Experimental performance analysis of a CO2 direct-expansion solar assisted heat pump water heater

Abstract In the past decades, combined solar energy with heat pump systems has been one of the very widespread ways to improve the efficiency of conventional heat pumps. In that way, this paper presents an experimental analysis of the influence of environmental conditions in a small size DX-SAHP operating with CO2 as refrigeration fluid to heat water. The variation of the water inlet temperature, the solar radiation flux and the relative humidity are analyzed and 88 different experimental results are presented. To support the analysis, a thermographic camera was also used. The results show that the gas cooler outlet pressure and temperature are strongly influenced by the water inlet temperature. As the water inlet temperature increases, the gas cooler outlet pressure and temperature increase, and consequently the COP decreases. The COP decreases in 45.8 % for the variation of the water inlet temperature to 15oC to 35 oC. The variation of the solar radiation of 876.9 W.m-2 to 30.17 W.m-2 presents a reduction of performance in about 30 %. For the same conditions for the water heat exchange, the pressure ratio of the cycle decreases with an augment on the solar radiation flux. In addition, the COP is higher in 6 % for a change in the mean relative humidity of 31.6 % to 55.8 %.