Date/Time: 04-23-2019 - Tuesday - 05:00 PM - 07:00 PM
Guo Jiang1 Shenghong Ju1 2 Junichiro Shiomi1 2 3

1, The University of Tokyo, Tokyo, , Japan
2, National Institute for Materials Science (NIMS), Tokyo, , Japan
3, Japan Science and Technology Agency (JST), Tokyo, , Japan

Due to the major concern on global warming and energy crises issues, radiative cooling which utilize the untapped cold space as potential heat sink, has attracted intense attention again with the newly development in nanotechnology. Many successful experiment results on radiative cooling have focused on blocking the solar energy absorption (0.4-4 μm) and maximizing the thermal radiation loss (>4 μm) to the surrounding simultaneously, which shows exciting temperature drop even under the direct sunshine. However, this design policy may suffer from absorbing too much downward atomspheric radiation in hot and humid regions, thus could be less effective in cooling. Selective emitter which only emit or absorb thermal energy within the 8-13 μm-atmospheric window can tap the most potential of the transparency of atmosphere and get lower stagnantion temperature. To our best knowledge, there is no reported work which shows perfect selective emittance property matching with the atmosphereic window.

In this work, we combined the traditional grating and multi-layer structure, aiming to get ideal optical response for radiative cooling purpose. The machine learning optimization was based on Bayesian algorithm and the optical response property of the structure was calculated by the rigorous coupled wave analysis (RCWA) method. For the design, the ideal optical response property should only have unity emittance in the 8-13 um wavelength range and unity reflectance outside the transparency window. We employed the typical polar material SiO2 as the grating material on the top to excite the surface phonon polariton. The multi-layer structure consists of Si3N4, Al2O3, SiC and Si with various thickness and sequence was used to manipulate the resonance position. We used silver thin film with sufficient thickness as substrate to totally reflect the solar energy. The periodic length, thickness and filling ratio of the top grating layer, the sequence and thickness of the multilayer structure were optimized by Bayesian algorithm. We obtain the optimal structure with nearly unity emittance only in the atmospheric window.

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