Materials

Super-white paint reflects 95.5 percent of sunlight to cool buildings

Super-white paint reflects 95....
Xiulin Ruan (left) and Joseph People (right) test out samples of the new cooling paint
Xiulin Ruan (left) and Joseph People (right) test out samples of the new cooling paint
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Joseph People takes a temperature reading of the new white paint
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Joseph People takes a temperature reading of the new white paint
Xiulin Ruan (left) and Joseph People (right) test out samples of the new cooling paint
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Xiulin Ruan (left) and Joseph People (right) test out samples of the new cooling paint
An infrared image of a P painted with the new reflective paint, clearly cooler than the surrounding surface
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An infrared image of a P painted with the new reflective paint, clearly cooler than the surrounding surface
An infrared image comparing the temperature of the new radiative cooling paint (left) and a commercial paint sample (right)
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An infrared image comparing the temperature of the new radiative cooling paint (left) and a commercial paint sample (right)
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Researchers from Purdue University have developed a new white paint that can reflect over 95 percent of sunlight, which can cool the surface even lower than the ambient temperature. Painted on a building, the new paint could help cut cooling costs and energy use.

It’s a basic fact of physics that white surfaces and objects reflect more light, and therefore remain cooler. Theoretically, that could be harnessed to help keep buildings cool – after all, air conditioning is one of the biggest guzzlers of energy in the summer months, and studies suggest that painting buildings white can cool entire cities.

Many different radiative cooling paints have been developed over the years, reflecting sunlight using glass, Teflon or, most commonly, titanium dioxide. But they all have their drawbacks.

For the new study, the Purdue researchers developed a new recipe. Instead of titanium dioxide, they used calcium carbonate fillers, which are much more abundant, cheaper, and absorb less ultraviolet light, so they stay cooler. The paint also contains a particle concentration of 60 percent and uses a variety of particle sizes, both of which help it scatter more sunlight.

In total, the new radiative cooling paint can reflect 95.5 percent of the light that strikes it. That’s better than many of the other paints in development, which manage between 80 and 90 percent, although it falls short of the record holder – the Teflon coating, which reflects 98 percent of light.

An infrared image comparing the temperature of the new radiative cooling paint (left) and a commercial paint sample (right)
An infrared image comparing the temperature of the new radiative cooling paint (left) and a commercial paint sample (right)

The team tested the new coating outdoors over two days, and showed that under direct Sun it remained 1.7 °C (3 °F) cooler than the ambient temperature, exhibiting a cooling power of 37 W/m2. At night, the paint dropped to 10 °C (18 °F) below the ambient temperature.

In another set of tests, the researchers painted some parts of a pattern with the new paint, and others with a regular white paint of the same thickness. Using an infrared camera, they could see the pattern clearly, thanks to the difference in temperature between the two materials.

The team says that the new paint could not only help cool buildings, but prevent outdoor electrical systems from overheating. But to ensure that it is commercially viable, the researchers next plan to investigate how well it lasts out in the elements.

"Our paint is compatible with the manufacturing process of commercial paint, and the cost may be comparable or even lower," says Xiulin Ruan, an author of the study. "The key is to ensure the reliability of the paint so that it is viable in long-term outdoor applications.”

The research was published in the journal Cell Reports Physical Science.

Source: Purdue University

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10 comments
paul314
If it drops that far below ambient temperature at night, could you run some kind of heat engine with it?
Ralf Biernacki
That's fantastic news, literally! Just think, if the surface of the paint is below ambient temperature, then you could cool the ambient air with it! Hasta la vista, global warming!
guzmanchinky
I need this for my camper van!
Johannes
Hey Ralf, ever heard of night sky cooling? Used centuries ago to make ice and as it's a physical phenomenon, it works in lots of situations. A conventional metal roof will cool several degrees below ambient at night if the sky's clear.
David V
Question : having just gone through a scorchingly hot summer here near Paris, if I painted my roof with this super white paint, I guess I would see a massive improvement to sleeping under my attic bedroom.
But what happens in winter ? How would my my traditional tiles behave ? They wouldn't absorb any heat during the day and maybe they would actually crack ?
Jean Hohl
What about the eyes of people and animals in the vicinity of those buildings on a clear, sunny day? It would be like staring at the sun. No consideration of that aspect of the paint's reflectivity is disturbing.
ljaques
Keep testing, Purdue! It seems like they're having reliability problems with it, so I hope they can overcome that. Good, reflective white paint is wonderful stuff. // Right you are, Ralf. Let's paint everything with it. Roofs, cars, lawns, streets, parking lots, everything! LOL
Dan
So who is selling it right now?
ScienceFan
There are white paints that are better than titanium oxide based paints. Also polypropylene spin foil is highly reflective. Absolutely wear sunglasses to avoid snow blindness. Unfortunately not UV resistent for more than a couple months. Dirt/dust is also an issue over time. Very reflective materials in IR are also poor emitters of heat. So in climates where it also gets cold in winter low temperature IR reflection is a very useful feature. That is more difficult that shorter wave lengths. And white roofs should somehow become fashionable...
Science Gal
Can anyone explain (in simple terms) how a material can have a temperature below ambient (or perhaps share a source that explains it)?