A Transparent Directional Thermal Emitter

Passive radiative cooling has been widely investigated; however, achieving effective cooling on windows remains a critical challenge, as they are the least energy-efficient building components with a limited sky view. Unlike horizontal surfaces, effective cooling on vertically oriented windows requires the rigorous mitigation of parasitic thermal radiation from the hot environment while simultaneously maintaining visible transparency. Here, we propose a transparent asymmetric directional emitter (TADE) with visible transmittance up to 0.83 and asymmetric emission characteristics, exhibiting high/low emissivity toward the sky/ground, respectively, in a vertical configuration. Field measurements demonstrate that at summer noon facing a hot ground, TADE-integrated glass achieves a maximum 4.8 °C reduction compared to regular glass. Global building energy simulations reveal that the directional thermal radiation characteristics of TADE can effectively reduce HVAC energy consumption in hot climates, providing a novel approach to enhancing the energy efficiency of building windows.