Purdue University engineers have created 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.
The researchers considered more than 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 different sizes, allowing the paint to scatter a wider range of wavelengths.
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 would not have to kick on.
“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,” said Xiulin Ruan, a Purdue professor of mechanical engineering.
“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,” said 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.
“An infrared camera gives you a temperature reading just like a thermometer would to judge if someone has a fever. These readings confirmed that our paint has a lower temperature than both its surroundings and the commercial counterpart,” Ruan said.
Commercial “heat rejecting paints” currently on the market reflect only 80%-90% of sunlight and cannot achieve temperatures below their surroundings.
The white paint that Purdue researchers created reflects 95.5% sunlight and efficiently radiates infrared heat.
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.
“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,” said Joseph Peoples, a Purdue Ph.D. student in mechanical engineering and a co-author of the work.
Purdue University is based in Indiana, USA.