Experimental investigation on a direct-expansion solar-assisted heat pump water heater using R290 with micro-channel heat transfer technology during the winter period

Abstract R290 (propane) is an environmental friendly refrigerant with zero ozone depletion potential (ODP) and low global warming potential (GWP). A direct-expansion solar-assisted heat pump (DX-SAHP) water heater system using R290 was designed and built to investigate its thermal performance, which comprised a constant-frequency rotary-type hermetic compressor with rated power of 471 W, a micro-channel solar collector/evaporator with surface area of 2.09 m2, a micro-channel condenser with effective heat-transfer area of 0.75 m2, and an electronic expansion valve. In order to evaluate the DX-SAHP system performance during the winter period, massive experiments have been carried out with a refrigerant charge of 350 or 410 g. Under these environmental parameters as follows: the average solar radiation intensity of 20-592 W•m−2, average ambient temperature of -3.0-12.2°C, and average wind speed of 0.01-1.15 m•s−1, while the 200 L water at 7.2-15.3°C was heated to 37.7-54.9°C, the average system coefficient of performance (COP) ranged from 2.12 to 4.43. Finally, the responses of the DX-SAHP system using R290 to variations in solar radiation intensity and ambient temperature are presented. The transient variations in the refrigerant mass flow rate, the electric power input to the compressor and the electronic expansion valve opening are also analyzed under different weather conditions.