Experimental and simulation investigation of propane as a refrigerent for heat pump water heaters

Heat pump water heaters (HPWHs) offer attractive benefits over traditionally used electric resistance type hot water systems as it is able to provide 2-3 times the efficiency. HPWHs however suffer from high upfront costs which in modern times have only been made worse by taxes imposed on commonly used hydrofluorocarbon (HFC) refrigerant. The imposed taxes are in recognition of the large amount heat trapped by HFCs compared to carbon dioxide and there is a general consensus that environmentally friendly refrigerant such as propane is the alternative. Experimental study of propane refrigerant (R290) in a HPWH was conducted and compared to R134a. The air sourced HPWH of approximately 3 kW heating capacity consisted of a rotary compressor, brazed plate condenser, thermal expansion valve, and fin and tube evaporator. In both cases both refrigerant charges were optimised for the testing unit. The results showed that R290 was able to achieve a 10% improvement at the ambient temperatures of 20°C and 30°C in terms of coefficient of performance (COP). However, the benefit of R290 refrigerant was not pronounced at 10°C ambient temperature as it achieved lower COP value than R134a. A theoretical analysis was conducted via an exergy analysis and simulation modelling to predict its improvements. The exergy analysis focused on quantifying exergy destruction to locate system inefficiencies so that effort can be concentrated on improving that area. The results identified that the compressor was the largest source of inefficiency. Simulation study showed reasonable agreement with the experimental results on the system performance.