Selectively Enhancing Solar Scattering for Direct Radiative Cooling through Control of Polymer Nanofiber Morphology

Abstract

Radiative cooling can alleviate urban heat island effects and passively improve personal thermal comfort. Among many emerging approaches, infrared (IR) transparent films and fabrics are promising because they can allow objects to directly radiate heat through bands of atmospheric transparency while blocking solar heating. However, achieving high solar reflectance while maintaining IR transmittance using scalable nanostructured materials requires control over the shape and size distribution of the nanoscale building blocks. Here, we investigate the scattering and transmission properties of electrospun polyacrylonitrile (PAN) nanofibers that feature spherical, ellipsoidal, and cylindrical morphologies. We find that nanofibers that have ellipsoidal beads exhibit the most efficient solar scattering, mainly due to the additive dielectric resonances of the ellipsoidal and cylindrical geometries, as confirmed through electromagnetic simulations. This favorable scattering decreases the amount of material needed to reach above 95% solar reflectance, which, in turn, enables high infrared transmittance (>70%) despite PAN's intrinsic IR absorption. We further show that these PAN nanofibers (nanoPAN) can enable cooling of surfaces with relatively low solar reflectance, which is demonstrated by covering a reference blackbody surface with beaded nanoPAN. During peak solar hours, this configuration lowers the temperature of the black surface by approximately 50 °C and is able to achieve as low as 3 °C below the ambient air temperature. More broadly, our demonstration using PAN, which is not as IR transparent as more commonly used polyethylene, provides a method for utilizing lower purity materials in radiative cooling.

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

“…Much work has been done for mid-infrared (MIR)-transparent radiative cooling textiles. [70][71][72] And a small thickness of < ~150 μm is usually needed to maintain enough MIR transmittance, limiting the solar reflectivity (discussed in the following section) and mechanical strength. On the other hand, the performance of MIR-emissive textiles does not depend on the spectrum of the underlying surface and thus relaxes restrictions on thicknesses.…”

Sustainable polymer materials for flexible light control and thermal management

“…In one study, polyacrylonitrile nanofibers were electrospun and fiber morphology was varied to control optical properties. 44 Fiber diameter varied between 0.2 and 1 μm as the fiber morphology varied from beaded to cylindrical. Large changes in solar reflectance and LWIR transmission were observed among the different nanofiber mats.…”

Electrospinning to Surpass White Natural Silk in Sunlight Rejection for Radiative Cooling

“…Recently, anisotropic fibrous media that are similar to ours have been studied for radiative cooling applications. [ 25–27 ] Li et al [ 25 ] used electrospun polyethylene oxide to have a mid‐IR emissivity spectrum desired for radiative cooling. For maximum solar rejection, they determined that a fiber diameter should be in the range of 0.5–1.2 μm, comparable to solar wavelengths, based on scattering efficiency calculations (the optimum diameter would have been 0.746 μm, see Section 10, Supporting Information).…”

“…The numerical study showed that a fiber diameter of 0.4–0.5 μm at a fiber fraction of 0.04 yielded the maximum radiative cooling under sunlight when the film thickness was below 1 mm. Kim et al [ 27 ] used electrospun polyacrylonitrile to render low solar transmissivity and high mid‐IR transmissivity. Ellipsoidal beadings on the fibers of a 0.067 μm in diameter increased solar reflectivity.…”