Journal of Thermal Science and Technology

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1 Bulletin of the JSME Vol.11, No.2, 2016 Journal of Thermal Science and Technology Expanded graphite/disodium hydrogen phosphate/sodium acetate trihydrate stabilized composite phase change material for heat storage Xu JI*, Haili LI*, Congbin LENG*, Ming LI*, Rongkang FAN* and Jiaxing LIU* * School of Energy and Environmental Science, Yunnan Normal University, Kunming, Yunnan,, China jixu@ynnu.edu.cn Received 28 September 2016 Abstract A stabilized composite phase change material for heat storage was synthesized by adding expanded graphite and disodium hydrogen phosphate to sodium acetate trihydrate with the method of vacuum adsorption. The effects of expanded graphite and disodium hydrogen phosphate were experimentally explored and the thermal properties of the composite material were characterized. The experimental results indicated that disodium hydrogen phosphate was an excellent nucleating agent for sodium acetate trihydrate. With addition of 1% disodium hydrogen phosphate, the supercooling degree of sodium acetate trihydrate decreased significantly from over 38 to about 0.5. The addition of expanded graphite was also helpful to ameliorate supercooling of sodium acetate trihydrate. Furthermore, phase separation of sodium acetate trihydrate could be effectively eliminated by adding expanded graphite. Compared with the disodium hydrogen phosphate/sodium acetate trihydrate composite material (without addition of expanded graphite), the heat storage/release time of the expanded graphite/disodium hydrogen phosphate/sodium acetate trihydrate composite material was shorten by 75.3%. With the optimal ingredient proportion of 8% expanded graphite, 1% disodium hydrogen phosphate and 91% sodium acetate trihydrate, the composite material became a stabilized 'solid-solid' phase change energy storage material with excellent thermal performance. Its thermal conductivity was greatly improved, and the phase change latent heat reached 233.5kJ/kg. The supercooling and phase separation phenomenon were no longer observed. Key words: Supercooling, Phase separation, Sodium acetate trihydrate, Expanded graphite, Composite phase change heat storage material 1. Introduction Nowadays, accompanied by massive utilization of fossil fuels, the conventional fossil energy is gradually exhausted, and severe environment pollution arise. Human beings have to resort to renewable energy which is clean and environmentally friendly, and endeavor to improve the energy utilization efficiency. However, renewable energy has the disadvantages of non-uniform distribution in time and space. Energy storage becomes essential for renewable energy to be utilized widely. Phase change energy storage/release technology attracts wide attention (Waqas and Din, 2013; Motahar et al., 2014; Tyagi et al., 2012; Zhang et al., 2013; Alkan et al., 2011) due to the large phase change latent heat and the constant temperature during heat storage and release. Salt hydrate, with high phase change latent heat, high thermal conductivity and low cost, is the most typical inorganic phase change material for energy storage. As a salt hydrate, sodium acetate trihydrate(ch 3 COONa 3H 2 O, SAT)has relative molecular mass, density, solubility, phase change temperature(melting point) and phase change latent heat of , 1.45g/cm 3, 762 g/l (20 ), 58 and 265 kj/kg, respectively. However, supercooling and phase separation prevent it to act as a desired phase change heat storage material. Many researches focused on resolving these problems. In 2011, Peng Hu (Hu et al., 2011) reported that supercooling was eliminated by adding 5% aluminium Paper No

2 nitride nanoparticles as nucleating agent and 4% carboxyl methyl cellulose (CMC) as thickening agent into sodium acetate trihydrate. The phase change temperature and the phase change latent heat were 52.5 and kJ/kg, respectively, and its dehydration temperature was also improved. Hye Kyoung Shin(Shin et al., 2015) investigated experimentally the effect of the quantity of expanded graphite and CMC added into sodium acetate trihydrate on thermal conductivity and phase change latent heat. The thermal conductivity was 1.85 W/mK when 2.5% expanded graphite and 5% CMC were added to sodium acetate trihydrate. The phase change latent heat decreased with increasing of expanded graphite and CMC. Xing Jin (Jin et al., 2014a) conducted differential scanning calorimetry (DSC) experiment and concluded that supercooling could be prevented when cooling temperature of sodium acetate trihydrate was reasonably controlled. B.M.L. Garay Ramirez (Ramirez et al., 2014) found that supercooling was reduced and 95% heat was recycled by adding 0.5% silver nano-particles and mixture of high concentration silica gel and CMC into sodium acetate trihydrate. Xing Jin(Jin et al., 2014b) and Li Jing (Li, 2005) also found that supercooling degree of sodium acetate trihydrate increased with increasing of the maximum temperature in heating process. Inagaki T. (Inagaki et al., 2014) investigated the relationship between critical supercooling degree and cooling rate of sodium acetate trihydrate. Li Weihua (Li et al., 2013) reported that thermal conductivity was improved to two times of the original one when 10% expanded graphite was added into sodium acetate trihydrate. The researches provided basis for resolving the problems of supercooling, phase separation and stability of sodium acetate trihydrate. Expanded graphite (EG) is a kind of loose and porous material (Gao et al., 2014; Xu et al., 2014), and is usually synthesized through high temperature expansion of natural flake graphite. EG is endowed with the characteristics of high porosity, large specific surface area (Hao et al., 2014), high thermal conductivity and good adsorption ability (Zhu, 2012). In this paper, expanded graphite/disodium hydrogen phosphate/sodium acetate trihydrate stabilized composite phase change material was synthesized by adsorbing molten sodium acetate trihydrate and disodium hydrogen phosphate with expanded graphite in vacuum condition. The stabilized composite phase change material eliminated supercooling, phase separation and poor stability of sodium acetate trihydrate, and had potential to become a prospective phase change material for heat storage. 2. Experimental materials and experimental procedure 2.1 Preparation of composite phase change materials The raw materials were as followings: Sodium acetate trihydrate (CH 3 COONa 3H 2 O, SAT), Tianjin City Damao Chemical Reagent factory, analytically pure; Disodium hydrogen phosphate (Na 2 PO 4 12H 2 O, DHP), Aladdin, analytically pure; Expanded graphite, expansion rate 2mL/g, granularity 80 mesh, Qingdao Jin Ri Lai Graphite Limited Company, industrial grade. A certain amount of expanded graphite and sodium acetate trihydrate according to the mass ratio of 4:96, 6:94, 8:92 and 10:90 were weight and treated by vacuum adsorption at the constant temperature of for 4 hours. Shaking violently and frequently the container was conducted during the adsorption process to promote the full adsorption. Expanded graphite/ sodium acetate trihydrate composite phase change materials were obtained after cooling. Utilizing the above preparation method, expanded graphite/disodium hydrogen phosphate/sodium acetate trihydrate composite phase change materials were prepared according to the mass ratio of 4:1:95, 6:1:93, 8:1:91 and 10:1:89, respectively. 2.2 Experimental setups and experimental procedure Cooling curves measurement Figure 1 showed the experimental setup for temperature curves of step cooling. With the heating temperature of and the cooling temperature of 15, the temperature curves for step cooling of sodium acetate trihydrate with addition of expanded graphite and disodium hydrogen phosphate were measured. 2

3 Fig. 1 Experimental setup for cooling curves Specific heat capacity and phase change latent heat of composite phase change material The inhomogeneous adsorption and mixing of composite phase change material, and the little sample quantity (below 10mg) might result to a relatively large error with DSC method. Therefore, the specific heat capacity and the phase change latent heat of these composite phase change materials were characterized by employing the instruments shown in Figure 2. The spiral electric heating pipe and the temperature sensor I were installed at the center of insulated cylinder. Startup of the electric heating pipes was controlled by the temperature controller. The electric energy consumed by the electric heating pipe was recorded by a precise watt-hour meter. Temperature sensor II was inserted at the center of sample tube, and the temperature changes of the samples were recorded by a data acquisition instrument. Fig. 2 Test instruments for specific heat capacity and phase change latent heat When the sample tube filled with phase change material was placed in the insulated cylinder, the temperature of phase change material was lower than the set temperature, so phase change material absorbed heat, and the temperature within insulated cylinder decreased. Electric heating pipes started to work when the temperature of insulated cylinder was lower than the set lower temperature limit. Heating stopped when the set upper temperature limit was reached. This process recycled continuously so that the temperature of phase change material was consistent with the set temperature. Power consumption of heating pipes was partly absorbed by phase change material and sample tube, and other part dissipated into the environment. Therefore, the specific heat capacity and phase change latent heat of phase change material could be calculated when the heat dissipating capacity at the corresponding temperature was measured. Phase change latent heat is equal to the difference between the power consumption of heating pipes and the heat dissipating capacity of the insulated cylinder, the sensible heat of the sample, as shown in Eq.(1). T2 Tf T2 H f 2 T 2 0 o s ( 1 0) l ( 1 0) T0 T0 Tf dm Q q dt c m dt c m m dt c m m dt (1) Where, H f is the phase change latent heat of composite phase change material, kj/kg, Q 2 is the power consumption of heating pipes and q T2 is the heat dissipating capacity of the insulated cylinder at T 2, J/min. T 0 and T 2 is the initial and the final temperature respectively,. T f is the melting point temperature of composite phase change material,. m 0 is 23

4 the mass of sample tube, kg. m 1 is the total mass of the sample and the sample tube, kg. c 0 is the specific heat capacity of glass (sample tube), c s and c l is respectively the specific heat capacity of composite phase change material in solid phase and liquid phase, kj/(kg K). The heat dissipating capacity of the insulated cylinder at temperature T could be achieved by Eq.(2). dq Q Qb Qa qt (2) dt t t t b a Where, Q is the dissipating power by the insulated cylinder at the temperature of T after a period time of t. Q a and Q b is respectively the meter reading at time of t a and t b, kw h, the coefficient of is the conversion coefficient from kw h to J. When the sample is heated from the initial temperature of T 0 to the final temperature of T 1 (T 1 <T f ), the time consuming is t 1. Power consumption of heating pipes is Q 1, the system heat dissipating capacity at T 1 is q T1, so the specific heat capacity of the sample in solid phase c s is, T1 T1 c ( m - m ) dt Q q dt c m dt T s 1 o 1 T1 0 o 0 T0 (3) When the sample is heated from T 2 to T 3 (T f <T 2 <T 3 ), the time consuming and the power consumption of heating pipes are t 3 and Q 3, respectively. The system heat dissipating capacity at T 3 is q T3. Then the specific heat capacity of the sample in liquid phase c l is, T3 T3 ( m - m ) cdt Q q dt c m dt T 1 o l 3 T3 0 o 2 T2 (4) Phase change latent heat H f, specific heat capacity in solid phase c s and specific heat capacity in liquid phase c l can be obtained from equations 1~4. 3 Experimental results and discussions 3.1 Supercooling improvement of sodium acetate trihydrate (SAT) With addition of disodium hydrogen phosphate (DHP) Figure 3 was the cooling curve of sodium acetate trihydrate with different addition of disodium hydrogen phosphate (Na 2 HPO 4, DHP). The additive amounts of DHP were 0, 0.5%, 1%, 1.5%, 2%, respectively % SAT+0.5% DHP 99% SAT+1% DHP 98.5% SAT+1.5% DHP 98% SAT+2% DHP % SAT+0.5% DHP 99% SAT+1% DHP 98.5% SAT+1.5% DHP 98% SAT+2% DHP Fig.3 Cooling curve of sodium acetate trihydrate with addition of disodium hydrogen phosphate In figure, without addition of disodium hydrogen phosphate, the molten sodium acetate trihydrate did not crystallize when its temperature decreased to below the theoretical crystallization temperature of 58. Even, the temperature platform for latent heat release and the temperature turning point did not appear until its temperature decreased to below 20. It demonstrated that the molten sodium acetate trihydrate did not crystallize and heat release in the temperature range, and the supercooling of the sodium acetate trihydrate exceeded 38. Supercooling of sodium acetate trihydrate were drastically ameliorated with addition of the nucleating agent of 24

5 disodium hydrogen phosphate. When 0.5% disodium hydrogen phosphate was added, the composite phase change material began to crystallize at Then the released heat led to the temperature rise to 54.6, which was still lower than the theoretical crystallization temperature of 58. It was due to that the crystallization process finished before the composite phase change material s temperature reached the theoretical crystallization temperature, and no further heat was released to elevate the material's temperature. When 1.0% disodium hydrogen phosphate was added, the supercooling degree of the composite phase change material was only 0.5, and the optimized nucleation effect was achieved. With addiction of 1.5% disodium hydrogen phosphate, the supercooling degree increased to 3.8. With addiction of 2.0% disodium hydrogen phosphate, the composite phase change material started to crystallize at 51.5, then the temperature rose back to 58. The supercooling degree was 7.5. Therefore, the pure sodium acetate trihydrate with the supercooling degree of over 38 was unsuitable as a phase change energy storage material. The supercooling degree of the SAT/DHP composite phase change material decreased firstly and then increased with increasing addictive amount of disodium hydrogen phosphate within a certain range. With addition of 1% disodium hydrogen phosphate, the optimized nucleation effect and supercooling amelioration was achieved With addition of expanded graphite (EG) Figure 4 was the cooling curve of sodium acetate trihydrate with different addition of expanded graphite (EG). The additive amounts of EG were 4%, 6%, 8%, 10%, respectively % SAT+4% EG 94% SAT+6% EG 92% SAT+8% EG 90% SAT+10% EG % SAT+4% EG 94% SAT+6% EG 92% SAT+8% EG 90% SAT+10% EG Fig.4 Cooling curve of sodium acetate trihydrate with addition of expanded graphite In figure, when 4% expanded graphite was added, the composite phase change material started to crystallize at 28. However, its supercooling degree was up to 30, thus afterwards its temperature could not rise to the theoretical crystallization temperature of 58. When 6% expanded graphite was added, the supercooling degree decreased to 18. When 8% expanded graphite was added, the supercooling degree decreased further to 14. It was at this condition that sodium acetate trihydrate in liquid form after phase transformation was thoroughly absorbed by the micro-pores with large surface area of expanded graphite. The phenomenon of liquid leakage disappeared during phase transformation and the problem of phase separation was solved. Therefore, the composite SAT/EG material became a solid-solid stabilized phase change energy storage material. When 10% expanded graphite was added, the supercooling degree was only 8, and the heat release time was also shortened. It meant that heat conductivity coefficient of SAT/EG composite material increased and the efficiency of heat storage/release was improved. Thus, the addition of expanded graphite was helpful to ameliorate the supercooling of sodium acetate trihydrate during phase change. The expanded graphite particles acted as the exogenous nucleating agent, and provided points of attachment for sodium acetate trihydrate to crystallize.although the nucleation effect of disodium hydrogen phosphate was better than the expanded graphite, with addition of expanded graphite, the phase-separation problem of sodium acetate trihydrate was solved. Meanwhile, both the thermal conductivity and the heat storage/release efficiency was improved Supercooling degree of expanded graphite/ disodium hydrogen phosphate/ sodium acetate 25

6 trihydrate (EG/DHP/SAT) composite phase change material Figure 5 was the cooling curve of the EG/DHP/SAT composite phase change material with the mass ratio of expanded graphite, disodium hydrogen phosphate and sodium acetate trihydrate as 4:1:95,6:1:93,8:1:91, 10:1:89, respectively % SAT+4% EG+1% DHP 93% SAT+6% EG+1% DHP 91% SAT+8% EG+1% DHP 89% SAT+10% EG+1% DHP % SAT+4% EG+1% DHP 93% SAT+6% EG+1% DHP 91% SAT+8% EG+1% DHP 89% SAT+10% EG+1% DHP Fig.5 Cooling curve of expanded graphite/disodium hydrogen phosphate/ sodium acetate trihydrate composite phase change material Compared with the cooling characteristics of EG/SAT composite materials in figure 4, with extra addition of 1% disodium hydrogen phosphate, the supercooling degree of the EG/DHP/SAT composite material with 4% EG decreased from 30 to 5. When the mass ratio of EG, DHP and SAT was 6:1:93, the supercooling degree decreased from 18 to 4.5. The heat storage/release 'platform' was prolonged. When the mass ratio of EG, DHP and SAT was 8:1:91, the supercooling was eliminated. When the mass ratio of EG, DHP and SAT was 10:1:89, the supercooling degree was within 0.5. Figure 6 showed the heat storage/release times of the composite phase change materials with different ingredient proportion. With increasing of addictive amount of expanded graphite, the heat storage/release time was shorten. However, the difference was relatively unobvious when the addictive amount of expanded graphite exceeded 6% % SAT+6% EG 99% SAT+1% DHP 93% SAT+6% EG +1% DHP Fig.6. Heat storage/release time of composite phase change material with different ingredient proportion Considering that the addictives without heat storage function should be as little as possible to achieve high phase change heat for the composite material, the optimal composition of expanded graphite/ disodium hydrogen phosphate/ sodium acetate trihydrate composite phase change material was 8% expanded graphite, 1% disodium hydrogen 26

7 phosphate and 91% sodium acetate trihydrate. With the recommended prescription, the composite material became stabilized 'solid-solid' phase change energy storage material. Its thermal conductivity was greatly improved. Compared with the DHP/SAT composite material (without addition of expanded graphite), the heat storage/release time was reduced by 75.3%, and the efficiency of heat storage/ release was significantly promoted. 3.2 Microstructure of expanded graphite/disodium hydrogen phosphate/sodium acetate trihydrate composite phase change material a b EG Sodium acetate tridrate c d EG Sodium acetate tridrate EG Sodium acetate tridrate a- expanded graphite, b-2times, c-0times, d-1000times Fig.7Micrograph of sodium acetate trihydrate composite phase change material Figure 7 was the micrograph of EG/DHP/SAT composite phase change material with the optimal proportion: 8% expanded graphite, 1% disodium hydrogen phosphate and 91% sodium acetate trihydrate. In Figure 7 (a) (Wang et al., 2009), expanded graphite was loose porous network structure. Its surface had numerous uneven-size semi-open holes and cracks, and the pore size was relatively large. Thus, expanded graphite had quite excellent adsorptive capacity. In Figure 7 (b-d), the sodium acetate trihydrate crystals were uniformly coated in porosities of the expanded graphite. Consequently, the liquid sodium acetate trihydrate after phase change was hardly to break away from the constraint of expanded graphite micro-pores. The phase separation problem of sodium acetate trihydrate was solved. Furthermore, the frame of expanded graphite with excellent thermal conductivity would improve the thermal conductivity of the composite phase change material. Also, acting as nucleating agent, the lamilated structure of expanded graphite provided a lot of binding sites for sodium acetate trihydrate to crystallize, which would decrease the crystallization driving force of sodium acetate trihydrate, and reduce supercooling. 3.3 Phase separation elimination of expanded graphite/disodium hydrogen phosphate/sodium acetate trihydrate composite phase change material In our experiments, the molten pure sodium acetate trihydrate divided into two layers: the white foams in the upper 27

8 and the clear liquid in the lower. In the process of cooling and crystallization, the lower layer was translucent crystalline. With a period of time's standing, the crystal would deposit on the bottom of the test tube. With the increasing time of heat storage/release recycle, crystals on the bottom became fewer and fewer, and the nucleating effect degraded. Finally, the phase change energy storage material lost its function. With addition of expanded graphite, sodium acetate trihydrate was absorbed in the porosities of expanded graphite. Thus, the sedimentation of the crystals was prevented. A small amount of liquid leakage was observed on the bottom of test tube when 6% expanded graphite was added, which indicated that slight phase separation still existed. No liquid leakage during phase change was observed when 8% or more expanded graphite was added. The molten sodium acetate trihydrate was completely absorbed by the abundant interspaces and porous surface of expanded graphite. Therefore, the phase separation was eliminated. The composite material became stabilized solid-solid phase change energy storage material. 3.4 Specific heat capacity and phase change latent heat of expanded graphite/disodium hydrogen phosphate/sodium acetate trihydrate composite phase change material Table 1 showed the experimental results of specific heat capacity and phase change latent heat for expanded graphite/disodium hydrogen phosphate/sodium acetate trihydrate composite phase change material at different proportion of composition. Table 1 Thermal physical properties of expanded graphite/ disodium hydrogen phosphate / sodium acetate trihydrate stabilized composite phase change material at different proportion of composition Samples SAT [%] EG [%] DHP [%] c s [kj/(kg K)] c l [kj/(kg K)] H f [kj/kg] (Cai, 2010) # # # # In Table, with increasing addictive amount of EG, both specific heat capacity (solid, liquid) and phase change latent heat of expanded graphite/ disodium hydrogen phosphate / sodium acetate trihydrate stabilized composite phase change material decreased gradually. The values of specific heat capacity (solid liquid) and phase change latent heat were appropriately linear with the amount of sodium acetate trihydrate. The thermal properties of composite phase change material mainly depended on the phase change material of sodium acetate trihydrate. Nucleating agent and expanded graphite had little effect on thermal properties of the composite materials. Both the specific heat capacity and the phase change latent heat of the composite material were somewhat lower than those of the pure substance. The phase change latent heat is kj/kg with the optimal composition of 8% expanded graphite, 1% disodium hydrogen phosphate and 91% sodium acetate trihydrate. 4. Conclusions (1) Disodium hydrogen phosphate was an excellent nucleating agent for sodium acetate trihydrate. The supercooling degree of sodium acetate trihydrate was reduced from over 38 to nearly 0.5 with addition of 1% disodium dihydrogen phosphate. (2) The addition of expanded graphite could ameliorate the supercooling of sodium acetate trihydrate. The supercooling degree of sodium acetate trihydrate decreased with increasing addictive amount of expanded graphite within a certain range. When 10% expanded graphite was added, supercooling degree of sodium acetate trihydrate was reduced to 14. Furthermore, the thermal conductivity of composite phase change material was improved and the phase separation problem of sodium acetate trihydrate was solved with addition of expanded graphite. Compared with the disodium hydrogen phosphate/sodium acetate trihydrate composite material (without addition of expanded graphite), the heat storage/release time of the EG/DHP/SAT composite material was shorten by 75.3%. (3) The optimal composition of stabilized composite phase change material is 8% expanded graphite, 1% disodium 28

9 hydrogen phosphate and 91% sodium acetate trihydrate. With the recommended prescription, the supercooling and phase separation phenomenon of the composite material were eliminated. The phase change latent heat of the stabilized composite phase change material reached 233.5kJ/kg. The composite material became a stabilized 'solid-solid' phase change energy storage material. Acknowledgements The present study was supported by National Natural Science Foundation, China (Grant Nos ). References Alkan, C., Sarı, A. and Karaipekli, A., Preparation, thermal properties and thermal reliability of microencapsulated n-eicosane as novel phase change material for thermal energy storage, Energy Conversion and Management, Vol.52, No.1(2011), pp Cai, L. Y., Thermal Performance and Application of Sodium Acetate Trihydrate as Phase Change Material. Hangzhou: Zhejiang University of Technology, (in Chinese) Gao, W., Gao, L. M. and Ding L. L., Preparation and characteristic of TiO 2 / expanded graphite composite material, Journal of Heilongjiang University of Science and Technology, Vol.5(2014), pp (in Chinese) Hu P., Lu, D. J., Fan, X. Y., Zhou, X. and Chen, Z.S., Phase change performance of sodium acetate trihydrate with AlN nanoparticles and CMC, Solar Energy Materials & Solar Cells, Vol.95, No.9(2011), pp Inagaki, T., Isshiki, T. and Li, Y. R., Study on the Mechanism of Coagulation and Cooling of Phase-Change Latent Heat Storage Material Sodium Acetate Trihydrate in a Horizontal Rectangular Container, Kagaku Kogaku Ronbunshu, Vol.40, No.5(2014), pp (in Japanese) Jin, X., Medina, M. A., Zhang, X. and Zhang, S., Phase-Change Characteristic Analysis of Partially Melted Sodium Acetate Trihydrate Using DSC, International Journal of Thermophysics, Vol.35, No.1(2014a), pp Jin, X., Zhang, S., Medina, M.A. and Zhang, X.S., Experimental study of the cooling process of partially-melted sodium acetate trihydrate, Energy and Buildings, Vol.76(2014b), pp Li, J. Study on phase change heat storage material and its phase change characteristics. Beijing: Beijing University of Technology, (in Chinese) Li, W. H., Mao, J. F., Li, J. T. and Li, Y., Study of enhancing heat conductivity of composite phase change material based on trihydrate sodium acetate, Journal of Building Materials, Vol.16, No.3(2013), pp Motahar, S., Nikkam, N., Alemrajabi, A. A., Khodabandeh, R., Toprak, M. S. and Muhammed, M., A novel phase change material containing mesoporous silica nanoparticles for thermal storage: a study on thermal conductivity and viscosity, International Communications in Heat and Mass Transfer, Vol.56 (2014), pp Ramirez, B.M.L. G., Glorieux, C., Martinez, E.S. M. and Cuautle, J.J.A. F., Tuning of thermal properties of sodium acetate trihydrate by blending with polymer and silver nanoparticles, Applied Thermal Engineering, Vol.62, No.2(2014),pp Shin, H.K., Park, M., Kim, H. Y. and Park, S.J., Thermal property and latent heat energy storage behavior of sodium acetate trihydrate composites containing expanded graphite and carboxymethyl cellulose for phase change materials, Applied Thermal Engineering, Vol.75(2015), pp Tyagi, V. V., Buddhi, D., Kothari, R. and Tyagi, S.K., Phase change material (PCM) based thermal management system for cool energy storage application in building: An experimental study, Energy and Buildings, Vol.51(2012), pp Wang, W., Yang, X., Fang, Y., Ding, J. and Yan J., Preparation and thermal properties of polyethylene glycol/expanded graphite blends for energy storage, Applied Energy, Vol.86, No.9(2009), pp Waqas, A. and Din, Z. U., Phase change material (PCM) storage for free cooling of buildings - a review, Renewable & sustainable energy reviews, Vol.18, No.2 (2013), pp Wei, X., Li, H., Wang, X., Zhang, Y. and Chen, H., Preparation and Characterization of High Rate Expandable Graphite. Guangzhou Chemical Industry, Vol.6(2014), pp (in Chinese) Xu, S., Xu, P.Y., Yin, Z.J. and Liu, K.Z., The Preparation and Application Status of Expanded Graphite in Environmental Protection, Guangdong Chemical Industry, Vol.41, No.20(2014), pp (in Chinese) Zhang, Z., Shi, G., Wang, S., Fang, X. and Liu, X., Thermal energy storage cement mortar containing n-octadecane/expanded graphite composite phase change material, Renewable Energy, Vol., No.3 (2013), pp. 29

10 Zhu, M.C., New expanded graphite-based composites: preparation, modification and their use in specified pollutants removal from aqueous solution, Shanghai: Donghua University, (in Chinese) 10 2