Aqueous double-layer paint of low thickness for sub-ambient radiative cooling

Radiative cooling may serve as a promising option to reduce energy consumption for space cooling. Radiative cooling paints provide a cost-effective and scalable solution for diverse applications and attract great attention, but the state-of-art cooling paints generally use non-eco-friendly organic solvents and need large thicknesses (>400 μm) to realize high performance, which leads to high cost and environmental issues in implementation. This work aims to address these challenges by developing eco-friendly aqueous paints with low thickness (below 150 μm) by adopting a double-layer design based on a complementary spectrum strategy. The structure consists of a wide bandgap top layer to scatter short-wavelength light and a bottom layer with high reflectance to visible and near-infrared (NIR) irradiation. Effects of different design factors are studied using numerical simulation and experiments to attain the optimal design. The resulting Y₂O₃-ZnO paints show a strong reflectance of 95.4 % and a high atmospheric window emissivity of 0.93 at a low thickness of 150 μm. Field tests in the subtropic humid climate of Hong Kong demonstrated sub-ambient cooling of 2 °C at noon and 4 °C at night without shielding convection. The paints also show high robustness and excellent resistance to water and UV light attacks, rendering them promising for large-scale applications.