Performance comparison between the combined R134a/CO₂ heat pump and cascade R134a/CO₂ heat pump for space heating

In this paper, the performance of combined R134a/CO₂ and cascade R134a/CO₂ systems for space heating was studied and compared under specific operating conditions. A mathematical model was developed for this purpose and was verified by experimental data. The effects of key parameters such as water feed temperature, water supply temperature and ambient temperature on the performance of the two systems were further investigated. Results showed that cascade and combined systems had different preferable application conditions. The cascade system performed better at low ambient temperatures while the combined system performed better under conditions of high ambient temperature and high hot water temperature differences between the system inlet and outlet. A correlation was established and an operating condition coefficient was proposed to determine which system should be used for any given set of working conditions.