Thermodynamic modeling of a residential heat pump for combined space and water heating using binary CO2-based mixtures

The residential sector answers for a considerable share of the energy consumption worldwide. An alternative to meet technical and environmental requirements of space heating and hot water demanded by this sector uses CO2 mixtures as a refrigerant in compression-based cycles. However, while CO2 is nonflammable and has low GWP and ODP, it performs poorly due to its low specific refrigerating effect and large throttling losses. Differently, mixtures with a low-GWP refrigerant and CO2 could enhance the performance of these cycles while attending the environmental requirements. Therefore, this study considered the use of a combined heat pump for space and water heating using binary mixtures of CO2 and several refrigerants. Through a computational model, three configurations were tested, along with different ratios of space heating to hot water demands, and different CO2 mass fractions on the mixture. The heat pump configuration with the best performance range was the one with three gas coolers, where the low temperature gas cooler preheats the water, the intermediate equipment heats the space, and the high temperature gas cooler heats the already preheated water to the operating temperature. For this arrangement, heat pumps primarily dedicated to space heating benefit from mixtures with low mass fractions of CO2, approximately 10%. In contrast, heat pumps primarily attending water heating perform better for high mass fractions of CO2 (similar to 90%). Furthermore, in this combined configuration, about 87% of the mixtures analyzed could have higher coefficient of performance than pure fluids, indicating an improvement of CO2 heat pumps.