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Fatty acid esters-based composite phase change materials for thermal energy storage in buildings
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 energy storage in buildings Ahmet Sarı a, b, *, Ali Karaipekli a, b a b
Department of Chemistry, Çankırı Karatekin University, 18200 Çankırı, Turkey Department of Chemistry, Gaziosmanpas¸a University, 60240 Tokat, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2011 Accepted 7 November 2011 Available online 22 November 2011
In this study, fatty acid esters-based composite phase change materials (PCMs) for thermal energy storage were prepared by blending erythritol tetrapalmitate (ETP) and erythritol tetrastearate (ETS) with diatomite and expanded perlite (EP). The maximum incorporation percentage for ETP and ETS into diatomite and EP was found to be 57 wt% and 62 wt%, respectively without melted PCM seepage from the composites. The morphologies and compatibilities of the composite PCMs were structurally characterized using scanning electron microscope (SEM) and Fourier transformation infrared (FTeIR) analysis techniques. Thermal energy storage properties of the composite PCMs were determined by differential scanning calorimetry (DSC) analysis. The DSC analyses results indicated that the composite PCMs were good candidates for building applications in terms of their large latent heat values and suitable phase change temperatures. The thermal cycling test including 1000 melting and freezing cycling showed that composite PCMs had good thermal reliability and chemical stability. TG analysis revealed that the composite PCMs had good thermal durability above their working temperature ranges. Moreover, in order to improve the thermal conductivity of the composite PCMs, the expanded graphite (EG) was added to them at different mass fractions (2%, 5%, and 10%). The best results were obtained for the composite PCMs including 5wt% EG content in terms of the increase in thermal conductivity values and the 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. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Fatty acid esters Composite PCM Diatomite Expanded perlite Thermal properties Thermal reliability
1. Introduction The energy demand to provide a comfortable environment for humans in buildings has continuously increased worldwide. But, the energy use for heating, cooling and air conditioning increase the level of greenhouse gas emissions and decrease fossil fuel sources [1,2]. Therefore, energy storage becomes a key issue in engineering application. Thermal energy storage plays an important role in an effective use of energy in buildings not only by reducing the mismatch between supply and demand but also improving the performance and reliability of energy systems. Among all of thermal energy storage methods (sensible, latent and thermochemical heat), latent heat thermal energy storage employing a phase change material (PCM) is particularly effective technique due to its advantages of high energy storage density and its isothermal operating characteristics [3e6]. * Corresponding author. Department of Chemistry, Gaziosmanpas¸a University, 60240 Tokat, Turkey. Tel.: þ90 356 2521616; fax: þ90 356 2521585. E-mail addresses: [email protected], [email protected] (A. Sarı). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.11.017
The application of PCMs in buildings is well known and has been subject to considerable interest since the first reported application in the 1940s [7,8]. Researches on the application of PCMs in buildings have been focused on three fields in recent years. The first one is the reduction of temperature fluctuations of lightweight buildings by increasing their thermal mass [9e11]. This is done by incorporation of PCM into building materials. The second one is the cooling of buildings through intermediate storage of cold from the night or other cheap cold sources. If the cold is for free, as with cold from night air, this is also called free cooling and very promising with respect to energy saving [12,13]. A third application field is for heat storage in space heating systems [14]. The PCMs can be used by integration with different building’s structure such as gypsum board, plaster, concrete, clay minerals or other wall-covering material. But, there are some difficulties in fabrication of building materials containing PCMs. One of them is incorporation of PCM in construction material. PCMs in building materials are usually enclosed in metallic or polymeric capsules. The encapsulation of the PCM is expensive and it may affect the mechanical strength of the building material as well as it may lead
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to seepage during the melting period of PCM. Therefore, it is needed to direct heat exchange between PCM and medium to provide higher thermal energy storage performance. From this point of view, the PCM based-building composites are promising materials since they enables no corrosion, quick heat transfer and offers a large heat storage density if building materials with high porosity are selected [15,16]. Many inorganic and organic PCMs and their mixtures as thermal energy storage materials have been studied recently for impregnating into common building materials [12,17e23]. Among the PCMs, fatty acids, their esters and mixtures as organic phase change materials has been recommended as energy storage materials due to their desirable thermal and heat transfer characteristics and the advantage of easily impregnation, or directly incorporation into conventional building materials [24e26]. Moreover, most of the fatty acid esters are commercially available, since they are already produced in large amounts for plastics, cosmetics, textile and other industries. However, only few ester compounds were used to obtain composite PCMs for energy storage in building applications [27e29]. In this study, some fatty acid ester-based composites were fabricated as novel potential PCMs for thermal energy storage in building applications. These composite PCMs were prepared by directly incorporation of erythritol tetrapalmitate and erythritol tetrastearate as fatty acid esters in diatomite and expanded perlite as building materials. The composites were characterized structurally by SEM and FTeIR analysis techniques. Thermal energy storage properties and thermal durability of the composite PCMs were determined by using DSC and TG analysis methods, respectively. Moreover, the thermal conductivities of prepared composite PCMs were improved by adding expanded graphite (EG) at mass fraction 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 method reported in our previous study [30]. The diatomite sample was supplied from BEGeTUG Industrial Minerals & Mines Company (Istanbul, Turkey). Expanded perlite (EP) was supplied from Izper Company (IzmireTurkey). The chemical composition of the diatomite and EP samples used as porous building material in preparation of the composites are given in Table 1. Perlite and diatomite samples were previously sieved by 150 mm-mesh sieve and dried at 105 C for 24 h before use. Expanded graphite (EG; thermal conductivity: 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/perlite composite PCMs were prepared by using direct impregnation method [31,32]. The composite PCMs were prepared by directly mixing the fatty acid ester in melted state with the building
Table 1 Chemical 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
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material directly. To determine the maximum holding ratios of the building materials for ETP for the fatty acid ester compounds, a series of composites with different mass fractions of ester compounds (10, 20, 30, 40, 50, 60, and 70% w/w) were prepared. The composite PCM was simultaneously heated during the impregnation process at a constant temperature above the melting temperature of the ester compound in order to test the exudation of fatty acid ester from the porous spaces. The ETP and ETS used as PCM could be retained as 57 wt % in diatomite and 62 wt % in EP without the leakage of melted PCM and therefore these composites were defined as form-stable composite PCMs. 2.3. Analysis methods The morphology of prepared ETP/diatomite, ETS/diatomite, ETP/ perlite and ETS/perlite composite PCMs were observed using a SEM instrument (LEO 440 model). The chemical compatibility between the components of the composites was investigated by FTeIR spectroscopy technique. FTeIR spectra were taken on a KBr disk at a frequency range of 4000e400 cm1 by using a FTeIR spectrophotometer (JASCO 430 model). Measurements of solideliquid phase change temperatures and latent heat capacities of composite PCM were carried out by using a DSC instrument (PerkineElmer Jade model) under nitrogen atmosphere and at a heating rate of 10 C/min. The maximum deviations in the measurements of phase change temperatures and latent heat values are found to be 0.3 C and 3.6 J/g, respectively. Certified Indium standard was used as a reference for temperature calibration of the instrument. Samples were measured in a sealed aluminum pan with a mass of about 5.0 mg. Melting point and freezing point were obtained by drawing a line at the point of maximum slope of the DSC peak. The latent heat was calculated by numerical integration of the peak using software of DSC instrument. The thermal durability of the composite PCMs was also determined by using PerkineElmer TGA7 thermal analyzer. In each case, the about 10 mg specimens were heated from 25 to 500 C using a linear heating rate of 10 C/min under nitrogen atmosphere. In addition, to increase the thermal conductivities of the prepared composite PCMs, expanded graphite (EG) was introduced into them at mass fraction of 2, 5 and 10 wt%. Thermal conductivity values of all composites were measured by using a KD2 thermal property analyzer. 2.4. Thermal cycling test Thermal cycling test was performed to determine thermal reliability of the composite PCMs in terms of the change in phase change temperatures and latent heat values after a large number of thermal cycling. The test was carried out consecutively up to 1000 thermal cycling using a thermal cycler (BIOER TCe25/H model). DSC and FTeIR analyses were repeated to determine the thermal and 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 prepared form-stable composite samples. It can be seen from these SEM micrographs that the diatomite and EP have porous structures, which allow adsorbing the ETS and ETP in liquid state. The SEM images of composite PCMs show that ETP and ETS were well retained into the diatomite and EP. The porous structures of the EP and diatomite provided the mechanical strength for the whole composites and prevented the seepage of the melted ETS and ETP
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Fig. 1. The SEM images of the diatomite, EP and the prepared composite PCMs.
due to the effect of capillary and surface tension force between the components of the composites. In this study, the maximum percentage of the ETP and ETS to be retained into EP and diatomite was determined as 57 wt % for ester/diatomite composites and 62 wt % for ester/EP composites. It was no observed PCM leakage from the composites with these combinations even when the ETP and ETS melt in the composites. 3.2. Chemical compatibility analysis The chemical compatibility among the components of the composites was characterized by evaluating specific interactions between fatty acid esters and the building materials the using FTeIR spectroscopy technique. Fig. 2 shows FTeIR spectra of the diatomite, EP, ETP, ETS and the composite PCMs. The significant absorption bands and wavenumbers obtained from the FTeIR spectra are also given in Table 2. In the spectrum of pure ETP and ETS, there are two characteristic strong absorption bands arising from the C]O and CeO groups, usually found in the 1800e1650 cm1 and 1310e1100 cm1 regions, respectively. The
peaks at 2917 and 2849 cm1 represent the stretching vibration of eCH3 and eCH2 group of the esters, respectively. The pure diatomite possesses the characteristic absorption bands at 3748, 1652, 1195, 1093, and 792 cm1. The band at 3748 cm1 is due to the free silanol groups (SiOeH) and the band at 1093 cm1 reflects the siloxane (eSieOeSie) group stretching. The peak at 792 cm1 represents SiOeH vibration. The pure EP has main absorption bands at 3417, 1635, 1045, 788 and 458 cm1 respectively. The absorption band at 3417 cm1 signifies the stretching vibration of functional group of SieOH. The peaks at 1045, 788 and 458 belong to the SieOeSi asymmetric stretching vibration, peaks of bending and stretching vibrations SieO functional group, respectively. The peak at 1635 cm1 is due to the vibration of the OH bond. On the other hand, as can be seen from Fig. 2, the FTeIR spectra of diatomite and EP after the incorporation of the ETP and ETS esters indicated new absorption peaks due to the characteristic absorption bands of the esters. Furthermore, there were no new peaks other than characteristic peaks of the esters and the building materials at FTeIR spectra of the composites. These results indicate
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Fig. 2. FTeIR spectra of ETP, ETS and the prepared composite PCMs.
Table 2 FTeIR 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 CeO CeH
e e e
e e e
OeH
1635
1739 1380 2919 2852 e
SieO SieOeSi SieOH
458 1093 3748
1635 788 458 1045 3417
1739 1373 2925 2856 e e e e
e e e
1739 1380 2929 2852 1635 796 468 1093 3448
1739 1382 2915 2850 1635 794 472 1095 3446
1739 1394 2921 2852 1635 796 472 1093 3446
1739 1396 2917 2848 1635 796 476 1093 3448
that there is no chemical interaction between the esters and the building materials. As also seen in Table 2, small shifts were observed in the absorption bands of some functional groups of the components of composites (CeH, CeO, OeH, SieOH and SieOeSi). These shifts can be due to weak physical interactions among the
functional groups mentioned. It can also be attributed to the capillary and surface tension forces between the esters molecules and the pores of diatomite and EP, which prevent the leakage of ETS and ETP in melted state from the composites during solideliquid phase transition [33]. Combined with the result of SEM images and
Fig. 3. The melting and freezing DSC curves of ETS, ETP and the prepared composite PCMs.
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Table 3 DSC data of the esters and the prepared composite PCMs. Sample name
Melting
Freezing
Onset temperature ( C) ETP ETS ETP/Diatomite ETS/Diatomite ETP/EP ETS/EP
21.9 30.4 19.6 29.8 19.8 30.1
0.3 0.3 0.3 0.1 0.3 0.2
Peak temperature ( C) 25.6 35.5 24.8 34.5 25.6 35.5
0.1 0.1 0.2 0.3 0.1 0.1
Latent heat of melting (J/g) 201.1 208.8 110.6 116.1 119.0 119.1
2.3 3.1 1.3 2.3 1.6 1.2
Melting
Freezing
Experimental value (kJ/kg) ETP/Diatomite ETS/Diatomite ETP/EP ETS/EP
110.6 116.1 119.0 119.1
1.3 2.3 1.6 1.2
Calculated value (kJ/g)
Experimental value (kJ/kg)
108.4 117.3 117.9 127.6
101.2 114.8 111.5 128.6
2.7 3.3 3.1 3.6
18.8 28.8 14.3 30.0 14.4 30.0
0.2 0.1 0.3 0.3 0.3 0.2
Peak temperature ( C) 15.7 26.7 12.0 27.1 11.7 26.4
0.1 0.1 0.1 0.1 0.2 0.3
Latent heat of freezing (J/g) 200.8 207.2 101.2 114.8 111.5 128.6
2.1 2.3 2.7 3.3 3.1 3.6
were measured to be 14.3 0.3 C, 14.4 0.3 C, 30.0 0.3 C, and 30.0 0.2 C, respectively. Although there are slightly changes in the phase change temperatures of the composites, they are very close to pure esters. These little changes in phase change temperatures of the esters in the composite are probably due to the weak interactions characterized by FTeIR analysis. The phase change temperature of PCMs using in building applications should be close to human comfort temperature (16e26 C). The ETS/diatomite and the ETS/EP composites have suitable melting and freezing temperature for thermal energy storage by applying the exterior wall of buildings whereas the phase change temperatures of the composite PCMs prepared by using the ETS are in the range of 29e32 C. Thus, if the composite PCMs including ETS are used in the fabrication of building walls, the new composite material can absorb heat from the surrounding air and solar radiation heat during day, then release heat to the surrounding air at night. Hence, the load of air conditioning system used to cooling in buildings may reduce. On the other hand, the latent heats of melting were measured as 110.6 1.3 J/g, 119.0 1.6 J/g, 116.1 2.3 J/g and 119.1 1.2 J/g for ETP/Diatomite, ETP/EP, ETS/Diatomite and ETS/EP, respectively as the 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. As also seen from Table 4, the measured latent heats of melting and freezing of the composite PCMs were close to the values calculated by multiplying the mass content of the ETP and ETS of the composites and their phase change enthalpies in pure state. However, the calculated values were slightly higher than measured
Table 4 Calculated latent heat capacities of the prepared composite PCMs. Composite PCM
Onset temperature ( C)
Calculated value (kJ/kg) 115.1 118.1 125.2 128.4
FTeIR spectroscopy, it can be concluded that the there are good compatibility between ester compounds and the studied building materials. 3.3. Thermal properties of the composite PCMs Thermal properties such as melting and freezing temperatures and latent heats are important parameters for a latent heat thermal energy storage system. Fig. 3 shows the typical DSC curves of the ETP, ETS and the composite PCMs. The melting and freezing temperatures obtained from the DSC curves were also given in Table 3. From these curves, the melting and freezing temperatures were determined as 21.9 0.3 C and 18.8 0.1 C for ETP and 30.4 0.1 C and 28.8 0.1 C for the ETS, respectively. The melting temperatures 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 5 Comparison of thermal energy storage characteristics of the prepared composite PCMs with those of some composite PCMs reported in literature. Composite PCM
Melting point ( C)
Freezing point ( C)
Latent heat (J/g)
Reference
Capricelauric acid/gypsum (26/74 w%) Emerest2326/gypsum Propyl palmitate/gypsum (25e30/70e75 w/w%) Dodecanol/gypsum (25e30/70e75 w/w%) Methyl PalmitateeStearate/wallboard (26.6/73.4 w%) Butyl stearate/gypsum (25e30/70e75 w/w%) Capricelauric acid þ fire retardant (25e30/70e75 w/w%)/gypsum Capricepalmitic acid/gypsum (25/75 w/w%) nenonadecane/cement (50/50 w/w%) Capricestearic acid/gypsum (25/75 w/w%) Capricemyristic acid/gypsum (25/75 w/w%) Capricemyristic acid/Vermicuilite (20/80 w/w%) Capricemyristic acid/Expanded perlite (55/45 w/w%) RT20/Montmorillonite (58/42 w/w%) PEG1000/Diatomite (50/50 w/w%) Decanoic/Dodecanoic acid/Diatomite CapricePalmitic acid/Attapulgite (35/65 w/w%) PEG1000/Cement (25/75 w/w%) ETP/Diatomite (57/43 w/w%) ETS/Diatomite (57/43 w/w%) ETP/EP (62/38 w/w%) ETS/EP (62/38 w/w%)
19.1 16.9 19.0 20.0 22.5 18.0 17.0 22.9 31.9 23.8 21.1 19.8 21.7 23.0 27.7 16.7 21.7 24.3 19.6 29.8 19.8 30.1
e 19.3 16.0 21.0 23.8 21.0 21.0 21.7 31.8 23.9 21.4 17.1 20.7 e 32.2 e e 27.9 14.3 30.0 14.4 30.0
35.2 35.0 40.0 17.0 41.1 30.0 28.0 42.5 69.1 49.0 36.2 27.0 85.4 79.3 88.0 66.8 48.2 23.9 110.6 116.1 119.0 119.1
[8] [27] [29] [29] [29] [34] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] Present Present Present Present
0.3 0.1 0.3 0.2
0.3 0.3 0.3 0.2
1.3 2.3 1.6 1.2
study study study study
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Fig. 4. The melting and freezing DSC curves of the prepared composite PCMs after thermal cycling.
values. These results can be probably caused by the physical interactions between the esters and the inner surface of pore of Daitomite and EP. In addition, Table 5 presents the comparison of energy storage properties of the prepared composite PCMs with those 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 have important latent heat thermal energy storage potential in buildings. 3.4. Thermal reliability of the composite PCMs The PCMs to be used for thermal energy storage have to be stable in terms of thermal and chemical after long term utility period in practice applications. Thus, the superior composite should be shown no or less change in its thermal properties and chemical structure after long term utility period. Therefore, thermal cycling test was performed to determine the change in thermal properties and chemical structure of the composite PCMs by DSC analysis and FTIR analysis, respectively before and after 1000 cycling. It is clearly seen from the DSC curves of the composite PCMs before and after 1000 thermal cycling (Fig. 4), both the endothermic and exothermic peaks belonging to the PCMs looks like each other. After repeated thermal cycling, the melting and freezing temperatures of composite PCMs changed to 0.05 C and 1.63 C for ETP/diatomite, 0.01 C and 1.92 C for ETP/EP, 0.04 C and 0.08 C for ETS/diatomite, 0.41 C and 0.12 C for ETS/EP. The little change observed in phase change temperatures are in negligible level for thermal energy storage applications. Therefore, it can be said that the composite PCMs have good thermal reliability in terms of the changes in their phase change temperatures.
Fig. 5. FTeIR spectra of the prepared composite PCMs after thermal cycling.
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wavenumber values of the composites did not changed after thermal cycling. These results mean that chemical structure of the composite PCMs was not affected by thermal cycling. Therefore, it is noteworthy noted that the composite PCMs have good chemical stability after 1000 thermal cycling. 3.5. Thermal stability of the composite PCMs The thermal stabilities of the composite PCMs were evaluated by TG analysis and the results TG curves are showed in Fig. 6. From the TG curves, it can be said that there are two steps in the degradation of ETP/Diatomite and ETP/EP composites. The first step is roughly from 81 to 150 C, corresponding to the evaporation of water from the diatomite or EP the second step observed in the range of 450e500 C is assigned to the degradation of ETP molecular chain. However, the TG curves of the composites containing ETS include one step. This situation can be explained as follows: The molecular volume of ETS is the bigger than ETS and so ETS completely coats the pores of the diatomite and EP. Even if water retains in the pores of diatomite and EP, the pores coated with ETS do not allow the water evaporation from the composite PCM. Therefore, in the composites with ETS, the step corresponding to the evaporation of water from the composites either cannot observe or overlap with degradation step of ETS. In addition, as can be seen from TG curves, the composite PCMs were not degraded or showed almost no weight loss at lower temperature than 100 C. This result means that the composite PCMs have good thermal durability in their working temperature range.
Fig. 6. TG curves of the prepared composite PCMs.
Table 6 The measured thermal conductivity values and energy storage properties of the composite PCMs. Material
Thermal conductivity (W m1 K1)
EG Diatomite EP ETP ETS ETP/Diatomite ETS/Diatomite ETP/EP ETS/EP
4.26 0.07 0.04 0.25 0.26 0.19 0.21 0.15 0.16
0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02
Melting point ( C)
Latent heat of melting (J/g)
e e e e e 19.6 29.8 19.8 30.1
e e e e e 110.6 116.1 119.0 119.1
0.3 0.1 0.3 0.2
3.6. Thermal conductivity of the PCMs
1.3 2.3 1.6 1.2
The rate of energy storage and release is substantially depending on the thermal conductivity of PCMs [46,47]. In order to improve the thermal conductivity of the composite PCMs, the expanded graphite (EG) with high thermal conductivity was added to them at different mass fractions (2%, 5%, and 10%). The measured thermal conductivity results of the composite PCMs before and after EG addition were presented in Table 6 and Table 7, respectively. As seen from Table 6, thermal conductivity values at 25 C were measured as 0.19 0.01, 0.21 0.01, 0.15 0.01, 0.16 0.02 W m1 K1 for ETP/Diatomite, ETS/Diatomite, ETP/EP and ETS/EP composite PCMs, respectively. On the other hand, as seen from Table 7, the thermal conductivity values of the composite PCMs were increased with the increase in the mass percent of EG. However, the increase percent in thermal conductivity values was found to be lower than the expected ones for 2 wt % and 10 wt % EG additive as compared with that obtained for 5 wt% EG additive. Moreover, the decrease amount in latent heat capacities of the composite PCMs after EG
On the other hand, after repeated 1000 thermal cycling, the latent 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 while the 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% for ETS/EP. As can be seen from these results, the changes in the latent heat values of the composite PCMs are irregular and in a reasonable level for composite PCMs, which will be used for thermal energy storage applications in buildings [8,17,34]. The chemical stability of the composite PCMs after repeated thermal cycling was investigated by FTeIR analysis. As clearly seen from shows the FTeIR spectra in Fig. 5, the peak shapes and the
Table 7 The measured thermal conductivity values of the composite PCMs and energy storage properties after EG addition in different mass fractions. Composite PCM
Thermal conductivity (W m1 K1)
Increase in thermal conductivity (%)
Melting point ( C)
Latent heat of melting (J/g)
ETP/Diatomite/EG(2 wt%) ETS/Diatomite EG(2 wt%) ETP/EP composite EG(2 wt%) ETS/EP composite EG(2 wt%) ETP/Diatomite/EG(5 wt%) ETS/Diatomite EG(5 wt%) ETP/EP composite EG(5 wt%) ETS/EP composite EG(5 wt%) ETP/Diatomite/EG(10 wt%) ETS/Diatomite EG(10 wt%) ETP/EP composite EG(10 wt%) ETS/EP composite EG(10 wt%)
0.23 0.01 0.25 0.01 0.19 0.02 0.20 0.01 0.32 0.01 0.33 0.02 0.26 0.01 0.28 0.02 0.36 0.02 0.38 0.01 0.31 0.02 0.35 0.01
20 18 24 22 68 57 73 75 90 81 106 120
19.8 0.3 29.9 0.1 19.9 0.3 30.3 0.2 19.8 0.3 29.9 0.1 19.9 0.3 30.3 0.2 19.8 0.3 29.9 0.1 19.9 0.3 30.3 0.2
109.5 0.2 115.4 0.3 118.4 0.2 118.1 0.1 108.2 2.2 114.3 1.7 117.2 1.2 116.6 1.4 108.2 2.2 114.3 1.7 117.2 1.2 116.6 1.4
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addition were determined to be higher than the theoretical expectation for especially 10 wt % EG additive relatively the 5 wt% EG additive. As seen from these results, the best results were obtained for the composite PCMs including with 5 wt% EG in terms of the increase percent in thermal conductivity values and the decrease amount in latent heat capacity. Therefore, by taking into account the increase percent in thermal conductivity values and the decrease amount in latent heat capacity, the optimum mass ratio of EG additive used to improve the thermal conductivity of the prepared composite PCMs was determined as 5wt%. 4. Conclusions In this study, ETP/Diatomite, ETS/Diatomite, ETP/EP and ETS/EP composite PCMs were prepared as novel composite PCMs for thermal energy storage applications in buildings. The composite PCMs were obtained by direct incorporation of the esters with building materials. The maximum ester content in diatomite and EP was found as 57 and 62 wt%, respectively. These form-stable composite PCMs were characterized by using SEM, FTeIR, DSC and TG analysis techniques. The SEM results showed that ETP and ETS were successfully retained into the pores of diatomite and EP used as supporting materials. The FTeIR results proved the availability of good chemical compatibility between the components of the composites. DSC analysis results indicated that the melting temperatures and latent heats of the prepared composite PCMs are in the range of 19.64e30.15 C and 110.60e119.19 J/g, respectively and these properties are suitable for thermal energy storage applications in buildings. The thermal cycling test revealed that the composite PCMs had good thermal reliability and chemical stability even after 1000 thermal cycles. The TG analysis results signified that the composites have good thermal durability above their working temperature range. Moreover, in order to improve the thermal conductivity of the composite PCMs, the expanded graphite (EG) with high thermal conductivity was added to them at different mass fractions (2%, 5%, and 10%). The best results were obtained for the composite PCMs including with 5wt% EG in terms of the increase percent in thermal conductivity values and the decrease amount in latent heat capacity. Acknowledgements Authors thank Altınay BOYRAZ (Erciyes University, Technology Research & Developing Center) for SEM and TG analysis. References [1] A.M. Khudhair, M.M. Farid, A review on energy conservation in building applications with thermal storage by latent heat using phase change materials, Energy Conversion and Management 45 (2004) 263e275. [2] I. Dincer, On thermal energy storage systems and applications in buildings, Energy and Buildings 34 (2002) 377e388. [3] B. Zalba, J.M. Marin, L.F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Applied Thermal Engineering 23 (2003) 251e283. [4] D. Rozanna, A. Salmiah, T.G. Chuah, R. Medyan, S.Y. Thomas Choong, M. Sa’ari, A study on thermal characteristics of phase change material (PCM) in gypsum board for building application, Journal of Oil Palm Research 17 (2005) 41e46. [5] L.F. Cabeza, G. Svensson, S. Hiebler, H. Mehling, Thermal performance of sodium acetate trihydrate thickened with different materials as phase change energy storage material, Applied Thermal Engineering 23 (2003) 1697e1704. [6] J. Zuo, W. Li, L. Weng, Thermal performance of caprylic acid/1-dodecanol eutectic mixture as phase change material (PCM), Energy and Buildings 43 (2011) 207e210. [7] V.V. Tyagi, D. Buddhi, PCM thermal storage in buildings: a state of art, Renewable and Sustainable Energy Reviews 11 (2007) 1146e1166. [8] L. Shilei, Z. Neng, F. Guohui, Eutectic mixtures of capric acid and lauric acid applied in building wallboards for heat energy storage, Energy Buildings 38 (2006) 708e711.
215
[9] A.M. Khudhair, M.M. Farid, Use of phase change materials for thermal comfort and electrical energy peak load shifting: experimental investigations, Proceedings of ISES World Congress 2007 vol. I e vol. V (2009) 283e288. [10] A.M. Borreguero, M. Carmona, M.L. Sanchez, J.L. Valverde, J.F. Rodriguez, Improvement of the thermal behaviour of gypsum blocks by the incorporation of microcapsules containing PCMS obtained by suspension polymerization with an optimal core/coating mass ratio, Applied Thermal Engineering 30 (2010) 1164e1169. [11] A. Pasupathy, L. Athanasius, R. Velraj, R.V. Seeniraj, Experimental investigation and numerical simulation analysis on the thermal performance of a building roof incorporating phase change material (PCM) for thermal management, Applied Thermal Engineering 28 (2008) 556e565. [12] B. Zalba, J.M. Marín, L.F. Cabeza, H. Mehling, Free-cooling of buildings with phase change materials, International Journal of Refrigeration 27 (2004) 839e849. [13] H. Mehling, L. Cabeza, M. Yamaha, Application of PCM for heating and cooling in Buıldings, Thermal Energy Storage for Sustainable Energy Consumption NATO Science Series 234 (2007) 323e348. [14] F. Kuznik, J. Virgone, J. Noel, Optimization of a phase change material wallboard for building use, Applied Thermal Engineering 28 (2008) 1291e1298. [15] M. Rady, Granular phase change materials for thermal energy storage: Experiments and numerical simulations, Applied Thermal Engineering 29 (2009) 3149e3159. [16] L. Shilei, Z. Neng, F. Guohui, Impact of phase change wall room on indoor thermal environment in winter, Energy and Buildings 38 (2006) 18e24. [17] D. Feldman, D. Banu, D. Hawes, Development and application of organic phase change mixtures in thermal storage gypsum wallboard, Solar Energy Material Solar Cells 36 (1995) 147e157. [18] M. Hadjieva, R. Stoykov, T. Filipova, Composite saltehydrate concrete system for building energy storage, Renewable Energy 19 (2000) 111e115. [19] H. Kaasinen, The absorption of phase change substances into commonly used building materials, Solar Energy Material Solar Cells 27 (1991) 173e179. [20] W. Streicher, L. Cabeza, A. Heinz, Inventory of phase change materials (PCM). IEA solar heating and cooling programme e task 32, Advanced Storage Concepts for Solar and Low Energy Buildings (2005). [21] L.F. Cabeza, C. Castellón, M. Nogués, M. Medrano, R. Leppers, O. Zubillaga, Use of microencapsulated PCM in concrete walls for energy savings, Energy and Buildings 39 (2007) 113e119. [22] C. Castellón, M. Medrano, J. Roca, L.F. Cabeza, M.E. Navarro, A.I. Fernández, A. Lázaro, B. Zalba, Effect of microencapsulated phase change material in sandwich panels, Renewable Energy 35 (2010) 2370e2374. [23] A. Hauer, H. Mehling, P. Schossig, M. Yamaha, L.F. Cabeza, V. Martin, F. Setterwall, International energy agency implementing agreement on energy conservation through energy storage, Annex 17 Final report (2005). [24] M. Natalia, R. Dimaano, T. Watanabe, L.F. Cabeza, G. Svensson, S. Hiebler, H. Mehling, The capricelauric acid and pentadecane combination as phase change material for cooling applications, Applied Thermal Engineering 22 (2002) 365e377. [25] A. Sar, Eutectic mixtures of some fatty acids for low temperature solar heating applications: thermal properties and thermal reliability, Applied Thermal Engineering 25 (2005) 2100e2107. [26] A. Sarı, Thermal characteristics of a eutectic mixture of myristic and palmitic acids as phase change material for heating applications, Applied Thermal Engineering 23 (2003) 1005e1017. [27] D. Feldman, D. Banu, DSC analysis for the evaluation of an energy storing wallboard, Thermochim Acta 272 (1996) 243e251. [28] M. Shapiro, Development of the enthalpy storage materials, mixture of methyl stearate and methyl palmitate, Subcontract Report to Florida Solar Energy Center (1989). [29] D. Feldman, D. Banu, D. Hawes, Low chain esters of stearic acid as phase change materials for thermal energy storage in buildings, Solar Energy Materials Solar Cells 36 (1995) 311e322. lu, A. Biçer, A. Karaipekli, Synthesis and thermal energy storage [30] A. Sarı, R. Erog properties of erythritol tetrastearate and erythritol tetrapalmitate, Chemical Engineering Technolology 34 (2011) 87e92. [31] D. Feldman, D. Banu, D. Hawes, E. Ghanbari, Obtaining an energy storing building material by direct incorporation of an organic phase change material in gypsum wallboard, Solar Energy Materials 22 (1991) 231e242. [32] H. Mehling, S. Hiebier, L.F. Cabeza, News on the Application of PCMs for Heating and Cooling of Buildings. Advanced Thermal Energy Storage Through Phase Change Materials and Chemical Reactions Feasibility Studies and Demonstration Project, 17, Third Workshop, IEA, ECES IA Annex, Tokyo, Japan, October 2002, 1e2. [33] D. Zhang, S. Tian, D. Xiao, Experimental study on the phase change behavior of phase change material confined in pores, Solar Energy 81 (2007) 653e660. [34] D.W. Hawes, D. Feldman, D. Banu, Latent heat storage in building materials, Energy Buildings 20 (1993) 77e86. [35] A. Karaipekli, A. Sarı, Capric acid and palmitic acid eutectic mixture applied in building wallboard for latent heat thermal energy storage, Journal of Scientific and Industrial Research 66 (2007) 470e476. [36] H. Li, X. Liu, G.Y. Fang, Preparation and characteristics of nenonadecane/ cement composites as thermal energy storage materials in buildings, Energy Buildings 42 (2010) 1661e1665. [37] A. Sarı, A. Karaipekli, K. Kaygusuz, Capric acid and stearic acid mixture impregnated with gypsum wallboard for low-temperature latent heat thermal energy storage, International Journal of Energy Research 32 (2008) 154e160.
216
A. Sarı, A. Karaipekli / Applied Thermal Engineering 37 (2012) 208e216
[38] A. Sarı, A. Karaipekli, K. Kaygusuz, Capric acid and myristic acid for latent heat thermal energy storage, Energy Sources Part A 30 (2008) 1498e1507. [39] A. Karaipekli, A. Sarı, Capricemyristic acid/vermiculite composite as formstable phase change material for thermal energy storage, Solar Energy 83 (2009) 323e332. [40] A. Karaipekli, A. Sarı, Capricemyristic acid/expanded perlite composite as form-stable phase change material for latent heat thermal energy storage, Renewable Energy 33 (2008) 2599e2605. [41] X. Fang, Z. Zhang, A novel montmorillonite-based composite phase change material and its applications in thermal storage building materials, Energy and Buildings 38 (2006) 377e380. [42] S. Karaman, A. Karaipekli, A. Sarı, A. Biçer, Polyethylene glycol (PEG)/ diatomite composite as a novel form-stable phase change material for thermal energy storage, Solar Energy Materials & Solar Cells 95 (2011) 1647e1653.
[43] M. Li, H. Kao, Z. Wu, J. Tan, Study on preparation and thermal property of binary fatty acid and the binary fatty acids/diatomite composite phase change materials, Applied Energy 88 (2011) 1606e1612. [44] M. Li, Z. Wub, H. Kao, Study on preparation, structure and thermal energy storage property of capricepalmitic acid/attapulgite composite phase change materials, Applied Energy 88 (2011) 3125e3132. [45] Hui Li, G.Y. Fang, Experimental investigation on the characteristics of polyethylene glycol/cement composites as thermal energy storage materials, Chemical Engineering and Technology 33 (2010) 1650e1654. [46] A. Sarı, A. Karaipekli, Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material, Applied Thermal Engineering 27 (2007) 1271e1277. [47] A. Mills, M. Farid, J.R. Selman, S. Al-Hallaj, Thermal conductivity enhancement of phase change materials using a graphite matrix, Applied Thermal Engineering 26 (2006) 1652e1661.
Contents lists available at SciVerse ScienceDirect
Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Fatty acid esters-based composite phase change materials for thermal energy storage in buildings Ahmet Sarı a, b, *, Ali Karaipekli a, b a b
Department of Chemistry, Çankırı Karatekin University, 18200 Çankırı, Turkey Department of Chemistry, Gaziosmanpas¸a University, 60240 Tokat, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 April 2011 Accepted 7 November 2011 Available online 22 November 2011
In this study, fatty acid esters-based composite phase change materials (PCMs) for thermal energy storage were prepared by blending erythritol tetrapalmitate (ETP) and erythritol tetrastearate (ETS) with diatomite and expanded perlite (EP). The maximum incorporation percentage for ETP and ETS into diatomite and EP was found to be 57 wt% and 62 wt%, respectively without melted PCM seepage from the composites. The morphologies and compatibilities of the composite PCMs were structurally characterized using scanning electron microscope (SEM) and Fourier transformation infrared (FTeIR) analysis techniques. Thermal energy storage properties of the composite PCMs were determined by differential scanning calorimetry (DSC) analysis. The DSC analyses results indicated that the composite PCMs were good candidates for building applications in terms of their large latent heat values and suitable phase change temperatures. The thermal cycling test including 1000 melting and freezing cycling showed that composite PCMs had good thermal reliability and chemical stability. TG analysis revealed that the composite PCMs had good thermal durability above their working temperature ranges. Moreover, in order to improve the thermal conductivity of the composite PCMs, the expanded graphite (EG) was added to them at different mass fractions (2%, 5%, and 10%). The best results were obtained for the composite PCMs including 5wt% EG content in terms of the increase in thermal conductivity values and the 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. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Fatty acid esters Composite PCM Diatomite Expanded perlite Thermal properties Thermal reliability
1. Introduction The energy demand to provide a comfortable environment for humans in buildings has continuously increased worldwide. But, the energy use for heating, cooling and air conditioning increase the level of greenhouse gas emissions and decrease fossil fuel sources [1,2]. Therefore, energy storage becomes a key issue in engineering application. Thermal energy storage plays an important role in an effective use of energy in buildings not only by reducing the mismatch between supply and demand but also improving the performance and reliability of energy systems. Among all of thermal energy storage methods (sensible, latent and thermochemical heat), latent heat thermal energy storage employing a phase change material (PCM) is particularly effective technique due to its advantages of high energy storage density and its isothermal operating characteristics [3e6]. * Corresponding author. Department of Chemistry, Gaziosmanpas¸a University, 60240 Tokat, Turkey. Tel.: þ90 356 2521616; fax: þ90 356 2521585. E-mail addresses: [email protected], [email protected] (A. Sarı). 1359-4311/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2011.11.017
The application of PCMs in buildings is well known and has been subject to considerable interest since the first reported application in the 1940s [7,8]. Researches on the application of PCMs in buildings have been focused on three fields in recent years. The first one is the reduction of temperature fluctuations of lightweight buildings by increasing their thermal mass [9e11]. This is done by incorporation of PCM into building materials. The second one is the cooling of buildings through intermediate storage of cold from the night or other cheap cold sources. If the cold is for free, as with cold from night air, this is also called free cooling and very promising with respect to energy saving [12,13]. A third application field is for heat storage in space heating systems [14]. The PCMs can be used by integration with different building’s structure such as gypsum board, plaster, concrete, clay minerals or other wall-covering material. But, there are some difficulties in fabrication of building materials containing PCMs. One of them is incorporation of PCM in construction material. PCMs in building materials are usually enclosed in metallic or polymeric capsules. The encapsulation of the PCM is expensive and it may affect the mechanical strength of the building material as well as it may lead
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to seepage during the melting period of PCM. Therefore, it is needed to direct heat exchange between PCM and medium to provide higher thermal energy storage performance. From this point of view, the PCM based-building composites are promising materials since they enables no corrosion, quick heat transfer and offers a large heat storage density if building materials with high porosity are selected [15,16]. Many inorganic and organic PCMs and their mixtures as thermal energy storage materials have been studied recently for impregnating into common building materials [12,17e23]. Among the PCMs, fatty acids, their esters and mixtures as organic phase change materials has been recommended as energy storage materials due to their desirable thermal and heat transfer characteristics and the advantage of easily impregnation, or directly incorporation into conventional building materials [24e26]. Moreover, most of the fatty acid esters are commercially available, since they are already produced in large amounts for plastics, cosmetics, textile and other industries. However, only few ester compounds were used to obtain composite PCMs for energy storage in building applications [27e29]. In this study, some fatty acid ester-based composites were fabricated as novel potential PCMs for thermal energy storage in building applications. These composite PCMs were prepared by directly incorporation of erythritol tetrapalmitate and erythritol tetrastearate as fatty acid esters in diatomite and expanded perlite as building materials. The composites were characterized structurally by SEM and FTeIR analysis techniques. Thermal energy storage properties and thermal durability of the composite PCMs were determined by using DSC and TG analysis methods, respectively. Moreover, the thermal conductivities of prepared composite PCMs were improved by adding expanded graphite (EG) at mass fraction 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 method reported in our previous study [30]. The diatomite sample was supplied from BEGeTUG Industrial Minerals & Mines Company (Istanbul, Turkey). Expanded perlite (EP) was supplied from Izper Company (IzmireTurkey). The chemical composition of the diatomite and EP samples used as porous building material in preparation of the composites are given in Table 1. Perlite and diatomite samples were previously sieved by 150 mm-mesh sieve and dried at 105 C for 24 h before use. Expanded graphite (EG; thermal conductivity: 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/perlite composite PCMs were prepared by using direct impregnation method [31,32]. The composite PCMs were prepared by directly mixing the fatty acid ester in melted state with the building
Table 1 Chemical 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
209
material directly. To determine the maximum holding ratios of the building materials for ETP for the fatty acid ester compounds, a series of composites with different mass fractions of ester compounds (10, 20, 30, 40, 50, 60, and 70% w/w) were prepared. The composite PCM was simultaneously heated during the impregnation process at a constant temperature above the melting temperature of the ester compound in order to test the exudation of fatty acid ester from the porous spaces. The ETP and ETS used as PCM could be retained as 57 wt % in diatomite and 62 wt % in EP without the leakage of melted PCM and therefore these composites were defined as form-stable composite PCMs. 2.3. Analysis methods The morphology of prepared ETP/diatomite, ETS/diatomite, ETP/ perlite and ETS/perlite composite PCMs were observed using a SEM instrument (LEO 440 model). The chemical compatibility between the components of the composites was investigated by FTeIR spectroscopy technique. FTeIR spectra were taken on a KBr disk at a frequency range of 4000e400 cm1 by using a FTeIR spectrophotometer (JASCO 430 model). Measurements of solideliquid phase change temperatures and latent heat capacities of composite PCM were carried out by using a DSC instrument (PerkineElmer Jade model) under nitrogen atmosphere and at a heating rate of 10 C/min. The maximum deviations in the measurements of phase change temperatures and latent heat values are found to be 0.3 C and 3.6 J/g, respectively. Certified Indium standard was used as a reference for temperature calibration of the instrument. Samples were measured in a sealed aluminum pan with a mass of about 5.0 mg. Melting point and freezing point were obtained by drawing a line at the point of maximum slope of the DSC peak. The latent heat was calculated by numerical integration of the peak using software of DSC instrument. The thermal durability of the composite PCMs was also determined by using PerkineElmer TGA7 thermal analyzer. In each case, the about 10 mg specimens were heated from 25 to 500 C using a linear heating rate of 10 C/min under nitrogen atmosphere. In addition, to increase the thermal conductivities of the prepared composite PCMs, expanded graphite (EG) was introduced into them at mass fraction of 2, 5 and 10 wt%. Thermal conductivity values of all composites were measured by using a KD2 thermal property analyzer. 2.4. Thermal cycling test Thermal cycling test was performed to determine thermal reliability of the composite PCMs in terms of the change in phase change temperatures and latent heat values after a large number of thermal cycling. The test was carried out consecutively up to 1000 thermal cycling using a thermal cycler (BIOER TCe25/H model). DSC and FTeIR analyses were repeated to determine the thermal and 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 prepared form-stable composite samples. It can be seen from these SEM micrographs that the diatomite and EP have porous structures, which allow adsorbing the ETS and ETP in liquid state. The SEM images of composite PCMs show that ETP and ETS were well retained into the diatomite and EP. The porous structures of the EP and diatomite provided the mechanical strength for the whole composites and prevented the seepage of the melted ETS and ETP
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Fig. 1. The SEM images of the diatomite, EP and the prepared composite PCMs.
due to the effect of capillary and surface tension force between the components of the composites. In this study, the maximum percentage of the ETP and ETS to be retained into EP and diatomite was determined as 57 wt % for ester/diatomite composites and 62 wt % for ester/EP composites. It was no observed PCM leakage from the composites with these combinations even when the ETP and ETS melt in the composites. 3.2. Chemical compatibility analysis The chemical compatibility among the components of the composites was characterized by evaluating specific interactions between fatty acid esters and the building materials the using FTeIR spectroscopy technique. Fig. 2 shows FTeIR spectra of the diatomite, EP, ETP, ETS and the composite PCMs. The significant absorption bands and wavenumbers obtained from the FTeIR spectra are also given in Table 2. In the spectrum of pure ETP and ETS, there are two characteristic strong absorption bands arising from the C]O and CeO groups, usually found in the 1800e1650 cm1 and 1310e1100 cm1 regions, respectively. The
peaks at 2917 and 2849 cm1 represent the stretching vibration of eCH3 and eCH2 group of the esters, respectively. The pure diatomite possesses the characteristic absorption bands at 3748, 1652, 1195, 1093, and 792 cm1. The band at 3748 cm1 is due to the free silanol groups (SiOeH) and the band at 1093 cm1 reflects the siloxane (eSieOeSie) group stretching. The peak at 792 cm1 represents SiOeH vibration. The pure EP has main absorption bands at 3417, 1635, 1045, 788 and 458 cm1 respectively. The absorption band at 3417 cm1 signifies the stretching vibration of functional group of SieOH. The peaks at 1045, 788 and 458 belong to the SieOeSi asymmetric stretching vibration, peaks of bending and stretching vibrations SieO functional group, respectively. The peak at 1635 cm1 is due to the vibration of the OH bond. On the other hand, as can be seen from Fig. 2, the FTeIR spectra of diatomite and EP after the incorporation of the ETP and ETS esters indicated new absorption peaks due to the characteristic absorption bands of the esters. Furthermore, there were no new peaks other than characteristic peaks of the esters and the building materials at FTeIR spectra of the composites. These results indicate
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Fig. 2. FTeIR spectra of ETP, ETS and the prepared composite PCMs.
Table 2 FTeIR 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 CeO CeH
e e e
e e e
OeH
1635
1739 1380 2919 2852 e
SieO SieOeSi SieOH
458 1093 3748
1635 788 458 1045 3417
1739 1373 2925 2856 e e e e
e e e
1739 1380 2929 2852 1635 796 468 1093 3448
1739 1382 2915 2850 1635 794 472 1095 3446
1739 1394 2921 2852 1635 796 472 1093 3446
1739 1396 2917 2848 1635 796 476 1093 3448
that there is no chemical interaction between the esters and the building materials. As also seen in Table 2, small shifts were observed in the absorption bands of some functional groups of the components of composites (CeH, CeO, OeH, SieOH and SieOeSi). These shifts can be due to weak physical interactions among the
functional groups mentioned. It can also be attributed to the capillary and surface tension forces between the esters molecules and the pores of diatomite and EP, which prevent the leakage of ETS and ETP in melted state from the composites during solideliquid phase transition [33]. Combined with the result of SEM images and
Fig. 3. The melting and freezing DSC curves of ETS, ETP and the prepared composite PCMs.
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Table 3 DSC data of the esters and the prepared composite PCMs. Sample name
Melting
Freezing
Onset temperature ( C) ETP ETS ETP/Diatomite ETS/Diatomite ETP/EP ETS/EP
21.9 30.4 19.6 29.8 19.8 30.1
0.3 0.3 0.3 0.1 0.3 0.2
Peak temperature ( C) 25.6 35.5 24.8 34.5 25.6 35.5
0.1 0.1 0.2 0.3 0.1 0.1
Latent heat of melting (J/g) 201.1 208.8 110.6 116.1 119.0 119.1
2.3 3.1 1.3 2.3 1.6 1.2
Melting
Freezing
Experimental value (kJ/kg) ETP/Diatomite ETS/Diatomite ETP/EP ETS/EP
110.6 116.1 119.0 119.1
1.3 2.3 1.6 1.2
Calculated value (kJ/g)
Experimental value (kJ/kg)
108.4 117.3 117.9 127.6
101.2 114.8 111.5 128.6
2.7 3.3 3.1 3.6
18.8 28.8 14.3 30.0 14.4 30.0
0.2 0.1 0.3 0.3 0.3 0.2
Peak temperature ( C) 15.7 26.7 12.0 27.1 11.7 26.4
0.1 0.1 0.1 0.1 0.2 0.3
Latent heat of freezing (J/g) 200.8 207.2 101.2 114.8 111.5 128.6
2.1 2.3 2.7 3.3 3.1 3.6
were measured to be 14.3 0.3 C, 14.4 0.3 C, 30.0 0.3 C, and 30.0 0.2 C, respectively. Although there are slightly changes in the phase change temperatures of the composites, they are very close to pure esters. These little changes in phase change temperatures of the esters in the composite are probably due to the weak interactions characterized by FTeIR analysis. The phase change temperature of PCMs using in building applications should be close to human comfort temperature (16e26 C). The ETS/diatomite and the ETS/EP composites have suitable melting and freezing temperature for thermal energy storage by applying the exterior wall of buildings whereas the phase change temperatures of the composite PCMs prepared by using the ETS are in the range of 29e32 C. Thus, if the composite PCMs including ETS are used in the fabrication of building walls, the new composite material can absorb heat from the surrounding air and solar radiation heat during day, then release heat to the surrounding air at night. Hence, the load of air conditioning system used to cooling in buildings may reduce. On the other hand, the latent heats of melting were measured as 110.6 1.3 J/g, 119.0 1.6 J/g, 116.1 2.3 J/g and 119.1 1.2 J/g for ETP/Diatomite, ETP/EP, ETS/Diatomite and ETS/EP, respectively as the 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. As also seen from Table 4, the measured latent heats of melting and freezing of the composite PCMs were close to the values calculated by multiplying the mass content of the ETP and ETS of the composites and their phase change enthalpies in pure state. However, the calculated values were slightly higher than measured
Table 4 Calculated latent heat capacities of the prepared composite PCMs. Composite PCM
Onset temperature ( C)
Calculated value (kJ/kg) 115.1 118.1 125.2 128.4
FTeIR spectroscopy, it can be concluded that the there are good compatibility between ester compounds and the studied building materials. 3.3. Thermal properties of the composite PCMs Thermal properties such as melting and freezing temperatures and latent heats are important parameters for a latent heat thermal energy storage system. Fig. 3 shows the typical DSC curves of the ETP, ETS and the composite PCMs. The melting and freezing temperatures obtained from the DSC curves were also given in Table 3. From these curves, the melting and freezing temperatures were determined as 21.9 0.3 C and 18.8 0.1 C for ETP and 30.4 0.1 C and 28.8 0.1 C for the ETS, respectively. The melting temperatures 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 5 Comparison of thermal energy storage characteristics of the prepared composite PCMs with those of some composite PCMs reported in literature. Composite PCM
Melting point ( C)
Freezing point ( C)
Latent heat (J/g)
Reference
Capricelauric acid/gypsum (26/74 w%) Emerest2326/gypsum Propyl palmitate/gypsum (25e30/70e75 w/w%) Dodecanol/gypsum (25e30/70e75 w/w%) Methyl PalmitateeStearate/wallboard (26.6/73.4 w%) Butyl stearate/gypsum (25e30/70e75 w/w%) Capricelauric acid þ fire retardant (25e30/70e75 w/w%)/gypsum Capricepalmitic acid/gypsum (25/75 w/w%) nenonadecane/cement (50/50 w/w%) Capricestearic acid/gypsum (25/75 w/w%) Capricemyristic acid/gypsum (25/75 w/w%) Capricemyristic acid/Vermicuilite (20/80 w/w%) Capricemyristic acid/Expanded perlite (55/45 w/w%) RT20/Montmorillonite (58/42 w/w%) PEG1000/Diatomite (50/50 w/w%) Decanoic/Dodecanoic acid/Diatomite CapricePalmitic acid/Attapulgite (35/65 w/w%) PEG1000/Cement (25/75 w/w%) ETP/Diatomite (57/43 w/w%) ETS/Diatomite (57/43 w/w%) ETP/EP (62/38 w/w%) ETS/EP (62/38 w/w%)
19.1 16.9 19.0 20.0 22.5 18.0 17.0 22.9 31.9 23.8 21.1 19.8 21.7 23.0 27.7 16.7 21.7 24.3 19.6 29.8 19.8 30.1
e 19.3 16.0 21.0 23.8 21.0 21.0 21.7 31.8 23.9 21.4 17.1 20.7 e 32.2 e e 27.9 14.3 30.0 14.4 30.0
35.2 35.0 40.0 17.0 41.1 30.0 28.0 42.5 69.1 49.0 36.2 27.0 85.4 79.3 88.0 66.8 48.2 23.9 110.6 116.1 119.0 119.1
[8] [27] [29] [29] [29] [34] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] Present Present Present Present
0.3 0.1 0.3 0.2
0.3 0.3 0.3 0.2
1.3 2.3 1.6 1.2
study study study study
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Fig. 4. The melting and freezing DSC curves of the prepared composite PCMs after thermal cycling.
values. These results can be probably caused by the physical interactions between the esters and the inner surface of pore of Daitomite and EP. In addition, Table 5 presents the comparison of energy storage properties of the prepared composite PCMs with those 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 have important latent heat thermal energy storage potential in buildings. 3.4. Thermal reliability of the composite PCMs The PCMs to be used for thermal energy storage have to be stable in terms of thermal and chemical after long term utility period in practice applications. Thus, the superior composite should be shown no or less change in its thermal properties and chemical structure after long term utility period. Therefore, thermal cycling test was performed to determine the change in thermal properties and chemical structure of the composite PCMs by DSC analysis and FTIR analysis, respectively before and after 1000 cycling. It is clearly seen from the DSC curves of the composite PCMs before and after 1000 thermal cycling (Fig. 4), both the endothermic and exothermic peaks belonging to the PCMs looks like each other. After repeated thermal cycling, the melting and freezing temperatures of composite PCMs changed to 0.05 C and 1.63 C for ETP/diatomite, 0.01 C and 1.92 C for ETP/EP, 0.04 C and 0.08 C for ETS/diatomite, 0.41 C and 0.12 C for ETS/EP. The little change observed in phase change temperatures are in negligible level for thermal energy storage applications. Therefore, it can be said that the composite PCMs have good thermal reliability in terms of the changes in their phase change temperatures.
Fig. 5. FTeIR spectra of the prepared composite PCMs after thermal cycling.
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wavenumber values of the composites did not changed after thermal cycling. These results mean that chemical structure of the composite PCMs was not affected by thermal cycling. Therefore, it is noteworthy noted that the composite PCMs have good chemical stability after 1000 thermal cycling. 3.5. Thermal stability of the composite PCMs The thermal stabilities of the composite PCMs were evaluated by TG analysis and the results TG curves are showed in Fig. 6. From the TG curves, it can be said that there are two steps in the degradation of ETP/Diatomite and ETP/EP composites. The first step is roughly from 81 to 150 C, corresponding to the evaporation of water from the diatomite or EP the second step observed in the range of 450e500 C is assigned to the degradation of ETP molecular chain. However, the TG curves of the composites containing ETS include one step. This situation can be explained as follows: The molecular volume of ETS is the bigger than ETS and so ETS completely coats the pores of the diatomite and EP. Even if water retains in the pores of diatomite and EP, the pores coated with ETS do not allow the water evaporation from the composite PCM. Therefore, in the composites with ETS, the step corresponding to the evaporation of water from the composites either cannot observe or overlap with degradation step of ETS. In addition, as can be seen from TG curves, the composite PCMs were not degraded or showed almost no weight loss at lower temperature than 100 C. This result means that the composite PCMs have good thermal durability in their working temperature range.
Fig. 6. TG curves of the prepared composite PCMs.
Table 6 The measured thermal conductivity values and energy storage properties of the composite PCMs. Material
Thermal conductivity (W m1 K1)
EG Diatomite EP ETP ETS ETP/Diatomite ETS/Diatomite ETP/EP ETS/EP
4.26 0.07 0.04 0.25 0.26 0.19 0.21 0.15 0.16
0.02 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02
Melting point ( C)
Latent heat of melting (J/g)
e e e e e 19.6 29.8 19.8 30.1
e e e e e 110.6 116.1 119.0 119.1
0.3 0.1 0.3 0.2
3.6. Thermal conductivity of the PCMs
1.3 2.3 1.6 1.2
The rate of energy storage and release is substantially depending on the thermal conductivity of PCMs [46,47]. In order to improve the thermal conductivity of the composite PCMs, the expanded graphite (EG) with high thermal conductivity was added to them at different mass fractions (2%, 5%, and 10%). The measured thermal conductivity results of the composite PCMs before and after EG addition were presented in Table 6 and Table 7, respectively. As seen from Table 6, thermal conductivity values at 25 C were measured as 0.19 0.01, 0.21 0.01, 0.15 0.01, 0.16 0.02 W m1 K1 for ETP/Diatomite, ETS/Diatomite, ETP/EP and ETS/EP composite PCMs, respectively. On the other hand, as seen from Table 7, the thermal conductivity values of the composite PCMs were increased with the increase in the mass percent of EG. However, the increase percent in thermal conductivity values was found to be lower than the expected ones for 2 wt % and 10 wt % EG additive as compared with that obtained for 5 wt% EG additive. Moreover, the decrease amount in latent heat capacities of the composite PCMs after EG
On the other hand, after repeated 1000 thermal cycling, the latent 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 while the 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% for ETS/EP. As can be seen from these results, the changes in the latent heat values of the composite PCMs are irregular and in a reasonable level for composite PCMs, which will be used for thermal energy storage applications in buildings [8,17,34]. The chemical stability of the composite PCMs after repeated thermal cycling was investigated by FTeIR analysis. As clearly seen from shows the FTeIR spectra in Fig. 5, the peak shapes and the
Table 7 The measured thermal conductivity values of the composite PCMs and energy storage properties after EG addition in different mass fractions. Composite PCM
Thermal conductivity (W m1 K1)
Increase in thermal conductivity (%)
Melting point ( C)
Latent heat of melting (J/g)
ETP/Diatomite/EG(2 wt%) ETS/Diatomite EG(2 wt%) ETP/EP composite EG(2 wt%) ETS/EP composite EG(2 wt%) ETP/Diatomite/EG(5 wt%) ETS/Diatomite EG(5 wt%) ETP/EP composite EG(5 wt%) ETS/EP composite EG(5 wt%) ETP/Diatomite/EG(10 wt%) ETS/Diatomite EG(10 wt%) ETP/EP composite EG(10 wt%) ETS/EP composite EG(10 wt%)
0.23 0.01 0.25 0.01 0.19 0.02 0.20 0.01 0.32 0.01 0.33 0.02 0.26 0.01 0.28 0.02 0.36 0.02 0.38 0.01 0.31 0.02 0.35 0.01
20 18 24 22 68 57 73 75 90 81 106 120
19.8 0.3 29.9 0.1 19.9 0.3 30.3 0.2 19.8 0.3 29.9 0.1 19.9 0.3 30.3 0.2 19.8 0.3 29.9 0.1 19.9 0.3 30.3 0.2
109.5 0.2 115.4 0.3 118.4 0.2 118.1 0.1 108.2 2.2 114.3 1.7 117.2 1.2 116.6 1.4 108.2 2.2 114.3 1.7 117.2 1.2 116.6 1.4
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addition were determined to be higher than the theoretical expectation for especially 10 wt % EG additive relatively the 5 wt% EG additive. As seen from these results, the best results were obtained for the composite PCMs including with 5 wt% EG in terms of the increase percent in thermal conductivity values and the decrease amount in latent heat capacity. Therefore, by taking into account the increase percent in thermal conductivity values and the decrease amount in latent heat capacity, the optimum mass ratio of EG additive used to improve the thermal conductivity of the prepared composite PCMs was determined as 5wt%. 4. Conclusions In this study, ETP/Diatomite, ETS/Diatomite, ETP/EP and ETS/EP composite PCMs were prepared as novel composite PCMs for thermal energy storage applications in buildings. The composite PCMs were obtained by direct incorporation of the esters with building materials. The maximum ester content in diatomite and EP was found as 57 and 62 wt%, respectively. These form-stable composite PCMs were characterized by using SEM, FTeIR, DSC and TG analysis techniques. The SEM results showed that ETP and ETS were successfully retained into the pores of diatomite and EP used as supporting materials. The FTeIR results proved the availability of good chemical compatibility between the components of the composites. DSC analysis results indicated that the melting temperatures and latent heats of the prepared composite PCMs are in the range of 19.64e30.15 C and 110.60e119.19 J/g, respectively and these properties are suitable for thermal energy storage applications in buildings. The thermal cycling test revealed that the composite PCMs had good thermal reliability and chemical stability even after 1000 thermal cycles. The TG analysis results signified that the composites have good thermal durability above their working temperature range. Moreover, in order to improve the thermal conductivity of the composite PCMs, the expanded graphite (EG) with high thermal conductivity was added to them at different mass fractions (2%, 5%, and 10%). The best results were obtained for the composite PCMs including with 5wt% EG in terms of the increase percent in thermal conductivity values and the decrease amount in latent heat capacity. Acknowledgements Authors thank Altınay BOYRAZ (Erciyes University, Technology Research & Developing Center) for SEM and TG analysis. References [1] A.M. Khudhair, M.M. Farid, A review on energy conservation in building applications with thermal storage by latent heat using phase change materials, Energy Conversion and Management 45 (2004) 263e275. [2] I. Dincer, On thermal energy storage systems and applications in buildings, Energy and Buildings 34 (2002) 377e388. [3] B. Zalba, J.M. Marin, L.F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Applied Thermal Engineering 23 (2003) 251e283. [4] D. Rozanna, A. Salmiah, T.G. Chuah, R. Medyan, S.Y. Thomas Choong, M. Sa’ari, A study on thermal characteristics of phase change material (PCM) in gypsum board for building application, Journal of Oil Palm Research 17 (2005) 41e46. [5] L.F. Cabeza, G. Svensson, S. Hiebler, H. Mehling, Thermal performance of sodium acetate trihydrate thickened with different materials as phase change energy storage material, Applied Thermal Engineering 23 (2003) 1697e1704. [6] J. Zuo, W. Li, L. Weng, Thermal performance of caprylic acid/1-dodecanol eutectic mixture as phase change material (PCM), Energy and Buildings 43 (2011) 207e210. [7] V.V. Tyagi, D. Buddhi, PCM thermal storage in buildings: a state of art, Renewable and Sustainable Energy Reviews 11 (2007) 1146e1166. [8] L. Shilei, Z. Neng, F. Guohui, Eutectic mixtures of capric acid and lauric acid applied in building wallboards for heat energy storage, Energy Buildings 38 (2006) 708e711.
215
[9] A.M. Khudhair, M.M. Farid, Use of phase change materials for thermal comfort and electrical energy peak load shifting: experimental investigations, Proceedings of ISES World Congress 2007 vol. I e vol. V (2009) 283e288. [10] A.M. Borreguero, M. Carmona, M.L. Sanchez, J.L. Valverde, J.F. Rodriguez, Improvement of the thermal behaviour of gypsum blocks by the incorporation of microcapsules containing PCMS obtained by suspension polymerization with an optimal core/coating mass ratio, Applied Thermal Engineering 30 (2010) 1164e1169. [11] A. Pasupathy, L. Athanasius, R. Velraj, R.V. Seeniraj, Experimental investigation and numerical simulation analysis on the thermal performance of a building roof incorporating phase change material (PCM) for thermal management, Applied Thermal Engineering 28 (2008) 556e565. [12] B. Zalba, J.M. Marín, L.F. Cabeza, H. Mehling, Free-cooling of buildings with phase change materials, International Journal of Refrigeration 27 (2004) 839e849. [13] H. Mehling, L. Cabeza, M. Yamaha, Application of PCM for heating and cooling in Buıldings, Thermal Energy Storage for Sustainable Energy Consumption NATO Science Series 234 (2007) 323e348. [14] F. Kuznik, J. Virgone, J. Noel, Optimization of a phase change material wallboard for building use, Applied Thermal Engineering 28 (2008) 1291e1298. [15] M. Rady, Granular phase change materials for thermal energy storage: Experiments and numerical simulations, Applied Thermal Engineering 29 (2009) 3149e3159. [16] L. Shilei, Z. Neng, F. Guohui, Impact of phase change wall room on indoor thermal environment in winter, Energy and Buildings 38 (2006) 18e24. [17] D. Feldman, D. Banu, D. Hawes, Development and application of organic phase change mixtures in thermal storage gypsum wallboard, Solar Energy Material Solar Cells 36 (1995) 147e157. [18] M. Hadjieva, R. Stoykov, T. Filipova, Composite saltehydrate concrete system for building energy storage, Renewable Energy 19 (2000) 111e115. [19] H. Kaasinen, The absorption of phase change substances into commonly used building materials, Solar Energy Material Solar Cells 27 (1991) 173e179. [20] W. Streicher, L. Cabeza, A. Heinz, Inventory of phase change materials (PCM). IEA solar heating and cooling programme e task 32, Advanced Storage Concepts for Solar and Low Energy Buildings (2005). [21] L.F. Cabeza, C. Castellón, M. Nogués, M. Medrano, R. Leppers, O. Zubillaga, Use of microencapsulated PCM in concrete walls for energy savings, Energy and Buildings 39 (2007) 113e119. [22] C. Castellón, M. Medrano, J. Roca, L.F. Cabeza, M.E. Navarro, A.I. Fernández, A. Lázaro, B. Zalba, Effect of microencapsulated phase change material in sandwich panels, Renewable Energy 35 (2010) 2370e2374. [23] A. Hauer, H. Mehling, P. Schossig, M. Yamaha, L.F. Cabeza, V. Martin, F. Setterwall, International energy agency implementing agreement on energy conservation through energy storage, Annex 17 Final report (2005). [24] M. Natalia, R. Dimaano, T. Watanabe, L.F. Cabeza, G. Svensson, S. Hiebler, H. Mehling, The capricelauric acid and pentadecane combination as phase change material for cooling applications, Applied Thermal Engineering 22 (2002) 365e377. [25] A. Sar, Eutectic mixtures of some fatty acids for low temperature solar heating applications: thermal properties and thermal reliability, Applied Thermal Engineering 25 (2005) 2100e2107. [26] A. Sarı, Thermal characteristics of a eutectic mixture of myristic and palmitic acids as phase change material for heating applications, Applied Thermal Engineering 23 (2003) 1005e1017. [27] D. Feldman, D. Banu, DSC analysis for the evaluation of an energy storing wallboard, Thermochim Acta 272 (1996) 243e251. [28] M. Shapiro, Development of the enthalpy storage materials, mixture of methyl stearate and methyl palmitate, Subcontract Report to Florida Solar Energy Center (1989). [29] D. Feldman, D. Banu, D. Hawes, Low chain esters of stearic acid as phase change materials for thermal energy storage in buildings, Solar Energy Materials Solar Cells 36 (1995) 311e322. lu, A. Biçer, A. Karaipekli, Synthesis and thermal energy storage [30] A. Sarı, R. Erog properties of erythritol tetrastearate and erythritol tetrapalmitate, Chemical Engineering Technolology 34 (2011) 87e92. [31] D. Feldman, D. Banu, D. Hawes, E. Ghanbari, Obtaining an energy storing building material by direct incorporation of an organic phase change material in gypsum wallboard, Solar Energy Materials 22 (1991) 231e242. [32] H. Mehling, S. Hiebier, L.F. Cabeza, News on the Application of PCMs for Heating and Cooling of Buildings. Advanced Thermal Energy Storage Through Phase Change Materials and Chemical Reactions Feasibility Studies and Demonstration Project, 17, Third Workshop, IEA, ECES IA Annex, Tokyo, Japan, October 2002, 1e2. [33] D. Zhang, S. Tian, D. Xiao, Experimental study on the phase change behavior of phase change material confined in pores, Solar Energy 81 (2007) 653e660. [34] D.W. Hawes, D. Feldman, D. Banu, Latent heat storage in building materials, Energy Buildings 20 (1993) 77e86. [35] A. Karaipekli, A. Sarı, Capric acid and palmitic acid eutectic mixture applied in building wallboard for latent heat thermal energy storage, Journal of Scientific and Industrial Research 66 (2007) 470e476. [36] H. Li, X. Liu, G.Y. Fang, Preparation and characteristics of nenonadecane/ cement composites as thermal energy storage materials in buildings, Energy Buildings 42 (2010) 1661e1665. [37] A. Sarı, A. Karaipekli, K. Kaygusuz, Capric acid and stearic acid mixture impregnated with gypsum wallboard for low-temperature latent heat thermal energy storage, International Journal of Energy Research 32 (2008) 154e160.
216
A. Sarı, A. Karaipekli / Applied Thermal Engineering 37 (2012) 208e216
[38] A. Sarı, A. Karaipekli, K. Kaygusuz, Capric acid and myristic acid for latent heat thermal energy storage, Energy Sources Part A 30 (2008) 1498e1507. [39] A. Karaipekli, A. Sarı, Capricemyristic acid/vermiculite composite as formstable phase change material for thermal energy storage, Solar Energy 83 (2009) 323e332. [40] A. Karaipekli, A. Sarı, Capricemyristic acid/expanded perlite composite as form-stable phase change material for latent heat thermal energy storage, Renewable Energy 33 (2008) 2599e2605. [41] X. Fang, Z. Zhang, A novel montmorillonite-based composite phase change material and its applications in thermal storage building materials, Energy and Buildings 38 (2006) 377e380. [42] S. Karaman, A. Karaipekli, A. Sarı, A. Biçer, Polyethylene glycol (PEG)/ diatomite composite as a novel form-stable phase change material for thermal energy storage, Solar Energy Materials & Solar Cells 95 (2011) 1647e1653.
[43] M. Li, H. Kao, Z. Wu, J. Tan, Study on preparation and thermal property of binary fatty acid and the binary fatty acids/diatomite composite phase change materials, Applied Energy 88 (2011) 1606e1612. [44] M. Li, Z. Wub, H. Kao, Study on preparation, structure and thermal energy storage property of capricepalmitic acid/attapulgite composite phase change materials, Applied Energy 88 (2011) 3125e3132. [45] Hui Li, G.Y. Fang, Experimental investigation on the characteristics of polyethylene glycol/cement composites as thermal energy storage materials, Chemical Engineering and Technology 33 (2010) 1650e1654. [46] A. Sarı, A. Karaipekli, Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material, Applied Thermal Engineering 27 (2007) 1271e1277. [47] A. Mills, M. Farid, J.R. Selman, S. Al-Hallaj, Thermal conductivity enhancement of phase change materials using a graphite matrix, Applied Thermal Engineering 26 (2006) 1652e1661.