Comparative study on the thermal performance of the battery two-phase direct CO₂ cooling system with parallel and half-series configuration for electric vehicles

Two-phase direct cooling is one of the most effective methods for battery thermal management in electric vehicles, but cooling results are strongly affected by the vapor quality state, even generating thermal runaway. This paper aims to reveal the system and battery cooling performance for two systems' configuration, which is the parallel and half-series of the battery sink with the cabin evaporator. The cooling performance sensitivity to the valve control was first studied. Then the two systems were compared under different ambient temperature, supply air temperature and battery heat power, to fully evaluate the battery cooling uniformity and comprehensive system performance. The battery performance including vapor quality and temperature along the flow direction was mainly analyzed. And the refrigerant flow nonuniformity at battery was varied to discuss the battery performance and its probability of thermal runaway. The results showed half-series configuration benefited little to COP and much to battery cooling performance compared with the parallel configuration. The superheat degree decreasing at cabin evaporator exit hardly deteriorate the battery cooling performance in the half-series configuration, which proved the control simplicity and reliability. Thus, the results could provide positive reference to estimate the novel system's ability against the thermal runaway under complicated environment.