A scalable microstructure photonic coating fabricated by roll-to-roll “defects” for daytime sub-ambient passive radiation cooling

The deep space’s coldness (~4K) provides a ubiquitous and inexhaustible thermodynamic resource to suppress the cooling energy consumption. However, it is nontrivial to achieve sub-ambient radiative cooling during daytime under strong direct sunlight, which requires rational and delicate photonic design for simultaneous high solar reflectivity (> 94%) and thermal emissivity. A great challenge arises when trying to meet such strict photonic microstructure requirements while maintaining manufacturing scalability. Herein, we demonstrate a rapid, low-cost, template-free roll-to-roll method to fabricate spike microstructured photonic nanocomposite coatings with Al2O3 and TiO2 nanoparticles embedded that possess 97.5% of solar reflectivity and 98.6% of thermal emissivity. A novel Roll-to-roll Defects Coefficient is proposed to predict the microstructure density. When facing direct sunlight at summer noon (806 W/m2 solar intensity), the meta-coatings show a radiative cooling power of 99.2 W/m2. Combined with the coatings’ superhydrophobic and contamination resistance merits, the potential 15.1% cooling energy saving capability is numerically demonstrated across the United States.