Radiative Cooling Walls with Asymmetric Emissivity for Building Envelope

Passive daytime radiative cooling (PDRC) is an attractive electricity-free approach to reducing energy consumption of buildings by reflecting sunlight [wavelengths (<i>λ</i>) ~ 0.3 to 2.5 μm] and emitting infrared radiation (IR) through the atmospheric transparency window (ATW: <i>λ</i> ~ 8 to 13 μm) to the cold sky. Current PDRC research focuses on roofs, but limited attention has been paid to the walls, which occupy a major portion of building envelopes. Unlike the roofs, the walls face both cold sky and hot ground, so either high emissivity or low emissivity is not the best solution. Here we develop a scalable cooling wall with asymmetric emissivity. The wall can simultaneously reflect the thermal radiation from the hot ground and remain emissive to the cold sky, to achieve further building cooling and even sub-ambient cooling, which are demonstrated by simulations and experiments. Such asymmetric emissivity leads to further cooling of ~100 W m^-2 than conventional walls at a ground temperature &gt; 50 °C, and shows temperature drops of 3.1 °C (peak) / 2.3 °C (daily average) compared to the control wall in the field tests. Taking the United States as an example, the proposed zigzag wall can work effectively over 29% area and benefit 45% population, providing up to 27 MJ m^-2 year-round saving per wall area for a typical midrise apartment building.