White Solar-Reflectance Paint Reduces Need for Air Conditioning

A team from Purdue University has developed an improved ultrawhite paint that, tests showed, reflected nearly 98% of sunlight — compared to 95.5% of sunlight that an earlier version of the paint reflected following its introduction in October 2020. Applied to, or coated on a surface, the paint also directs incoming infrared heat away from the surface.

The paint is poised to lessen the need for air conditioning; attempts to develop radiative cooling paint as an alternative to traditional air conditioners have been ongoing since the 1970s. It also achieved full functionality in cold weather (winter) environments, giving it use in other cooling applications.

Typical commercially available white paint gets warmer rather than cooler. Paints designed to reject heat reflect only 80% to 90% of sunlight, preventing them from making a coated surface cooler than its surroundings.

The solar reflectance capabilities of the paint stem directly from its whiteness; that quality is the result of two distinct compositional characteristics.

The first is a very high concentration of barium sulfate. The compound is similarly a primary component of white-reflectance coating(s) and is used in the manufacture of photo paper and certain cosmetics to make them white. According to Xiangyu Li, a postdoctoral researcher at MIT who worked on the project while earning an advanced degree at Purdue, barium sulfate functions to make objects highly reflective or, in other words, highly white.

A formulation made of calcium carbonate, commonly found in rocks and seashells, was a component of the team's previous whitest paint.

The second whitening trait of the new paint involves the different sizes of the compositional barium sulfate particles. The extent to which each particle scatters light depends on the size of that given particle. A wider variance of particle sizes allows the paint to scatter more of the light spectrum from the sun (in other words, broader spectral scattering capabilities).

“Although a higher particle concentration is better for making something white, you can't increase the concentration too much,” Li said. “The higher the concentration, the easier it is for the paint to break or peel off.”

The team said that if it were to cover a roof area of roughly 1000 sq ft with its paint, it could achieve an estimated cooling power of 10 kW. That value is more powerful than a central air-conditioning system in most houses, said Xiulin Ruan, a professor of mechanical engineering at Purdue.

Xiulin Ruan, a Purdue University professor of mechanical engineering, holds up his lab's sample of its whitest paint. Courtesy of Purdue University/Jared Pike.

In another test taking place outdoors in mid-winter (at an ambient temperature of 43 °F), the paint lowered a sample temperature by 18 °F. The result showed the barium sulfate paint to be effective in certain (and, potentially, any) outdoor conditions. Further, the researchers said, the technique they used to create the paint is compatible with the commercial paint fabrication process.

Patent applications for the paint formulation have been filed, according to a press release from Purdue.

The research was supported by the Cooling Technologies Research Center at Purdue University and the Air Force Office of Scientific Research through the Defense University Research Instrumentation Program.

The research was published in ACS Applied Materials & Interfaces (https://pubs.acs.org/doi/10.1021/acsami.1c02368).

*This article has been updated.