Research On Phase Change Material Based Thermal Management System For Cylindrical Lithium Ion Battery

Due to the dependence on fossil fuels in the development of human society,countries around the world are currently facing a series of environmental problems such as energy shortages,global warming,atmospheric pollution and acid rain.Moreover,the electrochemical energy storage power station and the new energy automobile industry have developed rapidly.Lithium-ion battery has attracted much attention as the most ideal power source at present,but its shortcomings of thermal stability restricted its further development and application.Therefore,it is very crucial to develop an efficient,low-cost,low-weight battery thermal management system to control battery temperature and temperature uniformity within the optimal range.The phase change material based thermal management system has the advantages of simple structure,strong shape adaptability and high latent heat,compared with air cooling,liquid cooling and heat pipe based thermal management systems.It can not only improve the temperature uniformity of battery pack during normal operation,but also can be used to preventing thermal runaway propagation.This paper combines two research methods of experiment and numerical simulation.And the influence of phase change temperature,discharge rate,battery arrangement,battery spacing,convection heat transfer coefficient,expanded graphite mass fraction on the paraffin/expanded graphite composite phase change material based battery thermal management system is studied.Finally,an optimization method which coupling optimization algorithm and numerical model are proposed to optimize the design of the phase change material based thermal management system.The research work in this paper can provide references for the practical application of lithium-ion battery thermal management.Since there is no final conclusion on the choice of phase transition temperature and there is currently no general model that can link the paraffin phase transition temperature to other thermophysical parameters.Firstly,the paraffin/expanded graphite composite phase change material based thermal management system is prepared.The effect of different discharge rates on the thermal management performance of the system is analyzed.And the heat dissipation performance of the natural convection system and different phase change material based thermal management systems are compared.The experimental results show that,the addition of phase change materials can effectively improve the heat dissipation performance of the system.And the higher the discharge rate,the more obvious the improvement effect.The greater the discharge rate of the battery,the greater the maximum temperature of the battery and the maximum temperature difference between the batteries.But after the solid-liquid phase change occurs,the difference between different rates will decrease.Due to the influence of phase change process,the maximum temperature difference between the batteries in the phase change material based thermal management system does not necessarily appear at the end of discharge.For the phase change material based thermal management system studied in this paper,the optimal phase change temperature is 48℃.When designing a phase change material based thermal management system,in addition to selecting the most suitable phase change temperature,the selection of other parameters is also important.Only the appropriate parameter combination can fully bring out the advantages of the phase change material based thermal management system.Because the current research on the phase change material based thermal management system mainly focuses on the improvement of the thermal conductivity of the phase change material and the combination with other thermal management systems.And the effect of the group structure on the thermal performance of the phase change material based thermal management system is rarely studied.Additionally,an electrochemical-thermal coupling model is developed based on COMSOL Multiphysics.And the effect of battery arrangement and battery spacing on the temperature distribution of the system is discussed numerically to optimize the design of the group structure.The simulation results show that for rectangular phase change material based thermal management systems,the parallel arrangement is superior to the non-parallel arrangement,due to higher utilization of phase change materials and better heat dissipation performance.During the discharge process,the maximum temperature difference curve in the phase change material based thermal management system will show some or all of the "rising-falling-rising-accelerating rising" trend.The falling and second rising trend are caused by the phase transition process in different regions of the system,and the accelerated rising is caused by the lack of latent heat.For the cylindrical battery phase change material based thermal management system studied in this paper,the optimal value of the cell spacing is between 4 mm and 5 mm.In order to achieve a balance between the thermal management performance and the module energy density.Finally,an optimization method which combines optimization algorithm and numerical model is proposed.It can be divided into two steps:single-parameter optimization and multi-parameter optimization.The research results show that increasing the external convection heat transfer coefficient can reduce the maximum temperature of the battery but the effect is not obvious,while reducing the external convection heat transfer coefficient can significantly improve the system temperature uniformity.If the expanded graphite mas fraction exceeds the optimal value,it may negatively affect the thermal management performance.And this problem can be improved by appropriately increasing the battery spacing.The single-parameter optimization results of battery spacing,convection heat transfer coefficient and expanded graphite mass fraction are 10 mm,0 W/(m2·K),and 10%,respectively;while the multi-parameter optimization results are 4.4 mm,0.8 W/(m2·K)and 10.9%,respectively.After multi-parameter optimization,the volume of the phase change material is reduced by 18.4%,and the maximum battery temperature is reduced by 2.2℃,which proves that the proposed optimization method is effective.