Radiative Cooling and Thermoregulation in the Earth's Glow using Directional Emitters

Buildings exchange large amounts of heat with the environment as longwave radiation. Typical building envelopes are omnidirectional emitters and absorbers of this heat. However, vertical surfaces of buildings see a thermally oriented environment – the cold sky above and a seasonally hot or cold terrestrial environment near and below the horizon. Controlling radiative heat flows to and from such environments can yield large energy savings and thermal comfort in buildings, but remains unattained because it entails avoiding seasonal terrestrial heat gains and losses, while emitting heat skywards via the radiative cooling mechanism. As a solution, we propose micropatterned, directionally emissive facades (DEFs) which, on walls and windows, can achieve a skywards net heat loss through the long-wavelength infrared (LW-IR, λ~8-13 μm) atmospheric transmission window, while blocking terrestrial heat exchange near and below the horizon. Such DEFs can radiatively cool buildings by up to 100 Wm^-2 in the summer, and reduce overcooling by 20 Wm^-2 or more in the winter, relative to typical envelopes. This novel passive thermoregulation may yield similar or much greater energy savings per m² than cool roofs, depending on the type of facade. Furthermore, DEFs may cool urban canyons by 1-2°C, reducing heat island effects, and in turn lowering building cooling loads. Our design represents a new platform for tremendous and untapped energy savings.