Experimental study on dehumidification and heating performance of CO₂ heat pump system for electric vehicles at low ambient temperatures (5 °C and 15 °C)

Efficient thermal management in electric vehicles (EVs) is critical for maintaining passenger comfort, particularly under cold and humid environmental conditions. Carbon dioxide (CO₂), as a natural, environmentally friendly, and thermodynamically efficient refrigerant, offers a promising alternative to conventional hydrofluorocarbon (HFC) refrigerants in vehicle heat pump systems. This study presents a dual-mode CO₂-based heat pump system specifically designed for EV applications, enabling flexible operation for both dehumidification and heating functions. Mode A utilizes an additional outdoor heat exchanger (ODHX) for refrigerant subcooling, enhancing dehumidification. Mode B eliminates ODHX, concentrating heat release in the gas cooler for superior heating output. A series of experimental investigations were conducted to assess system performance across a range of ambient temperatures and compressor speeds. Key performance metrics—including moisture extraction rate (MER), heating capacity, coefficient of performance (COP), and specific moisture extraction rate (SMER)—were evaluated under various operating conditions for both functional modes. The results reveal that at an ambient temperature of 5 °C, Mode B demonstrates superior heating performance, achieving 26.3–27.9 % higher heating capacity and a 26.3–27.6 % improvement in COP compared to Mode A, indicating its suitability for cold climate operation. Conversely, Mode A outperforms Mode B in mild-humid conditions (e.g., 15 °C), delivering 25.4–28.6 % higher MER and 26.3–29.2 % greater SMER, making it more effective for moisture control.