expanded graphite composite phase change thermal energy storage material

expanded graphite composite phase change thermal energy storage material

Energy Conversion and Management 47 (2006) 303–310 www.elsevier.com/locate/enconman Study on paraffin/expanded graphite composite phase change thermal ...

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Energy Conversion and Management 47 (2006) 303–310 www.elsevier.com/locate/enconman

Study on paraffin/expanded graphite composite phase change thermal energy storage material Zhengguo Zhang *, Xiaoming Fang The Key Lab of Enhanced Heat Transfer and Energy Conservation, Ministry of Education, Chemical Engineering College, South China University of Technology, Guangzhou 510640, PR China Received 19 April 2004; received in revised form 1 September 2004; accepted 14 March 2005 Available online 26 September 2005

Abstract A paraffin/expanded graphite composite phase change thermal energy storage material was prepared by absorbing the paraffin into an expanded graphite that has an excellent absorbability. In such a composite, the paraffin serves as a latent heat storage material and the expanded graphite acts as the supporting material, which prevents leakage of the melted paraffin from its porous structure due to the capillary and surface tension forces. The inherent structure of the expanded graphite did not change in the composite material. The solid–liquid phase change temperature of the composite PCM was the same as that of the paraffin, and the latent heat of the paraffin/expanded graphite composite material was equivalent to the calculated value based on the mass ratio of the paraffin in the composite. The heat transfer rate of the paraffin/expanded graphite composite was obviously higher than that of the paraffin due to the combination with the expanded graphite that had a high thermal conductivity. The prepared paraffin/expanded graphite composite phase change material had a large thermal storage capacity and improved thermal conductivity and did not experience liquid leakage during its solid–liquid phase change. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Phase change material; Thermal energy storage; Paraffin; Expanded graphite

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Corresponding author. Tel.: +86 20 87112997; fax: +86 20 87113735. E-mail address: [email protected] (Z. Zhang).

0196-8904/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2005.03.004

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1. Introduction Thermal energy storage plays an important role in an effective use of thermal energy and has applications in diverse areas, such as building heating/cooling systems, solar energy collectors, power and industrial waste heat recovery [1]. Among several thermal energy storage techniques, latent thermal energy storage is a particularly attractive technique that provides a high storage capacity per unit mass (and also per unit volume generally) and has the property of storing heat as the latent heat of fusion at a constant temperature, i.e. the phase change temperature. Phase change materials (PCMs) that are used as storage media in latent thermal energy storage can be classified into two major categories: inorganic compounds and organic compounds. Inorganic PCMs include salt hydrates, salts, metals and alloys, whereas organic PCMs are comprised of paraffin, fatty acids/esters and polyalcohols. Paraffin is taken as the most promising phase change material because it has a large latent heat and low cost and is stable, nontoxic and not corrosive [2]. However, paraffin waxes suffer from a low thermal conductivity (0.24 W m 1 K 1) and liquid leakage when they undergo the solid–liquid phase change. These drawbacks reduce the rate of heat storage and extraction during the melting and solidification cycles and restrict their wide applications, respectively. Presently, two methods are offered to enhance the thermal conductivity of the paraffin wax. One is to insert a metal matrix into the paraffin wax [3], and the other is to manufacture the latent thermal energy storage system with a fin configuration for the storage tubes [4]. Nevertheless, such metal structures lead to an increase in the weight and volume of the storage system. In order to prevent the liquid leakage during the solid–liquid phase change of the paraffin, a general solution is to encase the paraffin in spherical capsules or microcapsules, whose shells are usually made of polymers [5,6]. Another method is to prepare a polyethylene–paraffin compound as a form-stable, solid–liquid phase change material [7]. Although the encapsulation and the preparation of the form-stable PCMs can effectively prevent the liquid leakage, the polymer shell or the polymeric supporting material leads to a reduced thermal conductivity due to their low thermal conductivity coefficients. Therefore, it is of importance to prepare a novel composite phase change material with enhanced heat conductivity and without liquid leakage during its phase change. In this paper, a paraffin/expanded graphite composite phase change material with a good thermal conductivity was prepared by absorbing liquid paraffin into the pores of expanded graphite that has an excellent absorbability and a good thermal conductivity. The structure and properties of the prepared composite phase change material have been characterized.

2. Experimental 2.1. Materials Technical grade paraffin (melting point Tm = 48–50 °C) was used without further treatment. Expandable graphite (average particle size: 300 lm, expansion ratio: 200 ml/g) was supplied by the Qingdao Graphite Co. Ltd.

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2.2. Preparation of expanded graphite Firstly, the expandable graphite was dried in a vacuum oven at 65 °C for 16 h. Then, a steel crucible containing about 3–4 g of the expandable graphite was put into a muffle furnace, and a subsequent heat treatment was held at 800 °C for 40 s. The expansion occurred during the heat treatment. 2.3. Preparation of paraffin/expanded graphite composite PCM The paraffin was melted by heating it at 65 °C, and then, the expanded graphite was mixed into the liquid paraffin. After being filtered and dried, the paraffin/expanded graphite composite phase change material was obtained. 2.4. Characterizations of expanded graphite and paraffin/expanded graphite composite PCM The Brunauer–Emmet–Teller (BET) surface area and pore size distribution of the expanded graphite was measured using an auto N2 absorption instrument (ASAP 2010, Micrometrics, USA). The microstructures of the expanded graphite and the composite PCM were observed using a scanning electron microscope (SEM, Hitachi E1010). The density of the composite PCM was measured by a pycnometer method that was usually used for density measurements. The phase change temperature and latent heat of the composite PCM were measured using a differential scanning calorimeter (DSC2910, Texas Instrument Inc., USA) using N2, and the rate of raising the temperature was 5 °C/min. 2.5. Test of thermal storage performance Seven grams of the pure paraffin and 7 g of the paraffin/expanded graphite composite PCM were put into two glass test tubes with the same shell thickness and diameter, respectively. Two thermocouples were placed in the centers of the two test tubes, respectively. The two test tubes were put into a water bath at a constant temperature of 29 °C. After the temperature reached a balance, the two tubes were rapidly placed into another water bath at a constant temperature of 65 °C, where the paraffin and the composite PCM performed heat storage. After the heat storage was finished, the two tubes were put back into the water bath at 29 °C, where the paraffin and the composite PCM performed heat extraction. The temperature variations of the paraffin and the composite PCM during their heat storage and extraction processes were automatically monitored using a data acquisition/switch unit (Agilent 34970A) with a temperature measuring accuracy of ±0.1 °C.

3. Results and discussion 3.1. Pore size distribution and absorbability of expanded graphite In order to investigate the absorbability of the prepared expanded graphite, its pore size distribution and BET surface area were measured. The measurement results showed that the

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BET surface area of the expanded graphite was 21.564 m2/g. The pore size distribution of the expanded graphite is shown in Fig. 1. The prepared expanded graphite had a wide pore size distribution, which mainly consisted of mesopores and macropores. It is known that organic compounds are easily absorbed into mesopores and macropores of expanded graphites [8]. Therefore, the prepared expanded graphite had a high sorption capacity and can be used as a good matrix for expanded graphite based composites. Fig. 2 illustrates the amount of paraffin absorbed per gram of the expanded graphite at 65 °C. From the beginning of the sorption process to 300 s of the sorption time, the paraffin was very rapidly absorbed by the expanded graphite, and its mass ratio in the composite dramatically increased to 83%. In the range between 300 s and 4000 s, the sorption rate was still rapid. When the sorption time exceeded 4000 s, the mass ratio of the paraffin in the composite did not significantly increase with sorption time, suggesting that the sorption was close to a balance. In this paper, 3600 s was selected as the sorption time to prepare the composite PCM, in which there was 85.6 wt.% paraffin. 3.2. Microstructures of expanded graphite and composite PCM Fig. 3 shows the SEM micrographs of the expanded graphite and the composite PCM. It can be clearly seen from Fig. 3(a) that the expanded graphite has a worm-like appearance of its particles. As shown in Fig. 3(b), after the paraffin had been absorbed into the pores of the expanded graphite, the expanded graphite remained in the worm-like structure, and the absorbed paraffin exhibited a uniform distribution in the paraffin/expanded graphite composite PCM owing to the capillary force and the surface tension force of the porous expanded graphite. It had been measured by the pycnometer method that the density of the prepared paraffin/ expanded graphite composite PCM (85.6 wt.% paraffin) was 715.7 kg/m3 at 25 °C, which was less than that of the pure solid paraffin (891.2 kg/m3). The pycnometer method has an accuracy of ±2%. It can be assumed that a thermal storage unit where the paraffin/expanded graphite composite PCM was used as the storage medium will have less weight than a thermal storage unit

Fig. 1. Pore size distribution of the expanded graphite.

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Fig. 2. The amount of paraffin absorbed per gram of expanded graphite.

Fig. 3. SEM micrographs of the expanded graphite and the paraffin/expanded graphite composite PCM: (a) Expanded graphite; (b) paraffin/expanded graphite composite PCM.

based on pure paraffin as the storage medium when the two thermal storage units have the same volume.

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3.3. Thermal characteristics of composite PCM Fig. 4 shows the typical DSC curves of the pure paraffin used in the preparation of the composite PCM. These DSC curves present reference data to evaluate the changes in the thermal properties of the composite PCM, depending on the amount of paraffin. It can be seen from Fig. 4(a) 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 = 48.79 °C), and the minor peak at the left side of the main peak corresponds to the solid–solid phase transition of paraffin (transition temperature Tt = 30.81 °C). The DSC curve of the composite PCM including 85.6 wt.% paraffin was shown in Fig. 4(b). There are also two peaks (Tm = 48.93 °C, Tt = 31.85 °C) in the DSC curve of the composite PCM, and the thermal characteristics of the composite PCM are very close to those of the pure paraffin. This is because there is no chemical

Fig. 4. DSC curves of the paraffin and the paraffin/expanded graphite composite PCM: (a) Paraffin; (b) paraffin/ expanded graphite composite PCM.

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reaction between the paraffin and the expanded graphite in the preparation of the composite PCM. The latent heat 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. As shown in Fig. 4(a), the latent heat of the solid–solid and solid–liquid transition are 39.59 J/g and 149.1 J/g, respectively. So, the total latent heat of the pure paraffin is 188.69 J/g. As shown in Fig. 4(b), the latent heat of the solid–solid and solid–liquid transition are 46.55 J/g and 114.9 J/g, respectively. The total latent heat of the composite PCM is 161.45 J/g, that is, 85.56% as large 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 (85.6%). 3.4. Comparison on thermal storage performance of composite PCM with that of paraffin The heat storage and release curves of the paraffin and the composite PCM are shown in Fig. 5. From the beginning of the storage performance test to 140 s of the test time, the temperatures of the paraffin and the composite PCM are the same, 28.5 °C. As the test time increased further, the temperatures of the paraffin and the composite PCM went up, and the phase change from solid to liquid (thermal storage process) occurred. Until the temperatures of the paraffin and the composite PCM reached balance at 65 °C, it took 1040 s for the paraffin, whereas, it took only 760 s for the composite PCM to reach temperature balance, which was reduced 27.4% compared with that for the paraffin. It was obvious that the heat storage rate of the composite PCM was higher than that of the paraffin. It can also be seen from Fig. 5 that after the heat storage finished, it took 500 s for the paraffin to drop its temperature from 65 °C to 29 °C (heat release process) and only 240 s for the composite PCM, indicating that the time for the composite PCM was reduced 56.4% compared with that for the paraffin. It suggests that the heat release rate of the composite PCM was much higher than that of the paraffin. Therefore, it was concluded that the heat transfer rate of the composite PCM was obviously higher than that of the pure paraffin, which is due to the combination of the paraffin with the expanded graphite that had a high thermal conductivity. Moreover, because the heat transfer rate in the heat storage process was controlled by natural convection, whereas in the heat release process, it was controlled by thermal conduction, the increase in the conductivity coefficient of the PCM had a more significant enhancement effect on the heat transfer in the heat release process than in the heat storage process. Therefore,

Fig. 5. Heat storage and release curves of the paraffin and the composite PCM.

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the reduced time for the composite PCM in the heat release process was more than in the heat storage process. In addition, it was observed that liquid leakage did not occur while the composite PCM was performing the phase change from solid to liquid, owing to the fact that the paraffin was absorbed into the pores of the expanded graphite and held by the capillary force and the surface tension force of the porous expanded graphite.

4. Conclusions The prepared expanded graphite had a wide pore size distribution that mainly consisted of mesopores and macropores and possessed a high sorption capacity for the paraffin. The expanded graphite remained in a worm-like structure in the paraffin/expanded graphite composite PCM. The use of the composite PCM as a storage medium can reduce the weight of the thermal storage unit. The latent heat of the paraffin/expanded graphite composite PCM is equivalent to the calculated value based on the mass ratio of the paraffin in the composite. Liquid leakage did not occur during the solid–liquid phase change of the composite PCM. The heat transfer rate of the composite PCM was obviously higher than that of the pure paraffin owing to the combination with the expanded graphite that had a high thermal conductivity.

Acknowledgement This work was supported by the Science and Technology Project of Guangdong Province.

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