Performance of direct expansion photovoltaic-thermal evaporator assisted compression heat pump water heaters using a zeotropic mixture

•The R290/R600a mixture has improved compressor volumetric efficiency.•The R290/R600a zeotropic refrigerant mixture is proposed.•The R290/R600a has 3.5–7.9% higher COP than R134a.•DXPV-TE assisted CHPWH has lower environmental impact than electrical resistance water heater.•Payback duration was estimated as 1.221 years. The single-component refrigerant used in a direct expansion photovoltaic-thermal evaporator assisted compression heat pump water heater gets excess superheating, which results in uneven panel cooling and a loss in compressor volumetric efficiency. To tackle these issues, a binary zeotropic refrigerant mixture consisting of R290 and R600a (mass fraction of 0.6:0.4) with a temperature glide of 4–7 °C is proposed in this research. The performance parameters such as compressor volumetric efficiency, compressor power consumption, condenser heating capacity, photovoltaic power output, photovoltaic efficiency, coefficient of performance and solar energy absorption coefficient were predicted under the influence of solar irradiation, ambient wind speed, ambient temperature and ambient relative humidity. The results showed that the DXPV-TE using R290/R600a mixture has 3.8–10.3 % improved electrical power output than R134a by improving thermal uniformity of the panel. The R290/R600a mixture has 1.1–2.5 % lower compressor power consumption with 0.6–1.9 % higher relative compressor volumetric efficiency than R134a. Moreover, the R290/R600a mixture has a 3.5–7.9 % higher coefficient of performance than R134a because of 4.2–5.7 % higher condenser heating capacity. The proposed system using R290/R600a mixture has 88.34 % lower environmental impact than electrical resistance water heaters. The payback duration was estimated as 1.221 years compared to electrical-resistance water heaters.