Experimental and numerical evaluation of a seasonally reversible RC–PCM–SA composite external louver system for window thermal regulation

Abstract

Adaptive building envelopes that coordinate solar regulation, thermal storage, and radiative heat exchange remain challenging for year-round energy management. This study proposes a reversible radiative cooling–phase change material–selective absorber (RC–PCM–SA) external louver system for hot-summer and cold-winter regions. The RC layer provides solar reflection and radiative cooling in summer, the SA layer enhances solar heat utilization in winter, and the PCM layer provides thermal buffering. Scaled climatic experiments, Fluent simulations, and annual EnergyPlus simulations were conducted to evaluate thermal performance and energy-saving potential. Experiments demonstrated distinct seasonal thermal responses, while CFD analysis revealed reversed heat-transfer pathways between summer heat rejection and winter heat harvesting. Compared with conventional double glazing and a fixed external louver, the proposed system achieved additional annual energy savings through seasonal functional switching and thermal storage. Annual energy consumption decreased from 2774.98 to 2214.73 kWh, corresponding to a corrected net saving of 555.63 kWh/year and an energy-saving rate of 20.0 % after accounting for lighting and parasitic energy consumption.