Theoretical and experimental investigation of a combined R134a and transcritical CO₂ heat pump for space heating

In this paper, a novel combined R134a and transcritical CO₂ system was studied theoretically and experimentally for space heating. Experimental results showed that the ambient and space heating temperatures substantially affected the system performance. At the heating requirement with 50 °C in feed water temperature and 70 °C in supply water temperature, the heating capacity and coefficient of performance (COP) increased by 32.6% and 18.2%, respectively, as the ambient temperature increased from −20 °C to 0 °C. The system COP increased by up to 15% at fixed ambient condition as the feed and supply water temperatures decreased from 50 °C/70 °C to 40 °C/50 °C. A mathematical model was also developed and validated using experimental data. The model was used to investigate the performance improvement of the combined system in comparison to the standard transcritical CO₂ system. The comparison results showed that the combined system improved the COP by up to 22%.