Performance analysis of CO2 thermal management system for electric vehicles in winter

CO2 is well-suited for thermal management systems (TMSs) in electric vehicles, particularly in winter when both the cabin air and the battery require heating. However, due to the distinct heat exchange boundary conditions of these two heated components, traditional theories of optimal operation of CO2 cycle are not applicable. In this paper, the system characteristics of a CO2 TMS in the cabin-and-battery mixed heating mode are comprehensively investigated. The results show that there is a pseudo-optimal discharge pressure that maximizes the COPTMS, regardless of whether the system is operating in a transcritical or subcritical mode. In addition, besides the global optimal COPTMS, there may still be other local maximum points, which are determined by the CO2 flow distribution in both gas coolers and the COP rise rate of the battery cycle. Furthermore, this work offers a thorough investigation of the impact of essential factors on the pseudo-optimal discharge pressure and proposes an accurate prediction approach for the best control of the CO2 TMS. The CO2 TMS can still ensure a COPTMS above 2.0 to meet the thermal demands of both the cabin and the battery even under the challenging operating circumstances of −20 °C, proving the technology's great competitiveness.