Editing Radiative cooling (section)

== Applications ==

=== Climate change ===
{{Excerpt|Passive daytime radiative cooling|paragraphs=1-3}}[[File:Atmosfaerisk spredning.png|thumb|211x211px|Passive radiative cooling technologies use the [[infrared window]] of 8–13 μm to radiate heat into outer space and impede solar absorption.]]
=== Architecture ===
[[File:Roof-albedo.svg|thumb|240x240px|Different roof materials absorb more or less heat. A higher roof [[albedo]], or the whiter a roof, the higher its solar reflectance and heat emittance, which can reduce energy use and costs.]]
[[Cool roof]]s combine high solar reflectance with high [[Thermal emittance|infrared emittance]], thereby simultaneously reducing heat gain from the sun and increasing heat removal through radiation. Radiative cooling thus offers potential for passive cooling for residential and commercial buildings. Traditional building surfaces, such as paint coatings, brick and concrete have high emittances of up to 0.96.<ref>{{Cite web|url=https://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html|title=Emissivity Coefficients Materials|website=www.engineeringtoolbox.com|access-date=2019-02-23}}</ref> 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 thermal emittances of up to 0.96.<ref>{{Cite web|title=Find rated products – Cool Roof Rating Council|website=coolroofs.org|url=https://coolroofs.org/directory |access-date=2019-02-23}}</ref> 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 [[Long-wave infrared|long wavelength infrared radiation]] into space, and can achieve 5&nbsp;°C sub-ambient cooling under direct sunlight.<ref>{{Cite journal|last1=Raman |first1=Aaswath P.|last2=Anoma|first2=Marc Abou|last3=Zhu|first3=Linxiao|last4=Rephaeli|first4=Eden |last5=Fan|first5=Shanhui|date=November 2014|title=Passive radiative cooling below ambient air temperature under direct sunlight|journal=Nature|volume=515|issue=7528|pages=540–544|doi=10.1038/nature13883 |bibcode=2014Natur.515..540R|pmid=25428501|s2cid=4382732 }}</ref> 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.<ref>{{Cite journal|last1=Mandal|first1=Jyotirmoy|last2=Fu|first2=Yanke|last3=Overvig|first3=Adam |last4=Jia |first4=Mingxin|last5=Sun|first5=Kerui|last6=Shi|first6=Norman Nan|last7=Yu|first7=Nanfang|last8=Yang |first8=Yuan|date=19 October 2018|title=Hierarchically porous polymer coatings for highly efficient passive daytime radiative cooling|journal=Science|volume=362|issue=6412|pages=315–319|doi=10.1126/science.aat9513 |pmid=30262632|bibcode=2018Sci...362..315M|doi-access=free}}</ref> In experiments under direct sunlight, the coatings achieve 6&nbsp;°C sub-ambient temperatures and cooling powers of 96 W/m<sup>2</sup>.

Other notable radiative cooling strategies include dielectric films on metal mirrors,<ref>{{Cite journal|last1=Granqvist|first1=C. G.|last2=Hjortsberg|first2=A.|date=June 1981|title=Radiative cooling to low temperatures: General considerations and application to selectively emitting SiO films|journal=Journal of Applied Physics|volume=52|issue=6|pages=4205–4220|doi=10.1063/1.329270|bibcode=1981JAP....52.4205G}}</ref>  and polymer or polymer composites on silver or aluminum films.<ref>{{Cite journal|last=Grenier|first=Ph. |date=January 1979|title=Réfrigération radiative. Effet de serre inverse|journal=Revue de Physique Appliquée|volume=14|issue=1|pages=87–90 |doi=10.1051/rphysap:0197900140108700|url=https://hal.archives-ouvertes.fr/jpa-00244594/document}}</ref> Silvered polymer films with solar reflectances of 0.97 and thermal emittance of 0.96, which remain 11&nbsp;°C cooler than commercial white paints under the mid-summer sun, were reported in 2015.<ref>{{Cite journal|last1=Gentle|first1=Angus R.|last2=Smith|first2=Geoff B. |date=September 2015|title=A Subambient Open Roof Surface under the Mid-Summer Sun|journal=Advanced Science |volume=2|issue=9|pages=1500119|doi=10.1002/advs.201500119|pmc=5115392|pmid=27980975}}</ref> 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.<ref>{{Cite journal|last1=Gentle |first1=A. R.|last2=Smith|first2=G. B.|date=2010-02-10|title=Radiative Heat Pumping from the Earth Using Surface Phonon Resonant Nanoparticles|journal=Nano Letters|volume=10|issue=2|pages=373–379 |doi=10.1021/nl903271d|pmid=20055479|bibcode=2010NanoL..10..373G}}</ref><ref>{{Cite patent|inventor= Yu, Nanfang; Mandalal, Jyotirmoy; Overvig, Adam and Shi, Norman Nan |title=Systems and methods for radiative cooling and heating|gdate=2016-06-17|country=WO|number=2016205717A1}}</ref> In 2017, an example of this design with resonant polar silica microspheres randomly embedded in a polymeric matrix, was reported.<ref>{{Cite journal|last1=Zhai|first1=Yao|last2=Ma|first2=Yaoguang|last3=David|first3=Sabrina N.|last4=Zhao |first4=Dongliang|last5=Lou|first5=Runnan|last6=Tan|first6=Gang|last7=Yang|first7=Ronggui |last8=Yin |first8=Xiaobo|date=2017-03-10|title=Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling|journal=Science|volume=355|issue=6329|pages=1062–1066|bibcode=2017Sci...355.1062Z |doi=10.1126/science.aai7899|pmid=28183998|doi-access=free}}</ref> 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/m<sup>2</sup> under direct sunshine along with high-throughput, economical roll-to-roll manufacturing.

===Heat shields===
[[High emissivity coatings]] that facilitate radiative cooling may be used in [[Heat shield |reusable thermal protection systems]] (RTPS) in spacecraft and [[hypersonic]] aircraft. In such heat shields a high emissivity material, such as [[molybdenum disilicide]] (MoSi<sub>2</sub>) is applied on a thermally insulating ceramic substrate.<ref name="Shao2019"/> In such heat shields high levels of total [[emissivity]], typically in the range 0.8 - 0.9, need to be maintained across a range of high temperatures. [[Planck's law]] dictates that at higher temperatures the radiative emission peak shifts to lower wavelengths (higher frequencies), influencing material selection as a function of operating temperature. In addition to effective radiative cooling, radiative thermal protection systems should provide damage tolerance and may incorporate self-healing functions through the formation of a viscous glass at high temperatures.

===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 ===
{{see also|Yakhchāl}}
{{multiple image
| align             = right
| total_width       = 460
| image1            = Yakhchal_radiative_cooling.svg
| alt1              = Radiative cooling energy budget in a yakhchāl
| caption1          = Radiative cooling energy budget
| image2            = Yakhchal-kheshti.jpg
| alt2              = Ice pool beside the [[Meybod]] yakhchāl in Iran
| caption2          = Ice Pool beside the [[Meybod]] yakhchāl in 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.<ref name="icemaking_radiative">{{Cite web |title=Lesson 1: History Of Refrigeration, Version 1 ME |url=http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Ref%20and%20Air%20Cond/pdf/RAC%20%20Lecture%201.pdf |url-status=dead |archive-url=https://web.archive.org/web/20111216053420/http://nptel.iitm.ac.in/courses/Webcourse-contents/IIT%20Kharagpur/Ref%20and%20Air%20Cond/pdf/RAC%20%20Lecture%201.pdf |archive-date=2011-12-16 |publisher=[[Indian Institute of Technology Kharagpur]]}}</ref><ref>{{cite journal |year=1997 |title=XXII. The process of making ice in the East Indies. By Sir Robert Barker, F. R. S. in a letter to Dr. Brocklesby |journal=Philosophical Transactions of the Royal Society of London |volume=65 |pages=252–257 |doi=10.1098/rstl.1775.0023 |jstor=106193 |doi-access=free}}</ref><ref name=":2">{{Cite web |date=2016-04-04 |title=The Persian ice house, or how to make ice in the desert |url=https://www.fieldstudyoftheworld.com/persian-ice-house-how-make-ice-desert/ |access-date=2019-04-28 |website=Field Study of the World}}</ref>

In Iran, this involved making large flat [[Yakhchal#Ice pools|ice pools]], which consisted of a reflection pool of water built on a bed of highly insulative material surrounded by high walls.  The high walls provided protection against convective warming, the insulative material of the pool walls would protect against conductive heating from the ground, the large flat plane of water would then permit evaporative and radiative cooling to take place.