Cross-Linked Porous Polymeric Coating without a Metal-Reflective Layer for Sub-Ambient Radiative Cooling

Abstract

Passive daytime radiative cooling provides cooling without energy input. This method is eco-friendly, which is beneficial, considering the increasing problems of global warming and urban heat islands. A poly(vinylidene fluoride) (PVDF) and polyurethane acrylate (PUA) matte white coating was prepared via photo-initiated free-radical polymerization. The porous polymeric coating without a metal-reflective layer exhibited an average emissivity of 0.9333 in the atmospheric window and an average solar reflectance of 0.9336 in the direct AM1.5 solar spectrum (888 W m −2 in the 0.3−2.5 μm region). The radiative cooling power of the fabricated radiative cooler with a thickness of 518 μm was 94.2 W m −2 . Furthermore, the radiative cooler demonstrated radiative cooling performance during both daytime and nighttime in Seoul, Korea, and Chiang Mai, Thailand. The PVDF/PUA matte white coating without a silver reflector can prevent solar absorption caused by the oxidation of silver and reduce the light pollution caused by the metallic film because of the antiglare surface of the matte coating.

Cooling capacity evaluation of passive radiation cooling materials

“…The absorption power of 80 W/m 2 corresponds to that the radiation film has a transmittance of nearly 10%, so it is important to improve the sunlight reflection of the radiation film, otherwise the radiation film is difficult to obtain the cooling effect during the daytime. In this case, the temperature inside the test chamber was used as the environmental comparison temperature to evaluate the radiation cooling capacity in some articles [13][14][15][16]. As can be seen from figure 6 (b), for the case of 80 W/m 2 pad absorbing power, although the temperature at the center of the radiation film is greater than the external ambient temperature, it is still less than the temperature near the inner wall of the insulation chamber.…”

Iridescent Daytime Radiative Cooling with No Absorption Peaks in the Visible Range

“…The energy needs for cooling are increasing at a fast pace worldwide, with buildings alone accounting for ≈40% of the total global energy use and 28% of total CO 2 emission, according to statistics from the United States Department of Energy. [1] Similar needs can be found in the automotive sector, especially with the advent of electric vehicles whose batteries can be porous polymers, [16][17][18][19][20] and metamaterials. [21][22][23][24][25] Although many reported radiative coolers have achieved good cooling performance, these coatings are characterized by broadband, high reflectivity over the whole solar spectrum, being thus limited to a white or mirror-like appearance at visible wavelengths.…”

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.…”