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Thermal Properties of Paraffin Wax-based Composites Containing Graphite

Z. H. Raoa; G. Q. Zhanga a Faculty of Materials and Energy, Guangdong University of Technology, Guangzhou, China Online publication date: 13 December 2010

To cite this Article Rao, Z. H. and Zhang, G. Q.(2011) 'Thermal Properties of Paraffin Wax-based Composites Containing

Graphite', Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 33: 7, 587 — 593 To link to this Article: DOI: 10.1080/15567030903117679 URL: http://dx.doi.org/10.1080/15567030903117679

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Energy Sources, Part A, 33:587–593, 2011 Copyright © Taylor & Francis Group, LLC ISSN: 1556-7036 print/1556-7230 online DOI: 10.1080/15567030903117679

Thermal Properties of Paraffin Wax-based Composites Containing Graphite Z. H. RAO1 and G. Q. ZHANG1

Downloaded By: [South China University of Technology] At: 23:44 13 December 2010

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Faculty of Materials and Energy, Guangdong University of Technology, Guangzhou, China Abstract The paraffin/graphite composite phase change materials (PCMs) for battery thermal management system (BTMS) were prepared in thermostatic water bath. X-ray diffraction test shows that there is no chemical reaction between the paraffin and the graphite. The energy storage properties of the paraffin/graphite composite PCMs were characterized by Differential scanning calorimetry. The total latent heat was reduced with the increasing of the mass fraction of graphite. As the PCMs must have high thermal conductivity and be insulated, the proposed mass fraction of graphite is 20 wt% and the total latent heat is 89.6% as great as that of the pure paraffin, which may be the best for BTMS. Keywords battery thermal management system, graphite, paraffin wax, phase change material, thermal properties

Introduction Electric and hybrid electric vehicles are more energy efficient and cleaner than conventional vehicles (Pesaran, 2002). The automotive market demands high specific power and high specific energy density batteries to meet the operational needs of electric vehicles (Khateeb et al., 2004). High temperature has a strong effect on power batteries’ lifetime and eventually can lead to thermal failure. Nickel-metal hydride (NiMH) and lithiumion batteries have demonstrated superior performance compared to other power batteries. However, an excessive or uneven temperature rise in a module or pack significantly reduces its cycle life (Spotnitz et al., 2007). Therefore, a successful BTMS is required. Some traditional battery thermal management system (BTMS), such as air-cooling, make the overall system too bulky, complex, and expensive in terms of blower, fans, pumps, pipes, and other accessories, which add on to the system weight and parasitic power requirements (Mills and Hallaj, 2005; Chang et al., 2008; Harper and Brown, 1921; Bejan and Sciubba, 1992). Previous research on BTMS using a phase change materials (PCM) is a viable solution (Khateeb et al., 2004). A single PCM is not sufficient for high heat fluxes, so much attention has been paid to composite PCM, which exhibits many desirable characteristics, such as high heat of fusion; varied phase change temperature; negligible super-cooling; lower vapor pressure in the melt; chemically inert and stable; self-nucleating; no phase segregation; and commercial availability at reasonable cost (Hasnain, 1998; Peng and Fuchs, 2004; Fang and Zhang, 2006; Liu et al., 2006). In temperatures exceeding 50ıC, in particular, charging efficiency as well as battery life Address correspondence to Prof. Guoqing Zhang, Faculty of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, China. E-mail: [email protected]

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may deteriorate due to the heat (Sato and Yagi, 2000). The paraffin wax with large heat storage capacity was used (melting temperature Tm D 48–50ıC). However, low thermal conductivity is the major drawback of the paraffin wax as PCM, and some methods have been tried to enhance heat transfer (Velraj et al., 1999; Gong and Mujumdar, 1996; Bugaje, 1997), so it is very important to analyze the thermal properties of paraffin waxbased composites. In recent years, some studies were carried out to enhance the thermal conductivity of the paraffin wax. The most often used method is to insert particles with high thermal conductivity, such as metal particles, carbon nanotubes, and so on, into the paraffin (Wang et al., 2009; Fang et al., 2008). Xiao (Zhou et al., 2009) involved the preparation and characterization of form-stable paraffin/porous silica ceramic composite. Ho and Gao (2009) investigated experimentally thermophysical properties of nanoparticle-in-paraffin emulsion. Although these particle materials have high thermal conductivity, the density of them is different with paraffin wax. The density of metal structures is often higher than paraffin wax so that it strongly affects the properties such as materials that cannot be mixed evenly. Moreover, such metal structures lead to an increase in the weight of the BTMS. Carbon nanotubes have low density but increase the cost of the BTMS. Most of the previous literatures only provided the thermal conductivity or surface distribution of the materials (Wang et al., 2009). In particular, the information of thermal properties of a PCM at its phase change temperature is very useful for BTMS applications. The relationship between temperature and the thermal conductivity of the material is an important parameter for selecting a PCM. Furthermore, the studies regarding the internal structure of the PCM and whether there is a chemical reaction between the paraffin and the graphite in the preparations and operating in BTMS are limited in the literature. In this article, the paraffin wax/graphite composite PCM for BTMS was prepared. X-ray diffraction (XRD) experiments were performed directly on the samples. The energy storage properties of the paraffin/graphite composite PCMs were characterized by differential scanning calorimetry (DSC). The structure and thermal properties of the prepared material have been characterized.

Experimental Paraffin wax (industrial grade) with a melting temperature (Tm ) of 48–50ıC was purchased from Shanghai Yonghua Paraffin Co., Ltd., Shanghai, China. The graphite (modified natural graphite, MGS-1) with a good thermal conductivity was supplied by the Hunan Shanshan New Materials Co., Ltd., Changsha, China. Paraffin wax was used without further purification. First, the paraffin wax was melted at the temperature of 65ıC in the thermostatic water bath (501A, Hangzhou ZT Instrument Co., Ltd., Hangzhou, China). Later, the graphite was put into the melted liquid paraffin using a stir disruptor for at least 1 h. After the samples were dried at room temperature, the paraffin/graphite composite PCMs were obtained. The composites were prepared at a different mass fraction of graphite, !p D 7:7, 14.3, 20, and 25 wt%, respectively. XRD experiments were performed directly on the samples using an X-ray (Y-4Q, Dandong-ray Instruments Inc., Dandong, China) diffractometer at a rate of 2ı /min. The phase change temperature and latent heat (LH) of the composite PCMs were measured using a differential scanning calorimeter (DSC2910, Texas Instrument Inc., Dallas, TX) using N2 , calibrated with an indium standard in the range from room temperature to 80ı C, and the rate of raising the temperature was 10ı C/min.


Paraffin Wax-based Composites Containing Graphite

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Figure 1. XRD patterns of pure paraffin.

Results and Discussion Zhang and Fang (2006) have investigated the pore size distribution and surface microstructures of paraffin/expanded graphite composite phase change thermal energy storage material and found that the absorbed paraffin exhibited a uniform distribution in the paraffin/expanded graphite composite material. Figure 1 shows the XRD patterns of pure paraffin, and Figure 2 shows the XRD patterns of paraffin/graphite composite PCMs. The diffraction peak intensity of the composite PCM is evidently stronger than that of the

Figure 2. XRD patterns of paraffin/graphite composite PCMs.


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Figure 3. DSC curves of the pure paraffin.

pure paraffin wax. They have no new peak appearance. The reason is that the graphite itself is just a kind of supporting material; and it means that there is no chemical reaction between the paraffin and the graphite. Figure 3 shows the typical DSC curves of the pure paraffin. These DSC curves present reference data to evaluate the changes in the thermal properties depending on the amount of paraffin. It can be seen that there are two peaks in the DSC curve of the pure paraffin. The sharp or main peak represents the solid–liquid phase change of the paraffin (melting temperature Tm D 49:54ıC), and the minor peak at the left side of the main peak corresponds to the solid–solid phase transition of paraffin (transition temperature Tt D 31:21ıC). The latent heat of the solid–solid (LHS-S) and solid–liquid (LHS-L) transition are 33.29 and 147.1 J/g, respectively. Figure 4 shows the typical DSC curves of the paraffin/graphite composite PCMs (20 wt%). DSC analysis was conducted to investigate the influence of graphite addition on the thermal properties, including the melting temperatures and the latent heat storage

Figure 4. DSC curves of the paraffin/graphite composite PCMs.



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Figure 5. Tm and Tt variation of the composite PCMs.

capacities. The melting temperature is Tm D 49:59ıC, and the transition temperature is Tt D 31:98ı C. The latent heat of the solid–solid (LHS-S) and solid–liquid (LHS-L) transitions are 29.73 and 131.9 J/g, respectively. Figure 5 shows the Tm and Tt variation of the composite PCMs. The Tm and Tt of the composite PCMs are very close to those of the pure paraffin. This is because there is no chemical reaction between the paraffin and the graphite in the preparation of the composite PCMs. Figure 6 shows the LH variation of composite PCMs. The LH of the paraffin is obtained as the total area under the peaks of the solid–solid and solid–liquid transitions of the paraffin in the composite by numerical integration. The total latent heat of the pure paraffin is 180.39 J/g. But the total latent heat of the composite PCM (25 wt%) is 127.82 J/g, that is only 70.9% as large as that of the pure paraffin. The total latent

Figure 6. LH variation of the composite PCMs (LH-Sum D LHS-S C LHS-L).


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Figure 7. Temperature with discharge time (discharge current: 2,981 mA).

heat was reduced with increasing the mass fraction of graphite. The total latent heat of the composite PCM (20 wt%) is 161.63 J/g, which is 89.6% as great as that of the pure paraffin, and it is equivalent to the calculated value by multiplying the latent heat of the dispersed paraffin with its mass fraction. Figure 7 shows the changes of temperature on the surface of a monomer battery (SCNi-MH, 2,200 mAh, 22 mm diameter, 42.5 mm high) during discharge. The temperature of the battery with no cooling system exceeded 55ı C within 8 min. The temperature of the battery using the paraffin wax/graphite composite PCM (20 wt%) for BTMS, as expected, is controlled under 40ıC. It is believed that as the graphite is mixed into the paraffin wax, the thermal conductivity of composite PCMs increases.

Conclusions The paraffin/graphite composite PCMs for BTMS were prepared by a thermostatic water bath. XRD experiments were performed directly on the samples and it was found that there was no chemical reaction between the paraffin and the graphite. The energy storage properties of the paraffin/graphite composite PCMs were characterized by DSC and the result showed that the total latent heat was reduced with increasing the mass fraction of graphite. As the PCMs must have high thermal conductivity and cannot transmit electric current, the mass fraction of graphite, which is approximately 20 wt%, may be the best for BTMS.

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