A quasi-two-stage heat pump for efficient dual-temperature supply-space heating and domestic hot water

Heat pumps play a critical role in delivering space heating and domestic hot water in residential buildings. However, managing these two demands efficiently remains a technical challenge, especially when high water temperatures are required. This study introduces a dual-mode flexible heat pump operating in two distinct modes: during charging mode, it provides space heating while recovering heat from the refrigerant into an integrated latent thermal energy storage unit. During discharging mode, this stored energy supplies domestic hot water at higher temperatures. A performance analysis was carried out using refrigerants under different domestic hot water demand levels and space heating temperatures. Propane achieved a power reduction of 13.2% compared to a standard system at a 22% domestic hot water share. Further analysis showed that R1234yf delivered the highest power reduction of 14.3%, followed by R1234ze, propane, and isobutane, all within 13.1-13.6%. These maximum reductions were achieved at optimal storage temperatures of 18.0-26.0 °C depending on the refrigerant. The results demonstrate that controlling the storage temperature based on domestic hot water demand share and space heating temperature is essential for optimal performance. The proposed system presents a promising pathway to significantly reduce electricity consumption in residential heating without relying on electric backup heaters.