+/-0.4

Onset [ºC] 264.46+/-1.39 302.58+/-0.13 301.33+/-0.47 273.24+/-0.05

Peak [ºC] 273.75+/-0.04 303.93+/-0.3 302.86+/-0.52 271.41+/-0.23

Endset [ºC] 275.20+/-0.11 309.94+/-0.58 293.94+/-3.8 259.78+/-1.02

Table 2-17: Melting DSC analysis: comparison between pure NaNO3 vs mixtures

(3), (4), (5), (6) (20 ºC·min-1). Average values of three different samples.

Melting Latent Heat [J·g-1] Onset [ºC] Peak [ºC] Endset [ºC]

NaNO3 -176.75+/-1.95 302.58+/-0.13 303.93+/-0.30 309.94+/-0.58

NaNO3-NaCl-Na2CO3(3) -171.36+/-0.53 286.73+/-0.13 288.78+/-0.06 293.80+/-0.52

NaNO3-NaCl-Na2CO3(4) -181.78+/-2.46 286.36+/-0.11 288.85+/-0.18 294.11+/-0.62

NaNO3-NaCl-NaBr(5) -178.30 +/-0.80 286.31+/-0.14 289.02+/-0.30 294.78+/-0.50

NaNO3-NaCl-NaBr(6) -173.18 +/-2.08 286.41+/-0.10 289.45+/-0.175 295.26+/-0.50

The standard deviation of the measurements is small. Compared to pure NaNO3

the latent heat of the mixtures does not follow any significant trend, a reduction on

the latent heat is observed for the mixtures (3) and (6), while an increase is observed

for the mixtures (4) and (5). On the other hand, the results for the different

temperatures (onset, peak and endset) show practically the same value regardless

the composition tested. The onset melting temperature shows a reduction of 16.1 ºC

for the different compositions tested compared to the pure salt. Figure 2-18 shows

the heat flow curves of mixture (3) and (4) compared to NaNO3. The samples

behave as NaNO3 lowering their temperatures (onset, peak and enset).

Page 88: Pablo Giménez Gavarrell

Phase Change Materials

46

Figure 2-18: Heat flow curve of mixtures (3) and (4) and NaNO3 while heating at 20

ºCmin-1

Table 2-18: Freezing DSC analysis: comparison between pure NaNO3 vs mixtures

(3), (4), (5), (6) (20 ºC·min-1)

Freezing Latent Heat [J·g-1] Onset [ºC] Peak [ºC] Endset [ºC]

NaNO3 175.81+/-2.04 301.33+/-0.47 302.86+/-0.52 293.94+/-3.8

NaNO3-NaCl-Na2CO3(3) 168.77+/-0.56 284.56+/-0.58 286.28+/-0.15 279.91+/-0.30

NaNO3-NaCl-Na2CO3(4) 179.39+/-2.17 285.06+/-0.39 287.20+/-0.17 280.53+/-0.68

NaNO3-NaCl-NaBr(5) 176.09+/-0.10 284.05+/-0.40 286.05+/-0.42 279.01+/-0.36

NaNO3-NaCl-NaBr(6) 171.15+/-1.88 285.15+/-0.30 286.42+/-0.20 279.55+/-0.53

Similar observations can be done for the freezing results. Considering the

equipment accuracy in the measurement of the latent heat (+/- 5%), no trends

appears for this parameter. Similar reduction on the freezing onset temperature are

observed (-16 ºC) regardless the composition. The accuracy of the temperature

results is very high and the four mixtures shows the same value for the onset (286

°C) which is very different from the expected value from the phase diagrams.

-250

-200

-150

-100

-50

0

200 250 300 350 400

Heat

Flo

w [

mW

]

Temperature [ºC]

NaNO3

Mix(3)

Mix(4)

Page 89: Pablo Giménez Gavarrell

47

Thermal Energy Storage for High Temperature Applications

It seems that for the four compositions the peak amplitude (OnsetT–EnsetT) has

been shifted to a lower temperature without any other modification, neither latent

heat, nor shape. For NaNO3 the difference between onset and endeset temperature

(7.5 ºC) is very similar to the difference between the onset and endset temperature

for the four compositions. Therefore the stored energy is about the same as for pure

NaNO3 with an offset of -16 °C.

2.2.2.5. Other compositions

DurferritTM salt has been also characterized through DSC. Figure 2-19 shows the

results for four different samples tested during 5 thermal cycles (25-200ºC). The

integration of the initial heating peak on the first heating of each sample can be

used to determine the amount of water contained, assuming that the absorbed

energy is due to evaporation of water from the sample. Knowing the enthalpy of

evaporation of water, 2257 J·g-1, a water content of about ~0.26 wt. % has been

calculated for each of the sample.

Figure 2-19: Durferrit heat flow curves: phase change evaluation (left) and

variability between samples (right)

The salt freezing occurs in a narrow temperature range, while the heat absorbed in

the melting process is produced on a wider temperature range. The peaks

amplitude are ~11ºC and ~48 ºC for freezing and melting respectively. Freezing

Page 90: Pablo Giménez Gavarrell

Phase Change Materials

48

process starts at 121 °C with a clear peak. On the other hand, in order to melt the

salt completely it is required to increase the temperature up to 154 °C. The heat of

fusion of the salt is 22 J·g-1 considerably lower than the sodium and potassium

nitrate based compositions analyzed. Table 2-19 shows the average results for the

four samples analyzed. The standard deviation of the measurements is low. This

indicates the homogeneity of the salt.

Table 2-19: Average results for four Durferrit samples

Freezing Melting

Latent heat [J·g-1] 17.75 +/- 0.38 -21.80 +/- 1.00

Onset [ºC] 121.20 +/- 1.43 106.00 +/- 5.24

Peak [ºC] 117.00 +/- 1.60 142.56 +/- 1.12

Endset [ºC] 110.52 +/- 2.03 153.92 +/- 1.69

Page 91: Pablo Giménez Gavarrell

49

Thermal Energy Storage for High Temperature Applications

2.2.2.6. Nitrate results

Finally, the specific cost of the different compositions has been calculated with the

measurement results (Table 2-20).

Table 2-20: Specific cost analysis of the different mixtures calculated with the

experimental latent heat measurement. (*FactSage, **literature)

Estimated Experimental

PCM

Composition (wt. %)

€/kg €/kWh Melting

T [ºC]

LH

[J·g-1]

Onset

T[ºC]

LH

[J·g-1]

NaNO3 0.77 15.6 306* 172** 302.6 176.8

KNO3 1.08 40.4 335* 95** 332.9 96.4

(1) KNO3(91.44)-KCl

(7.32)-KBr (1.24) 1.05 51.4 306.7* - 319.5 73.5

(2) KNO3(80.7)-

KBr(11.9)-KCl(7.4)

(wt%)

1.13 51.1 342.0* 140** 321.4 79.9

(- ) KNO3(95.5)-KCl(4.5)

(wt%) 1.05 47.9 320** 74** 321.7 79.3

(3) NaNO3 (86.66)-

NaCl(8.27)-

Na2CO3(5.07)

0.69 14.6 358.9* - 286.7 171.4

(4) NaNO3 (93.11)-

NaCl(4.67)-

Na2CO3(2.22)

0.73 14.4 294.6* - 286.4 181.8

(5) NaNO3 (90)-

NaCl(2.64)-

NaBr(7.36)

0.84 17.0 290.5* - 286.3 178.3

(6) NaNO3 (89.20)-

NaCl(1.35)-

NaBr(9.45)

0.88 18.2 290.0* - 286.4 173.2

Page 92: Pablo Giménez Gavarrell

Phase Change Materials

50

Comparing the literature values with the results of this study, the following

conclusions can be made:

In general, the melting temperatures coincide with previously reported

data, except for mixtures (1), (2), and (3).

Mixtures (1) and (2) have surprisingly similar results (even though the

literature suggested otherwise) and present an off-eutectic behavior,

suggesting that it might be difficult to synthesize and/or measure the

theoretical eutectic.

None of the KNO3-based mixtures improved the latent heat with respect to

the main pure material and are more costly.

The NaNO3-based mixtures essentially have the same latent heat as the

main pure component, they all have lower melting temperatures slightly

lower than 300ºC, and mixtures (3) and (4) are slightly cheaper than the

pure salt.

As confirmed by the experimental measurements, the reduction on the melting

temperature observed in the sodium nitrate mixtures brings them out of the

desired temperature range. In addition to this effect, there is no remarkable latent

heat enhancement observed to justify their use. Moreover, the KNO3 ternary

mixtures show off-eutectic behavior or non-congruent melting which are not

favorable for a TES based on PCM, and all of them with a lower latent heat.

Durferrit has been also selected because of its extremely low melting point,

although it shows a low latent heat. This salt will be encapsulated as an initial test

in the 100-200 ºC temperature range, prior to higher temperature tests. It is

important to highlight the importance of testing the salts experimentally due to the

different discrepancies observed between previously reported temperatures and

those calculated through thermodynamic programs.

These three inorganic salts (NaNO3 (Tm 302 ºC), KNO3 (Tm 332 ºC) and Durferrit

(Tfreeze 121 ºC)) and two pure metals characterized (lead (Tm 315 ºC) and tin(Tm 179

ºC)) have been selected as PCM for the capsule manufacturing. The DSC measured

melting temperature and latent heat is presented in Figure 2-20. Lead, NaNO3, and

KNO3 having the melting temperatures in the range of interest; tin and Durferrit

will also be used because having a significant lower melting temperature with

similar materials can simplify some of the initial experimental measurements.

Page 93: Pablo Giménez Gavarrell

51

Thermal Energy Storage for High Temperature Applications

Figure 2-20: Latent heat and melting temperature of several PCM tested.

315ºC;

21kJ/kg

179ºC;

44 kJ/kg

332ºC;

96kJ/kg

302ºC;

177kJ/kg

121ºC;

18kJ/kg

0

25

50

75

100

125

150

175

200

100 130 160 190 220 250 280 310 340 370

Lat

ent

Hea

t (k

J/k

g)

Temperature [ºC]

Pb Sn KNO3 NaNO3 Durferrit

Page 94: Pablo Giménez Gavarrell

Phase Change Materials

52

Page 95: Pablo Giménez Gavarrell

53

3 MACRO ENCAPSULATION OF

PCM

acro-encapsulated phase change materials (PCM) are potentially an

interesting high energy density solution to store thermal energy near

isothermal conditions. They are usually implemented in a packed bed

latent heat storage system, consisting of a storage medium divided into small

encapsulated particles which increase the specific surface area to exchange heat

with the working fluid (synthetic oil, molten salts or steam). This technology is

expected to yield to a much more compact storage system compared to its sensible

heat storage counterpart.

3.1. Introduction and Main Objectives

The capsule-PCM is formed by a core material, the PCM, in charge of the storage of

energy using its phase change enthalpy, and a shell material surrounding the PCM,

M

Page 96: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

54

acting as an interface between the heat transfer fluid and the storage material

(Figure 3-1).

Figure 3-1: Schematic representation of a PCM-capsule

An elastic shell or a void space is required to accommodate the volumetric

expansion that most PCM suffer when they melt. The storage of thermal energy

using encapsulated PCM offers potential benefits whenever the capsule

accomplishes the following main requirements50:

i. The capsule has to meet the requirements of strength, corrosion

resistance, and thermal stability, giving mechanical integrity to the

capsule when the PCM is in liquid state.

ii. Act as barrier to protect the PCM from damaging interaction with the

surrounding HTF.

iii. Provide sufficient surface for an efficient heat transfer.

iv. Provide structural stability and easy handling.

The available encapsulating technologies change dramatically depending on the

temperature limits of the applications. There are a large number of micro-

encapsulation methods for low temperature PCM: physical methods (pan coating,

air-suspension, centrifugal extrusion, coacervation, spray drying…) and chemical

methods (interfacial polymerization, in situ polymerization, matrix

polymerization)51,52. Thin, sealed, and high molecular weight polymeric films are

commonly used to encapsulate these PCM maintaining the shape and preventing

PCM from leakage during the phase change process.

Shell

Core

PCM(solid)

Void

Shell

Core

PCM(liquid)

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55

Thermal Energy Storage for High Temperature Applications

On the other hand, high temperature (T>300ºC) encapsulation has been less

developed. For example, among the 43 heat transfer studies in capsules of various

geometries and in packed bed storage systems summarized in Regin et al.50 only 5

of them are dealing with high temperature materials. Cost-effectiveness, corrosion,

and thermal stability concerns are also important aspects that differ significantly at

high temperature applications.

The objectives of this chapter are to 1) develop a functional macro-encapsulation

concept for high temperature PCM (inorganic salts or metals) with melting

temperatures in the 300-450ºC range to be used as latent heat storage in direct

steam generation (DSG) solar thermal plants and to 2) develop an experimental rig

to test such concept (Figure 2-2).

Figure 2-2. Schematic representation of the PCM study with the contents of this

chapter marked in yellow.

The target operating conditions of steam in DSG solar thermal plants is T300-

320ºC at 100 bars. Ideally a latent heat storage is designed to be used in an

isothermal system (i.e. saturated steam at 312ºC, 100 bar), but there is interest in

PCM TES heat

exchanger system

Packed bed

Macro-encapsulation

Screen & characterize

PCMs

Screen Shell Materials

Model Single Capsule

Fabricate capsules

Test single

capsule

Compare Experiments

& Model

Evaluate performance & challenges

Tube & housing

Metal PCM

Double PCM

Page 98: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

56

exploring the use of a single storage system to also cover the superheated regime

(up to 450 ºC) for engineering simplicity. The goal is to design a concept that might

be valid for both saturated and superheated modules with slight modifications for

each case.

3.2. Background on High Temperature Encapsulation

An efficient and cost-effective PCM encapsulation has to be achieved for latent heat

packed beds to be reliable and economically attractive. The challenges in high

temperature encapsulation are mainly related to finding suitable, compatible

materials to exchange heat between heat transfer fluids and PCM working under

high temperature operating conditions.

One of the major barriers of the high temperature encapsulations is preventing the

shell rupture when the core PCM melts and expands in volume due to the

difference in density between the solid and liquid PCM. This phenomenon might

occur when the mechanical integrity is not intrinsic to the capsule, but depends on

the core material, for example, when the capsule is created as deposited layers

around a solid PCM. To avoid the overpressure created by the PCM’s melting (and

volume expansion), an empty space between the PCM and the shell or a flexible

shell is required. Since the 1990s, researchers have tried many different approaches

to encapsulate high temperature PCM. The main challenges and proposed

solutions are reviewed below.

In Mathur (2011)53 and Mathur & Kasetty (2012) (US Patent 20120018116 A1)54 two

different encapsulation methods are developed around different nitrate salts

(Figure 3-2). The first one consists of the creation of a void space between the shell

and the PCM through an organic sacrificial layer. The second approach introduces

agglomerated metal particles between the PCM and the shell. In the first case, the

thermal degradation of the sacrificial layer will leave enough empty space; in the

second case the agglomerated metal particles will act as a flexible layer absorbing

the volumetric expansion. The encapsulation method used in both cases is a

fluidized bed coating. The shell materials are identified as thermally stable after

several thousands of freeze-thaw cycles and compatible with the core PCM.

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57

Thermal Energy Storage for High Temperature Applications

Figure 3-2: KNO3 encapsulated particles53,54

Alam et al. 55 developed an innovative technique that does not require a sacrificial

layer to accommodate the volumetric expansion of the PCM upon melting. A non-

reactive polymer is used over the PCM (NaNO3 and NaNO3-KNO3) pellet followed

by deposition of a metal layer (non-vacuum metal deposition technique). The

polymer is not eliminated from the capsule during the manufacturing process but

remains, stable and non-reactive with the PCM at the operation temperature. The

authors validate this new manufacturing process by testing the capsules more than

2200 thermal cycles between 250 - 325 ºC. The integrity of the capsules is surprising

especially considering that the polymer used (Teflon) melts at 327 ºC, very close to

the upper temperature limit.

A thick Cr-Ni bi-layer is used in Zhang et al. 56 to encapsulate copper. A chromium

periodic-barrel electroplating method and nickel barrel-plating method is used.

The same PCM is also encapsulated in Maruoka et al. 57, where an intermediate

layer of carbon or ruthenium is introduced between the copper core and the nickel

shell. A carbon or ruthenium layer, in Mauroka at al.57, and chromium oxide layer,

in Zhang et al.56, are used as inhibition layers to avoid the direct solution between

copper and nickel, because they have complete solubility in the phase diagram at

the operating temperature. In the case of Al-Si alloys, Nomura et al. 58 successfully

encapsulated micro-spheres using stable α-Al2O3 shell with interesting self-

repairing properties oxidizing the core aluminum.

In the temperature range of ~400ºC Zhao et al. (2013)44 considered 2 types of

spherical capsules (20-50 mm in diameter): Zn encapsulated in Ni and eutectic salt

mixtures (NaCl - 43mol% MgCl2) in stainless steel capsules. These results are also

presented in the patent US2011/0259544 A159 which covers encapsulation materials

and PCM at temperatures higher than 400ºC for capsules with a nominal

dimension between 1-100 cm. The compatibility between the materials enables the

use of a 1018 carbon steel capsule for MgCl2-NaCl; a stainless steel 304 capsule for

Page 100: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

58

NaNO3 and nickel or 316 stainless steel for Zn encapsulation. Spherical and tubular

encapsulation and other PCM including NaNO3, KNO3, NaNO3-KNO3, MgCl2,

MgCl2-NaCl, MgCl2-KCl, NaCl-KCl, inorganic salts, and combinations are

proposed. The experimental study performed by Zhang H.L. et al. 60 would fit into

the patent claims, testing the binary system NaNO3-KNO3, although stainless steel

AISI 321 was used to encapsulate.

In Blaney et al.61 the same group analyses the capsule mechanical resistance due to

stresses from the thermal expansion and volume change of the PCM. The study

considers a three-dimensional finite element model simulating the stress

distribution in a spherical nickel shell of 250 μm thickness formed around a sphere

of Zn by electroless deposition, and a stainless steel cylindrical shell containing Zn.

The nickel shell produced by the electroless deposition process was not

demonstrated to be a feasible way of containing the molten zinc, presenting too

many potential modes of failure and being unlikely to survive multiple cycles of

heating and cooling.

Similar conclusions are found experimentally in Maruoka et al.36 in which lead

pellets are encapsulated in nickel through electroplating. Although the capsule film

shows enough strength when it is thick enough or the PCM diameter is small, the

stress analysis and observations after thermal cycling tests suggest that the coated

film is not uniform and an inactive weak layer exists.

Metallic spherical macro-capsules for high temperature PCM were studied already

in 1995 by Yagi et al. 34 to recover heat of industrial processes using a packed bed.

Six different PCM were considered: two inorganic salts: KNO3-NaNO3 eutectic and

NaCl; and four metallic materials: lead, aluminum and two Al-Si alloys.

Table 3-1: High temperature PCM candidates34

PCM KNO3-NaNO3

eutectic Pb

Al-Si

(87.4-12.6

wt. %)

Al-Si

(74.9-25.1

wt. %)

Al NaCl

Melting point

[ºC] 222 328 577 577 660 800

Latent Heat [J/g] 94 23 516 441*

(estimated) 397 482

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59

Thermal Energy Storage for High Temperature Applications

The heating and cooling by convection of a single metallic capsule containing PCM

in a nitrogen gas stream was conducted in Yagi et al.34. The metallic PCM material,

due to its high thermal conductivity, behaved as a thermally uniform material,

while in the melting process in molten salt PCM the thermal gradients inside the

capsule were important, for the capsule size tested (ϕ=4cm, thickness 2 mm). It is

important to mention that the PCM were poured into the hollow capsules in liquid

state. When the PCM freezes and contracts, a void space is created which leaves the

required empty space for subsequent melting and freezing cycles.

Solomon’s and Zhao’s theses62,63 developed metallic cylindrical macro-capsules

used to encapsulate some of the previous high temperature PCM (Table 3-1): salts

(NaNO3 and NaCl) and metals (Al). The thermal properties of the capsule are

tested in a home-made macro-DSC (Differential Scanning Calorimeter), analyzing

the specific heat, the latent heat, the energy stored, and the evolution of the storage

capacity when the capsules are exposed to different thermal cycling conditions.

Although the high thermal conductivity and latent heat of some metallic PCM

makes them an interesting solution for TES, interactions between the metallic PCM

and metallic shell material (e.g. alloying) can result in a reduction of the initial

latent heat storage capacity of the PCM. Aluminum (PCM)-stainless steel capsules

exposed to 720ºC for 500 and 1000 hours showed a 4.1% and 10.4% decrease in

storage capacity respectively.62 Also, in Zhao (2013)63 zinc and aluminum used as

PCM showed a drop in the storage capacity of 12% and 5% respectively after 7

heating/cooling thermal cycles (450ºC) for the zinc, and after 500h at 720ºC for the

aluminum. Therefore, novel encapsulation material such as non-reactive metals,

ceramic capsules or different non-reactive coatings must be developed to solve this

problem. The use of high temperature paints and sodium silicate to protect carbon

steel capsule from highly corrosive chloride mixture in Nath R. 64 is one of the

attempts to solve this problem.

The feasibility of PCM encapsulated in metallic cylinders has been re-evaluated in

recent publications (Figure 3-3). However, publications from the 90s use the same

technology. For this type of capsules, their filling is more easily controlled and,

consequently, the internal stresses caused by the PCM volumetric expansion can be

accommodated. The encapsulation material must be sufficiently thick to maintain

the structural integrity of the EPCM and it has to provide sufficient void space in

the manufacturing process for the PCM’s volumetric expansion.

Page 102: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

60

Figure 3-3: Encapsulated PCM: Encapsulated NaCl-MgCl2 (left)65; Encapsulated

MgCl2 (center)66; Encapsulated Ternary carbonate eutectic (lithium, sodium and

potassium carbonates) (right)67

The content of this extended review on high temperature encapsulated PCM’s

experimental studies is summarized in Table 3-2. Recently the number of

publications in this subject has risen dramatically, as solar thermal power plant

developments require new, more efficient energy storage solutions. There is only

one study of high temperature macro-encapsulation of PCM from the 1990s and

the rest are mainly investigations carried out in the past five years.

In a PCM-capsule, the amount of material that does not undergo a phase change

should be minimized because the objective is to store thermal energy primarily in

latent heat and not in sensible heat. Thus, capsule filling ratios and shell thickness

must be evaluated in detail. Considering spherical and (long) cylindrical capsules,

Figure 3-4 represents the ratio between the shell volume (Vshell) and capsule core

volume (Vcore) for different dimensionless PCM-radii (Rpcm/Rcapsule) based on

Equation 3-1 (a-b), considering that the PCM completely fills the capsule.

a) For a sphere:

b) For a cylinder:

Equation 3-1

Page 103: Pablo Giménez Gavarrell

61

Thermal Energy Storage for High Temperature Applications

Figure 3-4: Vshell/Vpcm ratio vs Rpcm/Rcapsule for spherical and cylindrical capsules

As Figure 3-4 shows, a spherical capsule with a lower PCM radius (Rpcm) than

0.87·Rcapsule will contain a shell volume higher than 50% of the PCM, where at least

one third of the total capsule material is not being used for latent heat storage. This

ratio would be Rpcm <0.8·Rcapsule for cylindrical capsules. This an arbitrary limit for

the ratio Rpcm/Rcapsuel where the volume of the shell material represents 50% of the

volume of PCM or lower, but it highlights the importance to maximized the ratio

Rpcm/ Rcapsule to minimize the shell volume for a given capsule diameter and

geometry. At the same time, the encapsulation material has to be sufficiently thick

to prevent the deformation of the capsule caused by stresses on the shell. Therefore,

this is an important consideration in the capsule design and subject to compromise.

Parameters such as the PCM/Capsule diameter ratio, which determines the

shell/core volume ratio, have been also included in Table 3-2.

Table 3-2: Summary of experimental studies on high temperature encapsulated

PCM. Spherical capsules are marked in blue, cylindrical capsules in black.

0

0.5

1

1.5

2

0 0.2 0.4 0.6 0.8 1

Vsh

ell/

Vp

cm

Rpcm/Rcapsule [-]

Sphere

Cylinder

Page 104: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

62

PC

M T

melt

(ºC

)

LH

(kJ/

kg

)

NaN

O3

306

172

PT

FE

(0.5

-0.7

mm

) –

Nic

kel

(10-8

m)

PT

FE

(0.5

-0.7

mm

)

27.4

31.4

8-

40.9

%250-3

26 (

2200 c

ycle

s)

250-3

26 (

1000 c

ycle

s)24.4

70.8

92

KN

O3

334

92

PT

FE

(0.5

-0.7

mm

) –

Nic

kel

(50-8

m)

280-3

50 (

110 c

ycle

s)

NaN

O3-

50w

t.%

KN

O3

222

120

(PT

FE

-FE

P)–

Nic

kel

180-2

42(1

000 c

ycle

s)

NaN

O3-

KN

O3-L

iNO

3

122

140

FE

P–

Nic

kel

100-1

44 (

440 c

ycle

s)

Nom

ura

et

al 5

82015

Al-

40 w

t.%

Si

573

247

(ME

PC

M)

α-A

l2O

3

0.0

407

0.0

022

-40.9

%500-8

00 (

10 c

ycle

s)0.0

363

0.8

92

Zhan

g, G

. et

al 5

62014

Cu

1083

53.2

(ME

PC

M)

Ch

rom

ium

–n

ickel

2.9

06

0.5

53

-320.8

%1050 -

1150 (

1000 c

ycle

s)1.8

0.6

19

Zhan

g, H

.L. e

t al

60

2014

NaN

O3-

40w

t.%

KN

O3

230

-Sta

inle

ss s

teel

AIS

I 321

75

1.5

-72

0.9

60

Zn

420

113

Sta

inle

ss s

teel

304

25.4

1.5

875

~20%

30.6

%

27-4

85 (

7 c

ycle

s,

-12.3

% S

tora

ge

Cap

acit

y)

22.2

30.8

75

Al

660

397.3

Sta

inle

ss s

teel

304

25.4

1.5

875

~20%

30.6

%

30-7

10 (

6 c

ycle

s)

710 (

500h

, -5

%

Sto

rage

Cap

acit

y)

22.2

30.8

75

NaN

O3

308

162.5

Car

bo

n s

teel

1018

50.8

1.5

875

~20%

13.8

%30-3

50 (

3 c

ycle

s)47.6

30.9

38

NaC

l-M

gCl 2

444

292

Sta

inle

ss s

teel

304

Car

bo

n s

teel

1018

25.4

1.5

875

~20%

30.6

%470 (

300h

)22.2

30.8

75

MgC

l 2714

454

Sta

inle

ss s

teel

304

25.4

1.5

875

~20%

30.6

%850 (

1000h

)22.2

30.8

75

NaC

l800

482

Sta

inle

ss s

teel

304

25.4

1.5

875

~20%

30.6

%30-8

50 (

3 c

ycle

s)22.2

30.8

75

Zhao

et

al 6

62013

MgC

l 2714

454

Sta

inle

ss s

teel

304L

(lo

w c

arb

on

sta

inle

ss s

teel

)25.4

1.5

875

20-3

0%

30.6

%33-7

50 (

60 c

ycle

s; 4

80h

) 22.2

30.8

75

NaN

O3

308

177.1

**C

arb

on

ste

el 1

018

50.8

1.5

875

18-2

0%

13.8

%300-4

70 (

50 c

ycle

s; 3

00h

)47.6

25

0.9

38

NaC

l -

43m

ol%

MgC

l 2444

292**

Sta

inle

ss s

teel

304

Car

bo

n s

teel

1018

25.4

50.8

1.5

875

18-2

0%

30.6

%

13.8

%

22.2

25

47.6

25

0.8

75

0.9

38

Ala

m e

t al

55

2015

2013

Zhen

g et

al

65

Au

tho

r(s)

Year

Th

erm

al Sta

bil

ity

T t

est

ed

(ºC

)

PC

M

dia

mete

r

(mm

)

Ex

tern

al

dia

mete

r

(mm

)

Sh

ell

thic

kn

ess

(mm

)

Vo

idSh

ell/

Co

re

Vo

lum

e r

ati

o

φ P

CM

/

φ C

ap

sule

Co

re m

ate

rial

Sh

ell m

ate

rial

Zhao

, W. 6

32013

Page 105: Pablo Giménez Gavarrell

63

Thermal Energy Storage for High Temperature Applications

PC

M T

melt

(ºC

)

LH

(kJ/

kg

)

Al

660

397

stai

nle

ss s

teel

304L

25.4

1.5

875

20-3

0%

30.6

%720 (

1000h

lea

ds

to

-10%

sto

rage

cap

acit

y)22.2

30.8

75

NaC

l800

430

stai

nle

ss s

teel

304L

25.4

1.5

875

20-3

0%

30.6

%850 (

1000h

)22.2

30.8

75

carb

on

ste

el

50.8

1.5

875

-13.8

%620-6

80 (

10 c

ycle

)47.6

30.9

38

carb

on

ste

el

Co

atin

gs (

Hig

h T

pai

nts

or

sod

ium

silic

ate

)

50.8

1.5

875

-13.8

%680 (

72h

)47.6

30.9

38

mild

ste

el c

oat

ed w

ith

seal

met

50.8

1.5

875

-13.8

%650-6

80 (

20 c

ycle

s)47.6

30.9

38

Pen

dya

la, S.

2012

NaN

O3

306**

*172**

*si

lico

n d

ioxid

e30.1

50.0

75

20-3

5%

1.5

%25-3

50 (

7 c

ycle

s)30

0.9

95

Ter

rafo

re2011

KN

O3

(In

org

anic

sal

ts)

333

92

met

allic-

cera

mic

2.1

69

0.2

92

-156.3

%300-8

00

1.5

85

0.7

31

0.5

50.0

25

33.1

%400

0.5

0.9

09

1.1

0.0

533.1

%1

0.9

09

3.1

0.0

510.3

%3

0.9

68

4.1

0.0

57.7

%4

0.9

76

carb

on

- n

ickel

(0.0

53 -

0.9

40m

m)

4.9

86

0.9

93

-25-1

100 (

50 c

ycle

s)3

0.6

02

Ruth

eniu

m-n

ickel

(1 -

0.9

40)m

m6.8

81.9

4-

30.4

36

KN

O3-N

aNO

3

eute

ctic

222

94

Lea

d328

23

Al-

12.6

Si (w

t.%

)577

516

Al -

25.1

Si (w

t.%

)577

441

Al

660

397

NaC

l800

482

NaN

O3

301.9

174.1

119.7

11.1

326.5

0%

44.1

%250-3

50 (

10 c

ycle

s)17.4

50.8

85

KN

O3

331.6

95.6

19.3

11.1

322.8

0%

45.3

%280-3

80 (

10 c

ycle

s)17.0

50.8

83

Durf

erri

t121.2

19.7

19.8

31.1

324.6

5%

43.8

%90-1

80 (

10 c

ycle

s)17.5

70.8

86

Sn

179.6

44.5

918.6

81.1

317.1

0%

47.2

%140-2

40 (

10 c

ycle

s)16.4

20.8

79

Pb

314

22.8

520.1

81.1

317.9

0%

42.8

%280-3

50 (

10 c

ycle

s)17.9

20.8

88

23**

*n

ickel

2016

Pre

sent

work

Gim

enez

-Gav

arre

ll, P

.

2002

Mar

uoka

et a

l 57

Yag

i &

Akiy

ama

34

1995

2003

Mar

uoka

& A

kiy

ama

36

Cu

1083

209

NaC

l -

56w

t% K

Cl

Nat

h, R

. 64

Solo

mon, L

.D.

62

2013

2012

-~

674

Au

tho

r(s)

Year

Th

erm

al Sta

bil

ity

T t

est

ed

(ºC

)

PC

M

dia

mete

r

(mm

)

Ex

tern

al

dia

mete

r

(mm

)

Sh

ell

thic

kn

ess

(mm

)

Vo

idSh

ell/

Co

re

Vo

lum

e r

ati

o

φ P

CM

/

φ C

ap

sule

Co

re m

ate

rial

Sh

ell m

ate

rial

Pb

Sta

inle

ss s

teel

40

2-

37.2

%

Bo

rosi

lica

te g

lass

36

0.9

00

328

Page 106: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

64

Spherical capsules (in blue) and cylindrical capsules (in black) studies are included

in Table 3-2. The latent heat of the PCM corresponds to the latent heat of the raw

material; (**) indicates the latent heat of the raw material measured using

MacroDSC; (***) indicates the latent heat of the raw material extracted from the

literature; and (MEPCM) indicates the latent heat of the microencapsulated PCM as

a whole. The melting temperature and latent heat of the different encapsulated

high temperature PCM reviewed are represented in Figure 3-5. For the existing

high temperature encapsulated PCM studies, there seems to be a casual and almost

linear correlation between the chosen PCM latent heat and melting temperature

values. The represented materials are expected to be the consequence to select the

most cost-effective PCM at each temperature.

Figure 3-5: High temperature PCM successfully encapsulated

Most of the capsules analyzed contain a void space left to accommodate the PCM’s

volumetric expansion. In cylindrical capsules this space is left during the filling

process. On the other hand, PCM made out of pellets, the void space is left by

controlling the compactness of the PCM powder. This void space can be included

0

100

200

300

400

500

600

0 200 400 600 800 1000 1200

Late

nt

Heat

[kJ/

kg

]

PCM Melting Temperature [ºC]

Metal

Chloride

Nitrate

Present work Metal

Present work nitrate

Page 107: Pablo Giménez Gavarrell

65

Thermal Energy Storage for High Temperature Applications

in the Shell/Core volume ratio in order to calculate the effective Shell/solid PCM

volume ratio by:

Shell-solid PCM volume ratio:

Equation 3-2

Figure 3-6 represents the shell-solid PCM volume ratio as a function of the external

diameter for different types of PCM. Metallic PCM are more often used for small

capsules, while nitrate salts are encapsulated using larger capsules. For capsules

diameter lower than 5 mm we can find shell/solid PCM volume ratios varying

from ~10% to ~300%. These large variations indicate the difficulties to create thin,

homogeneous, and resistant barriers around the core PCM. On the other hand, as

the capsule diameter increases the shell-solid PCM ratio seems to progressively

reduce yielding much more efficiently filled PCM capsules. The target dimensions

and filling ratios for the capsules developed in this study (20 mm external

diameter) are also included in Figure 3-5 and Figure 3-6 to show they are in line

with similar studies but present a higher filling ratio than other successful macro-

capsules.

Figure 3-6: Shell/solid PCM volume ratio vs. capsule external diameter.

1%

10%

100%

1000%

0 20 40 60 80

Sh

ell/

so

lid

PC

M v

olu

me

rati

o [

%]

External Diameter [mm]

Metal

Chloride

Nitrate

Present work Metal

Present work nitrate

Large variability

Page 108: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

66

3.3. Shell Material Selection

A successful encapsulation procedure must meet these three requirements: 1)

material compatibility with both: PCM and heat transfer fluid, 2) thermal

cycling/stability and 3) mechanical stresses under operating conditions, as

commented previously. It must also provide an adequate surface for heat transfer.

The problem is complex due to the combination of high temperature and high

pressure requirements, since the capsule has to exchange heat with water vapor at

T> 300ºC and P100 bar. To simplify the analysis, the thermal and pressure

problem have been decoupled. As a first step, the thermal problem can be

evaluated in a straightforward experiment, concentrating on the high temperature

aspect to characterize the melting/freezing process and analyze the thermal cycling

behavior. Once the thermal resistance is evaluated, the following step would be to

couple high temperature and pressure and analyze the capsule behavior under

such conditions.

Some potential shell materials can be highlighted (Figure 3-7) from the

experimental studies on high temperature PCM-Capsule previously summarized

in Table 3-2. Most of these studies have used stainless steel to encapsulate different

PCM, investigating additional intermediate layers to prevent or reduce possible

interactions between the PCM and the stainless steel shell. Different coatings are

also used as an additional barrier between the capsule and the heat transfer fluid. It

is important to mention the novelty of some polymers to successfully encapsulate

high temperature PCM around 325ºC.55

Page 109: Pablo Giménez Gavarrell

67

Thermal Energy Storage for High Temperature Applications

Figure 3-7: Shell materials used to encapsulate high temperature PCM. (*Present

work)

Based on some unsuccessful experiences trying to encapsulate salts with metals68

and metals with metals62,63, other potential shell candidates are explored.

Encapsulating metallic PCM with metals is difficult because the core and shell

materials can alloy during the encapsulation procedure or during thermal cycling.

Encapsulating salts with metals can accelerate container corrosion issues due to the

high working temperatures, requiring specific coatings for both the HTF-shell and

the shell-PCM interfaces. Some ceramic shells also require coatings due to their

porous nature which can lead to leakages. Although polymers could be ideal

candidate to encapsulate due to their flexibility, low cost, and variety of

encapsulation methods, when this analysis was performed there was no

commercial polymer meeting the requirements of thermal stability above 400 ºC.

In the search for a compatible, impermeable medium to both the HTF (steam) and

potentially corrosive core materials (inorganic salts), borosilicate was proposed and

discussed in collaboration with Prof. I. G- Loscertales from the University of

Málaga as shell material. Several authors have used borosilicate tubes for hydrogen

Nitrates

•Polymer (PTFE)

•Coating: Ni

•Polymer (PTFE-FEP)

•Coating: Ni

•Polymmer (FEP)

•Coating: Ni

•Stainless steel AISI 321

•Carbon steel 1018 with Intermediate Layer of SiO2

•Metal+Clay

•Organic binder

•Borosilicate glass*

Chlorides

•Stainless steel 304L

•Carbon steel 1018 with Intermediate Layer :

•High T paints

•Na2SiO3

Metals

•α-Al2O3

•Stainless steel 304L

•Ni

•Ni with Intermediate Layer of:

•Cr

•Carbon

•Ruthenium

•Borosilicate glass*

Page 110: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

68

encapsulation, which can withstand 400bar.69,70 It means that with an appropriate

geometry it is able to resist the required pressure. On the other hand, it is able to

withstand high temperatures (500ºC for short-term usage (<10h); 450ºC for long-

term usage (>10h)). Its high thermal shock resistance is well known (borosilicate,

commonly known as PyrexTM, is a common cookware glass) making it an

interesting shell candidate together with its thermal stability and non-reactivity.71

Considering that the PCM in question are salts (typically highly corrosive) and the

heat transfer fluid is high pressure steam, finding an impermeable,

thermally/chemically stable, and non-reactive shell compatible with both external

and internal materials is very challenging. A SWOT analysis (Strengths-

Weaknesses-Opportunities-Threats) is shown for the borosilicate capsule in Figure

3-8.

One of the added benefits of using a glass shell is its transparency, allowing the

visualization of the phase change process within the capsule. Up to this date, the

visual observation of the melting process has been only carried out with organic

low temperature PCM (mainly paraffin wax n-octadecane), such as Moore and

Bayazitoglu (1982), Revankar et al. (2007), Tan et al. (2008 and 2009)72–75.

Therefore, a transparent high thermal resistance glass sphere could allow a similar

analysis for high temperature PCM to complement the recently increasing number

of numerical studies. Thus, this study presents a unique contribution by

developing a new encapsulation method for high temperature PCM based on

borosilicate capsules. We take advantage of the non-reactivity of this material, its

high thermal resistance, and the optical properties of the shell and the PCM,

developing an experimental set up able to reach temperatures up to 400 ºC, high

enough to melt some inorganic salt mixtures and metals while varying the external

flow conditions.

Page 111: Pablo Giménez Gavarrell

69

Thermal Energy Storage for High Temperature Applications

Figure 3-8: SWOT (Strengths-Weaknesses-Opportunities-Threats) analysis

performed to evaluate potential of borosilicate as a PCM shell material.

Strengths

- Material compatibility/ non-reactivity

- Thermal stability

-Inexpensive

-Available raw materials

-Easy to coat

Weaknesses

- Risk of mechanical failure (fragile)

- Management of PCM volume expansion

-Low thermal conductivity

Opportunities

- Transparent allows testing visualization

- Can encapsulate highly reactive materials

Threats

- Unknown mass production process or cost

-Patentability

Page 112: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

70

3.4. Capsule Design

The capsule dimensions are a compromise between achieving a maximum

allowable size and system design requirements (as smaller capsules have better

heat transfer behavior but produce a larger pressure drop along the packed bed),

while also taking into consideration the availability of raw materials and ease of

manufacturing. This size (diameter of 20 mm diameter with 1 mm thickness) is in

the same range as contemporary theoretical modeling studies by Ramos-Archibold

et al.76,77 and other salt capsules as shown previously in Figure 3-6. Additional

considerations should be taken into account once the encapsulating concept is

proven such as optimizing the packing degree by using multiple size capsules, for

example.

Two different capsule shapes are initially considered for this study: spherical and

cylindrical. Spherical capsules present some advantages in terms of minimizing the

surface and, consequently, the shell material for a given PCM volume. Cylinders

seem a priori easier to fabricate and mass produce, but natural convection effects

might be important if the capsule aspect ratio is large and capsule orientation will

also have an influence on system behavior.

The main heat transfer mechanisms controlling the melting process are heat

conduction and natural convection. Regardless of the heat source and boundary

conditions (constant heat flux at the wall or constant wall temperature) during the

initial stages of the melting process heat conduction plays a dominant role, until a

significant volume of material is melted and buoyancy-driven flow becomes

important. The thermophysical properties of the PCM, the heat supplied, and the

capsule characteristics (shape, orientation, and size) will have an important effect in

the dynamic behavior of the convective dominated melting process.78 To minimize

orientation and geometry effects, this study will focus on spherical capsules.

One of the main challenges regarding PCM encapsulation is the management of

the PCM volume expansion during the melting process without breaking the

capsule due to internal stress. As mentioned above, different solutions have been

proposed to overcome this difficulty, such as a polymer sacrificial layer that is

burned off as part of the manufacturing procedure. In this study, the use of a void

space inside the capsule will be tested as a method to manage the PCM volume

changes during the solidification/melting process. The minimum void space

Page 113: Pablo Giménez Gavarrell

71

Thermal Energy Storage for High Temperature Applications

required for each PCM capsule depends on the difference in density between liquid

and solid PCM. The metallic and salt PCM tested in this study have a volume

expansion below 10% and 20% respectively (specific values are shown in Table

3-3). For 20 mm diameter capsules, an empty space of 4 mm is left on top of the

PCM. This void space will cover the volume expansion of the different PCM tested.

3.5. Capsule Manufacturing

The PCM candidates for the capsule manufacturing are lead (Pb), tin (Sn), NaNO3,

KNO3 and Durferrit. As mentioned above, lead, NaNO3, and KNO3 have melting

temperatures in the range of interest; tin and Durferrit will also be used because

having a significant lower melting temperature with similar materials can simplify

some of the initial experimental measurements.

The capsule manufacturing procedure that will be described is used as a proof of

concept to develop the conceptual design. For the capsule mass production a

different manufacturing technique would be required.

The process uses hollowed borosilicate cylinders as raw material. The cylinders are

exposed initially to an annealing treatment (550ºC 1h and slow cooling inside the

furnace). One end of the tube is sealed. A spherical shape is achieved by keeping

the sealed end at higher temperature than the borosilicate softening temperature

(>550ºC) and blowing through the open side (Figure 3-9). The diameter is

controlled manually during the process.

Figure 3-9: Shaping a spherical capsule performed at the University of Zaragoza –

glass blowing service.

Once the initial shape is achieved and cooled down, the capsule is inspected to

analyze the residual thermal stresses in the glass. Polarized light is used for this

Page 114: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

72

purpose. The transitional areas between the spherical shape and the cylinder

contain residual stresses which represent weak points for crack initiation (Figure 3-

10).

Figure 3-10: Pre-shaped capsules (left) and local stresses in the sphere (right)

A new annealing process at 550ºC is required to effectively remove these residual

thermal stresses (Figure 3-11). The idea is to prevent the breakage of the capsule by

thermal shock when high temperature liquid PCM is poured inside. A second

visual inspection after the annealing and before filling the capsules is performed

with polarized light to ensure that there are not residual thermal stresses on the

capsule.

Figure 3-11: Annealing treatment of the pre-shaped capsules performed at the

University of Zaragoza glass blowing service.

Page 115: Pablo Giménez Gavarrell

73

Thermal Energy Storage for High Temperature Applications

The weight of the initial pre-shaped capsule is recorded. The capsules are

preheated before filling. It avoids the salt freezing when it is poured in liquid state

and it also minimizes the thermal shock. The salt is melted and poured into the

capsule controlling its level visually and its weight (Figure 3-12). The final weight

of the PCM is adjusted.

Figure 3-12: Crucibles created for the capsule filling and filled capsule performed at

the University of Zaragoza glass blowing service.

The closure of the capsule is performed, with the PCM at room temperature, by

connecting a vacuum pump to the open side of the tube while melting the

intermediate narrow section created for this purpose as shown in Figure 3-13.

Figure 3-13: Capsule closure procedure performed at the University of Zaragoza

glass blowing service.

The negative pressure created pulls in the soft and viscous glass, sealing the

remaining aperture when it solidifies. The final shape achieved is a uniform sphere

except in the closure section where a small elongation can be observed. Finally, a

Page 116: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

74

new annealing process inside a muffle furnace at 550ºC is required to eliminate the

closure residual thermal stresses. This means that the different encapsulated salts

have to withstand the temperature and duration of this treatment without

degradation. Thermo-gravimetric tests (1h 550ºC) were performed using

hermetically sealed and pin holed aluminum crucibles on the inorganic salts. No

weight loss was observed in the pin holed crucibles tests. The hermetically sealed

crucibles filled with salt tested did not open during the experiment. These results

guarantee the thermal stability of the salts in the last annealing process on the filled

and closed capsules.

The total volume for a 20 mm spherical capsule is ~4,189 mm3. For a shell thickness

of 1 mm, the shell volume represents 27.1% of the total capsule volume. Each

capsule is filled up to 87.4% of its maximum capacity, which is ~3,054 mm3 (with

the PCM in liquid state) considering a perfect spherical capsule, leaving

approximately 4 mm in the radial direction on top of the capsules. The ratio

between the PCM volume and the total capsule volume is 87.4% *(100-27.1) = 63.7%

with the PCM in liquid state. Because each material has different volumetric

expansion coefficients, a constant PCM liquid volume corresponds to different

filling ratios in the solid state for each PCM. The calculated amount of PCM for

each capsule type and the percentage of capsule filling in solid state are shown in

Table 3-3.

Table 3-3: Solid and liquid PCM density and calculated amount of PCM added to

each borosilicate capsule type. (*measured)

ρliquid

[g/cm3]

ρsolid

[g/cm3]

Volumetric

expansion

[%]

Capsule filling

in solid state

[Volume %]

Weight of

PCM

in the

capsule [g]

Latent

Heat*

[J·g-1]

Storage

Density

[J/capsule]

NaNO3 1.9 2.26 18.9% 73.5% 5.07 176.8 896.4

KNO3 1.865 2.11 13.1% 77.2% 4.98 96.4 480.1

Sn 6.99 7.365 5.4% 82.9% 18.65 44.4 828.1

Pb 10.66 11.34 6.4% 82.1% 28.44 20.9 594.4

Furthermore, the capsule has to withstand the pressure from the internal air

expansion. The inside initial pressure at room temperature, after the capsule

Page 117: Pablo Giménez Gavarrell

75

Thermal Energy Storage for High Temperature Applications

closure, is under atmospheric pressure because of the vacuum pump connected.

However the degree of vacuum is not specifically measured or controlled, but

could be in future mass production processes. Hence, it is assumed that there is

some air remaining inside the capsule during the fabrication process. The internal

pressure increases because the volume occupied by the internal air is reduced

when the PCM expands during melting. Besides this effect, the change in

temperature from 25ºC to 550ºC also increases the gas internal pressure. The

pressures inside the capsule in the last annealing treatment (most unfavorable case)

can be estimated using the combined gas law, assuming that the spherical capsule

does not expand at all compared to the PCM that expands much more, mainly

because of the phase change volumetric expansion.

The fraction of the initial volume that is occupied by the PCM at room temperature

is denoted by %PCMvolume. The densities of each PCM in solid and liquid state are

shown in Table 3-3. The thermal expansion in liquid state has not been considered.

The initial (Tinitial) and final (Tfinal) temperatures are 293 and 823 K. In the worst-case

scenario, where the created vacuum is neglected, the initial gas pressure (Pinitial air)

in the capsule is 1 atm. As the PCM expands, the effective volume of the gas left in

the capsule is reduced resulting in an increased air pressures. Using ideal gas law

for a volume domain described in Blaney et al.61 the final air pressure is determined

using Equation 3-3:

Equation 3-3

Figure 3-14 shows the pressure inside the spherical capsule filled with different

amount of PCM in solid state and exposed to the annealing temperature (550ºC).

The theoretical pressure inside the capsules manufactured during the annealing

process has been represented also in the graph. The design criterion for the capsule

filling percentage (“capsule filling in solid state volume” from Table 3-3) was

chosen to ensure that in the worst case scenario (capsule completely full of air, no

vacuum) the internal pressure does not dramatically increase as shown by the

vertical slopes in the curves in Figure 3-14 for each PCM.

Page 118: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

76

Figure 3-14: Pressure inside the capsules for different PCM and different capsule

filling percentages (in solid state) at room temperature (worst-case scenario, no

vacuum inside). The dots represent the expected pressure inside the capsules

manufactured

It is worth mentioning that because the capsule is not a perfect sphere there is an

extra volume that will reduce the real pressure inside the capsule. This means that

the represented dots in Figure 3-14 overestimate the real pressure as long as salt

does not degrade 1) because there is some degree of vacuum inside and 2) because

the capsules are not perfectly spherical. Table 3-4 shows the list of capsules

manufactured without defects which can be seen in Figure 3-15 to Figure 3-17.

There is certain variability among capsules (diameter, thickness, filling, closure

gap) inherent to the manual blowing manufacturing process, which can be

minimized when the fabrication is later standardized.

0

2

4

6

8

10

12

14

16

0% 20% 40% 60% 80% 100%

Pre

ssu

re i

nsi

de

the

cap

sule

[b

ar]

Capsule filling (% of PCM)

NaNO3

KNO3

Sn

Pb

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77

Thermal Energy Storage for High Temperature Applications

Table 3-4: List of spherical PCM capsules manufactured included in this study

Number Material Diameter1

[mm]

Diameter2

[mm]

Diameter3

[mm]

Avg φ

[mm]

SD φ

[mm]

PCM

[g]

Borosilicate

[g]

Total

Weight

[g]

1 NaNO3 20.55 20.63 20.56 20.58 0.04 5.07 3.354 8.424

2 NaNO3 19.55 19.53 19.62 19.57 0.05 5.07 3.014 8.084

3 NaNO3 19 19.03 18.97 19.00 0.03 5.07 2.831 7.901

4 NaNO3 19.66 19.71 19.73 19.70 0.04 5.07 3.058 8.128

5 Durferrit 19.3 19.28 19.26 19.28 0.02 5.00 2.921 7.921

6 Durferrit 21.04 20.99 21.03 21.02 0.03 5.00 3.508 8.508

7 Durferrit 19.24 19.17 19.2 19.20 0.04 5.00 2.896 7.896

8 KNO3 19.32 19.35 19.4 19.36 0.04 4.98 2.946 7.926

9 KNO3 19.66 19.75 19.68 19.70 0.05 4.98 3.056 8.036

10 KNO3 18.73 19.02 18.91 18.89 0.15 4.98 2.796 7.776

11 Pb 19.88 19.93 19.88 19.90 0.03 28.44 3.123 31.563

12 Pb 19.88 19.83 19.77 19.83 0.06 28.44 3.099 31.539

13 Pb 20.79 20.8 20.82 20.80 0.02 28.44 3.432 31.872

14 Sn 18.66 18.68 18.7 18.68 0.02 18.65 2.731 21.381

Page 120: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

78

Figure 3-15: NaNO3 capsules after the last heat treatment (1-2-3-4)

Figure 3-16: Durferrit capsules after the last heat treatment, capsules (5-6-7)

(1)

(2)

(3)

(4)

NaNO3

(5) (6) (7)

Durferrit

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79

Thermal Energy Storage for High Temperature Applications

Figure 3-17: Capsules after the last heat treatment: lead capsules (11-12-13) (left

image); tin (14) and KNO3 (10) capsules (right image)

Finally, two capsules have been broken to analyze the variability of the capsule

thickness through nine measurements along the capsule. Table 3-5 shows the

results. The deviations from the average value are lower than 3.5%.

Table 3-5: Variability of the capsule thickness

Thickness

Capsule A

[mm]

Thickness

Capsule B

[mm]

1.13 1.11

1.07 1.12

1.12 1.13

1.18 1.05

1.08 1.17

1.17 1.13

1.07 1.17

1.11 1.20

1.22 1.21

Average [mm] 1.13 1.14

Standard Deviation (SD) [mm] 0.05 0.05

Standard Error of the Mean (SEM) [mm] 0.02 0.02

Confidence Intervals (95%) [mm] 1.09 - 1.17 1.10 - 1.18

Sn (14) KNO3 (10)

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Macro Encapsulation of PCM

80

3.6. Capsule Testing

3.6.1. Set-up Design

The experimental set-up to test a single PCM capsule was conceived in

collaboration with Prof. I. G. Loscertales from the University of Malaga, with initial

experiments performed at the installations of Yflow Sistemas y Desarrollos, S.L.

Further modifications to the set-up and the main tests were carried out at the

laboratories of Abengoa Research.

The experimental set-up consists of a glass cylinder 2 mm thick, 550 mm long, with

a 36 mm outer diameter, insulated with 15 mm of mineral wool and an air blow

heater connected to one end of the cylinder which is fed from a pressurized air line

at 5 bars. A volumetric flow meter and a pressure meter are connected between the

blow heater and the pressurized line to control the volumetric flow rate and air

density and hence the air mass flow is determined. The blow heater (BH) causes a

pressure drop, reflected in an increase in the upstream pressure. This increase of

pressure has to be considered for the mass flow calculation due to the

measured/controlled variable is the volumetric flow rate. Figure 3-18 shows the

correlation between the intermediate pressure and the volumetric flow rate.

The set up developed allows the test of borosilicate-PCM capsules analyzing the

melting and solidification process. The PCM-capsule is placed inside the tube using

a home-made sample holder consisting of stainless steel mesh (5x5 mm squared)

used to fix the position of the capsule in the tube. The capsule is exposed to the air

flow, melting or freezing the PCM depending on the stream temperature. Figure

3-19 shows a schematic representation of the set-up (dimensions in mm).

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81

Thermal Energy Storage for High Temperature Applications

Figure 3-18: Intermediated relative pressure before the blow heater vs. volumetric

flow rate (liters per minute, LPM)

Figure 3-19: Experimental set-up schematic. All dimensions are in mm.

Six thermocouples monitor the system to estimate the real gas temperature applied

to the capsule. Thermocouples T1 to T3 are placed 200 mm downstream from the

blow heater. The thermocouples T4 to T6 are located at 150 mm from the end of the

borosilicate tube. The capsule is placed 1cm downstream from thermocouples T1 to

T3. The temperature upstream the capsule can be estimated assuming linear

thermal profile by:

Equation 3-4

y = 0.00122x1.541969 R² = 0.99015

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 20 40 60 80 100

Inte

rmed

iate

Pre

ssu

re[b

ar]

LPM

36

2

550

20200 150

TC1,TC2,TC3 TC4,TC5,TC6

air

g

Page 124: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

82

Figure 3-20 shows an example of the estimated temperature around the capsule in

a freezing experiment using Equation 3-4, where T1-3 and T4-6 are the average

temperatures of thermocouples 1-3 and 4-6. The temperature of the air around the

capsule is estimated assuming linear thermal profile between the two groups of

thermocouples.

Figure 3-20: Measured temperature profile in the experimental set-up: average

temperature thermocouple 1 to 3, 4 to 6, and capsule estimated temperature.

The air crossing the blow heater is heated up by an electrical resistance. The power

supplied to the resistance is controlled modifying the input voltage through a

voltage transformer. The set-up schematic and images of the testing devices can be

seen in Figure 3-21 and Figure 3-22 respectively.

170

190

210

230

250

270

290

310

0 10 20 30 40 50 60 70 80 90 100 110 120 130

Tem

pera

ture

[ºC

]

Time [s]

Avg T1-3

Avg T4-6

Tcapsule

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83

Thermal Energy Storage for High Temperature Applications

Figure 3-21: Schematic representation of the experimental set-up installation

Figure 3-22: Initial experimental set-up installation performed at the laboratories of

Yflow Sistemas y Desarrollos, S.L. under the collaboration with Prof. I. G.

Loscertales from the University of Malaga.

The blow heater outlet temperature depends on the power supplied and the mass

flow. A detail characterization of the blow heater determining parameters such as

the temperature and mass flow limits of the installation and the response against a

step-wise change in voltage are required for the definition of the experimental

procedure. Figure 3-23 shows a schematic representation of the expected response

BH

air

g

IR

Camera

Video

Camera

Data

logger

TCs

V

IR

Camera

Video

Camera

Page 126: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

84

of the set-up. The step in voltage applied is shown in blue, the blow heater outlet

temperature in red and the estimated air temperature around the capsule using

Equation 3-4 in green.

Figure 3-23: Schematic of the expected thermal response of the set-up

There is a transient for the outlet temperature of the blow heater to reach a specific

outlet temperature and also a delay caused by the thermal inertia of the glass tube.

Therefore, the air around the capsule increases gradually until it reaches the blow

heater outlet temperature.

Figure 3-24 represents the experimental characterization of the flow parameters.

For each volumetric flow rate and voltage there is a stable temperature around the

capsule. It determines the initial and final voltage in order to apply a desired initial

and final temperature around the PCM-capsule for a specific volumetric flow rate.

Two different volumetric flow rates will have different heat transfer convection

coefficient around the capsule. In order to test with the same initial and final

temperatures for two different volumetric flow rates, different initial and final

voltages must be applied.

Capsule External

Temperature

(Experiment)

∆V set up

Tem

pera

ture

Time

Blow Heater Outlet temperature

Page 127: Pablo Giménez Gavarrell

85

Thermal Energy Storage for High Temperature Applications

Figure 3-24: Temperature vs. Voltage applied for different experiments performed

with the set-up at different volumetric flow rates (LPM, liters per minute). The

melting temperatures of two PCM (Durferrit, NaNO3) are shown as reference.

3.6.2. Experimental Procedure

The experimental procedure to melt/freeze a single capsule is the following:

1) place a PCM-capsule in the metallic sample holder

2) set the volumetric air flow rate (40-50-60 LPM) and the initial voltage,

heating the capsule from room temperature to the initial temperature

3) hold the initial temperature for 25 min as a homogenization

temperature step

4) heat up the air to the desired final temperature by changing the

applied voltage

5) hold at this temperature while the PCM melts

6) maintain this temperature for 25 min apply the initial voltage, cooling

down the capsule to its initial temperature, freezing the PCM

The procedure is repeated up to 10 times to ensure the capsule integrity to several

thermal cycles. The PCM melting/freezing process is observed through a

transparent window in the set-up and recorded by a digital camera during the

0

50

100

150

200

250

300

350

400

450

100 150 200 250 300

Tem

pera

ture

[ºC

]

Voltage [V]

40 LPM

50 LPM

60 LPM

80 LPM

NaNO3

Durferrit

Page 128: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

86

thermal cycle in order to determine:

the melting start and end time, and consequently the melting duration;

the freezing starting time

to examine any crack formation

In both types of experiments (melting and freezing) the thermocouples measured

temperature has been used to estimate the temperature around the capsule. This

applied temperature is adjusted to an exponential curve to simplify the boundary

condition form using Equation 3-5. This boundary condition will be used in the

capsule model.

Equation 3-5

Where, ∆T is the temperature step applied, and τ is the time where the measured

temperature has reached 63.2% of the temperature step. These parameters have

been calculated for each experiment, shown in Table 3-6 and Table 3-7. As an

example, Figure 3-25 shows the estimated temperature based on Equation 3-5

compared to the measured temperature in the experimental set-up for a freezing

and for a melting experiment.

Figure 3-25: Capsule temperature measured and approximated equation adjusted

to be used in the capsule model for two experiments

180

200

220

240

260

280

300

320

340

0 50 100 150 200 250 300 350

Tem

per

atu

re [º

C]

Time [s]

T capsule [ºC]

T model [ºC]

EXP 14 280

290

300

310

320

330

0 50 100 150 200 250

Tem

per

atu

re [º

C]

Time [s]

T capsule [ºC]

T model [ºC]

EXP 7

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87

Thermal Energy Storage for High Temperature Applications

The experiments are recorded simultaneously with both a visual and an infrared

(IR) camera (FLIR model SC7200 F/3 MW InSb) during the thermal cycling. The

infrared radiation comprises wavelength from 0.7 μm to 1000 μm. However, the

InSb detector can capture the spectral range of from 1.5 - 5.1 µm 79 covering the

Short-wavelength IR and Mid-wavelength IR.

The test duct and capsule shell are made out of borosilicate. The total optical

transmittance of borosilicate is higher than 90% for wavelengths 0.3 μm to 2.2 μm

for thicknesses between 0.7 to 5 mm. The transmittance of 1 mm borosilicate

thickness is higher than 40% for wavelengths between 0.3 μm to 3.5 μm (Figure

3-26). The temperature range between 25ºC and 300ºC does not change significantly

the transmittance, modifying slightly only the lower wavelength limit which is in

the UV range. The borosilicate is opaque for wavelength higher than 5 μm which is

out of the spectral range of the IR detector. Therefore, temperature differences

between the capsule and the test duct should be visible with this equipment.

Figure 3-26: Total optical transmittance of borosilicate.71

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Macro Encapsulation of PCM

88

The other important property to consider is the emissivity, which is ~0.9 for

borosilicate (Pyrex glass) for a temperature range from 25 ºC to 300 ºC.80 When the

capsule surface is at higher temperature than the tube surface, its radiation is

expected to be distinguished from the test duct’s emitted radiation. The emissivity

of lead and tin from 25ºC to 250°C80 is Pb = 0.05-0.1 (polished), 0.3 (oxidized), and

0.65-0.4 (rough lead) and Sn = 0.05-0.1 (for unoxidized tin). Due to the low

emissivity of these materials compared to the emissivity of borosilicate the

radiation detected by the IR camera is expected to be due to the borosilicate

capsule.

3.6.3. Experimental Results and Discussion

Several tests were performed without test duct to guaranty that the information

about the phase change process from the IR camera and visual camera match and

to assist evaluating the images using a borosilicate tube, where the radiation might

be attenuated. Figure 3-27 shows one of the freezing tests on a NaNO3 capsule

performed without a confined flow. There, a previously melted capsule is cooled

by blowing hot air at 100ºC over its top surface. Due to subsequent melting and

freezing cycles and the effect of gravity, the salt moves to the bottom of the capsule.

The freezing start time is determined by a visible change in salt color (Figure 3-27,

top) and by an abrupt change in slope of the IR temperature traces (Figure 3-27,

bottom). The solidification of the first initial layer of salt is a very rapid

phenomenon lasting less than 10 seconds, as shown in the graph in Figure 3-27 and

qualitatively in Figure 3-28. The formation of this first external solid layer is quite

uniform and almost concentrically, if it were not for the uneven distribution of salt

and existence of the void inside the capsule.

Page 131: Pablo Giménez Gavarrell

89

Thermal Energy Storage for High Temperature Applications

Figure 3-27: NaNO3 capsule freezing in free flow: Image A (top left, 400 s) shows a

completely liquid salt-borosilicate capsule, while image B (top right, 410 s) shows

the beginning of the salt crystallization process, as marked by a change in slope in

the IR camera temperature traces (bottom right). The different temperature traces

correspond to the locations numbered 1-5 in the IR image (bottom left).

A BA B

A B

Time

Tem

per

ature

Page 132: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

90

Figure 3-28: Sequential images of a NaNO3 capsule during the solidification process

as recorded with a visual camera (top) and an IR camera (bottom).

The addition of the borosilicate test duct attenuates the radiation from the capsule

and the acuteness that the phase change process causes on the radiation emitted by

the capsule surface and detected by the IR camera. However, it reduces the heat

losses and provides a more uniform and controlled heating environment. Figure

3-29 and Figure 3-30 show the freezing curves of two NaNO3 experiments, now

with the borosilicate test tube and a metallic sample holder. When the external

layer of the PCM freezes, the thermal curve (temperature vs. time) recorded by the

IR camera changes abruptly in slope (dT/dt). This change is clearer without a test

duct (Figure 3-27) but it can be also observed using the test duct (Figure 3-29 and

Figure 3-30). The initiation of the crystallization process in the salt can be observed

also visually by a change in color (opaqueness) and the formation of dendritic

shaped crystals (Figure 3-29 at 314 and 316 seconds) but unfortunately the end of

the process cannot be observed neither by the IR nor the visual camera.

Page 133: Pablo Giménez Gavarrell

91

Thermal Energy Storage for High Temperature Applications

Figure 3-29: NaNO3 capsule freezing with test duct: IR camera temperature trace

and video snapshots during the liquid-solid phase change highlighted in the red

area.

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

160

180

200

220

240

260

280

300

320

290 300 310 320 330 340 350 360

dT

/d

t [º

C/

s]

Tem

pera

ture

[ºC

]

Time [s]

T1

dT1/dt

Vid

eo C

amer

a im

ages

310 s 312 s 314 s 316 s 318 s 320 s

Page 134: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

92

Figure 3-30: NaNO3 capsule freezing with test duct

The identification of the melting process initiation and duration is slightly different

from the crystallization. Again, the process is more visible without the duct and

sample holder, in the free flow heating experiment (Figure 3-31) than with the

borosilicate test tube and metallic sample holder (Figure 3-32). The melting start

time corresponds to the first abrupt change in the temperature profile that appears

at ~115-120 seconds. Due to the low thermal diffusivity of NaNO3 allowing large

thermal gradients within the PCM, the end of the melting process cannot be

observed with the IR camera. However, the visual camera is used to estimate the

end of the melting process in NaNO3 capsules, because the liquid salt is translucent

and visibly different from the solid opaque salt.

The melting process inside the capsules shows clearly an unconstrained melting

behavior 74: the process is initially dominated by heat conduction across the shell

wall but as the PCM melts, it sinks to the bottom of the capsule (Figure 3-31 D,

Figure 3-32 C). Natural convection can be observed inside the capsule: the hotter

liquid PCM rises to the top as the cooler liquid and solid pieces remain at the

bottom.

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

160

200

240

280

320

360

280 290 300 310 320 330 340 350 360

dT

/d

t [º

C/

s]

Tem

pera

ture

[ºC

]

Time [s]

T1

dT1/dt

A

1

B B

C C

A

A B C

Page 135: Pablo Giménez Gavarrell

93

Thermal Energy Storage for High Temperature Applications

Figure 3-31: Capsule temperature trace and sequential images from IR camera (top

row) and visual camera (bottom row) of a NaNO3 capsule during the melting

process under free flow heating, showing: A melting starts at the upper border, B

melting extends over the complete capsule external surface, C, D, E during the

melting process, F completely liquid capsule after melting ends.

Page 136: Pablo Giménez Gavarrell

Macro Encapsulation of PCM

94

Figure 3-32: Visual and IR images at different stages of the melting process of a

NaNO3 capsule inside a borosilicate duct.

The IR camera temperature traces should be analyzed with care, because not all the

changes in slope indicate the beginning or end of the phase change process. For

example, abrupt capsule movements within the sample holder due to large pieces

suddenly melting (Figure 3-31 C, D), large bubbles or even the thermocouples

Melting

Melting

Melting

Time

Tem

per

ature

Time

Time

Time

Tem

per

ature

Tem

per

ature

Tem

per

ature

A

B B

C C

A

D D

B

C

A

D

Page 137: Pablo Giménez Gavarrell

95

Thermal Energy Storage for High Temperature Applications

moving can appear to have a similar change in capsule temperature. The only way

to definitively confirm that a modification in slope corresponds to a phase change

in these experiments is by comparing the IR data with the visual camera. Figure

3-31- Figure 3-33 show visual images during the complete melting process and

their corresponding temperature trace from the IR camera. At certain points there

are changes in slope during the melting process and it is sometimes difficult to

assess from the IR data only if it is indicating melting start times.

Figure 3-33: NaNO3 capsule melting with test duct recorded with visual and IR

camera. Slope changes in the IR temperature traces correspond to melting.

Vid

eo C

amer

a im

ages

Time

Tem

per

atu

re

120 s 130 s 140 s 150 s

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Macro Encapsulation of PCM

96

In the case of metallic PCM, the reflectivity of the surface changes when the PCM

melts (Figure 3-34). Small bubbles are formed every time the PCM freezes because

of the PCM contraction and they disappear when melting. This is the only

information available for metallic PCM with the video camera and unfortunately

this information is too ambiguous to be used to determine the phase change

process. However, the infrared images allow a clear determination of the phase

change process for both melting and freezing experiments as shown in Figure 3-35

for a Pb-capsule.

Figure 3-34: Tin (Sn) capsule. Change in reflectivity when melting. Completely

solid Sn (left), during the solid-liquid phase change (middle), and completely liquid

Sn (right).

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97

Thermal Energy Storage for High Temperature Applications

Figure 3-35: Determination of the phase change process for melting and freezing

experiments on the Pb-Capsule number 11 based on the IR camera’s temperature

curves.

Time

Tem

per

ature

FreezingMelting

A B C D E

A B C D E

A B C D E

Temperature

Temperature

Temperature

Heating Cooling

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Macro Encapsulation of PCM

98

Figure 3-36: Comparison of IR camera temperature traces from Pb and NaNO3

PCM capsules during sequential melting and solidification in free flow experiment.

Shadowed area represents the standard deviation due to capsule surface location.

Overall, for salt PCM capsules, the melting front is hard to trace with the IR

camera, firstly because of the camera’s resolution and secondly because what we

measure is a combination between the radiation emitted from the capsule surface

and the radiation emitted by the PCM. In contrast, the IR camera provides much

more useful information for metallic PCM capsules.

Due to the large difference in thermal conductivity of some of the PCM tested (salt

vs metal) the IR temperature trace behavior is dramatically different in each case

(Figure 3-36). For low conductivity PCM (salts), the capsule shell will see only a

change in slope when the phase change begins, as higher thermal gradients appear

inside the salt core which is slowly melting. For high conductivity PCM (metals),

the core melts quickly throughout the capsule radius keeping the overall capsule

isothermal during the process and, consequently, showing a flatter temperature

profile in the IR camera temperature traces. This happens during both the melting

and the freezing process. This explanation will later be confirmed by the model

analysis in Chapter 5, showing the different type of behavior depending on the

120

140

160

180

200

220

240

260

280

300

320

340

360

380

0 100 200 300 400 500

Tem

pera

ture

[ºC

]

Time [s]

Pb

NaNO3

PbNaNO3

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99

Thermal Energy Storage for High Temperature Applications

type of PCM but it can also be seen qualitatively in the comparison shown in

Figure 3-36. Here, a lead (Pb) and a salt (NaNO3) capsule are compared under the

same external heating (Thot=430ºC) and cooling (Tcold=100ºC) temperatures in the

free flow set-up, although the lead capsule is only heated for 280 s whereas the salt

capsule is heated for 380 s.

3.6.4. Melting Results

Table 3-6 shows the results for different melting experiments on NaNO3 capsules in

the test duct. The volumetric air flow in the set-up is adjusted to 40-50-60 LPM in

order to evaluate the effect of different convective heat transfer coefficients around

the capsule. Different initial and final voltages have been adjusted for each

experiment resulting in different initial and final temperatures and different

dimensionless melting temperature θ

. For each experiment the

starting time of the melting process and the end time have been estimated

analyzing the recorded images, calculating the melting duration time.

The analysis of the melting experiment number 6 (Table 3-6) on a NaNO3 capsule is

described in detail as an example. The initial capsule temperature measured by the

thermocouple is 234ºC while the initial temperature measured by the IR camera is

203ºC. This difference might be caused by the radiation absorbed by the

borosilicate tube. Analyzing the temperature vs. time curve of the IR images, a

small variation is observed in t~60 seconds since step in voltage (Figure 3-37). The

derivative of the temperature recorded by the IR camera is represented in green.

The small temperature slope variation (blue) is clearly observed with its derivative

(green) indicating that the melting process of the first layer of PCM has begun.

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Macro Encapsulation of PCM

100

Table 3-6: Melting experiments summary (NaNO3 Tm 302ºC)

Num Material

PCM

Q

[LPM]

T

Initial

[ºC]

T

Final

[ºC]

∆T

[ºC]

τ

[s]

Start

Melting

[s]

End

Melting

[s]

Melting

Duration

[s]

θm

1 NaNO3 40 204.9 368.9 164.0 51.4 170 830 660 59.2%

2 NaNO3 50 256.6 415.0 158.4 47.8 50 380 330 28.7%

3 NaNO3 50 266.6 387.1 120.5 51.8 50 450 400 29.4%

4 NaNO3 50 270.0 381.0 111.0 45.5 60 570 510 28.8%

5 NaNO3 40 270.6 404.9 134.3 43.6 70 425 355 23.4%

6 NaNO3 50 233.9 387.9 154.0 64.8 60 410 350 44.2%

7 NaNO3 60 282.1 329.0 46.9 46.6 40 601 561 42.4%

8 NaNO3 60 278.5 348.0 69.6 49.6 30 460 430 33.9%

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101

Thermal Energy Storage for High Temperature Applications

Figure 3-37: Thermocouple temperature (red), IR camera temperature (blue) and

derivative of the IR temperature (green).

The recorded images of the capsule melting confirm that the melting process

begins around 60 seconds after the step in voltage was applied (Figure 3-38). The

end melting time is estimated using the recoded video, when the capsule is

completely transparent.

-0.05

0

0.05

0.1

0.15

0.2

200

225

250

275

300

325

350

375

400

-20 0 20 40 60 80 100 120 140 160

dT

emp

erat

ure

/dt

[ºC

/s]

Tem

per

atu

re [

ºC]

Time [s]

T capsule (Thermocouple) T (IR) dT/dt (IR) Melting

Experiment 6

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Macro Encapsulation of PCM

102

Figure 3-38: Recorded images of the melting experiment number 6. At t~60 seconds

the melting process seems to begin, confirmed by the IR temperature.

Durferrit capsules have been also thermally cycled to confirm the capsule integrity

(Figure 3-39). The phase change of this salt extends to a wide temperature range, as

observed in the DSC measurements, consequently the beginning and end of the

phase change process is harder to estimate.

Figure 3-39: Example of a Durferrit capsule melting experiment.

3.6.5. Freezing Results

Similarly to the melting experiments, freezing experiments have been also

performed. Table 3-7 summarizes the results. The start of the freezing process can

be estimated visually from the video recording. Unfortunately, in this case the end

time of the freezing process cannot be estimated because when the first layer of salt

solidifies, it loses its transparency. Figure 3-40 and Figure 3-41 are examples of

freezing experiment on a Durferrit capsule and NaNO3 capsule respectively.

40 s 60 s Melting 120 s 240 s 400 s280 s

250s 270s 290s 310s230s

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103

Thermal Energy Storage for High Temperature Applications

Figure 3-40: Example of a Durferrit capsule freezing experiment

Figure 3-41: NaNO3 capsule freezing experiment (Number 15).

Time: 15s Time: 17s Time: 20s

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Macro Encapsulation of PCM

104

Table 3-7: Freezing experiments summary.*SF: start Freezing

Num Material

PCM

Q

[LPM]

T

Initial

[ºC]

T

Final

[ºC]

∆T

[ºC] τ [s]

Start

Freezing

[s]

θm

9 NaNO3 40 372.0 228.6 -143.4 37.8 80 48.8%

10 NaNO3 40 369.8 244.9 -124.9 37.1 75 54.3%

11 NaNO3 50 349.6 163.9 -185.7 32.5 50 25.6%

12 NaNO3 50 357.9 223.3 -134.6 41.2 70 41.5%

13 NaNO3 50 359.4 256.0 -103.5 46.5 90 55.5%

14 NaNO3 50 330.7 205.0 -125.7 49.5 45 22.8%

15 NaNO3 50 314.3 185.4 -128.9 45.7 20 9.5%

16 NaNO3 50 379.8 282.1 -97.7 38.0 110 79.6%

17 NaNO3 50 380.5 273.3 -107.2 39.7 105 73.2%

18 NaNO3 50 404.2 280.1 -124.1 38.9 130 82.4%

19 NaNO3 50 345.0 238.0 -107.0 46.5 80 40.2%

20 NaNO3 60 347.9 237.6 -110.3 39.8 80 41.6%

3.7. Conclusions

An extended literature review on high temperature encapsulation has been

performed in this chapter. Borosilicate as an encapsulating material is proposed

based on the different problems identified for high temperature PCM. It is

compatible with the PCM (salt and metal) evaluated and with the HTF (high

pressure steam). The capsules are designed, manufactured, and tested in an

experimental rig to qualitatively and quantitatively analyze the PCM melting and

solidification process. The phase change process is identified using a combination

of visual and infrared images. Changes in the radiation from the capsule, detected

by the IR camera and transformed to temperature curves, are clearly correlated

with the phase change process.

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105

Thermal Energy Storage for High Temperature Applications

For inorganic salt PCM, changes in the slope of the temperature curves from the IR

images facilitate estimating the melting start time and freezing start time,

complementing the information observed visually recorded with the video camera.

The melting start time is hard to appreciate with the video camera, but is more

easily estimated based on the IR temperature profiles. The melting end time is

clearly observed with the video camera as well as the freezing start time.

For metallic PCM, the visual images do not provide any useful information.

However, based on the IR information, the beginning and the end of the phase

change process (in both melting and freezing) can be estimated.

Comparing the different PCM cores, the IR temperature history traces also show a

qualitative difference: metallic PC due to their higher thermal conductivity allow

smaller gradients within the capsule; when the outer surface melts, the inner core

quickly reaches the melting temperature, resulting in a flat (almost isothermal)

profile during the melting process. Salt PCM, with lower thermal conductivity, take

a longer time to melt and present larger gradients within the capsules themselves.

Qualitatively, the melting process shows a typical unconstrained melting behavior

with natural convection inside the capsule and solid portions sinking to the bottom

during the phase change, as confirmed by the visual camera images. The freezing

of the most outer layer shows in contrast a more uniform, almost concentric

behavior, if it were not for the existing asymmetries in the capsule due to the void

and salt location.

The analysis of the melting start time and melting duration as well as the freezing

start time for NaNO3-capsules is performed in Chapter 5 comparing with

numerical simulations to further explain the different results and trends.

Important questions regarding the borosilicate shell capsules appeared during their

manufacturing and testing such as the possibility of mass production to reduce

fabrication costs and their fragility. However, the capsules tested show mechanical

integrity and thermal stability over 10-15 freeze-thaw cycles. Similar processes in

glass laboratory test tube manufacturing also suggest approaches to automate and

simplify the fabrication process.

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Macro Encapsulation of PCM

106

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107

4 SINGLE CAPSULE MODEL

n this section the heat transfer of a single capsule is analyzed numerically. A

qualitative and quantitative study is provided to help understand the phase

change process and the PCM-capsule behavior. The objective is twofold: 1) aid

in the design of the PCM capsules and 2) compare to the single capsule

experimental results to further validate the proof-of-concept (Figure 2-2).

I

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Single Capsule Model

108

Figure 2-2 Schematic representation of the PCM study with the contents of this

chapter marked in yellow.

4.1. PCM-capsule heat transfer model

The phase change inside a capsule is a highly complicated process; the

understanding of the phenomena and their simulation present several challenges50:

The motion of the solid-liquid interface (moving boundary problem)

makes it non-linear.

The heat transfer process at the interface is difficult to predict due to

buoyancy-driven natural convection in the fluid.

The thermal resistance between shell and solid PCM remains uncertain

and it is hard to quantify.

Some other phenomena may also appear during phase change which can

have a determinant influence in the heat transfer process such as the

volumetric expansion resulting in over-pressurization at melting, or the

creation of voids during solidification.

PCM TES heat

exchanger system

Packed bed

Macro-encapsulation

Screen & characterize

PCMs

Screen Shell Materials

Model Single Capsule

Fabricate capsules

Test single

capsule

Compare Experiments

& Model

Evaluate performance & challenges

Tube & housing

Metal PCM

Double PCM

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109

Thermal Energy Storage for High Temperature Applications

The enthalpy formulation of the energy equation, Equation 4-1, simplifies some of

the difficulties linked to the moving boundary problem:

Equation 4-1

The solution of Equation 4-1 requires knowledge of the enthalpy–temperature

functional dependency. This is the preferred method amongst authors, its major

advantage being that an explicit treatment of the moving boundary is not required,

which simplifies the phase change problem. Other advantages are:

The governing equation is similar to the single phase equation.

There is no condition to be satisfied at the solid–liquid interface as it

automatically obeys the interface condition.

Enthalpy formulation allows a mushy zone between the two phases.

Modeling the melting process of a material in a spherical enclosure has received

much attention by the research community in the past decades. Assis et al.81

explored numerically and experimentally the process of melting of paraffin wax

(PCM) in a spherical geometry, including a parametric study about the influence of

capsule radius and HTF temperature on melting process. The simulations provide

detailed phase and flow fields inside the system and incorporate such phenomena

as convection in the liquid phase, volumetric expansion due to melting, sinking of

the solid in the liquid, and close contact melting. The sinking solid phase and the

thin liquid layer between it and the shell are shown. Moore et al. in 1982 studied

the same phenomenon numerically and experimentally, investigating the melting

of n-octadecane in a glass spherical enclosure.72

Archibold et al.77 also analyzed numerically the heat transfer during melting of a

PCM inside a closed and uniformly heated spherical shell. The results were

compared with the experiments performed by Tan et al.75. In a similar study, the

same authors further developed the model to consider partially filled spherical

shells.76

Despite the high complexity of the melting process and, consequently, the

complexity of the different models reviewed which could be implemented, a

compromise between complexity and computational time is finally met by

implementing a finite differences scheme to solve a one-dimensional heat transfer

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Single Capsule Model

110

model for a PCM-capsule based on Zhao et al.44. The boundary fixing method is

used to solve the motion of the solid-liquid interface. The heat transfer within the

molten material is assumed to be purely a conduction-dominated problem. The

solid portion of the PCM is considered to remain at the center of the sphere,

neglecting the solid core motion away from the center due to the density

differences between the solid and liquid phases. Due to the symmetry it can be

reduced to a 1D problem, as it is assumed that the convective heat transfer

coefficient is uniform around the capsule (diffusion occurs only in radial direction).

Several modifications are introduced in the approach including a gradual

temperature boundary condition (see 4.5), adjustments to account for void space

inside the capsules, and the general resolution of the equations at the interface.

The variability in the capsule manufacturing and the uncertainty regarding the

convective heat transfer coefficient in the experimental set up make meaningless

trying to solve a more complex model for this first approach. On the other hand,

the model implemented has sufficient complexity to estimate the main capsule

design parameters, demonstrate their feasibility, and validate the proof-of-concept

experiments. It also allows the study of different factors such as the influence of the

particular PCM properties on the phase change times, the impact of different shell

characteristics, capsule size, and the effect of the convective heat transfer coefficient

on the charging/discharging process.

The problem to be solved is the heating/cooling of a spherical capsule of outer

diameter R1=20 mm, with a shell of thickness e= R1-R2= 1.1 mm. The material

representing the PCM core (r < R2) undergoes melting/solidification during this

process, which is tracked with the melting front situated at r=s(t) (Figure 4-1). The

position of the melting/solidification front is forced to move two nodes each time

step calculating the time step required.

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111

Thermal Energy Storage for High Temperature Applications

Figure 4-1: Schematic of the problem to be solved

Solid Liquid

S(t)

R2

R1

PCM

Shell

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Single Capsule Model

112

Governing Equations Dimensionless Discretization

Diffusion equation for a sphere, being j=1,

2, 3 the shell, the liquid PCM and the solid

PCM respectively.

Boundary conditions Dimensionless Discretization

is the capsule radius divided by the PCM

radius

is the thermal diffusivity, ‘j’ is 1-2-3 for shell,

liquid and solid respectively,

; is the thermal

conductivity [W (m K)-1]; is the density [kg·m-3], and

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113

Thermal Energy Storage for High Temperature Applications

is the specific heat [J (g K)-1]

L: latent heat [J kg-1]

Dimensionless temperature, and

dimensionless melting temperature. Tm is the melting

temperature; Tf is heat transfer fluid temperature and To

is the initial temperature.

Dimensionless radial position, and

dimensionless melting front position.

dimensionless time

dimensnsionless heat transfer coefficient

For charging process, initially:

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Single Capsule Model

114

The linear system that is solved is:

Where P, T and MB are:

·

+

·

P·θk + T = MB· θk+1

θk+1= MB-1·[P·θk + T]

θk+1= MB-1· P·[MB-1·(P·θk-1 + T) ] + MB-1 T

θk+1= MB-1 P·MB-1· P·θk-1 + MB-1·P· MB-1 T + MB-1 T

θk+1= (MB-1 P)2 ·θk-1 + (MB-1·P + I)· MB-1 T

θk+1= (MB-1 P)2 · (MB-1·[ P·θk-2 + T]) + (MB-1·P + I)· MB-1 T

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115

Thermal Energy Storage for High Temperature Applications

θk+1= (MB-1 P)3 ·θk-2 + (MB-1 P)2MB-1 T + (MB-1·P + I)· MB-1 T

θk+1= (MB-1 P)3 ·θk-2 + ((MB-1 P)2+MB-1·P + I)· MB-1 T

θk+1= (MB-1 P)k+1 ·θ0 + MB-1 T]

Where Ωi, Ai, Bi, Ci and Di are:

Ω Ω Ω

Ω

The sub-index ‘i’ indicates the shell (1), liquid (2) and solid (3) region for Ω, D and

B. On the other hand, for each i-row, A and C are calculated with their radial

position (Ri is the radial position of each node).

The matrices built when there is no phase change involved are unchanged. The

boundary conditions between heat transfer fluid-shell and the adiabatic conditions

at the center of the capsule are also unchanged during the whole problem.

However, the boundary condition between the storage material and the shell

depends on the state of the PCM (liquid or solid). K1 or K2 is used to build this

condition depending on having liquid PCM-shell or solid PCM-shell respectively.

Finally, in order to solve the phase change problem, the moving boundary

condition is introduced between the solid-liquid interface. The model uses an

iterative process to calculate the time step and the temperature profile at the

interface when it moves two nodes inwards. The temperature in the following

node is forced to be the melting temperature (melting front position) and the matrix

MB is divided in two square sub-matrixes solving the problem by blocks, obtaining

the temperature from the node 1 to the melting front ‘node-1’, and from the melting

front ‘node+1’ to the external node. The new matrices in order to solve the phase

change problem are:

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Single Capsule Model

116

·

+

·

4.2. Grid and time-step convergence

The convergence tests are conducted by using various numbers of nodal points in

radial direction and various time steps. For a 50 mm diameter NaCl-MgCl2 eutectic

PCM capsule the melting start time does not vary significantly with the time step

for ∆τ<0.002. Similarly, the melting duration time does not vary significantly for a

longer time step, ∆τ<0.05. Consequently, the most restrictive time step ∆τ=0.002 has

been selected for the simulations. Figure 4-2 shows the result of the time step

convergence test.

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117

Thermal Energy Storage for High Temperature Applications

Figure 4-2: Melting starting time and melting duration time as a function of time

steps.

The result for the spatial convergence test is displayed in Figure 4-3. For a number

of nodes N>455 the changes in melting start time and melting duration become

small (∆τ=0.002).

Compared to Zhao et al.44 (∆τ=0.01 and N=55), the time step and number of nodal

points selected in this study (∆τ=0.002 and N=455) are significantly shorter and

larger respectively. This is due to the fact that in this analysis the comparison has

been performed comparing times in seconds instead of minutes with only one

significant decimal value. This analysis compares also the start melting time, which

is an order of magnitude less than the melting duration.

1820

1822

1824

1826

1828

1830

1832

1834

1836

1838

1840

140

142

144

146

148

150

152

154

156

158

160

0.001 0.01 0.1

Melt

ing

tim

e D

ura

tio

n [

s]

Melt

ing

Sta

rt t

ime [

s]

∆τ

Start Melting

Melting Duration

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Single Capsule Model

118

Figure 4-3: Start melting and melting duration time as a function of mesh size.

4.3. Model Validation

The model implemented has been validated by obtaining the results presented in

Zhao et al. 201344 with the same material properties (Zn-Ni capsule) and

dimensions (50 mm diameter). In Zhao’s simulation the time for the PCM to reach

the melting temperature is 204 seconds, with a convective heat transfer coefficient

of h=95.77 Wm-2K-1. Table 4-1 compares the results.

1750

1760

1770

1780

1790

1800

1810

1820

1830

1840

1850

130

132

134

136

138

140

142

144

146

148

150

0 100 200 300 400 500

Melt

ing

tim

e D

ura

tio

n [

s]

Melt

ing

Sta

rt t

ime [

s]

Nodes

Start Melting

Melting Duration

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119

Thermal Energy Storage for High Temperature Applications

Table 4-1: Comparison Zhao et al. vs. present work Zn-Ni capsule (50 mm in

diameter)

Reference Point Zhao et al.44 Present work Difference (%)

Start phase change 204 s 202 s -1%

Melting front at R=0.5 650 s 649 s -0.1%

End phase change 744 s 750 s +0.8%

Melting process 540 s 548 s +1.5%

Phase change from solid to liquid starts at the zinc/nickel interface when the

temperature reaches the melting point in about 3.4 min (204 s) compared to 202 s in

the implemented model (error: -1%) (Figure 4-4). The interface will pass through

R=0.5 after approximately 10.8 min (650 s), and will reach to the center after about

12.4 min (744 s), compared to 649 s and 750 s in the implemented model (error: -

0.1% and +0.8%). The melting process takes 540 s compared to 548 s in the

implemented model (error +1.5%) (Figure 4-5 to Figure 4-7).

Figure 4-4: Temperature at a various radial locations as a function of time: Zhao et

al.44 (left) and present work (right). Ni-Zn capsule

0 200 400 600 800 1000 1200 1400 1600 1800320

340

360

380

400

420

440

460

480

500

520

T in

sid

e c

ap

su

le [

ºC]

Time [s]

Temperature for different radial positions

T Center

T Shell int

T Shell ext

T experiment

Page 162: Pablo Giménez Gavarrell

Single Capsule Model

120

Figure 4-5: Location of the interface as a function of time: Zhao et al.44 (left) and this

work (right). Ni-Zn capsule

The results for the steel & NaCl-MgCl2 (shell-core) 50 mm capsule diameter have

been also compared in Table 4-2. The deviations from the reported results are lower

than 1.5%.

Table 4-2: Comparison Zhao et al. vs. present work Steel-NaCl-MgCl2 capsule

Reference Point Zhao et al. Present work Difference (%)

Start phase change 144 s 146 1.4%

End phase change 1992 s 1978 -0.7%

Melting process 1848 s 1832 -0.8%

0 100 200 300 400 500 6000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Lo

cati

on

of

inte

rface (

Scalin

g)

Time [s]

Page 163: Pablo Giménez Gavarrell

121

Thermal Energy Storage for High Temperature Applications

Figure 4-6: Temperature at a various radial locations as a function of time: Zhao et

al.44 (left) and This work (right). Steel NaCl-MgCl2 capsule.

Figure 4-7: Location of the interface as a function of time: Zhao et al.44 (left) and this

work (right). Steel-(NaCl-MgCl2) capsule.

4.4. Material properties

The physical properties for this analysis are presented in Table 4-3. The borosilicate

properties have been extracted from Schott Catalog71, at 300ºC when measured or

extrapolated as in Figure 4-8. When possible, measured values of the PCM

properties are used. The latent heat, the specific heat, and the melting temperatures

of the PCM (NaNO3, Sn, and Pb) have been measured while the thermal

0 500 1000 1500 2000 2500340

360

380

400

420

440

460

480

500

520

540

T in

sid

e c

ap

su

le [

ºC]

Time [s]

Temperature for different radial positions

T Center

T Shell int

T Shell ext

T experiment

0 200 400 600 800 1000 1200 1400 1600 1800 20000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1L

ocati

on

of

inte

rface (

Scalin

g)

Time [s]

Page 164: Pablo Giménez Gavarrell

Single Capsule Model

122

conductivity and density are have been extracted from Zhao et al. (2012).82 Table

4-4 shows the properties of the HTF (air) used in the experiments to estimate the

convective heat transfer coefficient used in the model.

Table 4-3: Thermo-physical properties used in the model. (*Measured)

k

[W m-1 K-1]

(liquid/solid)

ρ

[kg m-3]

(liquid/solid)

Cp

[kJ kg-1 K-1]

(liquid/solid)

LH

[kJ kg-1]

Tm

[ºC]

NaNO3 0.5/0.5 1900/2260 1.650/1.4* 176.8* 302*

Sn 26/57 83 6990/7365 0.24/0.24* 44.4* 179*

Pb 15/29 83 10660 /11340 0.17/0.17* 20.9* 315.4*

Borosilicate71 -/1.55 -/ 2200 -/ 1.23 N/A N/A

Stainless Steel84 -/ 14.9 -/ 7900 -/ 0.477 N/A N/A

In order to approximate the model to the experiments a modification is introduced

to consider the void space in the capsule. The NaNO3 storage material densities

have been multiplied by (1-Voidsolid) = 73.5 %, for solid density, and by (1-Voidliquid)

= 87.4%, for the liquid density. As the model is one-dimensional, the void space in a

specific location of the capsule cannot be considered. However, by changing the

density we can at least consider a capsule which is able to store the same energy as

the real capsule. The Sn-capsule density has been reduced similarly in order to

approximate the latent heat storage capacity of this capsule to the latent heat of

NaNO3-capsule by multiplying the solid density by 88.4 % and the liquid density

by 93.1 %, consequently both capsules are able to store the same amount of energy

as latent heat and phase change times can be compared. The density of solid and

liquid lead has been multiply by 82.2 % and 87.4 % respectively in order to consider

the void space present in this capsule.

Page 165: Pablo Giménez Gavarrell

123

Thermal Energy Storage for High Temperature Applications

Figure 4-8: Borosilicate thermo-physical properties71 (Black dots extrapolated at

higher temperature)

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

0 50 100 150 200 250 300 350 400

Sp

ecif

ic h

eat

[J/

(g·K

)]

Th

erm

al

Co

nd

ucti

vit

y [

W/

(mK

)]

Temperature [ºC]

Page 166: Pablo Giménez Gavarrell

Single Capsule Model

124

Table 4-4 Properties of the HTF (air) used in the experiments to estimate the

convective heat transfer coefficient used in the model.85

Property Value

(200ºC)

Value

(300ºC)

Value

(400ºC)

Unit

Medium : Air Air Air

Pressure : 1 1 1 [ bar ]

Temperature : 200 300 400 [ Celsius ]

Density : 0.7356 0.6072 0.517 [ kg / m3 ]

Specific Enthalpy : 476 579.6 685.3 [ kJ / kg ]

Specific Entropy : 7.334 7.532 7.702 [ kJ / kg K ]

Specific isobar

heat capacity Cp

1.026 1.046 1.069 [ kJ / kg K ]

Isobar coefficient of

thermal expansion :

2.115 1.745 1.486 [ 10-3 (1 / K) ]

Heat conductance 37.95 44.09 49.96 [ 10-3 (W / m·K)]

Dynamic viscosity : 26.09 29.86 33.35 [ 10-6 (Pa s) ]

Kinematic viscosity : 35.468 49.177 64.507 [ 10-6 m2 / s]

Thermal diffusivity : 503 694.3 903.8 [ 10-7 m2 / s]

Prandtl-Number : 0.7051 0.7083 0.7137

Page 167: Pablo Giménez Gavarrell

125

Thermal Energy Storage for High Temperature Applications

4.5. Boundary condition

In the previous simulations, and those performed in Zhao’s investigation44,63 the

capsule starts at constant temperature and at t=0 a temperature step from 250 ºC to

500 ºC is applied, since the surrounding heat transfer fluid is “hot” at 500 ºC during

charging. Similarly, a step from 500 ºC to 250 ºC is used for discharging where the

surrounding heat transfer fluid is “cold” at 250 ºC. This boundary condition has

been used to validate the implemented model. However, this is not the boundary

condition applied in the experimental set-up. In the experiments the thermal inertia

of the system makes the temperature around the sphere change gradually and not

stepwise. Moreover, considering any industrial systems, which are expected to last

a long operation time, thermal shocks like the one simulated (∆T=200 ºC) are

usually avoided.

The numerical model has been adapted to the experimental boundary conditions.

The air temperature boundary condition has been modified to approximate to the

experimental conditions. Figure 4-9 shows the Start Melting (SM) time and End

Melting (EM) time for the implemented model compared to the expected times

based on the temperature boundary condition applied to the capsule in the

experiment.

Figure 4-9: Schematic of air temperature boundary condition: previous model vs.

experiment. SM: start melting, EM: end melting.

Page 168: Pablo Giménez Gavarrell

Single Capsule Model

126

The dimensionless temperature is normalized by the heat transfer fluid

temperature. However, the HTF temperature boundary condition applied to the

capsule is time-dependent in the experiments.

Boundary condition

Dimensionless boundary condition

In the experiments the temperature of the final air temperature around the capsule

has been used to define the dimensionless temperature, while the time-dependent

air temperature has been introduced as a new term as follows:

Equation 4-2

with

Equation 4-3

The analytical approximation of the experimental air temperature around the

capsule has been used in order to facilitate its implementation in the model. The

discretization of the new boundary condition will be as follows:

Page 169: Pablo Giménez Gavarrell

127

Thermal Energy Storage for High Temperature Applications

4.6. Results and Discussion

4.6.1. Effect of the new boundary condition on the phase change times

As explained above, in Zhao’s simulation44,63 the step in temperature is applied in

t=0 s. However, in the experimental set-up a gradually increasing temperature is

applied. This led to a modification in the boundary condition to adapt the

simulations to the experimental set-up. An example of the time evolution of the

capsule temperature and heat transfer fluid temperature for the modified

boundary condition can be seen in Figure 4-10. As Figure 4-10 shows, the PCM

starts melting when R=R2 reaches the melting temperature (300 ºC approximately).

The melting process duration is described by the constant temperature trace inside

the capsule (R=0) finishing around 465 seconds.

Figure 4-10: Example of the new boundary condition introduced in the

mathematical model and temperature evolution for different capsule radial

positions

250

260

270

280

290

300

310

320

330

340

350

0 100 200 300 400 500 600

Tem

pera

ture

fo

r d

iffe

ren

t ra

dia

l p

osi

tio

ns

[ºC

]

Time [s]

HTF

T Shell ext (R=R1)

T Shell int (R=R2)

T Center (R=0)

Start

Melting

Page 170: Pablo Giménez Gavarrell

Single Capsule Model

128

The change in the boundary condition has an important impact on the melting start

time and in the melting end time as shown in Figure 4-11, where both boundary

conditions are compared. As expected, the melting process is shifted, starting and

finishing earlier when a discrete temperature time step (i.e. Zhao´s boundary

condition) is applied. However, the phase change process almost remains

unchanged (Figure 4-12). It means that the new boundary condition only delays the

phase change process without any other modification. When the melting process

starts, the dimensionless HTF temperature is approximate ~0.9 for the new

boundary condition, not too different than Zhao’s boundary condition where the

dimensionless temperature is 1. Then, as the temperature applied to the system

when the melting process starts is similar, the melting process is almost identical

for both types of boundary conditions. This suggests, as we will see later on, that

the melting process is governed mainly by the PCM thermal properties and the

external flow conditions, which are kept constant for both simulations shown in

Figure 4-11. The simulation conditions are: NaNO3 –Borosilicate capsule with

r1=0.01 m, a capsule thickness of 0.0011 m, the number of nodes N = 451, time step

∆τ = 0.002, temperature step applied ∆T = 100 ºC and dimensionless melting

temperature θm = 0.5. Two different set-up time constants have been chosen: τexp =

0.0001 s for Zhao’s Boundary condition and τexp = 50 s used in the present work;

and two convective heat transfer coefficients, 50 and 150 W m-2 K-1. The simulation

is extended up to θcenter = θ (R=0) = 0.9.

Page 171: Pablo Giménez Gavarrell

129

Thermal Energy Storage for High Temperature Applications

Figure 4-11: Effect of the new boundary condition applied to the capsule in the

present work compared to Zhao’s boundary condition on the temperature profiles

at the center of the capsule (R=0) and at the shell-PCM interface (R=R2) for a fixed

hconv=150 Wm-2K-1.

The melt fraction is defined as the amount of molten PCM divided by the initial

PCM amount (Equation 4-4). The density change has been neglected.

Equation 4-4

Changing the boundary condition barely affects the melt fraction. As shows Figure

4-12, this small change is only noticeable when the convective heat transfer

coefficient is high.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600

Dim

en

sio

nle

ss T

em

pera

ture

θ

[-]

Time [s]

HTF BC Zhao

T (R=0) BC Zhao

T (R=R2) BC Zhao

HTF BC Gimenez

T (R=0) BC Gimenez

T (R=R2) BC Gimenez

Page 172: Pablo Giménez Gavarrell

Single Capsule Model

130

Figure 4-12: Effect of the new boundary condition applied to the capsule in the

present work compared to Zhao’s boundary condition on the temporal evolution of

the melt fraction for two different convective heat transfer coefficients (50 and 150

Wm-2K-1).

4.6.2. Effect of capsule size on phase change

For this analysis a NaNO3-Borosilicate capsule has been considered. The convective

heat transfer coefficient is fixed to 50 Wm-2K-1. Three different capsule radii (r1)

have been simulated: 0.01, 0.02 and 0.03 meters (r1, 2·r1, 3·r1). The ratio R2/R1 is

fixed for all cases to 0.89 (Table 4-5).

Figure 4-13 shows the temperature profiles, Figure 4-14 (left) the location of the

solid-liquid interface, and Figure 4-14 (right) the melt fraction vs. time for the three

radii simulated. The melting start time and the melting time duration increases as

the radius of the capsule increases. When the radius is increased (r1, 2·r1, 3·r1) the

surface area increases as well, but squared (1A, 4A, 9A). Since the convective heat

transfer coefficient is fixed, the input power increases in the same proportion as the

surface area. However, the amount of PCM has increased to the third power

(1·Volume, 8·Volume, 27·Volume) consequently the melting process takes

significantly longer time.

0%

20%

40%

60%

80%

100%

0 100 200 300 400 500 600 700

Melt

Fra

cti

on

Time [s]

BC Zhao h=50 BC Gimenez h=50 BC Zhao h=150 BC Gimenez h=150

Page 173: Pablo Giménez Gavarrell

131

Thermal Energy Storage for High Temperature Applications

Table 4-5: Melting start time and melting duration for three different capsule sizes.

Melting Start time [min] Melting Duration [min]

Radius x1 (0.01m) 2.7 10.0

Radius x2 (0.02m) 4.3 28.6

Radius x3 (0.03m) 5.6 55.1

Figure 4-13: Dimensionless temperature at the center of the capsule (R=0) and at the

shell-PCM interface (R=R2) for three different capsule radii.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 500 1000 1500 2000 2500 3000 3500 4000 4500

Dim

en

sio

nle

ss T

em

pera

ture

θ

[-]

Time [s]

HTF T (R=0) Radius x1 T (R=R2) Radius x1 T (R=0) Radius x2 T (R=R2) Radius x2 T (R=0) Radius x3 T (R=R2) Radius x3

Capsule Size ↑

Page 174: Pablo Giménez Gavarrell

Single Capsule Model

132

Figure 4-14: Location of the solid-liquid interface for three different capsule radii

(left) and melt fraction for three different capsule radii (right)

4.6.3. Effect of capsule shell thickness on phase change

For this analysis a NaNO3-Borosilicate capsule has been considered. The convective

heat transfer coefficient is fixed to 50 Wm-2K-1. Three different capsule thickness ‘e’

have been simulated: 0.0011, 0.0022 and 0.0033 meters (e1, 2·e1, 3·e1). It changes the

ratio R2/R1. The capsule external radius is fixed to 0.01 m. As the capsule thickness

increases maintaining the capsule radius, the amount of PCM is reduced.

Consequently the melting time is reduced. The melting start time is also reduced

slightly as the capsule thickness is increased because more material has to be

heated up before the PCM reaches its melting temperature, taking longer time

when the PCM is further away from the capsule surface (Figure 4-15 and 4-16).

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1000 2000 3000

Lo

cati

on

of

the i

nte

rfase

S

(t)

[-]

Time [s]

Radius x1

Radius x2

Radius x3

0%

20%

40%

60%

80%

100%

0 1000 2000 3000

Melt

Fra

cti

on

[%

] Time [s]

Radius x1

Radius x2

Radius x3

Capsule Size ↑

Capsule Size ↑

Page 175: Pablo Giménez Gavarrell

133

Thermal Energy Storage for High Temperature Applications

Figure 4-15: Effect of the capsule thickness on the capsule temperature: at the center

(R=0) and at the shell-PCM interface (R=R2).

Figure 4-16: Location of the solid-liquid interface vs. time for different capsule

thickness (left); Melt fraction vs. time for different capsule thickness (right)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 200 400 600 800 1000

Dim

en

sio

nle

ss T

em

pera

ture

θ

[-]

Time [s]

HTF T (R=0) Thickness x1 T (R=R2) Thickness x1 T (R=0) Thickness x2 T (R=R2) Thickness x2 T (R=0) Thickness x3

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700

Lo

cati

on

of

the i

nte

rfase

S(t

) [-

]

Time [s]

Thickness x1

Thickness x2

Thickness x3

0%

20%

40%

60%

80%

100%

0 100 200 300 400 500 600 700

Melt

Fra

cti

on

Time [s]

Thickness x1

Thickness x2

Thickness x3

Thickness↑

Thickness↑

Capsule Thickness ↑

Page 176: Pablo Giménez Gavarrell

Single Capsule Model

134

4.6.4. Effect of shell material on phase change times: borosilicate vs.

steel

The effect of the shell material on the melting process of a NaNO3-capsule is

evaluated by comparing two materials with very different thermal conductivity: a

metal shell (stainless steel) and a glass (borosilicate). The materials´ thermal

properties are shown in Table 4-6. Stainless steel has a significantly higher thermal

conductivity (approximately 10 times higher) and higher thermal diffusivity than

borosilicate. For this analysis, the capsule geometry (volume, shell thickness), type

of PCM material, and external conditions are kept constant. The simulation

conditions are: r1 = 0.01 m, capsule thickness 0.0011 m, number of nodes N = 451,

∆τ = 0.002, ∆T = 100 ºC, dimensionless melting temperature θm = 0.5, set-up time

constant τexp = 50 s, simulation until θcenter = 0.9.

Table 4-6: Shell material properties

k

[W/(mK)]

ρ

[kg/m3]

Cp

[kJ/(kgK)]

Cp

[kJ/(m3K)]

=k/Cp

[m2/s]

Borosilicate71 1.55 2200 1.23 2706 5.73·10-7

Stainless Steel84 14.9 7900 0.477 3768.3 3.95·10-6

The temperature history at various locations (center, shell-PCM interface and

capsule surface) of a NaNO3 capsule, comparing different shell materials

(borosilicate vs. steel) for fixed geometry and external flow conditions can be seen

in Figure 4-17. For a constant shell thickness, the stainless steel shell takes slightly

longer time (approximately +3%) to reach the melting temperature at the shell-

PCM interface R=R2. In other words, the melting start time is longer. This can be

explained because the volumetric energy density “ρ·Cp” [Jm-3K-1]) of steel is ~40%

higher than that of borosilicate. This means that to reach the same temperature for a

constant volume we need to apply more heat to the steel capsule. However, once

the capsule starts melting, the process is faster with the higher conductivity metallic

shell (about 2% faster), since the center R=0 finishes the isothermal segment (=m)

earlier. The borosilicate and stainless steel present ~3 and ~20 times higher thermal

Page 177: Pablo Giménez Gavarrell

135

Thermal Energy Storage for High Temperature Applications

diffusivity than NaNO3. Therefore the salt PCM, with lowest thermal diffusivity, is

the limiting factor for the melting duration. Consequently, changes on the shell

material with higher thermal diffusivities within the range evaluated, do not affect

the melting process significantly.

Figure 4-17: Dimensionless temperature for different radial positions: center (R=0),

shell-PCM interface (R=R2) and capsule surface (R=R1) for NaNO3-capsule with

different shell materials. Convective heat transfer coefficient around the capsule 150

W m-2 K-1.

The melting start time and the melting duration of a NaNO3 capsule, comparing

different shell materials (borosilicate vs. steel) and different convective heat transfer

coefficients are shown in Table 4-7. The stainless steel capsule delays the beginning

of the phase change process (longer melting start time) compared to a borosilicate

capsule due to its higher volumetric energy density, as explained before. For low

convective heat transfer coefficients this effect in the melting start time is slightly

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600

Dim

en

sio

nle

ss T

em

pera

ture

θ

[-]

Time [s]

HTF

T (R=0) NaNO3-Borosilicate

T (R=R2) NaNO3-Borosilicate

T (R=R1) NaNO3-Borosilicate

T (R=0) NaNO3-STEEL

T (R=R2) NaNO3-STEEL

T (R=R1) NaNO3-STEEL

Page 178: Pablo Giménez Gavarrell

Single Capsule Model

136

more noticeable.

On the other hand, the borosilicate capsule delays the melting process (Figure 4-18).

The differences on the melting duration seem more noticeable for high convective

heat transfer coefficient, although these differences are very small.

Table 4-7: Melting start time and melting time duration for different convective

heat transfer coefficients and different shell materials. NaNO3 as PCM.

Melting Start

time [s]

Melting time

Duration [s]

h=50

[W m-2 K-1]

h=150

[W m-2 K-1]

h=50

[W m-2 K-1]

h=150

[W m-2 K-1]

Borosilicate and NaNO3 163.6 85.6 599.5 379.6

Stainless steel and NaNO3 175.3 87.8 596.9 371.2

Figure 4-18 (left) represents the melt fraction versus time for the four cases

simulated (different shell material and convective heat transfer coefficient). As

explained, for a fixed geometry, changes in shell material are negligible within the

range of convective heat transfer coefficients simulated. The location of the melting

front s(t) is also compared in Figure 4-18 (right). The effect of changing the shell to a

higher conductivity material to reduce the overall melting process time is slightly

more noticeable at higher heat transfer coefficients as mentioned above.

Page 179: Pablo Giménez Gavarrell

137

Thermal Energy Storage for High Temperature Applications

Figure 4-18: Melt fraction vs time (left) and Location of the solid liquid interface

(right) for different shell materials and different convective heat transfer coefficients

around a NaNO3 capsule

The same analysis on the effect of the shell material has been performed using a

metallic PCM core instead. The melting start time and the melting duration of

encapsulated tin (Sn), comparing different shell materials (borosilicate vs steel) and

convective heat transfer coefficients are shown in Table 4-8. Similarly to the NaNO3

case, the melting start time and the melting duration for the Sn-capsule are not

affected significantly by the shell material type when the convective heat transfer

coefficient is fixed (Figure 4-19). As in the NaNO3 case, the melting starts slightly

earlier for a borosilicate capsule compared to the steel capsule as before, because of

its lower ‘ρCp’ and the melting duration is shorter for the stainless steel capsule

compared to the borosilicate capsule.

The differences on the melting start time are reduced as the convective heat transfer

coefficient is increased (Figure 4-20). On the other hand, the borosilicate shell slows

down the melting process by approximately 10 seconds for the convective heat

transfer simulated. It might be caused by the difference in thermal diffusivity. The

difference in melting duration increases slightly as the convective heat transfer

increases.

0%

20%

40%

60%

80%

100%

0 100 200 300 400 500 600

Melt

Fra

cti

on

[%

]

Time [s]

NaNO3-Borosilicate h=50

NaNO3-Borosilicate h=150

NaNO3-STEEL h=50

NaNO3-STEEL h=150

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600

Lo

cati

on

of

the i

nte

rfase

S(t

) [-

]

Time [s]

NaNO3-Borosilicate h=50

NaNO3-Borosilicate h=150

NaNO3-STEEL h=50

NaNO3-STEEL h=150

hconv↑ hconv↑

Page 180: Pablo Giménez Gavarrell

Single Capsule Model

138

Table 4-8: Melting start time and melting time duration for different convective

heat transfer coefficients and different shell materials. Tin (Sn) as PCM.

Melting Start time [s] Melting Duration [s]

h=50

[Wm-2K-1]

h=150

[Wm-2K-1]

h=50

[Wm-2K-1]

h=150

[Wm-2K-1]

Borosilicate and Sn(PCM) 144.91 80.64 298.94 128.65

Stainless steel and Sn(PCM) 156.95 83.49 289.07 118.06

Figure 4-19: Dimensionless temperature for different radial positions: center (R=0),

shell-PCM interface (R=R2) and capsule surface (R=R1) for a Sn-capsule with

different shell materials. Convective heat transfer coefficient around the capsule 150

Wm-2K-1.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150 200 250

Dim

en

sio

nle

ss T

em

pera

ture

θ

[-]

Time [s]

HTF T (R=0) Sn-Borosilicate T (R=R2) Sn-Borosilicate T (R=R1) Sn-Borosilicate T (R=0) Sn-STEEL T (R=R2) Sn-STEEL T (R=R1) Sn-STEEL

Page 181: Pablo Giménez Gavarrell

139

Thermal Energy Storage for High Temperature Applications

Figure 4-20: Location of the solid liquid interface vs time (left) and Melt fraction vs

time (right) in the Sn capsule for different shell materials and different convective

heat transfer coefficients around the capsule.

4.6.5. Effect of the latent heat on the phase change times

The effect of the latent heat on the phase change times is evaluated. The simulation

conditions are: NaNO3 – Borosilicate capsule with r1 = 0.01 m, a capsule thickness

of 0.0011 m, the number of nodes N = 451, time step ∆τ = 0.002, temperature step

applied ∆T = 100 º C and dimensionless melting temperature θm = 0.5. The new

boundary condition is used with τexp = 50 s. The convective heat transfer coefficient

is set to 150 Wm-2K-1. The simulation is extended till θcenter = 0.9. The latent heat of

NaNO3 has been used together with an ideal PCM with a latent heat half of this

value and another with two times this latent heat. The results are shown in Table

4-9 and Figure 4-21.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 Lo

cati

on

of

the i

nte

rfase

S(t

) [-

]

Time [s]

Sn-Borosilicate h=50

Sn-Borosilicate h=150

Sn-STEEL h=50

Sn-STEEL h=150

0%

20%

40%

60%

80%

100%

0 100 200 300 400

Melt

Fra

cti

on

Time [s]

Sn-Borosilicate h=50

Sn-Borosilicate h=150

Sn-STEEL h=50

Sn-STEEL h=150

hconv↑

hconv↑

Page 182: Pablo Giménez Gavarrell

Single Capsule Model

140

Table 4-9: Melting start time and melting time duration for three different latent

heats tested.

Melting Start

time [min]

Melting time

Duration [min]

0.5 ·Latent Heat 1.4 4.0

1.0 ·Latent Heat 1.4 6.3

2.0 ·Latent Heat 1.4 10.7

The melting start time is unchanged for the three cases because the latent heat does

not affect the sensible heat of the PCM-capsule. On the other hand, the latent heat

affects linearly the melting time duration as shown in Figure 4-21.

Figure 4-21: Melting time duration vs. latent heat times the latent heat of NaNO3

(left); Effect of the latent heat on melt fraction for three different latent heats tested

(right)

0

2

4

6

8

10

12

0.0 1.0 2.0

Melt

ing

tim

e d

ura

tio

n [

min

]

Multiples of the latent heat of NaNO3

0%

20%

40%

60%

80%

100%

0 200 400 600 800

Melt

Fra

cti

on

Time [s]

0.5·Latent Heat

1.0·Latent Heat

2.0·Latent Heat

Page 183: Pablo Giménez Gavarrell

141

Thermal Energy Storage for High Temperature Applications

4.6.6. Effect of PCM characteristics on phase change times

In this section the effect of the PCM material (Salt NaNO3 vs Metal Sn) on the

melting start time, melting duration, melt fraction, and location of the solid-liquid

interfaces as a function of time, as well as the temperature profiles in the PCM-

capsule are compared. It is interesting because the choice of material implies a

combination of properties frequently with opposing effects and not merely an

increase in thermal conductivity or an increase in latent heat. Figure 4-22 shows the

temperature evolution at 3 different radial positions for a Sn-Borosilicate capsule

and NaNO3-borosilicate capsule. As can be observed the metallic PCM does not

allow large thermal gradients within the capsule because of its high thermal

conductivity and diffusivity. The capsule surface closely follows the temperature

profile inside the capsule; the surface does not increase its temperature until the

phase change process has finished. On the other hand, the salt PCM shows large

thermal gradients within the capsule. The surface temperature increases

progressively until the PCM outer surface (R=R1) reaches the phase change

temperature. At this point, the shell temperature profile changes its slope, but the

surface and the PCM-shell interface keeps heating up, whereas any location inside

the PCM (0<R<R1) will show an isothermal segment corresponding to the phase

change, confirming the appearance of large gradients inside the PCM.

In the case of metallic PCM the phase change duration (melting start and end

times) could be estimated by measuring the capsule surface temperature. However,

only the melting start time could be estimated based on the NaNO3-capsule surface

temperature. This observation can be perfectly correlated with the infrared images

recorded in previous section, where the temperature traces of the capsule surface

show an isothermal melting segment in the case of a metallic PCM but not in the

case of a salt PCM capsule.

Page 184: Pablo Giménez Gavarrell

Single Capsule Model

142

Figure 4-22: Dimensionless temperature of the capsule at the center (R=0), PCM-

shell interface (R=R2) and the capsule surface (R=R1) for two different PCM

(NaNO3 and Sn) encapsulated with borosilicate. Convective heat transfer

coefficient 50 Wm-2K-1

Analyzing the differences in the melting process of Sn and NaNO3 borosilicate

capsule, comparing capsules with the same latent heat capacity, Figure 4-23 shows

how tin (PCM)-capsule, with a similar melting start time and a thermal diffusivity

more than 2 orders of magnitude higher than NaNO3, shows around half melting

time. The heat goes through the molten PCM until the phase change front that is

why the thermal diffusivity of the PCM is important and a low value slows down

the melting process.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 200 400 600 800 1000

Dim

en

sio

nle

ss T

em

pera

ture

θ

[-]

Time [s]

HTF

T (R=0) Sn-Borosilicate

T (R=R2) Sn-Borosilicate

T (R=R1) Sn-Borosilicate

T (R=0) NaNO3-Borosilicate

T (R=R2) NaNO3-Borosilicate

T (R=R1) NaNO3-Borosilicate

Sn Melting

NaNO3 Melting

Page 185: Pablo Giménez Gavarrell

143

Thermal Energy Storage for High Temperature Applications

Figure 4-23: Location of the solid-liquid interface vs. time (left) and melt fraction vs.

time (right) for two different PCM (NaNO3 and Sn) encapsulated with borosilicate.

Convective heat transfer coefficient 50Wm-2K-1.

Finally, the study has been extended to convective heat transfer coefficients far

from the experimental conditions, comparing the melting start time and melting

duration for borosilicate capsules using tin and sodium nitrate as a PCM. The

results are shown in Figure 4-24. As we increase the heat transfer coefficient the

two times (phase change start and duration) for the two types of capsules tend to

stabilize becoming independent of the convective heat transfer coefficient and

being only dependent on the diffusion of heat through the capsule. The melting

start time is almost identical for convective heat transfer coefficients higher than

100 Wm-2K-1. As the convective heat transfer increases, the melting duration of both

capsules tend to a constant value significantly lower for a tin-capsule compared to

a NaNO3-capsule.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 100 200 300 400 500 600 700 Lo

cati

on

of

the i

nte

rfase

S(t

) [-

]

Time [s]

NaNO3-Borosilicate

Sn-Borosilicate

0%

20%

40%

60%

80%

100%

0 100 200 300 400 500 600 700

Melt

Fra

cti

on

Time [s]

NaNO3-Borosilicate

Sn-Borosilicate

Page 186: Pablo Giménez Gavarrell

Single Capsule Model

144

Figure 4-24: Start melting and melting duration time for borosilicate capsules with

Sn and NaNO3 as PCM for different convective heat transfer coefficients.

4.6.7. Effect of experimental conditions on the phase change times

A parametric study has been performed on borosilicate capsules (20 mm in

diameter, 1.1mm thickness) with NaNO3 as PCM to evaluate the effect of changes

in the experimental conditions (temperature step applied ‘∆T’, convective heat

transfer coefficient ‘h’ and dimensionless melting temperature ‘θm’) on the phase

change times. For different values of the convective heat transfer coefficient, h (50-

75-100-200-300W/m2K), temperature step, ∆T (60-100-140ºC), (with 0.5 as

dimensionless melting temperature, i.e. the melting temperature falls in the center

of the temperature range), and different dimensionless melting temperatures

θm(0.25-0.5-0.75), (fixing the temperature step ∆T=100ºC), the start and end melting

times and melting duration are analyzed. The average time constant (τ=45.7s)

calculated from the experiments has been used in the simulation to represent the

real air temperature profile applied to the capsule.

0

2

4

6

8

10

12

0 250 500 750 1000

Tim

e [

min

]

Convective heat transfer coefficient h [Wm-2K-1]

Melting Duration NaNO3-Borosilicate

Start Melting NaNO3-Borosilicate

Melting Duration Sn-Borosilicate

Start Melting Sn-Borosilicate

Page 187: Pablo Giménez Gavarrell

145

Thermal Energy Storage for High Temperature Applications

The first parametric study modifies the convective heat transfer coefficient and the

temperature step applied, fixing the dimensionless melting temperature (θm) to 0.5.

The “melting start time” and the “melting duration” results are represented in

Figure 4-25.

Figure 4-25: Melting start time (left) and Melting duration (right) vs. heat transfer

coefficient for different temperature steps, fixed dimensionless melting

temperature θm= 0.5 and time constant τ=45.7 s.

The starting time for the melting process seems dependent only on the convective

heat transfer coefficient, and independent of the externally applied temperature

step (∆T). The time to reach the PCM the dimensionless melting temperature of 0.5

does not depend on the temperature step applied. On the other hand, the melting

duration depends on both: the heat transfer fluid and the temperature step. The

relative effect of the heat transfer coefficient on the melting duration is similar for

each temperature step. The melting duration is reduced almost by half when the

heat transfer coefficient is increased from 50 to 300Wm-2K-1. A similar reduction is

observed when the temperature step is increased from 60ºC to 140ºC.

50

70

90

110

130

150

170

190

50 100 150 200 250 300

Melt

ing

Sta

rt t

ime [

s]

Heat transfer coefficient [Wm-2K-1]

∆T 60ºC

∆T 100ºC

∆T 140ºC

50

150

250

350

450

550

650

750

850

950

50 100 150 200 250 300

Melt

ing

Du

rati

on

tim

e [

s]

Heat transfer coefficient [Wm-2K-1]

∆T 60ºC

∆T 100ºC

∆T 140ºC

∆T 220ºC

Page 188: Pablo Giménez Gavarrell

Single Capsule Model

146

The second parametric study modifies the convective heat transfer coefficient and

the non-dimensional melting temperature (θm = 0.25-0.50-0.75), fixing the

temperature step ∆T=100ºC, because the experiments start at different temperature

with different θm and it is expected that changes in the boundary conditions and

the θm affect significantly the melting times. The “melting start time” and the

“melting duration” results are represented in Figure 4-26.

Figure 4-26: Melting start time (left) and Melting duration time (right) for different

convective heat transfer coefficients ‘h’ and dimensionless melting temperature θm

for a fixed T=100ºC

In this case the melting start time depends on both the convective heat transfer

coefficient and the dimensionless melting temperature. There is a pronounced

reduction in the melting start time at low convective heat transfer coefficients (50-

100Wm-2K-1) compared to (h>100Wm-2K-1) when the dimensionless melting

temperature is reduced. The dimensionless melting temperature has an important

impact on the melting start time at 50Wm-2K-1 reducing its effect when the heat

transfer coefficient is increased. The relative effect of the heat transfer coefficient on

the melting start time is similar for each dimensionless melting temperature. An

0

50

100

150

200

250

300

350

50 100 150 200 250 300

Mel

tin

g S

tart

tim

e [s

]

Heat transfer coefficient

[Wm-2K-1]

θm=0.25

θm=0.50

θm=0.75

0

200

400

600

800

1000

1200

50 100 150 200 250 300

Mel

tin

g D

ura

tio

n t

ime

[s]

Heat transfer coefficient

[Wm-2K-1]

θm=0.25

θm=0.50

θm=0.75

Page 189: Pablo Giménez Gavarrell

147

Thermal Energy Storage for High Temperature Applications

average reduction of 62% in the melting start time is observed when the heat

transfer coefficient is increases for 50 to 300Wm-2K-1. Increasing the dimensionless

melting temperature (starting the experiments at higher initial temperature) by

+0.25 the average increase on the melting start time is +83.6%.

Similar effect on the melting duration compared to the melting start time is

observed. On average a 46% reduction on the melting duration time is observed

when the heat transfer coefficient is increased from 50 to 300Wm-2K-1. This

reduction is similar for the three dimensionless melting temperatures. For low θm

the effect on the melting time of the convective heat transfer coefficient is lower. For

high θm and for the convective heat transfer coefficient around ~50 Wm-2K-1,

expected in the experimental set-up, changes in the θm and the convective heat

transfer coefficient has an important effect on the melting time duration.

4.7. Comparison to experimental results

4.7.1. Convection correlation for experimental set-up

The heat transfer coefficient between the surface of the capsule and the air stream is

an important parameter for estimating the heat exchange rate. Although the

distribution of the local Nusselt number is affected by the air pattern around the

sphere, Whitaker’s Nusselt number correlation86 is often employed in practice for

estimating the average heat transfer coefficient between a spherical surface and a

free stream (Equation 4-5). This correlation is valid for 3.5 < Rep < 7.6E4 and 0.7 < Pr

< 380 with the fluid properties evaluated at the free stream temperature T∞, except

for μs which is evaluated at surface temperature.

Equation 4-5

However, the experimental set-up is far from an air free stream around a sphere.

The air is expected to be affected by the blockage ratio, defined as the ratio between

diameters (Dsphere/Dtube). The effect of blockage ratio on the heat transfer for a

centrally located sphere in a pipe water flow (Prandlt=5.15) was investigated in

Krishnan et al.87 using CFD simulations. To validate the model the authors

Page 190: Pablo Giménez Gavarrell

Single Capsule Model

148

compared the CFD predicted transient Nusselt numbers at a low blockage ratio

(BR=0.02) with the steady classical solution of Kramers (1946) Equation 4-6

(1<Rep<2000).

Equation 4-6

The proposed correlation presented in Equation 4-7 considers the effect of the

blockage ratio on the Nusselt number. The equation fitted the data for

and where refers to Kramers solution (Equation 4-6).

Equation 4-7

In order to evaluate the different correlations the mass and volumetric flow rate

around the sphere in the experimental set-up is required. The mass flow rate is

calculated using the volumetric flow rate and the intermediate pressure, which

depends on the volumetric flow rate. The mass flow rate, at atmospheric pressure

and at different temperatures, determines the air velocity in the tube. The blockage

ratio in the set-up is BR = Dsphere /Dtube =0.625 (Dsphere =0.02 m and Dtube =0.032 m).

Table 4-10 and Figure 4-27 show the Nusselt number, the Reynolds number, and

the convective heat transfer coefficient using the proposed correlations. For

different volumetric flow rate and temperature, the standard deviation among the

three convective coefficients calculated with different correlation is lower than

2.7%.

Page 191: Pablo Giménez Gavarrell

149

Thermal Energy Storage for High Temperature Applications

Figure 4-27: Convective heat transfer coefficient vs. Reynolds number (at 300ºC)

Table 4-10: Nusselt number and convective heat transfer coefficient using different

correlations

Q

[LPM]

Tsphere

[ºC]

h Whitaker

[W/(m2K)]

h Kramer

[W/(m2K)]

h Krishnan

[W/(m2K)]

Nup

Whitake

Nup

Kramer

Nup

Krishnan

Rep vtube

[m·s-1]

40 250 41.01 40.22 40.4 19.82 19.44 19.53 947.1 1.99

40 300 42.71 41.92 42.12 19.23 18.87 18.97 888.6 2.18

40 350 44.35 43.55 43.77 18.72 18.39 18.48 838.9 2.371

50 250 48.23 47.03 47.21 23.31 22.73 22.81 1313 2.758

50 300 50.2 49.01 49.2 22.6 22.07 22.15 1232 3.022

50 350 52.09 50.91 51.12 22 21.5 21.58 1163 3.286

60 250 55.74 53.99 54.16 26.94 26.09 26.17 1748 3.672

30

35

40

45

50

55

60

65

70

75

80

500 1000 1500 2000 2500 3000

Co

nv

ecti

ve

hea

t tr

ansf

er

coef

fici

ent

[W·(

m2·K

)-1]

Re

h Whitaker

h Kramer

h Krishnan

Page 192: Pablo Giménez Gavarrell

Single Capsule Model

150

60 300 57.98 56.25 56.44 26.11 25.33 25.41 1640 4.024

60 350 60.15 58.43 58.62 25.4 24.67 24.75 1548 4.375

70 250 63.58 61.14 61.3 30.73 29.55 29.63 2259 4.747

70 300 66.12 63.7 63.87 29.77 28.68 28.76 2120 5.201

70 350 68.56 66.15 66.34 28.95 27.93 28.01 2001 5.655

80 250 71.77 68.5 68.65 34.69 33.11 33.18 2854 5.997

80 300 74.61 71.36 71.52 33.6 32.13 32.2 2678 6.571

80 350 77.35 74.1 74.28 32.66 31.29 31.36 2528 7.144

It is important to note the limits of the Krishnan correlation (Rep≤500 and

0.02≤BR≤0.5), and Kramers correlation (1<Rep<2000) and take into account that the

experimental set-up conditions are slightly above the limits of the proposed

correlations Rep ≈ (1000-1500), BR=0.625. In Whitaker’s correlation, for a

temperature difference of 100ºC between the HTF and the capsule surface, the term

(μ∞/μs)1/4 ≈ 1.03 in the worst-case scenario, which means that this 3% correction can

be neglected. Another difference is the use of water (Pr=5.12) in Krishnan´s

simulation instead of air (Pr=0.7) in the experimental set-up. This means that the

thermal diffusivity is low compared with the momentum diffusivity for water,

compared to the case of air, with a higher thermal diffusivity than momentum

diffusivity, but this should not affect the validity of the correlation.

4.7.2. Model vs. Experiments

There are certain limitations that have to be considered when comparing the

experimental results to the simulations. Some of them are described below:

The implemented model is one-dimensional which means that it might be

more appropriate to describe the freezing process of a uniformly heated and

completely filled capsule, because it is expected to show a radial evolution of

the freezing front. Whereas the fabricated capsules have a small void (to

manage volume expansion during melting) and are not perfectly spherical,

Page 193: Pablo Giménez Gavarrell

151

Thermal Energy Storage for High Temperature Applications

introducing spatial asymmetries in the melting process.

Only on a two-dimensional model the specific location of the capsule void

space can be considered. The simplest way to introduce a first approximation

correction for this empty space is by calculating an effective density of the

PCM-void system. It this sense, the equivalent completely filled PCM capsule

with the new density will store the same energy as the real capsule.

The set-up air conditions are slightly out of the validity limits of the heat

transfer convection coefficient correlations. Even so, they have been used

because these correlations are the closest to the experiments that have been

found in the literature.

The effect of the capsule holder is not considered, and it is expected to increase

slightly the turbulence of the air and the convective heat transfer coefficient.

Wall effects have also not been considered.

The thermocouples are not in contact with the capsule; therefore their

temperature might be slightly overestimated leading to higher temperatures in

the model than in the experimental set-up.

The melting process can be considered a two-dimensional problem as can be

seen from the video recording where the density differences are not negligible.

The solid fraction of the PCM sinks to the bottom of the capsule since the

density of solid PCM is higher than that of liquid PCM. These differences in

density might accelerate the real melting process, and they are not considered

in the implemented one-dimensional model.

There is another expected deviation based on the capsule position in the set-up.

The heat transfer fluid flows around the capsule from the top to bottom in a

channel. The highest rate of heat transfer should occur at the top. However,

over subsequent melting/freezing cycles, due to liquid/solid PCM density

differences and convective buoyant flows inside the capsule, the void space

moves to the top. The presence of a void located at the top of the capsule acts

as an insulator and is expected to slow down the heat flux into the PCM.

The melting process is expected to be affected by the difference in density between

the solid and liquid PCM, as commented before. On the other hand, the freezing

process is expected to behave more like a radial evolution, more similar to the

implemented one dimensional model. However, in the case of NaNO3 capsules,

Page 194: Pablo Giménez Gavarrell

Single Capsule Model

152

the phase change duration can be estimated only for melting experiments because

in the freezing experiments the opacity of the external layer of frozen salt, the lack

of thermocouple inside the capsule, and the challenges in seeing a clear

temperature trace with the IR camera do not allow to estimate the end time for

freezing experiments.

In contrast, the experimental melting and freezing start times can be clearly

measured and compared with their corresponding times in the one-dimensional

model. In this case we can expect the opposite behavior: the phase change start

time in the melting experiments are expected to behave closer to the model than the

phase change start time in the freezing experiments. The reason is the following: in

the melting experiments, the PCM starts in solid state and because of the low

thermal diffusivity of the NaNO3, the simulations indicate that finding thermal

differences of ~30 ºC between the shell-PCM interface (at R=R2) and center of the

capsule (at R=0) is not unexpected.

On the other hand, in freezing experiments the PCM starts in molten state. Based

on literature values, we have used the same thermal conductivity for solid and

liquid NaNO3. However, the convection in the liquid PCM inside the capsule (not

considered in the model), might reduce the thermal gradients compare to the solid

PCM case. Then, we will need to cool down the capsule to a lower temperature to

start the freezing process in our experiments and this process is expected to take

longer.

In order to have an order of magnitude of this effect we have simulated two

NaNO3-capsules with two different values of liquid thermal conductivity: 0.5 and 5

Wm-1K-1. For h=50 and 150 Wm-2K-1 the melting start time are 154 and 77 seconds

respectively. The increase of an order of magnitude in the thermal conductivity of

the liquid phase, trying to simulate the expected convection inside the capsule,

increases the melting start time by only ~7 seconds. This means that the effect

described can be neglected and it is not expected to change significantly the results.

4.7.2.1. Reproducing different PCM qualitative behavior

Figure 4-28 shows the experimental infrared temperature curves for a Pb-

Borosilicate capsule (blue) and a NaNO3-Borosilicate capsule (brown). In the

metallic capsule the complete phase change process can be distinguished by the

Page 195: Pablo Giménez Gavarrell

153

Thermal Energy Storage for High Temperature Applications

roughly isothermal segments. On the other hand, in a NaNO3-capsule only the

beginning of the phase change can be seen with the IR camera, identified by a

change in slope temperature history data. These qualitative differences are clearly

reproduced in the model by looking at the outer shell surface temperature (R=R1)

(Figure 4-22, Figure 4-29, Figure 4-30) and are a consequence of the thermal

resistance of the different PCM.

Figure 4-28: Experimental infrared melting and freezing curves for a metallic (Pb,

blue) and Salt (NaNO3, brown) borosilicate capsule

120

140

160

180

200

220

240

260

280

300

320

340

360

380

0 100 200 300 400 500

Tem

pera

ture

[ºC

]

Time [s]

Pb

NaNO3

PbNaNO3

Page 196: Pablo Giménez Gavarrell

Single Capsule Model

154

Figure 4-29: Melting (left) and freezing (right) comparison: experimental infrared

temperature history curves (blue) for a metallic borosilicate capsule compared to

the model results (brown)

Figure 4-30: Melting (left) and freezing (right) comparison: experimental infrared

temperature history curves (orange) for a NaNO3 borosilicate capsule compared to

the model results (blue and green)

140

160

180

200

220

240

260

280

300

320

340

360

0 50 100 150 200 250 300

Tem

per

atu

re [

ºC]

Time [s]

Pb experiment

Pb model

Melting (Model)

Melting(Experiment)

140

160

180

200

220

240

260

280

300

320

340

360

0 20 40 60 80 100

Tem

pera

ture

[ºC

]

Time [s]

Pb experiment

Pb modelFreezing(Model)

Freezing(Experiment)

50

70

90

110

130

150

170

190

210

230

250

270

290

310

330

350

0 100 200 300 400 500

Tem

per

atu

re [

ºC]

Time [s]

NaNO3 Experiment

T_HTF Model

T(R=R1) NaNO3 Model

T (R=0) NaNO3 Model

Start melting (Exp)

Start melting (Model)

50

70

90

110

130

150

170

190

210

230

250

270

290

310

330

350

250 350 450 550

Tem

pera

ture

[ºC

]

Time [s]

NaNO3 Experiment

T_HTF Model

T (R=R1) NaNO3 Model

T (R=0) NaNO3 Model

Start freezing (Exp)

Start freezing (Model)

Page 197: Pablo Giménez Gavarrell

155

Thermal Energy Storage for High Temperature Applications

Different qualitative and quantitative observations can be made:

Different types of PCM based on their thermal conductivity and diffusivity

present different behavior: metallic PCM barely have any temperature

gradients inside the capsule and the shell closely follows the core temperature;

but salt PCM exhibit large temperature gradients between the outer shell and

inner PCM core.

As perfectly reflected in the model, only the phase change process can be

distinguished in metallic PCM based on the capsule surface temperature

history data (Figure 4-29). In salt PCM the surface temperature can be only

used to estimate the melting and freezing start time, not the end time due to

the existence of large temperature gradients inside the PCM (Figure 4-30). This

explains why the IR camera temperature traces (corresponding to irradiation of

the surface temperature of the capsules) only show the complete melting

process in metallic PCM and not in salt PCM capsules.

The melting starting time is observed at higher temperature than the melting

temperature of each material. The IR camera data is the capsule surface

temperature. Because of the low thermal conductivity of the capsule, when the

PCM in contact with the capsule reaches the melting temperature, the capsule

surface temperature has increased (~+10ºC for our experimental conditions). In

spite of the model´s simplicity, it is able to reproduce this observation.

The same phenomenon is produced in the freezing experiments but in the

opposite direction (freezing will occur at lower temperatures than the PCM

melting temperature); consequently the temperature of the start melting and

start freezing observed with the infrared camera should differ by the sum of

these two ∆T.

4.7.2.2. Freezing Experiments

In the freezing experiments, only the freezing start time can be used to compare to

the model. This time is calculated in the model as the time when the shell-PCM

interface reaches the phase change temperature. On the other hand, this time has

been estimated in the video recording of the freezing experiments as the time when

a NaNO3 solid (opaque) layer is completely created, which can be also identified

Page 198: Pablo Giménez Gavarrell

Single Capsule Model

156

based on the infrared images. It means that this time is not affected by the phase

change process because it has not started yet.

As the parametric study concluded, the freezing start time depends mainly on the

dimensionless melting temperature (

) and the convective heat transfer

coefficient (h), being independent of the temperature step (∆T = Tf - To) applied.

However, analyzing the different volumetric flow rates used in the experiments

and their calculated convective heat transfer coefficient it seems that the convective

heat transfer coefficient may only vary between 40 and 60 Wm-2K-1. The vast

majority of the experiments have been performed with ~50 Wm-2K-1 (50LPM).

Figure 4-31 represents the estimated freezing start time vs. the dimensionless

melting temperature for the freezing experiments. Freezing start time increases

with the dimensionless phase change temperature as expected. Unfortunately, with

the experiments performed do not have a large difference in flow rates to extract

experimental trends with the heat transfer coefficient.

Figure 4-31: Experimental freezing start time vs. dimensionless phase

change temperature for freezing experiments

In order to compare with the experimental results, we can simulate the freezing

process for the NaNO3-Borosilicate capsule, using average values for the set-up

0

20

40

60

80

100

120

140

0.0 0.2 0.4 0.6 0.8

Fre

ezin

g S

tart

tim

e [s

]

Dimensionless melting temperature θm [-]

Exp h=40 W/(m2 K)

Exp h=50 W/(m2 K)

Exp h=60 W/(m2 K)

Page 199: Pablo Giménez Gavarrell

157

Thermal Energy Storage for High Temperature Applications

condition (∆T and τexp), simulating different dimensionless melting temperatures

and different convective heat transfer coefficients, as shown in Figure 4-32.

Figure 4-32: Model and experimental results comparison: Freezing start time vs

dimensionless phase change temperature for freezing experiments

As the model suggests, freezing start time increases with increasing dimensionless

phase change temperature and with higher heat transfer coefficients. As the

convective heat transfer coefficient increases, the differences in freezing start time

for different connective heat transfer coefficients are reduced. The experimental

results at 50 Wm-2K-1 show the same qualitative behavior as the model, fitting

reasonably well to a linear curve. However, the experimental times behave closer to

curves with a convective heat transfer coefficient h between 100 and 150 Wm-2K-1,

even though the estimated experimental convective heat transfer coefficient is

significantly lower (50 Wm-2K-1). These findings indicate that the calculated

convective heat transfer correlation might underestimate this coefficient. This

0

50

100

150

200

250

300

350

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fre

ezin

g S

tart

tim

e [

s]

Dimensionless melting temperature θm [-]

Exp h=40 W/(m2 K) 40 LPM

Exp h=50 W/(m2 K) 50 LPM

Exp h=60 W/(m2 K) 60 LPM

Model h=50 W/(m2 K)

Model h=100 W/(m2 K)

Model h=150 W/(m2 K)

Page 200: Pablo Giménez Gavarrell

Single Capsule Model

158

deviation could be due to:

Inaccuracies measuring the volumetric flow rate

Higher heat transfer coefficient in the experiments due to turbulent flow, not

fully developed temperature profiles, higher heat losses than expected, the

presence of the capsule holder introducing any one of these effects, etc.

Underestimated experimental heat transfer coefficient due to simplifications

and the use of correlation out of its validity limits

Considering the number of experiments under the same conditions, the

inaccuracies/difficulties in measuring phase change times visually from a video

recording, and the simplicity of the model it is quite remarkable that the trend with

the dimensionless temperature can be experimentally reproduced even though the

heat transfer coefficient is underestimated.

4.7.2.3. Melting experiments

Similar to the freezing experiments, the melting start time has been represented vs.

the dimensionless melting temperature (Figure 4-33). The experiments show the

expected behavior of the effect of the convective heat transfer coefficient: lower heat

transfer coefficients tend to increase the melting start time, while higher convective

heat transfer coefficients will tend to start the melting process sooner. The trend

with the dimensionless melting temperature is also as expected: higher

lead to longer melting start times.

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159

Thermal Energy Storage for High Temperature Applications

Figure 4-33: Experimental melting start time vs. dimensionless melting

temperature.

The simulations for different heat transfer coefficients have been superposed to the

experimental results (Figure 4-34). The general trends are reproduced: melting start

times increase with higher and lower h and there is a greater slope for lower

heat transfer coefficients. However, as before with the freezing experiments, the

calculated experimental hconv is underestimated especially for h=50 Wm-2K-1. We can

observe that the experiments with an estimated convective heat transfer coefficient

of ~40 Wm-2K-1 fit the simulated curve for h=50 Wm-2K-1. However, as in the

freezing experiments, the experiments with an estimated convective heat transfer

coefficient of ~50 Wm-2K-1 fit simulated curves with ‘h’ between 100 and 150 Wm-2K-

1. The experiments with an estimated h~60 Wm-2K-1 fit simulated curves with

higher ‘h>150 Wm-2K-1 ’.

0

20

40

60

80

100

120

140

160

180

200

0.0 0.2 0.4 0.6 0.8

Mel

tin

g S

tart

tim

e [s

]

Dimensionless melting temperature θm [-]

Exp h=40 W/(m2 K) 40 LPM

Exp h=50 W/(m2 K) 50 LPM

Exp h=60 W/(m2 K) 60 LPM

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Figure 4-34: Model and experimental results comparison: Melting start time vs.

dimensionless melting temperature.

Finally, the phase change start time for melting and freezing experiments can be

combined in a single graph (Figure 4-35). We observe a clear tendency and

alignment for the experiments performed with an estimated convective heat

transfer coefficient of ~50 Wm-2K-1 for both, melting and freezing experiments. This

shows the symmetry of the problem when comparing heating and cooling

experiments. Results for different heat transfer coefficients have not been

represented.

0

20

40

60

80

100

120

140

160

180

200

0.0 0.2 0.4 0.6 0.8

Melt

ing

Sta

rt t

ime [

s]

Dimensionless melting temperature θm [-]

Model h=50 W/(m2 K)

Model h=100 W/(m2 K)

Model h=150 W/(m2 K)

Exp h=40 W/(m2 K)

Exp h=50 W/(m2 K)

Exp h=60 W/(m2 K)

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Thermal Energy Storage for High Temperature Applications

Figure 4-35: Experimental phase change start time vs. dimensionless phase

change temperature.

For melting experiments we can estimate the melting duration calculating the

difference between the melting start time (based on the infrared images and video

recording) and the end melting time (based on the video recording). Figure 4-36

represent the experimental melting duration vs. the temperature step applied in

each melting experiments for different dimensionless melting temperatures.

Different simulated curves have been included for the same dimensionless melting

temperature using 50 Wm-2K-1 as heat transfer fluid. Other convective heat transfer

coefficients have been also included as reference. As shown, the numerically

predicted melting time duration shows reasonably good agreement when

compared to the results obtained in the experiments for three of them (θm=0.23 &

h=40 and 2 out of 3 experiments of θm=0.29 & h=50) fitting perfectly to the

simulated curve. On the other hand, some experiments deviate from their

simulated curves. One of the experiments ‘θm=0.29 & h=50’ shows melting time

significantly longer than the simulation (+19%) and one of the experiments ‘θm=0.43

& h=50’ shows melting time significantly shorter than the simulation (-17.5%).

However, considering all the limitations exposed previously these deviations could

be considered reasonable.

The experiment ‘θm=0.59 & h=40’ would fit a simulated curve with a lower

convective heat transfer coefficient (h=30). It means that it shows longer time than

0

20

40

60

80

100

120

140

160

180

200

0.0 0.2 0.4 0.6 0.8 1.0

Ph

ase

ch

an

ge S

tart

tim

e [

s]

Dimensionless phase change temperature θm [-]

Freeze h=50 W/(m2 K)

Melt h=50 W/(m2 K)

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162

simulated with h=40. And the experiments with ‘h=60’ fit a simulated curve with a

higher convective heat transfer coefficient. This might indicate, as in the phase

change starting time analysis was pointed out, that the convective heat transfer

coefficient estimated for the experimental set-up underestimate the real convective

heat transfer coefficient.

Figure 4-36: Model and experimental results comparison: Melting duration time vs.

temperature step applied for different dimensionless melting temperatures and

convective heat transfer coefficients.

In Table 4-11 the experimental convective heat transfer coefficient that will make

the simulated curves fit the experimental melting start time and melting duration

has been estimated. The melting start times for all the melting experiments fit

curves with higher convective heat transfer coefficient. It means that the melting

start process seems to be faster than expected numerically and the experimental

convective heat transfer coefficient was underestimated. Experiments 2, 3 and 5

show faster melting start time than expected but they fit very well the melting

200

300

400

500

600

700

800

900

1000

40 60 80 100 120 140 160 180

Melt

ing

Du

rati

on

tim

e [

s]

∆T [ºC]

Model θm=0.60 & h=30

Exp θm=0.59 & h=40

Model θm=0.23 & h=40

Exp θm=0.23 & & h=40

Model θm=0.29 & h=50

Exp θm=0.29 & h=50

Model θm=0.43 & h=50

Exp θm=0.43 & h=50

Model θm=0.37 & h=140

Exp θm=0.34 & h=60

Exp θm=0.43 & h=60

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Thermal Energy Storage for High Temperature Applications

duration time. Experiment 1 and 4 follow the same trend: they fit simulated curve

with a higher convective heat transfer coefficient than the experimentally estimated

value for the melting start time but the opposite for the melting duration.

The melting start time and melting duration for the experiment 6 is shorter than

simulated. Consequently the experiment fits simulated curve with higher

convective heat transfer coefficient. Finally, experiments 7 and 8 follow the same

trend, the PCM reaches its melting temperature in a significantly shorter time, and

the melting duration takes a moderate shorter time to finish. This means that

simulations with higher ‘h’ than the experimentally estimated would fit the

experimental results.

Table 4-11: Estimated experimental convective heat transfer coefficient that will

make the simulations fit the experimental times.

Melting Experiment Number

(estimated experimental ‘h’)

Melting start time (‘h’ that makes simulation

fit the experiments)

Melting duration (‘h’ that makes simulation

fit the experiments)

1 (h=40 Wm-2K-1) h↑ (~50 Wm-2K-1) h↓ (~30 Wm-2K-1)

2 (h=50 Wm-2K-1) h↑ (~100 Wm-2K-1) Ok

3 (h=50 Wm-2K-1) h↑ (~100 Wm-2K-1) Ok

4 (h=50 Wm-2K-1) h↑ (~100 Wm-2K-1) h↓

5 (h=40 Wm-2K-1) h↑ (~50 Wm-2K-1) Ok

6 (h=50 Wm-2K-1) h↑ (~100 Wm-2K-1) h↑

7 (h=60 Wm-2K-1) h↑ (~350 Wm-2K-1) h↑ (~140 Wm-2K-1)

8 (h=60 Wm-2K-1) h↑ (~350 Wm-2K-1) h↑ (~140 Wm-2K-1)

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4.8. Double PCM solution

A packed bed solution with steam as the heat transfer fluid requires a costly high

pressure vessel to contain the heat exchanger, increasing the system cost

dramatically. Alternative HTF-storage heat exchanger solutions were also

examined as part of this study. A more complex yet economical solution consisting

of a “double-PCM” system is proposed in order to avoid direct heat exchange of

the capsules with high steam pressure. Changing the heat exchanger surface to

tubes instead of a packed bed vessel can enable further cost reductions.

This solution, represented schematically in Figure 4-37, has been covered by the

Patent PCT/ES2015/070452-WO/2015/18945088. The invention relates to a thermal

storage system which includes a container (1) in which several components shown

in Figure 4-37 are arranged: a) a set of capsules (3) which form a porous bed and

contain a phase-change material having high energy density and consisting of

inorganic salts; and b) a matrix (2) which consists of a metal phase-change material

having high thermal conductivity and located in the interstices of the capsules (3).

The combination of two phase-change materials provides enhanced effective

conductivity and high energy density in the energy storage system. The invention

also relates to the method for charging and discharging said system by using a

heat-transfer fluid which flows through heat-exchange tubes (4) that pass through

the container (1).

The proposed innovative thermal energy storage system is composed by two

specific materials with a matching temperature in the range of the high pressure

Rankine cycles (150bar). These PCM are: a metallic MgZn alloy and chloride

quaternary salt mixture (LiCl-KCl-LiCO3-LiF) encapsulated and submerged on the

metallic alloy (properties can be seen in Table 4-12).

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Thermal Energy Storage for High Temperature Applications

Figure 4-37: Double PCM TES solution

Table 4-12: Material properties

Material ρ [kg m-3] ∆Hf [J kg-1] Tm [ºC]

Mg(49)-Zn(51) 2850 155 342

LiCl-KCl-LiCO3-LiF 3584 375 340

The usage of two PCM materials with different nature (metallic and inorganic salts)

allows a combination of their properties and makes possible to increase the thermal

conductivity of the embodiment without losing energy density. The study of the

optimal fraction of each material in terms of conductivity, energy density and cost

as well as other design consideration as the capsule fabrication and materials, and

the experimental characterization of both mixtures is out of the scope of this thesis.

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4.9. Discussion and conclusion of encapsulated PCM as TES system

A new encapsulation method for high temperature Phase Change Materials (PCM)

is developed for Direct Steam Generation (DSG) applications in Solar Thermal

Power Plants. A solution consisting of borosilicate and NaNO3 as a shell and PCM

core respectively has been studied focusing on the thermal behavior of the PCM-

capsule system. The phase change properties of some inorganic salts and metal

alloys have been thermally characterized using conventional DSC. A novel

encapsulation procedure has been developed together with an experimental setup

aimed at analyzing through visual observation and with an infrared camera the

melting and freezing behavior of high temperature PCM. The infrared images have

been demonstrated as a feasible method to determine the complete phase change

process in metallic PCM –borosilicate shell capsules.

A one-dimensional finite difference heat transfer model simulates the phase

transition within a single capsule. The model has the objective of a) aiding and

guiding further PCM capsule designs and b) is used to validate the concept of

borosilicate shell capsules in the 300-400ºC range. Experimental conditions and

empirical correlations have been used to determine the average convective

coefficient around the sphere. This parameter and the measured capsule

surrounding air temperature have been used as a boundary condition in the model.

The melting start time, melting duration and freezing start time have been taken to

compare both experiments and simulations.

The comparison of the model to the experimental data can be performed thanks to

the transparency of the shell and the salt in liquid state, a property of the system,

which allows the study of the behavior of the capsules through visual observation,

being able to estimate the start and finish of the melting process in heating

experiments and the start of the freezing process in cooling experiments.

In spite of the many simplifications implemented, considering the complexity of

the moving boundary phase change problem plus the asymmetries of a partially

filled, almost spherical borosilicate capsule, the model accomplishes the proposed

goals reasonably well. It helps understand the effects of different parameters on the

phase change process of PCM-capsules such as the nature of the PCM (salt or

metal), the shell material nature (borosilicate or steel), the thickness and size of the

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Thermal Energy Storage for High Temperature Applications

capsule; as well as the effect of experimental conditions such as the convective heat

transfer coefficient, the temperature step applied or the dimensionless melting

temperature.

As a summary, the key points are:

Using borosilicate as a shell material instead of encapsulating with metals will

not significantly change the melting times, even though the thermal resistance

of the shell wall does increase. For thin shells and within the range of the

parameters evaluated in this study, melting times are much more sensitive to

the capsule heat transfer with its surroundings, i.e. the convective heat transfer

coefficient values.

The PCM´s thermo-physical properties will have a large influence on the

temperature profiles inside the capsules and, consequently, on the information

which can be obtained by non-invasive measuring techniques such as real-time

imaging (visible and IR). Highly conductive metallic PCM clearly show the

phase change process on the surface, as the temperature is almost uniform

inside the capsules. Inorganic salts, on the other hand, do not show the

isothermal phase change process on the external layers as large temperature

gradients and high thermal inertia exist inside the capsules. The model is used

to corroborate these experimental findings.

The dimensionless melting temperature θm = (Tm-To)/(Tf-To) has a great effect

on melting times and durations, comparable to the effect of high heat transfer

coefficients.

Finally, the numerically predicted phase change start times and phase change

duration show reasonably good agreement when compared to the results

obtained in the experiments. Also, the qualitatively comparison between

different types PCM (metallic vs. salt) is perfectly aligned with the numerical

results. The convective heat transfer coefficients in the experiments are

underestimated, but this result is not surprising considering the limitations of

the correlations and the real flow and temperature distribution complexity in

the experiments.

Once the thermal problem of a single capsule was solved, an innovative solution

was proposed to avoid the existence of a large storage tank under pressure as well

as to address the mechanical integrity of the capsule exposed to high pressure. This

solution, successfully patented, consists of the use of a molten metal surrounding

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168

the borosilicate capsules. In the storage process the metal and capsules act as PCM,

storing energy as latent heat and the metal matrix increases the effective thermal

conductivity of the composite.

In conclusion, the main goal was to develop an encapsulation or storage system

with the following properties:

Enough strength and mechanical integrity to hold the PCM inside during

melting and solidification, accommodating the volume expansion during the

phase change process: several melting and freezing cycles have been

performed successfully on different partially filled PCM-capsules.

A non-porous shell to prevent any molten PCM leakage: no leakage has been

observed

Stable at high temperatures and continuous freeze/thaw thermal cycling: tested

successfully

A good thermal conductor to effectively transfer heat from the heat transfer

fluid (HTF) to the PCM: the study performed with the numerical model

indicates small differences between a metallic and a borosilicate shell.

A non-reactive shell material to the molten PCM

A non-reactive shell to the HTF (high temperature, high pressure steam)

Low cost material.

The remaining challenges are mainly economical: the capsule cost (material

and fabrication) and optimal heat exchanger configuration (to avoid costly

pressurized steam tanks).

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169

5 NANO-ENHANCED HEAT

TRANSFER FLUID FOR THERMAL

ENERGY STORAGE

olten salts are commonly used as heat transfer fluids and thermal energy

storage media in concentrated solar power central receiver plants.

Systems of heliostats concentrate solar energy in the solar receiver. The

molten salt circulates through the receiver absorbing the energy as sensible heat,

increasing its temperature from 290 ºC to 565 ºC. The hot salt is stored in tanks to be

used later to heat water and produce steam in the steam generator, which is used to

produce electricity in a turbine. The amount of energy one is able to store in the

molten salt ‘Q’ is determined by:

Equation 5-1

Where ‘m’ is the amount of salt, ‘∆T’ is difference between the temperature of the

salt in the hot tank and the temperature of the salt in the cold tank; and ‘Cp’ is the

specific heat of the salt. Consequently, the specific heat is directly proportional to

the storage capacity. This means that any improvement in this property increases

the storage density, and it translates in an increase in the storage capacity. Other

M

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Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage

170

approaches to increase the storage capacity involve increasing the maximum

operation temperature, determined by the thermal decomposition of the salt and/or

reducing the minimum working temperature, limited by the salt freezing

temperature.

The aim of this chapter is to increase the storage capacity of the current TES

systems based on two molten salt tanks. To do so, the addition of nanoparticles to

the base fluid is investigated. Choi in 199589 was the first to use the term

“nanofluid” to refer to this colloidal suspension of nanometer-scale particles in a

fluid. Since then, nanofluids have been extensively studied due to reports of

enhanced thermal and physical properties, such as thermal conductivity and heat

capacity. Small amount of nanoparticles dispersed in the storage media resulted in

important improvements for the thermal energy storage systems, showing a huge

potential to reduce costs in current and future solar thermal power plants.

Moreover, the ability to develop the technical know-how and capability to produce

nanofluids with customizable thermal properties would have a wide variety of

other engineering applications. The effect of nanoparticles in other thermal

properties of the inorganic salt such as the latent heat is also investigated.

5.1. Introduction

5.1.1. Background on Nano-enhanced HTF-TES materials

Romanin & Fereres90 compiled a large number of published experimental heat

capacity data for nanofluids. The authors tried to clarify the magnitude, nature, and

cases where an enhancement of specific heat capacity can be expected and potential

theories to explain the nano-modified specific heat capacity. The different trends

found in this meta-analysis (29 references) are summarized as follows:

1. Overall, water and organic based nanofluids do not show an increase of

the specific heat capacity with respect to the base fluid value; in fact,

adding any type of nanoparticle to these fluids decreases the specific heat

capacity. This effect is more noticeable as particle loading is increased.

2. There is substantial evidence that significant (>20%) heat capacity

enhancement is possible in the case of molten salts with nanoparticles, in

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Thermal Energy Storage for High Temperature Applications

contrast to water based and organic based nanofluids.

3. The published data also shows that the majority of the reported

enhancement occurs at nanoparticle concentrations around 1% by mass. At

higher concentrations, there might be higher propensity for agglomeration

of the nanoparticles leading to the degradation of the thermal properties

and thermal performance.

Despite the large number of experimental data, only a few research groups have

been working on molten salt nanofluids to this date. The main groups currently

investigating the specific heat of nanofluids based on molten salts are presented in

Figure 5-1.

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172

Figure 5-1. Main research groups analyzing molten salt nanofluids (main author

marked in red)

D BanerjeeSince 2009(All fluids)

(Texas A&M University)

D Shin (University of

Texas at Arlington) H Tiznobaik

B Jo N SinghS Jung H Kwak

R DevaradjaneJ SeoB Dudda

Ian C. Nelson & Ponnappan, Rengasamy

University of

Perugia (Italy)

2013 (Solar Salt)Chieruzzi, ManilaCerritelli, Gian FMiliozzi, AdioKenny, José M

2015 (Potassium Nitrate)Chieruzzi, ManilaMiliozzi, AdioCrescenzi, TommasoTorre, LuigiKenny, José M

UniversitatJaume I,

Castellon (Spain)

2014 (Solar Salt)Patricia Andreu-Cabedo, Rosa Mondragon,Leonor Hernandez,Raul Martinez-Cuenca,Luis CabedoJ Enrique Julia

National TsingHua

University (Taiwan)

National ChiaoTung

University (Taiwan)

2013 (Solar Salt )Lu MC, Huang CH

Univ. of Leeds & Univ. of Birmingham (UK)

2015 (Solar Salt)Mathew Lasfargue, Qiao Geng, Hui Cao, Yulong Ding

Taiwan

Texas A&M University

(USA)2012 (Solar Salt&Carbonate)Michael Schuller, Frank Little, Darren Malik, Matt Betts, Qian Shao, Jun Luo, Wan Zhong, Sandhya Shankar, AshwinPadmanaban

2015 (Solar Salt)Michael Schuller, QianShao, Thomas Lalk,

MScThesis:D. MalikM. Betts

PhD. Thesis:M. Lasfargue

ChinaXi’an Jiaotong

University

2015 (Carbonate)Y.B. TaoC.H. LinY.L. He

2014 (Hitec Salt)Ho MX, Pan C

PhD ThesisD. ShinH TiznobaikMSc Thesis:B. Dudda

National Renewable

Energy Laboratory Colorado

(USA)

2011 (Nitrate, PAO)Anne K. Starace, Judith C. Gomez, Jun Wang, Sulolit Pradhan, and Greg C. Glatzmaier

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Thermal Energy Storage for High Temperature Applications

The Texas A&M University research group led by Professor D. Banerjee was the

first and more active group publishing specific heat enhancement of molten salt

based nanofluids. D. Banerjee participated in the first report of specific heat

capacity enhancement using Polyalphaolefin (PAO) as a based fluid in 2009

modified with exfoliated graphite (EG) nanoparticles.91 The specific heat capacity of

the nanofluid was found to be enhanced by 50% compared with PAO at 0.6 wt. %

nanoparticle concentration.

After this initial publication this group moved to a synthetic oil used in CSP plants

known as Therminol VP-192,93 and molten salt such as: chlorides (KCl-CaCl₂-LiCl)94

and BaCl2-NaCl-CaCl2-LiCl95, binary carbonates (Li2CO3-K2CO3 eutectic)96–103 , and

nitrates (NaNO3-KNO3 solar salt)104–106.

After Banerjees’ interesting initial results, Starace et al.107 tried to reproduce

Nelson’s original results with PAO modified with expanded graphite

nanoparticles. The authors re-tested this fluid as well as different combinations of

base fluids (ethylene glycol, water/eth. Glycol, Ca(NO3)2*4H2O, mineral oil) and

other nanoparticle types (fumed silica 20 and 40 nm, 50 nm SiO2, 100 nm Al2O3, 15

and 20 nm Fe@Fe3O4, 40 nm Bi, 100 nm aluminum nitride). However, they found

“no increase in heat capacity upon the addition of the particles larger than the

experimental error”. It is worth mentioning that even though 5-10% of the

measurements performed showed slightly higher Cp than 6% of the base fluid Cp,

the vast majority of their measurement (>90%) fall within +/-6% difference

compared to the base fluid, which the authors considered experimental error.

Despite this first unsuccessful attempt to reproduce Banerjee’s results the number

of publications of molten salt based nanofluids has increased year after year. Table

5-1 shows the number of publications on the specific heat of molten salt nanofluids

published since 2009 classified by base fluid regardless of the Cp result

(enhancement or not). The initial PAO and Therminol VP-1, a conventional heat

transfer fluid used in parabolic trough CSP plants, are also included. The

specific references are presented on Table 5-2.

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Table 5-1. Number of publications on the specific heat capacity of high temperature

nanofluids.

Year 2009 2010 2011 2012 2013 2014 2015 2016

PAO 1

1

Therminol VP-1

2

Carbonates

4 3 2 3 2 5

Nitrates

1 2 3 4 3 1

Chlorides

1 1

Adding up these numbers, Figure 5-2 shows on which materials these different

research groups have been working on. Among the different high temperature

base fluids modified with nanoparticles, it is interesting to note that roughly 50% of

the references investigate carbonate salts (specifically the binary system Li2CO3-

K2CO3)

Figure 5-2. Experimental studies (39) on molten salt nanofluids measuring the

specific heat capacity of the liquid salt

22

19

14

2

PAO

Therminol VP-1

Carbonate

Nitrate

Chloride

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Thermal Energy Storage for High Temperature Applications

A summary of these publications, with the nanoparticle concentration and specific

heat enhancement, is presented on Table 5-2.

Table 5-2. Review on molten salt nanofluids including the base fluid, nanoparticle

concentration and specific heat enhancement. Nitrate, when not specified, refers to

solar salt (NaNO3-KNO3 60-40 wt. %). Studies marked in blue correspond to

references that do not belong to Texas A&M University.

Year Base Fluid NP

[wt.%]

Enhancement

[%] Reference

2009 PAO 0.6 +34% Nelson et al. (2009)91

2011 PAO 1 +0% Starace et al.(2011)107

2010 Therminol VP-1 1 +5.4%

Shin et al. (2010)92

2010 Therminol VP-1 1 +5.41% Kwak et al. (2010)93

2010 Chlorides 1 +5% Shin & Banerjee (2010)94

2011 Chlorides 1 +14.5% Shin & Banerjee (2011b)95

2011 Nitrates 1 +20% Betts (2011)108

2012 Nitrates

Hitec XL 1 +34.6%

Devaradjane & Shin

(2012)109

2012 Nitrates 1 +25% Dudda & Shin (2012)104

2013 Nitrates 1 +28% Dudda & Shin (2013)105

2013 Nitrates 1 +22.4% Chieruzzi et al. (2013)110

2013 Nitrates 2 -3% Lu & Huang (2013)111

2014 Nitrates

(Li-Na-K)-NO3 1 +13% Seo & Shin (2014)112

2014 Nitrates 1 +25% Andreu-Cabedo et al.

(2014)113

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176

2014 Nitrates

Hitec 0.063 +19.9% Ho &Pan (2014)114

2014 Nitrates 1 +10% Xiao et al. (2014)115

2015 Nitrates

KNO3 1 +6.1% Chieruzzi et al. (2015)116

2015 Nitrates 0.78 +30.6% Schuller et al. (2015)117

2015 Nitrates 0.1 +10.5% Lasfargues et al. (2015)118

2016 Nitrates

Hitec XL 1 +19%

Devaradjane & Shin

(2016)119

2010 Carbonate 1.5 +75% & +100% Shin & Banerjee (2010)96

2010 Carbonate 1 +22.4% & +17% Shin et al. (2010)92

2010 Carbonate 2.5 +14.6% Kwak et al.(2010)93

2010 Carbonate 1 +15.7% Jo & Banerjee (2010)120

2011 Carbonate 1 +24% Shin & Banerjee (2011a)97

2011 Carbonate 1 23% Shin & Banerjee (2011c)121

2011 Carbonate 1 +21% Jo & Banerjee (2011)122

2012 Carbonate 1 +28.4% Tiznobaik & Shin (2012a)123

2012 Carbonate 1 +83% & +20% Tiznobaik & Shin (2012b)124

2013 Carbonate 1 +26% Tiznobaik & Shin (2013b)98

2013 Carbonate 1.5 118-124% (zoneA)

0% (zoneB) Shin & Banerjee (2013)99

2013 Carbonate 1 +26% Tiznobaik & Shin (2013a)100

2014 Carbonate 1 +32% Shin & Banerjee (2014)101

2014 Carbonate 0.1 +57% Jo & Banerjee (2014)125

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Thermal Energy Storage for High Temperature Applications

2015 Carbonate 1 22% Tiznobaik et al. (2015)103

2015 Carbonate 1 +15% (Solid) Shin & Banerjee (2015)102

2015 Carbonate 1 +22% Jo & Banerjee (2015a)126

2015 Carbonate 1 +29.3% Jo & Banerjee (2015b)127

2015 Carbonate 1.5 +18.6% Tao et al. (2015)128

The types of nanoparticles have not been included because nearly all enhancements

occur with a wide variety of nanoparticles. The different types of nanoparticles

used include different metal oxides: Al2O3, TiO2, CuO and SiO2 (in various forms);

and carbon based nanoparticles including carbon black (amorphous carbon), multi-

wall (MW) and single-wall (SW) carbon nanotubes (CNT), and graphite platelets or

exfoliated graphite. No significant trend was found when the enhancement was

represented vs. the normalized nanoparticle diameter or the particle specific

surface area.90

If we represent the different enhancements grouped by type of base fluid (Figure

5-3) we can observe that there are only two PAO references with contradictory

results. On the other hand, Therminol VP-1 shows a marginal +5.4% of

enhancement, considered within the experimental error in Starace et al.107.

Chloride salts are well known due to its hygroscopicity and corrosiveness, for this

reason they were discarded.

Finally, carbonate and nitrate base nanofluids seem to show higher potential Cp

enhancement compared to the other base fluids. Although carbonate salts have

shown the highest Cp enhancement among the different molten salts, its

applicability as HTF and TES in solar power plans seems complicated due to its

high melting temperature (near 500ºC).

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178

Figure 5-3. Specific heat enhancement vs. Nanoparticle concentration.

Another observation is that most of the references on carbonate nanofluid (95%;

18/19) come from the same research group. On the contrary, the number of groups

investigating nitrate base mixtures modified with nanoparticles is larger. This

could be because of the immediate applicability that this technology would have in

today’s thermal storage in conventional solar thermal power plants which widely

use nitrate salt mixtures. Consequently, the nitrate mixture known as “solar salt”

has been the selected material of study.

5.1.1.1. Theories behind Cp enhancement

Shin and Banerjee95 introduced three independent mechanisms to explain the

observed enhancement of the specific heat capacity values, which are itemized as

follows:

-20%

0%

20%

40%

60%

80%

100%

120%

140%

0 0.5 1 1.5 2 2.5 3

Sp

ecif

ic H

eat

En

han

cem

en

t [%

]

Nanoparticle Concentration [wt. %]

PAO

Therminol VP-1

Chlorides

Nitrates

Carbonate

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Thermal Energy Storage for High Temperature Applications

1. Enhanced specific heat of nanoparticle due to its reduced size (higher

specific surface area per unit mass)

This proposed mechanisms is based on some reports in the literature of

nanoparticle powders, such as Wang et al.129. In these studies nanoparticles show

larger heat capacities than their bulk counterparts. If nanoparticles have the same

crystal structure as their bulk counterparts, the nanoparticles have higher heat

capacities because of their much larger proportion of surface atoms. However, this

first mechanism was discarded 90,107 because an increase of the reported 25%

between the nanoparticle Cp compared to the heat capacity of the bulk material

would have a negligible effect on the nanofluid Cp for the small mass fractions

used in the literature. Then, the result in nanofluidCp/basefluidCp ratios should be

close to one, as they were measured in Starace et al.107 for mass fractions of around

1 wt. % and lower.

2. Enhanced thermal properties of a dense semi-solid liquid layer of

eutectic molecules formed on the nanoparticle surface.

This second proposed mechanism was again discarded in Starace et al.107. Although

it has been demonstrated with molecular dynamic simulations and experiments

that liquid metals Al–Al2O3 form atomic layers at the interface with a flat solid130,

the layering dissipates within 1 or 2 nm at most. This means that at small

nanoparticle loadings, the liquid fraction that forms this layer would be also small.

As a result, in order to induce a large change in the overall heat capacity of the

system: a) the layered structure would need to have a heat capacity several times

higher that of the bulk base fluid which is improbable because this layer is made

out of the base salt and nanopaticles; or b) the layered structure must be wider than

2 nm which contradicts the thickness shown experimental and numerically.

3. Additional heat storage in the interfacial interactions (interfacial thermal

resistance) between the condensed phase and the nanoparticle

lattice.95,121

Apart from these three initially postulated mechanisms to justify the specific heat

enhancement, in 2012 Banerjee’s group proposed several new mechanisms to

explain the same phenomenon.

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180

4. Heat storage mechanisms due to chemical interactions between the

nanoparticle and the solvent phase.131

5. The formation of a percolation network composed of semi-solid layering

of salt eutectic induced by the presence of the nanoparticles in a

significant part of the bulk solvent phase. The percolation network

would form a “web” nanostructure that also “traps” the rest of the

nanoparticles in the percolation network.124,131

In 2013 the same group moved to needle like nano-structures instead of percolation

networks.99,100 They claim that the nanoparticles induce the formation of needle-like

structures in molten salts (Figure 5-4).

In 2014 Shin and Banerjee101 accepted that there is no direct contribution of

nanoparticles on enhanced specific heat capacity (Mechanism 1), therefore the

chain-like nanostructures formed is mainly responsible of this enhancement. The

same year Shin et al. (2014)132 distinguish between fractal-like fluid nanostructures

formed by nanoparticles or formed by the molten salt:

a)

b)

Figure 5-4. a) Fractal-like fluid nanostructures formed by nanoparticles in a

conventional nanofluid. b) Fractal-like fluid nanostructures formed by separated

base molten salts in a molten salt nanofluid.132

Nanoparticle

Fluid molecules

Aggregated nanoparticles nanoparticle

Ionic compound

Aggregated ionic compounds

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Thermal Energy Storage for High Temperature Applications

Finally, Tiznobaik et al. in 2015103 proposed the existence of “secondary long range”

nanostructures which primarily dominate the level of enhancement of the specific

heat capacity values, which is less sensitive to the material composition of the

nanoparticle (for a similar size, shape and mass concentration of the nanoparticles).

Apart from all these mechanisms, Thoms (2012)133 postulated the possibility of a

reversible adsorption type interaction on the nanoparticle surface exceeding the

melting point of the base fluid. Two different desorption behaviors are

hypothesized which could be accompanied by a reduction on the latent heat

(Figure 5-5 I&II)

I II

Figure 5-5. Schematic representation of two possible predicted thermal behavior of

adsorbed layer in nanofluid: a) extended solid-liquid phase transition (left); b)

nanoporous substrate studies and experimental observation (right).133

Summarizing, since 2011 the TexasA&M group has postulated various different

mechanisms to justify the specific heat enhancement of nanofluids. It is worth to

mention that all the mechanisms proposed by the Texas A&M’s group are

supported mainly by SEM images, which are taken at room temperature (e.g.

solidified nanofluids). Consequently, there is no physical evidence that these

different “structures” exist at high temperature with the salts in molten state.

Finally, Thoms proposed the existence of a reversible adsorption layer around the

nanoparticle that could correlate specific heat with latent heat. Unfortunately, the

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latent heat is not reported in any of the Texas A&M’s papers, even though the

phase change of the nanofluid is produced prior to in any Cp measurement in

molten state. This means that we cannot validate Thoms’ hypothesis using Texas

A&M’s results.

5.1.2. Impact of a Cp enhancement on a CSTP plant cost

The molten salt used is a mixture of salts composed by sodium nitrate (NaNO3)

and potassium nitrate (KNO3) in the proportion 60-40 wt. %. In a CSTP plant the

molten salt system consists essentially of:

Molten salt storage tanks

Solar receiver

Interconnecting pipes

Heat exchanger in the steam generator system

Cold and hot pumps for salt

Two types of salt inventory must be considered: active and inactive. The active salt

inventory is the salt in charge of guaranteeing that the turbine can supply the

nominal power during the nominal storage hours. For a 15h molten salt TES and

50MWe solar thermal power plant, the active salt inventory would be 134:

Equation 5-2

Where t is the thermal efficiency converting thermal energy in the molten salt to

electricity ( t ~42%); ~ 1.513 KJ/(Kg K) is the average specific heat capacity of the

salt in the temperature range (290ºC to 565ºC). The active storage would be

approximately ~15450 tons of salt.

The inactive volume consists of the salt volume required for filling all the

equipment: the vapor generation system (~0.5% of the active salt), pipes (~1.5% of

the active salt and the solar receiver (~1% of the active salt). There is also an inactive

volume in the storage tanks required due to the minimum submergence pump

height and the ~1 m operating minimum level of the tanks. Summarizing, the

inactive volume might represent ~10.5% of the active salt.

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Thermal Energy Storage for High Temperature Applications

According to Konstantin et al.135 the TES system can represent ~9-10% of the total

cost of a CSP plant (including salt inventory, hot & cold tanks, foundations, pumps,

piping, NOx abatement system, melting system, and heat exchangers)

Quotes from different suppliers indicate the price of the raw KNO3 and NaNO3 to

be 860 $/ton and 444 $/ton. An additional 200 $/ton must be added as a transport

cost. This leads to ~810 $/ton of Solar Salt. Other reference values of the solar salt

suggest a price close to ~1 $/kg.12,136 Using the latest reference multiplied by the

total amount of salt required (active and inactive storage) results in a salt inventory

cost of ~17 M$.

As the salt inventory is about ~50% of TES cost (Figure 5-6) this would lead to a

TES cost of ~34 M$ and ~366 M$ of total CSTP plant cost, considering the TES

represents 9.3% of the total cost (Figure 5-7).

Figure 5-6. Break-Down TES cost of a 50MWe -TES 15h central receiver CSP

Plant.135

49%

17%2%

6%

13%

4% 9%Salt

Storage tanks

Insulation materials

Foundation

Heat Exchangers

Pumps

Balance of system

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184

Figure 5-7. Break-Down cost of a 50MWe -TES 15h central receiver CSTP Plant.135

If we could develop a nano-HTF TES material with a specific heat enhancement of

25% respect to the molten salts, the active salt inventory would be reduced by 20%.

According to the literature review it seems reasonable to assume that this

enhancement could be achieved by dispersing 1wt. % of nanoparticles within the

salt. With these assumptions, the breakeven price that makes both salt inventories

cost the same is 23 $/kg of nanoparticles (assuming that this cost includes the

nanofluid synthesis process cost). This means that a first cost reduction in the

reference salt inventory could be achieved by finding nanoparticle suppliers with a

cheaper price than the breakeven price.

However, the salt inventory cost would not be the only saving, since the TES size

would also be reduced. Figure 5-8 compares the conventional CSTP plant (50MW,

15h TES) with the same system using a hypothetical nano-HTF TES. As Figure 5-8

shows, even without considering any cost reduction in the salt inventory, the

reduction in the TES size represents a TES cost reduction of 9% and 0.9% of the

total CSP cost.

3.8%

31.3%

16.2%

1.7%

9.3%

12.4%

5.7%

6.4%

8.0%

5.2% Site Preparation

Heliostat Field

Reveiver System

Tower

TES

Power Block

Balance of Plant

EPC Contractors Engineering

Contingencies

Owners Costs

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Thermal Energy Storage for High Temperature Applications

Figure 5-8. Comparison between a conventional molten salt Tower (50MW, 15h

TES) CSP plant cost vs. the potential use of a nano-HTF TES with an increase of

25% of the base salt specific heat.

Both potential cost reductions (cost of the salt inventory and reduction in the TES

size) in a CSTP plant motivate the investigation of this technology and its potential

use.

5.1.3. Aim and objectives

The aim of this work is to explore and understand the mechanisms behind the

effect of the addition of nano-particles on the thermo-physical properties of nitrate

based salt mixtures. The specific objectives are:

a) Reproduce the different specific heat enhancements produced by

nanoparticles dispersed in solar salt (NaNO3-KNO3 60-40 wt. %)

b) Explore different synthesis procedures

c) Evaluate the effect of nanoparticles on other properties such as the phase

change characteristics

d) Evaluate the stability of nanofluid in molten state

CSPcost

TES

Saltcost Active

TEScost

TES 10% CSP cost

Inactive

Active

Salt Inventory

Othercosts

Salt 50% TES cost

InactiveNanoSaltcost

Othercosts

TEScost

TES

Other

Conventional CSP Nano-HTF TES CSP

Other

-9%

TES cost reduction of 9%

CSP cost reduction of 0.9%

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186

5.2. Materials and Methods

The materials used to produce the nanofluids are: NaNO3 (Spectrum Chemical,

min 99% Crystal, Reagent, A.C.S); KNO3 (Spectrum Chemical P1345 125gr min 99%

Crystal, Reagent, A.C.S); and SiO2 (10 and 30 nm) from Meliorum Technologies

Nanomaterials, 620 Park Ave, Rochester, NY 14607 10gr).

5.2.1. Synthesis of nanofluids

The initial synthesis procedure to prepare the salt mixture and the nanofluid is the

two-step method described in Dudda & Shin (2013)105 and schematized in Figure

5-9. The salt components and the nanoparticles in the form of powder are pre-dried

for 1h at 200ºC on a hot plate to ensure the absence of moisture. The three dry

components (nanoparticles, NaNO3, and KNO3) are then weighed and mixed in a

flask in the appropriate proportion. Milli-Q water is added as a solvent. The

dilution used for the salt is 10 mL of water per 100 mg of salt mixture. The amount

of salt synthesized per batch is 200 mg. The flasks (vials) are then sonicated for 200

minutes (Branson 3510, Branson Ultrasonics Co.) and the well-dispersed mixture is

evaporated on a hot plate at 200ºC inside a laboratory hood for 5-6 hours (Figure 5-

10), until the solvent water evaporates completely, leaving a powder-form

nanofluid (Figure 5-11).

The evaporated salt from the vial is scratched and mixed. The different vials with

salt are closed, hermetically sealed with paraffin and kept in a desiccator. Before

opening the vials for testing they are placed on a hot plate at 200 ºC for 30 minutes.

Then, the vial is opened and dried for another 30 min before testing.

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Thermal Energy Storage for High Temperature Applications

Figure 5-9. Schematic representation of the synthesis method

Figure 5-10. Neat and nanofluid solar salt water solution after sonication, and flasks

during solvent evaporation

Weigh & mix

components:

nanoparticles

salt

Add distilled

water

Sonicate

~200 min

Evaporate water in vial 5-6 h @ 200ºC Preheat 1 h @

200ºC

Prepare sample in crucible

Measure in Differential

Scanning Calorimeter

(DSC)

NaNO3

KNO3

SiO2

+ Water

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188

Figure 5-11. Powder-form nanofluid (1% SiO2 10nm) after synthesis, before and

after scratching ready for testing

5.2.2. DSC Measurements

The DSC Q20 and Q2000 from TA instruments have been used for the differential

scanning calorimeter analysis. The calibration procedure is performed by using the

melting temperatures and latent heat of standard certified reference materials (In,

Zn), at a heating rate of 10 ºC·min-1 (for the cell constant calibration).

In order to test the salt samples 11-18 mg are introduced in Tzero standard

aluminum crucibles. Modulated DSC is used for the specific heat capacity

measurement. The following thermal cycle is applied to the samples: the initial

temperature is 80 ºC; the samples are heated up from 80 ºC to 420 ºC at 20 ºC/min to

pre-melt the sample as recommended in Boettinger et al.137 for powder samples.

Then the sample is cooled down to 150 ºC at the same heating rate. At this point,

the modulation starts with the temperature amplitude of 0.5 ºC every minute at a

heating rate of 5 ºC/min up to 420 ºC, recommended by the equipment

manufacturer (TA instruments). Two minutes isothermal segments are added

before each dynamic segment. Nitrogen is used as inert gas during the thermal

program (50 ml/min).

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Thermal Energy Storage for High Temperature Applications

Figure 5-12. Schematic showing onset and peak temperatures, width at half peak

height, and latent heat as determined by the DSC tests.

The melting and freezing onset temperatures are estimated as the intersection point

between the baseline connecting the points before and after the transition and the

tangent at the point of largest slope on the heat flow DSC curve (Figure 5-12). The

latent heats of phase change are determined by numerical integration of the area

under the peaks. The temperature at which heat flow during phase changes

reaches the maximum (peak temperature) is also reported as representatives of the

phase change heat flow curve.

5.2.2.1. Measurement errors and statistical analysis

The different specific heat, latent heat, and onset temperatures are presented within

a confidence interval at 90% (α=0.1). Minimum and maximum values of each

measurement are calculated according to Equation 5-3 (exemplified for Cp)

Equation 5-3

Peak T heating

Hea

t in

(h

eati

ng

)H

eat o

ut

(co

oli

ng

)

Cooling

Heating

Peak T cooling

Width at½ height

Width at½ height

Onset T heating

Onset T cooling

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190

Being tα/2,n-1 the value of the t-Student for the sample and selected confidence

interval, s the standard deviation of the data and n the number of repetitions of the

measurement.

In order to verify that the base fluid and nanofluids have different average Cp

values (latent heat, onset temperatures, etc.) a hypothesis test is performed:

Equation 5-4

The sub index 1 and 2 refer to the nanofluid and base fluid respectively, s the

standard deviation of the data and n the number of samples measured. This

statistic is compared with the value of the t-Student (tα/2, n1+n2-2) for selected α (0.1)

and the degrees of freedom equal to the total number of measurements minus 2.

The rejection of the null hypothesis (equal mean values) is done when |ttest| > tα/2,

n1+n2-2. It assumes that the samples are independent, from normal populations and

with equal variance.

5.2.2.2. Sapphire correction

The modulation cell constant (K, typically close to 1), which is the ratio between the

true-certified specific heat value of the reference material (sapphire disc) and its

measured value is used to correct the specific heat capacity measurement of the salt

sample:

Equation 5-5

Equation 5-6

The TA software allows the introduction of one single cell constant value in order

to correct subsequent measurements. For measurements performed on a

temperature spam of 50ºC this correction has been found to be accurate enough.

However, the use of a single value correction might be not appropriate for

measurement performed on a temperature range higher than 100º C. Figure 5-13

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Thermal Energy Storage for High Temperature Applications

shows an example of how correcting by a fixed cell constant over a temperature

range might affect the results.

Figure 5-13. Specific heat of sapphire: theoretical (blue), measured (red) and

corrected (green) for the temperature range 100 - 250 ºC

In Figure 5-13 the blue line is the theoretical Cp value of sapphire and the red line is

the measured raw Cp value for the sapphire reference disc. To obtain the reversing

Cp constant in the temperature range 100 - 250 ºC, the ratio between the theoretical

Cp and the measured one is calculated and averaged in this temperature range.

This single value, required by the TA software, is used to correct subsequent

measurements. If we correct the sapphire measured using this value, the green line

is obtained, which might differ in the limits of the temperature range from the real

sapphire value even though it has been corrected. In this specific example, the

maximum deviations are -3.5% (at 100 ºC) and +3.2% (at 260 ºC). On the other hand,

if the raw data is corrected by a temperature dependent MDSC constant we do not

introduce any additional uncertainty on the measured data correction.

0.6

0.7

0.8

0.9

1

1.1

1.2

100 150 200 250 300 350 400

Sp

ecif

ic H

eat

[J/(

g K

)]

Temperature [ºC]

Theoretical

RAW Measure

Corrected Measure

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192

5.2.2.3. Other measurement considerations

Along this investigation different uncertainty sources in the DSC measurements

have been identified. Based on the lessons learned measuring the specific heat of

nitrate salts some difficulties and recommendations are highlighted regarding both

equipment and sample (nature and preparation) in order to obtain reliable

measurements.

Inherent to the material (molten salts):

Salt behavior: salts creep in the crucibles. Figure 5-14 shows nitrate salt

inside standard aluminum DSC crucibles after testing. It is observed that

the salt tends to move towards the crucible borders when it is melted. This

behavior is an important source of uncertainty and can lead to DSC sensor

contamination.

Figure 5-14. Position of nitrate salt inside the aluminum crucibles after testing in the

DSC: salts creep up the crucible walls away from the base center.

Moisture control: salts are tremendously hygroscopic, thus the ambient

moisture can affect the results. Preheating samples, working in a dry

environment or even readjusting sample mass to account for excess

moisture is recommended.

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Thermal Energy Storage for High Temperature Applications

The flatness of the reference and sample crucibles is extremely important

for Cp measurements. For example, Figure 5-15 shows, for some type of

crucibles, how the solidification of KNO3 curves the bottom of the

crucibles.

Figure 5-15 Standard 40μl aluminum crucible after testing KNO3

Inherent to the equipment:

Crucible selection: certain equipment requires testing with a pin hole in the

lid of the crucible. However, hermetically sealed crucibles are preferred to

minimize ambient moisture and salt creeping effects. Absorbed moisture

during sample preparation can increase the pressure inside the crucibles

and in some cases (depending on crucible type) can lead to sudden

crucible bulging or even rupture during testing. On the other hand, the salt

creeping behavior might result in salt leaving the crucible when pin-holed

crucibles are used. Both phenomena might result on salt spilling over the

sensor affecting both the equipment and measurement results.

For Cp measurements the flatness of the reference and sapphire crucibles

is very important. The use of pin-holed crucible is recommended to

guarantee the shape of the crucible.

Crucible geometry: shape and size is limited by the equipment

manufacturer, but certain geometries seem more prone to salt creeping

and spilling.

If the Cp is to be measured over a temperature range, the sapphire

correction, as mentioned above, must be performed using a temperature

dependent reversing modulated DSC constant by selecting the default cell

constant to 1 and manually correcting the measured data for each

temperature.

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194

Sapphire uncertainty: it is recommended to run reference materials, not

only sapphire but base fluid samples to guarantee that any effect is due to

the sample and not the equipment.

5.3. Specific heat of nitrate base nanofluids

5.3.1. Results

The initial nanofluid synthesized is solar salt (SS, NaNO3-KNO3 60-40 wt. %) with

1% of SiO2 (10 nm diameter) nanoparticles. Two batches of neat solar salt (without

nanoparticles) and two batches of nanofluid are synthesized following the

procedure described above; testing 3-4 crucibles from each batch (~8 samples in

total). By following the same procedure with the neat salt we try to eliminate any

effect the synthesis process might have on the salt.

Figure 5-16. Specific heat of neat and nanofluid solar salt in solid and liquid phase.

0

2

4

6

8

10

12

190 210 230 250 270 290 310 330 350 370 390

Re

v C

p [

J/(g

K)]

Temperature [ºC]

neat_SS

nf_ss_SiO2(10nm)

1

1.2

1.4

1.6

1.8

2

190 230 270 310 350 390

Re

v C

p [

J/(g

K)]

Temperature [ºC]

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Thermal Energy Storage for High Temperature Applications

A typical Cp curve can be seen in Figure 5-16 for two samples tested. The peak

indicates the solid-liquid transition. The specific heat capacity of the salts in both

solid (to the left of the peak) and liquid phase (to the right of the peak) is roughly

constant or slightly increasing with temperature. The average specific heat capacity

results of the solar salt base nanofluids and the neat solar salt for specific

temperatures are shown in Figure 5-17.

Figure 5-17. Average specific heat of neat solar salt vs. solar salt nanofluid (10 and

30 nm SiO2 nanoparticles at a concentration of 1 wt %)

Nanofluids with different particle size were tested. The nanofluid with 10 nm SiO2

particles does not show enhancement in the temperature range (260 – 400 ºC) of

study (Figure 5-17). The average Cp values match the average base solar salt Cp,

although according to Dudda’s results105 a +13% specific heat enhancement on

average could be expected for this particular salt+nanoparticle combination. The

authors also reported that a higher enhancement can be expected (+21% on

average) using 1% of 30 nm SiO2 nanoparticles, which were tested in the following

batches.

The initial results for this nanofluid (1% of SiO2 30 nm) were promising. They

showed little enhancement. However, as the number of samples increased (28

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

1.90

2.00

250 275 300 325 350 375 400

Sp

ecif

ic H

eat

[J/(

g·K

)]

Temperature [ºC]

Neat SS (Vial)

NF SS 1% SiO2(10nm) (Vial)

NF SS 1% SiO2(30nm) (Vial)

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196

crucible tested from 6 different batches) the average Cp also fell into the confidence

interval for the base salt as can be seen in Figure 5-17. Based on the hypothesis test

(“the average Cp value is different”), we cannot reject the null hypothesis of the

equality of the two means as shown by the results presented in Table 5-3.

Table 5-3. Hypothesis test between the base solar salt and solar salt nanofluid (1 wt.

% 30 nm SiO2 nanoparticles)

Temperature (260ºC) Neat SS (Vial) NF SS 1% SiO2 (30nm) (Vial)

Avg Cp [J/ (g K)] 1.44 1.50

Std Cp [J/ (g K)] 0.11 0.21

Number of samples 8 28

Confidence Interval (90%) 1.37 – 1.51 1.43 – 1.57

This indicates that the maximum difference of +4.1% on average between neat salt

and the nanofluid (30 nm SiO2) observed at 260 ºC does not represent a statistically

significant difference between the mean value of the base fluid and the nanofluid.

Moreover, this enhancement seems to disappear at higher temperatures.

The latent heat of the different fluids is shown in Figure 5-18. The latent heat of

nanofluids is expected to decrease linearly on a per-mass basis as particles not

contributing to phase change are added to the base fluid. Since both nanofluids

contain the same mass fraction of nanoparticles (1 wt. % regardless of the

nanoparticle size) one would expect the same reduction of the latent heat for both.

An average reduction on the latent heat of nanofluid is observed for both types of

nanoparticles with no significant difference among them. There are no differences

between the melting and freezing latent heat. The confidence interval is smaller for

the 30 nm nanofluid because a larger number of samples tested.

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Thermal Energy Storage for High Temperature Applications

Figure 5-18. Freezing (left) and melting (right) average latent heat of the neat salt

and nanofluids synthesized. Measurements performed at 20 ºC/min and 5 ºC/min

respectively.

The melting and freezing onset and peak temperatures are shown in Table 5-4. No

significant difference is observed between the temperatures of solar salt and

nanofluids. The temperatures include the confidence interval (90%).

Table 5-4. Onset and peak melting and freezing temperatures

Temperatures [ºC] Neat SS (Vial) NF SS 1%SiO2

(10nm) (Vial)

NF SS 1%SiO2

(30nm) (Vial)

Onset (melting) 216.7 +/- 1.2 216.3 +/- 0.8 216.3 +/- 0.4

Peak (melting) 224.8 +/- 0.3 224.2 +/- 0.4 224.3 +/- 0.2

Onset (freezing) 232.9 +/- 1.3 232.8 +/- 1.8 233.9 +/- 1.2

Peak (freezing) 218.5 +/- 0.6 217.6 +/- 0.7 218.1 +/- 0.3

At this point we decided to explore other nanoparticle concentrations and other

base fluid compositions within the same phase diagram Figure 5-19 and Figure

5-20 show the effect of 1% of SiO2 (10 nm) nanoparticles on different base fluid

compositions. There is not significant effect of the nanoparticles regardless the

composition. The confidence interval is slightly higher for the nanofluids compared

to the neat salts. This is a consequence of a larger variability of the measurements of

the nanofluid and higher number of samples tested for the base salt.

100

105

110

115

120

125

130

Lat

ent

Hea

t (f

reez

ing

) [J

/g] Neat SS (Vial)

NF SS 1% SiO2(10nm) (Vial)

NF SS 1% SiO2(30nm) (Vial)

100

105

110

115

120

125

130

Lat

ent

Hea

t (m

elti

ng

) [J

/g] Neat SS (Vial)

NF SS 1% SiO2(10nm) (Vial)

NF SS 1% SiO2(30nm) (Vial)

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Figure 5-19. Effect of 1% of SiO2 (10 nm diameter) nanoparticles on the specific heat

(average 260 – 400ºC for mixtures and 350 – 400ºC for pure components) for

different base fluid mixture compositions.

Figure 5-20. Neat and nanofluid (1% SiO2 10 nm in diameter) specific heat heat

(average over 260 – 400ºC for mixtures and 350 – 400ºC for pure components) vs.

base fluid composition (NaNO3 wt %) in a NaNO3-KNO3 mixture.

1.20

1.30

1.40

1.50

1.60

1.70

1.80

0% 1%

Sp

ecif

ic H

eat

[J/(

g·K

)]

Concentration of SiO2 (10 nm) [wt. %]

KNO3 Na-KNO3(45-55wt%) Na-KNO3(60-40wt%) NaNO3

1.20

1.30

1.40

1.50

1.60

1.70

1.80

0% 20% 40% 60% 80% 100%

Sp

ecif

ic H

eat

[J/(

g·K

)]

NaNO3 (wt %) on a NaNO3-KNO3 mixture

Neat Nanofluid

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The specific heat of the different base salt mixtures depends on the composition. A

higher Cp for sodium rich mixtures and lower Cp for potassium rich mixtures

could be expected. The different composition seems to follow the weight average

mixing rule between the pure components. Some of the composition deviates

slightly from the expected Cp based on its composition, although inside the

experimental error. The specific heat of the different compositions tested does not

show any statistically significant differences between the neat salt and nanofluid

for any of the compositions tested. The specific heat values are shown in Table 5-5

with their confidence interval.

Table 5-5 Specific heat of neat salt and nanofluid (1 wt. % SiO2 10 nm

nanoparticles) modifying the base fluid (average over 260 – 400ºC for mixtures and

350 – 400ºC for pure components)

Specific heat [J/(g K)]

SiO2 (10 nm) concentration 0% 1%

KNO3 1.32 +/- 0.03 1.34 +/- 0.07

Na-KNO3 (30-70 wt. %) 1.42 +/- 0.03 1.43 +/- 0.05

Na-KNO3 (45-55 wt. %) 1.41 +/- 0.10 1.40 +/- 0.11

Na-KNO3 (60-40 wt. %) 1.47 +/- 0.07 1.47 +/- 0.12

NaNO3 1.60 +/- 0.05 1.58 +/- 0.11

Different SiO2 (10 nm) nanoparticle concentrations (0 - 1 - 3 - 5 -10 wt. %) have been

tested using Na-KNO3 60-40 wt. % (solar salt) and 30-70 wt. %. The results are

shown in Figure 5-21. A reduction on the Cp is observed with increasing

nanoparticle concentration. A larger dispersion in results is also seen with

increasing nanoparticle loading. None of the fluids tested show anomalous Cp

enhancement for the different nanoparticle concentrations tested.

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Figure 5-21. Specific heat capacity vs. nanoparticle concentration by mass (10 nm

diameter SiO2 nanoparticles) for two different base fluids Na-KNO3 (60-40 wt. %,

solar salt) and Na-KNO3 (30-70 wt. %) (at 400ºC)

Figure 5-22 shows a similar study using 20-60 nm SiO2 nanoparticles. The lower

number of samples in this study is translated into larger confidence intervals.

Again no Cp enhancement is observed for the different nanoparticle concentrations

tested.

1.20

1.30

1.40

1.50

1.60

1.70

1.80

0% 1% 2% 3% 4% 5% 6% 7% 8% 9% 10%

Sp

ecif

ic H

eat

[J/(

g·K

)]

Concentration of SiO2 (10 nm)

Na-KNO3 (60-40 wt.)

Na-KNO3 (30-70 wt.)

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Thermal Energy Storage for High Temperature Applications

Figure 5-22. Na-KNO3 (60-40 wt. %, solar salt) neat and nanofluid, specific heat for

different SiO2 (20 - 60 nm in diameter) nanoparticles (0 – 0.5 – 1 – 3.21 – 5.35 wt. %)

at 400 ºC.

Note that the confidence intervals of the different nanofluis are larger because a

low number of samples (3-4) for each nanofluid have been tested.

Finally, the eutectic Na-KNO3 composition (45-55 wt. %) has been modified with

different types of nanoparticles: CuO (Io-li-tec nanomaterials, 40-80 nm, 99.9%) and

Al2O3 (Aldrich Chemistry, 13 nm, 99.8%). The specific heat results are shown in

Figure 5-23. The specific heat of CuO and Al2O3 nanofluids are 1.50 +/- 0.09 J/ (g K)

and 1.45 +/- 0.05 J/ (g K) compared to 1.41 +/- 0.10 J/ (g K) of the base fluid.

1.20

1.30

1.40

1.50

1.60

1.70

1.80

0% 1% 2% 3% 4% 5% 6%

Sp

ecif

ic h

eat

[J/(

g K

)]

Concentration of SiO2 (20-60 nm)

Solar Salt

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Figure 5-23. Effect of 1 wt. % of nanoparticles (CuO and Al2O3) on the specficic heat

of the eutectic NaNO3-KNO3 (45-55 wt. %) (average over 260 – 400ºC)

A not-statistically significant enhancement of +5.9 % and +2.6% are observed for

CuO and Al2O3 nanofluid respectively.

5.3.1.1. Scanning Electron Microscopy Characterization

The characterization of several samples has been performed using Scanning

Electron Microscopy (SEM) in order to analyzed structural changes in the

nanofluids produced by the addition of nanoparticles to the base salt. In SEM

measurements, a focused electron beam is scanned over the surface of the sample.

When the electrons strike it, different interactions can occur: emission of

backscattered electrons (BE), auger electrons (electron bombarded 2-50 keV),

secondary electrons (SE) and X-rays. All these signals can be detected revealing

morphology, chemical composition or microstructure information of the sample.

1.20

1.30

1.40

1.50

1.60

1.70

1.80

0% 1% CuO

(40-80 nm)

1% Al2O3

(13 nm)

Sp

ecif

ic H

eat

[J/(

g K

)]

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A Hitachi S5200 SEM equipped with a field emission gun (FEG), located at

“Instituto de Ciencia de Materiales de Sevilla” (ICMS) has been employed in this

study to analyze the microstructure of base salt and nanofluid samples (2 kV and 7

mm of working distance). The microscope is dotted with an energy dispersive X-

ray (EDX) spectroscopy Bruker X Flash Detector 4010 which allows quantifying the

composition. The EDX analysis has been performed at 10kV.

The microstructure of the neat salt NaNO3-KNO3 (30-70 wt. %) and nanofluids with

1 wt. % and 5 wt. % of SiO2 nanoparticles (10 nm in diameter) are shown in Figure

5-24 to Figure 5-28. As Kramer et al. indicated, a tendency towards clustering of

similar ions in the solid solutions can be observed. The flat solidus boundaries

between solid and liquid solutions (a horizontal solidus, observed in the phase

change diagram Figure 5-40) may indicate a eutectic with limited solid solution.138

Figure 5-24. SEM (left) and SEM-EDS analysis (right) of the neat NaNO3-KNO3 (30-

70 wt. %)

This composition shows well defined sodium and potassium crystals. The structure

of the nanofluid (1 wt. % of SiO2, Figure 5-25) is similar to the base salt, with

identified well-dispersed silica nanoparticles.

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Figure 5-25. SEM (left) and SEM-EDS analysis (right) of the nanofluid 1% SiO2 (10

nm) NaNO3-KNO3 (30-70 wt. %)

Figure 5-26. SEM images of the nanofluid 1% SiO2 (10 nm) NaNO3-KNO3 (30-70 wt.

%)

The addition of nanoparticles to the base salt does not modify its appearance. The

silica nanoparticles seem to be well dispersed and do not show clusters (Figure

5-26). However, Figure 5-27 and Figure 5-28 show that significant clusters are

formed for the same base salt composition with 5 wt. % of SiO2 nanoparticles.

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Thermal Energy Storage for High Temperature Applications

Figure 5-27. SEM (left) and SEM-EDS analysis (right) of the nanofluid NaNO3-

KNO3 (30-70 wt. %) with 5% SiO2 (10 nm)

Figure 5-28. SEM images of the nanofluid NaNO3-KNO3 (30-70 wt. %) with 5% SiO2

(10 nm)

From the SEM analysis, it is observed that while the NaNO3-KNO3 (30-70 wt. %)

nanomaterial with 5 wt. % of SiO2 contains significant amounts of agglomerated

nanoparticles (Figure 5-28), the nanofluid with 1 wt. % of SiO2 shows

homogeneously dispersed nanoparticles.

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The eutectic NaNO3-KNO3 (45-55 wt. %) nanofluid with 1wt. % of CuO

nanoparticles is shown in Figure 5-29. The sample, after being tested in the DSC

shows homogeneous nanoparticles dispersion, similar to the silica nanofluid in

Figure 5-25. Higher proportion of sodium crystals is also observed due to its higher

content in this specific base salt.

Figure 5-29. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic

NaNO3-KNO3 (45-55 wt. %) with 1% CuO

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5.3.2. Discussion

In the classical limit, where size effects and surface effects are neglected and the

nanoparticles and the surrounding fluid are in thermodynamic equilibrium, the

specific heat of a mixture would be a weighted average of the two materials. The

Cp of a nanofluid on a mass basis, in the classical limit, is thus given by Equation

5-7.

Equation 5-7

The specific heat of different common nanoparticles and the specific heat values of

the base salt used in this study are shown in Table 5-6 and Table 5-7 respectively.

Table 5-6. Properties of common nanoparticle materials (bulk properties).90

Material Specific Heat Capacity

[J/(g K)]

Density

[kg/m3]

Al2O3 0.88 3950

SiO2 0.68 - 0.73 2650

CuO 0.53 6310

If the bulk material properties are used, adding any of the materials listed in Table

5-6 (Cp < 1J/gK) to molten salts as a base fluid ( Cpliquid 1.5 J/gK) will decrease the

Cp of the resulting nanofluid according to Equation 5-7. If small particle loadings

are considered, the effective heat capacity should not change and remain close to its

base fluid value.

However, the specific heat of the nanoparticles themselves can change with respect

to their bulk material values. Several authors have attempted to measure the

nanoscale effect on solid Cp. An increase of about 25% with respect to its bulk

value seems like an upper limit of this potential enhancement.90 Small particles

have a large surface area, providing a higher number of surface atoms and

vibrational modes available. If we consider the specific heat of the nanoparticle

with the lowest specific heat and a possible increase due to nanoscale effects of up

to 25% (CuO Cp = 0.53 J/(g K)*1.25) and the specific heat of solar salt (1.47 J/(g K)),

the expected nanofluid Cp using Equation 5-7 would be 1.462, 1.454 and 1.446 J/(g

K) for 1, 2 and 3 wt. % of nanoparticles. The calculated nanofluid Cp compared to

the confidence intervals for the mean value of the base salt suggest that we would

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208

not be able to measure significant differences between the base salt and nanofluids

because the predicted change in Cp is lower than the uncertainty in the

measurements.

Six similar studies also used nitrates based nanofluid in their research, specifically

solar salt, and the two step nanofluid synthesis procedure. Their measured specific

heat of solar salt is shown in Table 5-7.

Table 5-7. Solar salt specific heat measurement in molten state by different authors.

(two step nanofluid synthesis procedure)

Solar Salt

Cp [J/(g K)] Enhancement [%]

Latent

Heat [J/g]

Dudda & Shin (2012)104 1.38 +/- 0.01 +19% (5 nm SiO2)

+25% (30 nm SiO2) -

Dudda & Shin (2013)105 1.47 +/- 0.02 +13% (10 nm SiO2)

+21% (30 nm SiO2) -

Chieruzzi et al. (2013)110 1.65 +0.8% (7 nm SiO2)

+22.4% (SiO2-Al2O3) 110

Lu & Huang (2013)111 1.59 +/- 0.03 -3% (13 nm Al2O3

at 2 wt.%) -

Andreu-Cabedo et al. (2014)113 1.48 +/- 0.09 +25% (12 nm SiO2) -

Schuller et al. (2015)117

1.47 +/- 0.04

(1.59 two

months later)

+30.6% (40 nm Al2O3

at 0.78 wt.%) -

Present work 1.47 +/- 0.07 0% (10 nm SiO2)

+4.5% (30 nm SiO2) 112 +/- 3

The discrepancies in the base salt Cp (Table 5-7) is a well known problem. This

problem has led to specific activities of the SolarPACES Thermal Energy Storage

Group (an international network of researchers and industry experts for the

development of concentrating solar thermal power systems and solar chemistry

technologies). The proposed activity was a Round Robin Test on the measurement

of specific heat capacity of solar salt in which we have participated. The results

have been published in the SolarPACES conference 2016, showing relative error

values among partners between 5 and 10% for Cp between 200 and 400ºC.

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The base salt measurement of the present work matches 3 of the 5 references. The

other two references show ~10% higher Cp of the base fluid. It is worth mentioning

that the solar salt measurement reported in Schuller et al. (2015)117 shows an

inexplicable enhancement of 8% after repeating the measurements 2 months later,

with the samples kept at room temperature under inert atmosphere. These

deviations shown between authors and even within the same investigation indicate

how difficult is to get reliable specific heat capacity measurements when working

with nitrates molten salts.

The other important observation is that only one investigation reports values of the

phase change properties of the salts. In every Cp measurements the salt is melted

and the data is usually recorded by the DSC. Therefore, the phase change

measurement could help in identifying outliers in the data or to develop a global

explanation of any anomalous specific heat measurement.

Our specific heat results using vial evaporated nanofluids (Figure 5-21) are aligned

with Lu & Huang (2013)111 results, where no enhancement was observed but they

differ from Dudda & Shin’s results (Table 5-7).

Chieruzzi’s results110 with for SiO2 (7 nm) nanofluid did not show enhancement

either. In fact, although the authors highlight in the abstract and conclusions the

huge Cp enhancement (+22.5%) observed in one type of nanofluid and one

nanoparticle concentration, their results show more Cp reductions than

enhancements (Table 5-8). In fact, the important reduction on Cp observed at 0.5

wt. % for different nanoparticle types (-19%, -7.6%, -15.7%) seems more remarkable

than the enhancements. Due to the absence of error bars in this study the precision

of their measurements cannot be evaluated in other to clarify if the differences in

Cp are statistically significant.

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Table 5-8. Chieruzzi et al. (2013)110 results for solar salt modified with different

nanoparticle types and concentrations.

Cp Enhancement [%]

Nanoparticle

concentration [wt. %]

SiO2

(7nm)

Al2O3

(13nm)

TiO2

(20nm) SiO2+Al2O3

0.50% -19.36% -7.65% -15.66% -7.46%

1.00% 0.79% 5.89% -6.31% 22.45%

1.50% -1.46% -3.52% -11.77% 1.52%

In the case of Lu & Huang (2013)111 their results might not be directly comparable

with this study because the nanoparticle concentration and nanoparticle type were

different (2 wt. % of alumina instead of silica). However, their results show a

monotonous decreasing tendency on the nanofluid specific heat with the

nanoparticle concentration (with lower Cp than the base salt) increases from 2 wt.

% to higher concentrations, which is similar to our results with silica nanoparticles

(Figure 5-21 and Figure 5-22).

Our latent heat results can be also compared with Chieruzzi (2013)110. Their results

shows an anomalous latent heat enhancement of +14.9% for solar salt nanofluid

with 1 wt. % of SiO2 (7nm) nanoparticles. However, a small reduction on the latent

heat has been measured in this research: -1.5 % and -0.4 % on average for 10 nm

and 30 nm silica nanofluid respectively at 1 wt. %, which is inside the experimental

error.

Summarizing, the synthesized nanofluids in this study do not show anomalous

behavior neither on the specific heat nor the latent heat. The specific heat results are

aligned with Chieruzzi et al. (2013)110 and against Dudda & Shin’s results104,105

regarding solar salt nanofluid modified with silica nanoparticles. The effect of

several parameters were evaluated by changing: a) the base salt composition, b) the

nanoparticle loading (weight fraction), c) the nanoparticle type (SiO2, Al2O3, CuO),

and d) the nanoparticle size (10 and 30 nm). In all these cases, except for large

particle loadings above 5% by wt. the specific heat capacity was modified within

the confidence intervals, showing no statistical difference in average with respect to

the neat salt.

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If we analyze carefully the synthesis procedure to prepare the nanofluids followed

in the different studies (Table 5-9), even though all of them use a two step

procedure, some differences can be identified among authors. These differences are

mainly related to the sonication time and the evaporation temperature.

Table 5-9. Differences in the solar salt nanofluid synthesis procedure among

researchers. Enhancement at 1wt. % of nanoparticles concentration if not specified.

Synthesis Procedure Enhancement

[%] Sonication

time

Evaporation

temperature

Evaporation

time

Dudda and Shin

(2012)104 200 min 200ºC 7 h

+19% (5 nm SiO2)

+25% (30 nm SiO2)

Dudda and Shin

(2013)105 200 min 200ºC 7 h

+13% (10 nm SiO2)

+21% (30 nm SiO2)

Chieruzzi et al.

(2013)110 100 min 200ºC 2 h

+0.8% (7 nm SiO2)

+22.4% (SiO2-Al2O3)

Lu and Huang

(2013)105 100 min 90ºC 12 h

-3% (13 nm Al2O3

at 2 wt.%)

Andreu-Cabedo et al.

(2014)113 5 min 100ºC 1 h +25% (12 nm SiO2)

Schuller et al.

(2015)117 120 min 90ºC 45 min

+30.6% (40 nm

Al2O3

at 0.78 wt.%)

Present work 200 min 200ºC 7 h 0%

If we compare the evaporation temperature and time, we identify two main

groups. On the one hand the groups with long evaporation time (most of them

using an evaporation temperature of 200ºC). On the other hand the groups with

shorts evaporation times and 100ºC of evaporation temperature. One of the key

factors to explain how reducing the hot plate temperature can lead to a shorter

evaporation time is the exposed surface area (Figure 5-30).

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Figure 5-30. Vial (left) vs. larger surface area evaporation receptacles such as glass

petri-dish (center) and steel pan (right, from Schuller et al. (2012)139). Larger surface

area leads to a shorter evaporation times.

The other two references (Andreu-Cabedo et al. (2014)113 and Schuller et al. (2015) 117) evaporate in a shorter time, even though the hot plate temperature was set to

100ºC instead of 200ºC. This might be possible by using a larger surface area

evaporation method. This evaporation method will be used in the following section

(5.3.3 Results new synthesis method).

5.3.3. Results new synthesis method

The synthesis procedure is modified after observing that, unlike Dudda’s

results104,105, the nanofluids synthesized using vial did not show Cp enhancement.

Shin et al. (2014)101 reported higher Cp enhancement when evaporating the

nanofluid-water solution using a glass petri-dish on a hot plate (100ºC) instead of a

vial, although they only tested the binary carbonate Li2CO3-K2CO3. The authors

suggest that the larger surface area of the petri-dish leads to a faster evaporation.

The nanoparticles are thought to be less agglomerated, which is hypothesized to

yield a higher Cp enhancement. The change in the synthesis procedure (lower

evaporation temperatures) is also motivated by the data in Table 5-9 from Andreu-

Cabedo et al. (2014)113 and Schuller et al. (2015)117.

In Jo & Banerjee (2015b) and Shin & Banerjee (2013)99,127 the authors differentiated

between 2 areas on the petri-dish after the evaporation: fine powder (type-A,

believed to have a well-dispered nanofluid) and coarse powder (type-B, suggested

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Thermal Energy Storage for High Temperature Applications

to have agglomerated nanoparticles) and selectively tested each area. We proceed

in the same way (Figure 5-31).

Figure 5-31. Different areas tested on a petri-dish. Solar salt nanofluid with 1wt. %

of SiO2 (10 nm)

The results of the different nanofluid types tested are shown in Figure 5-32. The

solar salt nanofluid synthesized with the new method shows different degrees of

enhancement depending on the selected nanofluid type. Type A nanofluid shows

an average +8.4% specific heat enhancement, while Type B nanofluids shows an

average -4.2% decrease.

Type A

Type B

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Figure 5-32. Solar salt nanofluid (1 wt. %, 10 nm SiO2 nanoparticles) specific heat

results vs. Temperature synthesized using petri-dish (PD) evaporation. Neat solar

salt is mixed and evaporated in a vial.

Table 5-10. New synthesis method nanofluids - Cp results

Specific heat Cp [J/gK]

Temperature

[ºC]

Neat SS

(Vial)

Type A NF SS

1% SiO2 (10nm) (PD)

Type B NF SS

1% SiO2 (10nm) (PD)

260 1.44 +/- 0.07 1.54 +/- 0.11 1.41 +/- 0.07

300 1.46 +/- 0.07 1.57 +/- 0.11 1.41 +/- 0.06

350 1.48 +/- 0.07 1.61 +/- 0.12 1.41 +/- 0.06

400 1.50 +/- 0.07 1.64 +/- 0.13 1.41 +/- 0.07

The same statistical significance test is performed to the difference of mean Cp

values for the neat fluid and the new synthesized nanofluids. The results indicate

that the +8.4% average enhancement (+9.6% at 400ºC) on the specific heat of the

0.50

0.75

1.00

1.25

1.50

1.75

2.00

250 275 300 325 350 375 400

Sp

ecif

ic H

eat

[J/(

g·K

)]

Temperature [ºC]

Neat SS (Vial)

Type A NF SS 1% SiO2 (10nm) (PD)

Type B NF SS 1% SiO2 (10nm) (PD)

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Type A nanofluid is not statistically significant ( ),

with n1 and n2 8 and 12 samples respectively (Table 5-11). This means that we

cannot reject the null hypothesis of equality of means between Type A nanofluid

and the base salt. In the case of Type B nanofluid the differences on the Cp mean

value are not statistically significant either.

Table 5-11. Type A nanofluid SS Vial 1%SiO2 (10 nm) vs neat solar salt

Temperature (400ºC) Neat SS Vial Type A NF SS Vial

1%SiO2 (10 nm)

Avg Cp [J/ (g K)] 1.50 1.64

Std Cp [J/ (g K)] 0.10 0.25

Number of samples 8.0 12.0

Confidence Interval (90%) 1.43 – 1.57 1.51 – 1.77

However, the specific heat difference between Type A and B is significant (

) (Table 5-12).

Table 5-12. Type A vs Type B nanofluid SS Vial 1%SiO2 (10 nm)

Temperature (400ºC) Type A NF SS Vial

1%SiO2 (10 nm)

Type B NF SS Vial

1%SiO2 (10 nm)

Avg Cp [J/ (g K)] 1.64 1.41

Std Cp [J/ (g K)] 0.25 0.11

Number of samples 12.0 9.0

Confidence Interval (90%) 1.51 – 1.77 1.33 – 1.48

5.3.4. Discussion new synthesis method

Based on the results shown in Figure 5-32 two different hypotheses are proposed

regarding the specific heat enhancement:

a) A composition shift during the synthesis process: the water solution and

evaporation step is modifying the composition of the base fluid by zones

(type A and B) which can affect the Cp based on Figure 5-20.

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216

b) Adsorption at the nanoparticle interface: a reversible adsorption type

interaction on the nanoparticle surface, acting as an extended solid-liquid

phase transition, exceeding the melting point of the base fluid.

As the samples are selectively tested, if the process is varying the base fluid

composition (hypothesis (a)), different areas with higher sodium and higher

potassium concentration might appear, with higher and lower Cp respectively

based on Figure 5-20. An artificial specific heat enhancement and reduction could

be produced in each nanofluid type. A change in composition should also affect the

measured latent heat.

We have discarded all the mechanisms involving “needle-like structures”, “nano-

structures” and “fractal-like nano-structures” proposed by Banerjee and Shin

(section 5.1.1.1Theories behind Cp enhancement) based on our SEM images, where

none of these structures have been observed. Most of these structures are observed

in carbonate salts instead of nitrate salts. Besides this, the SEM images shown in

Dudda & Shin (2013)105 relative to solar salt show some agglomerates, very similar

to Figure 5-29, and none of the other authors reporting specific heat enhancement,

such as Andre-Cabedo et al. and Chieruzzi et al. (2013 and 2015)110,113,116 show any

type of nano-structures.

On the other hand, mechanism (b), postulated by Thoms133, has not yet been

confirmed experimentally. It is hypothesized that in both desorption behaviors

(Figure 5-5 I&II) a reduction on the latent heat can be expected.

These two possible explanations should be combined with the expected effect of

nanoparticles added to the molten salts (Table 5-13). As mentioned above, adding

nanoparticles with a lower specific heat capacity than the base fluid should in

theory reduce the nanofluid heat capacity. A reduction on the latent heat should

also be expected because particles do not undergo any phase change. Regarding

the phase change temperatures, a reduction on the onset temperatures could be

anticipated because nanoparticles might act as impurities slightly reducing the

melting onset temperatures as well as they might act as a nucleation sites

increasing the freezing onset temperature. The specific heat and latent heat

reduction should fall within the experimental error because of the low nanoparticle

concentration tested (1 wt. %).

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Table 5-13. Possible explanation regarding the specific heat enhancement

Adding

nanoparticles

with lower

Cp

Composition

Shift (a) Adsorbed

layer (b) Na-rich NF |K-rich NF

Cp ↓ (less than 1%) ↑ ↓ ↑ or ↓

LH ↓ (~1%) ↑ ↓ ↓ and ↓

Onset T Melting ↓ ~↑ ~↓ ~

Onset T Freezing ↑ ↑ ↓ ~

If we analyze the latent heat of this nanofluid (Figure 5-33, Table 5-14) we observe a

little increase in Type A (with higher average Cp) nanofluid while a reduction on

Type B nanofluid (with lower average Cp).

Figure 5-33. Latent heat of Type A and B nanofluid vs. Neat solar salt

100

105

110

115

120

125

130

Lat

ent

Hea

t (m

elti

ng

) [J

/g]

Neat SS (Vial)

Type A NF SS 1% SiO2(10nm) (PD)

Type B NF SS 1% SiO2(10nm) (PD)

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Table 5-14. New synthesis method nanofluids – Solid-liquid phase change results

Neat SS

(Vial)

Type A NF SS

1% SiO2 (10nm) (PD)

Type B NF SS

1% SiO2 (10nm) (PD)

Mel

tin

g

Integral

(J/g) 111.9 +/- 3.5 112.6 +/- 2.0 108.5 +/- 1.2

Tonset

(ºC) 216.7 +/- 1.2 215.7 +/- 1.1 215.4 +/- 0.8

Tpeak

(ºC) 224.8 +/- 0.3 223.9 +/- 0.5 224.0 +/- 0.5

Fre

ezin

g

Integral

(J/g) 112.7 +/- 2.6 113.3 +/- 2.2 109.5 +/- 0.7

Tonset

(ºC) 232.9 +/- 1.3 235.1 +/- 1.1 232.5 +/- 1.3

Tpeak

(ºC) 218.5 +/- 0.6 220.9 +/- 3.5 222.8 +/-6.5

During the phase change analysis between neat solar salt and nanofluids Type A

and B a single property showed a statistically significant difference: Type A

nanofluid shows a slightly higher onset freezing temperature Table 5-15 than the

neat solar salt and Type B nanofluid (Figure 5-34). On the other hand, Type B

nanofluid shows no statistically significant lower freezing onset temperature with

respect to the base salt.

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Thermal Energy Storage for High Temperature Applications

Figure 5-34. Freezing onset temperature difference between Type A and B

nanofluid.

Table 5-15. Hypothesis testing for the onset freezing temperatures for neat solar

salt vs. type A and type B nanofluid

Hypothesis testing

Type A vs. Neat SS

(statistically significant difference)

The combination of these three results (slightly higher latent heat and higher

freezing onset temperature and slightly higher specific heat) suggests that Type A

nanofluid could contain a larger amount of NaNO3 than the base fluid. These

results suggest that the synthesis method is producing different composition salts

and that these differences are more obvious when the solvent evaporation takes

place quicker in a large surface area container. Based on these results, one can

hypothesize that the different components of the mixture are not precipitating

homogenously.

Finally, it is important to mention that based on these results it seems as though

selectively choosing samples from a Petri-dish can lead to erroneous and random

0

0.5

1

1.5

2

2.5

3

180 200 220 240 260 280 300 320

Hea

t F

low

[W

/g]

Temperature [ºC]

Type A

Type B

freezing

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220

results. Also, since the evaporation method affects the results, both base fluids and

nanofluids should be synthesized following the same procedure for a systematic

comparison. The same thought was shared by Lasfargues (2014)140 where the

author “removed the salt crystals from the petri-dish and both coarse and fine

grains (nanofluid types) were mixed together and tested”.

In contrast, selectively choosing samples (Banerjee and Shin’s synthesis and testing

procedure) seems to be accepted by the scientific community. In their publications

slight modifications on the synthesis protocol are usually introduced to synthesize

the nanofluid. However, the synthesis protocol of the base fluid used for

comparison never changes. For instance, Jo & Banerjee (2010)120 evaluate the effect

of the hot plate temperature on the evaporation time and the effect on the Cp on

the nanofluids; only one base fluid is taken as a reference synthesized with a single

hot plate temperature. Another example appears in Shin & Banerjee (2010)96 where

the effect of using a large Petri dish (high surface are to evaporate) in the synthesis

protocol is analyzed. They discovered phase segregation claiming that it has been

promoted by the dispersed nanoparticles. However, the binary carbonate salt used

as a reference is synthesized using a vial (longer evaporation time). In both

examples the authors are comparing fluids synthesized with a different protocol,

which in our opinion leads to erroneous results.

In an effort to improve the methods for a fairer comparison, a petri-dish

evaporation was used to synthesize the base salt and a new nanofluid (1% SiO2

10nm). The salt crystals from the complete petri-dish are removed and mixed

together before testing, as in Lasfargues (2014)140 to avoid bias from selectively

choosing the samples by evaporation areas. The results are shown in Figure 5-35.

On average the specific heat of the solar salt nanofluid with 1 wt. % of SiO2

nanoparticles is between 1.9 and 4.4% higher than the neat solar salt, both

synthesized with petri-dish and mixing completely before testing, although this

enhancement is within the confidence intervals.

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Thermal Energy Storage for High Temperature Applications

Figure 5-35. Specific heat and latent heat results of the base salt and nanofluid (1%

of SiO2 10nm) synthesized using petri-dish and mixing completely the entire batch

of salt before testing instead of selectively choosing samples.

Table 5-16. Onset melting and freezing temperatures values corresponding to the

tests shown in Figure 5-35

Temperatures [ºC] Neat SS (Vial) Neat SS

(PD All mixed)

NF SS 1%SiO2 (10nm)

(PD All mixed)

Onset (melting) 216.7 +/- 1.2 216.4 +/- 1.1 215.2 +/- 0.9

Onset (freezing) 232.9 +/- 1.3 233.9 +/- 1.6 233.5 +/- 1.0

The results are compared with the initial solar salt. The neat salt (with no particles)

using a different evaporation process in a larger surface area container (petri-dish

instead of vial) shows on average a higher specific heat capacity than the same salt

evaporated in a small vial. However, no statistically significant difference between

these two neat fluids and the nanofluid is observed. An expected slight reduction

on the latent heat is observed for the nanofluid (Table 5-17).

1.00

1.25

1.50

1.75

2.00

250 275 300 325 350 375 400

Sp

ecif

ic H

eat

[J/(

g·K

)]

Temperature [ºC]

Neat SS (Vial)

Neat SS (Petri Dish AllMixed)

NF SS 1% SiO2 (10 nm) (PD AllMixed)

100

105

110

115

120

125

130

Lat

ent

Hea

t (m

elti

ng

) [J

/g]

Neat SS (Vial)

Neat SS (Petri Dish AllMixed)

NF SS 1% SiO2 (10 nm) (PD AllMixed)

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Table 5-17.Maximum and minimum Cp neat solar salt (PD) sample compared to

the average values of the neat SS (vial)

Neat SS (Vial) Neat SS PD

Max Cp sample

Neat SS PD

Min Cp sample

Specific Heat (260ºC) 1.44 +/- 0.07 1.591 1.324

Onset (melting) 216.7 +/- 1.2 215.0 216.9

Latent Heat(melting) 111.9 +/-3.5 116.6 108.8

Onset (freezing) 232.9 +/- 1.3 236.2 232.7

Latent Heat(freezing) 112.7 +/- 2.6 115 112

Figure 5-36. Specific heat and latent heat results of the neat solar salt (vial) and the

neat solar salt synthesized with petri dish selecting the highest and lowest Cp

value.

Two samples with the highest and lowest Cp from a petri dish selectively chosen

are compared with the average values of the neat SS synthesized with petri and

mixed completely before testing (Table 5-17, Figure 5-36). We can observed the

large variation of the specific heat of the selectively chosen samples, which

coincides with a higher and lower latent heat and freezing onset temperature for

the higher and lower specific heat sample respectively. These results suggest that

0.50

0.75

1.00

1.25

1.50

1.75

2.00

250 275 300 325 350 375 400

Sp

ecif

ic H

eat

[J/(

g·K

)]

Temperature [ºC]

Neat SS (Vial)

Max Cp neat SS (PD)

Min Cp neat SS (PD)

100

105

110

115

120

125

130

Lat

ent

Hea

t (m

elti

ng

) [J

/g]

Neat SS (Vial)

Max Cp neat SS (PD)

Min Cp neat SS (PD)

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Thermal Energy Storage for High Temperature Applications

composition shifts can be produced along the different petri dish areas which affect

the specific heat as well as the phase change properties of each sample. The heat

flow curves of these two samples are shown in Figure 5-37.

Figure 5-37. Heat flow curves vs. temperature for the sample with the maximum

and minimum Cp synthesized with Petri dish for a representative test from the

data shown in Figure 5-36.

Summarizing, we have proceeded according to Banerjee & Shin’s procedure99,127 in

both the synthesis procedure and the testing procedure by evaporating the

dissolved nanofluids in a larger surface area container (petri-dish) and selectively

choosing different regions to test. The solar salt nanofluid has shown an

enhancement of +8.4% on average, which is not statistically significant when

compared to the neat salt synthesized and evaporated in a vial. This fluid shows

also a slightly higher latent heat and higher freezing onset temperature. These three

-1

-0.5

0

0.5

1

1.5

2

2.5

3

3.5

190 210 230 250 270 290 310

Hea

t F

low

[W

/g]

Temperature [ºC]

Min Cp neat SS (PD)

Max Cp neat SS (PD)

freezing

melting

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224

observations, combined with the NaNO3-KNO3 phase change diagram (Figure

5-40), suggest that Type A nanofluid might contain a higher NaNO3 proportion. On

the other hand, Type B nanofluid shows a small reduction on the specific heat and

latent heat that could suggest a higher KNO3 content.

We have also observed that the neat salt samples without nanoparticles selected

from different areas in the Petri dish can show higher specific heat depending on its

composition; these samples show slightly higher latent as well. When the neat salt

is synthesized with petri dish and mixed together before testing, a not-statiscally-

significant small enhancement is also observed.

We can compare our average Cp enhancement (Type A nanofluid, best-case

scenario) with Andreu-Cabedo et al. (2014)113 results. The same nanoparticle type

(SiO2) is used in both studies with a very similar nanoparticle size (12 nm vs. 10

nm). The nanopartcile concentration (1 wt. %) is also the same. Table 5-18 compares

the results.

Table 5-18. Specific heat results: present work vs. Andreu-Cabedo et al. (2014). Neat

solar salt vs. solar salt nanofluid (1 wt. % of SiO2 nanoparticles). *Type A

nanofluid.

Average Cp value (250-420ºC) Base salt

[J/ (g K)]

Nanofluid

[J/ (g K)]

Cp

Enhancement

[%]

Andreu-Cabedo et al. (2014)113 1.48 +/- 0.09 1.85 +/- 0.06 +25

Present work 1.47 +/- 0.07 1.59 +/- 0.12* +8.4

As in the previous section, our results do not show a statistically significant

enhancement on the specific heat for solar salt nanofluid with 1 wt. % of SiO2

nanoparticles, which disagrees with Andreu-Cabedo et al. (2014)113 showing an

average +25% enhancement on the same nanofluid.

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Thermal Energy Storage for High Temperature Applications

5.3.4.1. Solubility as a key factor

Our experimental results suggest that the synthesis process seems to separate the

salt components. This separation is more evident when using larger surface area

evaporation receptacles. This separation might be related to the solubility of each

salt component in water. This parameter has never been mentioned in any of the 33

reference using these synthesis methods (solution-sonication-evaporation).

The solubility is the quantity of solute (nitrate salts in this case) that dissolves in a

given amount of solvent (water in this case). It depends on the nature of the solute

and solvent, the amount of solute, the temperature and pressure of the solvent and

it is often expressed as the quantity of solute per 100 g of solvent at a specific

temperature.

Table 5-19. Solubility of nitrate salts in water.141 Salt Solubility in water [g/100g of water] 20g of water (100ºC)

NaNO3 149 (80°C) 176 (100°C) 35.2 g

KNO3 168.8 (80°) 243.6 (100°) 48.7 g

As can be seen in Table 5-19, the 20 ml of water used in the solution step is large

enough to dissolve 200 mg of salt. However, the amount of water is eliminated

progressively during the evaporation process. As the water is continuously

evaporated there is a point where there is not enough water to keep the salt

dissolved starting its precipitation on the receptacle. The precipitation of the salt is

not expected to be uniform, but it will start with the less soluble component, in this

case NaNO3.

This effect could explain why the different areas in Figure 5-31 show slightly

different properties. The fine powder areas might precipitate first, with higher

content of the less soluble component at evaporation temperature (80 ºC – 100 ºC)

which is NaNO3. At the end of the evaporation process the remaining salt (rich in

KNO3) precipitates and forms the so called “Type B” nanofluid which might

contain a larger proportion of KNO3, as the slightly lower specific heat, latent heat

and freezing onset temperature suggest.

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226

This is an important observation for nitrate base nanofluid, but it could be even

more important for carbonate base nanofluid because, while the solubility ratio

between NaNO3 and KNO3 is 1.38, in the case of carbonates the solubility ratio

between K2CO3 and Li2CO3 is 216.4. This means that lithium carbonate is 2 orders

of magnitude less soluble than K2CO3. Consequently, lithium salt will start

precipitating sooner forming reach lithium carbonate areas with very different

properties than the remaining salt, which might lead to larger Cp enhancements as

observed in Figure 5-3 for Banerjee and Shin Group’s results 99,121,127

Even though the effect described previously regarding composition shift during the

synthesis might explain the results observed during this investigation, it cannot

justify +20% Cp enhancements on average shown in the literature (which were

however not achieved in this research) or values above the Cp of pure NaNO3 (the

pure component with the highest Cp)

5.3.5. Conclusions

In this study the effect of SiO2 nanoparticles on the specific heat of solar salt has

been investigated. Through different experiments we have tried to replicate

different published studies with unsuccessful results:

Varying nanoparticle size on solar salt

Varying nanoparticle concentration on solar salt

Varying base fluid composition with silica nanoparticles

Varying nanoparticle type with the eutectic composition as base fluid

We have found no statistically significant evidence that the specific heat of molten

salt can increase by the addition of nanoparticles. The variation of the synthesis

process has lead to larger Cp enhancements (no statistically significant either).

However, the results suggest that the composition of the salt is not homogeneous

when the samples are selected based on different visual aspect of the salt in the

petri dish. The two-step water solution and evaporation synthesis is affecting the

salt composition of the nanofluids, explaining to a certain extent positive Cp

enhancement reports. This composition shift can be explained through the

difference in water solubility of each salt component which leads to a segregation

during the evaporation step.

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Thermal Energy Storage for High Temperature Applications

DSC measurements with molten salts should be performed with care, as

corrections such as those with reference materials are temperature specific and

should not be extended over large temperature ranges. Also, molten salts have

been shown to creep and escape certain (not well-sealed or pin-holed) crucibles,

contaminating the equipment sensors throughout testing. More frequent testing to

account for drifts in measurements and baseline references should be performed

when measuring molten salts.

5.4. Latent heat of nitrate base nanofluids

Based on the specific heat results we decided to explore the phase change

properties of the salt. In recent years there has been a dramatic increase in research

analyzing the effect of nanoparticle addition to heat transfer fluids and storage

materials. Nonetheless, there are not that many experimental studies focused on

the phase-change properties of nanofluids as a latent heat storage material.

Therefore, we decide to extend the investigation analyzing the effect of adding

different SiO2 nanoparticle concentrations on the phase change characteristics of

different NaNO3-KNO3 mixtures along its phase change diagram with the aim of

understanding and quantifying these effects.

The latent heat of fusion of nanofluids is expected to decrease linearly as particles

not contributing to phase change are added to the base fluid.142,143 Some of the

research on nanoparticle enhanced phase change materials (PCM) uses materials

that melt at relatively low temperatures (e.g. paraffin with a melting point around

60ºC) and therefore are not suited for CSP applications. Figure 5-38 summarizes the

effect on the latent heat of the addition of nanoparticles to low melting temperature

organic PCM.

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228

Figure 5-38. Reported effect of nanoparticles on the latent heat of fusion of organic

PCM

Although there is a spread of different results, the general trend indicates that

adding nanoparticles reduces the latent heat of the composite material. However,

the experimental reduction in latent heat generally exceeds the theoretical

predictions. For instance, the magnitude of the effect of Al2O3 on the phase change

properties of paraffin is not clear. Ho et al. (2009)144 shows a reduction on the latent

heat following the predicted curve while Teng et al. (2012)145 shows a larger

reduction on the same material and nanoparticle type.

On the other hand, high temperature molten salt composites, more appropriate for

CSP applications, have been less studied. Figure 5-39 summarizes the effect on the

latent heat of the addition of nanoparticles to high temperature inorganic PCM.

50%

60%

70%

80%

90%

100%

110%

120%

0% 2% 4% 6% 8% 10%

No

rmal

ized

Lat

ent

Hea

t [%

]

Nanoparticle concentration [wt.%]

Theory

Ho et al. Paraffin-Al2O3

Wu et al. Parraffin-CuO

Teng et al. Paraffin-Al2O3 or TiO2

Teng et al. Paraffin-SiO2 or ZnO

Yang et al. Paraffin-Si3N4

Harikrishnan et al. Oleic Acid-CuO

Yavari et al. 1-Octadecanol-Graphene

Parameshwaran et al. Organic ester-Ag

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Thermal Energy Storage for High Temperature Applications

Figure 5-39. Summary of the effect of nanoparticle on the latent heat of high

temperature inorganic PCM

As observed in Figure 5-39, both contradictory 15% enhancements and 10%

reductions are reported at a nanoparticle concentration of 1% wt. These anomalous

and opposite trends on the latent heat in molten salt nanofluid research also

motivates this investigation.

The solid-liquid transitions are dramatically different in the case of eutectic and off-

eutectic mixtures. As the NaNO3-KNO3 binary system phase diagram in Figure

5-40 shows, a eutectic mixture melts at a specific temperature (eutectic

temperature) whereas off-eutectic compositions will melt over a temperature

range. It is unknown if the effect of nanoparticles on the latent heat is the same on

each type of salt mixture. To answer this question, different compositions are tested

in this study, as specified in Table 5-20. The compositions prepared are the pure

components, the eutectic mixture (49 mol% NaNO3), and two off-eutectic mixtures:

a potassium rich mixture with 34 mol% NaNO3 and a sodium rich mixture with 64

mol% NaNO3 (this particular composition is the one previously investigated,

80%

85%

90%

95%

100%

105%

110%

115%

120%

0% 1% 2% 3%

No

rmal

ized

Lat

ent

Hea

t [%

]

Nanoparticle concentration [wt.%]

Theory

Tao et al. Carbonate+CarbonBased

Lasfargues et al. Nitrate+CuO

Lasfargues et al. Nitrate+TiO2

Chierruzzi et al. (2013) Nitrate+SiO2

Chierruzzi et al. (2013) Nitrate+Al2O3

Chierruzzi et al. (2013) Nitrate+TiO2

Chierruzzi et al. (2015) KNO3+SiO2

Chierruzzi et al. (2015) KNO3+Al2O3

Chierruzzi et al. (2015) KNO3+(SiO2+Al2O3)

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Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage

230

known as “solar salt”). The off-eutectic compositions are located symmetrically

from the eutectic point in the phase diagram. For the two off-eutectic compositions

studied, this melting range should theoretically start near or slightly above the

eutectic temperature (223ºC) and span about 25ºC.

Table 5-20. NaNO3-KNO3 compositions tested

NaNO3 KNO3

KNO3 0 mol% (0 wt.% ) 100 mol% (100 wt. %)

K-rich off-eutectic 34 mol% (30 wt.% ) 66 mol% (70 wt. %)

Eutectic 49 mol% (45 wt. %) 51 mol% (55 wt. %)

Na-rich off-eutectic 64 mol % (60 wt. %) 36 mol% (40 wt. %)

NaNO3 100 mol% (100 wt.% ) 0 mol% (0 wt. %)

Figure 5-40. Phase diagram NaNO3-KNO3 from Factsage43 highlighting the

compositions tested: a hypoeutectic at 34 mol% NaNO3, the eutectic at 49 mol%

NaNO3, a hypereutectic at 64 mol % NaNO3, and the pure components KNO3 and

NaNO3.

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Thermal Energy Storage for High Temperature Applications

The synthesis method described previously in section “5.2.1 Synthesis of

nanofluids”. It has not been modified in order to compare it with Chieruzzi et al.

(2013, 2015)110,116, which showed anomalous latent heat results using the same

synthesis procedure. The DSC method described in “5.2.2 DSC Measurement” has

been slightly modified by changing all the different heating rates to 5 ºC/min and

eliminating the modulation for the specific heat measurement (Figure 5-41). The

isothermal segments and inert gas have been maintained.

Figure 5-41. Example of DSC cooling and heating curve example, showing

temperature vs. time in blue plotted on the right axis and heat flow vs. time in

green plotted on the left axis.

For PCM applications, the mentioned eutectic is preferable than off-eutectic

compositions (such as solar salt) to limit the temperature range at which the

mixture solidifies. For HTF application it is interesting to have materials that melt

at lower temperatures and with low latent heat.

The latent heat of fusion and the onset temperature are compared between the five

NaNO3-KNO3 compositions and the effect of nanoparticle addition is evaluated in

each case.

0

50

100

150

200

250

300

350

400

450

-1.2

-0.8

-0.4

0

0.4

0.8

1.2

0 20 40 60 80 100 120 140 160 180

Tem

per

atu

re [

ºC]

Hea

t F

low

[W

/g]

Time [min]

Heat Flow Temperature

freezing melting melting

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232

5.4.1. Results

The heat flow curves obtained for each composition is shown in Figure 5-42. The

bottom curves (endothermic process, since H<0) correspond to the melting

process and top curves (exothermic, H>0 ) correspond to crystallization. The pure

components as well as the eutectic composition behave very similarly, melting in a

narrow temperature range, since they melt at a specific temperature, while the

phase change of off-eutectic compositions expands over a wider temperature range

corresponding with the “mushy” region (between the solidus and liquidus lines in

the phase diagram).

Figure 5-42. Heat flow curves vs. temperature for the different compositions tested.

The latent heat of fusion results of the five compositions tested agree very well with

other previous studies such as Rogers & Janz (1982)146 evaluating different the

phase change properties of NaNO3-KNO3 mixtures as can be observed in Figure

5-43. These results indicate that the latent heat of a mixture cannot be simply

-3.5

-2.5

-1.5

-0.5

0.5

1.5

2.5

3.5

180 200 220 240 260 280 300 320 340 360

Hea

t F

low

[W

/g]

Temperature [ºC]

KNO3

Na-KNO3 (30-70 wt.%)

Eutectic

Na-KNO3(60-40 wt.%)

NaNO3

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Thermal Energy Storage for High Temperature Applications

calculated as a linear weighted average of the pure component latent heats and

must be experimentally determined.

Figure 5-43. Latent heat of fusion results for different NaNO3-KNO3 mixtures

A minimum latent heat is observed around 25 - 34 mol% of NaNO3 and the latent

heat of the eutectic composition and the pure KNO3 are very close in both studies.

Once the latent heat of the different mixtures has been verified, the effect of

different SiO2 (10 nm) nanoparticle concentration on the latent heat is evaluated.

The results are presented in Figure 5-44, Figure 5-45, and Figure 5-46. Adding

nanoparticles to any KNO3-NaNO3 mixture decreases the latent heat. This

reduction in latent heat increases with particle loading. Normalizing the results by

the latent heat of the neat salt mixture (Figure 5-46) can indicate whether there are

differences in the behavior between compositions with a single melting

temperature (pure components or eutectic) and mixture compositions melting over

a temperature range.

80

100

120

140

160

180

200

0 20 40 60 80 100

Lat

ent

hea

t o

f m

elt

ing

[J/

g]

NaNO3 (mol %) in a NaNO3-KNO3 mixture

This study

Rogers & Janz (1982)

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Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage

234

Figure 5-44. Latent heat vs. nanoparticle concentration for different NaNO3-KNO3

mixtures

Figure 5-45. Latent heat vs. NaNO3 content in a Na-KNO3 mixture for different

nanoparticle concentrations

60

80

100

120

140

160

180

200

0% 1% 2% 3% 4% 5%

Lat

ent

Hea

t [

J/g

]

Nanoparticle concentration [%]

0% mol NaNO3

34%mol NaNO3

49%mol NaNO3

64%mol NaNO3

100%mol NaNO3

60

80

100

120

140

160

180

200

0 20 40 60 80 100

Lat

ent

Hea

t o

f M

elti

ng

[J/

g]

NaNO3 (%mol) in a NaNO3-KNO3 mixture

0 wt% SiO2

1 wt% SiO2

5 wt% SiO2

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Thermal Energy Storage for High Temperature Applications

Figure 5-46. Normalized latent heat vs. nanoparticle concentration for different

NaNO3-KNO3 mixtures. Theory corresponds to a simple mixing rule calculation.

5.4.2. Discussion

Theoretically the latent heat of fusion of composite materials is expected to linearly

decrease with particle addition due to the nanoparticles which do not undergo a

phase change. A simple mixing rule for the latent heat can be used as a first

approximation to compare the expected latent heat of the nanofluids with their

measured values (Figure 5-46). The nanoparticle mass fraction is calculated as

follows:

Equation 5-8

The nanoparticle mass fraction (w) and the latent heat (LH) of the base fluid is used

to predict the latent heat of the nanofluid:

Equation 5-9

where m stands for mass and subscripts nm, np, and bf denote nanomaterial,

nanoparticle, and base fluid, respectively.

80%

85%

90%

95%

100%

105%

0% 1% 2% 3% 4% 5%

No

rma

lize

d L

aten

t H

eat

[%]

Nanoparticle concentration [%]

Theory

0% mol NaNO3

34%mol NaNO3

49%mol NaNO3

64%mol NaNO3

100%mol NaNO3

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Nano-Enhanced Heat Transfer Fluid for Thermal Energy Storage

236

The results shown in Figure 5-46 indicate that adding SiO2 nanoparticles to the base

salt results in a reduction of the latent heat, as expected but larger than predicted by

mixing rule theory, and independently of the salt composition evaluated. The

latent heat of fusion of the nanofluid decreases as the nanoparticle concentration

increases. This is also observed for both: pure components and mixtures.

In Giménez & Fereres (2015)147 we considered the possibility that the nanoparticle

effect on latent heat could be a function of the base salt composition, since latent

heat essentially follows mixing theory predictions for the eutectic mixture but it

decreases slightly more than the predicted values when nanoparticles are added to

an off-eutectic composition. This difference is more evident as nanoparticle

concentrations are increased. The larger number of samples tested complementing

this study on the same salt compositions suggest a similar tendency, although the

confidence intervals are overlapped.

If we analyze the rate of nanofluid latent heat reduction with respect to the particle

concentration, we observe that it is maintained quite constant for the range of

nanoparticle concentration tested (0 wt % to 5 wt. %). The rate of nanofluid latent

heat reduction is in all cases higher than the expected theoretical.

The slope of the linear fit of the latent heat measurements for each composition is

shown in Table 5-21. It is interesting how KNO3 and the eutectic composition,

which showed a very similar latent heat, show also similar latent heat reduction on

average while pure NaNO3 and both off-eutectic composition behave similarly.

Table 5-21. Slope of the linear fit of the normalized latent heat vs. the nanoparticle

concentration.

LH [J/g] Slope

[% latent heat / % of nanoparticle]

KNO3 99.8 +/- 4.4 -1.705

K-rich off-eutectic 91.1 +/- 2.4 -2.344

Eutectic 98.6 +/- 3.2 -1.857

Na-rich off-eutectic 110.1 +/- 2.7 -2.659

NaNO3 184.0 +/- 6.0 -2.515

Theory - -1.000

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The results presented here disagree with previous studies on the same salts and

nanoparticle type and concentration. Chieruzzi et al (2013)110 reported an

anomalous latent heat enhancement of +14.8% for 64%mol of NaNO3 composition

at 1% for SiO2 nanoparticles. Similar enhancement was reported for the same

nanoparticle loading but different nanoparticle types such as Al2O3 (+15.54%). On

the other hand TiO2 nanoparticles showed lower enhancement (+4.77%). The same

authors in Chieruzzi et al. (2015)116 tested the same nanoparticle (SiO2) type and

concentration on pure KNO3 reporting +11.84% enhancement on the latent heat of

fusion. However, none of the results reported by these authors have been

replicated in this study. On the contrary, the opposite trend has been measured

showing reductions in latent heat instead of enhancements.

Lasfargues et al. (2015)118 also tested the binary mixture (64%mol of NaNO3)

reporting different behavior depending on the nanoparticle type. While TiO2

nanofluid showed a slight latent heat enhancement of +3.81% and +0.53% (for

0.1wt.% and 1wt.% respectively), different tendency was reported for CuO

nanofluid with +2.46% enhancement on latent heat for 0.1wt.%, but -4% reduction

for 1wt.% of nanoparticle loading. Unlike other authors, Lasfargues’ synthesis

method does not involve the use of water to dissolve the salt and disperse the

nanoparticle through sonication before re-crystallization, but using purely physical

mixing through the use of a ball-mill.

The latent heat enhancement in Lasfargues et al. (2015) 118 was justified with

trapped nanostructures inside tiny agglomerates raising the enthalpy of melting as

more energy would be required to melt the solid trapped within these structures. A

similar explanation has been used to also explain a reduction in latent heat, i.e.

nanoparticle clustering effect in Zabalegui et al. (2014)143, where the base fluid

inside the aggregated nanoparticle clusters increase the strained interface volume

of an ordered structure with weaker molecular bonds.

In the case of Chieruzzi et al. (2013 & 2015)107,113 different hypothesis were used to

explain the enhancement mechanism for the latent heat of nanofluids: one of the

hypothesis was the high specific surface energies associated with the high surface

area of the nanoparticles per unit volume. Other proposed hypothesis was the

possible existence of a layer of small agglomerates in the nanofluids in which the

nanoparticles would be trapped. The melting of the solid entrapped in these

agglomerates would require more energy to occur causing an increase of the latent

heat.

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Unlike Chieruzzi’s and Lasfergues’ research, the measurements performed in this

study do not show any anomalous (positive) enhancement of the latent heat. The

reduction on latent heat observed in this study is larger on average than expected

by the mixing rule theory. This trend was also reported with paraffin and other

organic based PCM where an interfacial liquid layering mechanism was suggested

as a possible explanation.143

5.4.2.1. Interfacial liquid layering

Zabalegui et al. (2014)143 also found a reduction in latent heat in excess of the

mixing rule predictions and attributed it to three potential explanations: 1)

interfacial liquid layering, 2) Brownian motion effects and 3) particle clustering.

Among the different possible mechanisms proposed in Zabalegui et al.143 to explain

this further reduction in latent heat through the weakening of molecular bond

structures the interfacial liquid layering is evaluated. Brownian motion is discarded

as a possible explanation because the Brownian diffusion time scales are two orders

of magnitude larger than the momentum relaxation time (i.e. it is too slow).

The larger reduction of the latent heat suggests that part of the base fluid does not

contribute to the phase change. A new term is introduced on the nanomaterial

latent heat to account for this additional reduction:

Equation 5-10

Where wnp is the nanoparticle mass fraction and wi is the mass fraction of the

interfacial layer. Based on the experimental results the ratio goes from 0.7 to

1.66 based on Table 5-21.

Equation 5-11

It has been established from both experimental studies and molecular dynamics

simulations that the width (∆) of the interfacial or densely packed layer (DPL) is no

more than 1–2 nm.148,149 Since attractive forces dissipate normal to the particle

surface, base molecules further away from the interface migrate shorter distances.

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For spherical nanoparticles and concentric semi-solid liquid layering:

Equation 5-12

For 10 nm SiO2 nanoparticles and ∆=1 nm the ratio = 0.728. The silica

nanoparticle density is ρnp=2.65 g/cm3.90 We can consider the interfacial liquid

layering as a semi-solid layer with an average density between the solid and liquid

density of the base fluid density, which depends on the composition.

Using the average density of this layer for each composition, calculated with the

volumetric additivity rule150, as well as the experimental ratio, we can

estimate the thickness (∆) of this interfacial layer for each composition. The solid

and liquid densities are 2.26 and 1.9 g/cm3 for NaNO3, respectively; and 2.11 and

1.865 g/cm3 for KNO3.

Table 5-22. Estimated thickness (∆) of the hypothetical interfacial liquid layer that

could explain the larger reduction on the latent heat observed in nitrate base

nanofluids.

∆i [nm]

KNO3 0.705 0.93 1.23

K-rich off-eutectic 1.344 1.77 2.02

Eutectic 0.857 1.12 1.42

Na-rich off-eutectic 1.659 2.15 2.33

NaNO3 1.515 1.93 2.15

The calculated thickness for each composition (Table 5-22) seems reasonable, not

exciding the suggested 1-2 nm. However, this calculation assumes that the

interfacial liquid layer does not contribute to latent heat. This means that the

calculated thickness is the minimum estimate.

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5.4.2.2. Relationship between an interfacial liquid layer with the Cp

The interfacial compressed liquid layer has been used to justify the anomalous

specific heat enhancement in molten salt nanofluids (mechanism 2 in section

5.1.1.1Theories behind Cp enhancement). The latent heat measurements performed

in this section show that this interfacial liquid layer could exist and could be the

cause of a larger reduction on the latent heat of nanofluids than expected based on

the mixing rule. If a 1-2 nm thick compressed liquid layer exists and one estimates

its contribution to the total nanofluid heat capacity as an additional term in

Equation 5-7, it would need to have an extraordinary heat capacity value given its

small mass. Considering that the solid salts have a lower Cp than the liquid salts,

one might anticipate that the compressed liquid layer (ordered liquid molecules)

will have intermediate properties between the solid and the liquid states. Hence, it

is difficult to understand how this small liquid layer, which should theoretically

have lower Cp than the liquid, can increase the overall Cp.

Moreover, in this investigation a negligible specific heat enhancement has been

measured which means that on the one hand: if this layer exists, it is not causing an

anomalous specific heat enhancement. On the other hand, if we have not measured

specific heat enhancement of nanofluids because the interfacial liquid layer has not

been formed, another hypothesis is needed justifying the larger reduction on the

latent heat measured.

5.4.3. Conclusions

The effect of nanoparticles on the latent heat of different NaNO3-KNO3 mixtures

has been analyzed. Understanding the solid-liquid phase transition process in

molten salt mixtures with and without nanoparticles can help explain the nanofluid

trends in the specific heat capacity in the liquid phase and is also very useful to

characterize the melting/crystallization process in nanoparticle-enhanced phase

change materials.

A reduction in the latent heat of fusion can be explained by the addition of

nanoparticles which do not physically contribute to the phase change, as they are

solid during the melting/solidification of the base fluid. The experimental results

suggest this hypothesis does not fully explain the measured reduction in latent

heat, as there is a larger than expected reduction for all the salt compositions and all

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the nanoparticle concentrations tested. The decrease in latent heat increases as

particle loading increases.

If an adsorption mechanism at the nanoparticle surface is taking place (i.e. a

compressed ordered liquid layer at the fluid/nanoparticle interface), the latent heat

should be further reduced. Using the experimental deviation from the simple

mixing rule accounting for an additional mass (particles) not changing phase, we

have estimated the thickness of an interface to be between 1-2 nm. This is in

agreement with reported measured values and could explain different (higher or

lower) heat capacity values from the base salt. This interfacial semi-solid or

compressed liquid layer would act similar to an extended phase transition during

the temperature range of test.

Nevertheless, if the nanofluid synthesis procedure is affecting the salt composition,

as demonstrated above, the latent heat should be normalized by a neat salt that has

undergone the same synthesis procedure to ensure comparable neat/nanofluid

mixtures.

5.5. Stability of the molten salt nanofluid

5.5.1. Motivation

The applicability of nanofluids at a commercial scale depends on the stability of

nanoparticles in the molten salts. Up to this date and to the author´s best

knowledge, this parameter has only been verified mainly by two different

observations:

1) Good dispersion of nanoparticles in the initial water solution during the

nanofluid synthesis process

2) Thermal cycling of the samples in the DSC

Most authors employing a two-step synthesis method with a solvent (i.e. water)

have shown the nanofluid (salt and nanoparticle mixture) is uniformly dispersed in

the solution during the initial production process.93,126 However, given the high

melting temperature range of these fluids (above 200ºC) and the unavailability of

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standard measurement equipment to test at temperatures above 90-120ºC,

researchers have not evaluated the dispersion stability of these molten salt

nanofluids in the liquid state. SEM images are typically shown, e.g. Shin & Banerjee

(2011b)95, of the solid state mixture, but it seems unlikely that the crystallized

mixture may describe the molten nanofluid.

Checking the repeatability of DSC tests during thermal cycling has also been used

to assess the stability of molten salt nanofluids. In Jo & Banerjee (2014)125 the

stability was tested by 5 times DSC measurements on the same crucible. As the heat

capacity of the nanomaterials were almost uniform after 5 times of melting in the

DSC the authors concluded that it was stable. Similarly Dudda & Shin (2012)104

performed several (4 - 6 times) thermo-cycling (from 140 ºC to 500 ºC) to ensure

repeatability of the measurements and stability. In Schuller et al.117 the stability was

verified by re-testing the DSC samples after 1-2 months.

In Shin & Banerjee (2011c)121 thermo-cycling experiments were conducted for

repeated measurements of the specific heat capacity by using multiple freeze-thaw

cycles of the nanofluids/ nano-composites, respectively. Transmission electron

microscopy of the nanomaterial samples were performed to confirm the stability of

the nanofluids by analyzing the state of aggregation of the nanoparticles before and

after the thermocycling experiments in the DSC. In Shin and Banerjee (2011b)95

Scanning Electron Microscopy images of SiO2 nanofluid were used before

melting/solidification in the DSC to verify the stability.

Through different experiments under high temperature operating conditions, we

demonstrate in this section that the stability of this nanofluid is not guaranteed by

these observations. Testing at high temperatures, as they should operate in an

industrial application, is necessary. Our hypothesis is that the appearance of a well-

dispersed nanofluid in the molten state should be similar to the nanofluid

dissolved in water after sonication, i.e. a homogeneous color mixture.

5.5.2. Materials and methods

Near spherical metal oxide nanoparticles are used to produce the nanoparticle

dispersions. The nanoparticle concentration is 1 wt %. The nanoparticle supplier

information is the following: SiO2 (Meliorum Technologies Nanomaterials, NY, 10

nm), CuO (Io-li-tec nanomaterials, 40-80 nm, 99.9%) and Al2O3 (Aldrich Chemistry,

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13 nm, 99.8%). Carbon nanotubes (CNT), produced by Iolitec nanomaterials (CNT

95+%. OD 10-20 nm, L: 5-15μm), have been also investigated using gum arabic as a

surfactant (0.25 wt. % with respect to salt).

The synthesis method is the one described previously in section “5.2.1 Synthesis of

nanofluids”. The eutectic NaNO3-KNO3 (49-51 mol %) has been selected for this

section because of its lower melting temperature. The amount of salt synthesized is

2000 mg per batch (200 ml of water for the solution). This amount of salt is a large

enough sample to be able to observe any stratification in the vials after several

melting and freezing cycles.

5.5.3. Results and discussion

Figure 5-47 shows the nanoparticle dispersions during the water—solution step

after sonicating the dissolved nanofluids. The main difference between the four

nanoparticles dispersions synthesized while they are dissolved in water is the color

of the sonicated solution. While SiO2 and Al2O3 make the salt-water solution

slightly whiter and less transparent, CuO presents brownish translucent solution.

On the other hand CNT shows an opaque black solution. After sonication the

nanofluids are stable and homogeneously dispersed.

Neat salt SiO2 Al2O3 CuO CNT

Figure 5-47. Neat salt and nanofluids (dissolved in water) after sonication.

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The nanofluid -water solution is then evaporated on a hot plate at 200ºC. Once it

has been completely dried, the salt crystals is scratched from the beaker and milled

in a mortar. The resulting nanoparticle-salt powder can be seen in Figure 5-48.

SiO2-nanofluid CuO-nanofluid

Figure 5-48. The nanofluid powder after solvent water evaporation is ground in a

mortar and placed in smaller vials for the thermal cycling tests

Figure 5-49. Solid-state nanofluid before melting, from left to right: neat salt

NaNO3-KNO3 eutectic, SiO2 nanofluid, Al2O3 nanofluid, CuO nanofluid, and CNT

nanofluid with gum arabic (GA) dispersions.

As mentioned above, the homogeneity and dispersion stability of these

nanoparticle suspensions has been typically assessed in previous studies by

ensuring the salt - nanoparticle - water solution is well dispersed during the first

step of the synthesis procedure and subsequent solid phase SEM images to assess

the nanofluid particle dispersion quality.

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The first melt of the same vials from Figure 5-49 can be seen in Figure 5-50.

Qualitative differences can be easily seen between each nanoparticle type colloid:

the SiO2 nanofluid seems to have the most homogeneous appearance, followed by

the Al2O3 nanofluid. On the contrary, the CuO mixtures seem clearly stratified even

during this initial melt. The CNT+salt solution presented an interesting

phenomenon, growing in volume as the salt is heated up and melted.

Figure 5-50. Molten state nanofluid during the first melting process on the hot

plate at 350ºC from left to right: neat salt NaNO3-KNO3 eutectic, SiO2 nanofluid,

Al2O3 nanofluid, CuO nanofluid, and CNT with gum arabic nanofluid.

Each of the nanofluid samples is then subjected to the same thermal cycle on hot

plate: each vial is heated until it is fully melted and then cooled down to solidify.

This process is repeated six times.

Figure 5-51. Difference between SiO2, Al2O3, and CuO nanofluids after 6

melting and freezing cycles.

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The different types of nanofluid behave very differently during this

melting/freezing thermal cycling process. Figure 5-51 shows the SiO2, Al2O3 and

CuO nanofluids fully melted where significant qualitative differences can be

observed after the six melting/freezing cycles.

Ideally one would expect to see a homogeneous single color fluid, similar to the

initial water solution after sonication during the synthesis process. If these

nanoparticles are well-dispersed, due to their small size, they might change the

color of the fluid but agglomeration should not occur. However, the observation of

the nanofluid in molten state (Figure 5-50) indicates that having a good dispersion

during the water solution step of the synthesis procedure is a necessary but not a

sufficient condition to ensure a well-dispersed colloid. Moreover, a clear convective

movement is observed in every single nanofluid in the molten state. Additionally,

in some cases, the nanoparticles appear to concentrate at the bottom of the vials

(Al2O3 and CuO nanofluids in Figure 5-51).

The SiO2 nanofluid is the only nanofluid tested able to maintain the nanoparticles

dispersed in the liquid state salt without showing any precipitate (it is not possible

to determine in the solid state since both SiO2 particles and solid salt are white).

However, the SiO2 nanoparticles appear to be fully aggregated forming 0.2-0.5 mm

visible clusters. The convective movement and the SiO2 aggregates can be seen in

Figure 5-52. Note that the average aggregate size does not vary between thermal

cycles in the SiO2 nanofluid case.

Figure 5-52. Molten SiO2-nanofluid on a hot plate at 350ºC after 2, 3, 5 and 6 (left to

right) thermal cycles.

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The Al2O3 nanofluid shows a precipitate at the bottom of the vial indicating the

impossibility of maintaining the nanoparticles in suspension.

Figure 5-53. Molten Al2O3-nanofluid (left) and in the hot plate at 350ºC. CuO

nanofluid (right) in molten state and solidified after several freeze/thaw cycles

showing nanoparticle stratification

Similarly to the previous oxide nanoparticles, CuO cannot be maintained

suspended the molten salt. Figure 5-53 (right) shows black CuO nanoparticles

precipitated at the bottom of the vial. Even after slightly stirring the molten

nanofluid with a needle some nanoparticles are still found at the bottom. Some

other micron-sized particle clusters are observed suspended within the salt.

The images presented indicate the inability of different types of nanoparticles at 1

wt % to be kept homogeneously dispersed in the molten eutectic sodium-

potassium nitrate mixture.

5.5.3.1. Scanning Electron Microscopy Characterization

The characterization of several samples has been performed using Scanning

Electron Microscopy (SEM). The samples extracted after the stability test show

significantly bigger clusters which correlate to the macroscopic observations.

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Figure 5-54. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic

NaNO3-KNO3 (45-55 wt. %) with 1% CuO after the stability test

Figure 5-55. SEM analysis of the nanofluid eutectic NaNO3-KNO3 (45-55 wt. %)

with 1% CuO after the stability test

Similarly, the eutectic NaNO3-KNO3 (45-55 wt. %) with 1 wt. % of SiO2 shows also

aglomeration of nanoparticles when the samples are observed after the stability

test. Their appearance resembles the previous nanofluid (Figure 5-28) with 5 wt. %

of SiO2 although with smaller silica clusters.

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Figure 5-56. SEM (left) and SEM-EDS analysis (right) of the nanofluid eutectic

NaNO3-KNO3 (45-55 wt. %) with 1 wt. % of SiO2 after the stability test

Figure 5-57. SEM analysis of the nanofluid eutectic NaNO3-KNO3 (45-55 wt. %)

with 1 wt. % of SiO2 after the stability test

The SEM analysis of the eutectic nanofluid NaNO3-KNO3 (45-55 wt. %) 1 wt. % of

SiO2 (Figures 5-56 and 5-57) and CuO (Figures 5-54 and 5-55) nanoparticles shows

the presence of micro-size clusters, which is aligned with the macroscopic

observation (sedimentation and visually noticeable aggregates), but it is in

opposition to the well dispersed nanoparticles analyzed once synthesized after

testing in the DSC. These observations suggest that the nanoparticle distribution

could be homogenous after the synthesis process for low nanoparticle

concentrations, and it might remain when testing in the DSC. However, if we melt

a larger batch of nanofluid (grams), the nanoparticles tend to agglomerate forming

micro-size clusters.

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5.5.4. Conclusions

A series of experiments analyzing the macroscopic behavior of the molten eutectic

NaNO3-KNO3 mixture adding 1wt. % of different nanoparticle types (CNT, SiO2,

CuO and Al2O3) have been performed. The objective was to investigate whether

having a homogenous dispersion in the water solution step of the synthesis process

is a sufficient condition to guarantee the dispersion stability once the solvent has

been removed. Experiments we performed to check if different types of

nanoparticles can be kept homogeneously dispersed in nitrate base molten salts

after several melting and freezing cycles. The enhancement of any thermophysical

property suggested in the nanofluid literature such as enhanced thermal

conductivity or enhanced specific heat requires a homogeneous dispersion of the

nanoparticles for any engineering application.

It appears that researchers assume that synthesizing well-dispersed nanofluid in

water will ensure a good dispersion of the nanoparticles in the salt once the water

solvent is evaporated. The experimental results indicate that this assumption is a

necessary but not sufficient condition to ensure a well-dispersed nanofluid in the

molten state.

The visual observation of the salt in liquid state showed that none of the nanofluids

tested are able to maintain the stability of the nanoparticle suspension. Once the

salt is melted for the first time the color of the molten salt changes. Depending on

the nanoparticle type, different degrees of stratification is observed: CNT at the top,

CuO and Al2O3 (bottom). In the case of silica, a whitish cloud is clearly observed

suspended within the salt. Even though the silica nanoparticles do not precipitate

they are far from being well-dispersed.

The qualitative results presented in this section call into question most of the recent

literature on high temperature molten salt nanofluids. The different appearance

than a homogeneous mixture might indicate that regardless the large number of

publications in the field its usefulness is far from being a reality.

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5.6. Summary and conclusions

This chapter analyzes the effect of nanoparticles on the specific heat capacity and

latent heat of nitrate molten salts with the goal of increasing the storage capacity of

solar thermal plants. The extensive literature from the past years is to this date still

unclear about the possibility and magnitude of potential specific heat capacity

enhancement mechanisms.

However, the measurements performed in this chapter do not support the

existence of an anomalous specific heat enhancement caused by the addition of

nanoparticles to nitrate base molten salts. No statistical evidence has been found

relative to the moderate enhancement measured in this research. This

enhancement, combined with other phase change parameters suggest that

composition shifts can be produced during the synthesis process. Moreover, the

large uncertainty in the molten salt specific heat capacity measurements (without

nanoparticles) questions the significance of some of the reported data.

The main contributions of this work are the following:

The two-step water solution synthesis process, extensively used in the

literature for these mixtures, leads to component segregation during the

solvent evaporation step. This composition shift effect is more noticeable when

the evaporation is faster or using containers with a larger surface area.

The solubility of the different salt components has been demonstrated to be

key property that might explain the Cp enhancement shown in the literature:

NaNO3 (component with a larger Cp) is less soluble in water and, therefore,

will start precipitating from the mixture first as the solvent water is evaporated.

Initial areas of salt+nanoparticle precipitation form more “homeogenously”

looking crystals, which have been described as less-agglomerated nanofluids

before. Thus, these areas contain a higher content of NaNO3 and will have

consequently a higher Cp, regardless of the nanoparticles added (for low

enough concentrations). This is demonstrated with measurements of neat salts

(without nanoparticles) showing “enhancement” by following different

evaporation processes without adding any particles.

Sapphire corrections should be performed carefully for specific heat capacity

measurements over a wide temperature range, as they can easily increase the

measurement error. DSC equipment calibration must be performed regularly

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and more frequently with molten salts, as these materials creep, escape the

crucibles and contaminate the DSC sensors during testing.

The latent heat of nanofluid has been observed to decrease in a larger amount

than the theoretically predicted, against limited existing investigations with

molten salts but aligned with extensive results from lower temperature phase

change materials with nanoparticles for increased thermal conductivity.

The existence of an interfacial liquid layer has been hypothesized as

responsible for this larger reduction in latent heat calculating the layer

thickness for each base fluid. The calculated layer thickness seems to be

reasonable, not discarding this hypothesis. However the phase transition of

this hypothesized compressed liquid layer has not been observed

experimentally.

Finally, and most importantly from an industrial impact point of view, this is

the first time the stability of molten salt nanofluids have been evaluated in the

liquid state at high temperatures (above 300ºC). The results show the inability

of the molten salt to keep different nanoparticle types dispersed, even though

the different steps during the synthesis procedure show homogeneity.

Some thermo-physical properties might be improved trough the addition of

nanoparticles to the base salt, some expected such as thermal conductivity when

high thermal conductivity particles are added and other unexpected enhancement

such as specific heat, which has not been reproduced along this investigation, both

desirable for thermal storage materials, the first one directly related to the heat

transfer rate, and the second one related to the amount of energy that can be stored

in solar thermal plants. However, the nanoparticle agglomeration and stratification

presented in this study is clearly indicating that more effort is needed to design a

stable, functional and realistic heat transfer nanofluid for any engineering

application.

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6 GENERAL CONCLUSIONS AND

FUTURE CHALLENGES

he aim of this thesis was to explore different techniques in order to increase

the storage capacity of the current TES systems in commercial concentrated

solar tower power plants. The two commercial TES technologies are: steam

accumulators and the two molten salt tanks. Both systems use sensible heat

storage: the first one in pressurized saturated liquid water and the second

increasing the temperature of molten salt. As alternatives to the current TES

technologies this thesis investigates the use of a) latent heat TES by encapsulating

phase change materials, and b) the modification of the thermo-physical properties

of molten salts through nanoparticle addition.

The use of phase change materials (PCM) has been explored in Chapters 2, 3 and 4

to increase the storage capacity of the steam accumulators. The goals accomplished

by this investigation are summarized as follow:

A wide material analysis has been performed starting from literature

values.

In the temperature range of interest the selected materials have been

T

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254

characterized experimentally using differential scanning calorimetry. The

importance of testing is highlighted due to discrepancies observed

between previously reported values and those calculated through

thermodynamic programs. Some compositions proposed as PCM

candidates have shown off-eutectic behavior without melting in a narrow

temperature range, making them undesirable for PCM applications. For

the thermo-economic comparison of materials, the measured latent heat

has been used. Since the different compositions evaluated have not shown

substantial benefits in terms of latent heat compared to their pure main

constituent and for the sake of simplicity during the encapsulation proof-

of-concept, pure salts (NaNO3 and KNO3) and certain metals (lead and tin)

were selected as PCM for subsequent sections.

The use of PCM requires the development of a functional container where the

phase change takes place and a heat exchanger design incorporating such

container to transfer heat back and forth to the heat transfer fluid (e.g. steam).

Here the use of a packed bed storage system is proposed, which requires the

design, development, and testing of capsules which has been performed in

Chapter 3.

An extended literature review of experimental macro-encapsulated PCM

studies has been performed identifying borosilicate as an interesting and

novel shell candidate because of its thermal properties, corrosion

resistance, and chemical compatibility with both the heat transfer fluid

(high pressure steam) and a wide variety of high temperature PCM (salts

and metals).

Different borosilicate capsules have been manufactured and filled with

different PCM materials. The designed capsules are in line with

contemporary studies in terms of size, temperature range, and shell/PCM

volume ratio. The appropriate degree of filling to minimize internal

pressure has been determined to calculate the maximum allowed amount

of PCM, improving the energy density of previously reported capsules.

An experimental set up has been developed to validate the capsule

concept by melting and freezing different capsules combining the

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information provided by a video camera and infrared camera to analyze

the phase change process. It is the first time the solid-liquid transition of

capsules of such sort has been visually analyzed at such temperatures and

through thermal imaging, describing the phase transition behavior

corresponding to the characteristics of each type of PCM.

A numerical model has been implemented (Chapter 4) to help understand the

influence of each capsule design parameter in the phase change time. The model is

capable of capturing the main physics taking place and is used to help understand

the experimental results. Simulations and experiments have been compared

qualitatively and quantitatively identifying some sources of uncertainty that could

explain the mismatch between them.

Finally, the addition of nanoparticles to improve the storage capacity of TES

systems based on molten salts has been investigated in Chapter 5. The initial goal

was to reproduce the specific heat enhancement reported in the literature by

several research groups. Unfortunately, the different nanoparticle concentrations,

sizes, types, and different base fluid compositions tested did not show any

remarkable and statistically significant enhancement of the specific heat of nitrate

salts. Modifications in the synthesis process have been made, discovering that the

water-solution evaporation step seems to separate salt in different regions with

slightly different compositions, which could explain slight modifications in the

specific heat and latent heat.

During this investigation some assumptions made in the synthesis process by

different research groups have been questioned. The water dissolution-sonication-

evaporation synthesis protocol, the most widely used, does not seem to be

appropriate because specific samples with higher (or lower) Cp than the average

could be selected even without nanoparticles. The ability of molten salt nanofluids

to keep the nanoparticles in suspension and well dispersed has been also

questioned. Scanning electron microscopy images did not show the presence of

“special” or “anomalous” nanostructures in the salt.

The latent heat of molten salt nanofluids has been also investigated. The latent heat

has been observed to decrease in a larger amount than the theoretically predicted,

against limited existing investigations with molten salts but aligned with extensive

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General Conclusions and Future Challenges

256

results from lower temperature phase change materials with nanoparticles for

increased thermal conductivity. Finally, we have hypothesized the existence of an

interfacial liquid layer as responsible for this larger reduction in latent heat

calculating the layer thickness for each base fluid.

Future challenges involve several aspects: on one hand, finding a scalable

encapsulation fabrication process for high temperature PCM using borosilicate. The

packed bed solution is, however, dependent on the development of cost-efficient

steam tanks to contain the capsules. On the other hand, a more efficient system to

exchange heat with high pressure steam is a casing and tube heat exchanger, which

minimizes the surface exposed to high pressure when the vapor circulates through

the tubes. Following this idea, the solution based on double PCM should be further

analyzed numerically and experimentally.

In the field of high temperature nanofluids a large amount of work is required in

order to apply this technology at a commercial scale such as:

a) Develop a novel, scalable synthesis process that does not require the

dissolution of the salt mixtures in water.

b) Guarantee the stability of the molten salt nanofluid at high temperatures

(e.g. in the liquid state) exploring different additives, since common

surfactants will thermally degrade at these temperatures.

One of the main challenges in this field is to end the controversy on the specific

heat enhancement of nanofluids. A Round Robin test could be proposed where the

same nanofluid is tested by different research groups.

Unfortunately, the corrosive nature of molten salts, their tendency to absorb any

ambient moisture, and the high testing temperatures of interest (above 300ºC)

makes experimentation with molten salt nanofluids extremely difficult. The lack of

standard laboratory equipment to test such high temperature colloids increases the

effort to measure and quantify the effect of such nanoparticles. Further advancing

in the understanding of ionic liquid based nanofluids, liquid at room temperature,

may help evaluating the possibilities of extraordinary enhancements not deducible

a priori from general principles.

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The optical properties of the nanoparticles with the transparency of the molten salts

could be used in other applications leading to several research projects. For

example, the evaluation of ceramic/quartz transparent solar receivers using nano-

modified heat transfer fluids could have potential advantages.

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General Conclusions and Future Challenges

258

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