High‐Strength Flexible Membrane with Rational Pore Architecture as a Selective Radiator for High‐Efficiency Daytime Radiative Cooling

Abstract

radiative cooling requires a radiator with spectral selectivity, which refers to high absorptivity/emittance only in the atmospheric window (8-13 µm) and an extremely high solar reflectance (0.3-2.5 µm). [2] Traditional solutions combine a shielding layer and a radiative layer [3] or a radiative layer and a reflective metal layer. [2b,4] However, conventional materials and their combinations have a limited cooling effect because of their inability to satisfy the stringent requirements for selectivity. In 2014, Raman et al. [4b] reported an advanced 1D nanophotonic device that can reflect 97% of solar irradiance and allow selective radiation in atmospheric windows. It is the first device to achieve absolute cooling, that is, ≈4.9 °C sub-ambient temperature at a solar intensity (I solar ) of 850 W m −2 . Subsequently, many precision photonic devices have been used for PDRC, such as 1D photonic films [5] and metasurfaces, [6] as well as the product of their combination. [1a,7] These photonic devices have resulted in high efficiencies in PDRC applications. Nonetheless, large-scale applications of these devices remain to be a challenge owing to the demand for manufacturing processes.Recently, PDRC designs based on porous polymer structures have attracted considerable attention because of their high cooling performance, simplicity, applicability, and economic efficiency. [8] For example, Yang et al. [8a] reported a porous poly (vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) coating that exhibit an average solar reflectance (R solar , 0.24-2.5 µm) and long-wave IR (LWIR) emittance (ε LWIR , 8-13 µm) of ≈96% and ≈97.3%, respectively, allowing for a sub-ambient temperature drops of ≈6 °C at a I solar of 890 W m −2 . Wu et al. [9] reported a hierarchically porous array polymethyl methacrylate film with a porosity of 60% and a close-packed micropore array on the surface combined with abundant random nanopores inside, which demonstrated excellent R solar (95%) and ε LWIR (98%), with sub-ambient cooling of ≈5.5 to ≈8.9 °C. The excellent sunlight reflection originates from the bimodal pore size distribution with nanopores centered at hundreds of nanometers and micropores centered at several microns and/or high porosity. However, there is a certain distance away from perfect spectral selectivity, [9,10] especially the less reflectivity in the mid-wave

Recent progress in organic-based radiative cooling materials: fabrication methods and thermal management properties

“…Shorter film formation times would therefore be desirable to minimize this effect. A promising spray-phase-separation strategy was applied by Wang et al , 61 in which the slurry is atomized into small droplets during the spraying process, and phase separation occurs thereon (Fig. 3d).…”

Biomimetic reconstruction of butterfly wing scale nanostructures for radiative cooling and structural coloration

“…13,19 Thus, based on the biological inspirations, we realized that we could capture the novelty of this work by reproducing the dual functionalities of the butterfly species by mimicking the wing scale nanostructures and further enhancing the radiative cooling and structural cooling capabilities through experimental optimization. It was reported that the porous structure contributes to the radiative cooling enhancement, [20][21][22] but these studies rely on phase separation methods to fabricate a porous film, which is not a precise method to control the pore size and thereby reproduce the porous structure of the butterfly wing scales. In contrast, the solution process to create an inverse opal structure offers a facile fabrication platform to precisely control the pore size and spatially distribute the pores because the size of the sacrificial template directly corresponds to the pore size of the inverse opal structure as much previous literature about inverse opal structures has demonstrated for other applications.…”

Superhydrophobic Porous Coating of Polymer Composite for Scalable and Durable Daytime Radiative Cooling

“…Daytime radiative cooling materials are designed to satisfy specific spectral selectivity, which means high solar reflectance in the visible–near-infrared wavelength range (0.3–2.5 μm) and high emissivity in the atmospheric transparency window (8–13 μm) . Photonic crystals, , multilayer radiators, , particles embedded in composite materials, − hierarchically porous structure materials, − and other types of radiators have been designed, which have promising cooling applications in buildings, − solar cells, − wearable clothes, − and water condensation …”