Research on coupling modeling and operational parameter optimization of solar-assisted CO2 heat pump heating system

Low ambient temperatures degrade heat-pump performance during the heating season in northern China, posing a challenge to efficient building heating in cold regions. This study develops and validates a coupled model of a solar-assisted carbon dioxide (CO 2 ) heat-pump heating system. Unlike previous studies that separately analyze heat-pump units, solar collectors, or heating terminals, the proposed model integrates solar-house heat collection, the CO 2 cycle, water-side heating, and indoor thermal balance, while accounting for inclined-surface irradiance, radiative exchange, and wind effects. Experimental validation yielded mean absolute percentage errors (MAPEs) of 4.78%, 1.13%, 8.73%, and 8.78% for return-water, room-air, solar-house-air, and supply-water temperatures, respectively. Based on the validated model, the effects of meteorological conditions and flow rates were analyzed, and response-surface optimization was performed to maximize the system coefficient of performance ( COP sys ). Higher irradiance and ambient temperature improved system performance, whereas a higher diffuse-radiation fraction and parallel wind speed reduced it. The high-efficiency operating region corresponded to air flow rates of 0.20–0.35 kg/s and circulating-water flow rates of 0.018–0.030 kg/s. These results demonstrate the importance of coordinating air-side and water-side flow rates and provide theoretical guidance for the efficient operation of solar-assisted CO 2 heat-pump heating systems in cold regions.