Performance Optimization and Analyses on Recuperative Heat Pump System Using CO₂/R1233zd(E)

Air-source heat pumps, as a new, energy-saving, and environmentally friendly technology, are widely applied in various heating applications, yet their reliable operational efficiency at low ambient temperatures is a concern. In this study, a simple, low-cost, and reliable heat pump system is constructed using safety and environmentally friendly binary refrigerant CO₂/R1233zd(E), and the system performances such as the coefficient of performance (COP), volumetric heating capacity, suction pressure, and exhaust pressure are obtained through optimization analyses at ambient temperatures from -30 ℃ to -10 ℃. By comparing the vapor-injection heat pump system and cascade heat pump system, it was deduced that a recuperative heat pump system using a binary refrigerant can maintain high efficiency at low ambient temperatures. At ambient temperatures ranging from -30 ℃ to -10 ℃, the COP of the recuperative heat pump system decreases by 9.8% to 20.6%, whereas the volumetric heating capacity increases by 3.5% to 56.1% compared to the R410A vapor-injection heat pump system. Moreover, the compressor pressure ratio and its variation with ambient temperature are lower than other heat pump systems. Owing to the simple structure of the recuperative system and the adoption of the safe and environmentally friendly binary refrigerant CO₂/R1233zd(E), the solution proposed in this study has unique advantages in heat pump applications in regions with low ambient temperatures.