Research Of Power Battery Thermal Management System Based On Phase Change Heat Dispassion

In recent years,environmental pollution and energy crisis have attracted worldwide attention in the development of new-energy vehicles,while the key of the new-energy vehicles is the power battery.Lithium ion battery(LIB)is the dominant power battery by now due to its excellent performance.However,LIB still has some safety problems,and there are many battery fire and explosion accidents around the world,which severely restrict the promotion and application of new-energy vehicles.Therefore,the thermal safety problem of LIB has become a hot research topic.The temperature of LIB is the key factor affecting its electrochemical performance and security.So in order to control the temperature of LIB in the optimal working temperature range,it is urgent to study the thermal management system of power battery.In this thesis,the thermal management system of power battery based on phase change heat dissipation is investigated by means of experiment and simulation,and the method of optimization design has been proposed.Firstly,a composite phase change material(PCM)based battery thermal management system(BTMS)is studied in the dynamic cycling of the battery pack,and the composite PCM is made of paraffin and expanded graphite.The experimental results indicate that there are two temperature peaks in the temperature curve of single battery during one charge/discharge cycle,while it disappears in the PCM system for the temperature buffering of PCM.The maximum temperature and temperature difference of the battery pack increase with the increasing of cycle rate both in the natural convection and PCM systems.Then the cooling performance of the PCM system is superior to that of the natural convection system,which is more apparent especially at a high cycling rate.In addition,properly increasing the laying-aside time between each step is beneficial to the cooling performance of the PCM system in the cycle.Besides,the PCM with a phase change temperature of 45? is recommended to be used in the real BTMS for its better performance in the dynamic cycling.Additionally,a composite board has been presented to enhance the heat dissipation capability and heat-insulation capability of the battery pack,which consists of three parts with a sandwich structure,containing a heat conducting shell,an insulation panel and PCM.Then a three-dimension thermal model of LIB in the composite PCM system has been established according to the theory of heat generation of the battery and phase change heat transfer,and four different modes are compared in detail to verify the thermal performance of the composite board under normal operating condition and thermal abuse condition.The numerical results show that the cooling performance of Mode 1(no space)and Mode 2(air gap)are basically the same,indicating that increasing the distance between batteries in enclosed space is useless to enhance the capacity of heat dissipation.Then Mode 3(heat sink board)has a better heat dissipation capability,but its heat-insulation capability is the second worst.Besides,Mode 4(composite board)generally improves the heat dissipation capability and uniformity of the temperature,meanwhile it can enhance the heat-insulation capability of the batterypack to prevent the thermal runaway propagation.Furthermore,increasing the latent heat of PCM can greatly improve the thermal performance of the composite board,and the PCM with a latent heat of 1125 kJ/kg and the phase change temperature between 303.15 K and 323.15 K is recommend to be used in the composite board based BTMS.Finally,the experimental apparatus of the composite board based BTMS was built up according to the previous design,and the reliability of the previous design was further verified by experiment and simulation.A thermo-electrochemical coupled model has been established to analyze the thermal characteristics of the battery and the thermal performance of the composite board.The results indicate that the heat dissipation capability of the BTMS with graphite film and composite board is the best,especially at a higher discharge current rate,the BTMS with graphite film alone takes the second place and the case with no BTMS shows the worst.In addition,the thermo-electrochemical model can accurately predict the single battery voltage during the whole discharge process within 3%of the deviation range,and it has sufficient accuracy to predict the maximum temperature of batteries with different BTMS.The composite board can effectively enhance the heat-insulating capability of the battery under thermal abuse condition,and the results of the thermal model are well consistent with the experimental data.In summary,the heat generation of battery in the dynamic cycling,the heat transfer process and key factors of the PCM based BTMS,and performance of the composite board have been investigated by experimental and numerical methods in this thesis.And the research methods and results can provide some theoretical guidance and reference for the design of the real power BTMS.