Sustainable and self-cleaning bilayer coatings for high-efficiency daytime radiative cooling

Passive daytime radiative cooling (PDRC) without additional energy input to cool objects by directly radiating energy into outer space and reflecting sunlight provides a promising pathway to replace current compression-based cooling systems. However, the state-of-art PDRC coating still suffers from contamination during long-term operation to maintain the high solar reflectance ((R) over bar (solar)) and thermal emittance ((epsilon) over bar (LWIR)) in the long-wavelength infrared (LWIR) atmospheric transmission window. Here, to balance the super-hydrophobicity with micro-pores and the high-efficiency PDRC performance with nano-pores, we report a sustainable and scalable method to prepare a self-cleaning bilayer porous coating to achieve a static water contact angle (CA) similar to 163 degrees, (R) over bar (solar), = 0.97 and (epsilon) over bar (LWIR) = 0.96. Results show that (R) over bar (solar) changes little after being rubbed and exposed to air. And (R) over bar (solar) only decreases slightly from 0.97 to 0.92 in muddy water, illustrating the excellent contamination resistance of the super-hydrophobic bilayer coating. Under direct sunlight of 774 W m(-2), the bilayer coating achieves an average temperature of 4.0 degrees C below the ambient temperature and 4.7 degrees C below the commercial coating temperature. This remarkable cooling performance can be achieved even after contamination, which can be 3.1 degrees C below the ambient temperature after muddy water treatment. The super-hydrophobic bilayer coating remains clean and dry outdoors since the dust or contaminant can be easily carried away by rainwater, indicating that energy-free and labor-free cleaning of the coatings by rain favors long-term practical applications.

Automated summary

This study reports a sustainable and scalable method to prepare a self-cleaning bilayer porous coating with super-hydrophobicity and high-efficiency passive daytime radiative cooling (PDRC) performance. The coating shows excellent contamination resistance and cooling effect, and can be easily cleaned by rainwater without energy consumption or labor.