Sub-ambient daytime radiative cooling based on continuous sunlight blocking

Parasitic solar heating of the radiative cooler remarkably counteracts the cooling effect generated by radiative cooling. Many previous works have contributed to improving the solar reflectivity of the cooler to reduce its solar absorption, including using highly reflective film or optimized scattering effect. Herein, an easy and passive sunlight blocking strategy based on geometrical optics is applied to reduce the solar heating power of the cooler by using a ring-like shield to prevent the propagation of direct sunlight, corresponding to decoupling the cooler from solar beam irradiance. A black paint-coated cooler with strong solar absorption of over 0.9 and a porous polytetrafluoroethylene (P-PTFE) cooler with a high solar reflection of over 0.9 are fabricated to demonstrate the feasibility of the sunlight blocking strategy. Results show that radiative cooling to 3.5C and 6.5C below ambient temperature under average solar irradiance of 500 W m- 2 is achieved by the black paint and P-PTFE, revealing that the proposed sunlight blocking strategy contributes to efficient sub-ambient daytime radiative cooling. Theoretical analysis not only proves the effectiveness of the sunlight blocking strategy but also reveals that cooler with selective emission within the atmospheric window is more sensitive to sunlight, as well as capturing the radiative cooling performance under different conditions.

Automated summary

The study proposes a simple and passive sunlight blocking strategy to reduce the solar heating power of a radiative cooler, achieving efficient sub-ambient daytime radiative cooling.