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

Passive radiative cooling (PRC) ofers signifcant potential to reduce energy consumption and carbon emissions associated with cooling. Among various approaches, paint-like systems present several advantages in terms of cost efectiveness, scalability, and ease of application. In this study, we report on a PRC system composed of a paint mixture modifed with 50 % glass bubbles (GB) and a commercial polypropylene–polyethylene–polypropylene (PP–PE–PP) flm, commonly used as a battery separator. The resulting material exhibits a solar refectance of 94 % and a broad emittance of over 95 % in the sky-transparent window (STW) from 8 휇m to 13 휇m. The addition of glass bubbles enhances the solar refectance of the base paint in the near-infrared wavelengths, while the nanoporous PP–PE–PP flm (NPF) topcoat improves refectance in the UV range, remains largely transparent in the IR, and renders the overall coating washable. The material was tested under realistic outdoor conditions, comparing the performance when the PP–PE–PP flm was directly applied onto the wet paint layer versus when it was used as a separate windshield enclosing the sample test chamber. Despite its high solar refectance, no radiative cooling was observed relative to ambient temperature during peak hours (solar irradiation > 600 W·m2 ). However, below this threshold, a temperature drop of −3 ◦C and a cooling power exceeding 100 W·m2 were observed. Notably, even when a visibly opaque convection shield was used, the confguration in which the PP–PE–PP flm sealed the sample slot resulted in signifcant overheating of the air pocket surrounding the sample during the day. This outcome suggests that experimental setups incorporating a windshield, commonly found in the literature, may introduce an artifcial overheating efect, leading to biased measurements of passive radiative cooling.