Thermal Energy Storage With Phase Change Material—a State-Of-The

8/10/2019 Thermal Energy Storage With Phase Change Materiala State-Of-The

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Sustainable Cities and Society 10 (2014) 87100

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Sustainable Cities and Society

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Thermal energy storage with phase change materialA state-of-theart review

Dan Nchelatebe Nkwetta, Fariborz Haghighat

Department of Building, Civil and Environmental Engineering, Concordia University, Montreal,Quebec, CanadaH3G 1M8

a r t i c l e i n f o

Keywords:

Phase change materials

Thermal energy storageHotwater tank

a b s t r a c t

Recently, thermal energy storage (TES) has received increasing attention for its high potential to meetcities need for effective and sustainable energy use. Traditionally, energy was stored in the form ofsensible heat which requires large volume ofstorage material. The storage volume can be significantly

reduced if energy is stored in the form oflatent heat and thus can benefit enormously practical appli-cations. The existing approaches in the design, integration and application of phase change materials

(PCMs) in domestic hot water tanks (HWT) and transpired solar collector (TSC) using water/air as theheat transfer media are reviewed. Crucial influencing factors are considered, including thermo-physical

properties of different PCMs, different configurations of PCMs in HWT and TSC, and the limitations ofeach technique. This paper also discusses the existing simulation, design tools and experimental studies

related to PCMs usage in HWT and central thermal storage. 2013 Elsevier B.V. All rights reserved.

1. Introduction

Building sector contributes immensely to the total energy con-sumption, particularly for its space conditioning and domestic hotwater.Energy useand emissionsresult from both directsources (onsite use of fossil-fuels) and indirect sources (heating, electricity,

cooling and energy embodied in different construction materi-als). Prez-Lombard, Ortiz, and Pout (2008) reported that primaryenergy has grown by 49% and CO2emissions by 43%, with an aver-age annual increase of 2% and 1.8%, respectively. Based on the

International Energy Agency (IEA) reports on energy consumptiontrendsand promoting energyefficiencyinvestments, it is estimatedthat the building sector in developed countries is consuming over40% of the globalenergywith 24% of greenhouse gasemissions. The

growing peak demand of todays energy consumption for heatingor cooling contributes significantlyto a portion of utility-wide totaldemand and may lead more often to brown or black outs. During

peak energy demand periods, the cost of generating, distributingand maintaining electricity by the utilitycompanies is higher com-pared to non-peak periods (Agyenim & Neil, 2010). This cost islikely to increase due to the increase demand of improved thermalcomfort and emerging techniques such as electronic gadgets and

electric cars. Moreover, it is estimated that every day, over 2 mil-lion people immigrate to cities and thus more mega cities packed

Corresponding author. Tel.: +1 514 848 2424x3192; fax: +1 514848 7965.

E-mail addresses: [email protected], [email protected]

(F. Haghighat).

with densely high-rise buildings are needed to accommodate thispopulation.

The highly packed built urban environment influences the heatdissipation (Urban Heat Island) and pollution (Urban PollutionIsland) due to the reduction of airflow, city ventilation (Haghighat& Mirzaei, 2011). Impact of urban heat island (UHI) and urban

pollution island (UPI) on mortality rate and heat related diseasesare extensively addressed in the literature (Hayhoe, Sheridan,Kalkstein, & Greene, 2010; Kinney, ONeill, Bell, & Schwartz, 2008).Hajat, Kovats, Atkinson, and Haines (2002) reported an increase of

3.34% in death for every 1 C temperature increase above 21.5 C.This implies that cities are expecting more fatalities during heatwaves, and preparing urban-wide programs to confront before-hand prognosis solutions (Ng, 2009). Energy generated from fossil

fuels is extensively used in buildings for domestic hot water, spaceheating and/or cooling applications resulting in millions of tonsof carbon dioxide (CO2), climate change and related greenhouse

gas emissions.The growing concern aboutenvironmental problemsand the high costs of new power plants calls for new approachesto building technologies to stop this growth in electricity (ASHRAEhandbook-HVAC Application, 1991). This has promoted the needfor a reduction in CO2emissions via significant increase in energy

efficiency of buildings. To offset related greenhouse gas emissions,renewable energy sources must make a significant contribution toglobal energy production, storage and usage of which solar energyis a major contributor (Kalogirou, 2004a,b). Renewable energy

resources have massive energy potential but are not always fullyaccessible, can be diffused, or are regional, variable and intermit-tent. To sustain economic growth, issues relating to the supply

2210-6707/$ seefrontmatter 2013 Elsevier B.V. All rightsreserved.

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88 D.N. Nkwetta, F. Haghighat / Sustainable Cities andSociety10 (2014) 87100

and efficient use of energy must be addressed in the design of

low energy buildings and sustainable cities. Nkwetta, Smyth, Lo,and Mondol (2008) reported that future energy systems requiresome combinations of retrofitting and adaptiveness, since no sin-gle source of energy is capable of optimizing for all applications.

To achieve the European Union targets of 2020, it is necessary toencourage the retrofitting of existing buildings using local incen-tives andpolicies(DallO, Galante, & Pasetti, 2012). Dall et al.(2012)provided a methodologyfor evaluatingthe potential energysavings

of retrofitting residential building stocksand reported that by usingthe envelope retrofitting alone it is possible to reduce the energyused in the residential sector by up to 24.8% by 2020. Rosa, Cumo,Garcia, Calcagnini, and Sferra (2012) reported an energy consump-

tion reduction of 1416% resulting from corrective interventions onthe buildings using recycled non-toxic local materials and reducedenvironmental impact throughout their life cycle.

Gu,Sun, andWennersten (2013) reported that theexisting tech-

nologies forrenewableenergyare not yetsufficiently economicallyefficient and thus prevent the replacement of fossil fuels. They alsoreported it is impossible to generate enough energy using solartechnology at the local sites and thus the needs not to overlook

energy efficiency. They concluded that the two main contributorsto domestic energy consumption in the city are the household

and transport energy uses. They recommended that energy issuesshould not be considered as single element (notwithstanding the

high performance) but be considered from a systems perspec-tive point of view. This is because the high performance of singlesystems may not produce the same efficiency/performance whenintegrated and functioning as a system. The economical and suc-

cessful application of renewable energy technologies to improveenergy use and energy efficiency of buildings and offset relatedgreenhouse gas emissions depends on efficient energy storageoptions.

2. The need for energy storage

In regions with extreme weather conditions, a lot of variationsin energy demand and consumption are related to domestic hotwater demand, space heating and/or cooling applications and varydrastically from dayto night as well as seasonally. Changing energy

demand and consumption results in peak and off-peak energyusage, leading to variation in energy prices offered by majority ofthe utility companies with higher electricity rates being imposedduring peak-power demand (reflecting the cost of electricity pro-

vided during peak periods) compared to off-peak power demand(Agyenim & Neil, 2010; Lacroix, 1999). Wang and Yang (2012)reported on enhancing the intelligence of the multi-zone build-ing during its operations using particle swarm optimization (PSO).

They concluded PSO to be useful for maintaining the high comfortlevel in a building environment when the total energy supply is in

a shortage. The mismatch between the energy supply and energyconsumed and the need to store excess energy that would other-

wise be wasted as well as shifting peak power demand calls for theneed forthermal energystorage for different application areas (hotwater, space heating and air-conditioning). Thermal energy stor-

age (TES) systems enable greater and more efficient use of thesefluctuating energy sources by matching the energy supply to theenergy demand. This would greatly help to achieve a substantialreduction in fossil-based energy utilization and subsequent reduc-

tion in UHI and UPI phenomena, and would help in the designof sustainable cities. Two common methods of storing thermalenergy are sensible and latent heat storage. While the majority ofpractical applications make use of sensible heat storage methods,

latent heat storage such as phase change materials (PCM) provides

much higher storage density, with very little temperature variation

during the charging and discharging processes and thus proving to

be efficient in storing thermal energy.Domestic hot water is mostly provided using electric or gas

heaters which are simple, but they have very low efficiency ofenergyusage. Long andZhu (2008), reported that electric resistance

water heaters are convenient for both installation and operation,but their overall efficiency in converting energy of fossil fuels toelectric energy and then to thermal energy is quite low and alsoresults in tons of greenhouse gas emission. The performance of

water heaters depends mostly on the position and the numberof the thermal elements, energy delivery to the fluid stream, theinlet/incoming water temperature, the size and the aspect ratio ofthe tank, flow rate, and the location of the inlet and outlet of the

water heater(Bourke& Bansal, 2012). Themainchallengeisthatthesystem should be designed to be efficient, compact and econom-ical with minimum impact on the environment. TES systems arewidely used forbuilding applicationsand could be easilyintegrated

with a solar or a heat pump system, or be charged with purchasedelectricity during off-peak periods. Techniques to improve the per-formance of TES using PCM have been investigated and includeimproving heat transfer through the application of fins, enhancing

thermal conductivity, application of tube-in-shell TES, and applica-tion of micro-capsulation (Agyenim, Eames,& Smyth, 2009; Akgun,

Aydin, & Kaygusuz, 2008; Cabeza, Ibnez, Sol, Roca, & Nogus,2006a). Energystorage doesnot only improve the performance and

reliability of energy systems butplaysan importantrole in conserv-ing the energy and reducing the mismatch between energy supplyand demand.

2.1. Applications and advantage of phase changematerials (PCM)

inHWT

Water has been used and is currently being used as a storage

medium (sensible heat storage) in most of the low temperatureapplications. In such systems, as the energy is stored in the storagemedium,the temperature of the storage material (water) increases.Latent thermal storage on the other hand, in which energy is

stored in the material due to phase change, has attracted consid-erable interest in recent times due to its operational advantages.Hasnain (1998a,b) reported that thermal energy storage technolo-gies can play an important role in re-shaping patterns of electricity

use for both hot water, and space heating and cooling. He fur-ther highlighted that thermal storage systems can be applied inmost buildings with significant heating needs, and thus electric-ity rates can allow thermal storage to be competitive with other

forms of heating. Domanski, El-Sayed, and Jaworski (1994) andFuqiao, Maidment, Missenden, and Tozer (2002) reported that PCMthermalstorage technology, due to its high latent heat storage den-sity and compactness, allows for greater flexibility in choosing a

location for the storage system. Some of the operational advan-tages; smaller temperature swing between day indoors and night

outdoors, smaller size and lower weight per unit of storage capac-ity with high energy storage density were reported (Fuqiao et al.,

2002). Regin, Solanki, and Saini (2008) provided a detailed classi-fication of the phase change process and reported that the storagecapacity depends on the PCM latent heat value and specific heat

capacity.In addition, these systems are not only reliable and flexible but

can reduce electrical demand and utilitycharges, use less and or atleast no more energy than conventional systems, and cost no more

than non-storage systems. According to Hasnain (1998a,b) the eco-nomic aspect of TES in buildings is easily noticed where coolingdemands significantly contribute to high demand charges. In thephase transformation of the PCM, the solidliquid phase change of

material is of interest in thermal energy storage applications due

to the high energy storage density and capacity to store energy as


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D.N. Nkwetta, F. Haghighat / Sustainable Cities andSociety10 (2014) 87100 89

latent heat at constant or nearconstant temperature.In solidliquid

transformation, there is generally a small change in volume com-pare to solidgas and liquidgas transformations which occupylarge volumes with high latent heats. The large changes in vol-ume turn to make the phase changes not of interest due to the

complexity and large system sizes (Hasnain, 1998a,b; Ibrahim andMarc, 2002). A complete review of different types of phase changematerial used, their characteristics and classification, merits anddemerits as well as experimental techniques undertaken to investi-

gate the behavior of PCMs in bothmelting and solidification phaseshas been reported (Agyenim, Neil, Eames, & Smyth, 2010; Cabeza,Ibnez, Sol, Roca, & Nogus, 2006a,b; Cabeza, Castell, Barreneche,De Gracia, & Fernndez, 2011; Dincer & Rosen, 2003; Kenisarin

& Mahkamov, 2007; Mohammed, Khudhair, Siddique, & Razack,2004; Verma, Varun, & Singal, 2008). Cabeza et al. (2011) furtherhighlighted the need for the development of PCM containers tobe directed toward demonstration of physical and thermal stabil-

ity, since PCM must be able to undergo repetitive cycles of heatingand cooling. According to Agyenim, Neil, et al. (2010) a number ofcompanies like Cristopia, RUBITHERM, TEAP, Climator, MitsubishiChemical and EPS Ltd are also involved in the commercialization of

PCMs.

2.2. PCM heat transfer enhancement and PCM selection criteria

One of the greatest barriers to the wide application of mostPCMs is related to very low thermal conductivity, and thus need-ing improve heat transfer techniques to increase the charging anddischarging rates. The development of latent heat thermal stor-

age system involves the understanding of phase change materials,heat exchangers and PCM containers materials. The rate of charg-ing and discharging of the PCM storage system depends on thetype of heat exchanging surface (Hasnain, 1998a). He reported that

thermal conductivity of the heat exchanger container material andeffective thermal conductivity of the PCM are important param-eters in the selection and usage of PCM. The melted fraction ofthe PCM depends on these parameters (Hasnain, 1998a) and the

poor performances of heat exchangers lead to insufficient flow ofheat from the latent thermal energy storage systems (Agyenim,Knight, & Rhodes, 2010; Hasnain, 1998a; Sharma, Sharma, Buddhi,& Won, 2006). The heat transfer has been improved by developing

and enhancing the performance of different types of heat exchang-ers such as direct contact heat exchanger with immiscible heattransfer fluid in the PCM (Farid & Yacoub, 1989), double pipe heatexchanger (Fath, 1991) and classical double pipe or shell-and-tube

heat exchanger in cylindrical capsule (Domanski et al., 1994).The charging (melting) and discharging (solidification) of PCM

RT35 with five different heat exchangers as heat storage, two dif-ferent flow rates and two different water inlet temperatures were

experimentally investigated(Medrano et al., 2009). Theyconcludedthat in theturbulentregimeReynoldsnumbers arebetter/desirable

forfasterphase change processes since it reduces the phase changetime to about half. The rate of heat transfer is reduced by the for-

mationof the solidification layer on the heat exchanger surface andincreasing the heat transfer fluid (HTF) flow rate and decreasingthe HTF inlet temperature improves the solidification rate (Farid,

Khudhair, Razack, & Al-Hallaj, 2004). Increasing the driving force(water inlet temperature and PCM phase change temperature)decreases the phase change time and consequently, increases theaverage phase change power. Heat conduction problems are dif-

ficult to solve due to variable properties and moving boundaries(El-Dessouky & Al-Juwayhel, 1997; Hale & Viskanta, 1978, 1980;Hasnain, 1998a; Kerslake & Ibrahim, 1993). They concluded thatin analyzing heat conduction problems for actual system design,

periodic solutions are preferable compared to the present methods

(exact, integral, transient and purely numerical).

Additionally, the melting time and temperature distribution in

technical grade PCM was reported with conclusion for the needof a detailed heat transfer study regarding the qualitative andquantitative data for effective design of heat exchangers. Theseauthors suggested that heat transfer study involving the use of dif-

ferent types of heat carrying tubes with or without the types ofextended surfaces integrated to at least a prototype heat storageunit should be further investigated. Sharma et al. (2006) studiedthe effect of thermo-physical properties of heat exchanger con-

tainer materials on the thermal performance of the storage system.They concluded that an increase in thermal conductivity of con-tainer material results in a decrease in the PCM complete meltingtime. Also they reported that the thickness of heat exchanger con-

tainer material on the melted fraction of the PCM is in-significant,and the initial PCM temperature does not have significant effect onthe melted fraction of the PCM. In addition, it was reported thatthe boundary wall temperature plays an important role during the

melting process and has a strong effect on the melted fraction.Nallusamy, Sampath, and Velraj (2007) reported that the poor

heat transfer rate during charging and recovery processes makesthe latent heat storage systems not in commercial use as much

as sensible heat systems (SHS). They pointed out the fact that thesolidliquid interface drifts away from the convectiveheat transfer

surface during the phase change, increasing the thermal resis-tance of the growing layer of solidified PCM. The instantaneous

heat stored during the initial charging period/process was reportedto be high and turn to decrease with time due to temperaturedrop resulting from decrease in temperature differential betweenthe HTF and the storage tank (Nallusamy et al., 2007). However,

the major advantage of a combined storage system is that duringincrease and uniform charging and discharging process, the PCMstarts melting and the heat stored remains almost uniform (due toconstant temperature difference between the HTF and the storage

tank) for a longer period, which will be useful for many practicalapplications. They also reported that the mass flow rate has a sig-nificant effect on the average charging rate for the cumulative heatstored. Agyenim and Neil (2008) provided thermo-physical prop-

erties of PCMs with melting temperatures ranging from 50 to 60 Csuitable to store heat of fusion at a constant or near constant tem-perature. They further investigated the use of store energy from anair source heat pump to take advantage of off-peak electricity tariff

and concluded that, the heat exchanger system used was not goodenough to achieve the high heat transfer rate. Thus, new designsof heat exchanger to improve heat transfer rate and temperature isimperative but still to be developed and extendedly tested.

El-Sawi, Haghighat, and Akbari (2013) investigated numericallythe effect of convective heat transfer on the melting rate of PCM ofa thermal performance of centralized latent heat thermal energystorage (CLHTES) system. They also through a parametric study

investigated the effect of the temperature, PCM phase change tem-perature range, and the temperature difference of the incoming

air and PCM melting temperature on thermal performance of theCLHTES. Zhou and Zhao (2011) and Tian and Zhao (2013) reported

that metal foams are considered to be a promising solution to theheat transfer enhancement of PCMs due to their excellent physicalcharacteristics like relatively high thermal conductivity, ultra-light

isotropicstructures with porosity in the range of 8597% and muchmore continuous matrix to easily transfer the heat to PCMs. Tianand Zhao (2011) reported the effect of different metal foams onthe heat transfer in PCMs. They pointed out, at the two-phase

zone and liquid zone, metal foams have larger flow resistance thatsuppressedthe natural convectionin PCMs. Theynevertheless, con-cluded that the rate of heat transferred through the metal foamsolid structure to the whole domain of PCMs was faster and the

overall heat transfer performance of the PCMmetal foam sample

was still superior compared to that of the pure PCM samples (thus


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the enhancement of heat conduction offsets or exceeds the natural

convection loss).Mehling and Cabeza (2008) reported that a very common prac-

tical situation is that the charging and discharging time of the PCMis usually limited and the heat needs to be absorbed or released

quickly. The heat transfer fluid rapidly transfers the heat to thePCMwhena storage systemis charged withonlya single-stagePCMwhich reduces the temperature differential between the PCM andheat transfer fluid due to reduced temperature of the heat transfer

fluid. The poor heat transfer result in the PCM melting rapidly atthe entrance part where the heat transfer fluid enters the storageand more slowly at the end of the storage where the heat transferfluid outflows and thus recommended the use of cascaded ther-

mal energy storage as a solution. They also reported that duringthe charging process in cascaded thermal energy storage, PCMswith lower melting temperature can be placed at the end of theheat exchanger. As such, the temperature differential can be large

enoughto ensureall thePCMsare melted.During thedischarge pro-cess, cascaded thermalenergystorage works efficiently andhas thepotential to solve the problem of PCM at the end of the storage notlikely to be used for latent heat storage since the temperature of

heat transfer fluid rises (Mehling and Cabeza, 2008). Michelsa andPitz-Paal (2007) reported a more uniform heat transfer fluid outlet

temperature during the discharging process and higher portion ofthe PCM are likely to run through the phase change process for cas-

caded latent heat storage compared to the traditional single-stagestorage system for parabolic trough solar power plants. They alsoreported higher energy utilization efficiency for cascaded thermalenergy storage compared to the traditional single-stage thermal

energy storage system.Tian and Zhao (2012) reported higher energy efficiency and

exergy efficiency of up to 30% and 23%, respectively for cascadedthermal energy storage compared to the traditional single-stage

thermal energy storage system. Furthermore, they reported thatmetal foam-enhanced cascaded thermal energystorage can furtherincrease heat transfer rate and the exergy transfer rate of cascadedthermal energystorage by 27timesand thus reduced melting time

by 6787%. The increase heat transfer rate of 27 times dependingon the properties of the metal-foam samples used with higher poredensity and lower porosity likely to achieve a better performance.Li-Wuet al.(2013) reported that the narrow temperaturevariations

(nearly isothermal) of PCMduring melting can help protectthe tar-geted devices from overheating since the PCMs serve as an energybuffer. This PCM energy buffer is capable of extracting heat fromthe hot spots on the devices before it can be dissipated efficiently

to the surroundings.Al-Hinti et al. (2010) reported the effect of water withdrawal

pattern on the performance of the PCM or the water temperaturein the tank. On using the water draw off pattern, the average water

temperature in the SHS tank drops from 71C to 51 C, while thePCM temperature drops by only 12 C from72 C t o 6 0 C (Al-Hinti

et al. , 2010). Based on the temperature differential between thetwo systems, they reported that the heat transfer rates from the

PCM to the water were higher compared to the SHS. Furthermore,they reported the water temperature in the tank to be at 44 C,12h later, and only 7 C less than the starting temperature follow-

ing the complete discharge. They concluded that the temperaturerecovery pattern andreduced temperature drop is attributed to thecontinuous release of heat from the PCM to the surrounding water.They also investigated the hot water withdrawn from the system

according to the pattern to simulate suggested domestic day-timeconsumption for a small family.

They concluded that withdrawal pattern has a limited effect onthe PCM or the water temperature in the tank due to the natural

stratification of water in the storage tank driven by density dif-

ferences. Furthermore, the withdrawn batches were taken during

the time of the day where solar radiation was available with the

inlet temperature to the tank from the collector plates being higherthan the temperature of the discharged water from the bottom ofthe tank. The effect on the average water temperature was notsignificant resulting from relatively small volume of water with-

drawn compared to the total volume of water in the tank (Al-Hintiet al., 2010). They concluded that day-time consumption of mod-erate amounts of hot water from the storage tank on sufficientlyspaced time intervals when solar radiation is available, does not

adversely affect the final water temperature or the overall perfor-mance of thesystem. In addition, theydemonstrated that in cases ofextreme consumption during evening hours, the existence of PCMcanpartially recover thetemperature ofwater,and thus resultingin

extending the effective operational time of the system. Research ondifferent PCMs (Agyenim, Knight, et al., 2010; Agyenim, Neil, et al.,2010; Sharma, Tyagi, Chen, & Buddhi, 2009; Zalba, Marin, Cabeza,& Mehling, 2003) have concluded the following selection criteria to

be the most important in selecting and using the PCM:

Easy availability in large quantities and low cost. Possess high latentheat offusionper unit mass resultingin higher

amount of energy storage with smaller amount of material. Melting point of the PCM can be selected to match the system

desired operating temperature range. High specific heat to provide additional significant sensible heat

storage effects. Non-flammable, non-explosive and non-poisonous. No or very small volume and temperature changes during the

transition. Chemically stable with no chemical decomposition and corrosion

resistance to construction materials.

2.3. Latent heat storage (PCM) in HWTwith water as a transfer

medium

Water is commonly being used as theheat transfer fluid in manyapplications. Waterheaters have considerable thermal energystor-

age that can be used to manage the power demand of electricalgrids (Lacroix, 1999). The use of HWT with PCM as storage hasthe potential to store energy during off-peak periods and to bere-used during peak periods, thus reducing or shifting peak load

demand (Nallusamy et al., 2007; Sharma et al., 2006) as well asreducing costs (capital investments related to peak power genera-tions for the utility companies and thus less expensive services forthe customers).

Different methods have been proposed for adding PCM into hotwater tanks such as adding PCM elements inside a standard waterstorage tank (Esen & Durmus, 1998; Mehling, Cabeza, Hippeli, &Hiebler, 2003) as well as adding of the PCM into the solar collec-

tor loop (Rabin, Bar-Niv, Korin, & Mikic, 1995). Adding PCM to thestorage tank would improve theavailability ofhot water to theend-

user due to more energy storage at the top surface and re-heatingof the top layer after a period of discharge as well as resulting in

smaller storage volume compared to sensible heat storage.Dermott and Frysinger (1979) and Kamimoto, Abe, Sawata, Tani,

and Ozawa (1985) reported that the high storage density of PCM

with small difference in temperature change at different phasetransformation can be of merits in the use of waste heat and forsolar application. Paraffin has been used as storage materials dueto its availability in large temperature range, safety, reliability,

cost and non-corrosiveness. However, only technical grade paraf-fin which are chemically inert and stable below 500 C, show littlevolume changes on melting and have low vapor pressure in themelt form may be used as PCMs in latent heat storage systems

(Sharma et al., 2006). Paraffin however shows some undesirable

properties like:low thermal conductivity, non-compatible with the


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Fig. 1. Common configurations of HWT with PCM (a) HWT with PCM modules (Cabeza et al., 2006b) (b) A cross-sectional view of a cylindrical heat storage tank combine

with PCM (Sharmaet al., 2009).

plastic container and moderately flammable which needs to be

partly eliminated by slightly modifying the wax and storage unit(Sharma et al., 2006).

Nallusamy et al. (2007) reported the temperature variation ofHTF during continuous and batchwise discharging processes for

both SHS system and combined storage system. They concludedthat batchwisedischarging of hotwateris advantageousfor theSHSsystemsince the wateroutlet temperatureremainsalmost constantat 70 C throughout the process, whereas in the case of continu-

ous discharging process the water outlet temperature decreasescontinuously withtime and it is suitable forlimited practical appli-cations. They concluded that thecombined use of sensible and LHS

results in correcting the disadvantage of variation in water outlettemperature experienced in the conventional SHS system. Cassedy(2000) reported that the use of PCM such as paraffin for thermalenergy storage at temperature of 50100 C proves to be chemi-cally stable with no corrosion. However, they concluded that the

systems may not be cost effective since the cost of the systemsintegrated with PCM almost doubles the cost of hot water sys-tems. Improving and retaining heat at the upper portion of thehot water tank (HWT) is critical since the hot water is with-drawn

from the upper part of the storage tank. The addition of PCM mod-ules into theHWT result in many advantages (Barba & Spiga, 2003;Cabeza et al., 2006b; Canbazoglu, Sahinaslan, Ekmekyapar, Aksoy,& Akarsu, 2005; Kousksou, Bruel, Cherreau, Leoussoff, & El Rhafiki,

2011; Nallusamy et al., 2006; Sharma et al., 2006, 2009; Sozen,

Vafai, & Kennedy, 1991) including:

Systems having much higher storage density in the upper por-

tion/layer of the HWT. Re-heating of the transitional layers being after partial withdraw

of the hot water. Time taken to heat or re-heat the water in the HWT will be

reduced due to the presence of the heat from the PCM. Increasesthe degreeof thermal stratification in theHWT andthus

effective in increasing peak demand shift, energy conservationand load management.

Improves the efficiency of storage as intermediate heat can beused to heat the colder lower layers.

Thermal stratification creates very low heat transfer in the ver-

tical axis and thus storing heat for longer periods of time at theupper part of the tank.

It is understood that HWT integrated with PCM is meritorious.

However, a good mastery of the temperature requirement of thesystems, melting and solidifying temperatures of PCMs, costs andcharacteristics of container carrying the PCM with heat exchangereffectiveness as well as tank configuration is imperative. Also, to

realize the full economic benefits of PCM in hot water tanks, thesystem must be well designed and sized based on the need of the

application.

2.3.1. Configuration of HWTwith PCM, modeling, simulation and

experimental study

Residential,commercial and industrial buildings arehigh energyconsumers for hot water and space heating and cooling require-ment.Differentconfigurations of HWT with PCMareavailable intheliterature (Cabeza et al., 2006b; Kousksou et al., 2011; Nallusamy

et al., 2006; Sharma et al., 2009). Fig. 1 shows two examples of themost common configurations.

Modeling, simulation and experimental studies have been usedfor investigating the integration and performance evaluation of

hot water tanks with phase change material. Bony and Citherlet(2007) developed a PCM model, using the TRNSYS type of water

tank storage (Type 860) for different shapes (plates, cylinders andspheres) and numbers of PCM modules in the tank. They carried

out comparisons of measurements and simulations results to vali-datethe model, taking into account the sub-cooling, hysteresis andconvection of theliquid part of the PCM and reported a good agree-

mentbetween themonitoreddata andsimulation results. They alsoreported the need to reduce the simulation time. Talmatsky andKribus (2008) developed a mathematical model which describesthe heat storage tank with PCM, collector, pump, controller and

auxiliary heater.They carried out annualsimulations for different sites, load pro-

files, different kinds of PCM and volume fractions, and concludedthat the use of PCM in the storage tank does not yield a signifi-

cant benefit in energy provided to the end-user. They supported

their results by thefact that there is increase heat losses during the


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night times resulting from the reheating of the water by the PCM.

They also reported that when the system is operating not close tothe melting point, the advantage of the latent heat is diluted by thelarge amount of energy stored as sensible heat and thus innovativedesign for the use of PCM is still to be further investigated.

Domestic electrical hot water cylinder incorporating encap-sulated phase change material (PCM) in 57 vertical pipes wasinvestigated (Cabeza et al., 2011). They used a validated numericalmodel to optimize the PCM distribution inside the water cylinder

under different hot water demand scenarios. They concluded thatusing PCM in electrical HWT allows the use of low cost electricityduring low peak periods and increases the thermal energy storagecapacity of the cylinder. It was further reported that, even though

the amount of waterinside the tank is reducedbecause of the tubescontaining the PCM, the PCM system resulted in stored heat by thePCM being rapidly realized to the water. This heat further provideslonger period of hot water availability during the first discharge

and the off-peak electricity was sufficient to fully melt the PCM inall the evaluated cases (Cabeza et al., 2011). Additionally, the sys-tems with the PCM have higher hot water discharge capacity withdemand coverage increasing from 40% to 55% in one case. Cabeza

et al. (2011) concluded that, the systems having large number ofsmall tubes canprovide hotwaterfor a longerperiod of timeduring

the initial discharge. However, the system will provide hot waterfor a limited period of time after the initial discharge. They high-

lightedthe fact thatpartof the heatstoredby thePCMis releasedtothe water during this first discharge resulting from the high area ofheat transfer between the tubes and the water. It was highlightedthat thesystems with less tubesof larger diameter store theheatin

the PCM for posterior demands. They however concluded that thePCMdistribution inside the tank must be defined depending on thetiming and quantity of hot water demand.

Cabezaet al.(2005, 2006a) presented themodelingof a domestic

hot water tank with a PCM module using TRNSYS, TYPE 60PCM.Fig.2 illustratesdifferentelements taken into account in each of thenode of the water tank and used for the energy balance equation(Cabeza et al., 2006a).

To model the one-dimensionally stratification temperature inthetank, Cabezaet al.(2006a) assumed that a stratified water-filledsensible energy storage tank consisted of N fully mixed equal vol-ume segments and determined the degree of stratification. They

investigated the re-heating and cooling effect of water surround-ing the PCM in a hot watertank and concluded that thePCM-watertank re-heated the water surrounding the PCM module when thewater temperature is lower than the PCM temperature. Regarding

the cooling effect, they reported that the layer not in contact withPCM module cools down faster compared to water at the upper

Fig. 2. Schematic representation of energy flows into a node (Cabeza et al., 2006a).

layer. In their analysis, the temperature sensor at the upper layer ofthe PCM-hot water tank showed a very similar pattern to the PCMtemperature curve with an increasing temperature due to latent

heat of the PCM during phase transition. They also investigated theeffect of time steps (6min, 3 min and 1 min) on the performance ofthe PCM. The conclusion from their investigation was that the timestepis a critical factor in evaluatingthe performance of PCMs in hot

water tank.In addition, they highlighted that the shorter the time step,

the more precise and accurate the simulation results. They com-pared themeasuredand simulatedtemperature valuesat theupper

layer of the storage tank and reported a good agreement. However,PCMs-hot water tank design optimization is still to be investigatedfor different climatic applications and installations to improve theperformance of the system. According to Cabeza et al.(2006b) inte-

gration of PCM in the hot water tank for a single-family systemin Lleida, Spain, increased the solar fraction by up to 8%. Mehling,Cabeza, Hippeli, and Hiebler (2002, 2003) investigated the perfor-mance of a PCM-module at the top of a stratified hot water tank.

They investigated two PCM modules using different melting tem-peratures compared to water based storage tank as illustrated in

Fig. 3. They reported that the advantages of the stratification werenot destroyed in the tank with PCM. Instead, the addition of a

PCM module at the top of the water tank resulted in higher stor-age density, allowing reheating of the transition layer after partialunloading and compensation of heat loss in the top layer for aconsiderable time.

Cabeza et al. (2006b) reported on an experimental evaluationof HWT with granular PCM compound (90%) with graphite (10%)added to the top of the storage tank to increase heat transfer rate.The overall aim was to evaluate the behavior of a PCM in a HWT

Fig. 3. Heat storeswith different temperature levels: left, waterbased; right, PCM based(Mehling et al., 2003).


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8/14


Table 1

Summary table for PCM-hot water tank (modeling and experimentation) reported in literature.

Publications/refs. Objectives Method used Conclusions

Sozen et al. (1991) Thermal energy storage characteristics of both

sensible (1% carbon-steel) and latent heat

storage packed bed.

Experimental Better p erformance f or t he l atent h eat s torage p acked

bed.

Barba and Spiga (2003) Behavior of encapsulated PCM with threedifferent geometrical configurations.

Experimental Spherical capsules y ielded t he l argest energy d ensityand themost rapid chargeand release times.

Canbazoglu et al. (2005) To compare PCM-hot water tank (sodium

thiosulfate pentahydrate) with conventionalhotwatertank.

Experimental A 2 .593.45 t imes p erformance e nhancement i n

storage time of hot water, mass of produced hotwaterand total heat accumulated in the tank with PCM.

Cabeza et al. (2005,

2006a,b)

Modelisation of a domestic hotwatertank

with a PCM module.

HWT with PCMmodules to evaluate the

behavior of a PCM in a HWT under real life

conditions to increase theperformance of the

system via enhancingthe storage performance.

TRNSYS, TYPE 60PCM

Experimental and

numerical

Validation of TRNSYS, TYPE 60PCM, theshorter the

time step, themore precise andaccuratethe

simulation results.

PCM re-heated the water surrounding the PCM module

faster, increase in temperature dueto phase change of

thePCM, increasing quantity of thePCM increases

energy density.

Jos et al. (2005) Performance of PCMhot water tank for asingle-family system.

Experimental The addition of PCM in the storage tank i ncreased thesolar fraction, improvement in energy storage and

performance of thehot watertank. Improve the

availability of hotwaterto theend-userand reheating

of thetop layer after a period of discharge.

Esen and Durmus (1998) PCMelementsinside a standard water storage

tank, PCM-module at thetop of a stratifiedhot

water tank.

Experimental Improvement i n energy s torage a nd p erformance o f

the hotwatertank,improve theavailability of hot

waterto theend-userand reheating of the toplayer

after a period of discharge.

Rabinet al.(1995) PCM into the solar collector Experimental The addition of PCM in the solar collector results in

improvement in energy storage and performance of

the hotwatertank,Improve theavailability of hotwaterto theend-userand reheating of the toplayer

after a period of discharge.

Cassedy (2000) Use of PCMsuch as paraffin forthermal energy

storage at temperature of 50100C.

Experimental Proves t o be c hemically s table w ith n o corrosion b ut

may notbe cost effectivesince the cost of thesystems

with PCM almost doubles thecost of hotwater

systems.

Dermott and Frysinger

(1979) and Kamimoto

et al.(1985)

PCM f or w aste h eat a nd s olar a pplications. Experimental High s torage d ensity o f PCM with s mall d ifference i n

temperature change at different phase transformation

can beof merits in the use of waste heatand for solar

application.

Mehling et al.(2002, 2003) PCM-module at thetop of a stratifiedhot watertank.

Experimental andnumerical simulation

Improvement in energy storage and performance ofthe hotwatertank.

Esen and Durmus (1998) Effects of thermal and geometric parameters

on themelting time of differentPCMs with

different tank configurations, thermalperformance of solar water heating systems

with cylindrical latent heat storage units

containing several PCMs.

Theoretical/numerically The whole PCM melting timedependsnotonlyon

thermal andgeometric parameters,but also on the

thermo-physical properties of the PCM.

Bedecarrats et al. (1996)

andSaitoh and Hirose

(1995)

A tank with pipes containing thefluid being

embedded in thePCM and spherical container

with PCMcompared to hot watertank.

Numerical and

experimental

Tank with PCM show better performance compared to

hotwater tank.

Long and Zhu(2008) Analysis of air sourceheat pump waterheaterwith PCM storage.

Numerical/experimental Heatpump water heaters with PCM using off-peakelectricity is much more effectivethan

electric-resistance heaters, needing smaller space

compared to air-source heat pump with heaters in hot

watertanks. It canbe concluded that present work

could provide guidelines for airsource heat pump

waterheater with PCMfor thermal storage.

Bony and Citherlet (2007) To develop a PCM model, using theTRNSYS

type of water tank storage (Type 860) for

different shapes (plates, cylinders and spheres)

andnumbersof PCMmodules in thetank,comparisons of measurementsand simulationsresults.

TRNSYS (Type 860) Validated the potential of the proposed model, taking

into account the sub-cooling, hysteresis and

convection of theliquid part of thePCM.

Talmatsky and Kribus

(2008)

To develop a model which describes the heat

storage tank with PCM, collector, pump,

controller and auxiliary heater.

Mathematical model in

MATLAB

Conduction and convection occurs. Concluded that the

use ofPCMin HWT does not yield significant benefit in

energy provided to the end-user.

Dincer and Rosen (2003) To evaluate the performance of thermal energy

storage systems using theconcept of exergy

analysis.

Based on secondlaw of

thermodynamics

Gives a closervaluein assessing and comparing the

thermal performance of energy storage systems.

Cabeza, Castell, et al.

(2008)and Cabeza, Sol,

et al.(2008)

To evaluate natural convection heat transfer

coefficients in phase change material (PCM)

modules with external vertical fins.

To compare study of twohot water storagetanks (one without PCMand onewith PCM).

Experimental-exergetic

and energetic concepts

Theuse of external finsin PCM modules reduces the

time neededto transfer theheat to thesurrounding

waterand theheat transfer process was directly

proportionalto thesize of thefins.IncludingPCM in the second coil heat exchanger

located at the upper portion of thetank,increase the

ratio of heat transfer surface to PCM volume compared

to cylindrical PCM modules.


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Table 1 (Continued)


Nallus amy eta l. (2006) Toinves tigate the performanceofp ackedb ed

latent heat thermal energy storage integrated

with solar water heating system.

Parametric and

experimental

They concluded that theuse of packed bed latent heat

storage reduces thesize of thestorage tank and batch

wise dischargeof hotwater from the tank is best

suitable for intermittent demand of hot water.

Long and Zhu (2008) To evaluate theuse of fin in thermal energystorage systems.

Experimental The use of fin improves the conductive coefficient of solidification and melting process.

De Gracia et al. (2011) To evaluate performance of domestic HWT

with PCMin 57 verticalpipes.

Experimental The use of PCM in HWT allowsthe use oflow cost

electricity during low peak periods and increasesthermal energy storage capacity.

Porisini (1988) To evaluate corrosion resistance of metallic

alloysto hydrated salts used as PCM & thermal

performance reliability of PCM after repeated

thermal cycling.

Experimental The use of stainless steel was the most

corrosion-resistant alloy to selected hydrated salts

with melting points rangeof 1532 C.

Li etal. (2011) Preparation, structure and thermal energy

storage property of capricpalmitic

acid/attapulgite form-stable PCM.

Experimental They c oncluded t hat t he C apric a cid a nd p almitic a cid

canbe absorbed uniformly into attapulgite and there

existed no chemical reaction between the

capricpalmitic acid and attapulgite

corrosion of the containercarrying the PCM, has also been of inter-est. Porisini (1988) reported on the corrosion resistance of metallicalloys to hydrated salts used as PCM for heat storage and the ther-mal performance reliability of PCM after repeated thermal cycling.

He concluded that stainless steel was the most corrosion-resistantalloy for selected hydrated salts showing good thermal stabilityafter repeated thermal cycling. Cabeza et al. (2002) carried outa corrosion resistance on different metals including copper and

aluminum in contact with molten salt hydrates. They concludedthat copper was resistant to calcium chloride and not to sodiumacetate while aluminum in contact with chlorides resulted in theformation of Al(OH)3 and was prone to localized pitting corro-

sion and resistant to sodium acetate. Farrell, Norton, and Kennedy(2006) reported that one of the draw backs of PCMs for thermalenergy storage is corrosion when they are in direct contact withmetal piping, plates or housings. They concluded that a conven-

tionalaluminum fin expanded copperheatpipeheat exchangerwillexperience galvanic corrosion and localized pitting corrosion whenimmersed in the PCMs. They further highlighted preventive meas-

ures for corrosion in copper and aluminum material and reportedthat theeffects of galvanic corrosioncan be reducedby using a largealuminum anode surface and a small copper pipe cathode.

Paraffin waxes have beenused in many applicationsdue to theirchemical stability, non-poisonous, no phase separation with only

a small change in volume during phase transformation with negli-gible degree of sub-cooling and excellent thermal stability (lack ofeffect of the cycleson its properties) as well as degrading of thermalbehavior due to contact with metals (Banaszek et al., 2000; Liu &

Chung, 2001; Neeper, 2000; Py & Mauran, 2001). Li, Wu, and Kao(2011) reported on the preparation, structure and thermal energystorage property of capricpalmitic acid. They concluded that thecapric acid and palmitic acid can be absorbed uniformly and there

existed no chemical reaction between the capricpalmitic acid and

PCM. A summary table for PCM-hot water tank (modeling andexperimentation) reported in literature is provided in Table 1.

2.4. The use of air as the transfer medium

Transpired solar collectors (TSC) using air as the heat transfermedium are used to provide the space heating requirement and

have proven to be highly efficient for pre-heating fresh air. TSC arecommonly classified as standalone or building integrated, glazedor unglazed one-pass or double-pass system with a perforatedabsorber layer.

2.4.1. Functioning of TSC

The perforated absorber surface is generally a metallic sheet

(usuallysteelor aluminum), which canbe integratedto thebuilding

facade and or roof integrated. The perforated absorber surface ofthe TSCs is heated using solar energy, which transmits thermalenergy to the ambient air. There is a cavity between the build-ing envelope or the back plate and TSC through which the heated

air is drawn into the building to provide the space heating. Toavoid over heating of the building during summer periods, a by-pass damper is used to help vent the heated air from the cavityto the exterior of the building. Shukla et al. (2012) reported on

different configuration of TSC and their performances, and theyconcluded that the flowrate, wind velocity,absorptivityand poros-ity are the most critical factors affecting the efficiency of thesecollectors. They further reported that integrated systems have sig-

nificant advantages over non-integrated systems such as an add-onto the building envelope which enhances durability of buildingenvelope and reduces the balance of system cost (BOS). The sys-tem is ideal to be used in the buildings in combination with other

technologies like PV, heat pump and PCM storage. Hybrid collec-tors (PV/T) have been investigated by different authors (Anderson,Duke, Morrison, & Carson, 2009; Delisle & Collins, 2007) and con-

cluded that thetechnology is attractive dueto thelow cost andhighefficiency. However, PCM integration potential with PV/T is still tobe investigated.

The use of air as heat transfer fluid in solar air collectors withPCMstorage hasshowninterest.Thishas resulted from thefact that

the stored heat from the mounted TSC with PCM can be releasedin to the buildings at night helping to extend the effective oper-ation time of the system. Also non-integrated systems often havehigher maintenance cost compared to integrated systems (Shukla

et al. , 2012). Goyal, Tiwari, and Garg (1998) carried out researchon the thermal energy storage with air collectors and classifiedthe air collectors based on their application as shown in Fig. 4.Stritih and Novak (2002) reported on the promising performance

in using stored heat from PCM storage in solar wall for heating and

ventilation.

2.4.2. Configuration,modeling, simulation and experimental

study of solar air collectors with PCM storage

Morrison and Abdel-Khalik (1978) developed a model forstudy-ing transient behavior of PCM and performances of solar heatingsystems using air and liquid as transfer medium. In conjunctionwith simulation techniques, they determined that the performance

of heating systems with latent and sensible heat storage dependson the storage size, collector quality and location.

The model developed by Morrison and Abdel-Khalik (1978) wasfurther extended by Vakilaltojjar and Saman (2001) to include a

PCM storage unit for space heating and cooling integrated with a

reverse cycle air conditioning system. The model was made-up of


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Air Heaters

Non-porous

With storageWithout storage

Latent heat

(PCM)

Porous Sensible heat

SandGround

Water

Concrete Br ick

Fig. 4. Classification of solar air heaters.

Adopted from Goyal et al.(1998).

several parallel layers of PCM slaps with air flowing through the

passage between the PCM slabs. In their analysis, they ignored thesensible heat and assumed heat transfer to occur only at a con-stant melting temperature with initial PCM temperature assumedequal to melting point. They reported that maximum sensible heat

transfer occur once the PCM melted completely with melting pointof 29 C and inlet air temperature of 40 C, respectively. Using a

two dimensional model, they reported that heat transfer occurringin the vertical direction resulted from the temperature difference

betweenthe airflowalong thesurface ofthe PCMand thePCM layer.In addition, the horizontal heat transfer was due to variation in

temperature of PCM along the horizontal plane directions.Jurinakand Abdel-Khalik (1979) presented an empirical method for sizing

PCM storage unit for solar air collectors at different climatic loca-tion during the heating period. They concluded that optimizationof solar air collector with PCM storage is necessary to achieve anybenefits. Belusko, Saman, and Bruno (2001) reported on the devel-

opment of mathematical models and a full scale prototype systemfor a roof integrated solar heating systems with PCM storage. ThePCM used was calcium chloride hexahydrate and they concludedthat the system ensured maximum use of solar energy, absorbed

and stores energy during the day and released the stored energy atnight to heat the living space.Numerical analysis of PCM thermal storage unit with melting

and freezing at varying wall temperature was reported by Halawa,

Bruno, and Saman (2007). They realized a very low heat transferrate during the final period of melting or freezing resulting fromsmall temperature differential between the air and the PCM. Dur-ing the heating mode, the heat stored/released during the final

period may not be useful due to an unsuitable air temperaturedelivered by the thermal storage unit. They concluded that for aproper design of such systems (PCM thermal storage with vary-ing wall air temperature), the following factors must be carefully

taken into account: range of outlet and inlet temperature and airflowrates, type ofPCMs,the match betweenenergy stored/releasedand comfort requirement.

A small unit of solar wall air collector with thermal energystorage option and building ventilation system was investigatedby Sokhansan and Schoenau (1991) and a payback period of 45years was reported. Tyagi et al. (2012) reported on the compar-ative experimental study of a typical solar air heater collector

with and without temporary heat energy storage (THES) material.Their evaluation based on energy and exergy analyses, found thatthe efficiencies in case of heat storage material/fluid are signifi-cantlyhigherthan that without THES.They further reported slightly

higher efficiencies in case of paraffin wax compared to hytherm oil.A seasonal thermal energy storage using paraffin wax as a PCMand flat plate solar air collectors was used in heating a green-house (Hseyin, 2005). He reported average net energy and exergy

efficiencies of 40.4% and 4.2%, respectively, and thus showing a

large difference (36.2%) in terms of energy and exergy efficien-

cies. He further concluded that exergyefficiency being the measureof the quality of energy is more significant and correctly reflectsthe thermodynamic and economic value of the storage operationthan energy efficiency and should be considered in the evaluation

and comparison of thermal energy storage systems. The need ofdesign and operational parameters in order to optimize the ther-

modynamic efficiency of thermal energystorage systemsas well asimproving the exergy efficiency by reducing exergy loss and aux-

iliary energy consumptions during the charging and dischargingprocesses was highlighted.

Hseyin and Aydn (2009) reported the analytical and experi-mental performance analysis of phase change material employed

to analyze the transient thermal behavior of the PCM storage unitduring the charge and discharge periods for greenhouse heating.The conclusion was that the solar air collector integrated with PCMcreated a 69 C temperature differential between the inside and

outside of the greenhouse, providing about 1823% of total dailythermal energy requirements of the greenhouse for 34h, in com-parison with the conventional heating device. It is requested thatfurther research be conducted on latentheat storage for greenhouse

applications and modeling of the heat storage systems to help inoptimizing the management of the heat storage systems. A sea-sonal thermal energy storage using paraffin wax as a PCM to heat agreenhouse of 180 m2 floor area was reported by Hseyin (2005).

An average daily rate of thermal exergy transferred and stored inthelatent heatstorage (LHS) unitwere111.2 W and 79.9 W, respec-tively.with average net energyand exergy efficiencies of 40.4% and4.2%, respectively.

Experimental results and two-dimensional theoretical mathe-matical model of PCM to analyze the transient thermal behaviorof the storage unit during the charge and discharge periods wasreported by Saman, Bruno, and Halawa (2005). They concluded

that the warm air from the roof integrated air collector is circu-lated through the spaces between the PCM layers (charging thestorage unit) and the stored heat is used to heat the ambient air

before being admitted to a living space. Within the living space asignificant warming effect is perceived during the initial periods ofdelivering air to the living space during the heating mode and thusconcluded that it is advantageous from the thermal comfort pointof view.

Alkilani, Sopian, Sohif, and Alghol (2009) examined indoor per-formance predicationfor outputair temperature resulting from thedischarge in a solar air collector with PCM storage with the goal toabsorb and store the solar energy. Their system illustrated in Fig. 5

consisted of a single-transparent glazed solar air collector, isolatedductand integrated PCMstorage unit. The storage unit was dividedinto a single row of cylinders containing the PCM with the cylin-ders placed in the cross flow of forced air stream. The paraffin wax

(PCM) with a mass fraction of 0.5% aluminum powered was used


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Table 2

Summary table forair collectors with PCMstorage reported in literature.


Morrison and Abdel-Khalik

(1978)

To studying transientbehavior of PCMand

performances of solar heating systems using

airand liquid as transfer medium.

Model and simulation

techniques

The storage size, collector quality andlocationhave an

effect on theperformanceof heating systems.

Jurinak and Abdel-Khalik(1979)

To size PCMstorage forsolarair collectors atdifferent location during the heating period.

Empirical The size of the PCM storage and solar collectors determinetheenergy collection and density of PCM storage canlead

to small storage tanks.

Sokhansan and Schoenau(1991)

Theuse of small unit of solar wall aircollectorwith thermal energy storage option and

building ventilation system.

Experimental The payback period was 45 years.

Vakilaltojjar and Saman

(2001)

To study a PCM storage unit for spaceheating

and cooling integrated with a reverse cycle air

conditioning system.

Model Heat transfer occurred in the vertical (due to temperature

difference) between airflow along thesurface of PCMand

PCMlayer andhorizontal (due to variation in temperature

ofPCM along the horizontal plane) directions.

Belusko et al.(2001) To study performance of full scale prototype

systemfor a roof integrated solar heating

systems with PCM storage.

Mathematical and

Experimental

The systemensures maximum usage of solar energy

absorbed.

Stritih and Novak (2002) To store heat from PCMstoragein solar wall

for heating and ventilation.

Experimental Promising p erformance i n the u se o f store h eat f rom P CM

storage in solar wall forheating and ventilation.

Alkilani et al.(2009) Indoor performance predication for output air

temperatureresulting from thedischarge in a

solar aircollector with PCMstorage.

Experimental Freezing t ime o f the P CM was i nversely p roportional t o the

mass flowrate.

Krishnananth and

Murugavel (2012)

Performance of a double-pass solar air

collector integrated with parrafin PCM storage

with aluminum capsules.

Experimental The s ystem d elivered higher temperature a ir t hrought o ut

theday with higherefficeicny recorded in theevening

hours.

Fatah (1994) To predict theperformanceof a solar air

collectorintegrated with PCMstorage with a

copper tubes absorber filled with PCM.

Experimental System s howed b etter p erformance o therwise t he d aily

average efficiencywas 26%and 63%.

Enibe (2002) Evaluated theperformanceof a singleglazed

natural convection solar air heater integrated

with paraffinwax as PCMstorage.

Experimental Peak a ir t emperature r ise o f 15 K with p eak c umulative

useful efficiencyof close to 50%. Thesystem is suitablefor

useas solar cabinet crop dryer formedicinal plantsand

aromatic herbs.

Enibe (2003) Transient thermal analysis of a natural

convection solar airheater integrated with a

paraffin type phase change material (PCM).

Experimental Day-long m aximum p redicted c umulative u seful a nd

overall efficiencies being 13 and 18%, respectively.

Tyagi et al.(2012) To compare experimentally a typical solar air

heatercollector with andwithout heat energy

storage.

Experimental The e fficiencies i n c ase o f h eat s torage m aterial/fluid are

significantly higher than that without THES.

Hseyin and Aydn (2009) To analyze thetransient thermal behavior ofthestorage unit duringthe chargeand

discharge periods for greenhouse.

Experimental Providing a bout 1 823% o f total d aily t hermal e nergyrequirementsof thegreenhouse for34h.

Samanet al.(2005) To analyze thetransient thermal behavior of

PCM storage unit duringthe chargeanddischarge periods.

Experimental/Theoretical

(mathematical)

Within theliving spacea significant warming effectis

perceived duringthe initial periods of delivering airto theliving space.

Kousksou et al.(2007) To understand the behaviorof thesystem

using eithersingle or multiplePCMs.

Numerical Performance of the latent thermal storage system can be

improved by thejudicious choice of themelting

temperatureof thePCM andthe use of multiple PCMs can

reduce the irreversibility inside the storage tank.

Halawa et al.(2007) To understand the analysisof PCM thermal

storage unit with melting andfreezing atvarying wall temperature.

Numerical Low heat transfer rate during the final period of melting or

freezing resulting from small temperature differentialbetween the airand thePCM.

Hseyin (2005) A seasonal thermal energy storage using

paraffin wax asa PCM and flat plate solar air

collectors in heating a greenhouse.

Experimental Reported a verage n et e nergy a nd e xergy e fficiencies o f

40.4%and 4.2%, respectively and thus showing a large

difference (36.2%) in terms of energy and exergy

efficiencies.

Fig. 5. Single-pass solar airheater integrated with PCM unit (Alkilani et al., 2009).

to improve the heat transfer with the conclusion that the freezingtime of the PCM was inversely proportional to the mass flow rate.

Krishnananth and Murugavel (2012) investigated the experi-mental performance of a double-pass solar air collector integratedwith parrafin PCM storage with aluminum capsules. They con-cluded that the solar air heater with paraffin as storage delivered

higher temperatures air throught out the day with higher effice-icny recorded in the evening hours. Three different option; PCMcapsule above absorber plate, PCM capsule below absorber plateand PCM capsule above back plate were further evaluated exper-

imentally by Krishnananth and Murugavel (2012) and conlcudedthat the systems with the PCM capsule placed above the absorberplate was the most efficient. Fatah (1994) researched on the per-formance of a solar air collector integrated with PCM storage with

an absorber made up of copper tubes filled with PCM with melt-ing temperatures of 51 and 43C. He concluded that this system


12/14


showed better performance otherwise the daily average efficiency

was 26% and 63%.Enibe (2002) contructed and evaluated the performance of a

single glazed natural convection solar air heater integrated withparaffin wax as PCM storage (under real life conditions). He

reported peak air temperature rise of 15K with peak cumulativeuseful efficiency of close to 50% and conlcuded that the system issuitable for use as solar cabient crop dryer for medicincal plants,aromatic herbs andother crops not needing exposure to direct sun-

light. Enibe (2003) reported on the transient thermal analysis of anatural convection solar air heater consisting of a single-glazed flatplate solar collector integrated with a paraffin type phase changematerial (PCM) energy storage subsystem and a rectangular enclo-

sure serving as theworking chamber. He concluded that the timeofpeak temperatureand overall temperatureprofiles for the absorberand heat exchanger plates,the heated air, and glazing surface werewithin 10 C. A maximum predicted airflow rate of 0.01kg s1, cor-

responding to a maximum inlet velocity of 0.33m s1 with theday-long maximum predicted cumulative useful and overall effi-ciencies of 13 and 18%, respectively were also reported. Table 2 is asummary of air collectors with PCM storage reported by different

authors in the literature.

3. Conclusion

The applications in which PCMs can be used are many and thusneeding different PCMs to be critical analysis. This review paperpresents different configurations, modeling, simulation and exper-

imental studies conducted forPCMs hot water tanks andTSC withPCMs storage. It helps to clarity the factors and most importantselection criteria to be taken into account when selecting and usingdifferent PCMs. PCMs usage has the potential to improve the stor-

age capacity, serve energy as well as shifting and smoothing peakpower demand when integrated for use in hot water tanks andspace heating.

Despite the merits reported by different researchers regarding

the use of PCM in HWTs, some few authors concluded in their find-ings that the use of PCM in HWT does not yield significant benefitin energy provided to the end-user and the systems may not becost effective since the cost of the systems integrated with PCM

almost doubles the cost of hot water systems. However, innova-tive system design, reduction in heat losses and further testingof such systems are to be further investigated. From a practicalapplication need, the effect of boundary wall temperature on the

melting process and melted fraction of the PCM modules in hotwater tanks has been given little attention due to the use of com-mercial aluminum bottles. Higher energy utilization efficiency andexergy efficiency of up to 30% and 23%, respectively, were been

reported for cascaded thermal energy storage compared to thetraditional single-stage thermal energy storage system. However,

further investigation with cascaded thermal energy storage in hotwater tanks is required.

Moreover, there is lack of clear selection criterion for the quan-tity of PCM to be included in the PCM modules of the thermalenergy storage for system optimization as well as complete cost

analysis of such systems. It is desirable to select the TES basedon the following criteria: cost, efficiency, environmental impact,life cycle cost, safety, and the required space. Further researchis needed to develop a procedure for comparison and to further

assess accurately the performance of these systems in real lifeconditions with different usage pattern under different climaticconditions. Transpired solar collectors with PCM storage have thepotential to improve the total daily thermal energy and reduce

energy mismatch between energy generation and use. However,

optima selection and design criteria to be taken in to account are;

PCMwith higher latent heat, improve collector quality andlocation

of the collector as they determine the energy collection and den-sity of PCM storage which can further reduce the size of the storagetanks and associated system costs. Optimal design and integrationreduces heat losses. Integrated transpired solarcollectors with PCM

storage systems have significant advantages over non-integratedsystemssuchas anadd-onto thebuildingenvelopewhichenhancesdurability of building envelope and reduces the balance of systemcost (BOS).

However, national and international test standards and com-monly acceptable modeling tools for different applications withPCMs are still not widely available making it difficult for com-parison to be made. To reduce and or solve this problem, model

development and integration, national and international test stan-dards for different applications with PCMs should be harmonized,made compatible to each other and widely available. Additionally,suitable design of heat exchangers to improve the heat transfer

rates, details on PCM stability and corrosion of PCM containersfor long term use, systematic approach in the design, integrationand evaluation of PCM integrated in HWT, used with solar air col-lectors and other storage tanks for space heating is to be further

investigated, in particular to large scale real life applications. Theimpact of urban residences on energy consumption and carbon

emissions concluded that the two main contributors to domes-tic energy consumption are the household and transport sectors.

Furthermore, energy issues should not be considered as single ele-ment notwithstanding the high performance as this may not beefficient but should be considered from a systems perspective.The optimization, control and automation of these systems and not

elements/components are further needed.

Acknowledgments

Theauthorswill like to appreciate theFQRNTfor providingfund-ing to the first author for a post-doctoral fellow during which thisresearch is conducted, and the Public Works and Government Ser-

vices Canada for its support.

References

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