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Purdue researchers Xiulin Ruan (left) and Joseph Peoples used infrared cameras to compare the cooling performance of white paint samples on the roof. (Photo credit: Jared Pike/Purdue)

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Researchers report that a new white paint can make the surface temperature 18 degrees Fahrenheit lower than the surrounding environment.

According to the researchers, the paint hardly absorbs solar energy, but radiates heat from the building, thus replacing the need for air conditioning. If the building does not heat up, the air conditioner does not have to be activated.

"It is very counterintuitive that the surface under direct sunlight is cooler than the area reported by the local weather station, but we have proven that it is possible," said Ruan Xiulin, a professor of mechanical engineering at Purdue University.

This paint not only radiates heat from the surface, but also radiates heat from the earth to deep space, where the heat travels indefinitely at the speed of light. In this way, heat will not be trapped in the atmosphere and cause global warming.

“We don’t transfer heat from the surface to the atmosphere. We just dump it all into the universe. The universe is an infinite radiator,” said Li Xiangyu, a postdoctoral researcher at MIT, who works as a doctoral student in Ruan’s laboratory. Participated in this project at the time.

Researchers say that if paint is applied to various surfaces, including roads, roofs and cars around the world, the surface of the earth will actually become colder due to this technology.

As reported by Cell Reports Physical Science, compared to commercial white paint, the new paint can maintain a lower temperature under direct sunlight and reflect more ultraviolet light.

Their evidence is the infrared camera images of the two paints they took in the roof experiment.

"Infrared cameras provide you with temperature readings, just like a thermometer tells whether someone has a fever. These readings confirm that our paint is cooler than the surrounding environment and commercial paint," Ruan said.

Commercial "insulation coatings" currently on the market only reflect 80%-90% of sunlight, and cannot reach temperatures lower than the surrounding environment. The new white paint reflects 95.5% of sunlight and effectively radiates infrared heat.

Developing paint formulations is not easy. This six-year study is based on attempts in the 1970s to develop radiant cooling coatings as a viable alternative to traditional air conditioners.

The researchers considered more than 100 different material combinations, narrowed them down to 10, and tested about 50 different formulations for each material. They found a formula made of calcium carbonate, an earth-rich compound commonly found in rocks and shells.

This compound used as a paint filler makes the performance of the formulation basically the same as that of commercial white paint, but the cooling performance is greatly enhanced. Due to their atomic structure, these calcium carbonate fillers have a so-called large "band gap" and therefore hardly absorb ultraviolet light. They also have a high concentration of particles of different sizes, enabling the coating to scatter a wider range of wavelengths.

According to the researchers' cost estimates, the production cost of this coating will be cheaper than its commercial alternatives, and it will save about $1 a day, which could have been used for air-conditioning for a single-story house of about 1,076 square feet.

"Your air conditioner starts mainly because the sun heats the roof and walls, making your house feel warmer inside. This paint basically creates free air conditioners by reflecting the sun and offsetting the heat inside the house," co-author, PhD in Mechanical Engineering Said Birth Joseph People.

People say that reducing air-conditioning also means using less coal energy, which may lead to a reduction in carbon dioxide emissions. Researchers are conducting further studies to evaluate these benefits.

Researchers are developing other paint colors that may have a cooling effect. The team filed an international patent application for the coating formulation through the Purdue Research Foundation Technology Commercialization Office.

Purdue University's Cooling Technology Research Center and Air Force Science Research Office funded this work through the National Defense University Research Instrument Program.

Original research DOI: 10.1016/j.xcrp.2020.100221

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