Thermal performance analysis and burning questions of refrigerant direct cooling for electric vehicle battery

Refrigerant direct cooling technology has been gaining attention due to its advantages of simple system composition and high cooling efficiency, but its temperature control performance does not have advantages over mature liquid cooling in industrial applications, reasons for which should be discovered. This paper proposes a novel discrete model based on an electro-thermal coupling method and thermal resistance network to analyze the local temperature control performance of refrigerant direct cooling and liquid cooling systems under real operating scenarios with acceptable computing time and accuracy. The results show that the phase change of the refrigerant in the refrigerant direct cooling plate significantly affects the temperature control performance. The vapour-liquid two-phase zone has a high heat transfer capability, which can dissipate heat well and maintain a small temperature difference with the maximum temperature of the battery reduced by 28.3% compared with the liquid cooling system. There is a serious heat transfer deterioration in the superheated gaseous zone, causing a significant increase in the local battery temperature, and the maximum temperature difference is much higher than that in the liquid cooling system. When the refrigerant switches from two-phase to single-phase under 2C charging condition, the heat transfer coefficient reduces by roughly 73.6%, which causes the accompanying battery unit temperature to rise by about 12 ∼ 14 °C. Besides, the low refrigerant temperature will cause a large temperature difference in the vertical direction of the battery, which restricts the application of the refrigerant direct cooling system in practice. When the evaporation pressure increases from 300 kPa to 500 kPa, the temperature difference will decrease from 14.1 °C to 8.8 °C which is a 37.6% reduction. Through an in-depth analysis of the local temperature distribution of battery units, two burning questions were identified which deteriorate the temperature control performance of the refrigerant direct cooling system and results can provide directions for performance improvement.