activated-attapulgiate composite as form-stable phase change material for thermal energy storage

Energy Conversion and Management 81 (2014) 306–311

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Stearic–capric acid eutectic/activated-attapulgiate composite as form-stable phase change material for thermal energy storage Shaokun Song a, Lijie Dong a, Shun Chen a, Haian Xie a, Chuanxi Xiong a,b,⇑ a State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China b School of Materials Science and Engineering, Wuhan Textile University, Wuhan 430073, PR China

a r t i c l e

i n f o

Article history: Received 30 December 2013 Accepted 20 February 2014 Available online 15 March 2014 Keywords: Fatty acid Eutectic Form-stable PCM Attapulgiate Thermal reliability

a b s t r a c t The aim of this research was to prepare a novel form-stable PCMs (FSPCM) for latent heat thermal energy storage (LHTES) in low temperature, by incorporating eutectic mixture of stearic-capric acid (S–C) into activated-attapulgite (a-ATP) which acted as supporting material in the composite. The a-ATP is openended tubular capillary with large specific surface area, which is beneficial for the adsorption of PCMs. The maximum mass fraction of stearic-capric binary fatty acid loaded in a-ATP is determined as high as 50 wt% without melted S–C seepage from the composite. The phase change temperatures and latent heats of FSPCM are measured to be 21.8 °C and 72.6 J/g for melting process, and 20.3 °C and 71.9 J/g for freezing process, respectively, indicating it has suitable phase change temperature and high latent heat storage capacity. Moreover, the S–C/a-ATP FSPCM shows good thermal and chemical reliability after 1000 times thermal cycling test, which is identified by differential scanning calorimetry (DSC) and Fourier transformation infrared (FTIR). Therefore, the S–C/a-ATP FSPCM is an effective LHTES building material to reduce energy consumption. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, energy shortage is becoming a global key issue, and thermal energy storage is playing an increasingly important role in reducing dependency on fossil fuels [1,2]. Latent heat thermal energy storage (LHTES) using phase change materials (PCMs) is considered to be the most effective and promising technique for solar energy heating and cooling applications due to its ability to provide a high storage density at nearly constant temperature [3]. Therefore, PCMs in buildings shift the peak of electric load to off peak load periods [4]. In recent years, extensive studies have been conducted in building energy conservation for decreasing the indoor temperature fluctuations, improving the indoor thermal comfort and saving electric energy [5–7]. Among the PCMs investigated, fatty acids are the mostly employed due to their desirable features, such as smaller volume change, negligible supercooling, low vapor pressure, good thermal and chemical stability, nontoxic and self-nucleating behavior [8,9]. ⇑ Corresponding author at: State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, and School of Materials Science and Engineering, Wuhan University of Technology, Wuhan 430070, PR China. Tel./fax: +86 27 87652879. E-mail address: [email protected] (C. Xiong). http://dx.doi.org/10.1016/j.enconman.2014.02.045 0196-8904/Ó 2014 Elsevier Ltd. All rights reserved.

Most of all, fatty acids show good compatibility with construction materials such as gypsum wallboard, plaster, concrete, clay minerals, and other wall covering materials. These are all-important characteristics to create building energy conservation materials with superior phase change properties [10]. Various methods have been suggested for the impregnation of PCMs into construction materials, such as microencapsulation [11] and FSPCM [12]. FSPCM, in which the PCM is dispersed in supporting material, are attracting increasing attention due to their low cost, easy fabrication, large latent heat, suitable thermal conductivity, the ability to keep form stable in phase change process, as well as thermal reliability over a long period of usage [13,14]. Attapulgite (ATP), also called as palygorskite, is a crystalline hydrated magnesium aluminum silicate with structural formula of Si8O20Mg5(Al)(OH)2(H2O)4
4H2O [15]. ATP has a specific surface area of about 300–600 m2/g, as well as unique layered, porous structure and tubular morphology [16]. To date, ATP has received extensive attention for its large adsorption capacity and relatively low price in modern science and technology, for instance in the disposal of sewage [17], the decolorization of cooking oil [18], the catalysis field [19], and the superabsorbent composite [20,21], etc. The adsorption capacity of ATP will be further enhanced when the coordinated and adsorbed water filled in zeolite channels is removed by calcinations [22]. Despite many studies on

307

S. Song et al. / Energy Conversion and Management 81 (2014) 306–311

ATP, a research on the application of thermal activated ATP (a-ATP) as building materials for thermal energy storage is still lacking, previously. In this study, the a-ATP was obtained by thermal treatment, and used as supporting material. Binary mixture of stearic acid (SA) and capric acid (CA) was compounded according their eutectic ratio. Then, S–C eutectic was absorbed into the pores of a-ATP with vacuum adsorption method. The prepared S–C/a-ATP as form-stable PCM was characterized in terms of chemical compatibility, thermal properties and thermal stability using scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transformation infrared spectroscope (FTIR) and thermal gravimetric (TG) analysis technique. 1000 melting/freezing cycles were performed additionally to test the thermal and chemical reliability of S–C/a-ATP. The results reveal that the S–C/a-ATP composite with the advantages of low cost, simple process, flame retardant, enhanced thermal stability, high latent heat, suitable phase change temperature, and good thermal reliability are potential materials in solar application for building energy conservation. 2. Experimental 2.1. Materials Stearic acid and capric acid (AR) were purchased from Sinopharm Chemical Regent Co. Ltd., China. The physical properties of stearic acid and capric acid are given in Table 1. Attapulgiate clay used in this study was received from Huaiyuan Mining Industrial Ltd. Co. of Jiangsu, China. 2.2. Preparation of S–C eutectic The phase change temperatures of SA and CA are higher for passive solar houses. In this sense, S–C eutectic is prepared to decrease the phase change temperature to a suitable value. The blending process of CA and SA is regarded as the ideal solution model and the eutectic ratio XA and melting point T can be calculated by Schroder’s equation: T = 1/(1/Tm Rln XA/Hm,A) [23]. Where Hm,A and Tm are the latent heat and melting point of component A, R is the gas constant. By calculation, the combination ratio of eutectic PCM was found to be 86 wt% CA and 14 wt% SA. Then, sample is weighed according to the proportion, and put in the water bath at a constant temperature of 70 °C until it melts completely. The sample is stirred for 30 min with a magnetic stirring apparatus at 300 rpm and then cooled to room temperature.

and put them in a vacuum oven with a constant temperature of 80 °C for 24 h. The sample with a maximum ratio of S–C and no melted S–C was observed meanwhile was optimum. The maximum S–C amount in a-ATP was found to be 50 wt% and this composite was named as form-stable PCM. 2.4. Characterization The chemical structure analysis of the samples is carried out on a FTIR spectrophotometer (Thermo Nicolet Nexus). The FTIR spectra were recorded from 400–4000 cm 1 with resolution of 2 cm 1, using KBr pellets. The morphologies of samples are observed by SEM (S-4800, Hitachi). The sample was coated by Ag before observation. The microstructures of the a-ATP and the composite PCM were observed using a TEM (H-Tecnai). The thermal properties of samples such as latent heat and phase change temperature are determined in a DSC instrument (Pyris 1 DSC, Perkin-Elmer). The analysis was carried out with a heating and cooling rate of 10 °C/ min in the range of 10  70 °C under nitrogen atmosphere. About 5 mg sample was used each time. To erase the thermal history, the second DSC run was recorded. The phase change temperature corresponds to the extrapolated onset temperature obtained by drawing a line at the point of maximum slope of the leading edge of the DSC peak and extrapolating baseline on the same side as the leading edge of the peak. The latent heat of phase change was calculated by numerical integration of the area under the peaks. TGA (NETZSCH STA 449F3) was used to evaluate the thermal stability of samples. The samples were heated from 50 °C to 800 °C at heating rate of 10 °C/min under N2 atmosphere. Besides, 1000 times repeated melting/freezing cycling was conducted additionally to determine the thermal reliability of FSPCM. 3. Results and discussion 3.1. FTIR analysis of S–C/a-ATP FSPCM FTIR spectroscopy is used to evaluate the probable interactions between a-ATP and S–C in the composite. The FTIR spectra of the aATP powder, S–C and S–C/a-ATP FSPCM are presented in Fig. 1. In the FTIR spectrum of ATP, the multiple absorption peaks at 3614, 3550, 3419 cm 1 are corresponded to AOH group. The peaks at 1032, 980, 796 cm 1 are all the characteristic absorption peak of

a-ATP

2.3. Preparation of S–C/a-ATP form-stable PCM 3419

Transmittance

Attapulgiate was activated in a muffle furnace at 400 °C for 2 h. Then, the S–C/a-ATP FSPCM was prepared through vacuum fusion absorption method [24,25]. It is sensible that the higher the mass ratio of S–C in composite is the larger latent heat the obtained FSPCM exhibits. In order to determine the maximum mass ratio of S–C that can be absorbed in composite, a series of binary systems of S–C and a-ATP with different mass ratio were prepared,

S-C

1710 S-C/a-ATP

Table 1 Thermal properties of SA and CA. Sample

Melting Melting point Tm (°C)

CA SA

31.5 68.7

3329

Freezing Latent heat DHm (J/g) 165.7 221.1

Freezing point Tf (°C) 30.6 66.4

Latent heat DHf (J/g) 163.5 219.2

4000

3500

1713

3000

2500

2000

1500

1000

-1

Wavenumber (cm ) Fig. 1. FTIR spectra of a-ATP, S–C and S–C/a-ATP FSPCM.

500

308

S. Song et al. / Energy Conversion and Management 81 (2014) 306–311

SiAO and SiAOASi, as well as AlAO and AlAOASi. In the spectrum of S–C, the multiple peaks at 2921 and 2852 cm 1 are attributed to the CAH stretching vibration of ACH3 and ACH2. The broad peak located at 3300–2800 cm 1 is caused by AOH stretching vibration, which usually overlaps with the ACH3 and ACH2 stretching vibration. The intensive peak at 1710 cm 1 is the characteristic absorption peak for stretching vibration of C@O group. Compared with the FTIR spectra of S–C and a-ATP, it can be found that the characteristic peaks of S–C and a-ATP also appear in that of S–C/a-ATP. Whereas, the C@O vibration peak of S–C shifts from 1710 cm 1 to 1713 cm 1, the AOH peak of a-ATP shifts from 3419 cm 1 to 3392 cm 1. These frequency changes reveal that there is no chemical interaction except strong hydrogen bond exists between carboxyl group of S–C and hydroxyl group of a-ATP. Therefore, the melted fatty acid molecules can be held easily in the surface and pores of a-ATP by these attractive forces. 3.2. Morphology characterization of the S–C/a-ATP FSPCM The morphologies of a-ATP, particle size and shape are crucial for achieving the desired properties. The surface morphology of a-ATP and S–C/a-ATP form-stable PCM is investigated by SEM analysis. As can be seen from Fig. 2(a), a-ATP has a fibrous morphology and its diameter is about 20–25 nm and length is 0.2–1 lm. The surface of a-ATP nanotube is full of trench. Therefore, a-ATP has high specific surface area and excellent absorbability, and organic compounds such as S–C can be easily absorbed in a-ATP by surface tension. Whereas, it can be found that the surface of a-ATP in composite becomes smooth and the diameter shows an apparent increment, to 30–50 nm. In addition, most a-ATP aggregates are broken down to primary particles, which implies that the S–C cover diminish the interfacial interaction between a-ATP nanotubes.

The results indicate that S–C has been absorbed onto the surface of a-ATP. Fig. 3 shows TEM images of a-ATP and S–C/a-ATP FSPCM. As can be seen that the a-ATP is open-ended nanotube structure, and the thickness of the wall is 8–10 nm. Whereas, the pore canal of a-ATP in the composite is fully filled, demonstrating that the melted S–C is absorbed into the canals of a-ATP by capillary force in the preparation. Besides, apparent diameter increment also can be observed, which is in accordance with SEM images. Because of the strong hydrogen bond, capillary force and surface tension, a-ATP can keep the melted S–C in composite when temperature rises above the melting point. Hence, the composite is form-stable PCM, and have advantage in direct-gain passive solar houses. 3.3. Thermal properties and thermal stability of the S–C/a-ATP FSPCM DSC analysis is conducted to investigate the thermal properties such as melting or freezing temperature and the latent heat storage capacities of FSPCM. The DSC curves of S–C eutectic and FSPCM are presented in Fig. 4. Only one solid–liquid phase change peak can be observed in the DSC thermograms of S–C and FSPCM, which is evidenced binary eutectic forming. From the DSC curves, the melting and freezing temperatures are found to be 24.1 °C and 20.7 °C for S–C. However, the melting and freezing temperatures of FSPCM decrease to 21.8 °C and 20.3 °C. The slightly decrease of phase change temperature is probably due to the strong interactions between S–C and a-ATP. These results are in good agreements with literatures [14,26]. The phase change temperature of the prepared FSPCM is in the range of indoor comfortable temperature (15– 25 °C). Besides, ATP is flame retardant construction material with large reserves in China, which has much lower price than active carbon, expanded graphite. Hence, it can be applied directly in

Fig. 2. SEM micrographs of a-ATP and S–C/a-ATP.

Fig. 3. TEM images of a-ATP (a) and S–C/a-ATP (b).

309

S. Song et al. / Energy Conversion and Management 81 (2014) 306–311

Endo 60

100

S-C 45

80

30

70

48.5%

15

Mass (%)

Heat Flow (mW)

ATP

90

S-C/a-ATP

0

-15

60 50

S-C/a-ATP

40 30

-30

20 -45 10 -60 Exo -20

-10

0

10

20

30

40

50

60

S-C

0

70

o

0

Temperature ( C)

100

200

300

400

500

600

o

Tempearture ( C )

Fig. 4. DSC curves of S–C and S–C/a-ATP FSPCM.

Fig. 5. TG curves of S–C, ATP, and S–C/a-ATP FSPCM.

the building materials for heat and cold storage in winter and summer to reduce energy consumption. On the other hand, the latent heats of melting and freezing are calculated as 161.35 J/g and 158.42 J/g for S–C eutectic and 72.6 J/g and 71.3 J/g for FSPCM. Although the latent heats of FSPCM are slightly lower than the theoretical value based on the mass fraction of S–C in composite, the FSPCM exhibits the acceptable latent heat capacities. Table 2 presents the comparison of the thermal properties of FSPCM prepared in this study with those of different FSPCMs available in literatures [27–33]. The results indicate that the S–C/a-ATP prepared in this study has suitable phase change temperature and the latent heat are as high as those of literatures, and therefore, it has an important LHTES potential in buildings. Thermal stability is a significant factor in evaluating the FSPCM for the applications of heat energy storage or thermal regulation [34]. The TG curves of S–C eutectic, ATP, and S–C/a-ATP are shown in Fig. 5. The TG curve of ATP clearly shows a 9.2% weight loss of dehydration of adsorbed water from 76 °C to 137 °C, which is attributed to the elimination of linked, zeolitic and crystallized water molecules. The removal of bounded water in ATP canals leading an increment of specific surface area [22,35]. A higher specific surface area of a-APT enhances the interaction between a-APT and S–C, which is helpful for the formation of a more intact network and also contributes to the improvement of adsorbability for a-ATP. In the TG curve of S–C, its decomposition starts at 150 °C and ends at 250 °C. According to the TG curve of S–C/aATP, the onset decomposition temperature of S–C has been postponed and the decomposition process of S–C has been prolonged from 197 °C to 384 °C. These results reveal that the decomposition of S-C is retarded in composite and the working thermal stability of the FSPCM is promoted. From the TG curve of the FSPCM, the maximum adsorption ratio of S–C is 48.5 wt%, consistent with

experimented value. Furthermore, the S–C/a-APT composite has good fire resistance compared with the polymer based FSPCM. The prepared FSPCM in this study is supposed to be applied in the solar energy storage for building energy conservation and the designed working temperature is below 80 °C, so the FSPCM can be used repeatedly. 3.4. Thermal reliability of the S–C/a-ATP FSPCM As we all know, a perfect PCM should not only have excellent heat storage ability but also should have good thermal and chemical reliability with respect to a large number of melting and freezing cycles. Therefore, thermal cycling test is conducted additionally because it is the most important parameter about PCM in its service life. The thermal properties of S–C/a-ATP FSPCM after repeated thermal cycling are evaluated by DSC. The DSC curves of FSPCM before and after 1000 thermal cycling are given in Fig. 6. The melting temperature of composite varies 0.39 °C and freezing temperature varies 0.57 °C after 1000 thermal cycling. Meanwhile, the latent heats of melting and freezing changed by 1.7% and 3.6% after thermal cycling, indicating there is no mass loss of S–C. The results mean that the variations of the phase change temperatures and latent heats are negligible for LHTES applications. Therefore, it can be concluded that the thermal properties of S–C/a-ATP FSPCM keep stable for at least 1000 cycling. Moreover, FTIR analysis is applied to monitor chemical structure change of S–C/a-ATP after thermal cycling. The FTIR spectra of cycled and uncycled FSPCM are shown in Fig. 7. As can be seen that the FTIR spectrum of cycled FSPCM, the shape and frequency values of all peaks are in accordance with those of uncycled FSPCM, indicating the chemical structure of S–C/a-ATP is not affected by

Table 2 Comparison of thermal properties of the composite prepared with that of some FSPCMs in literatures. FSPCM

Melting point (°C)

Freezing point (°C)

Latent heat (J/g)

Reference

Capric–myristic acid (20 wt%)/vermiculite RT20/montmorillonite Capric-stearic acid (25 wt%)/gypsum Capric acid (55 wt%)/expanded perlite n-nonadecane (50 wt%)/cement Lauric acid (60 wt%)/expanded perlite Stearic acid (60 wt%)/halloysite Capric-stearic acid (50 wt%)/activated-attapulgiate

19.8 20.8 23.8 31.8 31.9 44.1 53.5 21.8

17.1 – 23.9 31.6 31.8 40.9 49.21 20.3

27.0 53.6 49.0 98.1 69.1 93.4 93.9 72.6

[27] [28] [29] [30] [31] [32] [33] Present study

310

S. Song et al. / Energy Conversion and Management 81 (2014) 306–311

Acknowledgements

Endo 40

after 1000 cycling

30

We gratefully acknowledge the financial support of the National Natural Science Foundation of China (Nos. 51273157, 51173139, 51072151) and New Century Excellent Talents in University of Ministry of Education of China (No. NCET-10-0659).

Heat Flow (mW)

20 10 0 -10

uncycled

References

-20 -30 -40 Exo -20

-10

0

10

20

30

40

50

60

70

o

Temperature ( C) Fig. 6. DSC curves of S–C/a-ATP FSPCM before and after 1000 thermal cycling.

Transmittance

uncycled

4000

after 1000 cycling

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 7. FTIR spectra of the S–C/a-ATP FSPCM before and after thermal cycling.

repeated melting/freezing cycling. The FTIR results demonstrate that the S–C/a-ATP FSPCM is chemically stable for at least 1000 thermal cycling.

4. Conclusions In summary, the preparation and characterization of S–C/a-ATP FSPCM for LHTES is presented. The S–C and activated-ATP are chosen as PCM and supporting material, respectively. Activated-ATP is open-ended tubular capillary with large specific surface area, and hence S–C can be easily retained in the pore canals and on the surface of a-ATP due to the strong hydrogen bond, capillary and surface tension forces. The maximum adsorption ratio of S–C eutectic in composite is found as high as 50 wt% without seepage of melted S–C eutectic. The S–C/a-ATP composite melts at temperature of 21.8 °C with latent heat of 72.6 J/g, and freezes at temperature of 20.3 °C with latent heat of 71.3 J/g. Moreover, the composite shows negligible variations in latent heat storage capacity although it is subjected to 1000 melting/freezing cycling, indicating it can be used repeatedly in long term. The S–C/a-ATP FSPCM possesses advantages of low cost, simple process, flame retardant, enhanced thermal stability, high latent heat, suitable phase change temperature, good thermal and chemical reliability after a large number of thermal cycling. Therefore, the prepared S–C/a-ATP has important potential in the application of building material to reduce energy consumption.

[1] Liu C, Li F, Ma LP, Cheng HM. Advanced materials for energy storage. Adv Mater 2010;22:26–8. [2] Zhou D, Zhao CY, Tian Y. Review on thermal energy storage with phase change materials (PCMs) in building applications. Appl Energy 2012;92:593–605. [3] Li HR, Jiang M, Dong LJ, Xie HA, Xiong CX. Aqueous preparation of polyethylene glycol/sulfonated graphene phase change composite with enhanced thermal performance. Energy Convers Manage 2013;75:482–7. [4] Sun YJ, Wang SW, Xiao F, Gao D. Peak load shifting control using different cold thermal energy storage facilities in commercial buildings: a review. Energy Convers Manage 2013;71:101–14. [5] Wang X, Zhang YP, Xiao W, Zeng RL, Zhang QL, Di HF. Review on thermal performance of phase change energy storage building envelope. Chin Sci Bull 2009;54:920–8. [6] Jiang F, Wang X, Zhang YP. A new method to estimate optimal phase change material characteristic in a passive solar room. Energy Convers Manage 2011;52:2437–41. [7] Jiang F, Wang X, Zhang YP. Analytical optimization of specific heat of building internal envelope. Energy Convers Manage 2012;63:239–44. [8] Fang G, Li H, Liu X. Preparation and properties of lauric acid/silicon dioxide composites as form-stable phase change materials for thermal energy storage. Mater Chem Phys 2010;122:533–6. [9] Sari A. Thermal reliability test of some fatty acids as PCMs used for solar thermal latent heat storage applications. Energy Convers Manage 2003;44: 2277–87. [10] Wang L, Meng D. Fatty acid eutectic/polymethyl methacrylate composite as form-stable phase change material for thermal energy storage. Appl Energy 2010;87:2660–5. [11] Li HR, Jiang M, Dong LJ, Xie HA, Xiong CX. Facile preparation and thermal performances of hexadecanol/crosslinked polystyrene core/shell nanocapsules as phase change material. Polym Composite 2014. http://dx.doi.org/10.1002/ pc.22879. [12] Wang Y, Xia TD, Zheng H, Feng HX. Stearic acid/silica fume composite as formstable phase change material for thermal energy storage. Energy Build 2011;43:2365–70. [13] Chen KP, Yu XJ, Tian CR, Wang JH. Preparation and characterization of formstable paraffin/polyurethane composites as phase change materials for thermal energy storage. Energy Convers Manage 2014;77:13–21. [14] Trigui A, Karkri M, Krupa I. Thermal conductivity and latent heat thermal energy storage properties of LDPE/wax as a shape-stabilized composite phase change material. Energy Convers Manage 2014;77:586–96. [15] Bradley WF. The structural scheme of attapulgite. Am Miner 1940;25:405–10. [16] Galan E. Properties and application of palygorskite-sepiolite clays. Clay Miner 1996;31:443–53. [17] Feng L, Xu WH, Liu TF, Liu J. Heat regeneration of hydroxyapatite/attapulgite composite beads for defluoridation of drinking water. J Hazard Mater 2012;221:228–35. [18] Liu YF, Huang JH, Wang XG. Adsorption isotherms for bleaching soybean oil with activated attapulgite. J AM Oil Chem Soc 2008;85:979–84. [19] Li X, Ni C, Yao C, Chen C. Development of attapulgite/Ce1 xZrxO2 nanocomposite as catalyst for the degradation of methylene blue. Appl Catal B-Environ 2012;117:118–24. [20] Lai SL, Han WJ, Yuan D. Synthesis of P(AA–AM)/attapulgite clay SAR under microwave irradiation. J Macromol Sci A 2011;48:31–6. [21] Chen LF, Lang HW, Lu Y, Cui CH, Yu SH. Synthesis of an attapulgite clay@carbon nanocomposite adsorbent by a hydrothermal carbonization process and their application in the removal of toxic metal ions from water. Langmuir 2011;27:8998–9004. [22] Tang QG, Liang JS, Meng JP, Wang F. Effect of heat treatment on properties of mineral attapulgite. Adv Mater Res 2009;58:42–6. [23] Zhang YP, Su YH, Ge XS. Prediction of the melting temperature and the fusion heat of (quasi-) eutectic PCM. J China Univ Sci Technol 1995;25:474–8. [24] Chen M, Zheng S, Wu S. Melting intercalation method to prepare lauric acid/ Organophilic montmorillonite shape-stabilized phase change material. J Wuhan Univ. Technol-Mater Sci Ed 2010;25:674–7. [25] Mei D, Zhang B, Liu R, Zhang YT, Liu J. Preparation of capric acid/halloysite nanotube composite as form-stable phase change material for thermal energy storage. Sol Energy Mater Sol Cells 2011;95:2772–7. [26] Zhang D, Tian S, Xiao D. Experimental study on the phase change behavior of phase change material confined in pores. Sol Energy 2007;81:653–60. [27] Karaipekli A, Sari A. Capric–myristic acid/vermiculite composite as form-stable phase change material for thermal energy storage. Sol Energy 2009;83: 323–32.

S. Song et al. / Energy Conversion and Management 81 (2014) 306–311 [28] Fang X, Zhang Z, Chen Z. Study on preparation of montmorillonite-based composite phase change materials and their applications in thermal storage building materials. Energy Convers Manage 2008;49:718–23. [29] Sari A, Karaipekli A, Kaygusuz K. Capric acid and stearic acid mixture impregnated with gypsum wallboard for low-temperature latent heat thermal energy storage. Int J Energy Res 2008;32:154–60. [30] Sari A, Karaipekli A. Preparation, thermal properties and thermal reliability of capric acid/expanded perlite composite for thermal energy storage. Mater Chem Phys 2008;109:459–64. [31] Li H, Liu X, Fang G. Preparation and characteristics of n-nonadecane/cement composites as thermal energy storage materials in buildings. Energy Build 2010;42:1661–5.

311

[32] Sari A, Karaipekli A, Alkan C. Preparation, characterization and thermal properties of lauric acid/expanded perlite as novel form-stable composite phase change material. Chem Eng J 2009;155:899–904. [33] Mei D, Zhang B, Liu R, Zhang H, Liu J. Preparation of stearic acid/halloysite nanotube composite as form-stable PCM for thermal energy storage. Int J Energy Res 2011;35:828–34. [34] Rathod MK, Banerjee J. Thermal stability of phase change materials used in latent heat energy storage systems: a review. Renew Sust Energy Rev 2013;18:246–58. [35] Wang WJ, Zhang JP, Chen H, Wang AQ. Study on superabsorbent composite VIII. Effects of acid- and heat-activated attapulgite on water absorbency of polyacrylamide/attapulgite. J Appl Polym Sci 2007;103:2419–24.