Radiative Cooling: Principles, Progress, and Potentials

Abstract

The recent progress on radiative cooling reveals its potential for applications in highly efficient passive cooling. This approach utilizes the maximized emission of infrared thermal radiation through the atmospheric window for releasing heat and minimized absorption of incoming atmospheric radiation. These simultaneous processes can lead to a device temperature substantially below the ambient temperature. Although the application of radiative cooling for nighttime cooling was demonstrated a few decades ago, significant cooling under direct sunlight has been achieved only recently, indicating its potential as a practical passive cooler during the day. In this article, the basic principles of radiative cooling and its performance characteristics for nonradiative contributions, solar radiation, and atmospheric conditions are discussed. The recent advancements over the traditional approaches and their material and structural characteristics are outlined. The key characteristics of the thermal radiators and solar reflectors of the current state‐of‐the‐art radiative coolers are evaluated and their benchmarks are remarked for the peak cooling ability. The scopes for further improvements on radiative cooling efficiency for optimized device characteristics are also theoretically estimated.

Metal‐Loaded Dielectric Resonator Metasurfaces for Radiative Cooling

“…[1][2][3][4] Although various nanostructures with relatively strong cooling performance have been reported, due to their complicated structures and costly nanofabrication process, [3] scaling up these designs for practical applications are always challenging, let along integrating them with various platforms. With advance in nanofabrication technologies, especially designed radiative cooling nanostructures have attracted increasing research interests.…”

“…[8] These demonstrate relatively strong intrinsic emissivity within the IR atmospheric window, with a comprehensive summary of such studies presented by Granqvist. [8] In recent years, there has been renewed interests in radiative cooling, [1][2][3][4]9,10] or broadly speaking, selective thermal emission, [11][12][13] with significant progress facilitated by advancements in nanotechnology and nanofabrication. The present capabilities of nanofabrication coupled with advances in optical nanostructure design enable scientists to restructure the surface of an object at the nanoscale and dramatically change the electromagnetic absorptivity of the object in a given spectral range.…”

“…According to Kirchhoff's law of thermal radiation, the angular-and frequency-dependent emissivity of an object at thermal equilibrium is equal to its absorptivity. [1,2,4] Metamaterials have the advantage of supporting both local and lattice modes of resonance, which adds design flexibility to achieve desired emission wavelengths, bandwidth, and emission angular span for maximizing the cooling efficiency. Based on this principle, so far, various nanostructures have been proposed for radiative cooling.…”

Radiative sky cooling: fundamental physics, materials, structures, and applications

“…Its influence is illustrated in Figure 2: higher precipitable water vapor concentration leads to stronger absorption within the atmospheric window. Similarly, the atmospheric window dependence on RH was demonstrated in [41] by modeling transmittance characteristics during the summer solstice at two locations. The atmospheric transmittance within the 8-13 μm spectral window is higher in Perth than in Brisbane despite the close ambient temperatures, due to lower water vapor concentration in Perth than in Brisbane as reflected by the mean RH.…”

Passive cooling of surfaces

“…Due to the radiative cooling, a temperature from 15 o C to 25 o C which is below ambient temperature may be achieved by a blackbody. Spectral selective radiative materials have a high emittance (low reflectance) in the spectral region of the atmospheric window and a high reflectance in the remaining spectral ranges [10][11][12][13][14]. In this present work, silicon monoxide (SiO) films were deposited on highly reflective substrate (Al on glass).…”