Passive daytime radiative cooling

Passive daytime radiative cooling (PDRC) is a zero-energy building cooling method proposed as a solution to reduce air conditioning, lower urban heat island effect, cool human body temperatures in extreme heat, move toward carbon neutrality and control global warming by enhancing terrestrial heat flow to outer space through the installation of thermally-emissive surfaces on Earth that require zero energy consumption or pollution.^{[2][3][4][5][6][1][7][8][9]} In contrast to compression-based cooling systems that are prevalently used (e.g., air conditioners), consume substantial amounts of energy, have a net heating effect, require ready access to electricity and often require coolants that are ozone-depleting or have a strong greenhouse effect,^[10][11] application of PDRCs may also increase the efficiency of systems benefiting from a better cooling, such like photovoltaic systems, dew collection techniques, and thermoelectric generators.^[12][13]

PDRC surfaces are designed to be high in solar reflectance (to minimize heat gain) and strong in longwave infrared (LWIR) thermal radiation heat transfer through the atmosphere's infrared window (8–13 μm) to cool temperatures even during the daytime.^[14][15][16] It is also referred to as passive radiative cooling, daytime passive radiative cooling, radiative sky cooling, photonic radiative cooling, and terrestrial radiative cooling.^{[15][16][12][17]} PDRC differs from solar radiation management because it increases radiative heat emission rather than merely reflecting the absorption of solar radiation.^[18]

Some estimates propose that if 1–2% of the Earth's surface area were dedicated to PDRC that warming would cease and temperature increases would be rebalanced to survivable levels.^[19][16] Regional variations provide different cooling potentials with desert and temperate climates benefiting more from application than tropical climates, attributed to the effects of humidity and cloud cover on reducing the effectiveness of PDRCs.^[20][21][22] Low-cost scalable PDRC materials feasible for mass production have been developed, such as coatings, thin films, metafabrics, aerogels, and biodegradable surfaces.

PDRCs can be included in self-adaptive systems, 'switching' from passive cooling to heating to mitigate any potential "overcooling" effects in urban environments.^[3][23] They have also been developed in colors other than white, although there is generally a tradeoff in cooling potential, since darker color surfaces are less reflective.^[24][25] Research, development, and interest in PDRCs has grown rapidly since the 2010s, which has been attributed to a scientific breakthrough in the use of photonic metamaterials to achieve daytime cooling in 2014,^[26][12][27] along with growing concerns over energy use and global warming.^[28][29]

Passive daytime radiative cooling (PDRC) uses the coldness of outer space as a renewable energy source to achieve daytime cooling that can be used in many applications,^[30][31][32] such as indoor space cooling,^[33][34] outdoor urban heat island mitigation,^[35][36] and solar cell efficiency.^[37][38] PDRC surfaces are designed to be high in solar reflectance to minimize heat gain and strong in longwave infrared (LWIR) thermal radiation heat transfer.^[39] On a planetary scale, it has been proposed as a way to slow and reverse global warming.^[40][41] PDRC applications are deployed as sky-facing surfaces, similar to other renewable energy sources such as photovoltaic systems and solar thermal collectors.^[38] PDRC became possible with the ability to suppress solar heating using photonic metamaterials, first published in a study by Raman et al. to the scientific community in 2014.^[37][42] PDRC applications for indoor space cooling is growing with an estimated "market size of ~$27 billion in 2025."^[43]