Materials Chemistry and Physics 109 (2008) 459–464
Preparation, thermal properties and thermal reliability of capricacid/expanded perlite composite for thermal energy storage
Ahmet Sarı ∗, Ali KaraipekliDepartment of Chemistry, Gaziosmanpasa University, 60240 Tokat, Turkey
Received 9 September 2007; received in revised form 3 December 2007; accepted 7 December 2007
bstract
The aim of this research is the preparation of a novel form-stable composite PCM by incorporation of capric acid (CA) within the expandederlite (EP), characterization of the composite by SEM and FT-IR technique and determination of thermal properties and thermal reliability of theomposite PCM using DSC analysis. The maximum proportion of CA as phase change material (PCM) in the composite was found as 55 wt%.his composite was specified as form-stable because it does not allow the melted PCM leakage even when it is heated over the melting point of
A. Thermal properties of the form-stable CA/EP composite PCM were measured using DSC analysis. Thermal reliability of the composite PCMas investigated by thermal cycling test with respect to the changes in its thermal properties. Thermal conductivity of the composite PCM was
lso increased approximately as 64% by adding 10 wt% expanded graphite. 2007 Elsevier B.V. All rights reserved.
Thermal energy storage (TES) is one of the most effectiveethods for energy efficiency, energy savings, use of avail-
ble resources and renewable energies. Thermal energy can betored as sensible heat, latent heat, and thermo-chemical heat,r combination of these. Among the TES methods, latent heathermal energy storage (LHTES) method by phase change mate-ial (PCM) is one of the most preferred methods because of itsigh-storage density and property of storing heat at a constantemperature. A large number PCMs such as salt hydrates, paraf-n, non-paraffin organic acids, clathrates, eutectic organic and
norganic compounds have been investigated for utilization inHTES applications [1–5].
One of the promising LHTES applications is the use of PCMsn building to improve thermal comfort of indoor environment inhich people work and live [6,7]. Integration of building mate-
ials (gypsum wallboard, concrete, clay and etc.) with a PCM
rovides a new functional and effective composite building ele-ent which can affect significant energy savings [8–10]. There
re two common methods for incorporation of PCMs within con-
perties; Thermal reliability; Thermal energy storage
truction elements [11]. One is the incorporation of PCM intouilding materials during fabrication process. The other is thempregnation of building material in the liquid PCM. The secondne appears to be the most simple and economical procedure.owever, the composite PCM prepared using this method shoulde form-stable over several melting-freezing cycles from pointf view of preventing PCM leakage in melted state.
A great deal of candidates inorganic and organic PCMs andheir mixtures as LHTES materials have been studied recentlyor impregnating into common building materials [12–19].mong the PCMs, fatty acids, fatty acid esters and their mixturesave been attracted a great interest due to their good character-stics such as suitable phase change temperature, high latenteat capacity, congruently melting, non-toxicity, no super cool-ng, low vapour pressure, no or less volume change duringolid–liquid phase change, high thermal and chemical stabil-ty after long-term utility period [20–25]. In addition, they cane incorporated directly into conventional building materials6,13,15].
Many porous materials have been used in construction field.erlite is a glassy amorphous volcanic rock and can be expanded
o 7–16 times its original volume when it is heated at temper-ture range, 700–1200 ◦C [26,27]. Due to its low density andelatively low price, many commercial applications for perliteave developed. In the construction and manufacturing fields,
t is used in lightweight plasters and mortars, insulation, andeiling tiles. Expanded perlite (EP) is abundantly available inurkish markets. Considering all mentioned above, EP is one of
he most feasible candidates as an economical building materialor incorporation of fatty acids as PCMs. However, the principalroblem of EP is the low thermal conductivity. This probleman be partially solved by introduction of a material with highhermal conductivity into EP.
The objective of this paper was the preparation of CA/EPomposite PCM as novel form-stable composite PCM by incor-oration of CA within the EP, characterization of the compositey SEM and FT-IR technique and determination of thermalroperties and thermal reliability of the composite PCM usingSC analysis. Thermal conductivity of the form-stable compos-
te PCM was also increased by adding expanded graphite (EG)ith high thermal conductivity. In addition, the influence of the
hermal conductivity enhancement on heat storage and releaseates of the composite PCM was also evaluated experimentally.
. Experimental
.1. Materials
Capric acid (CA, 98% pure, m.p.: 32.14 ◦C) was purchased from Flukaompany. Expanded perlite (EP) was supplied from Izper Company (Izmir-urkey). The chemical constituent of the EP is given in Table 1. The EP wasried for 24 h and sieved by 150 �m-mesh sieve. The pore size distribution of theP was measured by means of mercury intrusion porosimeter (Quantachromeorporation, Poremaster 60 model). The micrographs of the EP and CA/EP
omposite PCM was taken by Scanning Electron Microscope (SEM, JEOL JWS-400). Expanded graphite (EG; thermal conductivity: 2–90 W m−1 K−1) used toncrease thermal conductivity of the composite PCM was supplied from Astasompany (Sivas, Turkey).
tcpwa
Fig. 1. (a) Vacuum impregnation set-up and (b)
y and Physics 109 (2008) 459–464
.2. Preparation of the form-stable composite PCM
In the preparation of CA/EP composite PCM, vacuum impregnation methodas conducted (Fig. 1a). To determine the maximum absorption ratio at whicho melted PCM seepage was observed, a series of CA/EP composites in differenteight proportions (30, 40, 50, 60, and 70% w/w) were prepared. The CA used
s PCM was absorbed into EP as much as 55% of total weight of the composite.his composite was described as form-stable composite PCM since it did notllow the melted CA seepage from the composite PCM. To promote the ther-al conductivity of prepared form-stable composite PCM, expanded graphite
EG) was introduced into the composite in mass fraction of 10 wt%. Thermalonductivities of the form-stable CA/EP and CA/EP/EG composite PCMs wereeasured by using hot-wire method [28,29].
.3. Thermal cycling test
Thermal cycling test was conducted to determine the thermal reliability oform-stable CA/EP composite PCM. The test was done consecutively up to 5000hermal cycling using the procedure given in literature [17,22,30,31]. Thermaleliability of the composite PCM with respect to thermal cycling number, 1000,000, and 5000 was evaluated using DSC analysis in terms of the changes in itshase change temperatures and latent heats.
.4. Melting and freezing performance test
The melting and freezing performances of the form-stable CA/EP andA/EP/EG composite PCMs were tested using experimental set-up shown inig. 1b. For the performance test, both composite PCMs were separately filled
nto glass cells with the same dimensions. One thermocouple with a precisionf ±0.1 ◦C was placed at the centres of composites for temperature measure-ent. Water with a constant temperature, 35 ◦C was circulated for the melting
rocess through the test cells. After melting process of the composite PCMs,hey were immediately subjected to solidification process by circulating waterith a constant temperature, 15 ◦C. The temperature variation during the melting
nd freezing periods was automatically recorded with a data logger at 1 min-nterval.
.5. Analysis methods
Thermal properties of the form-stable composite PCM such as melting andreezing temperatures and latent heats were measured using a DSC instru-ent (SETARAM DSC 131). The analyses were conducted by a heating
ate, 5 ◦C min−1 under a constant stream of argon at atmospheric pres-ure. The phase change temperatures (melting and freezing) were takens the intersection between the tangent to the maximum rising slope of
he peak and the extrapolated sample baseline. The latent heats of phasehange were determined by numerical integration of the area under theeaks. DSC measurements were repeated three times. The mean deviationas ±0.12 ◦C and ±0.42 J g−1 in the measurement of phase change temper-
tures and latent heat capacities, respectively. FT-IR spectra of form-stable
thermal performance measurement set-up.
A. Sarı, A. Karaipekli / Materials Chemistry and Physics 109 (2008) 459–464 461
Csd
3
3P
maoc
ttcp1grga
3c
ptPmCtcFs[
Fig. 2. Pore size distribution of EP.
A/EP composite before and after thermal cycling were obtained by a FT-IRpectrophotometer (JASCO 430). FT-IR analyses were carried out using KBrisk.
. Results and discussion
.1. Characterization of form-stable CA/EP compositeCM
The EP has pore structure consisted from mesopores andacropores, as seen Fig. 2. Organic compounds such as fatty
cids can be absorbed easily in these pores. The SEM imagesf EP given in Fig. 3a and b show that the CA is successfullyonfined into pores of EP.
FT-IR spectroscopy technique was used to characterize ofhe composite PCM by evaluation of the interactions betweenhe components of the composite. The FT-IR spectrum of theomposite was compared with that of CA (Fig. 4). The strongeak of carbonyl group ( C O) of CA at 1710 cm−1 shifted to698 cm−1 in case of composite. The vibration peak of hydroxyl
roup ( OH) of CA at 732 cm−1 also shifted to 720 cm−1. Theseesults are evidence of the interaction between the COOHroup of the CA and alkaline region (SiO2, Na2O, K2O, MgO,nd CaO) in EP [32,33].
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Fig. 3. The SEM micrographs of EP (×500 magnification
Fig. 4. FT-IR spectra of CA and form-stable CA/EP composite PCM.
.2. Thermal properties of CA and form-stable CA/EPomposite PCM
DSC analyses have been conducted to measure the thermalroperties of CA and form-stable CA/EP composite PCM. DSChermograms of the CA and the form-stable CA/EP compositeCM are shown in Fig. 5. From the DSC curves, temperatures ofelting and freezing were determined as 32.14 and 32.53 ◦C forA and 31.80 and 31.61 ◦C for the CA/EP composite, respec-
ively. The decreases in phase change temperatures of CA in theomposite are probably due to the interaction characterized byT-IR analysis. However, these interactions are physical and notrong because of the weak alkalinity of the expanded perlite33]. The weak interaction between the PCM and inner surface
f the porous material also leads to a depression of the phasehange temperatures of PCM in the porous materials accordingo the opinions of Radhakrishnan and Gubbins [34] and Rad-akrishnan et al. [35]. Moreover, the pore’s walls of EP and
s). (a) Before loading CA and (b) after loading CA.
462 A. Sarı, A. Karaipekli / Materials Chemistry and Physics 109 (2008) 459–464
Table 2Comparison of thermal properties of the form-stable CA/EP composite PCM with that of different composite PCMs in literature
PCM Melting point (◦C) Freezing point (◦C) Latent heat (J g−1) References
Capric-lauric acid + fire retardant/gypsum 17.0 21.0 28.0 [13]Dodecanol/gypsum 20.0 21.0 17.0 [13]Propyl palmitate/gypsum 19.0 16.0 40.0 [13]Butyl stearate/gypsum 18.0 21.0 30.0 [13]CA-LA/gypsum 19.1 – 35.2 [17]CA-PA/gypsum 22.9 21.6 42.5 [18]CA/EP 31.8 31.6 98.1 This studyC 31.5 96.3 This study
aolie
fpiCitP
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anncccetb
Ft
tacarr
A/EP/EG 31.6
dhesion forces between the CA molecules and the cell surfacef EP may affect the mechanism of CA crystallization as it isocated in the pores of EP. They may act primarily as nucleat-ng agent, but next they may hinder crystallization due to stericffects.
On the other hand, the melting and freezing temperatures oform-stable CA/EP composite PCM were suitable for LHTESurposes in buildings. On the other hand, the latent heats of melt-ng and freezing were found to be 156.40 and 154.24 J g−1 forA and 98.12 and 90.06 J g−1 for form-stable CA/EP compos-
te PCM. In addition, the latent heats of melting and freezing ofhe composite PCM are as high as those of different compositeCMs in literature (Table 2).
.3. Thermal reliability of the form-stable CA/EPomposite PCM
The composite PCMs must be stable as thermal in practicefter long-term utility period. Therefore, there should be no sig-ificant change in their thermal properties after the repeatedumbers of heating and cooling. Thermal cycling test wasonducted to determine the thermal reliability of form-stableomposite PCM. Fig. 6 shows the DSC thermograms for CA/EP
omposite PCM before and after thermal cycling. Thermal prop-rties of the composite PCM are given in Table 3. From thisable, it is possible to observe the effect of thermal cycle num-er on thermal reliability of the composite PCM with respect to
Fig. 5. DSC curves of CA, and form-stable CA/EP composite PCM.
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TTt
T
N
0135
ig. 6. DSC curves of the form-stable CA/EP composite PCM before and afterhermal cycling.
he changes in its thermal properties. After repeated 1000, 3000,nd 5000 thermal cycling, the melting temperature of compositehanged by −0.48, −2.64 and −1.55 ◦C as the freezing temper-ture of the composite changed by −0.25, −0.31 and −0.09 ◦C,espectively. The changes in phase change temperatures withepeated thermal cycling are in acceptable level for LHTESpplications. Therefore, it can be concluded that the form-stableomposite PCM shows good thermal reliability with respect tohe changes in phase change temperatures. On the other hand,fter repeated 1000, 3000, and 5000 thermal cycling, the latenteat value of melting changed by −1.2%, −4.8%, and −2.6%s the latent heat value of freezing changed by 4.8%, −1.9%,
nd 0.6%, respectively. The decreases in the latent heat capacityf the composite PCM are negligible for LHTES applications inuildings [17,22,30,31].
able 3he changes in thermal properties of the CA/EP composite PCM with respect
o thermal cycling number
hermal properties
o of thermal cycling Tm (◦C) �Hm (J g−1) Tf (◦C) �Hf (J g−1)
A. Sarı, A. Karaipekli / Materials Chemistry and Physics 109 (2008) 459–464 463
boiani
3c
ip(TtiacOa
Fc
FC
mio
PoacTttpi(aicc
4
Fig. 7. FT-IR spectra of the CA/EP before and after thermal cycling.
Fig. 7 shows the FT-IR spectra of CA/EP composite PCMefore and after thermal cycling. The shape and frequency valuesf all peaks did not changed after thermal cycling. This resultndicates that the chemical structure of the composite was notffected by repeated melting/freezing cycling. Therefore, it isoteworthy noted that the form-stable CA/EP composite PCMs stable chemically after 5000 thermal cycling.
.4. Thermal conductivity improvement of the form-stableomposite PCM
The heat storage and release performances of PCMs aremportant criteria for an effective LHTES system. The pre-ared CA/EP composite PCM had a low thermal conductivity0.087 Wm−1 K−1) due to low thermal conductivity of the EP.o improve the thermal conductivity, the EG was added to
he composite in mass fraction of 10%. Thermal conductiv-ty of CA/EP/EG composite in the solid state was measured
s 0.143 Wm−1 K−1 using hot-wire method. Therefore, thermalonductivity of the composite PCM was increased as about 64%.n the other hand, the melting temperature, freezing temperature
nd latent heat of melting of the CA/EP/EG composite PCM was
ig. 8. Melting temperature curves of CA/EP composite PCM and CA/EP/EGomposite PCM.
p5tPnoarimaTwCIs
Ca
ig. 9. Solidification temperature curves of CA/EP composite PCM andA/EP/EG composite PCM.
easured as 31.6 and 31.5 ◦C, and 96.3 kJ kg−1. These resultsndicated that the EG added to PCM had not significant effectn thermal properties of the PCM.
The improvement in thermal conductivity of the compositeCM with addition of EG was also confirmed by comparisonf the melting and freezing times of the composite PCMs withnd without EG. Figs. 8 and 9 show the melting and freezingurves of the CA/EP and CA/EP/EG composites, respectively.he melting and freezing times were taken as time elapsed from
he same starting temperature to the phase change tempera-ure which represents the finishing point of the phase changerocess. The melting process took 25 min for CA/EP compos-te whereas it took only 15 min for the CA/EP/EG compositeFig. 8). The freezing time took 42 min for the CA/EP compositend only 37 min for CA/EP/EG composite (Fig. 9). These resultsndicated that the heat storage and release rates of CA/EP/EGomposite PCM were obviously higher than that of the CA/EPomposite PCM due to high thermal conductivity of EG.
. Conclusions
A novel form-stable composite PCM was prepared by incor-oration of CA within the EP. The composite PCM including5 wt% was described as form-stable because it does not allowhe leakage of melted PCM. The form-stable CA/EP compositeCM was characterized by SEM and FT-IR spectroscopy tech-iques. The melting and freezing temperatures and latent heatsf form-stable composite PCM were determined using DSCnalysis to be 31.80 and 31.61 ◦C and 98.12 and 90.06 J g−1,espectively. The composite PCM had good thermal reliabilityn terms of the change in thermal properties after 5000 ther-
al cycling. Thermal conductivity of the composite PCM waslso increased approximately as 64% by addition of 10 wt% EG.he improvement in thermal conductivity of the composite PCMas also verified by evaluation of melting and freezing times ofA/EP and CA/EP/EG composite PCMs. Furthermore, the FT-
R analysis confirmed that the composite had good chemical
tability after 5000 thermal cycling.
Based on all results it was also concluded that the especially,A/EP/EG as novel form-stable composite PCM can be useds functional and effective building material that can improve
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64 A. Sarı, A. Karaipekli / Materials Ch
ndoor thermal comfort due to absorption of heat in conjunctionith melting of the PCM.
cknowledgements
Authors thank Dr Orhan UZUN and Fikret YILMAZ due toSC and SEM analyses, and the staff of Central Laboratory iniddle East Technical University for porosity measurement.
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