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 COP_TMS, regardless of whether the system is operating in a transcritical or subcritical mode. In addition, besides the global optimal COP_TMS, 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 COP_TMS 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.