Thermodynamic analysis of propane-based zeotropic mixtures in space heating and domestic hot water heat pumps

Propane is considered one of the most promising refrigerants for residential heat pumps due to its favorable thermodynamic properties and negligible climate impact. However, its use remains challenging in high-temperature applications due to reduced capacity and efficiency, as well as high compressor discharge temperatures. A viable strategy to mitigate these issues is the use of zeotropic mixtures, whose properties can be tailored to the specific application, thereby improving cycle efficiency. Building on this approach, the present study investigates the potential of propane-based zeotropic mixtures as working fluids for heat pumps in both space heating and domestic hot water production. A steady-state thermodynamic model of a single-stage vapor compression heat pump was developed to evaluate the performance of six low-global warming potential co-fluids blended with propane. Results show that mixtures can increase the coefficient of performance compared to pure propane, with butane blends achieving the best improvements in space heating (+7%) and CO2 blends in domestic hot water production (+11%). The use of co-fluids effectively reduced the compressor discharge temperature (up to 14 °C) but penalized the volumetric heating capacity. The analysis highlights the impact of superheating, subcooling, and mixture composition, showing that the highest coefficient of performance was achieved with a temperature match in the evaporator rather than in the other heat exchangers. Overall, propane-based zeotropic mixtures enhance performance while mitigating operational constraints, demonstrating their viability as low-global warming potential refrigerants for residential heat pumps.