Passive daytime radiative cooling (PDRC) (also passive radiative cooling, daytime passive radiative cooling, radiative sky cooling, photonic radiative cooling, and terrestrial radiative cooling[2][3][4][5]) is the use of unpowered, reflective/thermally-emissive surfaces to lower the temperature of a building or other object.[6]
Some estimates propose that dedicating 1–2% of the Earth's surface area to PDRC would stabilize surface temperatures.[16][3] Regional variations provide different cooling potentials with desert and temperate climates benefiting more than tropical climates, attributed to the effects of humidity and cloud cover.[17][18][19] PDRCs can be included in adaptive systems, switching from cooling to heating to mitigate any potential "overcooling" effects.[20][21] PDRC applications for indoor space cooling is growing with an estimated "market size of ~$27 billion in 2025."[22]
PDRCs leverage the natural process of radiative cooling, in which the Earth cools by releasing heat to space.[25][26][7] PDRC operates during daytime.[27] 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, proportional to the exposed surface area.[28][29]
PDRC applications are deployed as sky-facing surfaces.[14] Low-cost scalable PDRC materials with potential for mass production include coatings, thin films, metafabrics, aerogels, and biodegradable surfaces.
While typically white, other colors can also work, although generally offering less cooling potential.[30][31]
Research, development, and interest in PDRCs has grown rapidly since the 2010s, attributable to a breakthrough in the use of photonic metamaterials to increase daytime cooling in 2014,[4][32][15][33] along with growing concerns over energy use and global warming.[34][35] PDRC can be contrasted with traditional compression-based cooling systems (e.g., air conditioners) that consume substantial amounts of energy, have a net heating effect (heating the outdoors more than cooling the indoors), require ready access to electric power and often employ coolants that deplete the ozone or have a strong greenhouse effect,[36][37]
^ abWang, Tong; Wu, Yi; Shi, Lan; Hu, Xinhua; Chen, Min; Wu, Limin (2021). "A structural polymer for highly efficient all-day passive radiative cooling". Nature Communications. 12 (365): 365. doi:10.1038/s41467-020-20646-7. PMC7809060. PMID33446648. Accordingly, designing and fabricating efficient PDRC with sufficiently high solar reflectance (𝜌¯solar) (λ ~ 0.3–2.5 μm) to minimize solar heat gain and simultaneously strong LWIR thermal emittance (ε¯LWIR) to maximize radiative heat loss is highly desirable. When the incoming radiative heat from the Sun is balanced by the outgoing radiative heat emission, the temperature of the Earth can reach its steady state.
^ abcZevenhovena, Ron; Fält, Martin (June 2018). "Radiative cooling through the atmospheric window: A third, less intrusive geoengineering approach". Energy. 152: 27. Bibcode:2018Ene...152...27Z. doi:10.1016/j.energy.2018.03.084. S2CID116318678 – via Elsevier Science Direct. An alternative, third geoengineering approach would be enhanced cooling by thermal radiation from the Earth's surface into space." [...] "With 100 W m2 as a demonstrated passive cooling effect, a surface coverage of 0.3% would then be needed, or 1% of Earth's land mass surface. If half of it would be installed in urban, built areas which cover roughly 3% of the Earth's land mass, a 17% coverage would be needed there, with the remainder being installed in rural areas.
^"What is 3M Passive Radiative Cooling?". 3M. Archived from the original on 22 September 2021. Retrieved 27 September 2022. Passive Radiative Cooling is a natural phenomenon that only occurs at night in nature because all nature materials absorb more solar energy during the day than they are able to radiate to the sky.
^Wang, Tong; Wu, Yi; Shi, Lan; Hu, Xinhua; Chen, Min; Wu, Limin (2021). "A structural polymer for highly efficient all-day passive radiative cooling". Nature Communications. 12 (365): 365. doi:10.1038/s41467-020-20646-7. PMC7809060. PMID33446648. Accordingly, designing and fabricating efficient PDRC with sufficiently high solar reflectance (𝜌¯solar) (λ ~ 0.3–2.5 μm) to minimize solar heat gain and simultaneously strong LWIR thermal emittance (ε¯LWIR) to maximize radiative heat loss is highly desirable. When the incoming radiative heat from the Sun is balanced by the outgoing radiative heat emission, the temperature of the Earth can reach its steady state.