Passive daytime radiative cooling
Passive daytime radiative cooling (PDRC)—also called passive radiative cooling, daytime passive radiative cooling, radiative sky cooling, photonic radiative cooling, and terrestrial radiative cooling—is the use of surfaces that are highly reflective and have high thermal emissivity to reduce the temperature of a building or other object.
It has been proposed as a method of reducing temperature increases caused by greenhouse gases by reducing the energy needed for air conditioning, reducing the urban heat island effect, and human body temperatures.
PDRC can aid systems that are more efficient at lower temperatures, such as photovoltaic systems, dew collection devices, and thermoelectric generators.
Some estimates propose that dedicating 1–2% of the Earth's surface area to PDRC would stabilize surface temperatures. 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. PDRC can be included in adaptive systems, switching from cooling to heating to mitigate any potential overcooling effects. PDRC applications for indoor space cooling is growing with an estimated "market size of ~$27 billion in 2025".
PDRC surfaces are designed to be high in solar reflectance to minimize heat gain and strong in longwave infrared (LWIR) thermal radiation heat transfer matching the atmosphere's infrared window (8–13 μm). This allows the heat to pass through the atmosphere into space.
PDRC leverages the natural process of radiative cooling, in which the Earth cools by releasing heat to space. PDRC operates during daytime. On a clear day, solar irradiance can reach 1000 W/m2 with a diffuse component between 50 and 100 W/m2. The average PDRC has an estimated cooling power of ~100–150 W/m2.
PDRC applications are deployed as sky-facing surfaces. 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.
Research, development, and interest in PDRC has grown rapidly since the 2010s, attributable to a breakthrough in 2014 in the use of photonic metamaterials to increase daytime cooling, along with growing concerns over energy use and global warming. PDRC can be contrasted with conventional 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 layer or have a strong greenhouse effect.
Unlike solar radiation management, PDRC increases heat emission beyond simple reflection.