Combined Thermodynamic and Environmental Impact Assessment of Environmentally Preferable Refrigerants Used for Water Heaters Based on Water-to-Water Heat Pump

Water-to-water vapor-compression heat pumps offer an efficient solution for utilizing low-grade thermal energy for space heating and domestic hot water production. However, previous studies have generally focused on individual refrigerants or a limited number of alternatives, while systematic comparisons of different low-global-warming-potential (GWP) refrigerant families under consistent operating conditions remain limited. To address this gap, this study presents a combined thermodynamic and environmental assessment of potential alternatives to R134a using a numerical model validated against experimental data from an R134a water-to-water heat pump prototype. Candidate refrigerants from different chemical families were evaluated under identical operating conditions based on the cycle and system coefficients of performance (COP), thermodynamic quality factor, compressor pressure ratio, volumetric refrigerating capacity (VRC), and Total Equivalent Warming Impact (TEWI). The results identified R152a, R450A, and R513A as promising candidates for further evaluation, while several hydrocarbon and hydrocarbon-containing refrigerants exhibited favorable performance but require additional safety considerations. At a heat-source temperature of 10 °C and heat-sink temperatures of 40–60 °C, R152a exhibited the highest cycle COP, ranging from 5.45 to 3.53, compared with 5.30–3.33 for R134a, corresponding to improvements of approximately 2.8% at 40 °C and 6.0% at 60 °C. R152a and R430A also exhibited the lowest TEWI values, approximately 29.51 and 30.19 t CO₂-eq, respectively. The VRC analysis further demonstrated that comparable or higher COP does not necessarily imply compatibility with an existing R134a compressor because differences in vapor density and volumetric capacity may require different compressor displacement and operating characteristics. By integrating thermodynamic performance, compressor-related characteristics, and environmental indicators within a common assessment framework, this study provides a systematic basis for identifying promising low-GWP refrigerants for water-to-water heat pump applications. The results should therefore be interpreted as a theoretical screening framework for further refrigerant-specific evaluation rather than as evidence of direct drop-in compatibility with existing R134a systems.