Research On The Refrigerant-based Direct Cooling Thermal Management And Controlling Characteristics Of Power Battery Fast Charging

With the continuous development of rapid charging of electric vehicles,the characteristics of high battery heat and fast temperature rise in the fast charging process have gradually increased the requirements for battery thermal management technology.Therefore,the research content of this paper is to adopt the direct cooling thermal management system in the fast charging process of power battery,explore the cooperative control relationship of the main control factors in the direct cooling system,and optimize the temperature control strategy.Firstly,the research work starts from the establishment of the simulation model of the thermal management system.Based on the theory of heat transfer and thermodynamics,the independent numerical models of each element in the system are established respectively.The reliability of the simulation model is supported by the experimental verification of typical parameters such as temperature and pressure in the battery and the steam compression refrigeration cycle.The system plane physical coupling model was established by onedimensional simulation platform to realize the cyclic integration of direct cooling and heat management system.Secondly,in view of the choice of fast charging mode,by comparing the advantages and disadvantages of different charging modes,the widely used three-stage constant current and constant voltage charging method is selected.In view of the influence of the main control factors of direct cooling and heating management on the thermal management effect of the battery,the variation of thermal characteristic parameters with the change of compressor speed and condenser fan speed was investigated respectively.The results show that when the compressor speed increases in a linear trend,the change trend of cooling capacity is first fast and then slow.The cooling capacity has a great influence on the temperature difference between the battery modules.The improvement of the condenser fan speed can increase the cooling capacity,but when the speed is increased to meet the cooling capacity of the system,the improvement of the cooling capacity is small,and the impact on the temperature difference between modules is small,but it has a better effect on the system energy consumption than COP(Coefficient of Performance)optimization.The increase of condenser fan speed has obvious effect on reducing compressor power consumption,but has little effect on cooling responsiveness.The influence of the inlet air temperature of the condenser on the cooling capacity and COP of the system is greater at low fan speed and less at high fan speed.For battery charging and discharging process features and thermal management measure difference of thermal simulation,the results show that the discharge process of the compressor speed relative to the charging process of incremental percentage change as charging rate increased linearly,and condenser fan speed as the nonlinear quickly before they are slowly changing trend of curve,and change with the change rate is less than the compressor speed.Then,the synergistic control relationship of the main control factors under different load changing conditions in the process of fast charging was simulated and analyzed.The results show that the power consumption of the compressor was reduced and the power consumption of the condenser fan was increased by adjusting the speed of the condenser fan,and the maximum Energy Efficiency Ratio(EER)existed in the adjustment process.In the process of constant current charging,with the increase of charging ratio,the change rate of compressor speed at a certain temperature level is faster than that of charging ratio,and the change rate of condenser fan speed is greater than that of charging ratio,but less than that of compressor speed.The change of rotation speed is nonlinear.In the case of linear variable current charging,the rate of change of compressor speed is related to the range of change of charging current and the rate of change of current.The rate of change of compressor speed has a linear relationship with the rate of change of current,and its slope gradually increases with the increase of charging range,and the rate of increase gradually accelerates.The slope of the characteristic curve of condenser fan speed change rate is linear.The variation law of rotational speed was fitted mathematically.Simulation analysis was carried out on the speed following rule of compressor and condenser fan during the nonlinear variable current charging process,and the current change rate was tracked with the fitting relationship.The results showed that the fitting relationship could control the battery temperature level,and EER met the optimal value during the changing process of charging.Finally,a comprehensive temperature control strategy based on temperature threshold and charging current change rate was proposed for the temperature control method in the process of rapid charging.The results show that the time required for the battery to reach the equilibrium temperature can be adjusted by adjusting the gain coefficient when the battery is cooled at different temperatures and charging ratios.And the gain coefficient is improved.Under constant current charging condition,the strategy of constant asynchronous gain control for condenser fan and compressor speed can save 13.4% energy consumption.In the case of nonlinear variable current charging,the variable step gain strategy can save 16.6% energy consumption.