Review and selection of EPCM as TES materials for building applications · 2018. 12. 17. · Review and selection of EPCM as TES materials for building applications Karina Fullenkampa,

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International Journal of Sustainable Energy

ISSN: 1478-6451 (Print) 1478-646X (Online) Journal homepage: http://www.tandfonline.com/loi/gsol20

Review and selection of EPCM as TES materials forbuilding applications

Karina Fullenkamp, Macarena Montané, Gustavo Cáceres & Gerardo Araya-Letelier

To cite this article: Karina Fullenkamp, Macarena Montané, Gustavo Cáceres & GerardoAraya-Letelier (2018): Review and selection of EPCM as TES materials for building applications,International Journal of Sustainable Energy, DOI: 10.1080/14786451.2018.1543307

To link to this article: https://doi.org/10.1080/14786451.2018.1543307

Published online: 15 Nov 2018.

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Review and selection of EPCM as TES materials for buildingapplicationsKarina Fullenkampa, Macarena Montanéa, Gustavo Cáceresa and Gerardo Araya-Letelierb

aFacultad de Ingeniería y Ciencias, Universidad Adolfo Ibáñez, Santiago, Chile; bEscuela de Construcción Civil,Facultad de Ingeniería, Pontificia Universidad Católica de Chile, Santiago, Chile

ABSTRACTIn order to improve the thermal efficiency of building thermal energystorage (TES) systems, the feasibility of using encapsulated phasechange materials (EPCMs) as heat storage media is analysed in this work.Specifically, the finite element method is used to perform thermalbehaviour analyses of several EPCMs. These analyses include technicaland economic assessments in order to identify the best combination ofPCM and shell material, using as main parameters: thermal energystorage, heat transfer rate, materials cost, among others. The resultsshow that EPCMs composed by Na2SO4·6H2O as PCM and covered bystainless steel highlight as TES materials.

ARTICLE HISTORYReceived 18 June 2018Accepted 14 October 2018

KEYWORDSEncapsulated phase changematerials; buildingapplications; thermal energystorage materials

1. Introduction

Energy consumption in buildings has presented a fast increase mainly because of the rapid growthof the population, the human time spent indoors and the demand for building functions andindoor environmental quality (Cao, Dai, and Liu 2016). Worldwide, 60% of global energy con-sumption is due to heating and cooling (Su et al. 2017; Nejat et al. 2015). In addition, morethan 75% of the energy supply for heating and cooling is based on fossil fuels (European Commis-sion 16-2-2016). Particularly, the building sector is responsible for more than 40% of the finalglobal energy consumption (Nejat et al. 2015; Souayfane, Fardoun, and Biwole 2016). It is esti-mated that 85% of gas emissions in the building is caused by heating, cooling and lighting activities(I.E. Agency 2008).

Latent heat storage (LHS) for thermal energy storage (TES) in building applications highlights asa promising and sustainable solution to supply the high energy demand from the building sector,which has presented a dramatic increase over the past decades (Cao, Dai, and Liu 2016). LHSmaterials, also known as phase change materials (PCM), the highlight for TES building applicationmainly since they account with higher energy densities than the widely used sensible heat storagematerials (Sharma et al. 2015). Therefore, PCM usage allows to increase TES capacity and to reducethe TES system size (Khudhair and Farid 2004; Sharma et al. 2009; Zalba et al. 2003).

Nevertheless, PCM must be encapsulated for technical uses in almost all their applications,including buildings applications (Cabeza et al. 2011). According to Giro-Paloma et al. (2016),PCM needs to be encapsulated, to avoid the leakage in the liquid state when PCM is mixed withother materials, such as building construction materials (Pasupathy, Velraj, and Seeniraj 2008). Inte-gration of microencapsulated phase change materials into various construction materials have beenidentified as a key potential mechanism to minimise energy consumption in buildings (Su et al.2017). In direct TES systems, PCM encapsulation also contributes to avoid corrosion and improves

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CONTACT Karina Fullenkamp [email protected]

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thermal reliability (Zheng et al. 2013). In addition, since PCM account with low thermal conduc-tivity, encapsulation of PCM is an excellent mechanism to increase heat transfer rate of PCM char-ging and discharging cycles (Zhao and Zhang 2011; Zheng et al. 2013; Tyagi et al. 2011).

In addition to the energy demand increase, another consequence of the fast population growth isthe reduction of the available habitable space, which leads to increase the price and population den-sity of these places. As consequence of the buildings thermal energy consumption increase and avail-able habitable space decrease, the integration of EPCM in building applications highlights as anexcellent mechanism to increase energy density, reduce TES size and enhance the heat transferrate of TES materials.

Therefore, the main objective of the present work is to perform an assessment of encapsulatedphase change materials (EPCM) for low-temperature TES in buildings, in order to find a novelEPCM geometry configuration and material combination which increases the energy density ofthe TES material and, therefore, reduces the volume of the required amount of materials, sincespace is generally a scarce resource in buildings. In addition, this work also attempts to find lowprice EPCM novel composition, in order to increase the competitiveness of the TES technology.It is expected that this work contributes to improving building energy efficiency and reduceshuman fossil fuels consumption.

2. Phase change materials

Latent heat storage, sensible heat storage (SHS) and thermochemical heat storage (THS) are the threemethods used for TES. They are based on (i) phase change process, (ii) gradient of temperature and(iii) adsorption and absorption reactions, respectively (Yu, Wang, andWang 2013; N’Tsoukpoe et al.2009). Regarding the first two, PCMs have higher energy density than SHS materials (Sharma et al.2015) and, consequently, a smaller amount of materials is necessary to store the same amount ofthermal energy.

As consequence, smaller containers can be used, compared to containers based on SHS as TESmaterial, by using PCM as TES material. In fact, the energy density of PCM is in the range 300–500 MJ/m3, whereas the average TES capacity of the most employed SHS materials is approximately100 MJ/m3 (Tatsidjodoung, Le Pierrès, and Luo 2013), where the most practical available material iswater, which accounts 250 MJ/m3 of energy density using 60°C of temperature gradient (Inter-national Organization for Standardization 2007–12). Therefore, in terms of energy density andused space between these two methods, LHS is more promising than SHS.

Regarding LHS and THS methods, THS materials are characterised by insignificant heat loss andlong-term heat availability (IEA 2013). In addition, THS materials have approximately 1000 MJ/m3

of average energy density (Tatsidjodoung, Le Pierrès, and Luo 2013), which is considerably higherthan the other TES methods. Despite its advantages, THS has not been extensively researchedand has important disadvantages like poor long-term durability, weak chemical stability and com-plex reactors requirement for the specific chemical reaction (Tian and Zhao 2013). Then, due toTHS disadvantages, LHS is again the most promising option.

PCMs are highly popular due to their high heat capacity and their non-toxic properties, as con-sequence, they can be used in building applications as TES material (Guo and Zhang 2008; Kaizawaet al. 2008; Amin et al. 2017). Specifically, solid–liquid PCMs can store and release a relatively largequantity of heat within a narrow temperature range, without a large volume change (Lizana et al.2017). In addition, it has been shown that they are economically attractive (Hasnain 1998). Forexample, according to Morrison and Abdel-Khalik (1978) and Ghoneim (1989), the storage massfor rock (SHS) will be 7 times to that of paraffin 116 wax (LHS), 5 times to that of medicinalparaffin (LHS) and 8 times to that of Na2SO4·10H2O (LHS).

PCMs are classified as organic (non-paraffin, as fatty acids, esters and alcohols, andparaffin), inorganic (salts hydrates and metals) and eutectic compositions (Tatsidjodoung, LePierrès, and Luo 2013). The volumetric latent heat density of PCMs are in the ranges of

2 K. FULLENKAMP ET AL.

100–250 MJ/m3 for organic materials and 150-430 MJ/m3 for inorganic materials. Amongorganic materials, fatty acids, esters and alcohols are usually highly flammable and have lowthermal conductivity with diverse levels of toxicity (Sharma et al. 2009), which limits theirapplication in buildings. Therefore, paraffin, salt hydrates and eutectic mixtures are the mainPCMs used for building applications (Zhou, Zhao, and Tian 2012; Kalnæs and Jelle 2015;Mehling and Cabeza 2007):

. Paraffin (CnH2n+2) is an organic material, characterised by having approximately 170 MJ/m3 of

average latent heat capacity and about 0.2 W/m°C of average thermal conductivity (Zhou,Zhao, and Tian 2012).

. Salt hydrates are alloys of inorganic salts (AB) and water (H2O) with a chemical formula AB·H2O.They have relatively high latent heat capacity and thermal conductivity (approximately350 MJ/m3 and 0.5 W/m°C, respectively) (Zhou, Zhao, and Tian 2012). Moreover, salt hydratesare cheaper than paraffin (Oliver, Neila, and García-Santos 2012) and non-flammable (Kalnæsand Jelle 2015). Thus, they show a higher potential for applicability in buildings (Pereira daCunha and Eames 2016) albeit with some disadvantages such as their low thermal reliabilityfor long-operation periods (Sharma et al. 2009), phase segregation, sub-cooling and corrosiveness(Tatsidjodoung, Le Pierrès, and Luo 2013; Kalnæs and Jelle 2015; Hasnain 1998; Mehling andCabeza 2007).

. Eutectics are mixtures of inorganic PCMs (mostly hydrated salts) and/or organic PCMs. They havea melting temperature below those of the constitutive compounds (Oliver, Neila, and García-San-tos 2012). In addition, eutectic alloys present a congruent phase change without phase segregation(Tatsidjodoung, Le Pierrès, and Luo 2013).

According to Vasu et al. (2017), the most important characteristics of LHS TES materials are sum-marised in Table 1.

3. PCM buildings applications

In buildings, PCM is used to enhance the thermal comfort of lightweight constructions, especially tosolve overheating problems in summer (Schossig et al. 2005). Traditionally, wallboards have beenstudied as one of the best options to incorporate PCM into building walls (Gracia and Cabeza2015). Nevertheless, current experimented uses of PCMs for TES include solar water-heating sys-tems, solar air-heating systems, solar cookers, solar greenhouses for curing and drying processes,building acclimatization as in a PCM trombe wall, PCM wallboards, under-floor heating systemsand ceiling boards (Fernandes et al. 2012).

Table 1. Summary of the most important characteristics of LHS materials, adapted from Vasu et al. (2017).

Performance Parameter 20–40°C (paraffin)30–80°C (salt hydrates)

Storage density Moderate (with low-temperature interval):0.3–0.5 GJ/m3

Lifetime Often limited due to the TES materialCycling

Technology status Available commercially for sometemperatures and materials

Advantages Medium storage densitySmall volumesShort distance transport possibility

Disadvantages Low heat conductivityCorrosiveness of materialsSignificant heat losses (depending on the level of insulation)

INTERNATIONAL JOURNAL OF SUSTAINABLE ENERGY 3

PCM can be used into active and passive applications:

. For active applications, PCM is integrated into thermally activated building systems, TES com-ponents, small-scale TES units or large-scale TES systems. In building active applications, TESsystems are useful as energy management method, by reducing the peak load and improveefficiency by adjusting the operation range (Amin et al. 2017). Regarding active building appli-cations, the integration of the TES in the building can be done by using the core of the building(core, floor, and walls), external solar facades, suspended ceilings, ventilation system, PV systemsand water tanks. They highlight that the benefits of this technology are focused on the energy costsavings by using thermal energy stored during low-cost electricity tariff (Gracia and Cabeza 2015).For active solar residential applications, TES allows a continuous operation of solar energy tech-nologies, by reducing or overcome intermittency.

. Among passive applications, some promising options are PCM usage as building components orstructures, by including these materials during the buildings design and construction stages. Inbuilding passive applications, TES allows to reduce thermal energy consumption by integratinginto construction materials.

Vrachopoulos et al. (Vrachopoulos et al. 2013) investigated the performance of a test phasechange material chamber for passive solar applications and concluded that PCM does not operateas an insulating material but as a stabilizer of the temperature within the constructed test chamber.Giro-Paloma et al. (Giro-Paloma et al. 2016) performed an interesting review of PCM building appli-cations, where wallboards, floors, concrete, gypsum and other parts are integrated with PCMs.

According to Lin et al. (Lin et al. 2017), PCM in buildings applications improves energy efficiency,by decreasing energy consumption and improving the thermal comfort of buildings. They highlightthe excellent effect on energy conservation of gypsum boards PCMs integration. They also highlightthat PCM panels of CaCl2·6H2O/expanded graphite composite PCM for application in buildings areable to reduce the temperature fluctuation of the test room, showing a smaller range of temperaturecompared with the conventional room (Ye et al. 2017). Moreover, PCM-gypsum boards can be usedin both new and old buildings (Sharifi, Shaikh, and Sakulich 2017). Gracia and Cabeza (2015) dis-cussed PCM-wallboards supported by expanded graphite nanosheets improve thermal conductivity.It was reported that enhance thermal storage and energy distribution.

Shukla and Singh (2011) performed an analysis of PCM storage unit for night coolness storage inthe summer season. They concluded that the mass of the PCM and the length of the storage unit playa vital role in obtaining the comfort temperatures at certain periods of time during the day operation,but during nighttime operation, it is the air flow rate and night ambient temperature which decidesthat how much mass of PCM can be solidified in limited summer time of 8 h. It has also been foundthat four times of air flow rate will be required when compared with the air flow rate during the day-time so that it can be charged in limited night duration of 8 h and provides comfort temperature forabout 8 h during the daytime. More mass will require higher flow rates, which in turn certainlyaffects the electrical power consumption.

Quanying, Lisha, and Chen (2010) studied the thermal performance of shape-stabilised phasechange paraffin wallboard. They concluded that the surface temperature of the shape-stabilisedphase change wallboard is lower than common wallboard without PCM. The higher the contentof the shape-stabilised phase change, the lower the surface temperature of the wallboard, and thesmoother the temperature change.

Quanying et al. (Quanying, Ran, and Lisha 2012) studied the thermal energy storage properties ofparaffin and fatty acid binary systems. They report that paraffin mixtures and fatty acid mixtures aresuitable PCM used for the wall since it this suitable phase change temperatures, large phase changelatent heat and good thermal stability. They reported that thermal stabilities of paraffin and fatty acidmixtures are good after 500 thermal cycles and the change rates of fatty acid mixtures in the phasechange temperatures and phase change latent heats are less than that of paraffin mixtures. Quanying


et al. (2015) studied thermal energy storage properties of the capric acid–stearic acid binary systemand 48# paraffin–liquid paraffin binary system. They concluded that phase change temperature andlatent heat of 48# paraffin and liquid paraffin mixture decrease with the increase in the liquid paraffincontent and phase change temperature of capric acid and stearic acid mixture decreases and thenincreases with the increase in stearic acid content. They also concluded that the stability ofparaffin and fatty acid mixtures is all good, and the stability of fatty acid mixtures is better thanthat of paraffin.

Yan et al. (2011) studied the thermal properties of shape-stabilised fatty acid mixtures used forwallboard. They reported that the suitable mass content of the fatty acid binary mixtures in theshape-stabilised PCM is 70%. The shape-stabilised PCM has no leakage, and the phase-change latentheat is large. Yan, Zhang, and Li (2010) studied thermal properties of some mixtures of fatty acidsand liquid paraffin as PCMs used for energy-storing wallboard. Some of the most interestingconclusions are: these mixtures can be used in different energy storage fields since the meltingpoint varies according to the materials proportion. They notice that phase-change temperaturesand phase-change latent heats of the mixtures of liquid paraffin and fatty acids decrease with theincrease of the mass contents of liquid paraffin. They concluded that when the mass contents ofliquid paraffin are 60–80%, the phase-change temperatures of mixtures are 20.7–28.6°C, and thephase-change latent heats are 35–90 J/g, so they are suitable PCMs used for wallboard in buildings.

Zhang et al. (2007) presented several PCM building applications. They highlight that PCMsincorporated in building envelopes can increase thermal capacity of light building envelopes, thusreducing and delaying the peak heat load and reducing room temperature fluctuation. Moreover,together with a solar collector system, a PCM building component can store more solar thermalenergy during the day and discharge the heat during the night, thus maintaining the good thermalcomfort of the room. They also highlight the possibility of store heat with cheap electricity at nightand then discharge heat during the day, thus decreasing the space. Moreover, they discussed thatenvelope of the building offers large areas for passive heat transfer within every zone of the building,which would add thermal storage for passive solar heating as well as create an opportunity for ven-tilation cooling and time shifting of mechanical cooling loads. They noticed that except for theexpense of the PCM, little or no additional cost would be incurred compared with ordinary envelopecomponents.

3.1. Encapsulated phase change materials

Based on size, encapsulated phase change materials (EPCMs) can be classified as nano (below1000 nm), micro (from 1000 nm to 1000 μm) and macro (above 1000 μm) (Qian et al. 2015). Typi-cally, macro encapsulated PCM is packaged in tubes, pouches, spheres, panels or other receptaclesand then incorporated into building products. At the contrary, micro-encapsulated PCM particlesare enclosed in a thin, high molecular weight polymeric film which should be compatible withboth the PCM and the construction materials (Zhang et al. 2007).

Despite PCM benefits and applications, they also account with disadvantages as (Khan, Khan, andGhafoor 2016) low thermal conductivity, corrosive nature of PCMs towards its container material,phase segregations and sub-cooling, incongruent melting, volume variation during phase transitionand higher cost. Zhao and G. H. Zhang (2011) establish three limitations in using PCMs in buildingmaterials: PCMs may interact with the building structure and change the properties of the buildingmaterials; leakage of PCMs could be a problem over the life time of the structure; PCMs had poorheat transfer coefficients in the solid state. An interesting method to solve these problems is theencapsulation of the PCM with a high thermal conductivity material.

According to Zhao and Zhang (2011), PCMmicro encapsulation solved the mentioned problems,and heat transfer rate raised significantly due to a much larger heat exchange surface was offered bymicrocapsules. According to Zhang et al. (2007), the PCM must be properly contained or encapsu-lated. This container should be able to resist change temperatures envisaged, possible chemical


attacks by the contained materials and the pressure imposed by the thermal expansion during solidto liquid phase change transitions.

Then, PCM encapsulation provides a barrier between the PCM and the heat transfer medium,which contributes to prevent corrosion (Deng et al. 2017; Jacob and Bruno 2015; Qian et al.2015) and to mitigate sub-cooling and segregation problems during thermal cycling (Salunkheand Shembekar 2012). In addition, it ensures the sustainability of the composition of PCM by redu-cing the possibility of reaction with the surroundings, improves thermal and mechanical stability andchemical compatibility with hazardous PCMs that cannot be exposed to surrounding such as build-ing temperature control, food storage and blood transport applications (Salunkhe and Shembekar2012; Khan, Khan, and Ghafoor 2016). Moreover, encapsulation of PCM improves heat transferrate by increasing the heat transfer area (Liu, Saman, and Bruno 2012). It has been shown thatthe transfer speed is indirectly proportional to the size of the EPCM (Alam et al. 2015). Therefore,the size of the vessel is an important parameter to consider for heat transfer rate enhancement.

According to Cabeza et al. (2011), in addition to maintaining liquid PCM and avoiding changes inits composition through contact with the environment, both micro and macro-encapsulationimprove the compatibility between the TES material and the environment, creating a barrier andmaintaining the PCM inside the capsule. Moreover, micro-encapsulation also improves heat transferand cycling stability.

Beside EPCM benefits, PCM thermal expansion during the solid–liquid transition can conduce toshell cracking or incomplete melting process as result of the high pressure inside the capsule.Authors confirm that this problem can be solved (Pitié, Zhao, and Cáceres 2011; Parrado et al.2015; Lopez et al. 2010) by leaving sufficient space inside EPCM capsule during its fabrication orby using high shell thickness to prevent deformation could be essential (Lopez et al. 2010; Blaneyet al. 2013; Mehling and Cabeza 2007).

3.2. EPCM fabrication methods

There are physical or chemical methods to encapsulate a PCM. The former deal with a large amountand rough surface encapsulation as compared to chemical techniques. Meanwhile the latter results inbetter heat storage capacity than that of physical methods. The most interesting methods reported inthe literature for EPCM fabrication are (Zakir Khan, Khan, and Ghafoor 2016):

. In-situ polymerisation: capsule is formed by polymerisation of monomers added to a reactor. Inthis processes generally is used two immiscible liquids and organic intermediates that react witheach other to establish a solid pre-condensate (Zhao and Zhang 2011). This method is preferablesince no reactive agents are added to the core material and polymerisation occurs exclusively inthe continuous phase (Giro-Paloma et al. 2016). It was reported that a smaller capsule size andexcellent shell structure can be achieved using this technique.

. Interfacial polymerisation: capsule is created by polymerisation of reactive monomers (Giro-Paloma et al. 2016). It involves dispersing an organic phase (containing poly-functional mono-mers and/or oligomers) into an aqueous phase (containing a mixture of emulsifiers and protectivecolloid stabilisers) along with the material to be encapsulated (Zhao and Zhang 2011). The mainadvantages of this technique are the high reaction speed, mild reaction course, and also that itsproducts have low penetrability (Giro-Paloma et al. 2016).

. Coacervation: In simple coacervation, a desolvation agent is added for phase separation. In com-plex coacervation, macromolecular colloid rich coacervate droplets surround dispersed microcap-sule cores and form a viscous microcapsule wall, which is solidified with cross-linking agents(Zhao and Zhang 2011). Despite coacervation, the main advantages are versatile and efficient con-trol of the particle size, aldehydes hardener, difficult to scale-up and agglomeration highlight asmain disadvantages (Jamekhorshid, Sadrameli, and Farid 2014).


. Spray-drying: EPCM is obtained by dispersing the material of the PCM in a concentrated solutionof the material until the desired size of the microcapsule is achieved. This emulsion is pulverisedinto droplets and dried to keep PCM inside the capsule (Giro-Paloma et al. 2016). Some advan-tages of this method are equipment and know-how widely available, versatile, easy to scale-up andlow-cost commercial process. Nevertheless, this drying process requires a high temperature.Moreover, agglomeration of particles or remaining uncoated particles can be produced.

. Suspension polymerisation: capsules are obtained by multiple simultaneous mechanisms such asparticle coalescence and break up secondary nucleation, and the diffusion of monomer to theinterface (Sánchez-Silva et al. 2010).

. Emulsion polmerisation: capsules are obtained by mixing the polymer in an oiled system and add-ing an emulsifier. Then, a water/oil emulsion is created to generate a crosslinked system (chemi-cally, thermally, or in an enzymatic way). The emulsion polymerisation is finished by washing theemulsion to remove the oil and to create the isolated microcapsules (Giro-Paloma et al. 2016).

Commercial companies have found different ways to encapsulate their PCM. Nevertheless, thepotential use of microencapsulated PCM in various thermal control applications is limited tosome extent by their cost (L. F. Cabeza et al. 2011). For this reason, several studies have been per-formed and techniques have been developed, in order to reduce fabrication costs and increase cap-sule characteristics, such as reaching decrease EPCM size and increase thermal conductivity. Noticethat the lower size of capsules presents easy application, good heat transfer due to the increased heatexchange surface and no need for protection against destruction as an advantage (Zhang et al. 2007).Among all the methods to encapsulate PCMs, in-situ polymerisation technique is preferred overother techniques, since it is possible to achieve a smaller capsule size and excellent shell structure.

4. Material selection

The materials used for numerical analyses are selected according to the criteria presented in Table 2.The categories of the key design characteristics of EPCM for energy storage purposes are classified bythermal, physical, chemical, kinetic and economic.

Notice that mechanical properties are not considered as key design characteristic (e.g. thermalstress resistance, Poisson ratio and Young modulus), since the low working temperature of buildingsTES significantly reduces the probability of mechanical problems. The selected materials and theirproperties are presented in Tables 3 and 4.

Among all the used criteria, the most important restrictions to select EPCM composition are:

4.1. Availability of materials properties information

According to Lizana et al. (Lizana et al. 2017), more than 250 PCMs are currently available for TESapplications in the temperature range of –10 to + 120°C. But, despite the number of availablematerials, some of their thermophysical properties like thermal conductivity, specific heat, density,

Table 2. Key design characteristics of TES materials for storage purposes (Khan, Khan, and Ghafoor 2016).

Thermal Physical Chemical Economic Environmental

Suitable phase-changetemperature.

Low volume changesduring phase transition.

Life cycle: reversible phasechange process.

Available in largequantities.

Recyclingpotential.

High latent heat of fusiondensity.

High density. Nonhazardous Low cost andcost-effective.

Low CO2 trace.Low or non sub-cooling. Chemical compatibility

shell-PCM.Non-pollutant.

High specific heat. Low vapour pressure.High thermal conductivityand good heat transfer.

Complete melting andthermal stability.

Non-flammable. Non-corrosive.

Low pressure at operatingtemperature.

Non-explosive elements/compounds.


etc. have not yet been fully assessed (Lizana et al. 2017). As consequence, until the latter mentionedproperties are estimated, and while the properties of the material are still not available, it is compli-cated to study the thermal energy storage of the EPCM using mathematical models.

4.2. Accurate ranges of temperature

Applications like house space heating require temperatures below 50°C, while applications like elec-trical power generation require high-temperature TES systems above 175°C [22]. According toLizana et al. (Lizana et al. 2017), accurate temperatures are up to 21°C for cooling applications,between 22°C and 28°C for comfort applications and over 29°C for hot water and heating appli-cation. If a solar collector is used as a sustainable source of energy, it must be considered that themaximum temperature reached by it is above 100°C. Therefore, an accurate temperature rangefor the TES system is 0–80°C, which is also suitable for building heating needs (Lizana et al.2017). This is the key criterion for our selection, considering that not all PCM fusion/crystallisationtemperatures are compatible with this range. Moreover, since the main objective of this work isfinding new types and configurations of EPCM for building applications, in order to validate theresults of this work they are compared with the results of paraffin wax PCM tests performed byTatsidjodoung et al. (Tatsidjodoung, Le Pierrès, and Luo 2013). The range of temperature usedon (Tatsidjodoung, Le Pierrès, and Luo 2013) is 25–75°C. In order to perform this validation, themelting point of the selected PCM of this work must be between 25°C and 75°C. Waqas et al.(Waqas, Ali, and Din 2015) studied the performance analysis of phase-change material storageunit for both heating and cooling of buildings. They concluded that the appropriate melting point

Table 3. Thermo-mechanical properties of solid shell material.

Material Specific Heat (J/kg·K) Thermal Conductivity (W/m·K) Density (kg/m3)

SteelStainless steel (AISI 4340)

SM 1 475a 44.5a 7850a

MetalCopper

SM 2 384b 401b 8960b

aSvante Littmarck and Farhad Saeidi (1986).bCáceres et al. (2017).

Table 4. Thermo-mechanical properties of PCMs.

TypeName or Formula Assigned ID

SpecificHeat (J/kg·K)

Latent Heat(kJ/kg)

ThermalConductivity(W/m·K)

Density(kg/m3)

MeltingPoint (°C)

Paraffinn-Octadecane (C18H38)

PCM 1 2140 (s)a

2660 (l)a244 0.35 (s)a

0.15 (l)a814 (s)a

774 (l) a28a

ParaffinParaffin wax (CnH2n+2)

PCM 2 2900 (s) b

2140 (l) b174c 0.16 (s) c

0.34 (l)c730 (s) c

916 (l) c64 a

Salt hydrateCaCl2·6H2O

PCM 3 2200 (s)d

1420 (l)d125 d 1.09 (s) d

0.53 (l) d1710 (s) d

1562 (l) e29 c

Salt hydrateNa2SO4·10H2O

PCM 4 1930 (s)d

2800 (l)d180 d 0.56 (s) d

0.45 (l) c1485 (s) d

1280 (l) f32 d

Eutectic compositionMg(NO3)2·6H2O NH4NO3

PCM 5 2130 (s)d

2670 (l)d125d 0.59 (s) d

0.50 (l) d1672 (s) c

1515 (l) e52 c

Eutectic compositionLiNO3 MgNO3·6H2O

PCM 6 2380 (s)d

2900 (l)d180 d 0.70 (s)d

0.51 (l)d1713 (s) d

1590 (l) e72 d

Note: (s): solid. (l): liquid.aJankowski and McCluskey (2014).bAgarwal and Sarviya (2017).cTatsidjodoung, Le Pierrès, and Luo (2013).dPereira da Cunha and Eames (2016)eZalba et al. (2003).fMa, Bao, and Roskilly (2017).


at which the performance of the storage unit was maximised for both seasons was found to be ∼27.5°C. Nevertheless, if PCM with a melting point of 29°C is used instead of 27.5°C, the cooling capacitywill improve by 15%, while the heating capacity will drop only by 3%. Therefore, the melting point ofthe PCM (∼29°C), which provided maximum cooling capacity during the summer season, could bealso used for winter heating. But the melting point of the PCM which provided (∼21°C) maximumheating capacity could not be used for summer cooling.

4.3. Accurate thermal conductivity

The thermal conductivity of EPCMs can be considered one of the most important parameters inthermal energy storage applications as well as phase change temperature and latent heat capacity(Sarı and Biçer 2012). Heat transfer rate is a vital factor to determine the charging/discharging vel-ocity of the thermal energy storage system and, therefore, the capacity to respond to energy demand,and enhancing thermal conductivity is an effective approach to improve it (Lin et al. 2017).

The performance of the TES systems depends on the properties of the TES materials chosen(Frazzica and Freni 2017). Then, regarding the use of metals as PCM in the range of temperaturesof interest, poor availability and very high cost limit their application in buildings despite having ahigh thermal conductivity (Tatsidjodoung, Le Pierrès, and Luo 2013), and volumetric fusion heat dueto their high density (Sharma et al. 2009). Nevertheless, metals highlight as shell material for EPCM(Zhu et al. 2018; Parrado et al. 2015).

Previous studies, e.g. Cáceres et al. (2017) and Cáceres et al. (2016), have shown that the use ofcopper as shell material enhances significantly the heat transfer rate of the charging and dischargingprocess due to their high thermal conductivity. Moreover, copper has also great mechanical proper-ties, which is a vital factor in order to maintain EPCMs structure. Nevertheless, the use of metals forcoating PCMs could produce physic-chemical issues (e.g. corrosion effects) if shell materials areincompatible with PCMs or heat transfer fluids (HTF). Therefore, the next criterion is crucial forTES material durability.

4.4. Compatibility between PCM and shell material

A crucial factor for the EPCM material selection is the shell and PCM compatibility at the workingtemperature range. PCM corrosion effects in TES systems can conduce to mechanical damage, loss oftechnically surface properties, reduction of material thickness, contamination of fluids and containerperforation.

Previous studies have been performed to measure the corrosion effects between metals and salthydrate. These studies recommend the use of stainless steel to contain CaCl2·6H2O (Abhat 1983;Porisini 1988; Solé et al. 2015; Khan, Khan, and Ghafoor 2016) and also the use of copper to containCaCl2·6H2O (Abhat 1983; Cabeza et al. 2001; Khan, Khan, and Ghafoor 2016). Regarding EPCMfilled with paraffin, studies concluded that paraffin-based materials are usually compatible withmetals. Hawlader, Uddin, and Zhu (2002), conducted thermal analyses and thermal cycle tests onmicroencapsulated paraffin and found that the microencapsulated paraffin still kept its geometricalprofile and heat capacity after 1000 cycles.

Copper belongs to the group of relatively noble metals. In regular air environment, it passivates,forming an external protective layer. In connection with its remarkable thermal properties, it is oneof the main reasons why Cu is so widely used in industry. Although in an environment containingchlorides, sulphates or nitrides, the above mentioned passive film may get damaged, leading to theinitiation of pitting corrosion. To prevent this from happening, inhibitors can be applied, such asheterocyclic, aliphatic, aromatic organic substances, a solution of purine (PU) and adenine (AD)(Scendo 2008).

On the other hand, the stainless steel possesses the content of elements, such as Cr, Ni, and Mo,which promotes good corrosion resistance (Kruizenga and Gill 2014). Even after long-term exposure


to molten salts, its mechanical properties do not deteriorate. The stainless corrosion test was per-formed with NaNO3 and KNO3 environment for more than 4000 h and 500 thermal cycles at amaximum temperature of 565°C. Despite the fact that the exposure for multiple cycles of thermalloading definitely increases the corrosion rate, the total metal losses ranged only 5–16 µm (Bradshawand Goods 2001).

4.5. Economic feasibility

The average cost of commercially available PCMs is approximately 7.78 USD/kg, ranging from 49.85USD/kg to 4.49 USD/kg for wholesale orders [1]. The feasibility of latent heat applications could beimproved if cheaper PCMs were available, as materials represent approximately 75% of system costs(Campos-Celador et al. 2014).

The material costs studied in this work are presented in Table 5. Regarding fabrication cost, sinceit is a recent innovation, most of currently EPCM based technology developments are at a laboratoryor low scale. Since laboratory or low scale costs are not suitable to estimate the commercial EPCMscost at industrial fabrication level, it is difficult to found accurate information of EPCM cost fabrica-tion. Nevertheless, there are a few firms developing EPCM as TES material, but even having contactwith these suppliers, materials prices vary depending on many other parameters, like space locationor market positions. In order to improve the investigation, fabrication costs should be studied infuture works.

4.6. EPCM durability

In general, two processes of enclosing PCM material inside the metal capsule can be considered(Muñoz-Sánchez et al. 2015). One of them involves inserting the PCM into a preformed shell,while the second concerns the development of the external layer coating the PCM. One of themost challenging issues affecting these processes is the capability of joining. Regarding the twopotential shell materials, AISI 4340 steel possesses good malleability, ductility and formability,especially in the annealed state. It can be welded using fusion or resistance welding methods.Also, Cu can be easily joined by welding and by soft or hard soldering. Therefore, in terms of joining,both materials can be successfully applied as shell materials that can be connected hermetically inorder to prevent the leakage of PCM material.

The capsule dimensions also influence the fatigue behaviour of the system, caused mainly by thecyclic thermal expansion of the PCM. As elaborated in the previous work (Gustavo Cáceres et al.2017), both Cu and steel alloy capsules should withstand more than 106 working cycles. Moreover,they are characterised by a similar elongation at break parameter (up to 22%) (make it from 2018).Although the melting point of AISI 4340 steel is (1690°C) higher than this of Cu (1085°C), they bothcan be readily applied even for high-temperature PCMs.

Table 5. PCM and SM costs.

Material ID Name Formula Cost (USD/m3)

PCM 1 n-Octadecane C18H38 n.a.PCM 2 Paraffin wax CnH2n+2 129.9PCM 3 Calcium chloride hexahydrate CaCl2·6H2O 67.05PCM 4 Sodium sulphate decahydrate Na2SO4·10H2O 262.6PCM 5 Ammonium and Magnesium nitrates hexahydrate

(eutectic composition)NH4NO3 Mg(NO3)2·6H2O

1002.9

PCM 6 Lithium nitrate and Magnesium nitrate hexahydrate (eutecticcomposition)

LiNO3 MgNO3·6H2O 628.56

SM 1 Stainless steel AISI 4340 15700SM 2 Copper Cu 62720


In terms of thermal conductivity, copper (401 W/m K) is nearly 10 times higher than for stainlesssteel (44.5 W/m K), what indicates that the performance efficiency of Cu capsules should be morebeneficial than this of steel capsules. Nevertheless, the economic aspect should be also considered,as Cu is also nearly seven times more expensive than AISI 4340 steel.

5. Methodology for EPCM modelling

This section presents the model used to calculate the thermal energy storage capacity and rate of thepreselected EPCM. This is a modification of the thermal energy storage model from Cáceres et al.(2017).

5.1. Geometry characterisation

The geometry used in this work is composed of two concentric spheres. The dimensions for theexternal radius (R), internal radius (r) and thickness (T = R− r) used for the main analyses ofthis work are R = 1.1, r = 1.0 and T = 0.1 mm. These values are selected considering previousstudies under ideal conditions.

Additionally, three dimensions are used to assess how shell thickness definition affects some par-ameters like the energy density, cost and heat transfer rate of EPCMs. For this analysis, an internalradius takes three different values (r = 1.0, r′ = 0.8 and r′′ = 0.6), while the external radius is main-tained equal to R = 1.1 mm. Consequently, three shell thicknesses are analysed (T = 0.1, T ′ = 0.3and T ′′ = 0.5).

5.1.1. EPCM boundary conditionThe initial temperature inside the sphere is T0 = 25°C, i.e. all the sphere of EPCM, except where theradius is equal to R. While the temperature on the boundary is TR = 75°C, i.e. only where the radiusis equal to R. The inside of the EPCM will increase its temperature from T0 to TR. It is important tomention that to use the latent heat of the PCM, its melting point must be within this range. At thecontrary, to keep the structure of the EPCM, the melting point of the shell must be higher than thetemperature range of heating, to maintain the shell in its solid state.

5.1.2. Assumptions:

. Thermal: The shell is considered homogeneous and isotropic. The density (r), specific heat (Cp)and thermal conductivity (k) are constant values. The PCM in the solid state is homogeneous.

. Mechanical: The shell has a linear elastic behaviour. For the PCM in a liquid state, the liquidpressure within the shell is uniform. The PCM is non-deformable in the solid state.

. Interfaces: For the external interface, i.e. interface between EPCM core and EPCM external ambi-ent, temperature balance and flux continuity is assumed. For the internal interface, i.e. interfacebetween the EPCM shell and EPCM core, temperature and pressure balances are assumed.

5.2. Heat transfer modelling

5.2.1. Shell modellingThe equation to describe the conductive heat transfer is:

rCp∂T∂t

+ ∇ · (−k∇T) = Q, (1)

where r is the density (kg/m3), Cp is the specific heat capacity at constant pressure (J/(kg·K)), T isabsolute temperature in (K), t is the time in (s), −k∇T is the heat flux by conduction in (W/m2),


which depends on the temperature and the thermal conductivity, and Q represents energystorage (J).

5.2.2. PCM modellingThe next equation describes the heat transfer with phase changes of the PCM:

reqCpeq∂T∂t

+ ∇ · (−keq∇T) = Q, (2)

where req, keq and Cpeq are the equivalent values of density, thermal conductivity and heat capacity,respectively, and Q represents the energy storage.

These equivalent values depend on the PCM portion in the solid phase (u), the density (r phase i),thermal conductivity (kphase i) and specific heat capacity (Cp,phase i) of each phase, where i is equal to1 for the solid state or equal to 2 for the liquid state:

req = ur phase1 + (1− u)r phase2, (3)

keq = uk phase1 + (1− u)kphase2, (4)

Cpeq = 1r(ur phase1Cp,phase1 + (1− u)r phase2Cp,phase2)+ L

∂am∂T

, (5)

where the equivalent heat capacity (Cpeq) represents the energy obtained from the sensible and latentheat, L is the latent heat, i.e. enthalpy of fusion or crystallisation, and am is a variable to describe theamount of the PCM under a melting process:

am = 12(1− u)r phase2 − ur phase1ur phase1 + (1− u)r phase2

, (6)

Thus,∂am∂T

is the variation of the fraction of PCM that is in its melting process as a function of thevariation of temperature.

6. Results and discussion

The results for the thermal behaviour during the heat charging process of the twelve EPCM combi-nations presented on Table 6 are analysed and discussed in this section. This analysis is focused onthe next parameters: (1) EPCMs composition, energy density and material cost, (2) energy densityand heat transfer rate comparison, (3) EPCMs thermal energy storage evolution analyses, including

Table 6. EPCMs composition, energy density and material cost.

EPCM Composition Volumetric Energy Volumetric MaterialID PCM SM density (GJ/m3) cost (USD/m3)

1 n-Octadecane C18H38 (PCM 1) Stainless steel (SM1) 0.302 n.a.2 Paraffin wax CnH2n+2 (PCM 2) Stainless steel (SM1) 0.300 602.023 CaCl2·6H2O (PCM 3) Stainless steel (SM1) 0.345 534.734 Na2SO4·10H2O (PCM 4) Stainless steel (SM1) 0.454 676.475 Mg(NO3)2·6H2O NH4NO3 (PCM 5) Stainless steel (SM1) 0.467 782.166 LiNO3 MgNO3·6H2O (PCM 6) Stainless steel (SM1) 0.496 1169.507 n-Octadecane C18H38 (PCM 1) Copper (SM2) 0.303 n.a8 Paraffin wax (PCM 2) Copper (SM2) 0.301 1878.229 CaCl2·6H2O (PCM 3) Copper (SM2) 0.346 2784.6510 Na2SO4·10H2O (PCM 4) Copper (SM2) 0.455 2619.4411 Mg(NO3)2·6H2O NH4NO3 (PCM 5) Copper (SM2) 0.455 2775.9612 LiNO3 MgNO3·6H2O (PCM 6) Copper (SM2) 0.498 3290.32

Note: Volumetric material costs are calculated using R = 1.1 mm and r = 1.0 mm. Volumetric material costs exclude individual fab-rication costs. Notice that volumetric energy density does not consider space between EPCMs.


temperature and thermal storage evolution, and (4) volume and cost comparison for storing 1 GJ ofenergy.

In addition, a comparison between EPCM and SHS materials is presented, and the benefits of theEPCM novel TES material configuration are quantitatively analysed. In addition, an analysis of shellthickness variation effects is performed, using the results obtained by decreasing the internal radius(r) dimension. Finally, a first approach to model validation is performed.

6.1. EPCMs composition, energy density and material cost.

The results show that the cost of that EPCM covered by stainless steel (602–1169 USD/m3)is considerably lower than the cost of that EPCM covered by a copper shell (1878–3290 USD/m3).

In addition, the volumetric energy density of each pair of EPCM filled with the same PCM is simi-lar. Moreover, the volumetric energy density is the same in some cases, e.g. EPCM 4 (composed byPCM 4 covered by stainless steel shell) and EPCM 10 (composed by PCM 4 covered by the coppershell) report 0.45 GJ/m3.

6.2. Energy density and heat transfer rate comparison

Energy density and heat transfer rate are two interesting parameters, mainly since they are directlyrelated to decrease TES system size and improve TES system efficiency, respectively. According toFigure 1, EPCMs placed on the upper right corner of the figure (i.e. ‘EPCM n’, with-n = 4, 5, 6, 10, 11, 12) account with the highest energy density and the highest heat transferrate and, therefore, they could be an attractive alternative for the TES system. At the contrary,those materials placed on the lower left corner present the worst behaviour. This is the case ofparaffin-based EPCMs (i.e. ‘EPCM n’, with n = 1, 2, 7, 8). As consequence, the best EPCMmust be EPCM 6 or EPCM 12, since they have the highest energy densities and are part of thegroup of EPCMs with the highest heat transfer rate (between 0.500 and 0.525 J/s). Nevertheless,this judgment can change considering other aspects, like material costs.

Results show that for each pair of EPCM filled with the same material as PCM (e.g. EPCM 1 andEPCM 7), the EPCM covered by stainless steel (represented by grey shell EPCMs in Figure 1) alwaysaccounts with higher thermal energy storage capacity than the EPCM covered by copper (rep-resented by orange shell EPCMs in Figure 1). Nevertheless, the difference between EPCMs composedby the same PCM is almost negligible. In fact, the TES capacity difference between ‘EPCM (n)’ and‘EPCM (n+ 6)’ is always smaller than 1% (n = 1, 2, 3, 4, 5, 6).

Figure 1. Energy density vs heat transfer rate comparison.


Notice that in order to present clearly the results by avoiding show superposed curves, in some ofthe further analyses, only the first 6 EPCM of Table 6 are included in the figures. Since the cost ofEPCM covered by copper is higher than EPCM covered stainless steel, but the thermal behaviour issimilar, from a technical-economical perspective, the usage of stainless steel shell is always better. Asconsequence, only results of PCMs covered by stainless steel (EPCM 1–EPCM 6) will be shown inthe following figures, in order to simplify the results presentation.

Considering this, only one shell is used for this analysis in order to present clearly the thermalenergy storage evolution of EPCM with different PCMs and to save computational time. Theshell material selected for this comparison is stainless steel (SM 1), mainly since its price is consider-ably lower than copper price, it also accounts with chemical compatibility with all PCMs, and despitecopper has higher thermal conductivity, they do not present a high difference in terms of heat trans-fer rate.

6.3. Thermal energy storage evolution

Table 3 shows the results of the thermal energy storage evolution of the first six EPCMs presented inTable 6. They are obtained by solving the mathematical model presented in the previous section.Figure 2 presents thermal energy storage evolution of these EPCMs during 15 s of the heating pro-cess, since this is the time required by all EPCMs to reach thermal stability.

According to the results presented in Figure 2 and Figure 3, despite EPCM 3 shows the faster heattransfer process, the energy storage capacity is lower than EPCM 4, 5 and 6. According to Figure 2,those EPCMs with eutectic compositions as PCMs (i.e. EPCM 5 and EPCM 6) present the highestthermal energy storage capacity at 15 s, followed by those EPCM with salt hydrates as PCMs (i.e.EPCM 3 and EPCM 4). Finally, the worst behaviour is presented by those EPCM with paraffin asPCMs (i.e. EPCM 1 and EPCM 2).

Moreover, it is possible to notice that EPCMs with the same PCM category (i.e. paraffin materials,salt hydrates or eutectic compositions) have a similar evolution, except for those EPCMs with salthydrate as PCMs (i.e. EPCM 3 and EPCM 4), which show a high difference in terms of TES capacity.

Figure 2. Thermal energy storage evolution comparison between EPCMs covered by stainless steel.


In fact, while EPCM 4 TES evolution is similar to eutectic compositions based EPCMs, EPCM 3 isnear to paraffin-based PCMs.

According to the results presented in Figure 2, EPCM 6 has the highest thermal energy storagecapacity, reaching 2.5 J per capsule of EPCM.

6.4. Volume and cost of EPCMs comparison

In order to improve the accuracy of EPCM selection, technical-economic parameters are used toselect the best EPCM option. The best thermo-economic options are those EPCM placed on thelower left corner of the figure, which means that this group of EPCMs can storage 1 GJ in a lowerspace at a lower cost than the other EPCMs. Considering material costs and volume results, pre-sented on Table 7 and compared on Figure 4, EPCM 4 is the best option, since between the materialsgroup with lower volumetric per GJ, EPCM 4 accounts with the lowest price. Notice that to calculatevolume and cost per GJ, it was calculated how many joules can be stored by each EPCM, considering5.5 × 10−9 m3 as EPCMs volume and 25–75°C as the working temperature range.

6.5. Comparison between EPCM and SHS materials energy densities

In order to measure the benefits of this novel configuration, a thermal behaviour comparison is per-formed between three different parameters:

Figure 3. Time of charging process comparison, using different EPCMs.

Table 7. EPCMs cost and volume per GJ.

EPCM ID PCM ID SM ID Cost of energy (USD/GJ) Volume of energy (m3/GJ)

1 1 1 n.a. 3.312 2 1 7270.73 3.343 3 1 4855.71 2.904 4 1 3648.36 2.205 5 1 4004.23 2.146 6 1 5375.92 2.017 1 2 n.a. 3.308 2 2 23126.29 3.329 3 2 18492.68 2.8910 4 2 14255.83 2.2011 5 2 14408.73 2.2012 6 2 15260.29 2.01


(a) LHS + SHS energy density of EPCM selected as the best option (EPCM 4), which is composed bythe combination of PCM 4 and SM 1. Notice that this energy density considers sensible andlatent heating of the PCM and sensible heating of the SM.

(b) SHS energy density of the core material of the EPCM selected as best options: Na2SO4 10H2O.Notice that the properties used are PCM 4 properties presented in Table 4. Notice that it is notconsidered its latent heat of fusion on this energy density calculation.

(c) SHS energy density of a common TES material used for building TES active system: Paraffinwax. Notice that the properties used are the PCM 2 properties presented in Table 4. Noticethat it is not considered its latent heat of fusion on this energy density calculation.

Figure 5 shows the thermal behaviour comparison between (a) LHS and SHS EPCM capacity ofthe best option selected: EPCM 4; (b) SHS of the core material of the best options selected: Na2SO4-·10H2O (PCM 4 properties at Table 4); (c) Energy density of sensible heat storage of a common TESmaterial used for building TES active system: Paraffin wax (PCM 2 properties at Table 4). Table 8shows the main comparison parameters of these materials.

Figure 4. Volume and cost to store 1 GJ of thermal energy comparison, using R = 1.1 and r = 1.0 mm.

Figure 5. Thermal behaviour comparison.


These results show that (a) and (c) show similar energy cost, even when (a) accounts more than 2times the volumetric energy density of (c). This occurs because of the volumetric material cost of (a)more than duplicate the cost of (c).

Finally, the use of SHS might be more convenient than the use of EPCMs as TES media dependingon the materials composition. This is due to the high cost of capsule materials. Then, EPCM could bealways more convenient than SHS materials if another type of cover material were used.

6.6. Thickness variation effects

In order to optimise the core/coat ratio, parameters like EPCMs heat transfer rate, energy storage andmaterials cost are used as selection criteria. Obviously, as the amount of shell material increases, theheat transfer rate also increases, due to the high thermal conductivity of the layer materials, as it ispresented in Figure 6. However, a thicker shell implies less amount of PCM and, therefore, the ther-mal energy storage capacity of the EPCM diminishes, as it is presented in Table 9.

Results show that it is possible to decrease the heat transfer process time by 40%, by increasingthickness three times. Nevertheless, it also means increase the material cost five times. Even more,an increment of 5 times the thickness implies a reduction of 70% of the time to complete theheat transfer process, but it also means increase the material cost five times. As consequence, it ishighly not recommended to increase the thickness of the capsule.

6.7. Mathematical model accuracy discussion

In order to pre-validate the model used in this work, the results obtained are compared with the lit-erature. Tatsidjodoung, Le Pierrès, and Luo (2013) review the thermal energy storage technologies

Table 8. Main parameters of TES materials comparison.

Case Materials Type of storageVolumetric energydensity (GJ/m3)

Volumetric materialcost (USD/m3) Energy cost (USD/GJ)

(a) PCM 4 / SM 1 SHS and LHS of PCM 4 / SHS of SM 1 0.45 1488.23 3648.36(b) PCM 4 SHS Na2SO4·10H2O 0.27 67.05 248.33(c) PCM 2 SHS paraffin wax 0.17 628.56 3697.41

Figure 6. EPCM 4 thermal energy storage evolution for different r and T values.


suitable for building applications with a particular interest in heat storage materials. The paper pro-vides an insight into recent developments on materials, their classification, their limitations andpossible improvements for their use in buildings.

According to Tatsidjodoung, Le Pierrès, and Luo (2013), 3.2 m3 of TES material are necessary tostore 1 GJ using paraffin wax as TES material. On the other side, considering the results of this paper,3.1995 m3 of TES material are necessary to store 1 GJ in the same paraffin wax.

Considering this difference, the model used presents a 0.015% of error. This is a useful infor-mation as first validation attempt, but experimental validation with concept tests at prototypelevel is expected to be included in future works.

7. Conclusions

The technical and economic feasibility of 12 EPCM combinations (presented in Table 6) is analysedand discussed in the present work. Considering the presented results, it is possible to conclude that:

(a) Regarding the SM discussion, the materials cost of those EPCM covered by stainless steel (602–1169 USD/m3) is considerably lower than the cost of those EPCM covered by a copper shell(1878 to 3290 USD/m3). In addition, the volumetric energy density of each pair of EPCMfilled with the same PCM are similar, or even the same. Therefore, regarding the shell materials,stainless steel is rather than copper.

(b) Regarding the PCM discussion, those EPCMs with eutectic compositions as PCMs present thehighest thermal energy storage capacity, followed by those EPCM with salt hydrates as PCMs.Finally, the worst behaviour is presented by those EPCM with paraffin as PCMs.

(c) Between all the EPCM filled with stainless steel, despite EPCM 5 and EPCM 6 present a higherenergy density than EPCM 4, they also represent an excessive cost increment. Therefore, the bestthermo-economic option is EPCM 4, composed by Na2SO4·10H2O as the phase change materialand stainless steel as the shell material.

(d) Regarding the LHS vs SHS discussion, the use of EPCM 4 can increase the energy density of TESmaterial in 34% and 78% compared to Na2SO4·10H2O and paraffin as SHS, respectively. There-fore, the thermal benefit of EPCM compared to SHS is notable. Nevertheless, EPCM usage isconvenient in some cases but not in all cases, due to the high cost of the core material.

(e) Regarding the thickness variation effects discussion, despite the increase in the heat transfer rate,it is not recommended to increase the capsule thickness, since increasing the thickness of theshell materials implies an excessive cost increment.

For future works, the cost reduction of the total TES system should be analysed, by adding man-ufacturing costs into the analysis, including low-cost shell materials. One of the main targets of thiswork is to analyse the technical and economic feasibility of using EPCM. Then, the target of futureworks should be on the assessment of the technical and economic feasibility of using EPCM in realbuilding applications.

Acknowledgements

The authors also would like to acknowledge to Krzysztof Naplocha and Anna Dmitruk.

Table 9. Parameter variation of EPCM 4 when different dimensions are used.

Dimensions Volumetric material Volumetric energy Time of thermal Cost of energyR r T cost (USD/m3) density (GJ/m3) stability at 75°C (s) (USD/GJ)

1.1 1.0 0.1 1488.23 0.45 10 3648.361.1 0.8 0.3 4330.58 0.30 6 14429.271.1 0.6 0.5 7172.94 0.23 3 30608.66


Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was supported in Chile by the projects CONICYT/FONDECYT/1151061, CONICYT/FONDAP/15110019(SERC-CHILE), ACCUSOL (ERANET-Lac, Cod. No. ELEC2015/T06-0523), Universidad Adolfo Ibáñez and byTHEMSYS Initiative (UAI-Earth).

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Abstract1. Introduction2. Phase change materials3. PCM buildings applications3.1. Encapsulated phase change materials3.2. EPCM fabrication methods

4. Material selection4.1. Availability of materials properties information4.2. Accurate ranges of temperature4.3. Accurate thermal conductivity4.4. Compatibility between PCM and shell material4.5. Economic feasibility4.6. EPCM durability

5. Methodology for EPCM modelling5.1. Geometry characterisation5.1.1. EPCM boundary condition5.1.2. Assumptions:

5.2. Heat transfer modelling5.2.1. Shell modelling5.2.2. PCM modelling

6. Results and discussion6.1. EPCMs composition, energy density and material cost.6.2. Energy density and heat transfer rate comparison6.3. Thermal energy storage evolution6.4. Volume and cost of EPCMs comparison6.5. Comparison between EPCM and SHS materials energy densities6.6. Thickness variation effects6.7. Mathematical model accuracy discussion

7. ConclusionsAcknowledgementsDisclosure statementReferences

Documents

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