Research On Thermal Management Performance Of Battery Modules Based On Composite Phase Change Material

The rapid development of the new energy industry has made electric vehicles popular,and batteries have attracted much attention as common energy storage and power devices.However,the heat accumulation of the battery during operation will affect the chargedischarge performance,cycle life and safety of the battery,so it is necessary to design a thermal management system to improve the temperature of batteries.There are three types of common battery thermal management cooling systems: air cooling,liquid cooling and phase change material(PCM)cooling.PCM cooling has become a research hotspot due to its advantages of high heat dissipation efficiency,good temperature control uniformity,and no need for additional energy.However,common PCM have problems such as low thermal conductivity and narrow heat absorption window.In this paper,three kinds of PCM with different melting points,octadecane,eicosane and lauric acid,were mixed in different proportions,and the heat absorption and release capacities of different samples were compared by heat flow curves.Finally,a composite phase change material(CPCM)with a wide phase change absorption window of 25-45 ℃ was obtained.And built a CPCM battery module with aluminum foam as the skeleton to verify its performance.Meanwhile,the thermal management performance of the CPCM battery module under air-cooled forced convection and water-cooled conditions was studied by simulation.The specific work contents are as follows:(1)CPCM was prepared by mixing PCM with octadecane,eicosane and lauric acid,and its heat absorption and release properties were analyzed by heat flow curves.Finally,a CPCM with wide phase transition window was obtained.The CPCM was combined with aluminum foam,and the lithium-ion battery module was built by 18650 batteries.The battery module was charged and discharged at 0.5C,1C and 2C at ambient temperatures of25 ℃,35 ℃ and 45 ℃,and the module was charged and discharged for a long time.Temperature and temperature difference were used as the evaluation criteria for thermal management performance.The results showed that the maximum temperature of the battery module could be reduced by 5.9 ℃ compared with that of ordinary battery modules.The temperature difference of the battery pack can be reduced by 4.1 ℃,that is,the temperature rise and temperature difference are reduced by 50% and 80% respectively,and the temperature of the battery module is still stable at about 40 ℃ after 50 cycles,indicating that the thermal management scheme has a good temperature control effect,and the CPCM also has good stability.(2)Based on the basic principles of battery heat generation and heat transfer,the heat generation and heat transfer model of lithium-ion battery unit and battery module were established.The temperature changes and voltage changes during the working process of the battery were obtained through simulation,and the accuracy of the model was verified by comparison with the experimental data.The temperature change of the battery module during discharge was analyzed.The temperature of the middle No.5 battery was the highest,and gradually decreased to the peripheral temperature in the form of concentric circles,and the temperature of the outermost battery was the lowest.(3)Based on the heat flow curve of CPCM,the heat generation and heat transfer model of the CPCM battery module was established in the simplified model of double-layer material stacking.The experimental data were compared with the simulation data,and the results showed that the model had high accuracy.According to the simulation results,compared with the ordinary battery module,the CPCM battery module can effectively reduce the operating temperature of the battery module and balance the temperature difference in the battery module.(4)CPCM-air-cooled and CPCM-water-cooled models were built,and the effects of different water flow rates and air flow rates on the thermal management performance of the battery module were studied.It was found that for the packaged battery module,air-cooled heat dissipation did not play an obvious cooling role,because the cooling air can only contact the outermost layer of the battery module.The temperature of the battery in the middle of the battery module did not change significantly,but the decrease of the temperature of the surrounding battery led to the increase of the temperature difference in the battery module.Because of its good thermal conductivity,CPCM group can still control the temperature difference between batteries at about 2 ℃ under the air cooling heat dissipation.Due to the thermal conductivity of the battery itself,the temperature difference first increases and then decreases with the increase of the airflow velocity.Due to the direct contact with the battery,the heat generated by the battery can be taken away in time,and the temperature of the battery module is controlled below 35 ℃,and the temperature difference is controlled at about 3 ℃.By combining the water cooling and CPCM structure,the temperature of the battery pack can be controlled below 30 ℃.The use of lower flow rate of water can be consistent with the control water-cooled module in the larger water flow of thermal management effect.