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Energy and Buildings 51 (2012) 73–83

Contents lists available at SciVerse ScienceDirect

Energy and Buildings

j ourna l ho me p age: www.elsev ier .com/ locate /enbui ld

reparation and thermal energy storage properties of building material-basedomposites as novel form-stable PCMs

hmet Sarı ∗, Alper Bic eraziosmanpas a University, Department of Chemistry, 60240 Tokat, Turkey

r t i c l e i n f o

rticle history:eceived 1 March 2012eceived in revised form 2 April 2012ccepted 7 April 2012

eywords:uilding materialatty acid ester

a b s t r a c t

In this study, ten kinds of composite phase change materials (PCMs) were prepared by impregnation ofxylitol penta palmitate (XPP) and xylitol penta stearate (XPS) esters into gypsum, cement, diatomite, per-lite and vermiculite via vacuum adsorption method. The form-stable composite PCMs were characterizedby using SEM and FT-IR, DSC and TG analysis techniques. The maximum impregnation ratio of both XPPand XPS into gypsum, cement, perlite, diatomite, and vermiculite were found to be 22, 17, 67, 48 and42 wt%, respectively. The DSC results showed that the melting temperatures and latent heat capacitiesof the composite PCMs varied from 20 ◦C to 35 ◦C and from 38 J/g to 126 J/g. TG investigations revealed

omposite PCMhermal propertieshermal energy storage

that the composite PCMs had excellent thermal durability above their working temperature ranges. Thethermal cycling test also exhibited that the composite PCMs had good thermal reliability and chemicalstability. In addition, thermal conductivities of the composite PCMs were increased by addition of EG inmass fraction of 10%. All of the conclusions indicated that among the prepared composite PCMs, espe-cially perlite and diatomite based-PCMs are potential candidates for energy storage applications such as

in bu

solar heating and cooling

. Introduction

Solar energy is an effective renewable energy source becausef its intermittent property in nature. Hence, the storage ofolar energy is a fundamental method for regulating the time-iscrepancy between the energy supply and demand. Generally,olar energy can be thermally stored as sensible heat, latent heat,eversible reaction heat, or combination of these [1]. Among these,atent heat storage by using phase change materials (PCMs) areromising method due to the advantages of high heat storage den-ity, a narrow temperature change, and requiring small size-system2]. In such type energy storage system, PCM can store and releasearge amounts of energy at a nearly constant temperature duringolid–liquid or vice phase change process. Several PCMs have beenested scientifically and industrially in many applications, such asn energy-efficient building materials [3–5], solar energy storageystems [6,7], greenhouses [8,9], temperature regulating textiles10,11], transportation packaging of temperature sensitive materi-ls [12], and heat management of electronics [13].

PCMs can be used as composite materials in buildings. The

uilding composite PCM will provide thermal storage distributedll through the building, allowing passive solar design and off-eak cooling in traditional frame constructions with a typical low

∗ Corresponding author. Tel.: +90 356 2521616; fax: +90 356 2521585.E-mail addresses: ahmet.sari@gop.edu.tr, asari@gop.edu.tr (A. Sarı).

378-7788/$ – see front matter © 2012 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.enbuild.2012.04.010

ildings since they had relatively higher latent heat capacity.© 2012 Elsevier B.V. All rights reserved.

thermal mass [14]. The performance of the building composite PCMwill depend on several factors: the melting point or temperaturerange of the PCM confined into the composite, the latent capac-ity per unit mass of the composite PCM, the preparation methodof the composite PCM, the direction of the wall prepared with thecomposite PCM, climatic conditions, direct solar gains, etc. [1]. Thefrequency of internal air temperature swings of a building envelopecan be decreased by using building composite PCM and the indoorair temperature of building envelope can be brought closer to thedesired temperature for a longer period of time [4].

The idea on the increase of thermal comfort in buildings by usingPCMs directed the researchers to build up new types of compos-ite materials and investigate their potential for minimizing energyconsumptions in buildings. The incorporation of some organicPCMs with construction materials such as gypsum board, plaster,concrete or other wall covering material has been studied exten-sively [15]. The heat storage capacity and structural stability of acomposite consisted of an inorganic salt hydrate and porous con-crete were investigated prepared [16]. Moreover, in recent yearsseveral organic PCMs such as paraffin, fatty acid, fatty acid eutecticmixture, fatty acid ester were incorporated with various buildingmaterials like cement, gypsum, vermiculite, attapulgite, diatomite,expanded perlite, activated montmorillonite, silica fume and clay

mineral [17–39]. The main reason for preferring the organic PCMsis their chemical compatibility with the studied building materials.On the other hand, fatty acids esters are reported as suitable PCMsfor preparation of energy storing composites due to their good

74 A. Sarı, A. Bic er / Energy and Buildings 51 (2012) 73–83

Table 1Chemical compositions of the building materials used in this study.

Building material SiO2 Al2O3 Fe2O3 CaO MgO K2O H2O CaSO4 Other

Gypsum – – – – – – 6.6 93.4 –Cement 20.05 4.95 3.71 62.74 1.06 0.67 0.95 – 4.14Diatomite 92.8 4.2 1.5 0.6 0.3 0.67 – – 0.5Perlite 71.0–75.0 12.5–18.0 0.1–1.5 0.5–0.2 0 0.03–0.5 4.0–5.0 – –

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hermophysical properties, thermal reliability and the advantagef directly integration with construction materials. However, theumber of works on the fatty acid ester-based building compositeCMs is limited [18,19,36].

Xylitol penta palmitate (XPP) and xylitol penta stearate (XPS)re proper PCMs for thermal energy storage applications in termsf their suitable melting temperatures (20.30 ◦C and 34.48 ◦C) andatent heats (164.46 kJ/kg and 193.91 kJ/kg). Thus, these ester com-ounds can be considered as promising PCMs in fabrication ofovel form-stable building composites. On the other hand, gypsumnd cement are conventional construction materials used exten-ively in building applications. In addition, perlite, vermiculite andiatomite are porous, ultra-lightweight, environmentally safe and

ow-cost building materials. Therefore, these materials are con-iderably appropriate for the development of novel form-stableuilding composite PCMs.

The present study is focused on the investigation of thermalnergy storage potential of ten kinds of form-stable compos-te PCMs prepared by impregnation of XPP and XPS esters withypsum, cement, diatomite, perlite and vermiculite via vacuumdsorption method. The form-stable PCMs were characterizedtructurally and morphologically by using scanning electron micro-cope (SEM) and Fourier transformation infrared (FT-IR) analysisechniques. Thermal energy storage properties, thermal reliabilitynd thermal durability of the composite PCMs were determined bysing differential scanning calorimetry (DSC) and thermogravime-ry (TG) analysis. In addition, thermal conductivity of the compositeCMs was increased by addition of EG in mass fractions of 10%.

. Experimental

.1. Materials

Xylitol penta stearate (XPS) and xylitol penta palmitate (XPP)ere synthesized by Fischer esterification reaction as reported in

ur prior study [40,41]. Gypsum and cement were supplied fromhe Sias Company (Sivas, Turkey) and AS company (Burdur, Turkey),espectively. Diatomite, perlite and vermiculite were supplied byEG–TUG Industrial Minerals & Mines Company (Istanbul, Turkey),

zper Company (Izmir, Turkey) and Agrekal Company (Antalya,urkey), respectively. These materials were dried at 105 ◦C during4 h before use. Table 1 shows the chemical compositions of theuilding materials providing by the manufacturer company.

.2. Preparation of form-stable composite PCMs

Ten kinds of building composite PCMs, XPP/gypsum,PP/cement, XPP/perlite, XPP/diatomite, XPP/vermiculite,PS/gypsum, XPS/cement, XPS/perlite, XPS/diatomite andPS/vermiculite were prepared by using vacuum adsorptionethod [20,22]. In order to obtain the form-stable composite

CM, the composites were prepared at different mass fractionsf the esters between 10 and 80%. The adsorption process wasontrolled by using a heating apparatus under a constant vacuumondition. The PCM (XPP or XPS ester) was heated in a flask over

0 16.0–35.0 1.0–6.0 – – 0.2–1.2

its melting temperature and then added gradually to a weightedquantity of the building material. The adsorption process wasmaintained at 70 mbar during 60 min. After reaching maximumadsorption the composite was set aside to draw off excessive PCMand left for drying for 48 h. The final form-stable composite wasweighted and the mass fraction of PCM was calculated. In addition,in order to test PCM exudation from the porous spaces, eachcomposite was simultaneously heated at a constant temperatureabove the melting temperature of the ester compound confinedinto the building material. The composite that did not show esterleakage in liquid state was defined as form-stable compositePCM.

2.3. Characterization of the form-stable composite PCMs

The morphology and microstructures of the studied buildingmaterials and the prepared composite PCMs was investigated usinga LEO 440 model SEM instrument. The chemical structures of thecomposite PCMs were characterized by using a FT-IR spectropho-tometer (JASCO 430 model). The spectral analyses were carried outusing KBr pellets between 400 and 4000 cm−1 wavenumber.

Phase change temperatures and latent heat values of the com-posite PCMs were measured using a DSC instrument (Perkin ElmerJADE model) at 5 ◦C/min heating rate and under flowing nitrogen.All DSC measurements were repeated three times for each sample.The accuracy of enthalpy and temperature data was determined as±5% and ±0.01 ◦C, respectively.

Thermal durability of the prepared composite PCMs was deter-mined by using a TG analyzer (Perkin-Elmer TGA7 model) undernitrogen atmosphere at a constant heating rate of 10 ◦C/min. Theprepared composite PCMs were subjected to a thermal cycling testconsisted of consecutive 1000 melting and freezing cycles by usinga thermal cycler (BIOER TC-25/H model). Thermal reliability andchemical stability of the composite PCMs after thermal cycling testwere evaluated by using DSC and FT-IR analysis. Furthermore, inorder to increase the thermal conductivity of the prepared compos-ite PCMs, expanded graphite (EG) with high thermal conductivitywas added to the composites at the mass fraction of 10%. Thermalconductivities of the composite PCMs were measured at 25 ◦C byusing a KD2 thermal property analyzer.

3. Results and discussion

3.1. Morphology of form-stable composite PCMs

A series of composites were prepared at different mass frac-tions of XPP and XPS esters ranged from 10 to 80 (w/w) to attainmaximum adsorption ratio. The mixtures that did not exhibit anyseepage of PCM throughout the heating period were recognized asform-stable composite PCM with the maximum combination ratio.The maximum fractions for both XPP and XPS impregnated into

gypsum, cement, perlite, diatomite and vermiculite were found tobe 22, 17, 67, 48 and 42 wt%, respectively. As clearly seen fromthese data, the XPP/perlite and XPS/perlite composites include thehighest PCM ratio.

A. Sarı, A. Bic er / Energy and Buildings 51 (2012) 73–83 75

F um, (X ermic

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ig. 1. The photograph images of the (a) gypsum, (b) XPP/gypsum, (c) XPS/gypsPS/diatomite, (k) perlite, (l) XPP/perlite, (m) XPS/perlite, (n) vermiculite, (o) XPP/v

The SEM images and photograph images of the form-stableuilding composite materials were shown in Figs. 1 and 2, respec-ively. As clearly seen from the photograph images in Fig. 1, theomposites did not show any leakage of PCM in melted sate. More-ver, as seen from Fig. 2(a, d, g, k, and n), gypsum, cement, perlite,iatomite, and vermiculite have porous structures consisted ofough and accidental micropores. The SEM images of the compositeCMs in Fig. 2(b, c, e, f, h, i, l, m, o, and p) also show that XPP andPS were homogenously dispersed into the porous networks of theuilding materials. The SEM results indicated that the composites

ad good mechanical strength due to their multiple porous struc-ures of the building materials and maintained their form-stabletates during the PCM leakage test due to capillary and surfaceension forces between the components of the composites.

d) cement, (e) XPP/cement, (f) XPS/cement, (g) diatomite, (h) XPP/diatomite, (i)ulite, and (p) XPS/vermiculite.

3.2. FT-IR analysis of the form-stable composite PCMs

The composite PCMs were characterized by FT-IR spectroscopyto investigate the chemical compatibility between the esters andthe building materials. Fig. 3(a and b) shows the FT-IR spectra ofXPS, XPP esters, gypsum, cement, perlite, diatomite, vermiculite,and the form-stable composite PCMs. As can be seen in Fig. 3(a),The C O stretching vibration bands for both esters are observedat 1743 cm−1 and 1739 cm−1 as the stretching vibration bands ofC O groups was monitored at 1176 cm−1 and 1180 cm−1, respec-

tively. The peaks in range of 2923–2854 cm−1 are identified assymmetric and asymmetric stretching peaks of C H groups ofthe esters. From Fig. 3(b), the following bands were recorded forcement: calcium hydroxide bands (3646 cm−1), molecular water

76 A. Sarı, A. Bic er / Energy and Buildings 51 (2012) 73–83

F ent, (( (p) X

(8aocaF31dbt

npra

ig. 2. The SEM images of the (a) gypsum, (b) XPP/gypsum, (c) XPS/gypsum, (d) cemk) perlite, (l) XPP/perlite, (m) XPS/perlite, (n) vermiculite, (o) XPP/vermiculite, and

3440–3446 and 1619–1632 cm−1), carbonate phases (1410–1500,65–885 and 705–717 cm−1), sulphates phases (1112–1153 cm−1),nhydrous calcium silicates (520–540 and 455–468 cm−1). More-ver, for gypsum the main bands seen at 1150 cm−1 and 1619 cm−1

an be attributed to the stretching vibration of OH group in waternd bending vibration of S O group in CaSO4. As also seen fromig. 3(b) the stretching vibration bands observed in the range of400–3500 cm−1 and the bending vibration bands in the range of560–1720 cm−1 correspond to the OH group of water in perlite,iatomite and vermiculite. Moreover, the absorption bands seen inetween 990 cm−1 and 1150 cm−1 represent the stretching vibra-ion of Si O group of perlite, diatomite and vermiculite.

On the other hand, as seen from Fig. 3(c and d), after the impreg-

ation of XPS and XPP the FT-IR spectra of the cement, gypsum,erlite, diatomite, and vermiculite included new absorption peakselated with the characteristic peaks of the esters. Any new peakspart from the characteristic peaks of the esters and the building

e) XPP/cement, (f) XPS/cement, (g) diatomite, (h) XPP/diatomite, (i) XPS/diatomite,PS/vermiculite.

materials were not observed in FT-IR spectra of the composites.This means that there is no chemical interaction between the estersand the building materials. In addition, when compared the FT-IRresults of the esters with that of the composite PCMs, some littleshifts can be observed in especially characteristic bands of the com-posites. These results may be due to the physical interactions suchas capillary and surface tension forces between the components ofcomposites preventing the leakage of the esters during the heat-ing processes of compounds. The similar results were reported fordifferent building composites [31,32].

3.3. Thermal properties of the esters and the form-stablecomposite PCMs

DSC is an accepted as suitable method for measuring the thermalenergy storage properties of a PCM since it prevents uncertaintyabout phase change temperatures, enthalpies, and subcoolings. The

A. Sarı, A. Bic er / Energy and Buildings 51 (2012) 73–83 77

Fig. 3. FT-IR spectra of (a) XPP and XPS esters, (b) building materials, (c) the form-stable composites with XPP content and (d) the form-stable composites with XPS content.

Fig. 4. The DSC curves obtained for the melting and freezing of XPP and XPS.

78 A. Sarı, A. Bic er / Energy and Buildings 51 (2012) 73–83

Fig. 5. The DSC curves for the melting and freezing of form-stable composites with XPP content.

Fig. 6. The DSC curves for the melting and freezing of form-stable composites with XPS content.

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

Material Phase change temperature formelting (◦C)

Latent heat of melting (J/g) Phase change temperature forfreezing (◦C)

Latent heat of freezing (J/g)

On-set Peak On-set Peak

XPP 20.30 26.10 164.46 20.35 17.25 162.45XPS 34.48 42.29 193.91 30.74 26.74 191.97XPP/gypsum 20.25 23.96 43.44 20.14 18.01 37.14XPP/cement 20.51 23.16 27.55 20.31 18.72 24.01XPP/perlite 20.62 26.63 106.60 20.22 16.77 104.80XPP/diatomite 20.61 25.65 77.43 20.30 17.39 78.64XPP/vermiculite 20.45 24.50 59.56 20.17 17.93 57.38XPS/gypsum 34.43 37.23 39.35 31.54 29.30 41.73XPS/cement 34.28 38.02 38.28 31.49 29.01 32.63XPS/perlite 34.04 39.60 125.73 31.35 27.64 123.63XPS/diatomite 34.70 40.11 86.81 32.31 29.08 87.75XPS/vermiculite 34.50 39.53 77.44 30.72 27.14 72.64

A. Sarı, A. Bic er / Energy and Buildings 51 (2012) 73–83 79

Table 3Comparison of thermal energy storage properties of the prepared composite PCMs with that of some composite PCMs in the literature.

Composite PCM Melting point (◦C) Freezing point (◦C) Latent heat (J/g) Reference

Lauric–stearic acid/gypsum 34.0 – 50.30 [17]ETP/cement 21.96 14.50 37.20 [19]ETS/cement 32.23 29.83 35.96 [19]ETP/gypsum 21.62 14.49 42.29 [19]ETS/gypsum 32.30 29.45 43.26 [19]Capric–palmitic acid/gypsum 22.90 21.70 42.50 [21]Capric–stearic acid/gypsum 23.80 23.90 49.0 [22]Capric–lauric acid/diatomite 16.74 – 66.80 [28]Capric–palmitic acid/attapulgite 21.71 – 48.20 [28]n-Nonadecane/cement 31.90 31.80 69.10 [29]RT20/montmorillonite 23.0 – 79.30 [30]Paraffin/expanded perlite 28.11 – 147.92 [34]n-Hexadecane/Na–montmorillonite 17.0 14.0 126.0 [35]ETP/diatomite 19.60 14.3 110.60 [36]ETP/perlite 19.80 14.4 119.0 [36]ETS/diatomite 29.8 30.0 116.10 [36]ETS/perlite 30.10 30.0 119.10 [36]XPP/gypsum 20.25 20.35 43.44 Present studyXPP/cement 20.51 30.74 27.55 Present studyXPP/perlite 20.62 20.14 106.60 Present studyXPP/diatomite 20.61 20.31 77.43 Present studyXPP/vermiculite 20.45 20.17 59.56 Present studyXPS/gypsum 34.43 20.30 39.35 Present studyXPS/cement 34.28 20.22 38.28 Present study

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XPS/perlite 34.04

XPS/diatomite 34.70

XPS/vermiculite 34.50

SC curves of XPP, XPS and the form-stable composite PCMs dur-ng heating and subsequent cooling are presented in Figs. 4–6,espectively. As clearly seen from Fig. 4, the XPP and XPS esterselt at 20.35 ◦C and 34.48 ◦C (on-set values), respectively as they

reeze at 20.30 ◦C and 30.74 ◦C (on-set values), respectively. Theseata showed that these esters were noticeably suitable for thermalnergy storage applications in buildings with respect to the climateonditions. The on-set and peak temperature values regarding withelting and freezing of the composite PCMs were also given in

able 2. As also seen from Fig. 5 and Table 2, the on-set melting tem-eratures of XPP/gypsum, XPP/cement, XPP/perlite, XPP/diatomite,PP/vermiculite, were measured as 20.25, 20.51, 20.62, 20.61, and0.45 ◦C, respectively while the on-set freezing temperatures wereeasured as 20.14, 20.31, 20.22, 20.30, and 20.17 ◦C for these

omposites, respectively. On the other hand, from Fig. 6, the on-et melting temperatures of XPS/gypsum, XPS/cement, XPS/perlite,PS/diatomite and XPS/vermiculite were determined to be 34.43,4.28, 34.04, 34.70, and 34.50 ◦C whereas the on-set freezing tem-eratures were determined to be 31.54, 31.49, 31.35, 32.31 and0.72 ◦C for the same composites, respectively. When compared

he phase change temperatures of the composite PCMs with that ofhe XPP and XPS esters, it can be seen small discrepancies. It maye due to the physical interactions identified as capillary forces andurface tension forces. The FT-IR spectroscopy results also verified

able 4he measured DSC data of the prepared form-stable composite PCMs after 1000 thermal

Composite PCM Phase change temperature formelting (◦C)

Latent heat of melting

On-set Peak

XPP/gypsum 19.95 23.88 41.72

XPP/cement 20.35 23.70 27.33

XPP/perlite 20.81 26.81 98.08

XPP/diatomite 20.36 24.50 68.70

XPP/vermiculite 20.57 24.33 55.65

XPS/gypsum 30.93 35.02 35.53

XPS/cement 31.43 35.61 35.17

XPS/perlite 31.42 37.12 121.42

XPS/diatomite 31.15 35.94 83.51

XPS/vermiculite 31.20 36.70 71.13

1.54 125.73 Present study1.49 86.81 Present study0.72 77.44 Present study

this idea. Moreover, by considering the phase change temperaturesof the composite PCMs it can be also concluded that the preparedten kinds of composite PCMs can be used as energy storage materialfor solar space heating and cooling applications.

On the other hand, from Figs. 5 and 6, the latent heats of melt-ing and freezing were found to be 164.46 and 162.45 J/g for XPPester and 193.91 and 191.97 for XPS ester and 43.44 and 37.14 J/gfor XPP/gypsum, 27.55 and 24.01 J/g for XPP/cement, 106.60 and104.80 J/g for XPP/perlite, 77.43 and 78.64 J/g for XPP/diatomite,and 59.56 and 57.38 J/g for XPP/vermiculite. In addition, the latentheats of melting and freezing were determined as 39.35 and41.73 J/g for XPS/gypsum, 38.28 and 32.63 J/g for XPS/cement,125.73 and 123.63 J/g for XPS/perlite, 86.81 and 87.75 J/g forXPS/diatomite, and 77.44 and 72.64 J/g for XPS/vermiculite. Thesevalues make the prepared composites appropriate PCMs for ther-mal energy storage utility in buildings. Moreover, compared to theother composite PCMs especially, the composites of XPP and XPSesters with perlite and diatomite can be taken into account the bestproper composite PCMs due to their higher latent heat values. Inaddition, the theoretical latent heat values of the composite PCMs

were calculated by using the following equation:

�Hcomp = PCMmf% × �HPCM (1)

cycles.

(J/g) Phase change temperature forfreezing (◦C)

Latent heat of freezing (J/g)

On-set Peak

20.28 18.29 42.8720.26 18.55 25.5420.47 16.96 98.0020.42 18.02 65.0620.46 17.93 50.7630.92 28.72 34.5730.81 28.30 33.0130.97 27.29 118.8831.37 27.97 82.2830.84 27.55 68.94

8 nd Buildings 51 (2012) 73–83

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Table 5TG data of XPP, XPS esters and the prepared form-stable composite PCMs.

Material Degradation temperature (◦C) Weight loss (%)

XPP First step: 79–271 First step: 48.2Second step: 271–452 Second step: 51.8

XPS First step: 58–306 First step: 46.8Second step: 306–519 Second step: 53.2

XPP/gypsum First step: 45–278 First step: 12.3Second step: 278–387 Second step: 11.6Third step: >510 Third step: 76.1

XPP/cement First step: 45–210 First step: 7.2Second step: 210–336 Second step: 10.1Third step: >544 Third step: 82

XPP/perlite First step: 80–258 First step: 24.9Second step: 258–554 Second step: 53.9Third step: >554 Third step: 21.2

XPP/diatomite First step: 45–290 First step: 25.3Second step: 290–438 Second step: 18.4Third step: >524 Third step: 56.3

XPP/vermiculite First step: 55–241 First step: 10.9Second step: 241–529 Second step: 28.1Third step: >529 Third step: 61.0

XPS/gypsum First step: 50–234 First step: 8.0Second step: 234–572 Second step: 18.4Third step: >572 Third step: 73.7

XPS/cement First step: 82–376 First step: 6.2Second step: 376–563 Second step: 12.4Third step: >563 Third step: 81.4

XPS/perlite First step: 96–276 First step: 23.9Second step: 276–549 Second step: 40.5Third step: >549 Third step: 35.6

XPS/diatomite First step: 57–282 First step: 18.8Second step: 282–499 Second step: 29.1Third step: >499 Third step: 52.1

XPS/Vermiculite First step: 72–330 First step: 22.2

0 A. Sarı, A. Bic er / Energy a

In this equation, �Hcomp, PCMmf% and �HPCM denote the theo-etical latent heats of the composite PCM, the mass fraction (%) ofCM (XPP or XPS esters) hold by the building material and the mea-ured latent heat values of the esters, respectively. For the latenteat values of melting, the differences between the measured andalculated values were found as −9.97% for XPP/gypsum, −0.91%or XPP/cement, 2.67% for XPP/perlite, 1.08% for XPP/diatomite,.91% for XPP/vermiculite, 6.46% for XPS/gypsum, −9.63% forPS/cement, 2.64% for XPS/perlite, 5.94% for XPS/diatomite, 3.99%

or XPS/vermiculite. Moreover, the differences regarding with theatent heat of freezing were found as −5.36 for XPP/gypsum, 10.94%or XPP/cement, 4.02% for XPP/perlite, −1.70% for XPP/diatomite,.16% for XPP/vermiculite, −0.19% for XPS/gypsum, −2.41% forPS/cement, 3.30% for XPS/perlite, 3.96% for XPS/diatomite, 9.02%

or XPS/vermiculite. As seen from these values, the latent heatalues measured for both melting and freezing processes of theomposite PCMs are close to the theoretical values.

On the other hand, in Table 3, the energy storage prop-rties of the prepared form-stable composite PCMs wereompared with that of other composites in the literature17,19,21,22,28–30,34–36]. By considering the these data, it can beppreciably noted that among the prepared composite PCMs, espe-ially XPP/perlite, XPS/perlite, XPP/diatomite, and XPS/diatomite,ave relatively higher latent heat capacity than the most of theomposite PCMs in the literature.

.4. Thermal reliability and chemical stability of the form-stableomposite PCMs

A building composite PCM should be stable thermally and chem-cally even it is subjected to a large number of thermal cyclingrocesses. In this study, thermal reliability and chemical stabil-

ty of the prepared composite PCMs after repeated 1000 meltingnd freezing cycles were evaluated by using DSC and FT-IR anal-sis. The DSC results obtained after 1000 thermal cycles wereiven in Table 4. As seen from this table, after thermal cycling,he change in the on-set melting and on-set freezing tempera-ures of the composite PCMs were found to be −0.3 ◦C and 0.14 ◦Cor XPP/gypsum, −0.16 ◦C and −0.05 ◦C for XPP/cement, 0.19 ◦Cnd 0.25 ◦C for XPP/perlite, −0.25 ◦C and 0.12 ◦C for XPP/diatomite,.12 ◦C and 0.29 ◦C for XPP/vermiculite. In similar way the changesere found as −3.5 ◦C and −0.62 ◦C for XPS/gypsum, −2.85 ◦C and0.68 ◦C for XPS/cement, −2.62 ◦C and −0.38 ◦C for XPS/perlite,2.89 ◦C and −0.94 ◦C for XPS/diatomite, −3.5 ◦C and −0.12 ◦C forPS/vermiculite. These little changes mean that that the preparedomposite PCMs had good thermal reliability with respect to theirhase change temperatures.

In addition, by comparing the latent heats of melting in Table 4ith the data obtained before thermal cycling, the changes were

ound to be −3.94% for XPP/gypsum, −0.79% for XPP/cement,7.99% for XPP/perlite, −11.27% for XPP/diatomite, −6.56% forPP/vermiculite, −9.70% for XPS/gypsum, −8.12% for XPS/cement,3.42% for XPS/perlite, −3.80% for XPS/diatomite, −8.14% forPS/vermiculite. Moreover, the changes in latent heat valuesegarding with the freezing processes was determined to be 15.42%or XPP/gypsum, 6.37% for XPP/cement, −6.48% for XPP/perlite,17.26% for XPP/diatomite, −11.53% for XPP/vermiculite, −17.15

or XPS/gypsum, 1.16% for XPS/cement, −3.84% for XPS/perlite,6.23% for XPS/diatomite, −5.09% for XPS/vermiculite. These

esults indicated that the prepared composite PCMs had good ther-al reliability after repeated 1000 thermal cycling in terms of the

hange in their latent heat values.

On the other hand, the chemical stability of the form-stable com-

osite PCMs after thermal cycling was also investigated by FT-IRpectroscopy analysis. As clearly seen from Fig. 7(a and b) there isot available any change in the shapes of the peak and deviation

Second step: 330–530 Second step: 18.4Third step: >530 Third step: 59.4

in the peak positions before and after thermal cycling. This meansthat the form-stable composite PCMs had good chemical stabilityafter thermal cycling.

3.5. Thermal durability of the form-stable composite PCMs

The thermal durability property is one of the most importantparameters for a composite PCM used in thermal energy storageapplications because it should be durable over its working tem-peratures. The thermal durability limits of the prepared compositePCMs were investigated by TG analysis. TG curves of XPP, XPSesters and the prepared form-stable composite PCMs were shownin Fig. 8(a–c) and the obtained data were also given in Table 5. Asshown in Fig. 8(a) and Table 5, the weight loss processes of XPPand XPS were carried out by two steps. For the XPP ester, the firststep corresponding to about 48% weight loss is between 79 ◦C and271 ◦C and the second one corresponding to about 52% weight lossis laid between 271 ◦C and 452 ◦C. As similar, at the first degradationstep in range of 58–306 ◦C XPS ester show as about 47% weight losswhile it lost its weigh part as about 53% in the range of 306–519 ◦C.As also seen from Fig. 8(b and c) and Table 5, all the form-stablecomposite PCMs degrade at three steps. The first two belong to theester compounds confined into the building materials. Moreover,

the amounts of total weight loss regarding with the first and secondsteps of the composite PCMs are very close to those of the compositePCMs. The final step is attributed to the degradation or evaporationof the metal oxides and water contents of the building material. As

A. Sarı, A. Bic er / Energy and Buildings 51 (2012) 73–83 81

Fig. 7. FT-IR spectra of the form-stable composite PCMs after thermal cycling.

Fig. 8. TG curves of (a) XPP and XPS esters, (b) the form-stable composites with XPP content, and (c) the form-stable composites with XPS content.

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lso seen from Table 4 the initial degradation temperatures regard-ng with the first steps of all the composite PCMs varied from 45 ◦Co 96 ◦C while the initial degradation temperatures regarding withhe second steps ranged from 210 ◦C to 376 ◦C. These results indi-ated that the initial degradation temperatures obtained for bothhe first and second steps of the composite PCMs showed a slightecrease in comparison with those of the ester compounds. How-ver, they are much higher than the phase change temperatures ofhe composite PCMs. Therefore, based on the TG results it can beoncluded that the prepared composite PCMs have good thermalurability above their working temperature range, 20–35 ◦C.

.6. Thermal conductivity of the PCMs

The thermal conductivity of PCMs can be considered one of theost important parameters in thermal energy storage applications

s well as phase change temperature and latent heat capacity. Thehermal energy transfer ratio of PCMs depends on this parameterecause it has a significant effect on the rates of energy storage andhe release of PCM [42,43]. In this study, the thermal conductivityf the form-stable composite PCM was increased by additionf expanded graphite (EG) in 10% mass fraction. The thermalonductivity values of XPP/gypsum, XPP/cement, XPP/perlite,PP/diatomite, XPP/vermiculite, XPS/gypsum, XPS/cement,PS/perlite, XPS/diatomite and XPS/vermiculite were measured as.21, 0.22, 0.11, 0.10, 0.11, 0.13, 0.16, 0.10, 0.11, and 0.12 W/mK.fter the addition of EG (10 wt%), thermal conductivity of the com-osites were measured as 0.28, 0.26, 0.14, 0.14, 0.16, 0.17, 0.20,.14, 0.15, and 0.16 W/mK, respectively. These results revealedhat thermal conductivity of the composite PCMs were increaseds 33, 18, 27, 40, 46, 31, 25, 40, 36, 33%, respectively after the EGddition.

On the other hand, the DSC analysis was performed to inves-igate the effect of EG additive on the thermal energy storageroperties of the composite PCMs. After ED addition, the melt-

ng temperatures of the composite PCMs decreased slightly rangedrom 0.01 ◦C to 1.3 ◦C as the decrease in the latent heat values ofhe composite PCMs was only between 5.4% and 8.6%.

. Conclusions

This study is focused on the preparation and properties of noveluilding material-based composite PCMs for thermal energy stor-ge and the conclusions are as follows:

1) Ten kinds of composite PCMs, XPP/gypsum, XPP/cement,XPP/perlite, XPP/diatomite, XPP/vermiculite, XPS/gypsum,XPS/cement, XPS/perlite, XPS/diatomite and XPS/vermiculitewere prepared as novel-form stable building composite PCMby using vacuum adsorption method.

2) The maximum mass percentages of XPP and XPS esters con-fined into gypsum, cement, perlite, diatomite and vermiculitewere determined as 22, 17, 67, 48 and 42 wt%, respectively.The composites with these combination ratios were identi-fied as form-stable PCMs which have good mechanical strengthbecause of capillary and surface tension forces between com-ponents of the composites.

3) The prepared form-stable composite PCMs were investigatedmorphologically by using SEM analysis techniques. The SEMresults indicated that the XPP and XPS esters were distributedhomogenously into the porous structures of the building mate-

rials. The form-stable composite PCMs were also characterizedby FT-IR spectroscopy and the results showed that there is nochemical interaction between the esters and the building mate-rials.

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ildings 51 (2012) 73–83

(4) The thermal energy storage properties of composite PCMs weremeasured by DSC analysis technique. The DSC results showedthe melting temperatures and latent heat capacities of the com-posite PCMs varied from 20 ◦C to 35 ◦C and from 38 J/g to 126 J/g.Especially the composites of XPP and XPS esters with perlite,vermiculite and diatomite can be taken into account the bestsuitable composite PCMs since they had relatively higher latentheat capacity than the others.

(5) The DSC and FT-IR results obtained after 1000 thermal cyclingtest demonstrated that the composite PCMs had good thermalreliability and chemical stability.

(6) TG analysis results exhibited that the composite PCMs hadexcellent thermal durability above their operating temperatureranges.

(7) Thermal conductivities of the composite PCMs were improvedby addition of EG in mass fraction of 10%. The increase amountin thermal conductivities of the composite PCMs was between18% and 46%. Moreover, the effect EG addition on the energystorage properties of the composite PCMs was insignificant.

Acknowledgement

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

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