| In the background of excessive consumption of traditional fossil energy and environmental pollution problems are becoming more and more prominent,the development of new energy vehicles has become the new course of the automotive industry.As the power core of electric vehicles,the overheating of lithium batteries under complex operating conditions can seriously affect the safety of vehicles.Therefore,an efficient,low-cost Battery Thermal Management System(BTMS)that can control the battery temperature and temperature uniformity in the best range is essential.In this paper,the thermal performance of Phase Change Material(PCM)thermal management system for Li-ion batteries and its optimization scheme are investigated from the perspective of heat dissipation safety of 18650 type Li-ion batteries using a combination of experiments and simulations.According to the application requirements of BTMS,inorganic PCM sodium acetate trihydrate was selected as the substrate and modified for the experiments.A certain amount of nucleating agent disodium hydrogen phosphate dodecahydrate(DSP)and thickening agent carboxymethyl cellulose(CMC)were added to solve the subcooling and phase separation defects unique to inorganic hydrated salts;subsequently,urea(urea)and expanded graphite(EG)were added to lower the phase transition temperature and enhance the thermal conductivity of PCM.The composite PCM prepared by the above method was subjected to heating-cooling cycle experiments and thermal property tests.The composite PCM was applied to the monolithic BTMS test device,and experiments were conducted to analyze the effect of different discharge multipliers on the thermal management performance of the system.The results showed that the best thermal performance of the composite PCM was achieved when 5wt% of DSP,3wt%of CMC and 8wt% of urea were added;the temperature reduction of the PCM-based BTMS compared to the battery monoblock was more than 15 K,indicating that the system achieved significant effect on battery temperature control.A mathematical model of cell-PCM phase change heat transfer is established and solved numerically,and the experimental results are used to verify the reliability of the numerical model.The effects of PCM thermal conductivity,fin type and convective heat transfer coefficient of the system shell on the variation of cell temperature are studied.The effect of fins on the enhanced heat transfer of BTMS is compared and analyzed.The effects of cooling water flow rate and inlet and outlet mode in the PCM cooling-water cooling composite thermal management method on the heat transfer and distribution pattern in the battery module are discussed.The results show that the correlation between cell temperature variation and PCM dosage is the largest when the thickness of PCM layer is 10 mm;the larger the volume of fins and the contact area with the cell,the lower the cell temperature extremes;with the enhancement of natural convection,the peak temperature and PCM melting rate inside the single BTMS gradually decrease;the fins placed in the module can effectively dissipate the internal heat accumulation and improve its temperature uniformity;increasing the cooling water flow rate inside the water-cooled plate In addition,compared with the inlet and outlet of cooling water on the same side,the opposite side inlet and outlet can effectively alleviate the poor cooling effect of the battery at the end of the water cooling channel,and the maximum temperature difference in the module can be reduced to 3.4K,which shows a better synergistic effect of this hybrid thermal management mode. |
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