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

Passive radiative cooling (PRC) offers significant potential to reduce energy consumption and carbon emissions associated with cooling. Among various approaches, paint-like systems present several advantages in terms of cost effectiveness, scalability, and ease of application. In this study, we report on a PRC system composed of a paint mixture modified with 50% glass bubbles (GB) and a commercial polypropylene–polyethylene–polypropylene (PP–PE–PP) film, commonly used as a battery separator. The resulting material exhibits a solar reflectance of 94% and a broad emittance of over 95% in the sky-transparent window (STW) from 8 μm to 13 μm. The addition of glass bubbles enhances the solar reflectance of the base paint in the near-infrared wavelengths, while the nanoporous PP–PE–PP film (NPF) topcoat improves reflectance 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 film 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 reflectance, 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 configuration in which the PP–PE–PP film sealed the sample slot resulted in significant 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 artificial overheating effect, leading to biased measurements of passive radiative cooling.