People living in hot climates have for centuries painted buildings white to keep them cool. White or silver coatings reflect the sun’s heat-producing infrared rays. But they can also create glare, and not everyone wants buildings with a white façade or roof.
Researchers at Columbia University have now made a two-layer cooling paint that can be any color. The simple, inexpensive paint reported in the journal Science Advances could be used to help cool buildings, data centers, and vehicles.
Keeping indoor spaces cool consumes an immense amount of energy. Cooling consumes around 15 percent of total household electricity usage in the United States, the authors write in the paper. If building and vehicle surfaces could reflect the sun’s heat, it would reduce the cooling burden on air-conditioners, bringing down their energy use and the cost of running them.
The researchers put the two layers of the cooling paint one by one on a surface. First comes a solution of a Teflon-like polymer in acetone and water. When the researchers coat it on the surface, the acetone and water evaporate, leaving behind an ultra-thin layer of the polymer with pores that reflect infrared light. On top of this polymer layer, which appears white, the researchers put a coat of commercial paint of different colors.
When sun shines on the paint, the top layer absorbs visible light of certain wavelengths depending on the color of the paint. The bottom layer, meanwhile, reflects the infrared radiation.
As a test, the researchers painted objects with coatings of different colors, and with regular commercial paint of the same colors. Placed outdoors in strong sunlight, the coating kept the objects anywhere from 3°C to up to 15.6°C cooler.
The paint was also durable. It did not degrade after being placed in an oven at 60°C for a month. The low-cost, scalable coating is “a practical and efficient solution to cooling colored objects in a green and energy-saving manner,” the team writes.
Source: Yijun Chen et al. Colored and paintable bilayer coatings with high solar-infrared reflectance for efficient cooling. Science Advances, 2020.