Note to reporters: A video about this research is available on YouTube. The paper is openly available on the journal website. Infrared images of paint, photos of researchers measuring the cooling performance of paint samples, b-roll and other multimedia are available through Google Drive. Journalists visiting the campus should follow the visitor health guidelines.

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

Update: Researchers at Purdue University have created the whitest paint on record. read more.

West Lafayette, Indiana-What if paint can cool the building to the point where it doesn't need air conditioning?

Engineers at Purdue University have created a white paint that keeps the surface 18 degrees Fahrenheit lower than the surrounding environment—almost like a refrigerator, but consumes no energy.

According to the researchers, the paint absorbs almost no solar energy and radiates heat away 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. A video about the project is available on YouTube.

"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 did his PhD on this project. . Student in Ruan Lab.

Researchers say that if this coating 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.

In a paper published on Wednesday (October 21) in Cell Reports Physical Science, the researchers showed that compared with commercial white paint, the paint they developed can maintain a lower temperature and reflect more under direct sunlight. More ultraviolet rays.

Their evidence is the infrared camera images of the two paints taken during 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 white paint created by Purdue University researchers reflects 95.5% of sunlight and effectively radiates infrared rays.

It is not easy to develop such coating formulations. 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, reduced them 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 sunlight and offsetting the heat inside the house," Purdue University Ph.D. Joseph Said the people. A student of mechanical engineering and a co-author of the work.

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. For further discussions on this intellectual property, please contact DHR Sarma at dhrsarma@prf.org and refer to the tracking code 2018-RUAN-68168-03.

This research was supported by the Purdue University Cooling Technology Research Center and the Air Force Scientific Research Office through the National Defense University Research Instrument Program (authorization number FA9550-17-1-0368).

Purdue University is a leading public research institution dedicated to developing practical solutions to today's toughest challenges. Purdue University is ranked as the fifth most innovative university in the United States by U.S. News and World Report, offering world-changing research and otherworldly discoveries. Purdue is committed to practical and online real-world learning, providing a transformative education for everyone. Committed to affordability and accessibility, Purdue University has frozen tuition and most of its fees at 2012-13 levels, allowing more students than ever to graduate without debt. See https://purdue.edu/ to learn how Purdue University never stops pursuing the next great leap. 

Writer and media contact: Kayla Wiles, 765-494-2432, wills5@purdue.edu

Source: Ruan Xiulin, ruan@purdue.edu

Journalists visiting campus: Journalists should follow the Protect Purdue Agreement and the following guidelines:

Full-day sub-ambient radiant cooling in commercial coatings with high quality factor

Li Xiangyu, Joseph Peoples, Zhifeng Huang, Zixuan Zhao, Jun Qiu and Xiulin Ruan

Radiation cooling is a passive cooling technology that works by reflecting sunlight and emitting radiation in the skylight. Although very ideal, it has not yet achieved full-day sub-ambient radiant cooling in similar commercial single-layer particle matrix coatings. Here, we demonstrated full-day sub-ambient radiant cooling in CaCO3-acrylic coatings by using a large band gap CaCO3 filler, a high particle concentration of 60% and a wide size distribution. Our paint shows a high solar reflectance of 95.5% and a high normal emissivity of 0.94 in the sky window. Field tests show that the cooling power at noon exceeds 37 W/m2, and the surface temperature is more than 1.7°C lower than the ambient temperature. It is recommended to use the quality factor RC to compare the cooling performance that is not affected by weather conditions. The standard RC of our paint is 0.49, which is the best radiant cooling performance. At the same time, it has the advantages of convenient paint form, low cost and compatibility with commercial paint manufacturing processes.

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