A comparative study of R134a and propane (R290) as refrigerants in heat pump water heaters

In the context of decarbonization efforts, heat pump water heaters (HPWHs) offer an attractive solution over conventional electric resistance type hot water systems due to their 2-3 times higher efficiency. However, the high manufacturing costs and taxes associated with using hydrofluorocarbon (HFC) refrigerant make HPWHs expensive. As a means to tackle this issue, this paper explores the use of propane (R290) as a refrigerant in HPWHs. The study involves an experimental comparison of R290 and R134a, with refrigerant charges optimized for the unit at different ambient temperatures. This current work demonstrates that R290 achieves a 10% improvement in coefficient of performance (COP) at ambient temperatures beyond 20°C. However, at 10°C ambient temperature, the study shows that R290 offers no advantage over R134a, and the COP is lower. These results indicate that the compressor is the largest source of inefficiency, and this aligns well with experimental results on system performance. Additionally, simulation tests using compressors designed for R290 did not predict better COP values than the test unit. Overall, the study suggests that R290 is a viable refrigerant option for HPWHs, but further research is necessary to optimize it’s use.