Abstract
The passive radiative cooling approach refers to the physical process that pumps heat into outer space via the atmospheric window (8–13 μm) without energy input. The ability to continuously adjust the emissivity of thermal emitters in the sky window while maintaining high reflectivity in the solar spectrum remains a challenge. In order to achieve this task, a novel design referred to as double-layer nanoparticle-based coating is proposed. Our proposed emitter is appropriate for both high solar reflection and strong mid-infrared emissivity. The bottom and top layers are Al2O3 embedded with Ni nanoparticles and a super-hydrophilic TiO2-SiO2 layer. The bottom layer is designed to achieve high emissivity in “the atmospheric transparency window”. The top layer is designed to block solar illumination and to favor an enhanced cleanability of the coated design. Our double-layer coating as an optical solar reflector has excellent solar irradiation (0.96) and is strongly emissive (0.97) across the “full sky window” at room temperature. Furthermore, a detailed numerical energy study has been performed, evaluating the temperature reduction and the radiative cooling performance under different conditions. The proposed simple coating can be used as an efficient radiative cooler on a large scale for energy conservation and thermoelectric devices.
Highlights
Radiative heat dissipating from Earth to outer space is a universal research topic that can have a vital impact on global energy consumption
Passive radiative cooling is a promising mechanism to maintain the temperature of Earth’s surface, which can usually be applied to spacecraft thermal control, thermal management of buildings, thermoelectric cooling, and cooling of solar cells [1,2,3,4,5]
Most of these radiative materials are not able to achieve both high solar reflection and strong mid-infrared thermal radiation emissivity only in the atmospheric window to avoid exchanging the thermal radiation with the atmosphere [7]
Summary
Radiative heat dissipating from Earth to outer space is a universal research topic that can have a vital impact on global energy consumption. Great efforts have been made to achieve a high cooling performance by enhancing the solar radiation reflectance and emitting strongly mid-infrared thermal energy via the famous atmospheric transparency window Their daytime passive radiative cooler consisted of porous anodic aluminum oxide deposed on the Al substrate back reflector, resulting in 99% reflectance of solar illumination and an average emissivity of about 95% in the mid-infrared atmospheric window. This composite structure shows a potential cooling power density of 64 W/m2 at ambient air temperature and experimentally provides a potential of cooling 2.6 ◦ C below the ambient. The spectral radiative properties of our proposed cooler are calculated using the transfer matrix formulation, considering the effect of the filling ratios, incidence angles, and the coating thicknesses
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