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Applied Thermal Engineering 37 (2012) 208e216

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Applied Thermal Engineering

journal homepage: www.elsevier .com/locate/apthermeng

Fatty acid esters-based composite phase change materials for thermal energystorage in buildings

Ahmet Sarı a,b,*, Ali Karaipekli a,b

aDepartment of Chemistry, Çankırı Karatekin University, 18200 Çankırı, TurkeybDepartment of Chemistry, Gaziosmanpasa University, 60240 Tokat, Turkey

a r t i c l e i n f o

Article history:Received 14 April 2011Accepted 7 November 2011Available online 22 November 2011

Keywords:Fatty acid estersComposite PCMDiatomiteExpanded perliteThermal propertiesThermal reliability

* Corresponding author. Department of Chemistry60240 Tokat, Turkey. Tel.: þ90 356 2521616; fax: þ90

E-mail addresses: ahmet.sari@gop.edu.tr, asari@go

1359-4311/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.applthermaleng.2011.11.017

a b s t r a c t

In this study, fatty acid esters-based composite phase change materials (PCMs) for thermal energystorage were prepared by blending erythritol tetrapalmitate (ETP) and erythritol tetrastearate (ETS) withdiatomite and expanded perlite (EP). The maximum incorporation percentage for ETP and ETS intodiatomite and EP was found to be 57 wt% and 62 wt%, respectively without melted PCM seepage from thecomposites. The morphologies and compatibilities of the composite PCMs were structurally character-ized using scanning electron microscope (SEM) and Fourier transformation infrared (FTeIR) analysistechniques. Thermal energy storage properties of the composite PCMs were determined by differentialscanning calorimetry (DSC) analysis. The DSC analyses results indicated that the composite PCMs weregood candidates for building applications in terms of their large latent heat values and suitable phasechange temperatures. The thermal cycling test including 1000 melting and freezing cycling showed thatcomposite PCMs had good thermal reliability and chemical stability. TG analysis revealed that thecomposite PCMs had good thermal durability above their working temperature ranges. Moreover, inorder to improve the thermal conductivity of the composite PCMs, the expanded graphite (EG) wasadded to them at different mass fractions (2%, 5%, and 10%). The best results were obtained for thecomposite PCMs including 5wt% EG content in terms of the increase in thermal conductivity values andthe decrease amount in latent heat capacity. The improvement in thermal conductivity values of ETP/Diatomite, ETS/Diatomite, ETP/EP and ETS/EP were found to be about 68%, 57%, 73% and 75%, respectively.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

The energy demand to provide a comfortable environment forhumans in buildings has continuously increased worldwide. But,the energy use for heating, cooling and air conditioning increasethe level of greenhouse gas emissions and decrease fossil fuelsources [1,2]. Therefore, energy storage becomes a key issue inengineering application. Thermal energy storage plays an impor-tant role in an effective use of energy in buildings not only byreducing the mismatch between supply and demand but alsoimproving the performance and reliability of energy systems.Among all of thermal energy storage methods (sensible, latent andthermochemical heat), latent heat thermal energy storageemploying a phase change material (PCM) is particularly effectivetechnique due to its advantages of high energy storage density andits isothermal operating characteristics [3e6].

, Gaziosmanpasa University,356 2521585.

p.edu.tr (A. Sarı).

All rights reserved.

The application of PCMs in buildings is well known and has beensubject to considerable interest since the first reported applicationin the 1940s [7,8]. Researches on the application of PCMs inbuildings have been focused on three fields in recent years. The firstone is the reduction of temperature fluctuations of lightweightbuildings by increasing their thermal mass [9e11]. This is done byincorporation of PCM into building materials. The second one is thecooling of buildings through intermediate storage of cold from thenight or other cheap cold sources. If the cold is for free, as with coldfrom night air, this is also called free cooling and very promisingwith respect to energy saving [12,13]. A third application field is forheat storage in space heating systems [14].

The PCMs can be used by integration with different building’sstructure such as gypsum board, plaster, concrete, clay minerals orother wall-covering material. But, there are some difficulties infabrication of building materials containing PCMs. One of them isincorporation of PCM in construction material. PCMs in buildingmaterials are usually enclosed in metallic or polymeric capsules.The encapsulation of the PCM is expensive and it may affect themechanical strength of the building material as well as it may lead

A. Sarı, A. Karaipekli / Applied Thermal Engineering 37 (2012) 208e216 209

to seepage during the melting period of PCM. Therefore, it isneeded to direct heat exchange between PCM and medium toprovide higher thermal energy storage performance. From thispoint of view, the PCM based-building composites are promisingmaterials since they enables no corrosion, quick heat transfer andoffers a large heat storage density if building materials with highporosity are selected [15,16].

Many inorganic and organic PCMs and their mixtures as thermalenergy storage materials have been studied recently for impreg-nating into common building materials [12,17e23]. Among thePCMs, fatty acids, their esters andmixtures as organic phase changematerials has been recommended as energy storage materials dueto their desirable thermal and heat transfer characteristics and theadvantage of easily impregnation, or directly incorporation intoconventional building materials [24e26]. Moreover, most of thefatty acid esters are commercially available, since they are alreadyproduced in large amounts for plastics, cosmetics, textile and otherindustries. However, only few ester compounds were used toobtain composite PCMs for energy storage in building applications[27e29].

In this study, some fatty acid ester-based composites werefabricated as novel potential PCMs for thermal energy storage inbuilding applications. These composite PCMs were prepared bydirectly incorporation of erythritol tetrapalmitate and erythritoltetrastearate as fatty acid esters in diatomite and expanded perliteas building materials. The composites were characterized struc-turally by SEM and FTeIR analysis techniques. Thermal energystorage properties and thermal durability of the composite PCMswere determined by using DSC and TG analysis methods, respec-tively. Moreover, the thermal conductivities of prepared compositePCMs were improved by adding expanded graphite (EG) at massfraction of 5%.

2. Experimental

2.1. Materials

Erythritol tetrapalmitate (ETP) and erythritol tetrastearate (ETS)used as PCMs in this study were synthesized by using the methodreported in our previous study [30]. The diatomite sample wassupplied from BEGeTUG Industrial Minerals & Mines Company(Istanbul, Turkey). Expanded perlite (EP) was supplied from IzperCompany (IzmireTurkey). The chemical composition of the diato-mite and EP samples used as porous building material in prepara-tion of the composites are given in Table 1. Perlite and diatomitesamples were previously sieved by 150 mm-mesh sieve and driedat 105 �C for 24 h before use. Expanded graphite (EG; thermalconductivity: 4.26 W/m K; assay: �99.99%; particle size: <45 mm)was supplied from the Fluka Company.

2.2. Preparation of the composite PCM

The ETP/diatomite, ETS/diatomite, ETP/perlite and ETS/perlitecomposite PCMs were prepared by using direct impregnationmethod [31,32]. The composite PCMs were prepared by directlymixing the fatty acid ester in melted state with the building

Table 1Chemical compositions of the diatomite and the expanded perlite (EP).

Material Constituent (ratio %)

SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O

Diatomite 92.8 4.2 1.5 0.6 0.3 e e

EP 71.0e75.0 12.5e18.0 0.1e1.5 0.5e0.2 0.03e0.5 2.9e4.0 4.0e5.0

material directly. To determine the maximum holding ratios of thebuilding materials for ETP for the fatty acid ester compounds,a series of composites with different mass fractions of estercompounds (10, 20, 30, 40, 50, 60, and 70% w/w) were prepared.The composite PCM was simultaneously heated during theimpregnation process at a constant temperature above the meltingtemperature of the ester compound in order to test the exudation offatty acid ester from the porous spaces. The ETP and ETS used asPCM could be retained as 57 wt % in diatomite and 62 wt % in EPwithout the leakage of melted PCM and therefore these compositeswere defined as form-stable composite PCMs.

2.3. Analysis methods

Themorphology of prepared ETP/diatomite, ETS/diatomite, ETP/perlite and ETS/perlite composite PCMs were observed using a SEMinstrument (LEO 440 model). The chemical compatibility betweenthe components of the composites was investigated by FTeIRspectroscopy technique. FTeIR spectra were taken on a KBr disk ata frequency range of 4000e400 cm�1 by using a FTeIR spectro-photometer (JASCO 430 model).

Measurements of solideliquid phase change temperatures andlatent heat capacities of composite PCM were carried out by usinga DSC instrument (PerkineElmer Jade model) under nitrogenatmosphere and at a heating rate of 10 C/min. The maximumdeviations in the measurements of phase change temperatures andlatent heat values are found to be�0.3 �C and�3.6 J/g, respectively.Certified Indium standard was used as a reference for temperaturecalibration of the instrument. Samples were measured in a sealedaluminum pan with a mass of about 5.0 mg. Melting point andfreezing point were obtained by drawing a line at the point ofmaximum slope of the DSC peak. The latent heat was calculated bynumerical integration of the peak using software of DSC instru-ment. The thermal durability of the composite PCMs was alsodetermined by using PerkineElmer TGA7 thermal analyzer. In eachcase, the about 10 mg specimens were heated from 25 to 500 �Cusing a linear heating rate of 10 �C/min under nitrogen atmosphere.In addition, to increase the thermal conductivities of the preparedcomposite PCMs, expanded graphite (EG) was introduced into themat mass fraction of 2, 5 and 10 wt%. Thermal conductivity values ofall composites were measured by using a KD2 thermal propertyanalyzer.

2.4. Thermal cycling test

Thermal cycling test was performed to determine thermal reli-ability of the composite PCMs in terms of the change in phasechange temperatures and latent heat values after a large number ofthermal cycling. The test was carried out consecutively up to 1000thermal cycling using a thermal cycler (BIOER TCe25/H model).DSC and FTeIR analyses were repeated to determine the thermaland chemical stability of the composite PCM after thermal cycling.

3. Results and discussion

3.1. Morphology analysis of composite PCMs

Fig. 1 shows the SEM images of the diatomite, EP and preparedform-stable composite samples. It can be seen from these SEMmicrographs that the diatomite and EP have porous structures,which allow adsorbing the ETS and ETP in liquid state. The SEMimages of composite PCMs show that ETP and ETS were wellretained into the diatomite and EP. The porous structures of the EPand diatomite provided the mechanical strength for the wholecomposites and prevented the seepage of the melted ETS and ETP

Fig. 1. The SEM images of the diatomite, EP and the prepared composite PCMs.

A. Sarı, A. Karaipekli / Applied Thermal Engineering 37 (2012) 208e216210

due to the effect of capillary and surface tension force between thecomponents of the composites. In this study, the maximumpercentage of the ETP and ETS to be retained into EP and diatomitewas determined as 57 wt % for ester/diatomite composites and62 wt % for ester/EP composites. It was no observed PCM leakagefrom the composites with these combinations even when the ETPand ETS melt in the composites.

3.2. Chemical compatibility analysis

The chemical compatibility among the components of thecomposites was characterized by evaluating specific interactionsbetween fatty acid esters and the building materials the usingFTeIR spectroscopy technique. Fig. 2 shows FTeIR spectra of thediatomite, EP, ETP, ETS and the composite PCMs. The significantabsorption bands and wavenumbers obtained from the FTeIRspectra are also given in Table 2. In the spectrum of pure ETP andETS, there are two characteristic strong absorption bands arisingfrom the C]O and CeO groups, usually found in the1800e1650 cm�1 and 1310e1100 cm�1 regions, respectively. The

peaks at 2917 and 2849 cm�1 represent the stretching vibration ofeCH3 and eCH2 group of the esters, respectively.

The pure diatomite possesses the characteristic absorptionbands at 3748, 1652, 1195, 1093, and 792 cm�1. The band at3748 cm�1 is due to the free silanol groups (SiOeH) and the band at1093 cm�1 reflects the siloxane (eSieOeSie) group stretching. Thepeak at 792 cm�1 represents SiOeH vibration. The pure EP hasmainabsorption bands at 3417, 1635, 1045, 788 and 458 cm�1 respec-tively. The absorption band at 3417 cm�1 signifies the stretchingvibration of functional group of SieOH. The peaks at 1045, 788 and458 belong to the SieOeSi asymmetric stretching vibration, peaksof bending and stretching vibrations SieO functional group,respectively. The peak at 1635 cm�1 is due to the vibration of theOH bond.

On the other hand, as can be seen from Fig. 2, the FTeIR spectraof diatomite and EP after the incorporation of the ETP and ETSesters indicated new absorption peaks due to the characteristicabsorption bands of the esters. Furthermore, there were no newpeaks other than characteristic peaks of the esters and the buildingmaterials at FTeIR spectra of the composites. These results indicate

Table 2FTeIR absorption bands and assignments of the diatomite, EP, ETP, ETS and the composite PCMs.

Group Diatomite EP ETP ETS ETP/Diatomite ETS/Diatomite ETP/EP ETS/EP

C]O e e 1739 1739 1739 1739 1739 1739CeO e e 1373 1380 1380 1382 1394 1396CeH e e 2925

285629192852

29292852

29152850

29212852

29172848

OeH 1635 1635788

e e 1635796

1635794

1635796

1635796

SieO 458 458 e e 468 472 472 476SieOeSi 1093 1045 e e 1093 1095 1093 1093SieOH 3748 3417 e e 3448 3446 3446 3448

Fig. 2. FTeIR spectra of ETP, ETS and the prepared composite PCMs.

A. Sarı, A. Karaipekli / Applied Thermal Engineering 37 (2012) 208e216 211

that there is no chemical interaction between the esters and thebuilding materials. As also seen in Table 2, small shifts wereobserved in the absorption bands of some functional groups of thecomponents of composites (CeH, CeO, OeH, SieOH and SieOeSi).These shifts can be due to weak physical interactions among the

Fig. 3. The melting and freezing DSC curves of E

functional groups mentioned. It can also be attributed to thecapillary and surface tension forces between the esters moleculesand the pores of diatomite and EP, which prevent the leakage of ETSand ETP in melted state from the composites during solideliquidphase transition [33]. Combined with the result of SEM images and

TS, ETP and the prepared composite PCMs.

Table 3DSC data of the esters and the prepared composite PCMs.

Sample name Melting Freezing

Onset temperature(�C)

Peak temperature(�C)

Latent heat ofmelting (J/g)

Onset temperature(�C)

Peak temperature(�C)

Latent heat offreezing (J/g)

ETP 21.9 � 0.3 25.6 � 0.1 201.1 � 2.3 18.8 � 0.2 15.7 � 0.1 200.8 � 2.1ETS 30.4 � 0.3 35.5 � 0.1 208.8 � 3.1 28.8 � 0.1 26.7 � 0.1 207.2 � 2.3ETP/Diatomite 19.6 � 0.3 24.8 � 0.2 110.6 � 1.3 14.3 � 0.3 12.0 � 0.1 101.2 � 2.7ETS/Diatomite 29.8 � 0.1 34.5 � 0.3 116.1 � 2.3 30.0 � 0.3 27.1 � 0.1 114.8 � 3.3ETP/EP 19.8 � 0.3 25.6 � 0.1 119.0 � 1.6 14.4 � 0.3 11.7 � 0.2 111.5 � 3.1ETS/EP 30.1 � 0.2 35.5 � 0.1 119.1 � 1.2 30.0 � 0.2 26.4 � 0.3 128.6 � 3.6

Table 4Calculated latent heat capacities of the prepared composite PCMs.

Composite PCM Melting Freezing

Experimentalvalue (kJ/kg)

Calculatedvalue (kJ/g)

Experimentalvalue (kJ/kg)

Calculatedvalue (kJ/kg)

ETP/Diatomite 110.6 � 1.3 108.4 101.2 � 2.7 115.1ETS/Diatomite 116.1 � 2.3 117.3 114.8 � 3.3 118.1ETP/EP 119.0 � 1.6 117.9 111.5 � 3.1 125.2ETS/EP 119.1 � 1.2 127.6 128.6 � 3.6 128.4

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FTeIR spectroscopy, it can be concluded that the there are goodcompatibility between ester compounds and the studied buildingmaterials.

3.3. Thermal properties of the composite PCMs

Thermal properties such as melting and freezing temperaturesand latent heats are important parameters for a latent heat thermalenergy storage system. Fig. 3 shows the typical DSC curves of theETP, ETS and the composite PCMs. The melting and freezingtemperatures obtained from the DSC curves were also given inTable 3. From these curves, the melting and freezing temperatureswere determined as 21.9 � 0.3 �C and 18.8 � 0.1 �C for ETP and30.4� 0.1 �C and 28.8� 0.1 �C for the ETS, respectively. The meltingtemperatures were measured to be 19.6 � 0.3 �C, 19.8 � 0.3 �C,29.8 � 0.1 �C, and 30.1 � 0.2 �C for ETP/diatomite, ETP/EP, ETS/diatomite and ETS/EP, respectively as their freezing temperatures

Table 5Comparison of thermal energy storage characteristics of the prepared composite PCMs w

Composite PCM Melting poi

Capricelauric acid/gypsum (26/74 w%) 19.1Emerest2326/gypsum 16.9Propyl palmitate/gypsum (25e30/70e75 w/w%) 19.0Dodecanol/gypsum (25e30/70e75 w/w%) 20.0Methyl PalmitateeStearate/wallboard (26.6/73.4 w%) 22.5Butyl stearate/gypsum (25e30/70e75 w/w%) 18.0Capricelauric acid þ fire retardant (25e30/70e75 w/w%)/gypsum 17.0Capricepalmitic acid/gypsum (25/75 w/w%) 22.9nenonadecane/cement (50/50 w/w%) 31.9Capricestearic acid/gypsum (25/75 w/w%) 23.8Capricemyristic acid/gypsum (25/75 w/w%) 21.1Capricemyristic acid/Vermicuilite (20/80 w/w%) 19.8Capricemyristic acid/Expanded perlite (55/45 w/w%) 21.7RT20/Montmorillonite (58/42 w/w%) 23.0PEG1000/Diatomite (50/50 w/w%) 27.7Decanoic/Dodecanoic acid/Diatomite 16.7CapricePalmitic acid/Attapulgite (35/65 w/w%) 21.7PEG1000/Cement (25/75 w/w%) 24.3ETP/Diatomite (57/43 w/w%) 19.6 � 0.3ETS/Diatomite (57/43 w/w%) 29.8 � 0.1ETP/EP (62/38 w/w%) 19.8 � 0.3ETS/EP (62/38 w/w%) 30.1 � 0.2

were measured to be 14.3 � 0.3 �C, 14.4 � 0.3 �C, 30.0 � 0.3 �C, and30.0 � 0.2 �C, respectively. Although there are slightly changes inthe phase change temperatures of the composites, they are veryclose to pure esters. These little changes in phase change temper-atures of the esters in the composite are probably due to the weakinteractions characterized by FTeIR analysis.

The phase change temperature of PCMs using in buildingapplications should be close to human comfort temperature(16e26 �C). The ETS/diatomite and the ETS/EP composites havesuitable melting and freezing temperature for thermal energystorage by applying the exterior wall of buildings whereas thephase change temperatures of the composite PCMs prepared byusing the ETS are in the range of 29e32 �C. Thus, if the compositePCMs including ETS are used in the fabrication of building walls, thenew composite material can absorb heat from the surrounding airand solar radiation heat during day, then release heat to thesurrounding air at night. Hence, the load of air conditioning systemused to cooling in buildings may reduce.

On the other hand, the latent heats of melting were measured as110.6 � 1.3 J/g, 119.0 � 1.6 J/g, 116.1 � 2.3 J/g and 119.1 � 1.2 J/g forETP/Diatomite, ETP/EP, ETS/Diatomite and ETS/EP, respectively asthe latent heats of freezing were measured as 101.2 � 2.7 J/g,111.5 � 3.1 J/g, 114.8 � 3.3 J/g, and 128.6 � 3.6 J/g, respectively. Asalso seen from Table 4, the measured latent heats of melting andfreezing of the composite PCMs were close to the values calculatedby multiplying the mass content of the ETP and ETS of thecomposites and their phase change enthalpies in pure state.However, the calculated values were slightly higher than measured

ith those of some composite PCMs reported in literature.

nt (�C) Freezing point (�C) Latent heat (J/g) Reference

e 35.2 [8]19.3 35.0 [27]16.0 40.0 [29]21.0 17.0 [29]23.8 41.1 [29]21.0 30.0 [34]21.0 28.0 [34]21.7 42.5 [35]31.8 69.1 [36]23.9 49.0 [37]21.4 36.2 [38]17.1 27.0 [39]20.7 85.4 [40]e 79.3 [41]32.2 88.0 [42]e 66.8 [43]e 48.2 [44]27.9 23.9 [45]14.3 � 0.3 110.6 � 1.3 Present study30.0 � 0.3 116.1 � 2.3 Present study14.4 � 0.3 119.0 � 1.6 Present study30.0 � 0.2 119.1 � 1.2 Present study

Fig. 4. The melting and freezing DSC curves of the prepared composite PCMs after thermal cycling.

A. Sarı, A. Karaipekli / Applied Thermal Engineering 37 (2012) 208e216 213

values. These results can be probably caused by the physicalinteractions between the esters and the inner surface of pore ofDaitomite and EP. In addition, Table 5 presents the comparison ofenergy storage properties of the prepared composite PCMs withthose of some composite PCMs reported in literature[8,27,29,34e45]. Base on this table it is noteworthy that the ETP/Diatomite, ETS/Diatomite, ETP/EP and ETS/EP composite PCMs haveimportant latent heat thermal energy storage potential inbuildings.

Fig. 5. FTeIR spectra of the prepared composite PCMs after thermal cycling.

3.4. Thermal reliability of the composite PCMs

The PCMs to be used for thermal energy storage have to bestable in terms of thermal and chemical after long term utilityperiod in practice applications. Thus, the superior composite shouldbe shown no or less change in its thermal properties and chemicalstructure after long term utility period. Therefore, thermal cyclingtest was performed to determine the change in thermal propertiesand chemical structure of the composite PCMs by DSC analysis andFTIR analysis, respectively before and after 1000 cycling.

It is clearly seen from the DSC curves of the composite PCMsbefore and after 1000 thermal cycling (Fig. 4), both the endo-thermic and exothermic peaks belonging to the PCMs looks likeeach other. After repeated thermal cycling, themelting and freezingtemperatures of composite PCMs changed to 0.05 �C and �1.63 �Cfor ETP/diatomite, 0.01 �C and 1.92 �C for ETP/EP, 0.04 �Cand�0.08 �C for ETS/diatomite, 0.41 �C and�0.12 �C for ETS/EP. Thelittle change observed in phase change temperatures are in negli-gible level for thermal energy storage applications. Therefore, it canbe said that the composite PCMs have good thermal reliability interms of the changes in their phase change temperatures.

Fig. 6. TG curves of the prepared composite PCMs.

Table 6The measured thermal conductivity values and energy storage properties of thecomposite PCMs.

Material Thermal conductivity(W m�1 K�1)

Meltingpoint (�C)

Latent heat ofmelting (J/g)

EG 4.26 � 0.02 e e

Diatomite 0.07 � 0.01 e e

EP 0.04 � 0.01 e e

ETP 0.25 � 0.01 e e

ETS 0.26 � 0.01 e e

ETP/Diatomite 0.19 � 0.01 19.6 � 0.3 110.6 � 1.3ETS/Diatomite 0.21 � 0.01 29.8 � 0.1 116.1 � 2.3ETP/EP 0.15 � 0.01 19.8 � 0.3 119.0 � 1.6ETS/EP 0.16 � 0.02 30.1 � 0.2 119.1 � 1.2

A. Sarı, A. Karaipekli / Applied Thermal Engineering 37 (2012) 208e216214

On the other hand, after repeated 1000 thermal cycling, thelatent heat value of melting changed by �2.4% for ETP/Diatomite,3.5% for ETP/EP, �2.9% for ETS/Diatomite and 1.3% for ETS/EP whilethe latent heat value of freezing changed by 5.8% for ETP/Diatomite, �4.3% for ETP/EP, 3.4% for ETS/Diatomite and 1.6% forETS/EP. As can be seen from these results, the changes in the latentheat values of the composite PCMs are irregular and in a reasonablelevel for composite PCMs, which will be used for thermal energystorage applications in buildings [8,17,34].

The chemical stability of the composite PCMs after repeatedthermal cycling was investigated by FTeIR analysis. As clearly seenfrom shows the FTeIR spectra in Fig. 5, the peak shapes and the

Table 7The measured thermal conductivity values of the composite PCMs and energy storage p

Composite PCM Thermal conductivity(W m�1 K�1)

Incrcon

ETP/Diatomite/EG(2 wt%) 0.23 � 0.01 20ETS/Diatomite EG(2 wt%) 0.25 � 0.01 18ETP/EP composite EG(2 wt%) 0.19 � 0.02 24ETS/EP composite EG(2 wt%) 0.20 � 0.01 22ETP/Diatomite/EG(5 wt%) 0.32 � 0.01 68ETS/Diatomite EG(5 wt%) 0.33 � 0.02 57ETP/EP composite EG(5 wt%) 0.26 � 0.01 73ETS/EP composite EG(5 wt%) 0.28 � 0.02 75ETP/Diatomite/EG(10 wt%) 0.36 � 0.02 90ETS/Diatomite EG(10 wt%) 0.38 � 0.01 81ETP/EP composite EG(10 wt%) 0.31 � 0.02 106ETS/EP composite EG(10 wt%) 0.35 � 0.01 120

wavenumber values of the composites did not changed afterthermal cycling. These results mean that chemical structure of thecomposite PCMswas not affected by thermal cycling. Therefore, it isnoteworthy noted that the composite PCMs have good chemicalstability after 1000 thermal cycling.

3.5. Thermal stability of the composite PCMs

The thermal stabilities of the composite PCMswere evaluated byTG analysis and the results TG curves are showed in Fig. 6. From theTG curves, it can be said that there are two steps in the degradationof ETP/Diatomite and ETP/EP composites. The first step is roughlyfrom 81 to 150 �C, corresponding to the evaporation of water fromthe diatomite or EP the second step observed in the range of450e500 �C is assigned to the degradation of ETP molecular chain.However, the TG curves of the composites containing ETS includeone step. This situation can be explained as follows: The molecularvolume of ETS is the bigger than ETS and so ETS completely coatsthe pores of the diatomite and EP. Even if water retains in the poresof diatomite and EP, the pores coated with ETS do not allow thewater evaporation from the composite PCM. Therefore, in thecomposites with ETS, the step corresponding to the evaporation ofwater from the composites either cannot observe or overlap withdegradation step of ETS. In addition, as can be seen from TG curves,the composite PCMs were not degraded or showed almost noweight loss at lower temperature than 100 �C. This result meansthat the composite PCMs have good thermal durability in theirworking temperature range.

3.6. Thermal conductivity of the PCMs

The rate of energy storage and release is substantially dependingon the thermal conductivity of PCMs [46,47]. In order to improvethe thermal conductivity of the composite PCMs, the expandedgraphite (EG) with high thermal conductivity was added to them atdifferent mass fractions (2%, 5%, and 10%). The measured thermalconductivity results of the composite PCMs before and after EGadditionwere presented in Table 6 and Table 7, respectively. As seenfrom Table 6, thermal conductivity values at 25 �C were measuredas 0.19 � 0.01, 0.21 � 0.01, 0.15 � 0.01, 0.16 � 0.02 W m�1 K�1 forETP/Diatomite, ETS/Diatomite, ETP/EP and ETS/EP composite PCMs,respectively. On the other hand, as seen from Table 7, the thermalconductivity values of the composite PCMswere increased with theincrease in the mass percent of EG. However, the increase percentin thermal conductivity values was found to be lower than theexpected ones for 2 wt % and 10 wt % EG additive as compared withthat obtained for 5 wt% EG additive. Moreover, the decreaseamount in latent heat capacities of the composite PCMs after EG

roperties after EG addition in different mass fractions.

ease in thermalductivity (%)

Melting point (�C) Latent heat ofmelting (J/g)

19.8 � 0.3 109.5 � 0.229.9 � 0.1 115.4 � 0.319.9 � 0.3 118.4 � 0.230.3 � 0.2 118.1 � 0.119.8 � 0.3 108.2 � 2.229.9 � 0.1 114.3 � 1.719.9 � 0.3 117.2 � 1.230.3 � 0.2 116.6 � 1.419.8 � 0.3 108.2 � 2.229.9 � 0.1 114.3 � 1.719.9 � 0.3 117.2 � 1.230.3 � 0.2 116.6 � 1.4

A. Sarı, A. Karaipekli / Applied Thermal Engineering 37 (2012) 208e216 215

addition were determined to be higher than the theoreticalexpectation for especially 10 wt % EG additive relatively the 5 wt%EG additive. As seen from these results, the best results were ob-tained for the composite PCMs including with 5 wt% EG in terms ofthe increase percent in thermal conductivity values and thedecrease amount in latent heat capacity. Therefore, by taking intoaccount the increase percent in thermal conductivity values andthe decrease amount in latent heat capacity, the optimum massratio of EG additive used to improve the thermal conductivity of theprepared composite PCMs was determined as 5wt%.

4. Conclusions

In this study, ETP/Diatomite, ETS/Diatomite, ETP/EP and ETS/EPcomposite PCMs were prepared as novel composite PCMs forthermal energy storage applications in buildings. The compositePCMs were obtained by direct incorporation of the esters withbuildingmaterials. Themaximum ester content in diatomite and EPwas found as 57 and 62 wt%, respectively. These form-stablecomposite PCMs were characterized by using SEM, FTeIR, DSCand TG analysis techniques. The SEM results showed that ETP andETS were successfully retained into the pores of diatomite and EPused as supporting materials. The FTeIR results proved the avail-ability of good chemical compatibility between the components ofthe composites. DSC analysis results indicated that the meltingtemperatures and latent heats of the prepared composite PCMs arein the range of 19.64e30.15 �C and 110.60e119.19 J/g, respectivelyand these properties are suitable for thermal energy storageapplications in buildings. The thermal cycling test revealed that thecomposite PCMs had good thermal reliability and chemical stabilityeven after 1000 thermal cycles. The TG analysis results signifiedthat the composites have good thermal durability above theirworking temperature range. Moreover, in order to improve thethermal conductivity of the composite PCMs, the expandedgraphite (EG) with high thermal conductivity was added to them atdifferent mass fractions (2%, 5%, and 10%). The best results wereobtained for the composite PCMs including with 5wt% EG in termsof the increase percent in thermal conductivity values and thedecrease amount in latent heat capacity.

Acknowledgements

Authors thank Altınay BOYRAZ (Erciyes University, TechnologyResearch & Developing Center) for SEM and TG analysis.

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