The energy efficiency of conventional CO2 systems used for heating is low. Therefore

cascade and subcooling systems can be adopted to improve the performance of CO2 systems. In this study

the thermodynamic models of a baseline CO2 system (BASE)

R410A vapor injection system

cascade system

indirect dedicated mechanical subcooling CO2 system (IDMS)

and direct dedicated mechanical subcooling CO2 system (DDMS) are established. The performances of the system with different heating terminals are optimized and analyzed. The results show that when the supply and return water temperatures are 65 ℃ and 40 ℃

respectively (the heating terminal is the traditional designed radiator) and the ambient temperature is -20–20 ℃

the COP of the DDMS system is improved by 3.8–20.9% compared with that of the baseline CO2 system. The CO2 subsystem plays a dominant role in the DDMS system. In contrast

for the IDMS system

the subcooling subsystem is more important for most cases concerning hot water production. A significant increase in system efficiency can be achieved by adding a relatively small vapor compression refrigeration cycle for the DDMS system. In the CO2 heat pump system

when the ambient temperature is above -15 ℃

the exergy efficiency of the DDMS system is higher than that of other systems. Compared to the baseline CO2 system

the exergy efficiency is improved by 19.3%–28.2%. For the cases with ambient temperature below -15 ℃

the CO2/R1234yf cascade system shows the highest exergy efficiency.