Scalable and sustainable hierarchical-morphology coatings for passive daytime radiative cooling

Passive daytime radiative-cooling materials (characterized by a high solar reflectance and thermal emittance) exhibit a cooling effect under direct sunlight with zero energy consumption, thereby decreasing the demand for air conditioning. Although various well-designed radiative-cooling materials have been reported to date, their syntheses are environmentally harmful and unsuitable for large-scale operation (as they involve complicated, high-cost, or solution-processed methods). In this study, a hierarchical-morphology coating for large-scale radiative-cooling applications was constructed by a one-step, inexpensive, solution-free, and environmentally friendly strategy. The hierarchical morphology (comprising nanospheres and micropores randomly dispersed throughout a polymer matrix) was fabricated through simple mechanical stirring (without the use of templates); no solvents or by-products were produced during the manufacturing process. The optimal coating showed high emissivity (95.1%) in the atmospheric-window band, strong solar reflectivity (94.0%), and a cooling power of 62.94 W m-2 (according to field tests). Moreover, covering the roof of a model with the as-prepared hierarchical-morphology coating reduced the average roof temperature by 11.5 celcius (according to outdoor tests). According to simulations, the coating enabled annual cooling-energy-consumption savings in the range of 14.5-41.2% for typical buildings located in different climatic regions, indicating high potential as an energy-saving building-envelope material.Graphical AbstractA one-step, scalable and sustainable strategy has been developed to fabricate hierarchical-morphology coatings for passive daytime radiative cooling.