A flexible hybrid-energy heat pump using efficient ionic liquids for sustainable solar cooling

Heat pumps have been widely adopted as an effective solution for sustainable building cooling. Thermally-driven absorption heat pumps can significantly reduce electricity consumption; however, they are hindered by issues such as low reliability, limited applicability, low efficiency, and crystallization limitations. To address these challenges, this study introduces a hybrid-energy heat pump (HEHP) that offers enhanced flexibility, enabling a gradual transition from an absorption cycle to a compression cycle, utilizing novel refrigerant/ionic liquids (ILs) as working fluids to eliminate crystallization constraints. The cycle performance of H2O/ILs, NH3/ILs, HFC/ILs, and HFO/ILs is compared, with NH3/[DMIM][DMP] identified as the most suitable alternative due to its high electrical coefficient of performance (COPele) of 19.2 and its significantly high compactness. The HEHP cycle employing NH3/[DMIM][DMP] is designed for Hong Kong, with the energy efficiency of the solar absorption sub-cycle ranging from 0.31 to 0.50. As the solar collector area rises, the COPele rises from 6.8 to 19.8, while the unit cooling potential decreases from 1.75 kWh/m2/day to 0.64 kWh/m2/day. The levelized cooling cost initially decreases before increasing, reflecting the interplay between higher initial costs and reduced operation costs, with the lowest value (0.075 USD/kWh) occurring at a solar collector area of 600 m2. With a relative improvement in demand met ratio of 27.7% to 47.5% and a reduction in electricity consumption of 39.4–110.0 MWh/year, the HEHP cycle demonstrates both high efficiency and flexible applicability for sustainable building cooling.