Charging Requirements and Unstable Characteristics of Automotive CO₂ Thermal Management Systems

Transcritical CO₂ thermal systems have emerged as leading solutions to addressing challenges such as "range anxiety in winter" and pronounced greenhouse effects associated with the working fluid in electric vehicle thermal systems. Nevertheless, the intricate interplay of transcritical cycles in the varied scenarios of electric vehicles introduces complexity, with performance and operational stability intricately linked to the refrigerant charge. This study conducts simulations to investigate the variability in refrigerant charging requirements for transcritical CO₂ thermal management systems under diverse operating conditions. We specifically examined the impact of three critical factors, namely ambient temperature, indoor airflow rate, and outdoor air velocity, on refrigerant requirements in different modes and their underlying mechanisms. In the heat pump mode, the demand for refrigerant charge increases with ambient temperatures and wind speed and decreases with cabinet air flow rate, with changes of 18.6%, 18.9%, and 6.16%, respectively. In the cooling mode, the refrigerant charge requirement decreases with ambient temperatures and cabinet air flow rate and increases with outdoor wind speeds, with changes of 7.03%, 7.85%, and 2.27%, respectively. In situations of nonoptimal charging, potential alterations in the interaction between system control variables and target variables contribute to system instability. This necessitates adjustments to refrigerant distribution to mitigate instability under specific operating conditions. The research outcomes hold substantial reference value for the optimization of electric vehicle air-conditioning accumulator designs, enhancement of energy efficiency, and improvement of overall thermal comfort.