Experimental study on charge optimization of R32/R1270 and its alternative performance in the quasi-two-stage compression heat pump for district heating

The effect of refrigerant charge on the system performance of quasi-two-stage compression heat pump (QCHP) with replacement refrigerant of R32/R1270 (15/85 wt%) (B for short) for district heating was studied under nominal conditions based on the self-designed QCHP test system and the feasibility of replacing R22 with B was discussed under variable evaporation temperature conditions. The results indicated that the refrigerant charge showed distinct impact on the COPh, heating capacity and discharge temperature of the system using B. The system with B at the optimal charge of 1.2 kg (Bopt for short) achieved the maximum heating capacity of 7.455 kW and the maximum COPh of 3.35 at nominal conditions. The COPh and heating capacity of both systems with Bopt and R22 had similar trends with the change in evaporating temperature, while their discharge temperatures showed different changes from the heating capacity. Compared to those of R22 system, Bopt system showed improvement in COPh and heating capacity of 9.73%~17.54% and 50.84%~53.40% respectively in the evaporating temperature of -15~-5 ?, with the distinctly lower discharge temperature. Thus, Bopt has remarkable replacement advantages in the QCHP. The lower discharge temperature highlights the outstanding benefits of Bopt system in the reliable heating at lower ambient temperature and longer service life. This study provides reliable reference data for refrigerant replacement in heat pumps and low-carbon heating.