Passive daytime radiative cooling

A 2019 study reported that "widescale adoption of radiative cooling could reduce air temperature near the surface, if not the whole atmosphere". Costs for global implementation have been estimated at $1.25 to $2.5trillion, or about 3% of annual gross world product (global GDP), with expected economies of scale. 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 (typical rooftop) 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: • Advancing toward a carbon-neutral future and achieving net-zero emissions. • Alleviating electrical grids and renewable energy sources from devoting electric energy to cooling. • Reducing greenhouse gas emissions by replacing fossil fuel energy use devoted to cooling. • Reducing local and global temperature increases associated with global warming. • Reducing thermal pollution of water resources. • Reducing water consumption for wet cooling processing. == Other geoengineering approaches ==

PDRC has been claimed to be more stable, adaptable, and reversible than stratospheric aerosol injection (SAI). 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 ==

(shown in orange) and minimizes the absorption of solar radiation (shown in yellow). The basic measure of PDRC is its solar reflectivity (in 0.4–2.5 μm) and heat emissivity (in 8–13 μm), 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". 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 (APM) 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 (up to 120W/m2), while colder regions and those with high humidity or cloud cover generally have less. India, and across Europe. Deserts experiencing broadband summertime terrestrial heat gains and wintertime losses, selective emitters can achieve seasonal thermoregulation and energy savings. Since dew formation appears less likely on vertical surfaces even in exceptionally humid environments, the thermoregulatory benefits of selective emitters will likely persist in both humid and dry operating conditions. Another response is to make hydrophobic self-cleaning PDRC materials. Scalable and sustainable hydrophobic materials that avoid VOCs can repel rainwater and other liquids. Wind Wind may alter the efficiency of passive radiative cooling surfaces and technologies. A 2020 study proposed using a "tilt strategy and wind cover strategy" to mitigate wind effects. The researchers reported regional differences in China, noting that "85% of China's areas can achieve radiative cooling performance with wind cover" whereas in northwestern China wind cover effects would be more substantial. Bijarniya et al. similarly proposes the use of a wind shield in areas susceptible to high winds. == Materials and production ==

PDRC surfaces can be made of various materials. However, for widespread application, PDRC materials must be inexpensive, available for mass production, and applicable in many contexts. Most research has focused on coatings and thin films, which tend to be more available for mass production, less expensive, and more applicable in a wider range of contexts, although other materials may provide potential for specific applications. PDRC research has identified more sustainable material alternatives, even if not fully biodegradable. A 2023 study reported that "most PDRC materials now are [substances] which will cause excessive emissions by consuming fossil fuels and go against the global carbon neutrality goal. Environmentally friendly bio-based renewable materials should be [...] ideal [for] PDRC systems". Multilayer and complex structures Advanced photonic materials and structures, such as multilayer thin films, micro- and nanoparticles, photonic crystals, metamaterials, and metasurfaces, have been reported as potential approaches. However, while multilayer and complex nano-photonic structures have proven successful in experimental scenarios and simulations, a 2022 study reported that widespread application "is severely restricted because of the complex and expensive processes of preparation". A 2022 study stated that coatings generally offer "strong operability, convenient processing, and low cost, which have the prospect of large-scale utilization". Dye of the desired color is coated on the polymer. Compared to traditional dye in porous polymer, in which the dye is mixed in the polymer, the new design can cool more effectively. A 2018 study reported significantly lowered coating costs, stating that "photonic media, when properly randomized to minimize the photon transport mean free path, can be used to coat a black substrate and reduce its temperature by radiative cooling". This coating could "outperform commercially available solar-reflective white paint for daytime cooling" without expensive manufacturing steps or materials. Films Neocerambyx gigas exhibited 95% solar irradiance and 96% emissivity.Many thin films offer high solar reflectance and heat emittance. However, films with precise patterns or structures are not scalable "due to the cost and technical difficulties inherent in large-scale precise lithography" (2022), The polyacrylate hydrogel film from the 2022 study has broader applications, including potential uses in building construction and large-scale thermal management systems. This research focused on a film developed for hybrid passive cooling. The film uses sodium polyacrylate, a low-cost industrial material, to achieve high solar reflectance and high mid-infrared emittance. A significant feature of this material is its ability to absorb atmospheric moisture, aiding evaporative cooling. This tripartite mechanism allows for efficient cooling under varying atmospheric conditions, including high humidity or limited access to clear skies. The study reported exceptional ability to facilitate radiative cooling. The fabric achieved 94.5% emissivity and 92.4% reflectivity. This combination of high emissivity and reflectivity is central to its cooling capabilities, significantly outperforming traditional fabrics. Additionally, the fabric's mechanical properties, including strength, durability, waterproofness, and breathability, confirmed its suitability for clothing. Aerogels Aerogels offer a potential low-cost material scalable for mass production. Some aerogels can be considered a more environmentally friendly alternative to other materials, with degradable potential and the absence of toxic chemicals. Aerogels can be useful as thermal insulation to reduce solar absorption and parasitic heat gain to improve the cooling performance of PDRC. Nano bubbles Pigments absorb light. Soap bubbles show a prism of different colors on their surfaces. These colors result from the way light interacts with differing thicknesses of the bubble's surface, termed structural color. One study reported that cellulose nanocrystals (CNCs), which are derived from the cellulose found in plants, could be made into iridescent, colorful films without added pigment. They made films with blue, green and red colors that, when placed under sunlight, were an average of nearly 7 °F cooler than the surrounding air. The film generated over 120W⋅m−2 of cooling power. Biodegradable surfaces Many proposed radiative cooling materials are not biodegradable. A 2022 study reported that "sustainable materials for radiative cooling have not been sufficiently investigated". == Applications ==

Passive daytime radiative cooling has "the potential to simultaneously alleviate the two major problems of energy crisis and global warming" PDRC has been applied for indoor space cooling, outdoor urban cooling, solar cell efficiency, power plant condenser cooling, among other applications. while emissions from air conditioning account for "13.7% of energy-related emissions, approximately 52.3 EJ yearly" However, this can be significantly reduced with the mass production of low-cost PDRCs for indoor space cooling. A multilayer PDRC surface covering 10% of a building's roof can replace 35% of air conditioning used during the hottest hours of daytime. In cities, PDRCs can produce significant energy and cost savings. In a study on US cities, Zhou et al. found that "cities in hot and arid regions can achieve high annual electricity consumption savings of >2200kilowatt-hours (kW⋅h), while <400kW⋅h is attainable in colder and more humid cities", ranking from highest to lowest by electricity consumption savings as follows: Phoenix (~2500kW⋅h), Las Vegas (~2250kW⋅h), Austin (~2100kW⋅h), Honolulu (~2050kW⋅h), Atlanta (~1500kW⋅h), Indianapolis (~1200kW⋅h), Chicago (~1150kW⋅h), New York City (~900kW⋅h), Minneapolis (~850kW⋅h), Boston (~750kW⋅h), Seattle (~350kW⋅h). The growing interest and rise in PDRC application to buildings has been attributed to cost savings related to "the sheer magnitude of the global building surface area, with a market size of ~$27billion in 2025", as estimated in a 2020 study. Even when installed on roofs in highly dense urban areas, broadband radiative cooling panels lower surface temperatures at the sidewalk level. A 2022 study assessed the effects of PDRC surfaces in winter, including non-modulated and modulated PDRCs, in the Kolkata metropolitan area. A non-modulated PDRC with a reflectance of 0.95 and emissivity of 0.93 decreased ground surface temperatures by nearly and with an average daytime reduction of . Solar cell efficiency declines 0.4–0.5% for every 1 °C increase in temperature. Personal thermal management Personal thermal management (PTM) employs PDRC in fabrics to regulate body temperatures during extreme heat. While other fabrics are useful for heat accumulation, they "may lead to heat stroke in hot weather". A 2021 study claimed that "incorporating passive radiative cooling structures into personal thermal management technologies could effectively defend humans against intensifying global climate change". Wearable PDRCs can come in different forms and target outdoor workers. Products are at the prototype stage. Although most textiles are white, colored wearable materials in select colors may be appropriate in some contexts. A review reported that supplementing the air-cooled condenser for radiative cooling panels in a thermoelectric power plant condenser achieved a 4096kWhth/day cooling effect with a pump energy consumption of 11kWh/day. == Side effects ==

Jeremy Munday writes that although "unexpected effects will likely occur", PDRC structures "can be removed immediately if needed, unlike methods that involve dispersing particulate matter into the atmosphere, which can last for decades". Stratospheric aerosol injection "might cause potentially dangerous threats to the Earth's basic climate operations" that may not be reversible, preferring PDRC. As per Chen et al., "to overcome this issue, dynamically switchable coatings have been developed to prevent overcooling in winter or cold environments". Low-cost PDRC colored paint coatings, which reduce glare and increase the color diversity of PDRC surfaces, have also been developed. While some of the surface's solar reflectance is lost in the visible light spectrum, colored PDRCs can still exhibit significant cooling power, such as a coating by Zhai et al., which used a α- coating (resembling the color of the compound) to develop a non-toxic paint that demonstrated a solar reflectance of 99% and heat emissivity of 97%. == History ==

's ability to cool its body temperature in extreme heat inspired early PDRC research. Nocturnal passive radiative cooling has been recognized for thousands of years, with records showing awareness by the ancient Iranians, demonstrated through the construction of Yakhchāls, since 400 B.C.E. PDRC was hypothesized by Félix Trombe in 1967. The first experimental setup was created in 1975, but was only successful for nighttime cooling. Further developments to achieve daytime cooling using different material compositions were not successful. In 2024, Nissan introduced a paint that lowers car interior temperatures by up to 11.7 K (21 °F) in direct sunlight. It involves two types of particles, each operating at a different frequency. One reflects near-infrared light. The second converts other frequencies to match the infrared window, radiating the energy into space. == See also ==