Nanophotonic porous structures for passive daytime radiative cooling

Owing to of its freedom from energy inputs, radiative cooling is emerging as a frontier in renewable energy research. Recent advances in nanophotonic structures have enabled scalable passive daytime radiative cooling (PDRC), such as porous polymer films, which can spontaneously reflect solar irradiance and emit heat to ultra-cold outer space. However, there is still limited fundamental understanding of nanophotonic structures underlying the cooling effects of porous polymer-based PDRC films. Hence, we performed optical simulations validated by experiments to quantify how the spatial distribution of nanopores in polymer film of various thickness affect its PDRC performance at different solar incidence angles. Our simulation results showed that influence of spatial distribution of pores on PDRC performance replies significantly on solar reflectance. A gradient distribution of pores with random pattern can remarkably enhance solar reflectance under direct sunlight up to 153.4%, while maintaining a slight variation of thermal emittance <4.4%. With such optimized spatial distribution of pores in polymer, daytime radiative cooling power increased 39.4W/m2 under direct sunlight in comparison with randomly distributed pores. These findings indicate that the spatial distribution of nanopores is critical both for demonstrating the cooling effect and for the rational design of nanophotoni…