Solar Energy Materials & Solar Cells 101 (2012) 114–122

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Solar Energy Materials & Solar Cells

0927-02

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Thermal energy storage properties and thermal reliability of some fatty acidesters/building material composites as novel form-stable PCMs

Ahmet Sarı n, Alper Bic-er

Gaziosmanpas-a University, Department of Chemistry, 60240 Tokat, Turkey

a r t i c l e i n f o

Article history:

Received 16 August 2011

Received in revised form

11 January 2012

Accepted 21 February 2012Available online 19 March 2012

Keywords:

Fatty acid ester

Building material

Composite PCM

Thermal properties

Thermal energy storage

48/$ - see front matter & 2012 Elsevier B.V. A

016/j.solmat.2012.02.026

esponding author. Tel.: þ90 356 2521616; fa

ail addresses: ahmet.sari@gop.edu.tr, asari061

a b s t r a c t

In this study, thermal energy storage properties and thermal reliability some fatty acid esters/building

material composites as novel form-stable phase change materials (PCMs) were investigated. The form-

stable composite PCMs were prepared by absorbing galactitol hexa myristate (GHM) and galactitol

hexa laurate (GHL) esters into porous networks of diatomite, perlite and vermiculite. In composite

PCMs, fatty acid esters were used as energy storage materials while diatomite, perlite and vermiculite

were used as building materials. The prepared composite PCMs were characterized using scanning

electron microscope (SEM) and Fourier transformation infrared (FT-IR) analysis techniques. The SEM

results proved that the esters were well confined into the building materials. The maximum mass

percentages of GHM adsorbed by perlite, diatomite and vermiculite were determined as 67, 55 and

52 wt%, respectively as they were found for GHL to be 70, 51 and 39 wt%, respectively. Thermal

properties and thermal stabilities of the form-stable composite PCMs were determined using

differential scanning calorimetry (DSC) analysis. The DSC results showed that the melting temperatures

and latent heat values of the PCMs are in range of about 39–46 1C and 61–121 J/g. The thermal cycling

test revealed that the composite PCMs have good thermal reliability and chemical stability. TG analysis

revealed that the composite PCMs had high thermal durability property above their working

temperature ranges. Moreover, the thermal conductivities of the PCMs were increased by adding the

expanded graphite (EG) in mass fraction of 5%. Based on all results, it was also concluded that the

prepared six composite PCMs had important potential for thermal energy storage applications such as

solar space heating and cooling applications in buildings.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Thermal energy storage (TES) is considered as one of mostimportant energy technologies. TES can be used for the utilizationof renewable energy sources and waste heat. An increasingattention has been paid to use of this essential technique forthermal applications ranging from heating to cooling, particularlyin buildings [1,2]. Latent heat storage using phase change mate-rial (PCM) is the most attractive choice for TES applicationsbecause of its advantages of storing and releasing large amountsof energy at a fixed temperature point (or within a smalltemperature range) during phase change process of PCM fromsolid to liquid or vice versa [3–5].

When PCM is used in buildings, the heat indoors is stored byPCM above its melting point and then, the stored heat is releasedat a lower temperature than its melting point at night [6,7].PCMs are used for two purposes in buildings. One is to store the

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x: þ90 356 2521585.

@hotmail.com (A. Sarı).

time-dependent and intermittent solar energy that does notnecessarily match the energy need for building space heating atall times. The other is to shift heating or cooling load of buildingfrom peak to off peak electricity periods. Therefore, the powergeneration management is improved and significant economicbenefit can be achieved.

The thermal comfort of a building can increase by decreasingthe frequency of internal air temperature swings so that indoorair temperature is closer to the desired temperature for a longerperiod of time [8–13]. The idea regarded with the improvement ofthe thermal comfort in building using PCMs directed theresearchers to develop new kinds of composite materials andapply them for TES in building envelopes. Over the past twodecades, the incorporation of PCMs with construction materialshas been investigated as potential technology for minimizingenergy consumptions in buildings. The several PCMs have beenimplemented in gypsum board, plaster, concrete or other wallcovering material for TES purpose. Feldman et al. [14–16] carriedout an extensive research on the stability of some organiccompounds and their compatibility with common constructionmaterials. Hawes et al. [17] prepared a form-stable composite

A. Sarı, A. Bic-er / Solar Energy Materials & Solar Cells 101 (2012) 114–122 115

PCM that consists of sodium thiosulphate pentahydrate andporous concrete. They investigated heat storage capacity andstructural stability of the composite after a large number ofthermal cycling. Some building composite PCMs were preparedand investigated their TES properties: fatty acid ester/cement andfatty acid ester/gypsum [18], PEG/diatomite [19], capric–myristicacid/vermiculite [20], some eutectic mixtures of fatty acids/expanded vermiculite [21], capric–palmitic acid/gypsum [22],lauric acid/expanded perlite [23], paraffin/expanded perlite [24],capric–stearic acid/gypsum [25] capric acid/expanded perlite[26], and capric–myristic acid/gypsum [27], capric-lauric acid/gypsum [28–30], fatty acids/diatomite [31], capric–palmitic acid/attapulgite [32], n-nonadecane/cement [33], paraffin and fattyacids/clay [34–37].

Considering the chemical compatibility with building materi-als, most of the researches preferred organic PCMs. Among thesegroup, fatty acids esters are promising PCMs in the preparation ofenergy storing composite PCMs because of their good thermo-physical properties, thermal reliability and the advantage ofdirectly incorporation into conventional building products. How-ever, only few esters have been used for this purpose so far[18,38,39].

Galactitol hexa myristate (GHM) and galactitol hexa laurate(GHL) are favorable organic PCMs for TES applications in terms ofsuitable their melting temperatures of 45.98 and 40.21 1C andlatent heats of 172.80 and 157.62 kJ/kg, respectively. Thus, theyare appropriate PCMs for fabrication of building compositematerials. Perlite has high porosity, low sound transmission, highfire resistance, a large surface area, low moisture retention, a verylow density. Vermiculite is porous, non-toxic, light, and readilyavailable natural mineral. Therefore, it is used in many commer-cial applications such as construction, thermal acoustic insulation,agricultural, and horticultural. Diatomite has light weight, highporosity, high purity, multi-shape, rigidity, and inert property.These clay minerals are also environmentally safe, ultra-light-weight building material and abundantly available in Turkishmarkets. Therefore, perlite, diatomite and vermiculite clays aresuitable and economical building materials for the preparationform-stable composite PCMs.

In this study, six novel composite PCMs, GHM/perlite, GHM/diatomite, GHM/vermiculite, GHL/perlite, GHL/diatomite andGHL/vermiculite were prepared and characterized in terms ofchemical compatibility using SEM and FT-IR techniques. Thermalenergy storage properties, thermal reliability and thermal dur-ability of the composite PCMs were determined using DSC and TGanalysis techniques. Moreover, the thermal conductivities of thecomposite PCMs were increased by adding expanded graphite inmass fraction of 5%.

2. Experimental

2.1. Materials

Galactitol hexa myristate (GHM) and galactitol hexa laurate(GHL) were synthesized by esterification reaction of galactitolwith myristic and lauric acids. These esters were synthesized

Table 1Chemical compositions (wt%) of the building materials used in this study.

Building material SiO2 Al2O3 Fe2O3

Diatomite 92.8 4.2 1.5

Perlite 71.0–75.0 12.5–18.0 0.1–1.5

Vermiculite 38.0–46.0 10.0–16.0 6.0–13.0

using the Fischer esterification method reported in previous study[40]. Diatomite, perlite and vermiculite were supplied by BEG-TUG Industrial Minerals & Mines Company (Istanbul, Turkey),Izper Company (Izmir–Turkey) and Agrekal Company (Antalya,Turkey), respectively. Table 1 shows the chemical compositions ofthese materials. The samples were sieved by 150 mm-mesh sieveand dried at 105 1C for 24 h before use.

2.2. Preparation of form-stable composite PCMs

GHM/perlite, GHM/diatomite, GHM/vermiculite, GHL/perlite,GHL/diatomite and GHL/vermiculite composites were preparedusing the vacuum impregnation method [19,36]. The process wasto be continued for 90 min at 65 kPa. Air was allowed to enter theflask again to force the liquid ester compounds to penetrate onto thepores of the clay materials. A series of composite including PCM atdifferent mass fractions (10, 20, 30, 40, 50, 60, 70, 80 wt%) wereprepared. In order to observe the PCM leakage from the composites,the composite PCMs were simultaneously heated during theimpregnation process above the melting temperatures of the PCM.The maximum absorption ratios of the clay samples for the esterswere determined. The composites with the maximum compositionratios were then sealed as form-stable composite PCMs.

2.3. Characterization of the form-stable composite PCMs

The morphology of the form-stable composite PCMs wasinvestigated using a LEO 440 model SEM instrument. The com-posite PCMs were characterized chemically using a JASCO 430model FT-IR spectrophotometer. The spectral data were obtainedusing KBr pellets at wavenumber range of 400–4000 cm�1.Thermal properties of the composite PCMs were measured usinga Perkin Elmer JADE model DSC instrument at 5 1C min�1 heatingrate and under nitrogen atmosphere. The accuracy level in themeasurement of enthalpy and temperature was determined to be75% and 70.01 1C, respectively. The thermal durability of thecomposite PCMs was also determined using Perkin–Elmer TGA7thermal analyzer. The TG analyses were performed at a heatingrate of 10 1C/min and under nitrogen atmosphere.

The thermal cycling test was performed to evaluate thermalreliability of form-stable composite PCMs with respect to thechange in phase change temperatures and latent heat capacityafter the accelerated melting-freezing cycles repeated for 1000times. This process was carried out using a thermal cycler (BIOERTC-25/H model). DSC and FT-IR analysis were also repeated toprove the thermal and chemical stability of the composite PCMsafter thermal cycling.

3. Results and discussion

3.1. Microstructures of form-stable composite PCMs

Fig. 1 shows the SEM images of diatomite, perlite, vermiculiteand composite PCMs. As seen in Fig. 1, diatomite, perlite, andvermiculite have rough and accidental microstructures. The SEMimages of the composite PCMs also show that GHM and GHL were

CaO MgO K2O Other

0.6 0.3 0.67 0.5

0.5–0.2 0 0.03–0.5 4.0–5.0 –

1.0–5.0 16.0–35.0 1.0–6.0 0.2–1.2

Fig. 1. SEM images of the (a) Perlite, (b) GHM/perlite, (c) GHL/perlite, (d) Diatomite, (e) GHM/diatomite, (f) GHL/diatomite, (g)Vermiculite, (h) GHM/vermiculite and

(i) GHL/vermiculite.

A. Sarı, A. Bic-er / Solar Energy Materials & Solar Cells 101 (2012) 114–122116

dispersed into the porous networks of diatomite, perlite andvermiculite used as supporting materials. The results indicated thatthese multiple porous structures into the clay samples providedgood mechanical strength to the composites and also the leakagebehavior of PCM melted in the composite during heating processwas prevented by the capillary and surface tension forces. Themaximum mass percentages of GHM confined into perlite, diatomiteand vermiculite were determined as 67, 55 and 52 wt%, respectivelyas they were found for GHL to be 70, 51 and 39 wt%, respectively.The PCM leakage in melted state was not observed until these masspercentages even when the composites were heated over themelting points of the ester compounds. Therefore, final productswere defined as form-stable composite PCMs.

3.2. FT-IR analysis of the composite PCMs

The chemical interactions between the fatty acid esters andthe building materials were characterized by FT-IR spectroscopy

analysis. Fig. 2(a) and (b) shows FT-IR spectra of the perlite,diatomite, and vermiculite, GHM, GHL and the composite PCMs.As shown from Fig. 2(a), GHM and GHL esters have two maincharacteristic peaks regarding to C¼O and C–O groups observedat 1702–1706 cm�1 and 1465–1471 cm�1, respectively. Thepeaks at 2850–2919 cm�1 range represent the stretching vibra-tion of –CH3 and –CH2 groups of the esters, respectively. Thepeaks observed at 3300–3500 cm�1 range and 1550–1639 cm�1

range correspond to the stretching vibration and bending vibra-tion of OH groups of water molecules in perlite, diatomite andvermiculite. In addition, the peaks at 950–1100 cm�1 range showthe stretching vibration of Si–O group of the building materials.

On the other hand, when compared the FT-IR spectra of thecomposite PCMs (Fig. 2b) with that of the building materials(Fig. 2a), some new absorption peaks attracted the attention. Sothese characteristics peaks are regarding with the ester com-pounds confined into building materials. Moreover, there werelittle shifts in some characteristic FT-IR bands. For example, the

Fig. 2. FT-IR spectra of GHM and GHL and the form-stable composite PCMs.

Fig. 3. DSC curves obtained for the melting and freezing of GHM and GHL.

A. Sarı, A. Bic-er / Solar Energy Materials & Solar Cells 101 (2012) 114–122 117

stretching vibration band of C¼O group of the esters at 1702–1706 cm�1 range shifted to 1737–1739 cm�1 range in case of thecomposites. The stretching band of C–O groups of the esters cm�1

shifted from 1465–1471 cm�1 range to 1380–1388 cm�1 rangeafter the impregnation process. The stretching vibration band ofSi–O groups of the building materials at 950–1100 range shiftedto 1000–1150 cm�1 range in case of composite PCMs. Moreover,any new peak was not seen at FT-IR spectra of the compositesbesides the characteristic peaks of the esters and the buildingmaterials. These results confirm that there is no chemical inter-action between the esters and the building materials and theinteractions between the components of the composites arephysical in nature [21,34,36].

3.3. Thermal properties of the esters and the form-stable

composite PCMs

The DSC curves of GHM, GHL and the form-stable compositePCMs are presented in Figs. 3 and 4 respectively. Thermal energystorage data obtained from the DSC curves were given also inTable 2. The melting temperatures of GHM and GHL esters weremeasured to be 45.98 1C and 40.21 1C, respectively as the freezingtemperatures were measured to be 44.28 1C and 36.42 1C,

respectively. These results indicated that GHM and GHL estercompounds were suitable for solar space heating and coolingapplications in buildings with respect to the climate conditions.As also seen from Table 2, the melting temperatures of GHM/perlite,GHM/diatomite and GHM/vermiculite were determined as 43.83 1C,45.86 1C and 44.90 1C, respectively while the freezing temperatureswere determined as 43.27 1C, 44.63 1C and 44.26 1C, respectively.On the other hand, the melting temperatures of GHM/perlite, GHM/diatomite and GHM/vermiculite were measured as 39.53 1C,39.03 1C and 39.48 1C whereas their freezing temperatures weremeasured as 35.55 1C, 37.85 1C and 35.19 1C, respectively. There aresmall differences in phase change temperatures of the compositePCMs when compared with that of the pure GHL and GHM esters.However, the phase change temperatures of the composite PCMsare very close to that of pure esters. The little changes are probablydue to the physical interactions (capillary forces between the wallof pores into the building material and ester molecules) character-ized by FT-IR analysis. Based on the melting temperatures of thecomposite PCMs it can be also concluded that the prepared sixcomposite PCMs can be used as thermal energy storage material forsolar space heating and cooling applications.

The latent heats of melting and freezing were found to be121.13 J/g and 118.67 J/g for GHM/perlite, 96.21 J/g and 92.46 J/g

Fig. 4. DSC curves for the melting and freezing of form-stable composite PCMs.

Table 2The measured DSC data of the esters and the prepared form-stable composite PCMs.

Material Melting temperature (1C) Latent heat of melting (J/g) Freezing temperature (1C) Latent heat of freezing (J/g)

GHM 45.98 172.80 44.28 168.75

GHL 40.21 157.62 36.42 154.13

GHM/perlite 43.83 121.13 43.27 118.67

GHM/diatomite 45.86 96.21 44.63 92.46

GHM/vermiculite 44.90 83.75 44.26 81.13

GHL/perlite 39.53 119.45 35.55 114.36

GHL/diatomite 39.03 63.08 37.85 61.14

GHL/vermiculite 39.48 61.38 35.19 59.22

Table 3Comparison of measured latent heats with calculated ones for the prepared composite PCMs.

Sample name Melting Freezing

Experimental value (J/g) Calculated value (J/g) Difference(%) Experimental value (J/g) Calculated value (J/g) Difference(%)

GHM/perlite 121.13 115.08 4.99 118.67 113.06 4.72

GHM/diatomite 96.21 95.04 1.21 92.46 92.51 0.05

GHM/vermiculite 83.75 88.99 5.89 81.13 87.75 8.16

GHL/perlite 119.45 110.33 7.63 114.36 107.89 5.66

GHL/diatomite 63.08 89.84 24.04 61.14 78.60 28.56

GHL/vermiculite 61.38 72.50 18.12 59.22 60.11 1.50

A. Sarı, A. Bic-er / Solar Energy Materials & Solar Cells 101 (2012) 114–122118

for GHM/diatomite, 83.75 J/g and 81.13 J/g for GHM/vermiculite,respectively. The latent heats of GHL/perlite, GHL/diatomite GHL/vermiculite were measured as 119.45, 63.08 and 61.38 J/g formelting process and 114.36, 61.14 and 59.22 J/g for freezing process,respectively (Table 2). The latent heat values of the composites makethem suitable PCMs for thermal energy storage purposes in build-ings. Especially, GHM/perlite and GHL/perlite composites are

considered as the best appropriate PCMs for TES applications dueto their higher latent heat values compared with that of the others.

On the other hand, Table 3 presents the comparison of themeasured latent heat values of the composite PCMs with thetheoretical values calculated using the following equation:

DHcomp ¼ ZDHPCM ð1Þ

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In Eq. (1), DHcomp, Z and DHPCM represent the theoretical latentheat values of the composite PCM, the mass percentage of GHMand GHL in the composites and the latent heats of the GHM andGHL measured by the DSC. As also seen from Table 3, themeasured latent heats measured for the melting and freezingprocesses of the composite PCMs were close to the theoreticalvalues calculated by using Eq. (1). In addition, Table 4 shows thecomparison of energy storage properties of the prepared compo-site PCMs with that of some composite PCMs in literature[17–19,21–26,31–33,36]. By considering these data it is remark-ably noted that the prepared composite PCMs in this study have

Table 4Comparison of thermal energy storage properties of the prepared composite PCMs wit

Composite PCM Melting point (1C)

Lauric–stearic acid (38 wt%)/gypsum 34.0

ETP/cement 21.96

ETS/cement 32.23

ETP/gypsum 21.62

ETS/gypsum 32.30

PEG1000/diatomite 27.7

Capric–palmitic acid/vermiculite 23.51

Capric–stearic acid/vermiculite 25.64

Capric–palmitic acid (25 wt%)/gypsum 22.9

Lauric acid/expanded perlite 41.8

Paraffin/expanded perlite 42.27

Capric–stearic acid (25 wt%)/gypsum 23.8

Capric acid/expanded perlite 31.8

Decanoic–dodecanoic acid/diatomite 16.74

Capric–palmitic acid/attapulgite 21.71

n-nonadecane(50 wt%)/cement 31.9

Lauric acid (60 wt%)/expanded perlite 44.1

Lauric acid/expanded fly ash 41.3

Lauric acid/expanded clay 41.7

GHM/perlite 43.83

GHM/diatomite 45.86

GHM/vermiculite 44.90

GHL/perlite 39.53

GHL/diatomite 39.03

GHL/vermiculite 39.48

Fig. 5. DSC curves for the melting and freezing of for

important TES potential for decreasing the load of heating,ventilating, and air conditioning in buildings.

3.4. Thermal reliability of the form-stable composite PCMs

A composite PCM should maintain its TES properties even it issubjected to a large number of thermal cycling. In this sense,thermal cycling test was conducted to determine the changes inTES properties of the composite PCMs. The DSC results obtainedafter 1000 thermal cycles were given in Fig. 5 and Table 5. Afterthermal cycling, the melting and freezing temperatures changed

h that of some composite PCMs in literature.

Freezing point (1C) Latent heat (J/g) Reference

– 50.3 [17]

14.50 37.20 [18]

29.83 35.96 [18]

14.49 42.29 [18]

29.45 43.26 [18]

32.2 87.09 [19]

22.15 72.05 [21]

24.90 71.53 [21]

21.7 42.5 [22]

– 91.7 [23]

40.79 87.40 [24]

23.9 49.0 [25]

31.6 98.1 [26]

– 66.8 [31]

– 48.2 [32]

31.8 69.1 [33]

40.9 93.4 [36]

– 38.7 [36]

– 46.7 [36]

43.87 121.13 Present study

45.97 96.21 Present study

44.90 83.75 Present study

34.55 119.45 Present study

36.85 63.08 Present study

33.49 58.38 Present study

m-stable composite PCMs after thermal cycling.

Table 5The measured DSC data of the prepared form-stable composite PCMs after 1000 thermal cycles.

Composite PCM Melting temperature (1C) Latent heat of melting (J/g) Freezing temperature (1C) Latent heat of freezing (J/g)

GHM/perlite 45.71 116.48 41.12 112.13

GHM/diatomite 47.26 90.58 45.09 90.87

GHM/vermiculite 48.34 79.36 46.01 76.42

GHL/perlite 38.52 112.46 34.74 106.57

GHL/diatomite 38.37 60.55 36.71 60.38

GHL/vermiculite 38.03 57.48 34.13 57.78

Fig. 6. FT-IR spectra of the form-stable composite PCMs after thermal cycling.

Fig. 7. TG curves of GHM/perlite, GHM/diatomite and GHM/vermiculite composites.

Fig. 8. TG curves of GHL/perlite, GHL/diatomite and GHL/vermiculite composites.

A. Sarı, A. Bic-er / Solar Energy Materials & Solar Cells 101 (2012) 114–122120

as 1.88 1C and �2.15 1C for GHM/perlite, 1.40 1C and 0.92 1C forGHM/diatomite, 3.44 1C and 1.75 1C for GHM/vermiculite,�1.01 1C and �0.81 1C for GHL/perlite, 0.66 1C and �1.14 1C forGHL/diatomite, 1.45 1C and �1.06 1C for GHL/vermiculite. Theresults showed that the changes in phase change temperatures ofthe composite PCMs are in insignificant level for TES applications.Therefore, it can be concluded that the composite PCMs havegood thermal reliability with respect to the changes in their phasechange temperatures.

On the other hand, after repeated 1000 thermal cycling thelatent heat values of melting changed as �3.8% for GHM/perlite,�5.8% for GHM/diatomite, �5.2% for GHM/vermiculite, �5.9%for GHL/perlite, �4.0% for GHL/diatomite, �6.6% for GHL/vermi-culite while the latent heat values of freezing changed as �5.5%for GHM/perlite, �1.7% for GHM/diatomite �5.8% for GHM/vermiculite, �6.8% for GHL/perlite, �1.2% for GHL/diatomite,�2.4% for GHL/vermiculite. As can be seen from these resultsthe changes in the latent heats values of the composite PCMs afterrepeated thermal cycling were in reasonable level for TESapplications.

The chemical stability of the composite PCMs after repeatedthermal cycling was also investigated by FT-IR analysis. Whencompared the FT-IR results given in Fig. 6 with the FT-IR resultsgiven in Fig. 2(b), it can be seen that the peak positions andshapes regarding with the characteristics groups of the compositePCMs are not changed before and after thermal cycling. Theseresults mean that any degradation in chemical structure of thecomposite PCMs was not occurred after thermal cycling.

3.5. Thermal stability of the composite PCMs

The thermal stabilities of the GHM/perlite, GHM/diatomite,GHM/vermiculite, GHL/perlite, GHL/diatomite and GHL/vermicu-lite were evaluated by TG analysis. The TG curves were shown inFigs. 7 and 8. As seen from the curves, the weight loss processes ofall composites consist of two steps. The first step was carried outin the temperature range of 76–294 1C for the compositesincluding GHM ester as it varies in the range of 83–362 1C forthe composites including GHL ester. The weight loss percentregarding with the first step of the TG curves were determinedas 65.6, 56.9 and 51.2 wt% for GHM/perlite, GHM/diatomite,GHM/vermiculite and 71.6, 48.4 and 38.7 wt% for GHL/perlite,GHL/diatomite and GHL/vermiculite. These results are approxi-mately equal to the mass percent of the esters confined into thebuilding materials. Therefore, it can be concluded that the firststep of the curves corresponds to the evaporation of estercompound in the composite and the other step is attributed to

Table 6The measured thermal conductivity values (at 25 1C) and energy storage properties before and after EG addition.

Material Thermal conductivity (W m�1 K�1) Thermal energy storage properties

Before EG addition After 5 wt% EG addition Melting point (1C) Latent heat of melting (J/g)

EG 4.26 – – –

Diatomite 0.07 – – –

Perlite 0.04 – – –

Vermiculite 0.06 – – –

GHM 0.11 – – –

GHL 0.10 – – –

GHM/perlite 0.15 0.22 43.28 117.13

GHM/diatomite 0.14 0.20 45.86 93.14

GHM/vermiculite 0.19 0.24 44.76 80.21

GHL/perlite 0.12 0.20 39.24 114.45

GHL/diatomite 0.17 0.24 39.14 61.12

GHL/vermiculite 0.16 0.22 39.32 59.76

A. Sarı, A. Bic-er / Solar Energy Materials & Solar Cells 101 (2012) 114–122 121

the degradation or evaporation of the metal oxides and watercontents of the building material. In addition, the initial tempera-tures regarding with the first step of the curves are much higherthan the phase change temperatures of the composites. Thisresult means that they have good thermal durability over theirworking temperature range, 39–46 1C.

3.6. Thermal conductivity of the PCMs

The energy storing and releasing rates of a PCM used for TES issubstantially depending on its thermal conductivity [41,42]. Forthis aim, thermal conductivity values of the composite PCMs wereincreased by adding the expanded graphite (EG) to them at massfraction of 5%. As seen from Table 6, thermal conductivity valueswere measured as 0.15, 0.14, 0.19, 0.12, 0.17, 0.16 Wm�1 K�1 forGHM/perlite, GHM/diatomite, GHM/vermiculite, GHL/perlite,GHL/diatomite, and GHL/vermiculite, respectively. After 5 wt%EG addition, thermal conductivity values of the composite PCMswere measured as 0.22, 0.20, 0.24, 0.20, 0.24, 0.22 Wm�1 K�1,respectively. These results showed that thermal conductivityvalues of GHM/perlite, GHM/diatomite, GHM/vermiculite, GHL/perlite, GHL/diatomite, and GHL/vermiculite were increased asabout 47%, 43%, 26%, 67%, 41% and 38%, respectively. In addition,DSC analysis was performed to investigate the effect of EGadditive on energy storage properties of the composite PCMs. Asseen from Table 6, after EG addition the changes in phase changetemperatures and latent heat values of the composite PCMs canbe considered as insignificant for TES applications.

4. Conclusions

In this study, some fatty acid esters/building material compo-sites were prepared as novel form-stable PCMs for TES applica-tions in buildings. The prepared composite PCMs werecharacterized by using SEM, FT-IR, and DSC analysis techniques.The maximum mass percentages of GHM confined into perlite,diatomite and vermiculite were determined as 67, 55 and 52 wt%,respectively while the maximum mass percentages of GHL in thecomposites were determined as 70, 51 and 39 wt%, respectively.The DSC results showed that the melting temperatures and latentheat values of the composite PCMs with GHM and GHL contentswere in the range of about 39–46 1C and 61–121 J/g. Especially,GHM/perlite and GHL/perlite composites can be considered as thebest suitable PCMs for TES applications due to their higher latentheat values compared with that of the other composite PCMs. TheFTIR and DSC analysis performed after thermal cycling testrevealed that the composite PCMs have good thermal reliability

and chemical stability. The TG analysis results signified that thecomposites have good thermal durability above their workingtemperature range. Moreover, the thermal conductivity values ofthe composite PCMs were increased with EG addition at massfraction of 5%. It was also concluded that the prepared GHM/perlite, GHM/diatomite, GHM/vermiculite, GHL/perlite, GHL/dia-tomite, and GHL/vermiculite composites can be taken intoaccount as promising PCMs for solar space heating and coolingapplications because of their good energy storage properties,thermal reliability and improved thermal conductivity.

Acknowledgments

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

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