Comparative analysis of CO₂ and propane heat pumps for water heating: seasonal performance of air and hybrid solar-air systems

This study numerically compares the seasonal heating performance (SCOP) of three 15 kW heat pumps using natural refrigerants: two with carbon dioxide and one with propane. The propane system is an air-source heat pump (R290-AHP). The carbon dioxide systems include an air-source heat pump (R744-AHP) and a dual-source solar-air heat pump (R744-SAHP) equipped with finned-coil and photovoltaic-thermal evaporators that can work simultaneously. In the case of a transcritical carbon dioxide cycle, the use of several low-temperature sources is a promising solution to improve the performance of the system by enhancing the exploitation of renewable energy sources. Since the efficiency of air and solar-based systems is related to weather conditions and location, there is the need for accurate models to evaluate the SCOP. In this work, a numerical model has been developed to design the three 15 kW heat pumps and assess the SCOPs under variable space heating and domestic hot water demand profiles, using climatic data for Rome and Strasbourg. The results indicate that the R290-AHP consistently achieves the highest SCOP, while the R744-AHP performs the lowest. The R744-SAHP overperforms the R744-AHP by approximately 4 % regardless of heating demand characteristics. In particular, the results show that the performance of the three heat pumps is significantly influenced by the distribution of the thermal load throughout the day. Specifically, when the thermal load is concentrated during daylight hours, the heat pumps can operate at a higher SCOP, especially for the R744-SAHP, and also increase, up to 44 %, the self-consumed photovoltaic energy produced.