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Radiative cooling is the process by which a body loses heat by thermal radiation.
Terrestrial radiative coolingEdit
Earth's energy budgetEdit
In the case of the Earth-atmosphere system radiative cooling is the process by which long-wave (infrared) radiation is emitted to balance the absorption of short-wave (visible light) energy from the sun.
The exact process by which Earth loses heat is rather more complex than often portrayed. In particular, convective transport of heat, and evaporative transport of latent heat are both important in removing heat from the surface and redistributing it in the atmosphere. Pure radiative transport is more important higher up in the atmosphere. Diurnal and geographical variation further complicate the picture.
The large-scale circulation of the Earth's atmosphere is driven by the difference in absorbed solar radiation per square meter, as the sun heats the Earth more in the Tropics, mostly because of geometrical factors. The atmospheric and oceanic circulation redistributes some of this energy as sensible heat and latent heat partly via the mean flow and partly via eddies, known as cyclones in the atmosphere. Thus the tropics radiate less to space than they would if there were no circulation, and the poles radiate more; however in absolute terms the tropics radiate more energy to space.
Nocturnal surface coolingEdit
Radiative cooling is commonly experienced on cloudless nights, when heat is radiated into space from the surface of the Earth, or from the skin of a human observer. The effect is well-known among amateur astronomers, and can personally be felt on the skin of an observer on a cloudless night. To feel the effect, one compares the difference between looking straight up into a cloudless night sky for several seconds, to that of placing a sheet of paper between one's face and the sky. Since outer space radiates at about a temperature of 3 kelvins (-270 degrees Celsius or -450 degrees Fahrenheit), and the sheet of paper radiates at about 300 kelvins (room temperature), the sheet of paper radiates more heat to one's face than does the darkened cosmos. 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. Note that it is not correct to say that the sheet "blocks the cold" of the night sky; instead, the sheet is reflecting heat to one's face and radiating back the heat of one's face that it just absorbed to not exceed the air temperature too much.
Kelvin's estimate of the Earth's ageEdit
The term radiative cooling is generally used for local processes, though the same principles apply to cooling over geological time, which was first used by Kelvin to estimate the age of the Earth (although his estimate ignored the substantial heat released by radioisotope decay, not known at the time).
Radiative cooling is one of the few ways an object in space can give off energy. In particular, white dwarf stars are no longer generating energy by fusion or gravitational contraction, and have no solar wind. So the only way their temperature changes is by radiative cooling. This makes their temperature as a function of age very predictable, so by observing the temperature, astronomers can deduce the age of the star.
Nocturnal ice making in Early India and IranEdit
In India before the invention of artificial refrigeration technology, ice making by nocturnal cooling was common. The apparatus consisted of a shallow ceramic tray with a thin layer of water, placed outdoors with a 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 by radiation upwards. Provided the air was calm and not too far above freezing, heat gain from the surrounding air by convection was low enough to allow the water to freeze. A similar technique was used in Iran as well.
Cool roofs combine high solar reflectance with high infrared emittance, thereby simultaneously reducing heat gain from the sun and increasing heat removal through radiation. Radiative cooling thus offers immense potential for supplementary passive cooling to residential and commercial buildings. Traditional building surfaces, such as paint coatings, brick and concrete have high emittances of up to 0.96. Consequently, they radiate heat into the sky to passively cool buildings at night. If made sufficiently reflective to sunlight, these materials can also achieve radiative cooling during the day.
The most common radiative coolers found on buildings are white cool-roof paint coatings, which have solar reflectances of up to 0.94, and a thermal emittances of up to 0.96. The solar reflectance of the paints arises from optical scattering by the dielectric pigments embedded in the polymer paint resin, while the thermal emittance arises from the polymer resin itself. However, because typical white pigments like 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 give solar reflectance of 0.96-0.99 and thermal emittance of 0.97. In experiments under direct sunlight, the coatings achieve 6°C sub-ambient temperatures and cooling powers of 96 W/m2.
Other notable radiative cooling strategies include dielectric films on metal mirrors, and polymer or polymer composites on silver or aluminum films. In 2014, researchers developed a multi-layer thermal photonic structure which selectively emits long wavelength infrared radiation into space, and can achieve 5°C sub-ambient cooling under direct sunlight. 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, have been also been reported. Researchers have also explored designs with dielectric silicon dioxide or silicon carbide particles embedded in polymers that are translucent in the solar wavelengths and emissive in the infrared. In 2017, an example of this design with resonant polar silica microspheres randomly embedded in a polymeric matrix, was reported. The material is translucent to sunlight and has infrared emissivity of 0.93 in the infrared atmospheric transmission window. 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.
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