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Experimental study on the thermal performance of graphene and exfoliated graphite sheet for thermal energy storage phase change material
Thermochimica Acta 647 (2017) 15–21
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Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Experimental study on the thermal performance of graphene and exfoliated graphite sheet for thermal energy storage phase change material Xia Liu, Zhonghao Rao ∗ School of Electric Power Engineering, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 5 July 2016 Received in revised form 12 November 2016 Accepted 16 November 2016 Available online 18 November 2016 Keywords: Phase change material Paraffin Graphene Exfoliated graphite sheet
a b s t r a c t In this paper, in order to enhance the thermal performance of paraffin in thermal energy storage, the graphene and exfoliated graphite sheet have been mixed with the paraffin to prepare composites. After the stability tests of dispersion, the scanning electron microscope (SEM), Fourier transform infrared spectroscope (FTIR), thermal conductivity analysis, differential scanning calorimeter (DSC) and thermogravimetric analyzer (TG) are applied to characterize the microstructure, chemical structure, thermal conductivity, enthalpy and stability of the materials, respectively. The results indicated that the thermal conductivity was able to be increased greatly with graphene or exfoliated graphite sheet. With the same mass fraction, the thermal conductivity of paraffin/graphene composite was greater than that of paraffin/exfoliated graphite sheet composite. Furthermore, when the mass fraction of graphene or exfoliated graphite sheet varied between 0–2.0 wt.%, the enthalpy of phase change material (PCM) composites rose firstly and then dropped down. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Phase change material (PCM) is able to be used in the thermal energy storage (TES), which has received great attention in recent years [1–4]. Besides, the temperature of PCM varies a little during the phase change process, including the melting process (heat absorbing) and solidification process (heat releasing). PCM can be classified into two major categories: inorganic PCM and organic PCM [5]. There are several kinds of inorganic PCM, including salts, salts hydrates, metals and alloys. Fatty acids/esters, polyalcohol and paraffin are treated as organic PCM [5]. However, resulting from the low thermal conductivity, most of the PCMs can’t reach the demand of TES. Consequently, the thermal performance of PCM is necessary to be enhanced. There are three methods to enhance the thermal conductivity of PCM: a. adopt enhanced heat transfer surface [6,7]; b. enhance the uniformity of the heat transfer process [8,9]; c. improve PCM thermal performance. The thermal properties of PCM are able to be enhanced by adding additives with high thermal conductivity, such as metal (e.g. nano-metal) [10–12], oxide class(e.g. titanium dioxide) [13,14] and graphite class (e.g. natural graphite) [15–17].
∗ Corresponding author. E-mail address: [email protected] (Z. Rao). http://dx.doi.org/10.1016/j.tca.2016.11.010 0040-6031/© 2016 Elsevier B.V. All rights reserved.
To enhance the thermal conductivity of shape-stabilized PCM effectively, Zhang et al. [18] have prepared nine kinds of composite PCM. They found that adding graphite could enhance the thermal properties mostly. Furthermore, when graphite content was 20 wt.%, the thermal conductivity was increased to 0.482 W m−1 K−1 , which was increased by as large as 221.4% [18]. In addition, with the same percentage of graphite (20 wt.%), the thermal conductivity of lithium nitrate composites has been increased by 426.53% [19]. Vitorino et al. [20] proved that the thermal properties of composites was approximately linear growth with graphite content. Among the organic PCMs, the paraffin is of importance to TES since its excellent properties: large latent heat, no phase segregation, low cost, non-toxic and no corrosive [21–23]. Furthermore, no degradation of thermal properties was observed after several melting/solidification cycles [13]. However, the thermal conductivity of paraffin is low as well. In order to enhance the thermal performance of paraffin, two kinds of material with high thermal conductivity can be considered as additives, the graphene and exfoliated graphite sheet. As one of the graphite, exfoliated graphite sheet and graphene have attracted more and more attentions due to their excellent thermal properties, Tao et al. [24] presented that graphene was the best material to enhance the thermal performance. When the graphene was 0.1 wt.%, 0.5 wt.%, 1.0 wt.%, 1.5 wt.%, 2.5 wt.%, the thermal conductivity has been enhanced by about
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8.01%, 15.54%, 26.11%, 27.73%, respectively. Fan et al. [25] stated when the mass fraction of graphene was 3 wt.%, the thermal conductivity was 2 times higher. In Ref. [26], the thermal conductivity of palmitic acid/graphene nanoplatelets composite has been increased by 800% compared to that of original material. Furthermore, Silakhori et al. [27] proved that the thermal conductivity of palmitic acid/polypyrrole/graphene nanoplatelets form-stable PCM was 38.7% higher than that of palmitic acid particles and 34.3% higher than that of palmitic acid/polypyrrole form-stable PCM. Zeng et al. [28] added 7.87 wt.% of the exfoliated graphite sheet to the palmitic acid/polyaniline composite, and the thermal conductivity was 237.5% higher. Kalaitzidou et al. [29] reported that when the volume fraction of exfoliated graphite sheet was 25%, the thermal conductivity had reached 6 times of the raw material. Moreover, Xiang and Drzal [30] proved that the thermal conductivity was associated with the particle size of exfoliated graphite nanoplatelets. The comparisons of thermal properties between paraffin/exfoliated graphite sheet composite and paraffin/graphene composite have been considered in this paper. The PCM composites are prepared in five kinds of mass ratio. The tests of morphology structure, chemical structure, latent heat and thermal conductivity were investigated in detail. 2. Experiments 2.1. Materials The paraffin, which was purchased from Shanghai Joule wax Co., Ltd, China, was chosen as PCM. Two kinds of material with great thermal conductivity have been chosen as additives. One was the graphene (3000–5000W m−1 K−1 ), which was obtained from Ninjing SCF Nanotech., Ltd, China (purity exceeding 99.5%). Another was the exfoliated graphite sheet (300–400 W m−1 K−1 ) by home-made, which was prepared from expanded graphite (EG). Furthermore, the EG was formed from the graphite powders (obtained from Qingdao Tianheda Graphite Co., Ltd, China) through microwave method. During the preparation of exfoliated graphite sheet, the absolute ethyl alcohol (C2 H5 OH, analytically pure, Shanghai Zhongqin Chemical reagent Co., Ltd, China) was used as solvent. The surfactant A (derived from Shanghai Lengfeng Chemical reagent Co., Ltd, China) and surfactant B (purchased from Tianjin kermel Chemical reagent co., Ltd, China) were applied to increase the stability of composites. 2.2. Preparations of the composites In this paper, EG was used to form the exfoliated graphite sheet by ultrasonic oscillations, which was prepared from graphite powder through microwave method for 1 min. The EG was then added to the mixture of absolute ethyl alcohol and deionized water with the ratio of 3:2. After that, the solution was under ultrasonic vibration for 50 h. Finally, the filtration, washing, drying of solution were conducted to form exfoliated graphite sheet. The preparation of paraffin/exfoliated graphite sheet composites were similar with that of paraffin/graphene composites. First of all, the paraffin was baked in the drying oven for 6 h. And then the paraffin was heated until completely melting (liquid phase) at thermostatic water bath. The melting state was kept for 30 min. Then the appropriate surfactant A, surfactant B and high-thermalconductivity materials were added into the molten paraffin. Based on the work of Tao et al. [31], the effects of surfactant on enthalpy could not be ignored. In order to remove the effect during the comparisons, the ratio of paraffin, surfactant A and surfactant B was kept as 100: 3: 3. Then, the variation of enthalpy was only resulted
from the additives. The mixture was stirred with a magnetic stirrer for 3 h. Finally the composite material was natural cooled in room temperature. 2.3. Characterizations The morphologies of EG, exfoliated graphite sheet, graphene and composite material were investigated by means of a scanning electron microscope (SEM, FEI Quanta TM 250). Before the examinations, all samples were coated with a thin layer of gold. Fourier transform infrared spectroscope (FTIR) were applied to determine the chemical structure of composites. Thermal conductivity of composites at 20 ◦ C was measured by means of steady-state heat flow method using the thermal conductivity tester (DRX III, Xiangtan Instrument Manufacturing Co., Ltd, China), where the error was ±3%. The latent heat of paraffin, paraffin/graphene composites and paraffin/exfoliated graphite sheet composites were obtained by differential scanning calorimeter (DSC, Q100, Ninjing dazan institute of electrical and mechanical, China), which worked in a nitrogen atmosphere with a linear heating rate of 5 ◦ C min−1 , the nitrogen flow rate was 60 ml min−1 . The thermal stability of paraffin, paraffin/graphene composites and paraffin/exfoliated graphite sheet composites were obtained by thermogravimetric (TG) analysis, in a nitrogen atmosphere with a linear heating rate of 10 ◦ C min−1 . Furthermore, the nitrogen flow rate was 80ml min−1 and the test temperature range was 30 ◦ C–600 ◦ C. 3. Results and discussions 3.1. The morphology structures of additives In this section, the morphology structures of the graphene and exfoliated graphite sheet have been analyzed, as presented in Fig. 1. It could be observed from Fig. 1(a) that the raw expanded graphite was similar to worm-like structure. Moreover, a large amount of extremely thin flake in worm-like expanded graphite could be clearly observed at higher magnification imaging. Fig. 1(b) showed the morphology structure of exfoliated graphite sheet, which was analogous to a scrap of paper. The morphology structure of graphene was shown in Fig. 1(c). Comparing with present exfoliated graphite sheet, the larger layer-structure and tortuosity were obtained for graphene. 3.2. The dispersion stability of composites In order to achieve better stable dispersion of graphene and exfoliated graphite sheet in paraffin, the composite surfactant was added to the composites. Fig. 2 showed the liquid dispersion stability of paraffin/graphene composites at 75 ◦ C hot water bath which ensured that composites was kept in liquidus phase. Three tubes were filled with different mass fraction of paraffin/graphene composites. Moreover, the ratio of paraffin: Surfactant A: Surfactant B was kept as 100:3:3. Obviously, no stratification appeared until 305 min at high temperature. This indicated that adding surfactant was able to increase the stability of composites. Moreover, more than 50 times cyclic heating and cooling tests have been conducted to ensure the dispersion stability and thermal properties. 3.3. The morphology structures of composites Fig. 3 presented the SEM photographs of the composites. Fig. 3(a) and (b) were the morphology structures of paraffin/graphene composites. Fig. 3(c) and (d) were the morphology structures of paraffin/exfoliated graphite sheet composites. It was easily obtained that graphene and exfoliated graphite sheet were uniformly distributed in the paraffin from Fig. 3(a) and (c). The
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Fig. 1. The microstructure of monomer: (a) expanded graphite; (b) exfoliated graphite sheet; (c) paraffin.
Fig. 2. Stability experiments of composites.
graphene and exfoliated graphite sheet constructed as framework that enhanced the heat transfer. Fig. 3(b) and (d) proved that the graphene and exfoliated graphite sheet and paraffin were strongly integrated without any microcracks or loose interfaces. 3.4. Chemical compatibility Chemical compatibility between the paraffin and graphene and exfoliated graphite sheet were examined by the FTIR in this section.
Fig. 4 demonstrated that the FTIR spectrums of the paraffin, paraffin/graphene composites and paraffin/exfoliated graphite sheet composites. It could be obtained that the characteristic absorption bands of the paraffin were 727 cm−1 , 883 cm−1 , 1169 cm−1 , 1648 cm−1 , 1744 cm−1 , 2850 cm−1 and 2920 cm−1 . The absorption bands denoted the rocking vibration of CH2 (727 cm−1 ), strong C H and CH2 (883 cm−1 ), bending vibration of CH (1169 cm−1 ), stretching vibration of C C in alkene (1648 cm−1 ), asymmetric stretching vibration of C O in ester (1744 cm−1 ), symmetric
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Fig. 3. The microstructure of the composites: (a) paraffin/graphene composites (×2400); (b) paraffin/graphene composites (×16000); (c) paraffin/exfoliated graphite sheet composites (×2400); (d) paraffin/exfoliated graphite sheet composites (×10000).
3.5. Thermal conductivity The thermal conductivities of the composites and pure paraffin were presented in Fig. 5. It was noted that the thermal conductivity of composites increased with the fraction of additives, as shown in Fig. 5(a). Fig. 5(b) showed the relative thermal conductivity D, which was computed from:
D=
Fig. 4. FTIR spectra of (a) paraffin/graphene composites; (b) pure paraffin and (c) paraffin/exfoliated graphite sheet composites.
stretching vibration of CH2 (2850 cm−1 ) and asymmetric stretching vibration of CH3 (2920 cm−1 ). Characteristic absorption bands of graphite class material were very weak comparing with that of paraffin. Consequently, the characteristic absorption bands of graphite class material (in the composites) was not obvious. However, characteristic absorption bands of the composites included all the bands observed from paraffin, without any additional bands as well. The results explained that the paraffin and graphene or exfoliated graphite sheet were chemically inert to each other.
com − p p
(1)
where denoted the thermal conductivity. The subscripts, “com” and “p”, represented the composites and paraffin, respectively. When the mass fraction of graphene were 0.2 wt.%, 0.5 wt.%, 1.0 wt.%, 1.5 wt.%, 2.0 wt.%, the thermal conductivities were 0.33 W m−1 K−1 , 0.35 W m−1 K−1 , 0.37 W m−1 K−1 , 0.41 W m−1 K−1 and 0.46 W m−1 K−1 (D = 13.8%, 20.7%, 27.6%, 41.4% and 58.6%), respectively. With the same mass fraction, the thermal conductivities of paraffin/exfoliated graphite sheet composites were 0.31 W m−1 K−1 , 0.32 W m−1 K−1 , 0.34 W m−1 K−1 , 0.38 W m−1 K−1 and 0.41 W m−1 K−1 (D = 6.9%, 10.3%, 17.2%, 31.0% and 41.4%), respectively. It could be concluded that graphene enhanced the thermal conductivity more greatly than exfoliated graphite sheet. It was because that the lamellar structure of graphene was larger than that of exfoliated graphite sheet, as shown in Fig. 1.
3.6. Enthalpy The enthalpy was calculated by integral from the DSC curves, as presented in Fig. 6. Tm represented the temperature of melting
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Fig. 5. (a) Thermal conductivity and (b) D of the composites. Fig. 7. The DSC curves for (a) paraffin/graphene composites and (b) paraffin/exfoliated sheet graphite composites.
Fig. 6. The schematic of enthalpy computing.
peak. Ts and Te are the starting point and end point of solid-liquid phase change. The enthalpy was computed from [32]:
Te (Q (Ts → Tm → Te ) − Q (Ts → Te )) dT
H=
(2)
The DSC curves of the composites and paraffin were presented Fig. 7. The melting temperature Teo , peak temperature Tm of materials were shown in Table 1. It could be obtained that the composites presented lower melting point than that of pure paraffin. Resulting from high thermal conductivities of exfoliated graphite sheet and graphene, the heat transfer rate of PCM from outside to inside was accelerated and the phase change temperature was decreased as well [33]. The actual measured enthalpy of composites were presented in Table 1. The melting enthalpy of pure paraffin was 135.65 J g−1 . When the additive were mixed into the paraffin with mass fraction of 0.2%, 0.5%, 1.0%, 1.5%, 2.0%, the melting enthalpies were 141.15 J g−1 , 146.85 J g−1 , 162.89 J g−1 , 155.89 J g−1 and 137.85 J g−1 for paraffin/graphene composites, respectively. With the same mass fraction, the enthalpies of paraffin/exfoliated graphite sheet were 141.61 J g−1 ,163.28 J g−1 , 169.02 J g−1 , 161.17 J g−1 and 143.83 J g−1 , respectively. Since the graphite and exfoliated graphene sheet were without latent heat during the test temperature range, adding the graphene and exfoliated graphite sheet into paraffin was able to reduce the enthalpy. In order to further investigate the influence of additives, the re-evaluated melting enthalpy was defined as follows [30]: Hcom = Hp (1 − G )
(3)
Ts
where Q denoted the heat flow.
where Hp denoted the re-evaluated melting enthalpy of paraffin. G was the mass fraction of additive. The re-evaluated melt-
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Table 1 The enthalpy and phase change temperature of composites. Materials
Melting point Teo (◦ C)
Melting peak Tm (◦ C)
Melting enthalpy H(J g−1 )
Re-evaluated enthalpy Hcom (J g−1 )
Pure paraffin Paraffin with graphene(0.2%) Paraffin with graphene(0.5%) Paraffin with graphene(1.0%) Paraffin with graphene(1.5%) Paraffin with graphene(2.0%) Paraffin with exfoliated graphite (0.2%) Paraffin with exfoliated graphite (0.5%) Paraffin with exfoliated graphite (1.0%) Paraffin with exfoliated graphite (1.5%) Paraffin with exfoliated graphite (2.0%)
39.3 39.0 39.0 38.9 38.5 38.3 39.0 39.0 38.9 38.5 38.3
45.3 44.8 44.5 44.8 44.9 44.6 44.8 44.5 44.8 44.9 44.6
135.65 141.15 146.85 162.89 155.05 137.85 141.61 163.28 169.02 161.17 143.83
135.65 141.43 147.59 164.54 157.41 140.66 141.89 164.10 170.73 163.62 146.76
Table 2 Thermal stability of paraffin composites. Mass fraction (%)
0 0.2 0.5 1.0 1.5 2.0
Paraffin/exfoliated graphite composites weight loss (wt.%)
paraffin/graphene composites weight loss (wt.%)
200 ◦ C
250 ◦ C
300 ◦ C
200 ◦ C
250 ◦ C
300 ◦ C
2.93 1.15 1.20 1.34 1.48 1.57
10.45 5.72 6.16 5.75 7.05 6.8
34.92 22.46 26.13 23.62 29.52 27.45
2.93 3.37 2.63 3.47 3.40 3.54
10.45 10.19 10.36 10.10 9.86 10.16
34.92 32.52 32.62 32.09 31.33 32.24
ing enthalpy of materials were shown in Table 1 as well. When the mass fraction of additive was 1.0%, the re-evaluated melting enthalpy was the largest both for two kinds of composite, which were 164.54 J g−1 , 170.73 J g−1 for paraffin/graphene composite and paraffin/exfoliated graphite sheet composite, respectively. With the increase of mass fraction, the enthalpy increased firstly and then decreased for both paraffin/graphene and paraffin/exfoliated graphite sheet composites. The variation trend of enthalpy via mass fraction of additives mainly resulted from two impact factors. With the intermolecular forces between the additives (graphene or exfoliated graphite sheet) and paraffin, the enthalpy of composites were increased (Factor 1). Another impact factor was the non-melting enthalpy of additives, which would dropped down the melting enthalpy of composites (Factor 2). When the mass fraction was lower than 1%, the melting enthalpy was mainly influenced by Factor 1. After rising the mass fraction, the effect of Factor 2 got larger and then the melting enthalpy decreased. The largest enthalpy located at the mass fraction of 1%. The same phenomenon could be obtained in Ref. [5,34]. In addition, the influence of exfoliated graphite sheet on the enthalpy was larger than that of graphene. With the smaller particles, the intermolecular forces became larger. As shown in Fig. 2, the particle size of exfoliated graphite sheet was smaller than that of graphene, which led to that the enthalpy of paraffin/exfoliated graphite sheet composites was greater than that of paraffin/graphene composites. 3.7. Thermal stability The results of thermal stability of paraffin and composites were shown in Fig. 8. Fig. 8(a) showed the thermal analysis curves of paraffin/exfoliated graphite composites. It could be obtained that the overall trend of mass loss of composites was consistent with that of pure paraffin. When the temperature reached 150 ◦ C, the composites mass began to drop. The weight of composites lost seriously from 250 ◦ C to 340 ◦ C. After 350 ◦ C, mass loss reduced very slowly, since composites have been completed thermal decomposition. In the meantime, the gas of decomposition evaporated away and less volatile substances left. The TG analysis curves of paraf-
Fig. 8. The TG results of (a) paraffin/graphite sheet composites and (b) paraffin/graphene composites.
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fin/graphene composites were presented in Fig. 8(b), whose trend was similar to that of paraffin/exfoliated graphite sheet composites. Table 2 exhibited the loss of mass fraction of composites at 200 ◦ C, 250 ◦ C and 300 ◦ C, respectively. At the same temperature, declining quantity of paraffin/exfoliated graphite sheet composites was lower than that of pure paraffin. At 200 ◦ C, the mass loss of paraffin/graphene composites was higher than paraffin’s. At 250 ◦ C and 300 ◦ C, the weight decrease of the paraffin/graphene composite was lower compared to pure paraffin. This demonstrated that the exfoliated graphite sheet has reinforced thermal inertia of paraffin, and graphene had a little influence on thermal inertia of paraffin. 4. Conclusions In this work, two kinds of composite phase change material have been prepared, the paraffin/graphene composites and paraffin/exfoliated graphite sheet composites. Furthermore, the mass fractions of graphene and exfoliated graphite sheet varied between 0–2.0 wt.%. The results demonstrated that: (1) The graphene and exfoliated graphite sheet and paraffin were strongly integrated without any microcracks or loose interfaces. The paraffin and graphene or exfoliated graphite sheet were chemically inert to each other. (2) Both graphene and exfoliated graphite sheet greatly improved the thermal conductivity of paraffin. However, with the same mass fraction, the effects of graphene was larger than that of exfoliated graphite sheet. (3) The composites presented lower melting point than that of pure paraffin. Furthermore, the enthalpy of composites rose firstly and then dropped down with the increase of mass fraction. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. U1407125). References [1] A.M. Khudhair, M.M. Farid, A review on energy conservation in building applications with thermal storage by latent heat using phase change materials, Energy Convers. Manage. 45 (2004) 263–275. [2] D. Zhou, C.Y. Zhao, Y. Tian, Review on thermal energy storage with phase change materials (PCMs) in building applications, Appl. Energ. 92 (2012) 593–605. [3] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renew. Sustain. Energ. Rev. 13 (2009) 318–345. [4] M.M. Farid, A.M. Khudhair, S.A.K. Razack, S. Al-Hallaj, A review on phase change energy storage: materials and applications, Energy Convers. Manage. 45 (2004) 1597–1615. [5] S. Kim, L.T. Drza, High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets, Sol. Energy Mater. Sol. Cells 93 (2009) 136–142. [6] Y.B. Tao, Y.L. He, Z.G. Qu, Numerical study on performance of molten salt phase change thermal energy storage system with enhanced tubes, Sol. Energy 86 (2012) 1155–1163. [7] P. Atkin, M.M. Farid, Improving the efficiency of photovoltaic cells using PCM infused graphite and aluminium fins, Sol. Energy 114 (2015) 217–228. [8] Y.B. Tao, Y.L. He, Y.K. Liu, W.Q. Tao, Performance optimization of two-stage latent heat storage unit based on entransy theory, Int. J. Heat Mass Transf. 77 (2014) 695–703. [9] T.K. Aldoss, M.M. Rahman, Comparison between the single-PCM and multi-PCM thermal energy storage design, Energy Convers. Manage. 83 (2014) 79–87. [10] X. Xiao, P. Zhang, M. Li, Preparation and thermal characterization of paraffin/metal foam composite phase change material, Appl. Energ. 112 (2013) 1357–1366.
21
[11] P. Zhang, Z. Meng, H. Zhu, Y. Wang, S. Peng, Experimental and numerical study of heat transfer characteristics of a paraffin/metal foam composite PCM, Energy Proced. 75 (2015) 3091–3097. [12] S.C. Lin, H.H. Al-Kayiem, Evaluation of copper nanoparticles −Paraffin wax compositions for solar thermal energy storage, Sol. Energy 132 (2016) 267–278. [13] A. Babapoor, G. Karimi, Thermal properties measurement and heat storage analysis of paraffinnanoparticles composites phase change material: comparison and optimization, Appl. Therm. Eng. 90 (2015) 945–951. [14] M. Nourani, N. Hamdami, J. Keramat, A. Moheb, M. Shahedi, Thermal behavior of paraffin-nano-Al2O3 stabilized by sodium stearoyl lactylate as a stable phase change material with high thermal conductivity, Renew. Energ. 88 (2016) 474–482. [15] F. Frusteri, V. Leonardi, S. Vasta, G. Restuccia, Thermal conductivity measurement of a PCM based storage system containing carbon fibers, Appl. Therm. Eng. 25 (2005) 1623–1633. [16] P. Jana, V. Fierro, A. Pizzi, A. Celzard, Thermal conductivity improvement of composite carbon foams based on tannin-based disordered carbon matrix and graphite fillers, Mater. Des. 83 (2015) 635–643. [17] Y. Zhong, S. Li, X. Wei, Z. Liu, Q. Guo, J. Shi, L. Liu, Heat transfer enhancement of paraffin wax using compressed expanded natural graphite for thermal energy storage, Carbon 48 (2010) 300–304. [18] Y. Zhang, J. Ding, X. Wang, R. Yang, K. Lin, Influence of additives on thermal conductivity of shape-stabilized phase change material, Sol. Energy Mater. Sol. Cells 90 (2006) 1692–1702. [19] M. Lachheb, A. Adili, F. Albouchi, F. Mzali, S. Ben Nasrallah, Thermal properties improvement of Lithium nitrate/Graphite composite phase change materials, Appl. Therm. Eng. 102 (2016) 922–931. [20] N. Vitorino, J.C.C. Abrantes, J.R. Frade, Gelled graphite/gelatin composites for latent heat cold storage, Appl. Energ. 104 (2013) 890–897. [21] A. Sharma, V. Tyagi, C. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renew. Sustain. Energy Rev. 13 (2009) 318–345. [22] B. Zalba, J.M. Marı´ın, L.F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Appl. Therm. Eng. 23 (2003) 251–283. [23] M.M. Farid, A.M. Khudhair, S.A.K. Razack, S. Al-Hallaj, A review on phase change energy storage: materials and applications, Energy Convers. Manage. 45 (2004) 1597–1615. [24] Y.B. Tao, C.H. Lin, Y.L. He, Preparation and thermal properties characterization of carbonate salt/carbon nanomaterial composite phase change material, Energy Convers. Manage. 97 (2015) 103–110. [25] L.-W. Fan, Z.-Q. Zhu, Y. Zeng, Q. Lu, Z.-T. Yu, Heat transfer during melting of graphene-based composite phase change materials heated from below, Int. J. Heat Mass Transf. 79 (2014) 94–104. [26] M. Mehrali, S.T. Latibari, M. Mehrali, T.M. Indra Mahlia, H.S. Cornelis Metselaar, M.S. Naghavi, E. Sadeghinezhad, A.R. Akhiani, Preparation and characterization of palmitic acid/graphene nanoplatelets composite with remarkable thermal conductivity as a novel shape-stabilized phase change material, Appl. Therm. Eng. 61 (2013) 633–640. [27] M. Silakhori, H. Fauzi, M.R. Mahmoudian, H.S.C. Metselaar, T.M.I. Mahlia, H.M. Khanlou, Preparation and thermal properties of form-stable phase change materials composed of palmitic acid/polypyrrole/graphene nanoplatelets, Energy Build. 99 (2015) 189–195. [28] J.-L. Zeng, S.-H. Zheng, S.-B. Yu, F.-R. Zhu, J. Gan, L. Zhu, Z.-L. Xiao, X.-Y. Zhu, Z. Zhu, L.-X. Sun, Z. Cao, Preparation and thermal properties of palmitic acid/polyaniline/exfoliated graphite nanoplatelets form-stable phase change materials, Appl. Energ. 115 (2014) 603–609. [29] K. Kalaitzidou, H. Fukushima, L.T. Drzal, Multifunctional polypropylene composites produced by incorporation of exfoliated graphite nanoplatelets, Carbon 45 (2007) 1446–1452. [30] J.L. Xiang, L.T. Drzal, Investigation of exfoliated graphite nanoplatelets (xGnP) in improving thermal conductivity of paraffin wax-based phase change material, Sol. Energy Mater. Sol. Cells 95 (2011) 1811–1818. [31] Y.B. Tao, C.H. Lin, Y.L. He, Effect of surface active agent on thermal properties of carbonate salt/carbon nanomaterial composite phase change material, Appl. Energ. 156 (2015) 478–489. [32] R. Al-Shannaq, M. Farid, S. Al-Muhtaseb, J. Kurdi, Emulsion stability and cross-linking of PMMA microcapsules containing phase change materials, Sol. Energy Mater. Sol. Cells 132 (2015) 311–318. [33] B.E. Jebasingh, Exfoliation of graphite by solar irradiation and investigate their thermal property on capric–myristic–palmitic acid/exfoliated graphite composite as phase change material (PCM) for energy storage, J. Energy Storage 5 (2016) 70–76. [34] S. Shaikh, K. Lafdi, K. Hallinan, Carbon nanoadditives to enhance latent energy storage of phase change materials, J. Appl. Phys. 103 (2008).
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Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Experimental study on the thermal performance of graphene and exfoliated graphite sheet for thermal energy storage phase change material Xia Liu, Zhonghao Rao ∗ School of Electric Power Engineering, China University of Mining and Technology, Xuzhou 221116, China
a r t i c l e
i n f o
Article history: Received 5 July 2016 Received in revised form 12 November 2016 Accepted 16 November 2016 Available online 18 November 2016 Keywords: Phase change material Paraffin Graphene Exfoliated graphite sheet
a b s t r a c t In this paper, in order to enhance the thermal performance of paraffin in thermal energy storage, the graphene and exfoliated graphite sheet have been mixed with the paraffin to prepare composites. After the stability tests of dispersion, the scanning electron microscope (SEM), Fourier transform infrared spectroscope (FTIR), thermal conductivity analysis, differential scanning calorimeter (DSC) and thermogravimetric analyzer (TG) are applied to characterize the microstructure, chemical structure, thermal conductivity, enthalpy and stability of the materials, respectively. The results indicated that the thermal conductivity was able to be increased greatly with graphene or exfoliated graphite sheet. With the same mass fraction, the thermal conductivity of paraffin/graphene composite was greater than that of paraffin/exfoliated graphite sheet composite. Furthermore, when the mass fraction of graphene or exfoliated graphite sheet varied between 0–2.0 wt.%, the enthalpy of phase change material (PCM) composites rose firstly and then dropped down. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Phase change material (PCM) is able to be used in the thermal energy storage (TES), which has received great attention in recent years [1–4]. Besides, the temperature of PCM varies a little during the phase change process, including the melting process (heat absorbing) and solidification process (heat releasing). PCM can be classified into two major categories: inorganic PCM and organic PCM [5]. There are several kinds of inorganic PCM, including salts, salts hydrates, metals and alloys. Fatty acids/esters, polyalcohol and paraffin are treated as organic PCM [5]. However, resulting from the low thermal conductivity, most of the PCMs can’t reach the demand of TES. Consequently, the thermal performance of PCM is necessary to be enhanced. There are three methods to enhance the thermal conductivity of PCM: a. adopt enhanced heat transfer surface [6,7]; b. enhance the uniformity of the heat transfer process [8,9]; c. improve PCM thermal performance. The thermal properties of PCM are able to be enhanced by adding additives with high thermal conductivity, such as metal (e.g. nano-metal) [10–12], oxide class(e.g. titanium dioxide) [13,14] and graphite class (e.g. natural graphite) [15–17].
∗ Corresponding author. E-mail address: [email protected] (Z. Rao). http://dx.doi.org/10.1016/j.tca.2016.11.010 0040-6031/© 2016 Elsevier B.V. All rights reserved.
To enhance the thermal conductivity of shape-stabilized PCM effectively, Zhang et al. [18] have prepared nine kinds of composite PCM. They found that adding graphite could enhance the thermal properties mostly. Furthermore, when graphite content was 20 wt.%, the thermal conductivity was increased to 0.482 W m−1 K−1 , which was increased by as large as 221.4% [18]. In addition, with the same percentage of graphite (20 wt.%), the thermal conductivity of lithium nitrate composites has been increased by 426.53% [19]. Vitorino et al. [20] proved that the thermal properties of composites was approximately linear growth with graphite content. Among the organic PCMs, the paraffin is of importance to TES since its excellent properties: large latent heat, no phase segregation, low cost, non-toxic and no corrosive [21–23]. Furthermore, no degradation of thermal properties was observed after several melting/solidification cycles [13]. However, the thermal conductivity of paraffin is low as well. In order to enhance the thermal performance of paraffin, two kinds of material with high thermal conductivity can be considered as additives, the graphene and exfoliated graphite sheet. As one of the graphite, exfoliated graphite sheet and graphene have attracted more and more attentions due to their excellent thermal properties, Tao et al. [24] presented that graphene was the best material to enhance the thermal performance. When the graphene was 0.1 wt.%, 0.5 wt.%, 1.0 wt.%, 1.5 wt.%, 2.5 wt.%, the thermal conductivity has been enhanced by about
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8.01%, 15.54%, 26.11%, 27.73%, respectively. Fan et al. [25] stated when the mass fraction of graphene was 3 wt.%, the thermal conductivity was 2 times higher. In Ref. [26], the thermal conductivity of palmitic acid/graphene nanoplatelets composite has been increased by 800% compared to that of original material. Furthermore, Silakhori et al. [27] proved that the thermal conductivity of palmitic acid/polypyrrole/graphene nanoplatelets form-stable PCM was 38.7% higher than that of palmitic acid particles and 34.3% higher than that of palmitic acid/polypyrrole form-stable PCM. Zeng et al. [28] added 7.87 wt.% of the exfoliated graphite sheet to the palmitic acid/polyaniline composite, and the thermal conductivity was 237.5% higher. Kalaitzidou et al. [29] reported that when the volume fraction of exfoliated graphite sheet was 25%, the thermal conductivity had reached 6 times of the raw material. Moreover, Xiang and Drzal [30] proved that the thermal conductivity was associated with the particle size of exfoliated graphite nanoplatelets. The comparisons of thermal properties between paraffin/exfoliated graphite sheet composite and paraffin/graphene composite have been considered in this paper. The PCM composites are prepared in five kinds of mass ratio. The tests of morphology structure, chemical structure, latent heat and thermal conductivity were investigated in detail. 2. Experiments 2.1. Materials The paraffin, which was purchased from Shanghai Joule wax Co., Ltd, China, was chosen as PCM. Two kinds of material with great thermal conductivity have been chosen as additives. One was the graphene (3000–5000W m−1 K−1 ), which was obtained from Ninjing SCF Nanotech., Ltd, China (purity exceeding 99.5%). Another was the exfoliated graphite sheet (300–400 W m−1 K−1 ) by home-made, which was prepared from expanded graphite (EG). Furthermore, the EG was formed from the graphite powders (obtained from Qingdao Tianheda Graphite Co., Ltd, China) through microwave method. During the preparation of exfoliated graphite sheet, the absolute ethyl alcohol (C2 H5 OH, analytically pure, Shanghai Zhongqin Chemical reagent Co., Ltd, China) was used as solvent. The surfactant A (derived from Shanghai Lengfeng Chemical reagent Co., Ltd, China) and surfactant B (purchased from Tianjin kermel Chemical reagent co., Ltd, China) were applied to increase the stability of composites. 2.2. Preparations of the composites In this paper, EG was used to form the exfoliated graphite sheet by ultrasonic oscillations, which was prepared from graphite powder through microwave method for 1 min. The EG was then added to the mixture of absolute ethyl alcohol and deionized water with the ratio of 3:2. After that, the solution was under ultrasonic vibration for 50 h. Finally, the filtration, washing, drying of solution were conducted to form exfoliated graphite sheet. The preparation of paraffin/exfoliated graphite sheet composites were similar with that of paraffin/graphene composites. First of all, the paraffin was baked in the drying oven for 6 h. And then the paraffin was heated until completely melting (liquid phase) at thermostatic water bath. The melting state was kept for 30 min. Then the appropriate surfactant A, surfactant B and high-thermalconductivity materials were added into the molten paraffin. Based on the work of Tao et al. [31], the effects of surfactant on enthalpy could not be ignored. In order to remove the effect during the comparisons, the ratio of paraffin, surfactant A and surfactant B was kept as 100: 3: 3. Then, the variation of enthalpy was only resulted
from the additives. The mixture was stirred with a magnetic stirrer for 3 h. Finally the composite material was natural cooled in room temperature. 2.3. Characterizations The morphologies of EG, exfoliated graphite sheet, graphene and composite material were investigated by means of a scanning electron microscope (SEM, FEI Quanta TM 250). Before the examinations, all samples were coated with a thin layer of gold. Fourier transform infrared spectroscope (FTIR) were applied to determine the chemical structure of composites. Thermal conductivity of composites at 20 ◦ C was measured by means of steady-state heat flow method using the thermal conductivity tester (DRX III, Xiangtan Instrument Manufacturing Co., Ltd, China), where the error was ±3%. The latent heat of paraffin, paraffin/graphene composites and paraffin/exfoliated graphite sheet composites were obtained by differential scanning calorimeter (DSC, Q100, Ninjing dazan institute of electrical and mechanical, China), which worked in a nitrogen atmosphere with a linear heating rate of 5 ◦ C min−1 , the nitrogen flow rate was 60 ml min−1 . The thermal stability of paraffin, paraffin/graphene composites and paraffin/exfoliated graphite sheet composites were obtained by thermogravimetric (TG) analysis, in a nitrogen atmosphere with a linear heating rate of 10 ◦ C min−1 . Furthermore, the nitrogen flow rate was 80ml min−1 and the test temperature range was 30 ◦ C–600 ◦ C. 3. Results and discussions 3.1. The morphology structures of additives In this section, the morphology structures of the graphene and exfoliated graphite sheet have been analyzed, as presented in Fig. 1. It could be observed from Fig. 1(a) that the raw expanded graphite was similar to worm-like structure. Moreover, a large amount of extremely thin flake in worm-like expanded graphite could be clearly observed at higher magnification imaging. Fig. 1(b) showed the morphology structure of exfoliated graphite sheet, which was analogous to a scrap of paper. The morphology structure of graphene was shown in Fig. 1(c). Comparing with present exfoliated graphite sheet, the larger layer-structure and tortuosity were obtained for graphene. 3.2. The dispersion stability of composites In order to achieve better stable dispersion of graphene and exfoliated graphite sheet in paraffin, the composite surfactant was added to the composites. Fig. 2 showed the liquid dispersion stability of paraffin/graphene composites at 75 ◦ C hot water bath which ensured that composites was kept in liquidus phase. Three tubes were filled with different mass fraction of paraffin/graphene composites. Moreover, the ratio of paraffin: Surfactant A: Surfactant B was kept as 100:3:3. Obviously, no stratification appeared until 305 min at high temperature. This indicated that adding surfactant was able to increase the stability of composites. Moreover, more than 50 times cyclic heating and cooling tests have been conducted to ensure the dispersion stability and thermal properties. 3.3. The morphology structures of composites Fig. 3 presented the SEM photographs of the composites. Fig. 3(a) and (b) were the morphology structures of paraffin/graphene composites. Fig. 3(c) and (d) were the morphology structures of paraffin/exfoliated graphite sheet composites. It was easily obtained that graphene and exfoliated graphite sheet were uniformly distributed in the paraffin from Fig. 3(a) and (c). The
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Fig. 1. The microstructure of monomer: (a) expanded graphite; (b) exfoliated graphite sheet; (c) paraffin.
Fig. 2. Stability experiments of composites.
graphene and exfoliated graphite sheet constructed as framework that enhanced the heat transfer. Fig. 3(b) and (d) proved that the graphene and exfoliated graphite sheet and paraffin were strongly integrated without any microcracks or loose interfaces. 3.4. Chemical compatibility Chemical compatibility between the paraffin and graphene and exfoliated graphite sheet were examined by the FTIR in this section.
Fig. 4 demonstrated that the FTIR spectrums of the paraffin, paraffin/graphene composites and paraffin/exfoliated graphite sheet composites. It could be obtained that the characteristic absorption bands of the paraffin were 727 cm−1 , 883 cm−1 , 1169 cm−1 , 1648 cm−1 , 1744 cm−1 , 2850 cm−1 and 2920 cm−1 . The absorption bands denoted the rocking vibration of CH2 (727 cm−1 ), strong C H and CH2 (883 cm−1 ), bending vibration of CH (1169 cm−1 ), stretching vibration of C C in alkene (1648 cm−1 ), asymmetric stretching vibration of C O in ester (1744 cm−1 ), symmetric
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Fig. 3. The microstructure of the composites: (a) paraffin/graphene composites (×2400); (b) paraffin/graphene composites (×16000); (c) paraffin/exfoliated graphite sheet composites (×2400); (d) paraffin/exfoliated graphite sheet composites (×10000).
3.5. Thermal conductivity The thermal conductivities of the composites and pure paraffin were presented in Fig. 5. It was noted that the thermal conductivity of composites increased with the fraction of additives, as shown in Fig. 5(a). Fig. 5(b) showed the relative thermal conductivity D, which was computed from:
D=
Fig. 4. FTIR spectra of (a) paraffin/graphene composites; (b) pure paraffin and (c) paraffin/exfoliated graphite sheet composites.
stretching vibration of CH2 (2850 cm−1 ) and asymmetric stretching vibration of CH3 (2920 cm−1 ). Characteristic absorption bands of graphite class material were very weak comparing with that of paraffin. Consequently, the characteristic absorption bands of graphite class material (in the composites) was not obvious. However, characteristic absorption bands of the composites included all the bands observed from paraffin, without any additional bands as well. The results explained that the paraffin and graphene or exfoliated graphite sheet were chemically inert to each other.
com − p p
(1)
where denoted the thermal conductivity. The subscripts, “com” and “p”, represented the composites and paraffin, respectively. When the mass fraction of graphene were 0.2 wt.%, 0.5 wt.%, 1.0 wt.%, 1.5 wt.%, 2.0 wt.%, the thermal conductivities were 0.33 W m−1 K−1 , 0.35 W m−1 K−1 , 0.37 W m−1 K−1 , 0.41 W m−1 K−1 and 0.46 W m−1 K−1 (D = 13.8%, 20.7%, 27.6%, 41.4% and 58.6%), respectively. With the same mass fraction, the thermal conductivities of paraffin/exfoliated graphite sheet composites were 0.31 W m−1 K−1 , 0.32 W m−1 K−1 , 0.34 W m−1 K−1 , 0.38 W m−1 K−1 and 0.41 W m−1 K−1 (D = 6.9%, 10.3%, 17.2%, 31.0% and 41.4%), respectively. It could be concluded that graphene enhanced the thermal conductivity more greatly than exfoliated graphite sheet. It was because that the lamellar structure of graphene was larger than that of exfoliated graphite sheet, as shown in Fig. 1.
3.6. Enthalpy The enthalpy was calculated by integral from the DSC curves, as presented in Fig. 6. Tm represented the temperature of melting
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Fig. 5. (a) Thermal conductivity and (b) D of the composites. Fig. 7. The DSC curves for (a) paraffin/graphene composites and (b) paraffin/exfoliated sheet graphite composites.
Fig. 6. The schematic of enthalpy computing.
peak. Ts and Te are the starting point and end point of solid-liquid phase change. The enthalpy was computed from [32]:
Te (Q (Ts → Tm → Te ) − Q (Ts → Te )) dT
H=
(2)
The DSC curves of the composites and paraffin were presented Fig. 7. The melting temperature Teo , peak temperature Tm of materials were shown in Table 1. It could be obtained that the composites presented lower melting point than that of pure paraffin. Resulting from high thermal conductivities of exfoliated graphite sheet and graphene, the heat transfer rate of PCM from outside to inside was accelerated and the phase change temperature was decreased as well [33]. The actual measured enthalpy of composites were presented in Table 1. The melting enthalpy of pure paraffin was 135.65 J g−1 . When the additive were mixed into the paraffin with mass fraction of 0.2%, 0.5%, 1.0%, 1.5%, 2.0%, the melting enthalpies were 141.15 J g−1 , 146.85 J g−1 , 162.89 J g−1 , 155.89 J g−1 and 137.85 J g−1 for paraffin/graphene composites, respectively. With the same mass fraction, the enthalpies of paraffin/exfoliated graphite sheet were 141.61 J g−1 ,163.28 J g−1 , 169.02 J g−1 , 161.17 J g−1 and 143.83 J g−1 , respectively. Since the graphite and exfoliated graphene sheet were without latent heat during the test temperature range, adding the graphene and exfoliated graphite sheet into paraffin was able to reduce the enthalpy. In order to further investigate the influence of additives, the re-evaluated melting enthalpy was defined as follows [30]: Hcom = Hp (1 − G )
(3)
Ts
where Q denoted the heat flow.
where Hp denoted the re-evaluated melting enthalpy of paraffin. G was the mass fraction of additive. The re-evaluated melt-
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Table 1 The enthalpy and phase change temperature of composites. Materials
Melting point Teo (◦ C)
Melting peak Tm (◦ C)
Melting enthalpy H(J g−1 )
Re-evaluated enthalpy Hcom (J g−1 )
Pure paraffin Paraffin with graphene(0.2%) Paraffin with graphene(0.5%) Paraffin with graphene(1.0%) Paraffin with graphene(1.5%) Paraffin with graphene(2.0%) Paraffin with exfoliated graphite (0.2%) Paraffin with exfoliated graphite (0.5%) Paraffin with exfoliated graphite (1.0%) Paraffin with exfoliated graphite (1.5%) Paraffin with exfoliated graphite (2.0%)
39.3 39.0 39.0 38.9 38.5 38.3 39.0 39.0 38.9 38.5 38.3
45.3 44.8 44.5 44.8 44.9 44.6 44.8 44.5 44.8 44.9 44.6
135.65 141.15 146.85 162.89 155.05 137.85 141.61 163.28 169.02 161.17 143.83
135.65 141.43 147.59 164.54 157.41 140.66 141.89 164.10 170.73 163.62 146.76
Table 2 Thermal stability of paraffin composites. Mass fraction (%)
0 0.2 0.5 1.0 1.5 2.0
Paraffin/exfoliated graphite composites weight loss (wt.%)
paraffin/graphene composites weight loss (wt.%)
200 ◦ C
250 ◦ C
300 ◦ C
200 ◦ C
250 ◦ C
300 ◦ C
2.93 1.15 1.20 1.34 1.48 1.57
10.45 5.72 6.16 5.75 7.05 6.8
34.92 22.46 26.13 23.62 29.52 27.45
2.93 3.37 2.63 3.47 3.40 3.54
10.45 10.19 10.36 10.10 9.86 10.16
34.92 32.52 32.62 32.09 31.33 32.24
ing enthalpy of materials were shown in Table 1 as well. When the mass fraction of additive was 1.0%, the re-evaluated melting enthalpy was the largest both for two kinds of composite, which were 164.54 J g−1 , 170.73 J g−1 for paraffin/graphene composite and paraffin/exfoliated graphite sheet composite, respectively. With the increase of mass fraction, the enthalpy increased firstly and then decreased for both paraffin/graphene and paraffin/exfoliated graphite sheet composites. The variation trend of enthalpy via mass fraction of additives mainly resulted from two impact factors. With the intermolecular forces between the additives (graphene or exfoliated graphite sheet) and paraffin, the enthalpy of composites were increased (Factor 1). Another impact factor was the non-melting enthalpy of additives, which would dropped down the melting enthalpy of composites (Factor 2). When the mass fraction was lower than 1%, the melting enthalpy was mainly influenced by Factor 1. After rising the mass fraction, the effect of Factor 2 got larger and then the melting enthalpy decreased. The largest enthalpy located at the mass fraction of 1%. The same phenomenon could be obtained in Ref. [5,34]. In addition, the influence of exfoliated graphite sheet on the enthalpy was larger than that of graphene. With the smaller particles, the intermolecular forces became larger. As shown in Fig. 2, the particle size of exfoliated graphite sheet was smaller than that of graphene, which led to that the enthalpy of paraffin/exfoliated graphite sheet composites was greater than that of paraffin/graphene composites. 3.7. Thermal stability The results of thermal stability of paraffin and composites were shown in Fig. 8. Fig. 8(a) showed the thermal analysis curves of paraffin/exfoliated graphite composites. It could be obtained that the overall trend of mass loss of composites was consistent with that of pure paraffin. When the temperature reached 150 ◦ C, the composites mass began to drop. The weight of composites lost seriously from 250 ◦ C to 340 ◦ C. After 350 ◦ C, mass loss reduced very slowly, since composites have been completed thermal decomposition. In the meantime, the gas of decomposition evaporated away and less volatile substances left. The TG analysis curves of paraf-
Fig. 8. The TG results of (a) paraffin/graphite sheet composites and (b) paraffin/graphene composites.
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fin/graphene composites were presented in Fig. 8(b), whose trend was similar to that of paraffin/exfoliated graphite sheet composites. Table 2 exhibited the loss of mass fraction of composites at 200 ◦ C, 250 ◦ C and 300 ◦ C, respectively. At the same temperature, declining quantity of paraffin/exfoliated graphite sheet composites was lower than that of pure paraffin. At 200 ◦ C, the mass loss of paraffin/graphene composites was higher than paraffin’s. At 250 ◦ C and 300 ◦ C, the weight decrease of the paraffin/graphene composite was lower compared to pure paraffin. This demonstrated that the exfoliated graphite sheet has reinforced thermal inertia of paraffin, and graphene had a little influence on thermal inertia of paraffin. 4. Conclusions In this work, two kinds of composite phase change material have been prepared, the paraffin/graphene composites and paraffin/exfoliated graphite sheet composites. Furthermore, the mass fractions of graphene and exfoliated graphite sheet varied between 0–2.0 wt.%. The results demonstrated that: (1) The graphene and exfoliated graphite sheet and paraffin were strongly integrated without any microcracks or loose interfaces. The paraffin and graphene or exfoliated graphite sheet were chemically inert to each other. (2) Both graphene and exfoliated graphite sheet greatly improved the thermal conductivity of paraffin. However, with the same mass fraction, the effects of graphene was larger than that of exfoliated graphite sheet. (3) The composites presented lower melting point than that of pure paraffin. Furthermore, the enthalpy of composites rose firstly and then dropped down with the increase of mass fraction. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. U1407125). References [1] A.M. Khudhair, M.M. Farid, A review on energy conservation in building applications with thermal storage by latent heat using phase change materials, Energy Convers. Manage. 45 (2004) 263–275. [2] D. Zhou, C.Y. Zhao, Y. Tian, Review on thermal energy storage with phase change materials (PCMs) in building applications, Appl. Energ. 92 (2012) 593–605. [3] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renew. Sustain. Energ. Rev. 13 (2009) 318–345. [4] M.M. Farid, A.M. Khudhair, S.A.K. Razack, S. Al-Hallaj, A review on phase change energy storage: materials and applications, Energy Convers. Manage. 45 (2004) 1597–1615. [5] S. Kim, L.T. Drza, High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets, Sol. Energy Mater. Sol. Cells 93 (2009) 136–142. [6] Y.B. Tao, Y.L. He, Z.G. Qu, Numerical study on performance of molten salt phase change thermal energy storage system with enhanced tubes, Sol. Energy 86 (2012) 1155–1163. [7] P. Atkin, M.M. Farid, Improving the efficiency of photovoltaic cells using PCM infused graphite and aluminium fins, Sol. Energy 114 (2015) 217–228. [8] Y.B. Tao, Y.L. He, Y.K. Liu, W.Q. Tao, Performance optimization of two-stage latent heat storage unit based on entransy theory, Int. J. Heat Mass Transf. 77 (2014) 695–703. [9] T.K. Aldoss, M.M. Rahman, Comparison between the single-PCM and multi-PCM thermal energy storage design, Energy Convers. Manage. 83 (2014) 79–87. [10] X. Xiao, P. Zhang, M. Li, Preparation and thermal characterization of paraffin/metal foam composite phase change material, Appl. Energ. 112 (2013) 1357–1366.
21
[11] P. Zhang, Z. Meng, H. Zhu, Y. Wang, S. Peng, Experimental and numerical study of heat transfer characteristics of a paraffin/metal foam composite PCM, Energy Proced. 75 (2015) 3091–3097. [12] S.C. Lin, H.H. Al-Kayiem, Evaluation of copper nanoparticles −Paraffin wax compositions for solar thermal energy storage, Sol. Energy 132 (2016) 267–278. [13] A. Babapoor, G. Karimi, Thermal properties measurement and heat storage analysis of paraffinnanoparticles composites phase change material: comparison and optimization, Appl. Therm. Eng. 90 (2015) 945–951. [14] M. Nourani, N. Hamdami, J. Keramat, A. Moheb, M. Shahedi, Thermal behavior of paraffin-nano-Al2O3 stabilized by sodium stearoyl lactylate as a stable phase change material with high thermal conductivity, Renew. Energ. 88 (2016) 474–482. [15] F. Frusteri, V. Leonardi, S. Vasta, G. Restuccia, Thermal conductivity measurement of a PCM based storage system containing carbon fibers, Appl. Therm. Eng. 25 (2005) 1623–1633. [16] P. Jana, V. Fierro, A. Pizzi, A. Celzard, Thermal conductivity improvement of composite carbon foams based on tannin-based disordered carbon matrix and graphite fillers, Mater. Des. 83 (2015) 635–643. [17] Y. Zhong, S. Li, X. Wei, Z. Liu, Q. Guo, J. Shi, L. Liu, Heat transfer enhancement of paraffin wax using compressed expanded natural graphite for thermal energy storage, Carbon 48 (2010) 300–304. [18] Y. Zhang, J. Ding, X. Wang, R. Yang, K. Lin, Influence of additives on thermal conductivity of shape-stabilized phase change material, Sol. Energy Mater. Sol. Cells 90 (2006) 1692–1702. [19] M. Lachheb, A. Adili, F. Albouchi, F. Mzali, S. Ben Nasrallah, Thermal properties improvement of Lithium nitrate/Graphite composite phase change materials, Appl. Therm. Eng. 102 (2016) 922–931. [20] N. Vitorino, J.C.C. Abrantes, J.R. Frade, Gelled graphite/gelatin composites for latent heat cold storage, Appl. Energ. 104 (2013) 890–897. [21] A. Sharma, V. Tyagi, C. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renew. Sustain. Energy Rev. 13 (2009) 318–345. [22] B. Zalba, J.M. Marı´ın, L.F. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Appl. Therm. Eng. 23 (2003) 251–283. [23] M.M. Farid, A.M. Khudhair, S.A.K. Razack, S. Al-Hallaj, A review on phase change energy storage: materials and applications, Energy Convers. Manage. 45 (2004) 1597–1615. [24] Y.B. Tao, C.H. Lin, Y.L. He, Preparation and thermal properties characterization of carbonate salt/carbon nanomaterial composite phase change material, Energy Convers. Manage. 97 (2015) 103–110. [25] L.-W. Fan, Z.-Q. Zhu, Y. Zeng, Q. Lu, Z.-T. Yu, Heat transfer during melting of graphene-based composite phase change materials heated from below, Int. J. Heat Mass Transf. 79 (2014) 94–104. [26] M. Mehrali, S.T. Latibari, M. Mehrali, T.M. Indra Mahlia, H.S. Cornelis Metselaar, M.S. Naghavi, E. Sadeghinezhad, A.R. Akhiani, Preparation and characterization of palmitic acid/graphene nanoplatelets composite with remarkable thermal conductivity as a novel shape-stabilized phase change material, Appl. Therm. Eng. 61 (2013) 633–640. [27] M. Silakhori, H. Fauzi, M.R. Mahmoudian, H.S.C. Metselaar, T.M.I. Mahlia, H.M. Khanlou, Preparation and thermal properties of form-stable phase change materials composed of palmitic acid/polypyrrole/graphene nanoplatelets, Energy Build. 99 (2015) 189–195. [28] J.-L. Zeng, S.-H. Zheng, S.-B. Yu, F.-R. Zhu, J. Gan, L. Zhu, Z.-L. Xiao, X.-Y. Zhu, Z. Zhu, L.-X. Sun, Z. Cao, Preparation and thermal properties of palmitic acid/polyaniline/exfoliated graphite nanoplatelets form-stable phase change materials, Appl. Energ. 115 (2014) 603–609. [29] K. Kalaitzidou, H. Fukushima, L.T. Drzal, Multifunctional polypropylene composites produced by incorporation of exfoliated graphite nanoplatelets, Carbon 45 (2007) 1446–1452. [30] J.L. Xiang, L.T. Drzal, Investigation of exfoliated graphite nanoplatelets (xGnP) in improving thermal conductivity of paraffin wax-based phase change material, Sol. Energy Mater. Sol. Cells 95 (2011) 1811–1818. [31] Y.B. Tao, C.H. Lin, Y.L. He, Effect of surface active agent on thermal properties of carbonate salt/carbon nanomaterial composite phase change material, Appl. Energ. 156 (2015) 478–489. [32] R. Al-Shannaq, M. Farid, S. Al-Muhtaseb, J. Kurdi, Emulsion stability and cross-linking of PMMA microcapsules containing phase change materials, Sol. Energy Mater. Sol. Cells 132 (2015) 311–318. [33] B.E. Jebasingh, Exfoliation of graphite by solar irradiation and investigate their thermal property on capric–myristic–palmitic acid/exfoliated graphite composite as phase change material (PCM) for energy storage, J. Energy Storage 5 (2016) 70–76. [34] S. Shaikh, K. Lafdi, K. Hallinan, Carbon nanoadditives to enhance latent energy storage of phase change materials, J. Appl. Phys. 103 (2008).