Abstract
This work theoretically demonstrates a quasi-periodic selective multilayer emitter for sub-ambient daytime radiative cooling. In the design process, by inserting well-defined materials with different refractive-index profiles in suitable layers, there are absorption bands at different regions and the absorption bands are enhanced and broad in atmospheric transparency windows. Combined with the interference effects of the surface and the high reflectivity of Ag, the solar reflectance of the emitter is maximal in the solar spectrum. The influence of different nonradiative heat exchanges and the ambient air temperatures on the cooling performance of the multilayer emitter are calculated. At the same time, the mechanism of radiative cooling is analyzed. All the results show that the proposed emitter can effectively achieve sub-ambient daytime radiative cooling. Due to the superior durability and mechanical robustness of the multilayer emitter, it may be a key element in the realization of energy-efficient radiative cooling devices.
Highlights
Passive radiative cooling is a ubiquitous process in which a surface can dissipate heat via thermal radiation
Outer space behaves as a blackbody, with a temperature of −270 ○C,1 and the typical terrestrial temperature is about 27 ○C
In 2014, Raman et al.30 reported a selective radiative emitter consisting of seven alternating layers of hafnium oxide (HfO2) and silicon dioxide (SiO2) on top of a 200-nm-thick silver (Ag) back reflector, resulting in an average emittance in the atmospheric transparency window of about 65% and 97% reflection of solar radiation
Summary
Passive radiative cooling is a ubiquitous process in which a surface can dissipate heat via thermal radiation. In 2014, Raman et al. reported a selective radiative emitter consisting of seven alternating layers of hafnium oxide (HfO2) and silicon dioxide (SiO2) on top of a 200-nm-thick silver (Ag) back reflector, resulting in an average emittance in the atmospheric transparency window of about 65% and 97% reflection of solar radiation They experimentally demonstrated that the photonic structure could cool about 4.9 ○C below the ambient temperature under direct sunlight (Psolar = 860 W/m2) and had a net radiative cooling power of 40.1 ± 4.1 W/m2. Chae et al. proposed a selective inorganic-based multilayer emitter with a four layered structure of 276-nm-thick SiO2, 312-nm-thick Si3N4, 1312-nm-thick Al2O3, and 200-nm-thick Ag, which is suitable for sub-ambient radiative cooling They theoretically demonstrated that the average emittance in the atmospheric transparency window was
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