Effects of refrigerant charge and structural parameters on the performance of a direct-expansion solar-assisted heat pump system

•A method calculating the refrigerant mass charge in DX-SAHPWH system is described.•The refrigerant charge distribution characteristics is investigated.•The system performance under varied structural parameters is obtained.•The optimum refrigerant charge is 1.65–1.75 kg.•The optimum solar collector area is 6.0 m2.

The direct-expansion solar-assisted heat pump (DX-SAHP) is widely studied as a refrigeration system, which can supply hot water for domestic use during the whole year. The system refrigerant charge and structure parameters are believed to have a great effect on the cycling thermal performance. The refrigerant mass charge including two-phase and single-phase in heat exchangers and pipes is calculated with distributed and lumped parameter approach mathematical models, respectively. Based on the system simulation program, the refrigerant distribution characteristics and system performance under varied structural parameters are obtained. The mathematical calculation results show that the 70%–80% refrigerant charge exists in the condenser and collector; the optimum refrigerant charge, solar collector area, condenser pipe length and condenser internal diameter for the system are 1.65–1.75 kg, 6.0 m2, 70 m and 9 mm, respectively. In the optimum parameters, the better system performance and feasible cost can be achieved.