(546h) Electrospun Infrared-Transparent Solar-Reflecting Nanofibers for Direct Radiative Cooling

Radiative cooling is a promising way to alleviate urban heat island effects and decrease the energy required for occupant thermal comfort. Among many emerging approaches, the use of infrared-transparent films and fabrics that allow objects to directly radiate heat through bands of atmospheric transparency while blocking solar irradiation are particularly well suited for seasonal and other temporary uses. These covers have been widely based on polyethylene due to its simple chemistry and consequently intrinsically low IR absorption. Here, we investigate the effects of polymer nanofiber morphology on the scattering and transmission properties of polyacrylonitrile (PAN) based films. By controlling the electrospinning process, a beaded fiber morphology can be fabricated which features ellipsoidal beads connected via thin cylindrical fibers. These beaded fibers exhibit greater solar scattering than either spherical or cylindrical polymer structures, as confirmed using electromagnetic simulations. These noteworthy scattering properties decrease the amount of material needed to reach above 95% solar-weighted total reflectance, enabling high infrared transmittance (> 70%) despite PAN’s intrinsic absorption in the 8-13 µm range. When scaled up and tested outdoors, the beaded nanofiber PAN film (nanoPAN) decreases the temperature of an unpolished PDMS-coated aluminum sheet by 7°C by shielding it solar radiation while allowing it to radiate heat to space. Heat transfer modeling agrees with the outdoor experiments and predicts temperature drops exceeding 10°C below ambient under standard sky conditions with an optimized beaded nanofiber film. This work on tailoring the morphology of polymer nanofibers paves the way for unconventional materials and lower-purity feedstocks to be used as solar filtering covers in direct radiative cooling applications.