OSCILLATING HEAT PIPE COUPLED WITH PHASE CHANGE MATERIAL (OHP/PCM) USED FOR BATTERY THERMAL MANAGEMENT

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1 Proceedings of the Asian Conference on Thermal Sciences 217, 1st ACTS March 26-3, 217, Jeju Island, Korea ACTS-23 OSCILLATING HEAT PIPE COUPLED WITH PHASE CHANGE MATERIAL () USED FOR BATTERY THERMAL MANAGEMENT Zhonghao Rao 1*, Jiateng Zhao 1 ACTS-P23 1 China University of Mining and Technology, No.1,Daxue Road, Xuzhou , China * Presenting and Corresponding Author: raozhonghao@cumt.edu.cn ABSTRACT In order to enhance the thermal performance of battery thermal management (BTM) system within electric vehicle (EV), a novel method for coupling oscillating heat pipe (OHP) and phase change materials (PCM) was proposed in this work. The aim for the PCM coupled with OHP is to combine the high thermal conductivity of OHP with the high latent heat capacity of PCM, where the PCM can serve as a thermal buffer or thermal container and the OHP can transport the heat to the buffer or from the buffer to the place where necessary duly. The experimental platform was constructed and the coupled thermal management system were tested under different conditions, including different heating powers, charge and discharge. The results mainly showed that the temperature difference between the heating section and PCM in the copper/pcm case is about 6 times larger than the case. The maximum temperature difference of the PCM reaches to about 16ºC after about 4s which is 8 times larger than that of the module approximately. The OHP in the coupling module could continue oscillating for a long time in the discharge process. The heat release rate of module is faster than that of the copper/pcm module and the duration can be reduced by about 1 minutes as the PCM temperature reduces from 63 ºC to 33 ºC KEYWORDS: Oscillation heat pipe, Phase change material, Coupled, Battery thermal management 1. INTRODUCTION With the continuous progress of society and economy, energy and environmental problems have become increasingly prominent. Energy saving and emission reduction has become increasingly urgent. The development of new energy vehicles is a promising direction. The development of new energy vehicles can reduce fossil energy use and promote the development of new energy, thereby reducing pollutant emissions. Being one of the four core technologies in electric vehicle, battery thermal management become the focus and hotspot in recent years, which can be divided into three parts according to the medium, including air-based cooling, liquid-based cooling and phase change materials (PCM) based cooling [1, 2]. The potential of PCM used in thermal management is tremendous. PCM acted as heat buffer mainly storing latent heat. However, PCM has the limitation of low thermal conductivity, so the heat absorbed in it is hard to be transmitted outside, which greatly limits the application of PCM[3]. In order to solve this problem, Fin or fin coupled with heat pipe are now generally utilized in many devices to transfer heat out. As a kind of good heat transfer media, oscillation heat pipe (OHP) could also compensate the disadvantage of low thermal conductivity for PCM and connect PCM with the outside. Comparing to traditional heat pipe, OHP has the advantage of low cost, large heat transfer limit, more superior performance, simple design and small size. In order to understand the characteristic of coupling structure, experimental investigation on the heat storage and release process of the coupling module under different conditions is carried out in this paper. 2. EXPERIMENTAL PROCESS AND RESULTS 2.1 EXPERIMENTAL DETAILS In this experiment, the test platform for the coupling module has been improved on the basis of previous work. Fig.1shows the photograph of experimental system, the schematic of a coupling module and the measurement points. The system includes an OHP, an energy storage tank filled with PCM, the heating system (heating wire and a DC power supply), the cooling system (low-constant temperature bath, a water-cooled block) and the data acquisition system (a computer, AGILENT acquisition/switch unit and OMEGA K-type thermocouples). The measuring points includes the temperature of the evaporation section (T1~T6), the temperature of the condensation section (T7~ T12), the inner temperature of the PCM in 1

2 energy storage tank (T13~T18) and the temperature of cooling water (T19 and T2). Thermal insulation of the whole experimental system is carried out by covering ceramic fiber cotton inside and aluminum foil outside. Heating Wire T3 C T19 A B T6 T9 PCM T2 T12 T5 T8 T11 T1 T4 EG/paraffin 7% T7 T1 T2 T13 T14 Clip plane: A T15 T16 B T17 T18 C Fig. 1 The photograph of experimental system and the schematic of a coupling module and the measurement points The structure, the dimension, filling ratio, working medium, the production process and method of the three-turn OHP utilized in this experiment and the thermal performance of the OHP can be seen in our previous work[4, 5]. The thermal resistance of the OHP and the DSC testing curve of EG/paraffin composite can be seen in Fig.2 and, respectively. The detail operation condition of the OHP-PCM coupling module can be seen in Table 1.The temperature of PCM in the energy storage tank can be simplified as the arithmetic mean of the six measuring points in the storage tank, which can be described as: =. The average temperature of heating section and cooling section are = and =, respectively. The ( ).The maximum temperature difference among the = heat discharge from the PCM can be calculated as: measuring points can be calculated as the formula, =, which is utilized to reflected the temperature uniformity of PCM. 15W 3W W Thermal Resistance( C W ) m=65.6 ºC.9 2. DSC/J g H=142.67J/g ( Temperature/ºC Fig. 2 Thermal resistances of the CLOHP under different heating powers and angles and DSC test of paraffin/eg composite[4] Table 1 Operation condition of the coupling module. Parameters Heating power Water flow rate Angle (α) PCM Charge None 15W/3W/W 9 EG/Paraffin Discharge W 32L/min 9 2

3 2.2 RESULTS AND DISCUSSION In the charge process, the tube embedded in the energy storage tank acts as the role of cooling region, where the heat absorbed can be stored in the PCM. Fig.3 shows the temperature variation of PCM ( ), heating section and cooling section for two different modules in the charge process which contain OHP and copper line, respectively. It can be seen from Fig.3 that the OHP can work continuously with the increase of PCM temperature. The temperature difference between the heating section and PCM became more stable, about 12ºC. As the OHP is replaced by copper line with the same shape, the experiment result shown in Fig.3 presents that the temperature gradient from the heating section to the cooling section of the cooper line is much larger than that of OHP. What is also obvious is that the temperature difference between the heating section and PCM is about 6 times larger than the former case. Pressure difference and temperature difference are the main driving force of heat transfer for the two cases, reflected in the temperature oscillation and the large temperature difference, respectively. It can be concluded from Fig.3,(c) and (d) that the heating power and the duration time do not follow a linear correlation, as the average temperature of PCM raise to the same value. When it raising to 63ºC, the duration time are 95s, 365s and 17s as the heating power are 15W, 3W and W, respectively. The temperature difference between the heating section and PCM under power of 15W and W both also turn to the relatively stable state, about 7ºC and 23ºC, respectively. The amplitude of temperature oscillation decreasing with the raise of power can reflects the increase of oscillation frequency indirectly. All the phenomenon shows that the OHP can work well in the coupling module and reduce the working temperature in the powerconstant condition, which may prolong the material life of components W W (c) 15W (d) W Fig.3 Temperature variation of two modules in the charge process Fig.4 exhibits the maximum temperature difference of PCM ( ) for two modules in the charge process when the heating power is 3W. The of the module is quite small and about 2 ºC, which present good temperature uniformity in the PCM. Compared with the case, of the copper/pcm module is much larger at the same time and two obvious rising process appears. It reaches to about 16ºC after about 4s which is 8 times larger than that of the module approximately. The reason might be that there exists large temperature gradient along the axial direction of the copper line, but the temperature of OHP is fluctuant without a stable temperature gradient. In the discharge process, the heat absorbed in the PCM can be transferred outside through the OHP. Fig.5 presents the temperature variation and temperature difference of the coupling module during the discharge process. The OHP in the coupling module could continue oscillating for a long time, which can be seen in the region on the left of the dotted line in Fig.5. With the decreasing of PCM temperature, the pulsation phenomenon in the OHP gradually abates and disappears. The thermal characteristic of OHP, such as start-up temperature and thermal resistance, has a great influence on the heat release performance, for the reason that the temperature decreases in the discharging process, determining the working state of the OHP. The module and the copper/pcm module are also compared in the discharge process. As shown in Fig. 5, the heat release rate of module is faster than that of the copper/pcm module. The duration can be reduced by about

4 minutes for the module when the temperature of PCM reduces from 63 ºC to 33 ºC. It can be deduced that the duration can be further shorten by reducing the start-up temperature and thermal resistance of OHP. 2 3W T pcm / C 8 4 Fig.4 The maximum temperature difference of PCM for two modules in the charge process T2-T Fig. 5 Temperature variation and temperature difference of the coupling module during discharge process 3. CONCLUSIONS The coupling module was contributed in this paper. The expanded graphite/paraffin composite material used in the experiment was prepared and tested, and the charge and discharge characteristic of the coupling module was investigated experimentally under different operating conditions. The copper/pcm module is also tested as contrast. The main conclusions can thus be summarized as follows: (1) The temperature difference between the heating section and PCM in the copper/pcm case about 6 times larger than the case. (2) The of the module is quite small and about 2 ºC in the charge process. Compared with the case, of the copper/pcm module is much larger at the same time. It reaches to about 16ºC after about 4s which is 8 times larger than that of the module approximately. (3) The OHP in the coupling module could continue oscillating for a long time in the discharge process. The heat release rate of module is faster than that of the copper/pcm module and the duration can be reduced by about 1 minutes when the temperature of PCM reduces from 63 ºC to 33 ºC ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No ) NOMENCLATURE Temperature ( ) Temperature difference ( ) Thermal resistance ( /W) H Latent heat (kj/kg) Q Heat (W) k Thermal conductivity (W/(m )) 4

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