| With the large consumption of fossil energy and the increasingly serious problems of environment around the world,people are more and more eager for sustainable energy and clean energy.By virtue of the advantages of low energy consumption and zeros emissions,electric vehicles have attracted more and more attention from government and automobile manufactures.Power batteries are the core energy storage components of electric vehicles,which will continuously heating up during operation.If the heat can not be released in time,it will cause capacity degradation,shorten the life span of batteries,and even cause fire in serious cases.In order to improve the performance,life span and thermal safety of the battery packs,a phase change material thermal management system coupled with liquid cooling is designed for battery packs,and the numerical simulation and optimization are carried out.Phase change material thermal management system is the most promising battery temperature control technology at present.By absorbing or releasing a huge amount of latent heat during phase change period,this system can keep the battery pack near the phase change temperature for a long time.By mixin g the phase change material with additives,the composite phase change material with high thermal conductivity can be made,and can effectively improve the temperature uniformity and heat dissipation efficiency of the battery pack.The liquid cooling therm al management system has the highest cooling efficiency at present.The heat generated by the battery can be efficiently removed by the flow of coolant with high specific heat capacity.This kind of system has been widely used in the market.This study com bines these two kinds of thermal management methods to give full play to their respective advantages and make up for each other's shortcomings.Firstly,the thermophysical properties of cylindrical lithium batteries and composite phase change materials are studied.By simplifying the model,the expression formula of equivalent thermophysical parameters for cylindrical lithium batteries is obtained,and the anisotropic thermal conductivity based on cylindrical coordinate system is obtained according to the thermal resistance series-parallel model.By putting forward hypotheses,the expression formula of thermophysical parameters for different composite phase change materials are obtained,which provides an important basis for subsequent study.Secondly,the layout of batteries,the layout of liquid cooling pipelines,the selection of composite phase change materials and the starting-up time of the liquid cooling system are studied.A thermal management system consisting of twenty-four 18650 NCM lithium batteries is designed.A novel control strategy of active liquid cooling system is proposed,which only start-up at the charging stage.This strategy can maintain the best temperature uniformity of the battery pack during the driving stage,and would not consume additional energy to power the thermal management system.Thirdly,the numerical method for calculating the lithium battery's heat generation rate is summarized,and the heat generation rate of 1.5Ah 18650 NCM lithium battery at 5C charge-discharge rate is obtained.Then,the finite element model of the thermal management system is established.The initial conditions and boundary conditions are established for the numerical simulation.Finally,according to the simulation results,the influences of coolan t flow rate,mixing ratio and density of composite phase change materials on the thermal management performance are investigated.Then,the advantages and di sadvantages of the thermal management system are analyzed.On the premise that the maximum temperature and temperature difference of battery pack do not exceed the allowable value,the system is optimized based on lightweight design and energy-saving design. |
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