Influence of subcooling on performance of direct-expansion solar-assisted heat pump

•A new DX-SAHP system with subcooling control is formed by adding two components.•A quasi-dynamic mathematical model of the new DX-SAHP system has been developed.•The simulated results are compared with experimental data of the original system.•The maximum heating COP of the new system occurs with degree of subcooling at 4 K.•The system has better performance with degree of subcooling in range of 2–5 K. As an essential operation parameter of a direct-expansion solar-assisted heat pump (DX-SAHP) system, the degree of subcooling has influence on the thermal performance of the system. Based on the original experimental setup, a throttling valve and a liquid receiver are added at the outlet of the condenser to control degree of subcooling, forming a new system. A quasi-dynamic mathematical model of the new system has been developed to analyze the effect of subcooling on the system performance, and the iterative criterions for the model convergence are degree of superheat and degree of subcooling. The simulation results show that, the heating capacity per unit refrigerant mass is sensitive to the variation of degree of subcooling at the lower degree of subcooling, while the compressor power per unit refrigerant mass is sensitive to that at the higher degree of subcooling. The maximum heating coefficient of performance (COP) of the new system occurs when the degree of subcooling remains constant at 4 K. In the four typical days, the heating COP values of the new system with a constant degree of subcooling of 4 K are 9.13%, 14.35%, 9.42% and 5.73% higher than that of the original system, respectively. Moreover, the parametric study shows that the DX-SAHP system can achieve better performance in term of heating COP by controlling the degree of subcooling in the range of 2–5 K.