Gravity-Driven Optically Tunable Window for All-Climate Building Energy Efficiency

Optically tunable windows enhance building energy efficiency by dynamically regulating solar transmission and mid-infrared (MIR) emissivity. However, windows relying on dynamic materials such as VO2, WO3, and hydrogels face intrinsic limitations, including limited tunability, narrow spectral modulation, low visible transmittance, and high haze. Here, instead of using dynamically materials, we propose a gravity-driven optical switching mechanism that enables broadband spectral modulation across the solar spectrum (0.3 to 2.5 μm) and the MIR range (4 to 20 μm). It achieves high visible transmittance in both cooling (62%) and heating modes (81%), along with a solar modulation capability of 44% and an emissivity modulation of 70% (0.24 in heating mode and 0.94 in cooling mode). Outdoor experiments demonstrated that the developed window can reduce indoor temperature by approximately 7.3 ℃ on sunny days in Hong Kong. Global-scale energy modeling further confirmed its superior energy-saving performance across diverse climatic regions, demonstrating potential reductions in building space heating and cooling load of up to 0.95 GJ m-2 yr-1 compared to conventional low-e glazing. With high stability and scalability, this novel structurally driven switching mechanism may offer practical implementations for next-generation sustainable buildings.