Nanoporous Film Layers to Enhance the Performance of Passive Radiative Cooling Paint Mixtures

Abstract Passive radiative cooling (PRC) technology promises to reduce a significant fraction of our energy needs and carbon footprint associated with cooling. Among different approaches, paint-like systems present several advantages in cost-effectiveness, scalability, and application ease. We report on a PRC system consisting of a paint mixture modified with 50 %wt glass bubbles and a top polypropylene-polyethylene-polypropylene (PP-PE-PP) film typically used as a battery separator. The resulting material exhibits a solar reflectance of 94 % and an absorption (evaluated by Attenuated Total Reflectance spectroscopy) with a peak at 10 µm of 60 %. The addition of the glass bubbles and the top film increases the paint reflectance over the solar spectrum and the UV, thanks to the nanoporous PP-PE-PP film (NPF), while allowing most thermal radiation through, shielding from convection, and making the coating more easily washable. The material has been tested under realistic outdoor conditions, checking the different performance obtained when sticking the PP-PE-PP film directly onto the wet paint layer or by using it as a separate windshield enclosing the sample test chamber. Despite its high solar reflectance, no radiative cooling is observed with respect to ambient temperature during the peak hours (solar irradiation >500 W m −2). Below this threshold, a temperature drop of −3 °C an…