Daytime radiative cooler using porous TiO₂: new approach

Abstract

Passive daytime radiative cooling remains a topic of intense interest that has gained great attention recently, mainly due to its important applications, such as electronic heat dissipation, solar cells, and photo-thermal technologies. In order to achieve the daytime radiative cooling for thermal management applications, a new smart shield should be designed to have both a low absorptivity in the solar range of 0.3–2.5 µm and high emissivity in the atmospheric transparency windows of 8–13 µm and 16–26 µm. In this work, we propose and analyze a new, inexpensive, and highly scalable double-layer coating to reach an efficient passive daytime radiative cooling. Double-layer coatings consisting of a top T i O 2 porous adsorbent layer and a bottom providing excellent transmittance in the solar spectrum are achieved properly by borosilicate-crown glass (BK7). We have shown that the proposed device preserves an average absorptivity/emissivity well below 7% in the entire solar spectrum and almost 95% in the atmospheric transparency windows. Besides, our design allows for sub-ambient temperature drops of 45° and diurnal cooling power output of 133 W / m 2 , even in the case of solar irradiance up to 1000 W / m 2 . The cooling performance persists under high humidity, even when taking the non-radiative heat exchange progress into account. The results revealed that the proposed design can be easily applied to a large area and promotes a significant step towards achieving large scale application in solar cells and related systems.

Flexible Daytime Radiative Cooling Enhanced by Enabling Three-Phase Composites with Scattering Interfaces between Silica Microspheres and Hierarchical Porous Coatings

“…In most of these application scenarios, daytime radiative cooling (DRC) is urgently needed rather than nocturnal cooling, yet it is also more challenging since solar radiation generally far exceeds thermal emission during the day. Many advanced photonic structures have already been developed toward maximizing both the solar reflectivity and thermal emissivity simultaneously, such as multilayered films, − nanoparticle-based composite materials, − and porous media coatings. − ,− For example, Zhai et al fabricated a kind of nonporous two-phase (i.e., microsphere-phase and polymer-phase) hybrid metamaterials in a roll-to-roll manner, which exhibits a strong emissivity (∼0.93) in the mid-infrared regime but a high transmittance in the visible range; with the silver coating deposited as a back reflector, the solar reflectivity can be increased to ∼96% and thus yielding a cooling power of 93 W/m 2 at noontime . Although silver and other metal mirrors are widely employed in the radiative coolers, it is generally expensive and prone to cause light pollution in practical use due to specular reflection.…”

Transparent Glass Surfaces with Silica Nanopillars for Radiative Cooling

“…Dielectric/metal multilayer coatings − and periodic or random distribution of micro/nano structures supporting surface phonon polariton (SPhP) excitations − on a surface were proposed to increase the emissivity in the ATW. Porous surfaces − and paints , are additional examples of randomly distributed elements for PRC. Although these designs are optimized for PRC applications, most of them are related to surfaces that present a color, scatter and/or have a high reflectivity in the visible range, these properties being incompatible with displays and other applications where transparency (i.e., neutral color transmission and low haze) is a requirement.…”