Thermodynamic performance of booster heat pumps using low-GWP zeotropic mixtures

Booster heat pumps (BHPs) are used in ultra-low-temperature district heating systems to locally raise the temperature for domestic hot water production. The efficiency and environmental performance of BHPs are therefore critical for next-generation district heating networks. However, no clear consensus has yet been reached on the most suitable refrigerants for these systems. Earlier studies have shown that low-GWP refrigerant mixtures can theoretically improve the coefficient of performance compared with conventional fluids such as R134a. In this study, we investigate how using zeotropic mixtures influences the thermal behavior of a BHP. The zeotropic mixture R444A and R456A were examined to determine whether their temperature glide could provide a better match between the water and refrigerant temperature profiles. The analysis was carried out using a numerical system model validated against experimental data for the zeotropic mixture R32/R1234yf. The results indicate that BHP cycles employing refrigerant mixtures can enhance both COP and exergetic efficiency when an appropriate glide match is achieved between the refrigerant and the heat source/sink during DHW preparation. Under the studied conditions, using R456A in a booster heat pump yields a COP of 5.1 and an exergetic efficiency of 47.4%.