Investigation of the Heating Performance of a Solar-Assisted Inter-Cooling Air Source Heat Pump System

Abstract

This paper proposes a novel solar-assisted inter-cooling air source heat pump (SCAS–HP) system that integrates a solar collector/evaporator into the intermediate injection branch to utilize solar energy for increasing the refrigerant flow rate at intermediate pressure. A numerical model of the system was developed and validated against the literature data, and then employed to investigate performance under varying operating conditions. Three configurations were examined—two-stage throttling (T–SCAS–HP), single-stage throttling (S–SCAS–HP), and parallel evaporators (P–SCAS–HP)—and their performance was compared with a conventional vapor injection air source heat pump (VI–ASHP) across solar radiation intensities of 10–1000 W/m2, outdoor air temperatures of −20 °C to 10 °C, and outlet water temperatures of 35 °C and 55 °C. Hourly performance, economics, and CO2 reduction were further assessed for Lhasa, Beijing, and Harbin. The results show that the COPh improvement of SCAS–HP over VI–ASHP rises with increasing solar radiation intensity. Furthermore, the solar radiation threshold at which the COPh of SCAS–HP begins to exceed that of VI–ASHP lies within the range of 100–200 W/m2. Moreover, the COPh increase rate of SCAS–HP rises with larger solar collector area and higher outdoor air temperature, but declines as the outlet water temperature rises. Among the three configurations, T–SCAS–HP outperformed S–SCAS–HP and P–SCAS–HP by 0.37 and 0.42 in COPh at outlet water temperatures of 35 °C and 55 °C, respectively. In typical-day simulations, the COPh showed a relative increase of 50.9% compared to that of the VI-ASHP system, corresponding to an absolute rise from 3.73 to 5.62 with a 130 m2 collector at 35 °C outlet water temperature. The energy saving rate ranked highest in Lhasa, followed by Beijing and Harbin. The shortest payback period (5.6 years) was achieved in Harbin with a 130 m2 collector area at the outlet water temperature of 55 °C. The proposed system is particularly suitable for large heating demand and long heating seasons in cold climates with abundant solar resources.