Colored, Daytime Radiative Coolers with Thin‐Film Resonators for Aesthetic Purposes

Abstract

sunlight. The features of an accurate radiative model, strong selective emission in the atmospheric transparency window, and broadband high reflectance to solar irradiation are each formulated individually and then configured collectively to accomplish this outcome. Because all of the successful examples of daytime radiative coolers possess high solar reflectivity, they are white or silver in color and are thus not visually appealing, [5][6][7]9] thereby restricting the possible installation locations and limiting their net cooling capacity. Although previous efforts have been paid to incorporate colors into the radiative cooler, [26] the research only dealt with theoretical calculations without experimental demonstrations, and structural optimization of colored radiative coolers has not been performed. Here, we present concepts and strategies for daytime radiative cooling systems that involve comparable attention to engineering design but with the goal of achieving systems that offer aesthetically desired colors and patterns and functional purposes, thus enabling more widespread installation. The experimental demonstration exhibits subambient cooling behaviors under a clear sky while preserving its color. The approaches reported here can address application concepts for wearable electronic devices whose operational temperature is lowered by radiative cooling. Figure 1a exhibits a schematic of a decorative colored passive radiative cooler (CPRC) for aesthetic purposes featuring areas with subtractive primary colors (i.e., cyan, magenta, and yellow) on a silvery background, where the latter area represents a conventional daytime radiative cooler. The CPRC consists of a SE comprising a bilayer of SiO 2 (650 nm) and Si 3 N 4 (910 nm), whose thicknesses are defined by extensive numerical optimization; and a metal reflector comprising an Ag film (100 nm) deposited on a silicon substrate (Figure 1b, left). Additional photonic nanostructures were inserted below the SE to generate vivid colors at specific desired areas (Figure 1b, right), which comprised a thin-film resonator composed of a metal-insulator-metal (MIM) structure. The MIM structure determined each color via interference in the 1D stacked layers, where the color generation was precisely controlled by tuning the thickness of the insulator layer (i.e., SiO 2 cavity) in the MIM.In this study, the MIM structure was chosen as the colorant structure because it provided minimal loss of solar reflectance and a narrow spectral width compared with other additive color filters such as metal gratings and 1D photonic crystals (1D PhC),Recently developed approaches in passive radiative cooling enable daytime cooling via engineered photonic structure layouts. However, the use of these daytime radiative coolers is restricted owing to their nonaesthetic appearance resulted from strong solar reflection. Therefore, this article introduces a colored passive radiative cooler (CPRC) capable of generating potential cooling power, based on a thin-film optical resonator embedded in ...

Colored and paintable bilayer coatings with high solar-infrared reflectance for efficient cooling

“…Furthermore, the white or silvery glare from these designs can harm human eyes. Colored radiative coolers (CRCs) have been explored to address this issue (29)(30)(31)(32)(33)(34). In a CRC, part(s) of the visible spectrum (VIS; 0.4 to 0.74 m) is selectively absorbed to exhibit the desired color, while other solar wavelengths, in particular, the near-to-short wavelength infrared (NSWIR; 0.74 to 2.5 m), are reflected ( Fig.…”

“…However, existing CRCs are limited in either their performance or their scope. For instance, multilayer photonic CRCs (29,30) have a high cooling performance but currently are rather expensive and difficult to apply on buildings or cars, which have various shapes, sizes, and textures (35,36). Colored paints containing TiO 2 and colorants (31,34), on the other hand, are scalable but usually absorb NSWIR wavelengths to become hot under sunlight.…”

A polydimethylsiloxane-coated metal structure for all-day radiative cooling

“…These pioneering works demonstrated the potential to realize daytime radiative cooling with no electricity consumption [15][16][17][18][19][20] . This technology can be used to assist climate control in buildings and significantly reduce energy usage [21][22][23][24][25][26][27] . Therefore, enhanced radiative cooling technology represents a new research topic with a significant impact on the energy sustainability .…”

Ideal spectral emissivity for radiative cooling of earthbound objects

“…The atmosphere acts as a selective window to electromagnetic waves, transmitting most of the visible sunlight and blocking harmful ultraviolet and X-rays 1 . Recently, the transparency window in the long-wavelength infrared, which ranges from 8 to 13 μm, has received great attention because it allows for below-ambient cooling without energy consumption [2][3][4][5][6][7][8][9][10][11][12][13][14][15] . The principle of this spontaneous cooling is based on thermal radiation in the transparency window being able to transfer heat between an object on the Earth's surface and the cold outer space without being blocked by the atmosphere.…”

“…This in turn relies on the fact that the outer space appears "cold" and reflects the young, finite-sized, and expanding nature of our universe. Even under direct sunlight, below-ambient cooling has been experimentally achieved by enhancing the reflectivity in the solar spectrum and the emissivity in the transparency window using various methods 3 , 7 , 9 – 12 , 15 . However, it remains unproven whether the 8.0–13.0 μm range is the exact optimal range to realize the maximum temperature drop versus ambient temperature, and the fundamental limit of the achievable cooling temperature has not been theoretically derived yet.…”