Low-temperature performance investigation of a direct integrated thermal management system based on CO₂ heat pump for electric vehicles

Due to the low-temperature heating performance, the CO₂ heat pump would be one of the most advanced technologies in the development of electric vehicles thermal management. Moreover, thermal efficiency advantages allow direct heating technology to develop in electric vehicles. To investigate the dynamic and steady performance of a CO₂ heat pump based integrated thermal management system (TMS), a multi-component coupling model was established and tested under the World Light Vehicle Cycle (WLTC) at low ambient temperature. Firstly, results indicated that cabin heating in mixing ventilation had an optimal discharge pressure of 62 bar. Secondly, between heating the cabin and battery in parallel and in series, preheating in series improves system performance by maintaining the subcritical cycle and keeping the battery temperature uniform with the latent heat transfer of refrigerant in the cold plate. The condensing temperature should be 18 °C to ensure a subcritical cycle and CO₂ quality at the outlet of the cold plate should be zero to prevent subcooling guaranteeing the temperature uniformity of the battery. Thirdly, for waste heat recovery, between absorbing from the motor only and absorbing from the motor and environment, the former had higher COP (coefficient of performance). The discharge pressure should be 48 bar to maintain the high COP of the motor with acceptable cooling rates. Finally, a mode-switchable and in-mode optimization combined control strategy was able to achieve the cabin, battery, and motor heat controls under the continuous WLTCs at an ambient temperature of −15 °C.