Preparation and thermal energy storage properties of building material-based composites as novel form-stable PCMs

Energy and Buildings 51 (2012) 73–83

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Preparation and thermal energy storage properties of building material-based composites as novel form-stable PCMs Ahmet Sarı ∗ , Alper Bic¸er Gaziosmanpas¸a University, Department of Chemistry, 60240 Tokat, Turkey

a r t i c l e

i n f o

Article history: Received 1 March 2012 Received in revised form 2 April 2012 Accepted 7 April 2012 Keywords: Building material Fatty acid ester Composite PCM Thermal properties Thermal energy storage

a b s t r a c t In this study, ten kinds of composite phase change materials (PCMs) were prepared by impregnation of xylitol penta palmitate (XPP) and xylitol penta stearate (XPS) esters into gypsum, cement, diatomite, perlite and vermiculite via vacuum adsorption method. The form-stable composite PCMs were characterized by using SEM and FT-IR, DSC and TG analysis techniques. The maximum impregnation ratio of both XPP and XPS into gypsum, cement, perlite, diatomite, and vermiculite were found to be 22, 17, 67, 48 and 42 wt%, respectively. The DSC results showed that the melting temperatures and latent heat capacities of the composite PCMs varied from 20 ◦ C to 35 ◦ C and from 38 J/g to 126 J/g. TG investigations revealed that the composite PCMs had excellent thermal durability above their working temperature ranges. The thermal cycling test also exhibited that the composite PCMs had good thermal reliability and chemical stability. In addition, thermal conductivities of the composite PCMs were increased by addition of EG in mass fraction of 10%. All of the conclusions indicated that among the prepared composite PCMs, especially perlite and diatomite based-PCMs are potential candidates for energy storage applications such as solar heating and cooling in buildings since they had relatively higher latent heat capacity. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Solar energy is an effective renewable energy source because of its intermittent property in nature. Hence, the storage of solar energy is a fundamental method for regulating the timediscrepancy between the energy supply and demand. Generally, solar energy can be thermally stored as sensible heat, latent heat, reversible reaction heat, or combination of these [1]. Among these, latent heat storage by using phase change materials (PCMs) are promising method due to the advantages of high heat storage density, a narrow temperature change, and requiring small size-system [2]. In such type energy storage system, PCM can store and release large amounts of energy at a nearly constant temperature during solid–liquid or vice phase change process. Several PCMs have been tested scientifically and industrially in many applications, such as in energy-efficient building materials [3–5], solar energy storage systems [6,7], greenhouses [8,9], temperature regulating textiles [10,11], transportation packaging of temperature sensitive materials [12], and heat management of electronics [13]. PCMs can be used as composite materials in buildings. The building composite PCM will provide thermal storage distributed all through the building, allowing passive solar design and offpeak cooling in traditional frame constructions with a typical low

∗ Corresponding author. Tel.: +90 356 2521616; fax: +90 356 2521585. E-mail addresses: [email protected], [email protected] (A. Sarı). 0378-7788/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2012.04.010

thermal mass [14]. The performance of the building composite PCM will depend on several factors: the melting point or temperature range of the PCM confined into the composite, the latent capacity per unit mass of the composite PCM, the preparation method of the composite PCM, the direction of the wall prepared with the composite PCM, climatic conditions, direct solar gains, etc. [1]. The frequency of internal air temperature swings of a building envelope can be decreased by using building composite PCM and the indoor air temperature of building envelope can be brought closer to the desired temperature for a longer period of time [4]. The idea on the increase of thermal comfort in buildings by using PCMs directed the researchers to build up new types of composite materials and investigate their potential for minimizing energy consumptions in buildings. The incorporation of some organic PCMs with construction materials such as gypsum board, plaster, concrete or other wall covering material has been studied extensively [15]. The heat storage capacity and structural stability of a composite consisted of an inorganic salt hydrate and porous concrete were investigated prepared [16]. Moreover, in recent years several organic PCMs such as paraffin, fatty acid, fatty acid eutectic mixture, fatty acid ester were incorporated with various building materials like cement, gypsum, vermiculite, attapulgite, diatomite, expanded perlite, activated montmorillonite, silica fume and clay mineral [17–39]. The main reason for preferring the organic PCMs is their chemical compatibility with the studied building materials. On the other hand, fatty acids esters are reported as suitable PCMs for preparation of energy storing composites due to their good

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

SiO2

Al2 O3

Fe2 O3

CaO

MgO

K2 O

H2 O

Gypsum Cement Diatomite Perlite Vermiculite

– 20.05 92.8 71.0–75.0 38.0–46.0

– 4.95 4.2 12.5–18.0 10.0–16.0

– 3.71 1.5 0.1–1.5 6.0–13.0

– 62.74 0.6 0.5–0.2 1.0–5.0

– 1.06 0.3 0 0.03–0.5 16.0–35.0

– 0.67 0.67 4.0–5.0 1.0–6.0

6.6 0.95 – – –

thermophysical properties, thermal reliability and the advantage of directly integration with construction materials. However, the number of works on the fatty acid ester-based building composite PCMs is limited [18,19,36]. Xylitol penta palmitate (XPP) and xylitol penta stearate (XPS) are proper PCMs for thermal energy storage applications in terms of their suitable melting temperatures (20.30 ◦ C and 34.48 ◦ C) and latent heats (164.46 kJ/kg and 193.91 kJ/kg). Thus, these ester compounds can be considered as promising PCMs in fabrication of novel form-stable building composites. On the other hand, gypsum and cement are conventional construction materials used extensively in building applications. In addition, perlite, vermiculite and diatomite are porous, ultra-lightweight, environmentally safe and low-cost building materials. Therefore, these materials are considerably appropriate for the development of novel form-stable building composite PCMs. The present study is focused on the investigation of thermal energy storage potential of ten kinds of form-stable composite PCMs prepared by impregnation of XPP and XPS esters with gypsum, cement, diatomite, perlite and vermiculite via vacuum adsorption method. The form-stable PCMs were characterized structurally and morphologically by using scanning electron microscope (SEM) and Fourier transformation infrared (FT-IR) analysis techniques. Thermal energy storage properties, thermal reliability and thermal durability of the composite PCMs were determined by using differential scanning calorimetry (DSC) and thermogravimetry (TG) analysis. In addition, thermal conductivity of the composite PCMs was increased by addition of EG in mass fractions of 10%. 2. Experimental 2.1. Materials Xylitol penta stearate (XPS) and xylitol penta palmitate (XPP) were synthesized by Fischer esterification reaction as reported in our prior study [40,41]. Gypsum and cement were supplied from the Sias Company (Sivas, Turkey) and AS company (Burdur, Turkey), respectively. Diatomite, perlite and vermiculite were supplied by BEG–TUG Industrial Minerals & Mines Company (Istanbul, Turkey), Izper Company (Izmir, Turkey) and Agrekal Company (Antalya, Turkey), respectively. These materials were dried at 105 ◦ C during 24 h before use. Table 1 shows the chemical compositions of the building materials providing by the manufacturer company.

CaSO4 93.4 – – –

Other – 4.14 0.5 – 0.2–1.2

its melting temperature and then added gradually to a weighted quantity of the building material. The adsorption process was maintained at 70 mbar during 60 min. After reaching maximum adsorption the composite was set aside to draw off excessive PCM and left for drying for 48 h. The final form-stable composite was weighted and the mass fraction of PCM was calculated. In addition, in order to test PCM exudation from the porous spaces, each composite was simultaneously heated at a constant temperature above the melting temperature of the ester compound confined into the building material. The composite that did not show ester leakage in liquid state was defined as form-stable composite PCM. 2.3. Characterization of the form-stable composite PCMs The morphology and microstructures of the studied building materials and the prepared composite PCMs was investigated using a LEO 440 model SEM instrument. The chemical structures of the composite PCMs were characterized by using a FT-IR spectrophotometer (JASCO 430 model). The spectral analyses were carried out using KBr pellets between 400 and 4000 cm−1 wavenumber. Phase change temperatures and latent heat values of the composite PCMs were measured using a DSC instrument (Perkin Elmer JADE model) at 5 ◦ C/min heating rate and under flowing nitrogen. All DSC measurements were repeated three times for each sample. The accuracy of enthalpy and temperature data was determined as ±5% and ±0.01 ◦ C, respectively. Thermal durability of the prepared composite PCMs was determined by using a TG analyzer (Perkin-Elmer TGA7 model) under nitrogen atmosphere at a constant heating rate of 10 ◦ C/min. The prepared composite PCMs were subjected to a thermal cycling test consisted of consecutive 1000 melting and freezing cycles by using a thermal cycler (BIOER TC-25/H model). Thermal reliability and chemical stability of the composite PCMs after thermal cycling test were evaluated by using DSC and FT-IR analysis. Furthermore, in order to increase the thermal conductivity of the prepared composite PCMs, expanded graphite (EG) with high thermal conductivity was added to the composites at the mass fraction of 10%. Thermal conductivities of the composite PCMs were measured at 25 ◦ C by using a KD2 thermal property analyzer. 3. Results and discussion 3.1. Morphology of form-stable composite PCMs

2.2. Preparation of form-stable composite PCMs Ten kinds of building composite PCMs, XPP/gypsum, XPP/cement, XPP/perlite, XPP/diatomite, XPP/vermiculite, XPS/gypsum, XPS/cement, XPS/perlite, XPS/diatomite and XPS/vermiculite were prepared by using vacuum adsorption method [20,22]. In order to obtain the form-stable composite PCM, the composites were prepared at different mass fractions of the esters between 10 and 80%. The adsorption process was controlled by using a heating apparatus under a constant vacuum condition. The PCM (XPP or XPS ester) was heated in a flask over

A series of composites were prepared at different mass fractions of XPP and XPS esters ranged from 10 to 80 (w/w) to attain maximum adsorption ratio. The mixtures that did not exhibit any seepage of PCM throughout the heating period were recognized as form-stable composite PCM with the maximum combination ratio. The maximum fractions for both XPP and XPS impregnated into gypsum, cement, perlite, diatomite and vermiculite were found to be 22, 17, 67, 48 and 42 wt%, respectively. As clearly seen from these data, the XPP/perlite and XPS/perlite composites include the highest PCM ratio.

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Fig. 1. The photograph images of the (a) gypsum, (b) XPP/gypsum, (c) XPS/gypsum, (d) cement, (e) XPP/cement, (f) XPS/cement, (g) diatomite, (h) XPP/diatomite, (i) XPS/diatomite, (k) perlite, (l) XPP/perlite, (m) XPS/perlite, (n) vermiculite, (o) XPP/vermiculite, and (p) XPS/vermiculite.

The SEM images and photograph images of the form-stable building composite materials were shown in Figs. 1 and 2, respectively. As clearly seen from the photograph images in Fig. 1, the composites did not show any leakage of PCM in melted sate. Moreover, as seen from Fig. 2(a, d, g, k, and n), gypsum, cement, perlite, diatomite, and vermiculite have porous structures consisted of rough and accidental micropores. The SEM images of the composite PCMs in Fig. 2(b, c, e, f, h, i, l, m, o, and p) also show that XPP and XPS were homogenously dispersed into the porous networks of the building materials. The SEM results indicated that the composites had good mechanical strength due to their multiple porous structures of the building materials and maintained their form-stable states during the PCM leakage test due to capillary and surface tension forces between the components of the composites.

3.2. FT-IR analysis of the form-stable composite PCMs The composite PCMs were characterized by FT-IR spectroscopy to investigate the chemical compatibility between the esters and the building materials. Fig. 3(a and b) shows the FT-IR spectra of XPS, XPP esters, gypsum, cement, perlite, diatomite, vermiculite, and the form-stable composite PCMs. As can be seen in Fig. 3(a), The C O stretching vibration bands for both esters are observed at 1743 cm−1 and 1739 cm−1 as the stretching vibration bands of C O groups was monitored at 1176 cm−1 and 1180 cm−1 , respectively. The peaks in range of 2923–2854 cm−1 are identified as symmetric and asymmetric stretching peaks of C H groups of the esters. From Fig. 3(b), the following bands were recorded for cement: calcium hydroxide bands (3646 cm−1 ), molecular water

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Fig. 2. The SEM images of the (a) gypsum, (b) XPP/gypsum, (c) XPS/gypsum, (d) cement, (e) XPP/cement, (f) XPS/cement, (g) diatomite, (h) XPP/diatomite, (i) XPS/diatomite, (k) perlite, (l) XPP/perlite, (m) XPS/perlite, (n) vermiculite, (o) XPP/vermiculite, and (p) XPS/vermiculite.

(3440–3446 and 1619–1632 cm−1 ), carbonate phases (1410–1500, 865–885 and 705–717 cm−1 ), sulphates phases (1112–1153 cm−1 ), anhydrous calcium silicates (520–540 and 455–468 cm−1 ). Moreover, for gypsum the main bands seen at 1150 cm−1 and 1619 cm−1 can be attributed to the stretching vibration of OH group in water and bending vibration of S O group in CaSO4 . As also seen from Fig. 3(b) the stretching vibration bands observed in the range of 3400–3500 cm−1 and the bending vibration bands in the range of 1560–1720 cm−1 correspond to the OH group of water in perlite, diatomite and vermiculite. Moreover, the absorption bands seen in between 990 cm−1 and 1150 cm−1 represent the stretching vibration of Si O group of perlite, diatomite and vermiculite. On the other hand, as seen from Fig. 3(c and d), after the impregnation of XPS and XPP the FT-IR spectra of the cement, gypsum, perlite, diatomite, and vermiculite included new absorption peaks related with the characteristic peaks of the esters. Any new peaks apart from the characteristic peaks of the esters and the building

materials were not observed in FT-IR spectra of the composites. This means that there is no chemical interaction between the esters and the building materials. In addition, when compared the FT-IR results of the esters with that of the composite PCMs, some little shifts can be observed in especially characteristic bands of the composites. These results may be due to the physical interactions such as capillary and surface tension forces between the components of composites preventing the leakage of the esters during the heating processes of compounds. The similar results were reported for different building composites [31,32]. 3.3. Thermal properties of the esters and the form-stable composite PCMs DSC is an accepted as suitable method for measuring the thermal energy storage properties of a PCM since it prevents uncertainty about phase change temperatures, enthalpies, and subcoolings. The

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

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

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

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

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

XPP XPS XPP/gypsum XPP/cement XPP/perlite XPP/diatomite XPP/vermiculite XPS/gypsum XPS/cement XPS/perlite XPS/diatomite XPS/vermiculite

Phase change temperature for melting (◦ C) On-set

Peak

20.30 34.48 20.25 20.51 20.62 20.61 20.45 34.43 34.28 34.04 34.70 34.50

26.10 42.29 23.96 23.16 26.63 25.65 24.50 37.23 38.02 39.60 40.11 39.53

Latent heat of melting (J/g)

164.46 193.91 43.44 27.55 106.60 77.43 59.56 39.35 38.28 125.73 86.81 77.44

Phase change temperature for freezing (◦ C) On-set

Peak

20.35 30.74 20.14 20.31 20.22 20.30 20.17 31.54 31.49 31.35 32.31 30.72

17.25 26.74 18.01 18.72 16.77 17.39 17.93 29.30 29.01 27.64 29.08 27.14

Latent heat of freezing (J/g)

162.45 191.97 37.14 24.01 104.80 78.64 57.38 41.73 32.63 123.63 87.75 72.64

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

Melting point (◦ C)

Freezing point (◦ C)

Latent heat (J/g)

Reference

Lauric–stearic acid/gypsum ETP/cement ETS/cement ETP/gypsum ETS/gypsum Capric–palmitic acid/gypsum Capric–stearic acid/gypsum Capric–lauric acid/diatomite Capric–palmitic acid/attapulgite n-Nonadecane/cement RT20/montmorillonite Paraffin/expanded perlite n-Hexadecane/Na–montmorillonite ETP/diatomite ETP/perlite ETS/diatomite ETS/perlite XPP/gypsum XPP/cement XPP/perlite XPP/diatomite XPP/vermiculite XPS/gypsum XPS/cement XPS/perlite XPS/diatomite XPS/vermiculite

34.0 21.96 32.23 21.62 32.30 22.90 23.80 16.74 21.71 31.90 23.0 28.11 17.0 19.60 19.80 29.8 30.10 20.25 20.51 20.62 20.61 20.45 34.43 34.28 34.04 34.70 34.50

– 14.50 29.83 14.49 29.45 21.70 23.90 – – 31.80 – – 14.0 14.3 14.4 30.0 30.0 20.35 30.74 20.14 20.31 20.17 20.30 20.22 31.54 31.49 30.72

50.30 37.20 35.96 42.29 43.26 42.50 49.0 66.80 48.20 69.10 79.30 147.92 126.0 110.60 119.0 116.10 119.10 43.44 27.55 106.60 77.43 59.56 39.35 38.28 125.73 86.81 77.44

[17] [19] [19] [19] [19] [21] [22] [28] [28] [29] [30] [34] [35] [36] [36] [36] [36] Present study Present study Present study Present study Present study Present study Present study Present study Present study Present study

DSC curves of XPP, XPS and the form-stable composite PCMs during heating and subsequent cooling are presented in Figs. 4–6, respectively. As clearly seen from Fig. 4, the XPP and XPS esters melt at 20.35 ◦ C and 34.48 ◦ C (on-set values), respectively as they freeze at 20.30 ◦ C and 30.74 ◦ C (on-set values), respectively. These data showed that these esters were noticeably suitable for thermal energy storage applications in buildings with respect to the climate conditions. The on-set and peak temperature values regarding with melting and freezing of the composite PCMs were also given in Table 2. As also seen from Fig. 5 and Table 2, the on-set melting temperatures of XPP/gypsum, XPP/cement, XPP/perlite, XPP/diatomite, XPP/vermiculite, were measured as 20.25, 20.51, 20.62, 20.61, and 20.45 ◦ C, respectively while the on-set freezing temperatures were measured as 20.14, 20.31, 20.22, 20.30, and 20.17 ◦ C for these composites, respectively. On the other hand, from Fig. 6, the onset melting temperatures of XPS/gypsum, XPS/cement, XPS/perlite, XPS/diatomite and XPS/vermiculite were determined to be 34.43, 34.28, 34.04, 34.70, and 34.50 ◦ C whereas the on-set freezing temperatures were determined to be 31.54, 31.49, 31.35, 32.31 and 30.72 ◦ C for the same composites, respectively. When compared the phase change temperatures of the composite PCMs with that of the XPP and XPS esters, it can be seen small discrepancies. It may be due to the physical interactions identified as capillary forces and surface tension forces. The FT-IR spectroscopy results also verified

this idea. Moreover, by considering the phase change temperatures of the composite PCMs it can be also concluded that the prepared ten kinds of composite PCMs can be used as energy storage material for solar space heating and cooling applications. On the other hand, from Figs. 5 and 6, the latent heats of melting and freezing were found to be 164.46 and 162.45 J/g for XPP ester and 193.91 and 191.97 for XPS ester and 43.44 and 37.14 J/g for XPP/gypsum, 27.55 and 24.01 J/g for XPP/cement, 106.60 and 104.80 J/g for XPP/perlite, 77.43 and 78.64 J/g for XPP/diatomite, and 59.56 and 57.38 J/g for XPP/vermiculite. In addition, the latent heats of melting and freezing were determined as 39.35 and 41.73 J/g for XPS/gypsum, 38.28 and 32.63 J/g for XPS/cement, 125.73 and 123.63 J/g for XPS/perlite, 86.81 and 87.75 J/g for XPS/diatomite, and 77.44 and 72.64 J/g for XPS/vermiculite. These values make the prepared composites appropriate PCMs for thermal energy storage utility in buildings. Moreover, compared to the other composite PCMs especially, the composites of XPP and XPS esters with perlite and diatomite can be taken into account the best proper composite PCMs due to their higher latent heat values. In addition, the theoretical latent heat values of the composite PCMs were calculated by using the following equation:

Hcomp = PCMmf% × HPCM

(1)

Table 4 The measured DSC data of the prepared form-stable composite PCMs after 1000 thermal cycles. Composite PCM

XPP/gypsum XPP/cement XPP/perlite XPP/diatomite XPP/vermiculite XPS/gypsum XPS/cement XPS/perlite XPS/diatomite XPS/vermiculite

Phase change temperature for melting (◦ C) On-set

Peak

19.95 20.35 20.81 20.36 20.57 30.93 31.43 31.42 31.15 31.20

23.88 23.70 26.81 24.50 24.33 35.02 35.61 37.12 35.94 36.70

Latent heat of melting (J/g)

41.72 27.33 98.08 68.70 55.65 35.53 35.17 121.42 83.51 71.13

Phase change temperature for freezing (◦ C) On-set

Peak

20.28 20.26 20.47 20.42 20.46 30.92 30.81 30.97 31.37 30.84

18.29 18.55 16.96 18.02 17.93 28.72 28.30 27.29 27.97 27.55

Latent heat of freezing (J/g)

42.87 25.54 98.00 65.06 50.76 34.57 33.01 118.88 82.28 68.94

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In this equation, Hcomp , PCMmf% and HPCM denote the theoretical latent heats of the composite PCM, the mass fraction (%) of PCM (XPP or XPS esters) hold by the building material and the measured latent heat values of the esters, respectively. For the latent heat values of melting, the differences between the measured and calculated values were found as −9.97% for XPP/gypsum, −0.91% for XPP/cement, 2.67% for XPP/perlite, 1.08% for XPP/diatomite, 2.91% for XPP/vermiculite, 6.46% for XPS/gypsum, −9.63% for XPS/cement, 2.64% for XPS/perlite, 5.94% for XPS/diatomite, 3.99% for XPS/vermiculite. Moreover, the differences regarding with the latent heat of freezing were found as −5.36 for XPP/gypsum, 10.94% for XPP/cement, 4.02% for XPP/perlite, −1.70% for XPP/diatomite, 9.16% for XPP/vermiculite, −0.19% for XPS/gypsum, −2.41% for XPS/cement, 3.30% for XPS/perlite, 3.96% for XPS/diatomite, 9.02% for XPS/vermiculite. As seen from these values, the latent heat values measured for both melting and freezing processes of the composite PCMs are close to the theoretical values. On the other hand, in Table 3, the energy storage properties of the prepared form-stable composite PCMs were compared with that of other composites in the literature [17,19,21,22,28–30,34–36]. By considering the these data, it can be appreciably noted that among the prepared composite PCMs, especially XPP/perlite, XPS/perlite, XPP/diatomite, and XPS/diatomite, have relatively higher latent heat capacity than the most of the composite PCMs in the literature.

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

Degradation temperature (◦ C)

Weight loss (%)

XPP

First step: 79–271 Second step: 271–452

First step: 48.2 Second step: 51.8

XPS

First step: 58–306 Second step: 306–519

First step: 46.8 Second step: 53.2

XPP/gypsum

First step: 45–278 Second step: 278–387 Third step: >510

First step: 12.3 Second step: 11.6 Third step: 76.1

XPP/cement

First step: 45–210 Second step: 210–336 Third step: >544

First step: 7.2 Second step: 10.1 Third step: 82

XPP/perlite

First step: 80–258 Second step: 258–554 Third step: >554

First step: 24.9 Second step: 53.9 Third step: 21.2

XPP/diatomite

First step: 45–290 Second step: 290–438 Third step: >524

First step: 25.3 Second step: 18.4 Third step: 56.3

XPP/vermiculite

First step: 55–241 Second step: 241–529 Third step: >529

First step: 10.9 Second step: 28.1 Third step: 61.0

XPS/gypsum

First step: 50–234 Second step: 234–572 Third step: >572

First step: 8.0 Second step: 18.4 Third step: 73.7

XPS/cement

First step: 82–376 Second step: 376–563 Third step: >563

First step: 6.2 Second step: 12.4 Third step: 81.4

XPS/perlite

First step: 96–276 Second step: 276–549 Third step: >549

First step: 23.9 Second step: 40.5 Third step: 35.6

XPS/diatomite

First step: 57–282 Second step: 282–499 Third step: >499

First step: 18.8 Second step: 29.1 Third step: 52.1

XPS/Vermiculite

First step: 72–330 Second step: 330–530 Third step: >530

First step: 22.2 Second step: 18.4 Third step: 59.4

3.4. Thermal reliability and chemical stability of the form-stable composite PCMs A building composite PCM should be stable thermally and chemically even it is subjected to a large number of thermal cycling processes. In this study, thermal reliability and chemical stability of the prepared composite PCMs after repeated 1000 melting and freezing cycles were evaluated by using DSC and FT-IR analysis. The DSC results obtained after 1000 thermal cycles were given in Table 4. As seen from this table, after thermal cycling, the change in the on-set melting and on-set freezing temperatures of the composite PCMs were found to be −0.3 ◦ C and 0.14 ◦ C for XPP/gypsum, −0.16 ◦ C and −0.05 ◦ C for XPP/cement, 0.19 ◦ C and 0.25 ◦ C for XPP/perlite, −0.25 ◦ C and 0.12 ◦ C for XPP/diatomite, 0.12 ◦ C and 0.29 ◦ C for XPP/vermiculite. In similar way the changes were found as −3.5 ◦ C and −0.62 ◦ C for XPS/gypsum, −2.85 ◦ C and −0.68 ◦ C for XPS/cement, −2.62 ◦ C and −0.38 ◦ C for XPS/perlite, −2.89 ◦ C and −0.94 ◦ C for XPS/diatomite, −3.5 ◦ C and −0.12 ◦ C for XPS/vermiculite. These little changes mean that that the prepared composite PCMs had good thermal reliability with respect to their phase change temperatures. In addition, by comparing the latent heats of melting in Table 4 with the data obtained before thermal cycling, the changes were found to be −3.94% for XPP/gypsum, −0.79% for XPP/cement, −7.99% for XPP/perlite, −11.27% for XPP/diatomite, −6.56% for XPP/vermiculite, −9.70% for XPS/gypsum, −8.12% for XPS/cement, −3.42% for XPS/perlite, −3.80% for XPS/diatomite, −8.14% for XPS/vermiculite. Moreover, the changes in latent heat values regarding with the freezing processes was determined to be 15.42% for XPP/gypsum, 6.37% for XPP/cement, −6.48% for XPP/perlite, −17.26% for XPP/diatomite, −11.53% for XPP/vermiculite, −17.15 for XPS/gypsum, 1.16% for XPS/cement, −3.84% for XPS/perlite, −6.23% for XPS/diatomite, −5.09% for XPS/vermiculite. These results indicated that the prepared composite PCMs had good thermal reliability after repeated 1000 thermal cycling in terms of the change in their latent heat values. On the other hand, the chemical stability of the form-stable composite PCMs after thermal cycling was also investigated by FT-IR spectroscopy analysis. As clearly seen from Fig. 7(a and b) there is not available any change in the shapes of the peak and deviation

in the peak positions before and after thermal cycling. This means that the form-stable composite PCMs had good chemical stability after thermal cycling. 3.5. Thermal durability of the form-stable composite PCMs The thermal durability property is one of the most important parameters for a composite PCM used in thermal energy storage applications because it should be durable over its working temperatures. The thermal durability limits of the prepared composite PCMs were investigated by TG analysis. TG curves of XPP, XPS esters and the prepared form-stable composite PCMs were shown in Fig. 8(a–c) and the obtained data were also given in Table 5. As shown in Fig. 8(a) and Table 5, the weight loss processes of XPP and XPS were carried out by two steps. For the XPP ester, the first step corresponding to about 48% weight loss is between 79 ◦ C and 271 ◦ C and the second one corresponding to about 52% weight loss is laid between 271 ◦ C and 452 ◦ C. As similar, at the first degradation step in range of 58–306 ◦ C XPS ester show as about 47% weight loss while it lost its weigh part as about 53% in the range of 306–519 ◦ C. As also seen from Fig. 8(b and c) and Table 5, all the form-stable composite PCMs degrade at three steps. The first two belong to the ester compounds confined into the building materials. Moreover, the amounts of total weight loss regarding with the first and second steps of the composite PCMs are very close to those of the composite PCMs. The final step is attributed to the degradation or evaporation of the metal oxides and water contents of the building material. As

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

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

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also seen from Table 4 the initial degradation temperatures regarding with the first steps of all the composite PCMs varied from 45 ◦ C to 96 ◦ C while the initial degradation temperatures regarding with the second steps ranged from 210 ◦ C to 376 ◦ C. These results indicated that the initial degradation temperatures obtained for both the first and second steps of the composite PCMs showed a slight decrease in comparison with those of the ester compounds. However, they are much higher than the phase change temperatures of the composite PCMs. Therefore, based on the TG results it can be concluded that the prepared composite PCMs have good thermal durability above their working temperature range, 20–35 ◦ C. 3.6. Thermal conductivity of the PCMs The thermal conductivity of PCMs can be considered one of the most important parameters in thermal energy storage applications as well as phase change temperature and latent heat capacity. The thermal energy transfer ratio of PCMs depends on this parameter because it has a significant effect on the rates of energy storage and the release of PCM [42,43]. In this study, the thermal conductivity of the form-stable composite PCM was increased by addition of expanded graphite (EG) in 10% mass fraction. The thermal conductivity values of XPP/gypsum, XPP/cement, XPP/perlite, XPP/diatomite, XPP/vermiculite, XPS/gypsum, XPS/cement, XPS/perlite, XPS/diatomite and XPS/vermiculite were measured as 0.21, 0.22, 0.11, 0.10, 0.11, 0.13, 0.16, 0.10, 0.11, and 0.12 W/mK. After the addition of EG (10 wt%), thermal conductivity of the composites were measured as 0.28, 0.26, 0.14, 0.14, 0.16, 0.17, 0.20, 0.14, 0.15, and 0.16 W/mK, respectively. These results revealed that thermal conductivity of the composite PCMs were increased as 33, 18, 27, 40, 46, 31, 25, 40, 36, 33%, respectively after the EG addition. On the other hand, the DSC analysis was performed to investigate the effect of EG additive on the thermal energy storage properties of the composite PCMs. After ED addition, the melting temperatures of the composite PCMs decreased slightly ranged from 0.01 ◦ C to 1.3 ◦ C as the decrease in the latent heat values of the composite PCMs was only between 5.4% and 8.6%. 4. Conclusions This study is focused on the preparation and properties of novel building material-based composite PCMs for thermal energy storage and the conclusions are as follows: (1) Ten kinds of composite PCMs, XPP/gypsum, XPP/cement, XPP/perlite, XPP/diatomite, XPP/vermiculite, XPS/gypsum, XPS/cement, XPS/perlite, XPS/diatomite and XPS/vermiculite were prepared as novel-form stable building composite PCM by using vacuum adsorption method. (2) The maximum mass percentages of XPP and XPS esters confined into gypsum, cement, perlite, diatomite and vermiculite were determined as 22, 17, 67, 48 and 42 wt%, respectively. The composites with these combination ratios were identified as form-stable PCMs which have good mechanical strength because of capillary and surface tension forces between components of the composites. (3) The prepared form-stable composite PCMs were investigated morphologically by using SEM analysis techniques. The SEM results indicated that the XPP and XPS esters were distributed homogenously into the porous structures of the building materials. The form-stable composite PCMs were also characterized by FT-IR spectroscopy and the results showed that there is no chemical interaction between the esters and the building materials.

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