Spectrally Selective Inorganic-Based Multilayer Emitter for Daytime Radiative Cooling

Abstract

Daytime radiative coolers are used to pump excess heat from a target object into a cold exterior space without energy consumption. Radiative coolers have become attractive cooling options. In this study, a daytime radiative cooler was designed to have a selective emissive property of electromagnetic waves in the atmospheric transparency window of 8–13 μm and preserve low solar absorption for enhancing radiative cooling performance. The proposed daytime radiative cooler has a simple multilayer structure of inorganic materials, namely, Al2O3, Si3N4, and SiO2, and exhibits high emission in the 8–13 μm region. Through a particle swarm optimization method, which is based on an evolutionary algorithm, the stacking sequence and thickness of each layer were optimized to maximize emissions in the 8–13 μm region and minimize the cooling temperature. The average value of emissivity of the fabricated inorganic radiative cooler in the 8–13 μm range was 87%, and its average absorptivity in the solar spectral region (0.3–2.5 μm) was 5.2%. The fabricated inorganic radiative cooler was experimentally applied for daytime radiative cooling. The inorganic radiative cooler can reduce the temperature by up to 8.2 °C compared to the inner ambient temperature during the daytime under direct sunlight.

Photonics Empowered Passive Radiative Cooling

“…Multilayer thin film structures based on dielectric polar materials (SiO 2 , SiO, TiO 2 , Al 2 O 3 , Si 3 N 4 , and SiC et al) with phonon resonances in the atmospheric transparency window have been widely studied since the first experimental demonstration of daytime radiative cooling below ambient temperature under direct sunlight. [ 9–18 ] Such structures are amenable to scalable fabrication and experimental realization and thus are ideal structures for proof‐of‐principle demonstration. Usually, such an integrated planar structure consists of two parts.…”

“…A three‐layer structure of Al 2 O 3 /Si 3 N 4 /SiO 2 on the top of Ag mirror was optimized to achieve thermal emissivity up to 0.87(Figure 2d,e). [ 18 ] And, eight alternating layers of TiO 2 and SiO 2 on top of the Ag mirror were demonstrated to show less than 1% solar absorption. [ 11 ] Although those multilayer thin film structures show remarkable cooling performance, their practical viabilities are doubted because making such a structure usually involves the vacuum‐based thin film deposition systems, which are time consuming and expensive.…”

A Reinforcement Learning Method for Optical Thin-Film Design

“…In recent decades, significant fundamental advances combined with the spectacular progress of nano-scale fabrication methods have led to a broad range of innovations in the design of optical thin-film. Many applications, such as broadband filter [1][2] [3], solar absorber [4][5] [6], and radiative cooling device [7] [8], increasingly rely on the intricate nanostructure design for greater performance at target wavelengths. Most of researchers make such designs based on human intelligence to solve a fundamental photonic problem: choosing the best combination of materials and nano-structure of a layered optical thin-film.…”

Switchable diurnal radiative cooling by doped VO₂

“…The absorptivity of the object should be near zero in the ultraviolet (UV) to near-infrared (NIR) range, and should radiate its energy with near-unity efficiency in the wavelengths at which electromagnetic radiation can be transmitted through the atmosphere (the "atmospheric window"). To meet those conditions, structured materials such as photonic crystals 10 , micro/nanoporous structures 11−17 , multilayer 18,19 , and mixtures of dielectric particles 20−23 have been used for their ability to manipulate optical responses. However, the spectra of such media cannot be changed after fabrication.…”