Influence of refrigerant charge and condenser area on direct-expansion solar-assisted heat pump system for radiant floor heating

Abstract

As known, the refrigerant charge and condenser area have important influence on the thermal performance of a heat pump system. A test rig of a direct-expansion solar-assisted heat pump (DX-SAHP) system for radiant floor heating has been built, and the quasi-dynamic mathematical model of the system has also been established, which uses propane (R290) as the refrigerant. The deviations between the simulated results and experimental data are within 7.1 %, showing a good agreement. The analysis reveals that, when the refrigerant charge of the DX-SAHP system increases, the condensing pressure and the subcooling degree in the condenser increase, while the evaporating pressure decreases, resulting in the increase of the effect heat gain and the compressor power. The system coefficient of performance (COP) first increases to a maximum and then decreases with the refrigerant charge, and remains at a relative high level within a certain range near the optimal refrigerant charge. The optimal refrigerant charge is independent of solar insolation and ambient temperature, and decreases with initial water temperature in the hot water storage tank, while the corresponding subcooling degree is always in the 6–9 °C range. Furthermore, the system COP is also influenced by the condenser area. Both the maximum COP and corresponding optimal refrigerant charge are found to increase with the condenser area. As the condenser area increases from 0.553 to 1.393 m2, these two parameters increase by 8.95 % and 27.91 %, respectively.