Experimental investigations on the performance of a single-stage compound air-source heat pump using CO2/R600a in cold regions

Despite the wider applications of heat pump, conventional heat pumps face limitations in cold-region operations. To improve heating performance at low ambient temperatures, a single-stage compound air-source heat pump was innovatively built in this research. Through switching operating mode and regulating operating parameters, the performance of compound heat pump using CO2/R600a was experimentally investigated under different operating conditions. The influences of operating parameters on system performance were studied. Under supply/return water temperatures of 75℃/50℃, coefficient of performance (COP) of the compound heat pump varied from 1.834 to 2.574 when air inlet temperature varied from −30℃ to 0℃, and the thermodynamic perfection of the compound heat pump cycle was higher than 46% in experiment. The essential factor affecting performance was the irreversible loss in recuperator. Because of the effective utilization of the temperature glide of CO2/R600a, the compound heat pump was suitable for the condition with the supply/return water temperatures of 75℃/50℃. The energy efficiency and volumetric heating capacity of the compound heat pump using CO2/R600a were both higher than those of two-stage and cascade systems using R134a and R410A. The outstanding experimental results of the single-stage compound heat pump provide a promising and competitive solution for clean heating in cold regions, attributed to its environmental friendliness, simplicity and cost effectiveness.