In heat pumps

the use of environmentally friendly refrigerants can reduce damage to the ozone layer. Under the condition of low ambient temperature

based on the existing heat pump system

the CO2 transcritical cycle and mechanical subcooling were adopted. An R1234yf/CO2 mechanical subcooling heat pump system (MSHPS)

a CO2/R1234yf double air cooler mechanical subcooling heat pump system (DMSHPS)

and a CO2 two-stage compression triple air cooler heat pump system (TSCHPS) are proposed in this study. By establishing a thermodynamic model

the heating performance of the system using CO2 and R1234yf was analyzed. Furthermore

the performance of different systems was analyzed through the optimal discharge pressure

evaporating temperature

condensing temperature

and flow rate ratio of the auxiliary cycle and main cycle. In this study

R1234yf was replaced with R134a; the heating performances of R1234yf and R134a in the MSHPS and DMSHPS were compared and found to be similar.