A flexible film to block solar radiation for daytime radiative cooling

Abstract

Passive radiative cooling without external energy input or environmental pollution is promising to be a serviceable strategy for energy saving. However, intense solar radiation seriously limits the performance of daytime radiative cooling. Here, a flexible and practical approach to reducing solar absorption of the emitters with a spectrally selective film (SSF) was proposed. The solar reflectivity of the SSF is 93%, while the IR transmissivity in the atmospheric window is 87%. Particularly, the SSF is hydrophobic, providing the film with resistance to the potential condensation problems. By covering with the SSF, black emitters achieved a consecutive 24-h sub-ambient radiative cooling. The temperature of the emitter surface decreased by 21.4 °C comparing to the bare ones at noon, with an average temperature drop of 3.1 °C below the ambient temperature. And higher average sub-ambient temperature drop of 4.8 °C was achieved by the improved experiment with convection shield. Additionally, the cooling performance for SSF with various emitters was also discussed. Commercial ESR film can achieve a cooling power of ~100 W/m2 with the SSF in arid areas. And the national-scale modeling in China showed that 85% of China's areas can achieve daytime sub-ambient radiative cooling by integrating with the proposed film. Thus, the scalable SSF can relax the requirements of solar reflection for emitters, providing a viable solution for daytime radiative cooling with simpler and cheaper emitters.