Radiative cooling is one of the few ways an object in space can release energy. In particular, white dwarf stars no longer generate energy by fusion or gravitational contraction, and they have no solar wind. So the only way its temperature changes is by radiative cooling. This makes its temperature as a function of age very predictable, so by observing the temperature, astronomers can deduce the age of the star.[1][2]​.

Night ice making in early India and Iran

In India, before the invention of artificial refrigeration technology, ice making by night cooling was common. The apparatus consisted of a shallow ceramic tray with a thin layer of water, placed outdoors with clear exposure to the night sky. The bottom and sides were insulated with a thick layer of hay. On a clear night, the water would lose heat upward to radiation. As long as the air was still and not far above freezing, the heat gain from the surrounding air by convection was low enough to allow water to freeze.[3]​ A similar technique was used in Iran as well.[4]​.

Architecture

Cool roofs combine high solar reflectance with high infrared emission, simultaneously reducing heat gain from the sun and increasing heat removal through radiation. Therefore, radiative cooling offers immense potential for complementary passive cooling of residential and commercial buildings.[5] Traditional building surfaces such as paint coatings, bricks and concrete have high emissions of up to 0.96.[6] Consequently, they radiate heat into the sky to passively cool buildings at night. If made sufficiently reflective in sunlight, these materials can also achieve radiative cooling during the day.

The most common radiation coolers found in buildings are white cool roof paint coatings, which have solar reflectances of up to 0.94 and thermal emissions of up to 0.96.[7] The solar reflectance of paints arises from the optical dispersion of dielectric pigments embedded in the polymer paint resin, while the thermal emission comes from the polymer resin itself. However, because typical white pigments, such as titanium dioxide and zinc oxide, absorb ultraviolet radiation, the solar reflectances of paints based on such pigments do not exceed 0.95. Recently, researchers have developed paintable porous polymer coatings whose pores scatter sunlight to provide a solar reflectance of 0.96 and a thermal emission of 0.97.[8]​ In experiments under direct sunlight, the coatings achieve temperatures of 6 °C and a cooling power of 96 W/m².

Other notable radiative cooling strategies include dielectric films on metallic mirrors,[9]​ and polymer composites or polymers on silver or aluminum films.[10]​ In 2014, researchers developed a multilayer thermal photonic structure that selectively emits long-wavelength infrared radiation into space, and can achieve subambient cooling of 5 °C under direct sunlight.[11]​ Silver polymer films with solar reflectances of 0.97 and a thermal emission of 0.96, which stay 11 °C cooler than commercial white paints in the mid-summer sun.[12] Researchers have also explored designs with silicon dioxide (Silicon Oxide (IV)") dielectric or silicon carbide particles embedded in polymers that are translucent at solar wavelengths and emissive at infrared.[13]​[14]​ In 2017, an example of this design was reported with resonant polar silica microspheres randomly embedded in a polymer matrix.[15]​ The material is translucent to sunlight and has an infrared emissivity of 0.93 in the atmospheric infrared transmission window. When backed with a silver coating, the material achieved a midday radiative cooling power of 93 W/m² under direct sunlight along with high-performance, economical roll-to-roll manufacturing.

Radiative cooling is one of the few ways an object in space can release energy. In particular, white dwarf stars no longer generate energy by fusion or gravitational contraction, and they have no solar wind. So the only way its temperature changes is by radiative cooling. This makes its temperature as a function of age very predictable, so by observing the temperature, astronomers can deduce the age of the star.[1][2]​.

Night ice making in early India and Iran

In India, before the invention of artificial refrigeration technology, ice making by night cooling was common. The apparatus consisted of a shallow ceramic tray with a thin layer of water, placed outdoors with clear exposure to the night sky. The bottom and sides were insulated with a thick layer of hay. On a clear night, the water would lose heat upward to radiation. As long as the air was still and not far above freezing, the heat gain from the surrounding air by convection was low enough to allow water to freeze.[3]​ A similar technique was used in Iran as well.[4]​.

Architecture

Cool roofs combine high solar reflectance with high infrared emission, simultaneously reducing heat gain from the sun and increasing heat removal through radiation. Therefore, radiative cooling offers immense potential for complementary passive cooling of residential and commercial buildings.[5] Traditional building surfaces such as paint coatings, bricks and concrete have high emissions of up to 0.96.[6] Consequently, they radiate heat into the sky to passively cool buildings at night. If made sufficiently reflective in sunlight, these materials can also achieve radiative cooling during the day.

The most common radiation coolers found in buildings are white cool roof paint coatings, which have solar reflectances of up to 0.94 and thermal emissions of up to 0.96.[7] The solar reflectance of paints arises from the optical dispersion of dielectric pigments embedded in the polymer paint resin, while the thermal emission comes from the polymer resin itself. However, because typical white pigments, such as titanium dioxide and zinc oxide, absorb ultraviolet radiation, the solar reflectances of paints based on such pigments do not exceed 0.95. Recently, researchers have developed paintable porous polymer coatings whose pores scatter sunlight to provide a solar reflectance of 0.96 and a thermal emission of 0.97.[8]​ In experiments under direct sunlight, the coatings achieve temperatures of 6 °C and a cooling power of 96 W/m².

Other notable radiative cooling strategies include dielectric films on metallic mirrors,[9]​ and polymer composites or polymers on silver or aluminum films.[10]​ In 2014, researchers developed a multilayer thermal photonic structure that selectively emits long-wavelength infrared radiation into space, and can achieve subambient cooling of 5 °C under direct sunlight.[11]​ Silver polymer films with solar reflectances of 0.97 and a thermal emission of 0.96, which stay 11 °C cooler than commercial white paints in the mid-summer sun.[12] Researchers have also explored designs with silicon dioxide (Silicon Oxide (IV)") dielectric or silicon carbide particles embedded in polymers that are translucent at solar wavelengths and emissive at infrared.[13]​[14]​ In 2017, an example of this design was reported with resonant polar silica microspheres randomly embedded in a polymer matrix.[15]​ The material is translucent to sunlight and has an infrared emissivity of 0.93 in the atmospheric infrared transmission window. When backed with a silver coating, the material achieved a midday radiative cooling power of 93 W/m² under direct sunlight along with high-performance, economical roll-to-roll manufacturing.