Passive daytime radiative cooling

[14] Low-cost scalable PDRC materials with potential for mass production include coatings, thin films, metafabrics, aerogels, and biodegradable surfaces.

[26][27] 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][28][4][29] along with growing concerns over energy use and global warming.

[41] 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.

[42] 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.

[19] However, selective emitters face challenges in real-world applications that can weaken their performance, such as from dropwise condensation (common even in semi-arid climates) that can accumulate on even hydrophobic surfaces and reduce emission.

[46] 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.

[50] 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.

[5] For example, the cooling potential of most of southeast Asia and the Indian subcontinent is significantly diminished in the summer due to a dramatic increase in humidity, dropping as low as 10–30 W/m2.

However, recently, it has been shown that for vertical building facades experiencing broadband summertime terrestrial heat gains and wintertime losses, selective emitters can achieve seasonal thermoregulation and energy savings.

[30][55][56][57][58] A 2023 study reported that "most PDRC materials now are non-renewable polymers, artificial photonic or synthetic chemicals, which will cause excessive CO2 emissions by consuming fossil fuels and go against the global carbon neutrality goal.

[37] Similarly, a 2020 study reported that "scalable production of artificial photonic radiators with complex structures, outstanding properties, high throughput, and low cost is still challenging".

[62] A 2022 study stated that coatings generally offer "strong operability, convenient processing, and low cost, which have the prospect of large-scale utilization".

This scattering enhances both solar reflectance (more than 96%) and thermal emittance (97% of heat), lowering surface temperatures six degrees below the surroundings at noon in Phoenix.

[69] The polyacrylate hydrogel film[70] from the 2022 study has broader applications, including potential uses in building construction and large-scale thermal management systems.

This tripartite mechanism allows for efficient cooling under varying atmospheric conditions, including high humidity or given limited access to clear skies.

[74] For instance, 2023 study reported that a that "new flexible cellulose fibrous films with wood-like hierarchical microstructures need to be developed for wearable PDRC applications.

[84] Passive daytime radiative cooling has "the potential to simultaneously alleviate the two major problems of energy crisis and global warming"[1] along with an "environmental protection refrigeration technology.

"[36] PDRCs have an array of potential applications, but are now most often applied to various aspects of the built environment, such as building envelopes, cool pavements, and other surfaces to decrease energy demand, costs, and CO2 emissions.

[86] Even when installed on roofs in highly dense urban areas, broadband radiative cooling panels lower surface temperatures at the sidewalk level.

Since silicon solar cells have a maximum efficiency of 33.7% (with the average commercial panel reaching around 20%), the majority of absorbed power produces excess heat and increases the operating temperature.

"[91] A 2021 study claimed that "incorporating passive radiative cooling structures into personal thermal management technologies could effectively defend humans against intensifying global climate change.

[7] A concentrated solar plant (CSP) on the CO2 supercritical cycle at 550ᵒC was reported to produce 5% net output gain over an air-cooled system by integration with 14 m2 /kWe capacity radiative cooler.

[94] 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.

"[95] Stratospheric aerosol injection "might cause potentially dangerous threats to the Earth’s basic climate operations" that may not be reversible, preferring PDRC.

[2] Zevenhoven et al. state that "instead of stratospheric aerosol injection (SAI), cloud brightening or a large number of mirrors in the sky (“sunshade geoengineering”) to block out or reflect incoming (short-wave, SW) solar irradiation, long-wavelength (LW) thermal radiation can be selectively emitted and transferred through the atmosphere into space".

"[9] In 2022, Khan et al. concluded that "low-cost optically modulated" PDRCs are "under development" and "are expected to be commercially available on the market soon with high future potential to reduce urban heat in cities without leading to an overcooling penalty during cold periods.

[19][20] While, as per Khan et al., developing VO2 is difficult, their review found that "recent research has focused on simplifying and improving the expansion of techniques for different types of applications.

[19] Glare caused from surfaces with high solar reflectance may present visibility concerns that can limit PDRC application, particularly within urban environments at the ground level.

[27] 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.

[7] In the 1980s, Lushiku and Granqvist identified the infrared window as a potential way to access the ultracold outer space as a way to achieve passive daytime cooling.