Radiative Cooling for Energy Sustainability: From Fundamentals to Fabrication Methods Toward Commercialization

Abstract Radiative cooling, a technology that lowers the temperature of terrestrial objects by dissipating heat into outer space, presents a promising ecologically‐benign solution for sustainable cooling. Recent years witness substantial progress in radiative cooling technologies, bringing them closer to commercialization. This comprehensive review provides a structured overview of radiative cooling technologies, encompassing essential principles, fabrication techniques, and practical applications, with the goal of guiding researchers toward successful commercialization. The review begins by introducing the fundamentals of radiative cooling and the associated design strategies to achieve it. Then, various fabrication methods utilized for the realization of radiative cooling devices are thoroughly discussed. This discussion includes detailed assessments of scalability, fabrication costs, and performance considerations, encompassing both structural designs and fabrication techniques. Building upon these insights, potential fabrication approaches suitable for practical applications and commercialization are proposed. Further, the recent efforts made toward the practical applications of radiative cooling technology, including its visual appearance, switching capability, and compatibility are examined. By encompassing a broad range of topics, from fundamental principles to fabrication and applications, this review aims to bridge the gap between theoretical research and real‐world implementation, fostering the advancement and widespread adoption of radiative cooling technology.

Radiative cooling occurs by the energy exchange among the Earth, the Sun, and the universe.Any object with finite temperature continuously radiates thermal energy (Supplementary Box1); therefore, the Earth, the Sun, and the universe, with surface temperatures of around 300K, 5800K, and 3K, respectively, radiate thermal energy in the form of electromagnetic waves 2 .The Earth's temperature is determined by the balance between energy absorbed from the Sun (insolation) and energy emitted to the universe, modified by energy absorption by the atmosphere.
Consider energy exchange from an insulated surface to the universe through a clear sky (Supplementary Figure 1a).The net cooling power of the surface is the balance of energy flows of the absorption of insolation and atmospheric radiation, the emission of the surface, and the other heat losses.Considering all energy exchanges, the net cooling power  _ is given by 3 where   is the thermal radiation power emitted by the surface,   is the atmospheric radiation absorbed by the surface,   is the solar radiation absorbed by the surface, and  − is an additional energy loss arising from the non-radiative heat transfer between the surface and the ambient.Each power term will be discussed in detail in the following subsections, then strategies to achieve maximum radiative cooling will be presented.
Thermal radiation from the surface At temperature T, the total radiation power   () from a surface can be calculated by integrating thermal emission over all wavelengths and directions: where   is the area of the radiating surface,   (, , , ) is emissivity of the surface at wavelength , zenith angle  and azimuth angle  at surface temperature T,   (, ) is the thermal radiation of a blackbody (Supplementary Figure 1b and Supplementary Box1).The effects of T and  are negligible compared to the effects of  and .Therefore, we assume that the emissivity depends only on  and  (i.e., (, )).

Atmospheric radiation
The atmosphere that surrounds Earth is composed of various gas molecules, including nitrogen N2, oxygen O2, water vapor H2O, argon Ar, and carbon dioxide CO2.These molecules absorb energy (solar or radiated from the earth and environment) then reradiate it (i.e., electromagnetic waves) at ambient temperature Tamb.Thus, the spectral intensity of atmospheric thermal radiation is given as   (,   ) =   (, )  (,   ) , where   (, ) is the emissivity of the atmosphere and IB (,   ) is the spectral emissivity of a black body.Therefore, the atmospheric radiation power absorbed by the surface can be calculated as: where   (, ) is the absorptivity of the surface, which can be replaced by its emissivity   (, ) according to Kirchhoff's law of thermal radiation (Supplementary Box1).
Some gas species in the atmosphere have dominant emissive characteristics over several wavelength bands.For instance, emission and absorption by H2O are strong in the wavelengths at  ~ 6 μm, and CO2 has a strong emission band at 13 ≤  ≤ 16 μm.Overall, several molecular emission bands give rise to atmospheric emissivity   (, ), which is high over a broad wavelength range 0.3 ≤  ≤ 20 μm except for some bands in the AW at 8 ≤  ≤ 13 μm because very few electromagnetic waves are emitted by or absorbed by the atmosphere in this region (Fig. 1c).Coincidentally, the AW aligns with the peak wavelength (~9 μm) of blackbody radiation at a typical terrestrial ambient temperature (~300 K).The wavelength region thus provides a channel for radiative cooling, through which thermal radiation can propagate to the universe without being absorbed by the atmosphere.
(, ) significantly influences the effectiveness of radiative coolers (Eq.3).Therefore, factors that affect   (, ), and methods to measure it, should be understood. is a key factor that affects   (, ) 4,5 .Basically, as  increases,   (, ) increases because the atmospheric transparency decreases as the horizontal direction is approached.Slight changes in  result in negligible differences in   (, ) at small , but significant differences at large ; for example, angle-dependent emissivity   (, ) can be estimated from a given atmospheric emissivity   (, 0) in the zenith direction as 4 : (, ) also depends on other factors, including geographical location, climate, and weather conditions.Absorption by H2O vapor is dominant in the AW, so   (, ) is quite sensitive to atmospheric humidity (Fig. 1c).Therefore, many studies have quantified correlations of between   (, ), H2O vapor pressure and ambient temperature in different regional and seasonal conditions [6][7][8] .Practically,   (, ) can also be obtained from experimental measurements 7,9 or by using computer code 10,11 that were developed from models that account for different climates and regions.

Insolation
During the night, sub-ambient radiative cooling can be easily obtained 5,12,13 .However, during the day, absorption of sunlight by surfaces heats them and significantly counteracts radiative cooling 3,14 .Above the atmosphere, sunlight has a spectral distribution that is close to that of an ideal blackbody that has a surface temperature of 5778 K (Fig. 1b).However, while travelling through the atmosphere to the ground, the solar spectrum is unevenly attenuated by scattering and absorption, so the spectral solar irradiance   (, ) is very different from the blackbody spectrum 2 .  (, ) that considers the relative optical path length through the atmosphere can be calculated using models that consider the air mass (AM) coefficient.In particular, AM 1.5, which corresponds to  = 48.2°, is widely used to represent the average atmosphere at mid-latitudes.Thus, the solar irradiance power   absorbed by the surface can be calculated as where 0 ≤ r(,) ≤ 1 is the absorbance.In general, the absorbed solar power has the most significant effect on the daytime radiative cooling effect.Insolation at AM 1.5 can reach up to 1,000 W•m -2 , so if the radiative cooler absorbs only 10% of it, the additional heat flux of 100 W•m -2 may cancel out the overall cooling effect.Therefore, daytime radiative coolers must have minimal absorption at 0.3 ≤  ≤ 2.5 μm, where the insolation is concentrated.
Non-radiative heat transfer Conduction and convection are important non-radiative heat-exchange mechanisms (Box 1).They result from the interaction between the radiative coolers and surrounding environment, such as the wind and the temperature difference between the radiative coolers and the ambient.The non-radiative heat transfer  − can be mathematically expressed as where ℎ is the lumped heat-transfer coefficient that combines convection and conduction, and TS is the surface temperature of the object to be cooled. − can be beneficial for cooling applications if TS >   , as in solar cells 15 and power plants 16 .However, if TS <   , then  − acts counter to cooling.Methods to reduce this effect include using convection shields 17 and thermal insulation 18 .
ℎ can be determined empirically to estimate the loss to  − .For example, the effect of wind speed   on heat transfer coefficients at the outer surface can be expressed as a linear correlation [19][20][21] ℎ =  +   , where the fitting parameters of  and  are obtained experimentally 22 or theoretically 23 .
Ideal radiative cooling: broadband and selective emitter Knowledge of the four heat-exchange processes that determine the net cooling effect of the daytime radiative cooler has been exploited to design two types of ideal radiative coolers for the maximum radiative cooling effect: a broadband emitter that emits in all of the electromagnetic spectrum except at wavelengths emitted by the sun, and a selective emitter that emits electromagnetic waves only in AW.The two types of radiative cooler provide ideal radiative cooling under different temperature conditions.
Both radiative coolers provide the ideal radiative cooling under different temperature conditions.The broadband emitter is preferred in above-ambient situations, because it emits a significant amount of energy to outer space.However, this type of device can also absorb the spectral emission from surroundings according to Kirchhoff's law, and this process can be impede in sub-ambient cooling.Consequently, the net cooling potential of the broadband emitter drastically decreases as the ambient temperature decreases (Supplementary Figure 1d).
In contrast, a selective emitter could exhibit more cooling potential than a broadband emitter in sub-ambient conditions, because it emits selectively to minimize the incoming radiative flux (Supplementary Figure 1d).Therefore, maximum radiative cooling effect for different purposes and environmental conditions, requires choice of materials and structures that have appropriate ideal radiative cooling curves.