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halloysite nanotube composite as form-stable phase change material
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Solar Energy 86 (2012) 1142–1148 www.elsevier.com/locate/solener
Preparation and thermal energy properties of paraffin/halloysite nanotube composite as form-stable phase change material Jiangshan Zhang a,b, Xiang Zhang b,⇑, Yazhen Wan b, Dandan Mei b, Bing Zhang b,⇑ a
College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455002, PR China b School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, PR China Received 30 March 2011; received in revised form 15 November 2011; accepted 5 January 2012 Available online 20 February 2012 Communicated by: Associate Editor Halime Paksoy
Abstract Phase change materials (PCMs) have attracted extensively interests in solar storage. In the study, we prepared a new kind of composite PCM by impregnating paraffin (P) into halloysite nanotube. The as-prepared composite PCM was characterized by TEM, FTIR and DSC analysis techniques. The composite can absorb paraffin as high as 65 wt.% and maintain its original shape perfectly without any paraffin leakage after subjected to 50 melt–freeze cycles. The melting temperature and latent heat of composite (P/HNT: 65/35 wt.%) were determined as 57.16 °C and 106.54 J/g by DSC. Graphite was added into the P/HNT composite to improve thermal storage performance, and the melting time and freezing time of the composite were reduced by 60.78% and 71.52% compared with the composite without graphite, respectively. Due to its high adsorption capacity, high heat storage capacity, good thermal stability and simple preparation method, the composite can be considered as cost-effective latent heat storage material for practical application. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Paraffin; Halloysite nanotube; Phase change material; Thermal properties
1. Introduction Latent heat thermal energy storage (LHTES) using PCM has attracted interest in solar storage and utilization due to its ability to provide a high storage density at nearly isothermal conditions (Dincer and Dost, 2002; Zalba et al., 2003; Farid et al., 2004). So far, a large number of PCMs such as salt hydrates, paraffins, fatty acids and their mixtures have been widely investigated for LHTES (Farid et al., 2004; Sari et al., 2009b; Wang et al., 2010; Zhao et al., 2010). Among them, paraffin is taken as a promising PCM because of its proper melting temperature range, high latent heat capacity, good chemical and thermal stability, little or ⇑ Corresponding authors.
E-mail addresses: [email protected] (X. Zhang), zhangb@ zzu.edu.cn (B. Zhang). 0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2012.01.002
no supercooling during the phase transition, nontoxicity and noncorrosivity against metal containers (Cho and Choim, 2000; He and Setterwall, 2002; Trp, 2005). However, the melted paraffin has to be kept in a closed tank or container to prevent leaching during the phase transition. Therefore, special latent storage device or elements such as a heat exchanger or lots of containers to encapsulate the PCM are needed which increase the cost (Meng and Hu, 2008; Baye´ s-Garcı´a et al., 2010). Recently, this problem can be solved using shape-stabilized or form-stable PCM composite which can be prepared by encapsulation of PCM into a polymeric structure such as low-density polyethylene, high-density polyethylene (HDPE), poly(styrene–butadiene–styrene) (SBS), poly (methyl methacrylate) and styrene maleic anhydride copolymer (SMA) (Ye and Ge, 2000; Xiao et al., 2002; Sari, 2004;
J. Zhang et al. / Solar Energy 86 (2012) 1142–1148
Zhang et al., 2006; Krup et al., 2007; Cemil and Sari, 2008; Sari et al., 2008a; Wang and Duo, 2010). However, its application was hampered by high cost of encapsulation and easy flammability. There were very few literatures that aimed to use clays as absorbents to prepare the form-stable composite PCMs for thermal energy storage (Fang et al., 2008; Sari et al., 2008b; Karaipekli and Sari, 2009a). Compared with polymeric encapsulants, clays are readily obtainable, hardly flammable and much cheaper. Moreover, several advantages like high adsorption capacity, high heat storage capacity, good thermal stability and direct usability without extra encapsulation render the composite potential heat storage material for practical application. HNT is a two-layered aluminosilicate clay mineral, which is available in abundance in China as well as other locations around the world. It is chemically similar to kaolin, differing mainly in the morphology of crystals (Joussein et al., 2005). HNTs possess hollow nanotubular structure and large specific surface area. Their novel physical and chemical properties have provided an opportunity as absorbent to remove harmful ions from waste water (Luo et al., 2010). However, there is not any literature that aims to use halloysite as adsorbent to prepare form-stable composite PCM. In this study, a new form-stable PCM was prepared by absorbing paraffin into the pores of halloysite nanotubes. In order to improve the thermal storage performance we also introduced graphite into composite PCM. The asprepared composite has a high adsorption capacity of paraffin, high heat storage capacity and good thermal stability, which indicates that the composite can be used as a effective material to store solar energy in practical application. 2. Experiments 2.1. Materials Paraffin was used as a latent heat storage material. It was supplied by Chemical Reagent Co., Ltd. (Tianjin, China). Halloysite clay from Henan Province (China) was milled and sieved followed by oven dried at 373 K for 24 h. 2.2. Preparation of the form-stable composite PCM The HNT and P were mixed in the toluene solution. The mixture was irradiated ultrasonically for 90 min, and then stirred and refluxed under 80 °C for 2 h in a water bath. After the recovery of toluene by simple distillation, the mixture was dried at 80 °C. In order to find the maximum adsorption capacity of P, the P/HNT composites were prepared with the different weight ratios of 50:50, 60:40, 65:35 and 70:30 respectively. In order to increase thermal storage and release rates, graphite (5 wt.%) was added in the mixture at the beginning. The following steps were consistent with the same method mentioned above.
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2.3. Characterizations of HNTs and the paraffin/HNTs composite PCM The Brunauer–Emmet–Teller (BET) surface area and pore-size distribution of the HNT were measured by a specific surface area analyzer (Quantachrome NOVA4200e). The microstructures of the HNT and the composite PCM were observed using a transmission electron microscope (TEM, FEITECNAIG2). The form-stable composite PCM was characterized using the FT-IR spectroscopy (NEXUS FT-IR). The phase change temperature and latent heat of the composite PCM were measured using a differential scanning calorimeter (STA449C, NETZSCH) at a heating rate of 5 °C/min in a purified nitrogen atmosphere. 2.4. Test of thermal storage and release rates Thermal performance test was conducted using the con¨ zonur et al., 2006; stant temperature water bath method (O Karaipekli and Sari, 2009a). Two glass test tubes of 250 mm height and 40 mm inside diameter and glass thickness of 1 mm were used: one containing the form-stable P/ HNT composite as reference and the other containing the P/HNT composite with 5 wt.% graphite (P/HNT/G). Two thermocouples were placed in the centers of the test tubes, respectively. The test tubes were put into a water bath at a constant temperature of 40 °C. After the temperature reached balance, the two tubes were rapidly placed into another water bath at a constant temperature of 70 °C, where the composite PCMs performed process of heat storage. After the heat storage was finished, the composites were immediately subjected to solidification process at a constant temperature of 40 °C, where the composite PCMs performed process of heat extraction. The temperature variations of the composite during these periods were recorded automatically by a PC via data logger (Agilent 34970A) with a temperature measuring accuracy of ±1.5 °C at time intervals of 10 s. 3. Results and discussions 3.1. Morphology characterization of the form-stable PCM The photograph of the pure P, P/HNT (65/35 wt.%) and P/HNT/G (65/30/5 wt.%) slices at room temperature is shown in Fig. 1A(a–c). The original samples have the cylindrical shape with smooth surface. When heated to 70 °C above the phase change temperature of the paraffin (57 °C), the pure paraffin melt in Fig. 1B(a). However, the form-stable composite samples still kept in shape perfectly and had no liquid leakage on the surface after subjected to 50 melt–freeze cycles, shown in Fig. 1B(b and c). Therefore, the composite PCMs are thermally stable after melt–freeze cycles. The further experiments indicated that composite samples could keep in shape perfectly even after subjected to 500 melt–freeze cycles. Fig. 2 shows the TEM images of the original HNTs and the composite prepared in this work. The original HNTs
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Fig. 1. Images of the pure P (a), P/HNT (b) and P/HNT/G (c) samples at room temperature (A) and after heated at 70 °C (B).
have an average length of 0.5–1 lm, a diameter in the range of 30–40 nm (Fig. 2a). The inner diameter is more or less 30 nm, while the thickness of the wall is about 8–10 nm. Compared with natural nanotubes, the pores of the HNT in the composite are partly filled with paraffin (Fig. 2b). Due to its large and smooth unhindered pores in the nm range, HNT could keep the melted P in pores even over the melting temperature of the P. Hence, the composite maintains its solid shape without any seepage of the melted P. In order to investigate the absorbability of the HNT, the pore size distribution and BET surface area were measured.
Fig. 3 depicts the adsorption–desorption curves and the pore distribution curves of original HNT and P/HNT (65/ 35 wt.%) composite. The results show that the specific surface area of original HNT is 57.76 m2/g, while the specific surface area of P/HNT composite is drastically decreased to 19.67 m2/g (Fig. 3a). The peaks between 6 nm and 10 nm are observed in the pore distribution of the original HNT. However, for P/HNT composite, the peaks between 6 nm and 10 nm are completely disappeared (Fig. 3b). The dramatic decrease of surface area was caused by the filling of the nanotubes with P, which is consistent with our TEM observation result.
Fig. 2. The TEM images: (a) HNT and (b) form-stable P/HNT composite PCM.
J. Zhang et al. / Solar Energy 86 (2012) 1142–1148
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Fig. 3. Isothermal adsorption–desorption curves (a) and pore distribution curves (b) of HNT and P/HNT composite PCM.
3.2. FT-IR spectroscopy The composite PCM was characterized by FT-IR spectroscopy to investigate the specific interactions between P and HNT. The FT-IR spectra of halloysite nanotubes, paraffin and the form-stable P/HNT composite are shown in Fig. 4. The FT-IR spectra of halloysite nanotubes exhibits two Al–OH stretching bands at 3699 cm 1 and 3625 cm 1 (Fig. 4a) and a single Al–OH bending band at 910 cm 1 (Frost, 1995; Farmer and Russell, 1964). The 1094 cm 1 peak is assigned to stretching mode of apical Si–O, while the band at 1033 cm 1 is caused by the stretching vibrations of Si–O–Si. Moreover, the band observed at 534 cm 1 is deformation vibration of Al–O–Si. In FT-IR spectra of P (Fig. 4b), the peaks at 2920 cm 1 and 2850 cm 1 are caused by stretching vibration of C–H and the peaks at 1467 cm 1 and 724 cm 1 are attributed to the deformation vibration of –CH2 and –CH3 and the rocking vibration
Fig. 4. FT-IR spectra of HNT (a), P (b) and form-stable P/HNT composite PCM (c).
of –CH2, respectively. For the composite (Fig. 4c), the peaks assigned to halloysite at 3699, 3625, 1094, 910, 534 cm 1 and the peaks assigned to P at 2920, 2850, 1467, 724 cm 1 are still existed and no significant new peak is observed, which indicate that the composite is just a physical interaction between P and HNT. 3.3. Thermal properties of the form-stable composite PCMs The DSC curves of P and P/HNT composite are shown in Fig. 5. As can be seen in Fig. 5, two main transition peaks were clearly observed, in which the sharp or main peak should be attributed to the heterogeneously nucleated rotator–liquid transition which represents the solid–liquid phase change of the paraffin and the minor peak at the left side of the main peak to the homogeneously nucleated crystal–rotator transition corresponds to the solid–solid phase transition of paraffin (Fan et al., 2004, 2005; Zhang et al., 2005; Xie et al., 2006). These figures indicate that both paraffin and the composite PCMs exhibit similar thermal characteristics. This is because there is no chemical reaction between the paraffin and HNT in preparation of the form-stable composite PCMs. The thermal properties of the pure paraffin and both composite PCMs, such as transition temperatures, phase change temperatures and the latent heat obtained by the DSC measurements are summarized in Table 1. As indicated in Table 1, the phase change temperatures of P/HNT composite with the different weight ratios of P only decreased by about 1 °C. The decrease in the phase change temperatures can be attributed to the interaction between the paraffin molecules and the pore wall of HNT. Some researchers have found similar results in other areas (Radhakrishnan and Gubbins, 1999; Zhang et al., 2007). The latent heat of P for melting and freezing was found to be 174.20 J/g and 168.74 J/g, and 106.54 J/g and 101.58 J/ g for P/HNT (65/35 wt.%) composite, respectively.
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Fig. 5. DSC curves for heating (a) and cooling (b) of P and form-stable composite PCM.
Table 1 Thermal properties of the paraffin and the composite PCMs. P:HNT:G (wt%)
50:50:0 60:40:0 65:35:0 65:30:5 70:30:0 100:0:0
Melting
Solidifying
Transition temperature Tt,m (°C)
Melting temperature Tm (°C)
Melting latent heat DHm (J/g)
Transition temperature Tt,f (°C)
Freezing temperature Tf (°C)
Freezing latent heat DHf (J/g)
40.67 40.72 40.78 40.81 40.89 41.01
57.02 57.10 57.16 57.26 57.23 57.28
72.03 93.56 106.54 104.58 112.44 174.20
37.94 37.87 37.92 37.65 37.74 38.26
53.66 53.70 53.82 54.17 54.21 54.25
68.08 88.05 101.58 104.25 111.24 168.74
Table 2 Comparison of thermal properties of the composite prepared with that of some composite PCMs in literatures. Composite PCM
Melting point (°C)
Freezing point (°C)
Latent heat (J/g)
Reference
RT20 (58 wt.%)/montmorillonite Lauric acid (60 wt.%)/expanded perlite Capric–lauric acid (40 wt.%)/expanded vermiculite Capric–palmitic (40 wt.%) expanded vermiculite Capric–stearic acids (40 wt.%) expanded vermiculite P/HNT(40:60 wt.%)/HNTs P/HNT/G (35:60:5 wt.%)/HNTs
23.0 44.13 19.09 23.51 25.64 57.16 57.26
– 40.97 19.15 21.40 24.90 53.82 54.17
79.25 93.36 61.03 72.05 71.53 101.58 104.25
Fang et al. (2008) Sari and Karaipeklia (2009) Karaipekli and Sari (2010) Karaipekli and Sari (2010) Karaipekli and Sari (2010) Present study Present study
However, the composite had slightly liquid leakage increasing the content of P more than 65% when it was heated above the melting point of P. Therefore, the maximum mass fraction of P retained in HNT was 65 wt.%. The effect of graphite additive on thermal properties of the composite was investigated by DSC analysis. As can be seen in Table 1, phase change temperatures for melting and freezing were determined as 57.16 °C and 53.82 °C for P/HNT (65/35 wt.%) composite, and 57.26 °C and 54.17 °C for P/HNT/G (65/30/5 wt.%) composite, respectively. In addition, the latent heat of P/HNT (65/35 wt.%) composite for melting and freezing was found to be 106.54 J/g and 101.58 J/g, and 104.58 J/g and 104.25 J/g
for P/HNT/G (65/30/5 wt.%) composite, respectively. These results showed that graphite additive had no significant effect on phase change temperatures and latent heat of the composite PCM. The thermal energy storage properties of the synthesized composite PCM were compared with that of the different composite PCMs reported, as listed in Table 2. It was observed that the adsorption capacity of HNTs was similar to that of the montmorillonite and expanded perlite, while higher than that of vermiculite and expanded vermiculite (Fang et al., 2008; Sari and Karaipeklia, 2009; Karaipekli and Sari, 2010). This confirmed that halloysite had a comparably higher adsorption capacity for adsorbing phase
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As reported in literatures, the expanded graphite and carbon fiber had been used to improve the thermal conductivity of the composite PCMs, which ranged from 0.26 W/ mK to 1.0 W/mK with different amounts of carbon (Sari and Karaipeklia, 2009; Karaipeklia et al., 2007). Compared with these values of thermal conductivity, the P/HNT/G displayed a higher level of thermal conductivity (0.89 W/ mK) when the same amount of 5% carbon was added into the composite PCMs. 3.5. The improvement of the thermal storage and release rates
Fig. 6. Melting temperature curves of P/HNT and P/HNT/G composite PCM.
Fig. 7. Freezing temperature curves of P/HNT and P/HNT/G composite PCM.
change materials. The latent heat of the prepared P/HNT or P/HNT/G composite PCM is higher than that of other composite PCM. 3.4. Thermal conductivity improvement of the form-stable composite PCM The high thermal conductivity of PCM is benefits to increase the rates of heat stored and released during melting and crystallization processes. The thermal conductivity of PCM was measured by hot-wire technique. The prepared P/HNT composite PCM has a low thermal conductivity (0.56 W/mK) due to the low thermal conductivity of the capric acid (0.22 W/mK) (Sari et al., 2007). In order to improve the thermal conductivity, the graphite was added to the composite in mass fraction of 5%. The thermal conductivity of P/HNT/G (60/35/5 wt.%) composite in the solid state was measured as 0.89 W/mK. The thermal conductivity of the composite PCM is increased by about 59%.
Thermal performance test was conducted using the con¨ zonur et al., 2006; stant temperature water bath method (O Karaipekli and Sari, 2009a). It was reported that the solid– liquid phase change process of paraffin was fully finished in the range of 40–70 °C (Jin et al., 2008). Therefore, we investigated the effect of graphite additive on thermal storage performance of the composite in the range of 40–70 °C and the temperature curves for melting and freezing were shown in Figs. 6 and 7. As seen from Fig. 6, it took 510 s for P/HNT composite to raise the temperature from 40 °C to 57 °C. However, for the P/HNT/G (5 wt.%) composite only 200 s. The freezing time was also determined from the freezing curves in Fig. 7. For P/HNT composite, it took 380 s to cool the samples from 70 °C to 57 °C. However, for the P/HNT/ G (5%) composite it took only 110 s. When comparing the melting and freezing time of P/HNT composite with that of P/HNT/G (5 wt.%) composite, it is obvious that the melting and freezing time of the P/HNT/G (5 wt.%) composite PCM were reduced by 60.78% and 71.52%, respectively, which indicated that the thermal storage and release rates were greatly increased by introducing graphite. 4. Conclusions The P/HNT composite was prepared as a new kind of form-stable composite PCM. The composite can contain paraffin as high as 65 wt.% and maintain its original shape perfectly without any leakage of paraffin after subjected to 50 melt–freeze cycles. The melting temperature and latent heat of composite (P/HNT: 60/40 wt.%) were determined as 57.16 °C and 106.54 J/g by DSC. Graphite additive can increase the thermal storage and release rates of the composite and can reduce the melting and freezing time by 60.78% and 71.52%, respectively. The as-prepared composite has a high adsorption capacity of paraffin, high heat storage capacity and good thermal stability, which indicates that the composite can be used as an effective material to store solar energy in practical application. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 20871105), Henan Outstanding Youth Science Fund (No. 0612002400).
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References Baye´s-Garcı´a, L., Ventola`, L., Cordobilla, R., Benages, R., Calvet, T., Cuevas-Diarte, M.A., 2010. Phase change materials (PCMs) microcapsules with different shell compositions: preparation, characterization and thermal stability. Solar Energy Materials and Solar Cells 94, 1235–1240. Cemil, A., Sari, A., 2008. Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage. Solar Energy 82, 118–124. Cho, K., Choim, S.H., 2000. Thermal characteristics of paraffin in a spherical capsule during freezing and melting processes. International Journal of Heat Mass Transfer 43, 3183. Dincer, I., Dost, S., 2002. A perspective on thermal energy storage system for solar energy storage applications. International Journal of Energy Research 20, 547–557. Fan, Y.f., Zhang, X.X., Wang, X.C., Li, J., Zhu, Q.B., 2004. Supercooling prevention of microencapsulated phase change material. Thermochimica Acta 413, 1–6. Fan, Y.F., Zhang, X.X., Wu, S.Z., Wang, X.C., 2005. Thermal stability and permeability of microencapsulated n-octadecane and cyclohexane. Thermochimica Acta 429, 25. Fang, X., Zhang, Z., Chen, Z., 2008. Study on preparation of montmorillonite-based composite phase change materials and their applications in thermal storage building materials. Energy Conversion and Management 49, 718–723. Farid, M.M., Khudhair, A.M., Razack, S.A.K., 2004. A review on phase change energy storage: materials and applications. Energy Conversion and Management 45, 1597–1615. Farmer, V.C., Russell, J.D., 1964. The infra-red spectra of layer silicates. Spectrochimica Acta 20, 1149–1173. Frost, R.L., 1995. Modification of kaolinite surfaces through intercalation with potassium acetate. Clays and Clay Minerals 43, 191. He, B., Setterwall, F., 2002. High-capacity cool thermal energy storage for peak shaving-a solution for energy challenges in the 21st century. Energy Conversion and Management 43, 1709. Jin, Z.G., Wang, Y.D., Liu, J.G., Yang, Z.Z., 2008. Synthesis and properties of paraffin capsules as phase change materials. Polymer 49, 2903–2910. Joussein, E., Petit, S., Churchman, J., Theng, B., Righi, D., Delvaux, B., 2005. Halloysite clay minerals – a review. Clay Minerals 40, 383–426. Karaipekli, A., Sari, A., 2009. Capric–myristic acid/expanded perlite composite as form-stable phase change material for thermal energy storage. Solar Energy 83, 323–332. Karaipekli, A., Sari, A., 2010. Preparation, thermal properties and thermal reliability of eutectic mixtures of fatty acids/expanded vermiculite as novel form-stable composites for energy storage. Journal of Industrial and Engineering Chemistry 16, 767–773. Karaipeklia, A., Sari, A., Kaygusuz, K., 2007. Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications. Renewable Energy 32, 2201–2210. Krup, I., Mikova, G., Luyt, A.S., 2007. Phase change materials based on low-density polyethylene/paraffin wax blends. European Polymer Journal 43, 4695–4705. Luo, P., Zhao, Y.F., Zhang, B., Liu, J.D., Yang, Y., Liu, J.F., 2010. Study on the adsorption of neutral red from aqueous solution onto halloysite nanotubes. Water Research 44, 1489–1497. Meng, Q.H., Hu, J.L., 2008. A poly(ethylene glycol)-based smart phase change material. Solar Energy Materials and Solar Cells 92, 1260– 1268. ¨ zonur, Y., Mazman, M., Paksoy, H.O ¨ ., Evliya, H., 2006. MicroencapO sulation of coco fatty acid mixture for thermal energy storage with phase change material. International Journal of Energy Research 30, 741–749.
Radhakrishnan, R., Gubbins, K.E., 1999. Free energy studies of freezing in slit pores: an order-parameter approach using Monte Carlo simulation. Molecular Physics 96, 1249–1267. Sari, A., 2004. Form-stable paraffin/high density polyethylene composites as solid–liquid phase change material for thermal energy storage: preparation and thermal properties. Energy Conversion and Management 45, 2033–2042. Sari, A., Karaipeklia, A., 2009. Preparation, thermal properties and thermal reliability of palmitic acid/expanded graphite composite as form-stable PCM for thermal energy storage. Solar Energy Materials and Solar Cells 93, 571–576. Sari, A., Cemil, A., Karaipekli, A., 2007. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Applied Thermal Engineering 27, 1271–1277. ¨ nal, A., 2008a. Preparation, Sari, A., Cemil, A., Karaipekli, A., O characterization and thermal properties of styrene maleic anhydride copolymer (SMA)/fatty acid composites as form stable phase change materials. Energy Conversion and Management 49, 373–380. Sari, A., Karaipekli, A., Kaygusuz, K., 2008b. Capric acid and stearic acid mixture impregnated with gypsum wallboard for low-temperature latent heat thermal energy storage. International Journal of Energy Research 32, 154–160. Sari, A., Karaipekli, A., Alkan, C., 2009a. Preparation, characterization and thermal properties of lauric acid/expanded perlite as novel formstable composite phase change material. Chemical Engineering Journal 155, 899–904. Sari, A., Cemil, A., Karaipekli, A., Orhan, Uzun, 2009b. Microencapsulated n-octacosane as phase change material for thermal energy storage. Solar Energy 83, 1757–1763. Trp, A., 2005. An experimental and numerical investigation of heat transfer during technical grade paraffin melting and solidification in a shell-and-tube latent thermal energy storage unit. Solar Energy 79, 648. Wang, L.J., Duo, M., 2010. Fatty acid eutectic/polymethyl methacrylate composite as form-stable phase change materials for thermal energy storage. Applied Energy 87, 2660–2665. Wang, J.F., Xie, H.Q., Zhong, X., Li, Y.Y., Chen, L.F., 2010. Enhancing thermal conductivity of palmitic acid based phase change materials with carbon nanotubes as fillers. Solar Energy 84, 339–344. Xiao, M., Feng, B., Gong, K.C., 2002. Preparation and performance of shape-stabilized phase change thermal storage materials with high thermal conductivity. Energy Conversion and Management 43, 1039. Xie, B.Q., Shi, H., Jiang, S.C., Zhao, Y., Han, C.C., Xu, D.F., 2006. Crystallization behaviors of n-nonadecane in confined space. Observation of metastable phase induced by surface freezing. Journal of Physical Chemistry B 110, 14279. Ye, H., Ge, X.S., 2000. Preparation of polyethylene–paraffin compound as a form-stable solid–liquid phase change materials. Solar Energy Materials and Solar Cells 64, 37–44. Zalba, B., Marin, J.M., Cabeza, L.F., 2003. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Applied Thermal Engineering 23, 251–283. Zhang, X.X., Tao, X.M., Yick, K.L., Fan, Y.F., 2005. Expansion space and thermal stability of microencapsulated n-octadecane. Journal of Applied Polymer Science 97, 390. Zhang, Y.P., Yang, R., Lin, K.P., Di, H.F., Jiang, Y., 2006. Preparation, thermal performance and application of shape-stabilized PCM in energy efficient buildings. Energy and Buildings 38, 1262–1269. Zhang, D., Tian, S., Xiao, D., 2007. Experimental study on the phase change behavior of phase change material confined in pores. Solar Energy 81, 653–660. Zhao, C.Y., Lu, W., Tian, Y., 2010. Heat transfer enhancement for thermal energy storage using metal foams embedded within phase change materials (PCMs). Solar Energy 84, 1402–1412.
Solar Energy 86 (2012) 1142–1148 www.elsevier.com/locate/solener
Preparation and thermal energy properties of paraffin/halloysite nanotube composite as form-stable phase change material Jiangshan Zhang a,b, Xiang Zhang b,⇑, Yazhen Wan b, Dandan Mei b, Bing Zhang b,⇑ a
College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455002, PR China b School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, PR China Received 30 March 2011; received in revised form 15 November 2011; accepted 5 January 2012 Available online 20 February 2012 Communicated by: Associate Editor Halime Paksoy
Abstract Phase change materials (PCMs) have attracted extensively interests in solar storage. In the study, we prepared a new kind of composite PCM by impregnating paraffin (P) into halloysite nanotube. The as-prepared composite PCM was characterized by TEM, FTIR and DSC analysis techniques. The composite can absorb paraffin as high as 65 wt.% and maintain its original shape perfectly without any paraffin leakage after subjected to 50 melt–freeze cycles. The melting temperature and latent heat of composite (P/HNT: 65/35 wt.%) were determined as 57.16 °C and 106.54 J/g by DSC. Graphite was added into the P/HNT composite to improve thermal storage performance, and the melting time and freezing time of the composite were reduced by 60.78% and 71.52% compared with the composite without graphite, respectively. Due to its high adsorption capacity, high heat storage capacity, good thermal stability and simple preparation method, the composite can be considered as cost-effective latent heat storage material for practical application. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Paraffin; Halloysite nanotube; Phase change material; Thermal properties
1. Introduction Latent heat thermal energy storage (LHTES) using PCM has attracted interest in solar storage and utilization due to its ability to provide a high storage density at nearly isothermal conditions (Dincer and Dost, 2002; Zalba et al., 2003; Farid et al., 2004). So far, a large number of PCMs such as salt hydrates, paraffins, fatty acids and their mixtures have been widely investigated for LHTES (Farid et al., 2004; Sari et al., 2009b; Wang et al., 2010; Zhao et al., 2010). Among them, paraffin is taken as a promising PCM because of its proper melting temperature range, high latent heat capacity, good chemical and thermal stability, little or ⇑ Corresponding authors.
E-mail addresses: [email protected] (X. Zhang), zhangb@ zzu.edu.cn (B. Zhang). 0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.solener.2012.01.002
no supercooling during the phase transition, nontoxicity and noncorrosivity against metal containers (Cho and Choim, 2000; He and Setterwall, 2002; Trp, 2005). However, the melted paraffin has to be kept in a closed tank or container to prevent leaching during the phase transition. Therefore, special latent storage device or elements such as a heat exchanger or lots of containers to encapsulate the PCM are needed which increase the cost (Meng and Hu, 2008; Baye´ s-Garcı´a et al., 2010). Recently, this problem can be solved using shape-stabilized or form-stable PCM composite which can be prepared by encapsulation of PCM into a polymeric structure such as low-density polyethylene, high-density polyethylene (HDPE), poly(styrene–butadiene–styrene) (SBS), poly (methyl methacrylate) and styrene maleic anhydride copolymer (SMA) (Ye and Ge, 2000; Xiao et al., 2002; Sari, 2004;
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Zhang et al., 2006; Krup et al., 2007; Cemil and Sari, 2008; Sari et al., 2008a; Wang and Duo, 2010). However, its application was hampered by high cost of encapsulation and easy flammability. There were very few literatures that aimed to use clays as absorbents to prepare the form-stable composite PCMs for thermal energy storage (Fang et al., 2008; Sari et al., 2008b; Karaipekli and Sari, 2009a). Compared with polymeric encapsulants, clays are readily obtainable, hardly flammable and much cheaper. Moreover, several advantages like high adsorption capacity, high heat storage capacity, good thermal stability and direct usability without extra encapsulation render the composite potential heat storage material for practical application. HNT is a two-layered aluminosilicate clay mineral, which is available in abundance in China as well as other locations around the world. It is chemically similar to kaolin, differing mainly in the morphology of crystals (Joussein et al., 2005). HNTs possess hollow nanotubular structure and large specific surface area. Their novel physical and chemical properties have provided an opportunity as absorbent to remove harmful ions from waste water (Luo et al., 2010). However, there is not any literature that aims to use halloysite as adsorbent to prepare form-stable composite PCM. In this study, a new form-stable PCM was prepared by absorbing paraffin into the pores of halloysite nanotubes. In order to improve the thermal storage performance we also introduced graphite into composite PCM. The asprepared composite has a high adsorption capacity of paraffin, high heat storage capacity and good thermal stability, which indicates that the composite can be used as a effective material to store solar energy in practical application. 2. Experiments 2.1. Materials Paraffin was used as a latent heat storage material. It was supplied by Chemical Reagent Co., Ltd. (Tianjin, China). Halloysite clay from Henan Province (China) was milled and sieved followed by oven dried at 373 K for 24 h. 2.2. Preparation of the form-stable composite PCM The HNT and P were mixed in the toluene solution. The mixture was irradiated ultrasonically for 90 min, and then stirred and refluxed under 80 °C for 2 h in a water bath. After the recovery of toluene by simple distillation, the mixture was dried at 80 °C. In order to find the maximum adsorption capacity of P, the P/HNT composites were prepared with the different weight ratios of 50:50, 60:40, 65:35 and 70:30 respectively. In order to increase thermal storage and release rates, graphite (5 wt.%) was added in the mixture at the beginning. The following steps were consistent with the same method mentioned above.
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2.3. Characterizations of HNTs and the paraffin/HNTs composite PCM The Brunauer–Emmet–Teller (BET) surface area and pore-size distribution of the HNT were measured by a specific surface area analyzer (Quantachrome NOVA4200e). The microstructures of the HNT and the composite PCM were observed using a transmission electron microscope (TEM, FEITECNAIG2). The form-stable composite PCM was characterized using the FT-IR spectroscopy (NEXUS FT-IR). The phase change temperature and latent heat of the composite PCM were measured using a differential scanning calorimeter (STA449C, NETZSCH) at a heating rate of 5 °C/min in a purified nitrogen atmosphere. 2.4. Test of thermal storage and release rates Thermal performance test was conducted using the con¨ zonur et al., 2006; stant temperature water bath method (O Karaipekli and Sari, 2009a). Two glass test tubes of 250 mm height and 40 mm inside diameter and glass thickness of 1 mm were used: one containing the form-stable P/ HNT composite as reference and the other containing the P/HNT composite with 5 wt.% graphite (P/HNT/G). Two thermocouples were placed in the centers of the test tubes, respectively. The test tubes were put into a water bath at a constant temperature of 40 °C. After the temperature reached balance, the two tubes were rapidly placed into another water bath at a constant temperature of 70 °C, where the composite PCMs performed process of heat storage. After the heat storage was finished, the composites were immediately subjected to solidification process at a constant temperature of 40 °C, where the composite PCMs performed process of heat extraction. The temperature variations of the composite during these periods were recorded automatically by a PC via data logger (Agilent 34970A) with a temperature measuring accuracy of ±1.5 °C at time intervals of 10 s. 3. Results and discussions 3.1. Morphology characterization of the form-stable PCM The photograph of the pure P, P/HNT (65/35 wt.%) and P/HNT/G (65/30/5 wt.%) slices at room temperature is shown in Fig. 1A(a–c). The original samples have the cylindrical shape with smooth surface. When heated to 70 °C above the phase change temperature of the paraffin (57 °C), the pure paraffin melt in Fig. 1B(a). However, the form-stable composite samples still kept in shape perfectly and had no liquid leakage on the surface after subjected to 50 melt–freeze cycles, shown in Fig. 1B(b and c). Therefore, the composite PCMs are thermally stable after melt–freeze cycles. The further experiments indicated that composite samples could keep in shape perfectly even after subjected to 500 melt–freeze cycles. Fig. 2 shows the TEM images of the original HNTs and the composite prepared in this work. The original HNTs
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Fig. 1. Images of the pure P (a), P/HNT (b) and P/HNT/G (c) samples at room temperature (A) and after heated at 70 °C (B).
have an average length of 0.5–1 lm, a diameter in the range of 30–40 nm (Fig. 2a). The inner diameter is more or less 30 nm, while the thickness of the wall is about 8–10 nm. Compared with natural nanotubes, the pores of the HNT in the composite are partly filled with paraffin (Fig. 2b). Due to its large and smooth unhindered pores in the nm range, HNT could keep the melted P in pores even over the melting temperature of the P. Hence, the composite maintains its solid shape without any seepage of the melted P. In order to investigate the absorbability of the HNT, the pore size distribution and BET surface area were measured.
Fig. 3 depicts the adsorption–desorption curves and the pore distribution curves of original HNT and P/HNT (65/ 35 wt.%) composite. The results show that the specific surface area of original HNT is 57.76 m2/g, while the specific surface area of P/HNT composite is drastically decreased to 19.67 m2/g (Fig. 3a). The peaks between 6 nm and 10 nm are observed in the pore distribution of the original HNT. However, for P/HNT composite, the peaks between 6 nm and 10 nm are completely disappeared (Fig. 3b). The dramatic decrease of surface area was caused by the filling of the nanotubes with P, which is consistent with our TEM observation result.
Fig. 2. The TEM images: (a) HNT and (b) form-stable P/HNT composite PCM.
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Fig. 3. Isothermal adsorption–desorption curves (a) and pore distribution curves (b) of HNT and P/HNT composite PCM.
3.2. FT-IR spectroscopy The composite PCM was characterized by FT-IR spectroscopy to investigate the specific interactions between P and HNT. The FT-IR spectra of halloysite nanotubes, paraffin and the form-stable P/HNT composite are shown in Fig. 4. The FT-IR spectra of halloysite nanotubes exhibits two Al–OH stretching bands at 3699 cm 1 and 3625 cm 1 (Fig. 4a) and a single Al–OH bending band at 910 cm 1 (Frost, 1995; Farmer and Russell, 1964). The 1094 cm 1 peak is assigned to stretching mode of apical Si–O, while the band at 1033 cm 1 is caused by the stretching vibrations of Si–O–Si. Moreover, the band observed at 534 cm 1 is deformation vibration of Al–O–Si. In FT-IR spectra of P (Fig. 4b), the peaks at 2920 cm 1 and 2850 cm 1 are caused by stretching vibration of C–H and the peaks at 1467 cm 1 and 724 cm 1 are attributed to the deformation vibration of –CH2 and –CH3 and the rocking vibration
Fig. 4. FT-IR spectra of HNT (a), P (b) and form-stable P/HNT composite PCM (c).
of –CH2, respectively. For the composite (Fig. 4c), the peaks assigned to halloysite at 3699, 3625, 1094, 910, 534 cm 1 and the peaks assigned to P at 2920, 2850, 1467, 724 cm 1 are still existed and no significant new peak is observed, which indicate that the composite is just a physical interaction between P and HNT. 3.3. Thermal properties of the form-stable composite PCMs The DSC curves of P and P/HNT composite are shown in Fig. 5. As can be seen in Fig. 5, two main transition peaks were clearly observed, in which the sharp or main peak should be attributed to the heterogeneously nucleated rotator–liquid transition which represents the solid–liquid phase change of the paraffin and the minor peak at the left side of the main peak to the homogeneously nucleated crystal–rotator transition corresponds to the solid–solid phase transition of paraffin (Fan et al., 2004, 2005; Zhang et al., 2005; Xie et al., 2006). These figures indicate that both paraffin and the composite PCMs exhibit similar thermal characteristics. This is because there is no chemical reaction between the paraffin and HNT in preparation of the form-stable composite PCMs. The thermal properties of the pure paraffin and both composite PCMs, such as transition temperatures, phase change temperatures and the latent heat obtained by the DSC measurements are summarized in Table 1. As indicated in Table 1, the phase change temperatures of P/HNT composite with the different weight ratios of P only decreased by about 1 °C. The decrease in the phase change temperatures can be attributed to the interaction between the paraffin molecules and the pore wall of HNT. Some researchers have found similar results in other areas (Radhakrishnan and Gubbins, 1999; Zhang et al., 2007). The latent heat of P for melting and freezing was found to be 174.20 J/g and 168.74 J/g, and 106.54 J/g and 101.58 J/ g for P/HNT (65/35 wt.%) composite, respectively.
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Fig. 5. DSC curves for heating (a) and cooling (b) of P and form-stable composite PCM.
Table 1 Thermal properties of the paraffin and the composite PCMs. P:HNT:G (wt%)
50:50:0 60:40:0 65:35:0 65:30:5 70:30:0 100:0:0
Melting
Solidifying
Transition temperature Tt,m (°C)
Melting temperature Tm (°C)
Melting latent heat DHm (J/g)
Transition temperature Tt,f (°C)
Freezing temperature Tf (°C)
Freezing latent heat DHf (J/g)
40.67 40.72 40.78 40.81 40.89 41.01
57.02 57.10 57.16 57.26 57.23 57.28
72.03 93.56 106.54 104.58 112.44 174.20
37.94 37.87 37.92 37.65 37.74 38.26
53.66 53.70 53.82 54.17 54.21 54.25
68.08 88.05 101.58 104.25 111.24 168.74
Table 2 Comparison of thermal properties of the composite prepared with that of some composite PCMs in literatures. Composite PCM
Melting point (°C)
Freezing point (°C)
Latent heat (J/g)
Reference
RT20 (58 wt.%)/montmorillonite Lauric acid (60 wt.%)/expanded perlite Capric–lauric acid (40 wt.%)/expanded vermiculite Capric–palmitic (40 wt.%) expanded vermiculite Capric–stearic acids (40 wt.%) expanded vermiculite P/HNT(40:60 wt.%)/HNTs P/HNT/G (35:60:5 wt.%)/HNTs
23.0 44.13 19.09 23.51 25.64 57.16 57.26
– 40.97 19.15 21.40 24.90 53.82 54.17
79.25 93.36 61.03 72.05 71.53 101.58 104.25
Fang et al. (2008) Sari and Karaipeklia (2009) Karaipekli and Sari (2010) Karaipekli and Sari (2010) Karaipekli and Sari (2010) Present study Present study
However, the composite had slightly liquid leakage increasing the content of P more than 65% when it was heated above the melting point of P. Therefore, the maximum mass fraction of P retained in HNT was 65 wt.%. The effect of graphite additive on thermal properties of the composite was investigated by DSC analysis. As can be seen in Table 1, phase change temperatures for melting and freezing were determined as 57.16 °C and 53.82 °C for P/HNT (65/35 wt.%) composite, and 57.26 °C and 54.17 °C for P/HNT/G (65/30/5 wt.%) composite, respectively. In addition, the latent heat of P/HNT (65/35 wt.%) composite for melting and freezing was found to be 106.54 J/g and 101.58 J/g, and 104.58 J/g and 104.25 J/g
for P/HNT/G (65/30/5 wt.%) composite, respectively. These results showed that graphite additive had no significant effect on phase change temperatures and latent heat of the composite PCM. The thermal energy storage properties of the synthesized composite PCM were compared with that of the different composite PCMs reported, as listed in Table 2. It was observed that the adsorption capacity of HNTs was similar to that of the montmorillonite and expanded perlite, while higher than that of vermiculite and expanded vermiculite (Fang et al., 2008; Sari and Karaipeklia, 2009; Karaipekli and Sari, 2010). This confirmed that halloysite had a comparably higher adsorption capacity for adsorbing phase
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As reported in literatures, the expanded graphite and carbon fiber had been used to improve the thermal conductivity of the composite PCMs, which ranged from 0.26 W/ mK to 1.0 W/mK with different amounts of carbon (Sari and Karaipeklia, 2009; Karaipeklia et al., 2007). Compared with these values of thermal conductivity, the P/HNT/G displayed a higher level of thermal conductivity (0.89 W/ mK) when the same amount of 5% carbon was added into the composite PCMs. 3.5. The improvement of the thermal storage and release rates
Fig. 6. Melting temperature curves of P/HNT and P/HNT/G composite PCM.
Fig. 7. Freezing temperature curves of P/HNT and P/HNT/G composite PCM.
change materials. The latent heat of the prepared P/HNT or P/HNT/G composite PCM is higher than that of other composite PCM. 3.4. Thermal conductivity improvement of the form-stable composite PCM The high thermal conductivity of PCM is benefits to increase the rates of heat stored and released during melting and crystallization processes. The thermal conductivity of PCM was measured by hot-wire technique. The prepared P/HNT composite PCM has a low thermal conductivity (0.56 W/mK) due to the low thermal conductivity of the capric acid (0.22 W/mK) (Sari et al., 2007). In order to improve the thermal conductivity, the graphite was added to the composite in mass fraction of 5%. The thermal conductivity of P/HNT/G (60/35/5 wt.%) composite in the solid state was measured as 0.89 W/mK. The thermal conductivity of the composite PCM is increased by about 59%.
Thermal performance test was conducted using the con¨ zonur et al., 2006; stant temperature water bath method (O Karaipekli and Sari, 2009a). It was reported that the solid– liquid phase change process of paraffin was fully finished in the range of 40–70 °C (Jin et al., 2008). Therefore, we investigated the effect of graphite additive on thermal storage performance of the composite in the range of 40–70 °C and the temperature curves for melting and freezing were shown in Figs. 6 and 7. As seen from Fig. 6, it took 510 s for P/HNT composite to raise the temperature from 40 °C to 57 °C. However, for the P/HNT/G (5 wt.%) composite only 200 s. The freezing time was also determined from the freezing curves in Fig. 7. For P/HNT composite, it took 380 s to cool the samples from 70 °C to 57 °C. However, for the P/HNT/ G (5%) composite it took only 110 s. When comparing the melting and freezing time of P/HNT composite with that of P/HNT/G (5 wt.%) composite, it is obvious that the melting and freezing time of the P/HNT/G (5 wt.%) composite PCM were reduced by 60.78% and 71.52%, respectively, which indicated that the thermal storage and release rates were greatly increased by introducing graphite. 4. Conclusions The P/HNT composite was prepared as a new kind of form-stable composite PCM. The composite can contain paraffin as high as 65 wt.% and maintain its original shape perfectly without any leakage of paraffin after subjected to 50 melt–freeze cycles. The melting temperature and latent heat of composite (P/HNT: 60/40 wt.%) were determined as 57.16 °C and 106.54 J/g by DSC. Graphite additive can increase the thermal storage and release rates of the composite and can reduce the melting and freezing time by 60.78% and 71.52%, respectively. The as-prepared composite has a high adsorption capacity of paraffin, high heat storage capacity and good thermal stability, which indicates that the composite can be used as an effective material to store solar energy in practical application. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (No. 20871105), Henan Outstanding Youth Science Fund (No. 0612002400).
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References Baye´s-Garcı´a, L., Ventola`, L., Cordobilla, R., Benages, R., Calvet, T., Cuevas-Diarte, M.A., 2010. Phase change materials (PCMs) microcapsules with different shell compositions: preparation, characterization and thermal stability. Solar Energy Materials and Solar Cells 94, 1235–1240. Cemil, A., Sari, A., 2008. Fatty acid/poly(methyl methacrylate) (PMMA) blends as form-stable phase change materials for latent heat thermal energy storage. Solar Energy 82, 118–124. Cho, K., Choim, S.H., 2000. Thermal characteristics of paraffin in a spherical capsule during freezing and melting processes. International Journal of Heat Mass Transfer 43, 3183. Dincer, I., Dost, S., 2002. A perspective on thermal energy storage system for solar energy storage applications. International Journal of Energy Research 20, 547–557. Fan, Y.f., Zhang, X.X., Wang, X.C., Li, J., Zhu, Q.B., 2004. Supercooling prevention of microencapsulated phase change material. Thermochimica Acta 413, 1–6. Fan, Y.F., Zhang, X.X., Wu, S.Z., Wang, X.C., 2005. Thermal stability and permeability of microencapsulated n-octadecane and cyclohexane. Thermochimica Acta 429, 25. Fang, X., Zhang, Z., Chen, Z., 2008. Study on preparation of montmorillonite-based composite phase change materials and their applications in thermal storage building materials. Energy Conversion and Management 49, 718–723. Farid, M.M., Khudhair, A.M., Razack, S.A.K., 2004. A review on phase change energy storage: materials and applications. Energy Conversion and Management 45, 1597–1615. Farmer, V.C., Russell, J.D., 1964. The infra-red spectra of layer silicates. Spectrochimica Acta 20, 1149–1173. Frost, R.L., 1995. Modification of kaolinite surfaces through intercalation with potassium acetate. Clays and Clay Minerals 43, 191. He, B., Setterwall, F., 2002. High-capacity cool thermal energy storage for peak shaving-a solution for energy challenges in the 21st century. Energy Conversion and Management 43, 1709. Jin, Z.G., Wang, Y.D., Liu, J.G., Yang, Z.Z., 2008. Synthesis and properties of paraffin capsules as phase change materials. Polymer 49, 2903–2910. Joussein, E., Petit, S., Churchman, J., Theng, B., Righi, D., Delvaux, B., 2005. Halloysite clay minerals – a review. Clay Minerals 40, 383–426. Karaipekli, A., Sari, A., 2009. Capric–myristic acid/expanded perlite composite as form-stable phase change material for thermal energy storage. Solar Energy 83, 323–332. Karaipekli, A., Sari, A., 2010. Preparation, thermal properties and thermal reliability of eutectic mixtures of fatty acids/expanded vermiculite as novel form-stable composites for energy storage. Journal of Industrial and Engineering Chemistry 16, 767–773. Karaipeklia, A., Sari, A., Kaygusuz, K., 2007. Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications. Renewable Energy 32, 2201–2210. Krup, I., Mikova, G., Luyt, A.S., 2007. Phase change materials based on low-density polyethylene/paraffin wax blends. European Polymer Journal 43, 4695–4705. Luo, P., Zhao, Y.F., Zhang, B., Liu, J.D., Yang, Y., Liu, J.F., 2010. Study on the adsorption of neutral red from aqueous solution onto halloysite nanotubes. Water Research 44, 1489–1497. Meng, Q.H., Hu, J.L., 2008. A poly(ethylene glycol)-based smart phase change material. Solar Energy Materials and Solar Cells 92, 1260– 1268. ¨ zonur, Y., Mazman, M., Paksoy, H.O ¨ ., Evliya, H., 2006. MicroencapO sulation of coco fatty acid mixture for thermal energy storage with phase change material. International Journal of Energy Research 30, 741–749.
Radhakrishnan, R., Gubbins, K.E., 1999. Free energy studies of freezing in slit pores: an order-parameter approach using Monte Carlo simulation. Molecular Physics 96, 1249–1267. Sari, A., 2004. Form-stable paraffin/high density polyethylene composites as solid–liquid phase change material for thermal energy storage: preparation and thermal properties. Energy Conversion and Management 45, 2033–2042. Sari, A., Karaipeklia, A., 2009. Preparation, thermal properties and thermal reliability of palmitic acid/expanded graphite composite as form-stable PCM for thermal energy storage. Solar Energy Materials and Solar Cells 93, 571–576. Sari, A., Cemil, A., Karaipekli, A., 2007. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Applied Thermal Engineering 27, 1271–1277. ¨ nal, A., 2008a. Preparation, Sari, A., Cemil, A., Karaipekli, A., O characterization and thermal properties of styrene maleic anhydride copolymer (SMA)/fatty acid composites as form stable phase change materials. Energy Conversion and Management 49, 373–380. Sari, A., Karaipekli, A., Kaygusuz, K., 2008b. Capric acid and stearic acid mixture impregnated with gypsum wallboard for low-temperature latent heat thermal energy storage. International Journal of Energy Research 32, 154–160. Sari, A., Karaipekli, A., Alkan, C., 2009a. Preparation, characterization and thermal properties of lauric acid/expanded perlite as novel formstable composite phase change material. Chemical Engineering Journal 155, 899–904. Sari, A., Cemil, A., Karaipekli, A., Orhan, Uzun, 2009b. Microencapsulated n-octacosane as phase change material for thermal energy storage. Solar Energy 83, 1757–1763. Trp, A., 2005. An experimental and numerical investigation of heat transfer during technical grade paraffin melting and solidification in a shell-and-tube latent thermal energy storage unit. Solar Energy 79, 648. Wang, L.J., Duo, M., 2010. Fatty acid eutectic/polymethyl methacrylate composite as form-stable phase change materials for thermal energy storage. Applied Energy 87, 2660–2665. Wang, J.F., Xie, H.Q., Zhong, X., Li, Y.Y., Chen, L.F., 2010. Enhancing thermal conductivity of palmitic acid based phase change materials with carbon nanotubes as fillers. Solar Energy 84, 339–344. Xiao, M., Feng, B., Gong, K.C., 2002. Preparation and performance of shape-stabilized phase change thermal storage materials with high thermal conductivity. Energy Conversion and Management 43, 1039. Xie, B.Q., Shi, H., Jiang, S.C., Zhao, Y., Han, C.C., Xu, D.F., 2006. Crystallization behaviors of n-nonadecane in confined space. Observation of metastable phase induced by surface freezing. Journal of Physical Chemistry B 110, 14279. Ye, H., Ge, X.S., 2000. Preparation of polyethylene–paraffin compound as a form-stable solid–liquid phase change materials. Solar Energy Materials and Solar Cells 64, 37–44. Zalba, B., Marin, J.M., Cabeza, L.F., 2003. Review on thermal energy storage with phase change: materials, heat transfer analysis and applications. Applied Thermal Engineering 23, 251–283. Zhang, X.X., Tao, X.M., Yick, K.L., Fan, Y.F., 2005. Expansion space and thermal stability of microencapsulated n-octadecane. Journal of Applied Polymer Science 97, 390. Zhang, Y.P., Yang, R., Lin, K.P., Di, H.F., Jiang, Y., 2006. Preparation, thermal performance and application of shape-stabilized PCM in energy efficient buildings. Energy and Buildings 38, 1262–1269. Zhang, D., Tian, S., Xiao, D., 2007. Experimental study on the phase change behavior of phase change material confined in pores. Solar Energy 81, 653–660. Zhao, C.Y., Lu, W., Tian, Y., 2010. Heat transfer enhancement for thermal energy storage using metal foams embedded within phase change materials (PCMs). Solar Energy 84, 1402–1412.