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Coating Buildings With This Paint Might Cool Them Off Enough to Reduce the Need for Air Conditioning


Ruan Xiulin, Purdue University / Jared Pike

In an effort to slow global warming, Purdue University engineers have created the whitest paint yet. Cladding buildings in this paint may one day cool them enough to reduce the need for air conditioning, the researchers say.

In October, the team created an ultra-white paint that pushed the boundaries of what white paint can look like. Now they have gotten over it. Not only is the newer paint whiter, it can also keep surfaces cooler than the formulation the researchers had previously demonstrated.

“If you used this paint to cover a ceiling area of ​​about 1,000 square feet, we estimate that you could get a cooling power of 10 kilowatts. That’s more powerful than the central air conditioners most homes use, ”said Xiulin Ruan, a Purdue professor of mechanical engineering.

The researchers believe that this white may be the closest equivalent to the blackest black, “Vantablack,” which absorbs up to 99.9% of visible light. The new whiter paint formula reflects up to 98.1% of sunlight, compared to 95.5% of the sunlight reflected by the researchers’ previous ultra-white paint, and sends infrared heat away from a surface at the same time.

Typical commercial white paint gets warmer rather than cooler. Paints on the market that are designed to reject heat reflect only 80% to 90% of sunlight and cannot make surfaces colder than their surroundings.

What makes the whitest paint so white?

Two characteristics give the painting its extreme whiteness. One is the paint’s very high concentration of a chemical compound called barium sulfate, which is also used to make photographic paper and cosmetics white.

“We look at various commercial products, basically anything that’s white,” said Xiangyu Li, a postdoctoral researcher at the Massachusetts Institute of Technology who worked on this project, published in ACS“Like a Ph.D. from Purdue. student in the laboratory in Rouen. “We found that by using barium sulfate, you can theoretically make things really very reflective, which means they’re really very white.”

The second characteristic is that the barium sulfate particles have different sizes in the paint. The amount of light that each particle scatters depends on its size, so a wider range of particle sizes allows paint to disperse more of the sunlight spectrum.

“A high concentration of particles that are also of different sizes gives the paint the widest spectral dispersion, which contributes to the highest reflectance,” said Joseph Peoples, Ph.D. from Purdue. mechanical engineering student.

There is a bit of room to make the paint whiter, but not much without compromising the paint.

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

How the whitest paint is also the freshest

The whiteness of the paint also means that the paint is the coldest on record. Using high-precision temperature reading equipment called thermocouples, the researchers demonstrated outdoors that paint can keep surfaces 19 degrees Fahrenheit cooler than their ambient surroundings at night. It can also cool surfaces to 8 degrees Fahrenheit below its surroundings in strong sunlight during the noon hours.

The solar reflectance of the paint is so effective that it even worked in the dead of winter. During an outdoor test with an ambient temperature of 43 degrees Fahrenheit, the paint succeeded in reducing the temperature of the sample by 18 degrees Fahrenheit.

This white paint is the result of six years of research based on attempts dating back to the 1970s to develop radiative cooling paint as a viable alternative to traditional air conditioners.

The Ruan lab had considered more than 100 different materials, narrowed them down to 10, and tested around 50 different formulations for each material. His previous ultra-white paint was a formulation made from calcium carbonate, a soil-abundant compound commonly found in rocks and seashells.

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The researchers showed in their study that, like commercial paint, their barium sulfate-based paint can potentially withstand outdoor conditions. The technique the researchers used to create the paint is also compatible with the commercial paint manufacturing process.

Source: Xiulin Ruan, Purdue University

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