Scalable Aqueous Processing‐Based Passive Daytime Radiative Cooling Coatings

Abstract

Passive daytime radiative cooling (PDRC) can realize electricity‐free cooling by reflecting sunlight and emitting heat to the cold space. Current PDRC designs often involve costly vacuum processing or a large quantity of harmful organic solvents. Aqueous and paint‐like processing is cost‐effective and environmentally benign, thereby highly attractive for green manufacturing of PDRC coatings. However, common polymers explored in PDRC are difficult to disperse in water, let alone forming porous structures for efficient cooling. Here, a simple “bottom‐up” ball milling approach to create uniform microassembly of poly(vinylidene fluoride‐co‐hexafluoropropene) nanoparticles is reported. The micro‐ and nanopores among secondary particles and primary particles substantially enhance light scattering and results in excellent PDRC performance. A high solar reflectance of 0.94 and high emittance of 0.97 are achieved, making the coating 3.3 and 1.7 °C cooler than commercial white paints and the ambient temperature, under a high solar flux of ≈1100 W m−2. More importantly, the volatile organic compound content in the aqueous paint is only 71 g L−1. This satisfies the general regulatory requirements, which are critical to sustainability and practical applications.

Passive daytime radiative cooling: Fundamentals, material designs, and applications

“…To overcome this issue, an aqueous processing-based polymer coating with both excellent PDRC performance and low VOC content was reported by the assembly of nanoparticles into uniform microclusters. 69 T A B L E 1 (Continued)…”

Colorful surfaces for radiative cooling

“…However, due to strong scattering and diffraction within solar spectral range, most radiative cooling materials are white in color. [46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] For a colored radiative cooling structure, absorption in the visible range is inevitable. Therefore, one needs to consider the balance between solar heating and thermal radiation for colored radiative cooling materials.…”

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.…”