Experimental and numerical analysis of a novel self-adaptive heat pump system with a hydrogel-fin heat exchanger for heating and cooling applications

Solar-assisted heat pump (SAHP) systems have exhibited favorable heating performance, whereas their cooling operation remains limited in multifunctional applications. This limitation mainly arises from the conflicting role of solar irradiation, which promotes evaporator heat gain in heating mode but increases condenser thermal load in cooling mode. To overcome this contradiction, a novel self-adaptive SAHP with a hydrogel-fin heat exchanger (HF-SAHP) is proposed in this study. A self-prepared thermoresponsive hydrogel is integrated onto the solar collector surface. Experimental measurements showed that it transmitted 95.78% of solar irradiation before phase transition, but only 3.41% after phase transition. Hydrogel-based spectral regulation modulates solar absorption and reflection, addressing the opposite solar-energy demands of SAHP systems in the two modes. In addition, an integrally formed base-fin structure also realizes the synergistic utilization of air-source and solar energy. The HF-SAHP was experimentally investigated under heating and cooling modes. A mathematical model was established, validated, and used to evaluate the hydrogel effect. In heating mode, the system achieved a maximum COP of 6.19. In cooling mode, the post-transition hydrogel substantially resisted solar absorption, reducing the condenser heat-dissipation burden by an average of 1563.51 W. The condensing temperature and pressure were effectively suppressed, and the compressor power consumption was decreased by 26.53 W after the performance difference stabilized. Meanwhile, the average cooling power reached 1957.66 W. This study addresses the performance limitation of SAHP systems in cooling mode and provides guidance for the multifunctional applications.