| Power battery is the core component of electric vehicle,and its thermal safety has been concerned by the society.The advanced battery thermal management system can export the heat dissipated by the power battery in time,prevent the battery from dangerous conditions of heat accumulation and thermal runaway,prolong the battery life and ensure the safety of electric vehicles.In this paper,the application and theoretical research are carried out according to the different temperature control requirements of square battery and cylindrical battery,single battery and battery pack.A variety of high-performance composite phase change materials(CPCM)are prepared,and the reliability experiment and simulation analysis of battery thermal management system are carried out combined with liquid cooling technology.The main work in this paper is as follows:This paper analyzes the charge and discharge principle of lithium-ion battery,states the equation of electrochemical reaction process and the mechanism of battery heat generation and heat transfer,lists the calculation methods of thermophysical properties of battery and CPCM involved in battery thermal management system,and gives a number of numerical calculation control equations used in the coupling process of CPCM and liquid cooling,such as the corresponding energy,mass and momentum conservation equations of battery,CPCM and liquid cooling.CPCM,air cooling and liquid cooling control group,liquid cooling / CPCM coupling battery cooling module are designed,and UFD software temperature control simulation is carried out.It is found that under the condition of bare cell discharge,the battery pack is only suitable for working at low discharge rate,while the CPCM thermal management system has strong temperature control ability,light weight and good uniformity,but it can not meet the temperature control requirements of battery under high discharge rate.In the control group,the cooling effect of liquid cooling is better than that of flowing air,but the cooling uniformity of single liquid cooling system is poor.The coupling of liquid cooling and CPCM can combine the advantages of the two heat management methods,and the battery pack heat dissipation scheme using multiple curved pipes for flow liquid cooling and phase change material coupling heat exchange has a good temperature control effect.Aiming at the composite materials required in phase change materials and liquid cooling systems,paraffin wax(PW)/high density polyethylene(HDPE)/expanded graphite(EG)/nano-silver(NANO-AG),stearic acid-Palmitic acid-lauric acid(SA-PA-LA)/EG/carbon fiber(CF),PW/EG/copper powder(Cu)/silica gel(SR)and binary thermal conductive silicone grease,a total of four composite materials.Among them,two kinds of materials,PW/HDPE/EG/NANO-AG and SA-PA-LA/EG/CF,which are formed by hot pressing,are used for thermal management of square batteries.These two CPCMs have high and low melting points respectively and are suitable for different Battery temperature rise control for rate discharge.PW/EG/Cu/SR CPCM has good fluidity before molding,which is suitable for the application of cylindrical battery pack thermal management system with complex structure.Binary thermal grease can be used to improve the problems of large gaps,large interface heat transfer thermal resistance,and low heat dissipation efficiency in the battery thermal management system.After material characterization,it was found that the prepared PW/HDPE/EG/NANO-AG had a good degree of crystallization,the PW leakage rate in the CPCM after the test was lower than that of the PW/EG CPCM,and the thermal conductivity was also significantly improved;The eutectic mass ratios of the prepared SA-PA-LA ternary phase change materials were 29.6:7.4:63,respectively;the combination of the components of SA-PA-LA/EG/CF was physical adsorption.When the CF content in SA-PA-LA/EG/CF is 5%and the mass ratio of SA-PA-LA to EG is 91:9,SA-PA-LA/EG/CF has both low leakage rate and high thermal conductivity;The prepared PW/EG/Cu/SR CPCM has the characteristics of good fluidity and stable shape before and after molding.The addition of SR in CPCM can improve the anti-leakage ability of CPCM,and the heat absorption and release rate of PW/EG/Cu/SR is obviously better than that of PW/EG CPCM.In this paper,the thermal conductive silicone grease with three kinds of mixed fillers(AIN/Cu,AIN/CF and Cu/CF)was prepared.The thermal conductivity and volume resistivity of the thermal conductive silicone grease mixed with Cu/CF filler were the highest and the friction and wear of the thermal conductive silicone grease containing aluminum nitride(AIN)/CF on the interface were the lowest.In order to reduce the temperature rise of the battery and test the temperature control effect of the double-layer CPCM combined with high and low melting points,the charge and discharge experiments of the single battery under different working conditions were carried out.It is found that compared with the temperature control without phase change materials,the single-layer PW/HDPE/EG/nano-Ag CPCM has obvious temperature control effect,but the double-layer CPCM(PW/HDPE/EG/nano-Ag and SA-PA-LA/EG/CF)can further reduce the temperature rise of the battery and better control the battery temperature and temperature difference.After adding aluminum honeycomb into the double-layer CPCM,the double-layer CPCM will have better thermal conductivity Mechanical properties and cyclic temperature control characteristics.When PW/HDPE/EG/nano-Ag CPCM controls the temperature of the square battery pack,it can not meet the continuous charge and discharge demand of the battery with high rate.After the liquid cooling pipe is embedded in the CPCM,the temperature and temperature difference of the battery are significantly improved;Different liquid cooling pipe layout has an impact on the temperature control effect of the battery.When the number of cold water inlet and outlet is 4,the temperature of the battery module is low and the temperature difference is small;Increasing the power of the refrigeration compressor and increasing the flow of the variable displacement pump can reduce the overall temperature of the battery module,but the cooling effect of changing the compressor is better than the latter method,the cooling effect of the battery is the best when both the compressor and the variable displacement pump are started at the same time during high-intensity continuous charge and discharge.When studying the thermal management of cylindrical batteries,increasing the battery spacing can reduce the maximum temperature of batteries,and increasing the thermal conductivity of CPCM can balance the temperature difference between batteries;Changing the position of inlet and outlet of liquid cooling pipeline will greatly affect the temperature difference of battery,and changing the ambient temperature of battery pack will also affect the results of battery thermal management;The pipe cooling structure of 3 inlet / 3 outlet coupled by CPCM and liquid cooling meets the multiple charging cycle conditions of 1C discharge-1C charging,2C discharge-1C charging and 3C discharge-1C charging,and can control the battery temperature within 45 ℃. |
