Radiative cooling

Radiative cooling has been applied in various contexts throughout human history, including ice making in India and Iran,[3] heat shields for spacecraft,[4] and in architecture.

[6][7] It has since been proposed as a strategy to mitigate local and global warming caused by greenhouse gas emissions known as passive daytime radiative cooling.

The effect is blunted by Earth's surrounding atmosphere, and particularly the water vapor it contains, so the apparent temperature of the sky is far warmer than outer space.

The same radiative cooling mechanism can cause frost or black ice to form on surfaces exposed to the clear night sky, even when the ambient temperature does not fall below freezing.

The most common radiative coolers found on buildings are white cool-roof paint coatings, which have solar reflectances of up to 0.94, and thermal emittances of up to 0.96.

[28] Later researchers developed paintable porous polymer coatings, whose pores scatter sunlight to give solar reflectance of 0.96-0.99 and thermal emittance of 0.97.

[31] Silvered polymer films with solar reflectances of 0.97 and thermal emittance of 0.96, which remain 11 °C cooler than commercial white paints under the mid-summer sun, were reported in 2015.

When backed with silver coating, the material achieved a midday radiative cooling power of 93 W/m2 under direct sunshine along with high-throughput, economical roll-to-roll manufacturing.

High emissivity coatings that facilitate radiative cooling may be used in reusable thermal protection systems (RTPS) in spacecraft and hypersonic aircraft.

In such heat shields a high emissivity material, such as molybdenum disilicide (MoSi2) is applied on a thermally insulating ceramic substrate.

Before the invention of artificial refrigeration technology, ice making by nocturnal cooling was common in both India and Iran.

In India, such apparatuses consisted of a shallow ceramic tray with a thin layer of water, placed outdoors with a clear exposure to the night sky.