| With no exhaust emissions and excellent acceleration capacity,battery electric cars have become a viable scheme for the planet to get rid of the dilemma of energy and pollution.However,the thermal safety of power batteries has been a bottleneck for the further development of EVs.In this paper,for the lithium iron phosphate cylindrical battery commonly used in passenger car,a kind of composite phase change material(PCM)coupled with mini-channel cooling liquid battery heat dissipation structure of compact was designed.The performance of the structure was predicted by numerical simulations with the supplement of physical experiments.The key work of this paper including:(1)In order to establish the model of heat generating for single cell,the theory of monomer battery volume heat production formula was got by the analysis of the chemical reaction mechanism of lithium battery.The equivalent internal resistance of the lithium battery under different SOC and different ambient temperatures was measured through the mixed pulse experiment,and the heat generation model for the lithium iron phosphate cylindrical battery was completed.Then,the temperature rise experiment and thermal simulation of the single battery under different working conditions during natural convection heat dissipation were compared to verify the accuracy and reliability of the heat generation model established by the theory.(2)To analyze the heat generation characteristics of the monomer lithium iron phosphate battery,the steady-state and transient thermal simulation were done for the monomer cylindrical battery under different parameters,including discharge rate,ambient temperature and convective heat transfer conditions.The results indicate that the maximum temperature of the battery appears at the geometric center and is positively correlated with the change of discharge rate and ambient temperature.When the battery work in high ambient temperature and high rates steady state or transient discharge,only forced liquid cooling can meet the heat dissipation requirements of the battery,and the maximum temperature and temperature difference can be controlled within a reasonable range.(3)To investigate the heat dissipation performance of the PCM thermal management system,a PCM thermal management system used pure paraffin was designed,and the influence of different parameters on the heat dissipation performance of the system was analyzed.The results reveal that the paraffin has the capacity to absorb heat and manipulate the temperature rise of the battery,the utilization of paraffin is limited by its low thermal conductivity.Then,the thermal conductive performance of the PCM was improved by adding expanded graphite(EG),and the effects of the mass fraction of expanded graphite,the spacing between cells and the convection condition on the temperature field of the cell were analyzed.The results uncover that the latent heat properties of PCM can be better utilized when the mass fraction of expanded graphite is12% and the cell spacing is 4mm.When the battery is in continuous charging and discharging condition,only with forced liquid cooling can the latent heat characteristics of PCM be restored before the next cycle,so as to continue to play the temperature control function.(4)To maintain the latent heat characteristics of PCM,a coupled PCM/liquidcooling battery thermal management system was designed.Firstly,the system was verified by experiments,and then the temperature distribution of the battery was studied under different liquid cooling flow rate,liquid cooling temperature and mass fraction of expanded graphite when the number of liquid cooling channels was 4 and 20 respectively,and a control strategy was proposed.The results demonstrate that the maximum temperature and the maximum temperature difference of the battery can be controlled within 323.15 K and 5K respectively by resizing the liquid cooling flow rate during the continuous charge-discharge process,and the latent heat characteristics of the PCM can be maintained with less energy consumption. |
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