Performance and transient behavior of a hydrogel flat-plate/finned dual-source heat pump under multi-mode heating: An experimental study

Conventional solar-air dual-source heat pumps employ separate evaporators, complicating system coordination. This paper presents a direct-expansion heat pump with an integrally formed flat-plate/finned dual-source (FFDS) heat exchanger and a thermochromic hydrogel coating—transparent in winter for solar absorption and opaque in summer for solar rejection. Focusing on winter heating, multi-mode experiments (DHW-only, combined intermittent/continuous space heating) were conducted under various weather conditions. Through high-resolution transient measurements, the study innovatively reveals the generation and evolution mechanisms of sawtooth oscillations in temperatures and pressures under real-time EEV dynamic regulation, including phase shift relationships and the attenuation of superheat oscillation amplitude with increasing condensing pressure—a mechanistic explanation rarely addressed in the literature. Comparative experiments quantify the asymmetric sensitivity of evaporator- and condenser-side parameters to environmental disturbances: compared with sunny conditions, overcast conditions reduce evaporating temperature and pressure by 10.42 °C and 0.136 MPa, raise the pressure ratio from 3.71 to 5.13, and lower the average COP from 4.77 to 3.67, while the condenser side varies modestly. Performance evaluation demonstrates the system’s adaptability to different weather scenarios and user demands: even under nighttime continuous heating with no solar energy input, the system maintains stable operation with an average COP of 3.23. These findings deepen the understanding of transient dynamics and multi-mode performance responses of integrated dual-source heat pumps under EEV dynamic regulation, and provide empirical data and insights for the development of stable and reliable control strategies and integrated system designs.