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.
Highlights
The recent progress on radiative cooling reveals its potential for applications in highly efficient passive cooling
Gu Centre for Micro-Photonics Faculty of Science Engineering and Technology In Section 3, we review the materials and device designs used for nocturnal radiative cooling in earlier studies
The humidity is high in general and meaningful cooling is only achievable at winter season, where a nighttime cooling of only 1–6 °C below ambient temperature was reported under clear sky conditions.[40]
Summary
The Earth’s atmosphere has a highly transparent window in the infrared (IR) wavelength range between 8 and μm, i.e., the atmosphere’s radiative emission is very weak in that window. The atmospheric window falls within the peak thermal radiation of a black body defined by Plank’s law at the ambient temperature (at around 300 K). Along with the effect of the incoming atmospheric radiation, the cooling performance of a radiator depends on other factors, such as, the nonradiative (conductive and convective) heat gain from the surrounding media and the incoming solar radiation during the daytime. To achieve a state where Tss is significantly below Ta, the radiative emission within the 8–13 μm wavelength region must be maximized and the absorbed power from the incoming atmospheric radiation, nonradiative contributions, and absorption of solar power must be minimized.
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