Radiative cooling
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In the study of
Radiative cooling has been applied in various contexts throughout human history, including
Terrestrial radiative cooling
Mechanism
Infrared radiation can pass through dry, clear air in the wavelength range of 8–13 µm. Materials that can absorb energy and radiate it in those wavelengths exhibit a strong cooling effect. Materials that can also reflect 95% or more of sunlight in the 200 nanometres to 2.5 µm range can exhibit cooling even in direct sunlight.[9]
Earth's energy budget
The Earth-atmosphere system is radiatively cooled, emitting long-wave (
Convective transport of heat, and evaporative transport of latent heat are both important in removing heat from the surface and distributing 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
Nocturnal surface cooling
Radiative cooling is commonly experienced on cloudless nights, when heat is radiated into outer space from Earth's surface, or from the skin of a human observer. The effect is well-known among amateur astronomers.
The effect can be experienced by comparing skin temperature from looking straight up into a cloudless night sky for several seconds, to that after placing a sheet of paper between the face and the sky. Since outer space radiates at about a temperature of 3 K (−270.15 °C; −454.27 °F), and the sheet of paper radiates at about 300 K (27 °C; 80 °F) (around room temperature), the sheet of paper radiates more heat to the 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. The sheet does not block the cold, but instead reflects heat to the face and radiates the heat of the face that it just absorbed.
The same radiative cooling mechanism can cause
Kelvin's estimate of the Earth's age
The term radiative cooling is generally used for local processes, though the same principles apply to cooling over geological time, which was first
Astronomy
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.[10][11]
Applications
Climate change
The widespread application of
Currently the Earth is absorbing ~1 W/m2 more than it is emitting, which leads to an overall warming of the climate. By covering the Earth with a small fraction of thermally emitting materials, the heat flow away from the Earth can be increased, and the net radiative flux can be reduced to zero (or even made negative), thus stabilizing (or cooling) the Earth (...) If only 1%–2% of the Earth’s surface were instead made to radiate at this rate rather than its current average value, the total heat fluxes into and away from the entire Earth would be balanced and warming would cease.[13]
PDRCs mimic the natural process of radiative cooling, in which the Earth cools itself by releasing heat to outer space (Earth's energy budget), although during the daytime, lowering ambient temperatures under direct solar intensity.[14] On a clear day, solar irradiance can reach 1000 W/m2 with a diffuse component between 50-100 W/m2. The average PDRC has an estimated cooling power of ~100-150 W/m2.[15] The cooling power of PDRCs is proportional to the exposed surface area of the installation.[16]
Architecture
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.[18] 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. 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.
In 2014, researchers developed the first daytime radiative cooler using a multi-layer thermal photonic structure that selectively emits
Other notable radiative cooling strategies include dielectric films on metal mirrors,[21] and polymer or polymer composites on silver or aluminum films.[22] 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.[23] Researchers 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.[24][25] In 2017, an example of this design with resonant polar silica microspheres randomly embedded in a polymeric matrix, was reported.[26] 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.
Heat shields
James Webb Space Telescope
The James Webb Space Telescope uses radiative cooling to reach its operation temperature of about 50 K. To do this, its large reflective sunshield blocks radiation from the Sun, Earth, and Moon. The telescope structure, kept permanently in shadow by the sunshield, then cools by radiation.
Nocturnal ice making in early India and Iran
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. 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.[27][28][3]
In Iran, this involved making large flat
See also
- Heat shield
- Optical solar reflector, used for thermal control of spacecraft
- Passive cooling
- Radiative forcing
- Stefan–Boltzmann law
- Terrestrial albedo effect
- Urban heat island
- Urban thermal plume
References
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- ^ Mestel, L. (1952). "On the theory of white dwarf stars. I. The energy sources of white dwarfs". Monthly Notices of the Royal Astronomical Society. 112 (6): 583–597. .
- ^ "Cooling white dwarfs" (PDF). Physics Department, University of Patras.
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Passive radiative cooling utilizes atmospheric transparency window (8–13 µm) to discharge heat into outer space and inhibits solar absorption.
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- ^ "Emissivity Coefficients Materials". www.engineeringtoolbox.com. Retrieved 2019-02-23.
- ^ "Find rated products – Cool Roof Rating Council". coolroofs.org. Retrieved 2019-02-23.
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- ^ WO 2016205717A1, Yu, Nanfang; Mandalal, Jyotirmoy; Overvig, Adam and Shi, Norman Nan, "Systems and methods for radiative cooling and heating", issued 2016-06-17
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- Indian Institute of Technology Kharagpur. Archived from the original(PDF) on 2011-12-16.
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