Highly suppressed solar absorption in a daytime radiative cooler designed by genetic algorithm

Here, we report a selective multilayer emitter for eco-friendly daytime passive radiative cooling. The types of materials and thickness of up to 10 layers of the multilayer structure are optimized by a genetic algorithm. The passive radiative cooler is designed to mainly target low solar absorption, which allows sub-ambient cooling under direct sunlight. We used a custom objective function in the solar region to achieve high-performance daytime radiative cooling to minimize solar absorption. The designed structure minimizes solar absorption with an average absorptivity of 5.0% in the solar region (0.3-2.5 mu m) while strongly emitting thermal radiation with an average emissivity of 86.0% in the atmospheric transparency window (8-13 mu m). The designed and fabricated structure achieves daytime net cooling flux of 84.8 W m(-2) and 70.6 W m(-2), respectively, under the direct AM 1.5 solar irradiation (SI) (total heat flux of 892 W m(-2) in the 0.3-2.5 mu m wavelength region). Finally, we experimentally demonstrate a passive radiative cooling of the fabricated selective emitter through a 72-hour day-night cycle, showing an average and maximum temperature reduction of 3.1 degrees C and 6.0 degrees C, respectively. Our approach provides additional degrees of freedom by designing both materials and thickness and thereby is expected to allow high-performance daytime radiative cooling.

Automated summary

This study presents a selective multilayer emitter for eco-friendly daytime passive radiative cooling, designed to achieve sub-ambient cooling under direct sunlight. By optimizing material types and thickness and using a custom objective function, the structure achieves high-performance daytime radiative cooling in the solar region.