Visibly Transparent Radiative Cooler under Direct Sunlight

Abstract

Transparency is an important characteristic in practical applications of radiative cooling, but the transmitted sunlight trapped in an inner space is generally the main cause of the increasing temperature. A transparent radiative cooler that can lower a temperature during the daytime by transmitting visible light, reflecting near‐infrared (NIR) light, and radiating thermal energy through the atmospheric window is proposed. In contrast to transparent selective emitters that transmit most of the incoming solar irradiance under direct sunlight and opaque radiative coolers that reflect all solar energy, the proposed cooler achieves transparency and the cooling effect simultaneously by selectively blocking solar absorption in the NIR regime. Outdoor rooftop measurements confirm that the cooler can reduce i) the inner temperature of an absorbing system and ii) its own temperature when combined with commercially available paints. During daytime, the cooler provides a temperature reduction of a maximum 14.4 °C and 10.1 °C for the inner and own temperature, respectively, and of an average 5.2 °C and 4.3 °C, respectively, in comparison to the transparent selective emitter. The proposed cooler can reduce the temperature during the daytime while maintaining transparency, confirming its possibilities in practical applications such as passive diurnal cooling of vehicles or buildings and compatibility with current paint technologies for aesthetic use.

Tunable metasurfaces towards versatile metalenses and metaholograms: a review

“…10,11 Resonant effects such as plasmonic resonance, [12][13][14][15] Mie resonance, 16,17 and Fabry-Pérot resonance 18,19 have also been exploited. By exploiting these resources, optical elements can be highly miniaturized and various optical applications have been implemented, such as beam splitters, [20][21][22] absorbers, [23][24][25][26][27][28][29] metalenses, 30,31 metaholograms, [32][33][34][35][36][37][38][39][40] selective thermal emitters, [41][42][43] detecting devices, [44][45][46] and structural color. [47][48][49][50][51][52] The functionality and efficiency of metasurfaces have been continuously increased by improving the methods to design meta-atoms, and the development of their material composition.…”

Three-dimensional artificial chirality towards low-cost and ultra-sensitive enantioselective sensing

“…Furthermore, artificial chiral nanostructures can be utilized in various photonic applications such as chiral metamirrors, 81 metaholograms, 82–86 metalenses, 87,88 multi-mode OAM generators, 89 beam steering, 90 and color prints. 91 These applications can be further improved by analysis of current metasurface and metamaterials that use various optical responses such as Mie-resonance, 92–96 geometric phases, 97,98 nanogap plasmonics, 99,100 nonlinear optics, 101 Bragg-reflection, 102,103 and spin Hall effect. 104–108 Future research could also focus on how tuneable chirality and controllable chiroptical properties can be produced so that the chiral structure is not fixed and the degrees of freedom can be increased.…”

Accurately Quantifying Clear-Sky Radiative Cooling Potentials: A Temperature Correction to the Transmittance-Based Approximation

“…Models that yield the level of detail needed for such calculations are comparatively rare [1,5,13]. One model, which has achieved almost universal use in recent radiative cooling literature, is the transmittance-based cosine approximation [1,[14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29], which was first used as part of a more comprehensive model by Granqvist in 1981 [1]. This model assumes that the irradiance of the atmosphere originates from greenhouse gases, including water vapor, carbon dioxide and ozone, and calculates the spectral, angular sky irradiance based on an effective spectral angular emittance as follows:…”

“…Since the irradiance from ozone and absorptance/emittance of SiO films have little overlap and the SiO film has a narrowband emittance, such a choice is justifiable in that context. However, the approximation has since been used to calculate the radiative cooling potentials of ideal emitters and cooling powers of radiative coolers with different spectral emittances, leading to both a systematic underestimation of cooling potential and a related overestimation of performance [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29]. The MODTRAN hemispherical emittance, which is more accurate, should ideally be used instead.…”