Theoretical investigation on the combined and cascade CO2/R134a heat pump systems for space heating

In this study, comparison between combined and cascade CO₂/R134a heat pump systems were investigated theoretically. The characteristics of the R134a and CO₂ cycles in both combined and cascade systems were studied under different working conditions. The evaporation temperature in the R134a cycle and the discharge pressure in the CO₂ cycle were first evaluated in the combined and cascade systems. Results showed that the change of the R134a evaporation temperature in the cascade system was much larger than that in the combined system as the hot water supply temperature increased from 55 to 75 °C. The CO₂ discharge pressures in the cascade system were much lower than those in the combined system under all working conditions. The heating capacity in the two cycles was then examined in the two systems. The results showed that the CO₂ cycle played the major role in the combined system while the R134a cycle played the major role in the cascade system. The system COP comparison showed that the cascade system performed better under relatively low ambient and hot water supply temperatures whilst the combined system performed better under high hot water supply temperatures. The compressor displacement ratio comparison results showed that the combined system would be much more environmentally friendly in comparison to the cascade system.