Passive radiative cooling design with novel selectively grating structure under direct sunlight

Daytime radiators spontaneously transfer excess heat to the external space and become an attractive choice for radiative cooling technology with the advantages of no energy consumption and no pollutant generation. A grating selective structure with high reflectivity in the solar band and high absorptivity in the 8-13 mu m band was proposed for daytime radiative cooling in this paper. The structural parameters and constituent materials of the structure were optimally solved using the RCWA-PSO algorithm to maximize its net cooling power. It was shown that the grating selective structure composed of SiO2, AlON, and MgAl2O4 with a solar band absorptivity of 4.9% can achieve a net cooling power of 72.42 W/m2 under intense solar radiation, while achieving a net cooling power of 121.47 W/m2 at night. Its cooling performance is better than that of more than 80% of flat panel radiators. Additionally, the electromagnetic field characteristics study revealed that SiO2 is easier to excite the magnetic polaritons (MPs) resonance mechanism than AlON and MgAl2O4, but the coupled resonance effects of MPs and Surface Plasmon Polaritons (SPPs) are easily excited inside MgAl2O4. Thus, SiO2 and MgAl2O4 may be the preferred materials for radiators.