| In the face of increasing energy and environmental pressure,the development of new energy vehicles has become one of the important ways to relieve the pressure,among which electric vehicles have ushered in a research and development boom.As the power source of electric vehicles,the life and safety of the power battery determine the safety performance of the vehicle to a large extent.Timely heat dissipation of the battery pack can effectively improve its safety,so the thermal management system for battery has become the focus of research and development.In this paper,the battery thermal management system and the composite heat transfer of phase change materials(PCM)were studied for the electric vehicle power battery.The main research contents are as follows:1.For 18650 lithium-ion batteries,the heat generation power density model of battery cell at 1C~5C discharge rate was established based on Bernardi heat generation rate model,and the heat generation model was established.The average surface temperature of battery cell was calculated by simulation,and the calculation results were compared with test results to confirm the validity of heat generation model.2.Aiming at 5×5 battery module,a single cooling system with PCM was used to simulate and analyze its cooling performance under the condition of single discharge.The results show that the temperature of the battery pack fails to reach the paraffin phase change temperature when the battery is discharged at 1C rate,and the maximum temperature and temperature difference increase slowly.At the end of discharge,the maximum temperature is 309.28 k,and the temperature difference is 1.07k;when the battery is discharged at 2C~5C rate,the temperature reaches the paraffin phase change temperature.Under the endothermic effect of paraffin,the rising trend of the maximum temperature slows down,and the temperature difference drops.At the end of discharge,the maximum temperature is 314.75 k,315.82 k,317.79 k and 320.29 k,and the temperature difference is 0.24 k,0.15 k,0.09 k and 0.18 k,respectively.To sum up,under the 1C~5C discharge rate of single discharge condition,the maximum temperature is below 323 k,and the temperature difference is lower than 5K.The cooling performance of single cooling system with PCM under single discharge condition is good,which can meet the cooling demand of battery pack.3.The cooling effect of a single cooling system with PCM was evaluated under the condition of continuous charge and discharge cycle.The simulation results show that when the battery is charged and discharged at 1C rate,the maximum temperature of the battery starts to rise slowly after reaching the paraffin phase change temperature.After five charging and discharging cycles,the maximum temperature is 317.60 k,and the temperature difference is 2.66 k,which meets the cooling requirements of the battery.When the battery is charged at 1C rate and discharged at 2C~5C rate,the maximum temperature and temperature difference exceed the demand limit in the third cycle and the second cycle,respectively.The maximum temperature is 324.14 k,339.72 k,332.73 k and 345.08 k,and the temperature difference is 6.46 k,9.45 k,6.47 k and 8.52 k,respectively.The maximum temperature increases with the increase of discharge rate in the discharge stage,and decreases slightly in the charge stage.Paraffin lost its cooling capacity because of melting completely,and the phase change cooling system failed,resulting in the maximum temperature and temperature difference exceeding the demand limit.Therefore,it is considered to add liquid cooling structure to form a composite cooling system to improve the cooling performance of the system.4.A composite cooling system combining phase change cooling and liquid cooling was tried.Aiming at the condition of 1C rate charging and 3C rate discharging,the cooling performance of 5 continuous charging and discharging cycles is simulated and analyzed.The simulation results show that the maximum temperature of the battery rises slowly in the period of charge and discharge,and reaches the maximum at the end of the fifth discharge stage,which is 321.43k;the temperature difference decreases at the discharging stage,and rises at the charging stage,and the maximum temperature difference is 1.89 k at the end of the first charging stage.Compared with the single cooling system of PCM,the composite cooling system can well meet the temperature control requirements of the battery under this condition.5.The influence of coolant inlet velocity on the cooling performance of composite cooling system was studied.Under the condition of 1C rate charging and 3C rate discharging,the maximum temperature and temperature difference of battery pack were analyzed when the coolant flow rate increased from 0.1m/s to 0.4m/s.The results show that the maximum temperature is 324.98 k,322.23 k,321.43 k and 321.03 k,and the maximum temperature difference is 3.10 k,1.94 k,1.89 k and 1.85 k,respectively.The increase of the coolant flow rate will reduce the maximum temperature and temperature difference,but the decrease will slow down gradually,and the effect of increasing the coolant flow rate will be less and less obvious.Therefore,in order to reduce the power consumption caused by the coolant flow,a smaller coolant flow rate should be selected as far as possible on the premise that the system can meet the temperature control requirements of the battery pack.6.The cooling plate structure of the system was optimized,and the original series type cooling plate was changed to parallel type.The calculation shows that under the condition of 1C rate charge and 3C rate discharge,the composite cooling system with parallel cooling plate can reduce the maximum temperature of battery pack by 3.11 k and the maximum temperature difference by 1.26 k compared with the original system under the same condition.The optimized cooling plate structure can improve the cooling performance of the system. |
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