A hybrid H₂O/IL absorption and CO₂ compression air-source heat pump for ultra-low ambient temperatures

Heat pumps are promising for carbon neutrality. However, the existing vapor-compression heat pumps suffer from severe performance deterioration while the absorption heat pumps rely on toxic NH₃-based fluids in sub-zero conditions. Therefore, a novel hybrid H₂O/ionic liquid absorption and CO₂ compression air-source heat pump is proposed for ultra-low ambient temperatures. The verified property and cycle models are used for performance optimization, characterization, and comparison. The coupling temperature that oppositely affects the efficiencies of the two sub-cycles is optimized to be 6 °C. With the ambient temperature rising from −30 °C to 0 °C, the primary energy efficiency (PEE) increases from 1.064 to 1.256, higher by 18–40% compared to a gas boiler. The heating capacity stably ranges in 94.0–102.9 kW, significantly increasing the reliability in cold climate. As the driving temperature rises from 96 °C to 130 °C, the PEE slightly varies in 1.155–1.190, while the exergetic coefficient of performance decreases from 0.609 to 0.600, encouraging the use of low-temperature heat sources (e.g., 100 °C). Compared to existing heat pumps, the hybrid heat pump efficiently operates at −30 °C, showing the highest PEE for most sub-zero conditions with much slower heating capacity deterioration. The advantage is much higher under colder conditions.