Passive daytime radiative cooling


Passive daytime 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 ~$27billion in 2025".
PDRC surfaces are designed to be high in solar reflectance to minimize heat gain and strong in longwave infrared thermal radiation heat transfer matching the atmosphere's infrared window. 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 1000W/m2 with a diffuse component between 50 and 100W/m2. The average PDRC has an estimated cooling power of ~100–150W/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 that consume substantial amounts of energy, have a net heating effect, 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.

Implementation

A 2019 study reported that "widescale adoption of radiative cooling could reduce air temperature near the surface, if not the whole atmosphere". To address global warming, PDRC materials must be designed "to ensure that the emission is the atmospheric transparency window and out to space, rather than just the atmosphere, which would allow for local but not global cooling".
Desert climates have the highest radiative cooling potential due to low year-round humidity and cloud cover, while tropical climates have less potential due to higher humidity and cloud cover. Costs for global implementation have been estimated at $1.25 to $2.5trillion, or about 3% of annual gross world product, with expected economies of scale. Low-cost scalable materials have been developed for widescale implementation, although some challenges toward commercialization remain.
Some studies recommended efforts to maximize solar reflectance, or albedo, of surfaces, with a goal of thermal emittance of 90%. For example, increasing reflectivity from 0.2 to 0.9 has a much greater effect than improving an already more reflective surface, such as from 0.9 to 0.97.

Benefits

Studies have reported many PDRC benefits:
PDRC has been claimed to be more stable, adaptable, and reversible than stratospheric aerosol injection.
Wang et al. claimed that SAI "might cause potentially dangerous threats to the Earth's basic climate operations" that may not be reversible, and thus preferred PDRC. Munday noted that although "unexpected effects will likely occur" with the global implementation of PDRC, that "these structures can be removed immediately if needed, unlike methods that involve dispersing particulate matter into the atmosphere, which can last for decades".
When compared to the reflective surfaces approach of increasing surface albedo, such as through painting roofs white, or the space mirror proposals of "deploying giant reflective surfaces in space", Munday claimed that "the increased reflectivity likely falls short of what is needed and comes at a high financial cost". PDRC differs from the reflective surfaces approach by "increasing the radiative heat emission from the Earth rather than merely decreasing its solar absorption".

Function

The basic measure of PDRC is its solar reflectivity and heat emissivity, to maximize "net emission of longwave thermal radiation" and minimize "absorption of downward shortwave radiation". PDRC uses the infrared window for heat transfer with the coldness of outer space to radiate heat and subsequently lower ambient temperatures passively, i.e. with zero energy input.
PDRC mimics the natural process of radiative cooling, in which the Earth cools itself by releasing heat to outer space, although during the daytime lowering ambient temperatures under direct solar intensity. On a clear day, solar irradiance can reach 1000W/m2 with a diffuse component between 50 and 100W/m2. the average PDRC had a cooling power of ~100–150W/m2. Cooling power is proportional to the installation's surface area.

Measuring effectiveness

The most useful measurements come in a real-world setting. Standardized devices have been proposed.
Evaluating atmospheric downward longwave radiation based on "the use of ambient weather conditions such as the surface air temperature and humidity instead of the altitude-dependent atmospheric profiles" may be problematic since "downward longwave radiation comes from various altitudes of the atmosphere with different temperatures, pressures, and water vapor contents" and "does not have uniform density, composition, and temperature across its thickness".

Broadband emitters (BE) vs. selective emitters (SE)

Broadband emitters exhibit high emittance in both the atmospheric LWIR window and the solar spectrum, whereas selective emitters only emit longwave infrared radiation.
In theory, selective thermal emitters can achieve higher cooling power. However, selective emitters face challenges in real-world applications that can weaken their performance, such as from dropwise condensation that can accumulate on even hydrophobic surfaces and reduce emission. Broadband emitters outperform selective materials when "the material is warmer than the ambient air, or when its sub-ambient surface temperature is within the range of several degrees".
Each type can be advantageous for certain applications. Broadband emitters may be better for horizontal applications, such as roofs, whereas selective emitters may be more useful on vertical surfaces such as building façades, where dropwise condensation is inconsequential and their stronger cooling power can be achieved.
Broadband emitters can be made angle-dependent to potentially enhance performance. Polydimethylsiloxane is a common broadband emitter. Most PDRC materials are broadband, primarily due to their lower cost and higher performance at above-ambient temperatures.

Hybrid systems

Combining PDRC with other systems may increase cooling power. When included in a system combining thermal insulation, evaporative cooling, and radiative cooling consisting of "a solar reflector, a water-rich and IR-emitting evaporative layer, and a vapor-permeable, IR-transparent, and solar-reflecting insulation layer", cooling power was quadruple that of "a state-of-the-art radiative cooler". This could extend the shelf life of food by 40% in humid climates, and more than triple it in dry climates, without refrigeration. The system, however, requires water to maintain cooling power.
A dual-mode asymmetric photonic mirror consisting of silicon-based diffractive gratings could achieve all-season cooling, even under cloudy and humid conditions, as well as heating. The cooling power of an APM can exceed that of standalone radiative coolers by more than 80%. Under cloudy sky, it could achieve 8 °C more cooling and, for heating, 5.7 °C.

Climatic variations

The cooling potential of various areas varies primarily based on climate zones, weather patterns, and events. Dry and hot regions generally have higher radiative cooling power, while colder regions and those with high humidity or cloud cover generally have less. Cooling potential changes seasonally due to shifts in humidity and cloud cover. Studies mapping daytime radiative cooling potential have been done for China, India, and the United States and across Europe.

Deserts

Dry regions such as western Asia, north Africa, Australia and the southwestern United States are ideal for PDRC due to the relative lack of humidity and cloud cover across the seasons. The cooling potential for desert regions has been estimated as "in the higher range of 80–110W/m2" and as 120W/m2. The Sahara Desert and western Asia is the largest area on earth with such a high cooling potential.
The cooling potential of desert regions is likely to remain relatively unfulfilled due to low population densities, reducing demand for local cooling, despite tremendous cooling potential.