Can we really achieve an appreciable amount of cooling of a room just by using a particular type of paint? If that is really workable, then in this era of climate change, there cannot be anything better than that. In a paper published in October 2021, in the journal Cell Reports Physical Science, the researchers from Purude University have shown that compared to commercial white paint, the paint that they have developed can maintain a lower temperature under direct sunlight and reflect more ultraviolet rays.

The research work

Purdue University engineers have created a white paint that can keep surfaces up to 18 degrees Fahrenheit cooler than their ambient surroundings – almost like a refrigerator does, but without consuming energy.

According to the researchers, the paint would replace the need for air conditioning by absorbing nearly no solar energy and sending heat away from the building. Without the building heating up, air conditioning wouldn’t have to kick on.

Purdue researchers Xiulin Ruan (L) and Joseph Peoples use an infrared camera to compare the cooling performance of white paint samples on a rooftop…
Image: Purdue University, Photographer: Jared Pike

Briefing on the development, Xiulin Ruan, a Purdue Professor of Mechanical Engineering, said, “It’s very counterintuitive for a surface in direct sunlight to be cooler than the temperature your local weather station reports for that area, but we’ve shown this to be possible.”

Effect on environment

The paint would not only send heat away from a surface, but also away from earth into deep space, where heat travels indefinitely at the speed of light. This way, heat doesn’t get trapped within the atmosphere and contribute to global warming.

“It’s very counterintuitive for a surface in direct sunlight to be cooler…”
Xiulin Ruan, a Purdue Professor of Mechanical Engineering

Explaining the fact, Xiangyu Li, a Postdoctoral Researcher at the Massachusetts Institute of Technology who worked on this project as a Ph.D. student in Ruan’s lab, said, “We’re not moving heat from the surface to the atmosphere. We’re just dumping it all out into the universe, which is an infinite heat sink.”

According to the researchers, the earth’s surface would actually get cooler with this technology if the paint was applied to a variety of surfaces including roads, rooftops and cars all over the world.

Other relevant information

Developing this paint formulation wasn’t easy. The six-year study builds on attempts going back to the 1970s to develop radiative cooling paint as a feasible alternative to traditional air conditioners.

The researchers considered over 100 different material combinations, narrowed them down to 10 and tested about 50 different formulations for each material. They landed on a formulation made of calcium carbonate, an earth-abundant compound commonly found in rocks and seashells.

This compound, used as the paint’s filler, allowed the formulation to behave essentially the same as commercial white paint but with greatly enhanced cooling properties. These calcium carbonate fillers absorb almost no ultraviolet rays due to a so-called large “band gap,” a result of their atomic structure. They also have a high concentration of particles that are of different sizes, allowing the paint to scatter a wider range of wavelengths.

An infrared camera image shows that white radiative cooling paint developed by Purdue University researchers (left,
purple) can stay cooler in direct sunlight compared to commercial white paint…
Image: Purdue University, Photographer: Joseph Peoples

Comparative costing

According to the researchers’ cost estimates, this paint would be both cheaper to produce than its commercial alternative and could save about a dollar per day that would have been spent on air conditioning for a one-story house of approximately 1,076 square feet.

Detailing further on it, Joseph Peoples, a Purdue Ph.D. student in mechanical engineering and a co-author of the work, said, “Your air conditioning kicks on mainly due to sunlight heating up the roof and walls and making the inside of your house feel warmer. This paint is basically creating free air conditioning by reflecting that sunlight and offsetting those heat gains from inside your house.”

He further explained that cutting down on air conditioning also means using less energy produced by coal, which could lead to reduced carbon dioxide emissions. The researchers have further studies underway to evaluate these benefits. They are now working on developing other paint colours that could have cooling benefits.

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