Performance of Transcritical CO2 Air-source Heat Pump Heating System with Mechanical Subcooling Using Zeotropic Refrigerant

Abstract

In this study, a mechanical subcooling transcritical CO2 heat pump heating system using a zeotropic refrigerant is proposed. A thermodynamic model is developed for this system and compared with that using a pure refrigerant. The results indicate that the coefficient of performance employing R1234ze(E)/R601(60/40) with a large temperature glide is up to 2.45, which is 13.82% higher than that of the pure component, at an ambient temperature of -12 ℃ and supply/return water temperatures of 65°C/40°C. The discharge pressure of the CO2 system can be efficiently reduced, and a higher subcooling degree is achieved using a zeotropic refrigerant; hence, the throttling irreversible loss decreases. The optimal discharge pressure is reduced by 27.85% using R290/R601(70/30). The temperature match in the subcooling process can be efficiently improved, and the exergy efficiency is up to 14.09% higher than that of the pure refrigerant component when using R1234ze(E)/R601(60/40). The relatively large temperature glide and suitable concave-convex characteristic of the temperature-enthalpy curve are the two key principles for zeotropic refrigerant selection used for CO2 heat pump systems with mechanical subcooling. Finally, the utilization of R1234ze(E)/R601(60/40) is recommended.

Keywords

• heating
• heat pump
• mechanical subcooling
• temperature matching
• zeotropic refrigerant
• CO2