Researchers from the University of Colorado Boulder have developed a new material capable of cooling structures without water and energy consumption even if they are directly under sunlight.

In a study published in the journal Science, Xiaobo Yin and colleagues described their work, saying the new material can be an eco-friendly cooling method for thermoelectric power plants that require massive amounts of water and electricity to meet ideal temperatures for operating machinery.

According to the researchers, the metamaterial is applied as film on a surface, efficiently cooling the structure underneath by reflecting solar energy while letting the surface release its own heat at the same time as infrared thermal radiation.

A hybrid of glass and polymer, the new material is only as thick as 50 micrometers, which makes it just a little thicker than aluminum foil. And because it is thin, it can also be manufactured into rolls, allowing economical but large-scale use in both commercial and residential structures.

"We feel that this low-cost manufacturing process will be transformative for real-world applications of this radiative cooling technology," Yin said.

A New Cooling Material

The new material works by making the most of passive radiative cooling, which is the process by which an object sheds heat naturally as infrared radiation without using energy. Some level of natural cooling at night can be expected from thermal radiation and it is used in some areas to cool residential properties. Daytime cooling, however, is more challenging, what with the slightest amount of solar energy directly absorbed enough to render passive radiation ineffective.

The researchers' challenge then was to come up with a material that does two things at the same time: reflect incoming energy from the sun and provide a way for infrared radiation to escape. To meet their goal, Yin and colleagues embedded glass microspheres that visibly scatter but radiate infrared light into polymer film. A thin coating of silver was then added underneath to deliver optimal spectral reflectance.

Cooling Applications

According to Gang Tan, one of the study's authors, just 108 to 215 square feet of the new material on a rooftop can be effective in cooling down a single-family house during summer.

Aside from cooling power plants and buildings, however, it can also be used to maximize the life span and efficiency of solar panels. Yin said applying the new material to solar panels can cool down panels and recover a percent or two of solar efficiency. Though they are designed to harness energy from the sun, solar panels can heat up, which reduces their solar energy-conversion ability.

The researchers are also looking at agriculture and aerospace as potential areas of application. A patent application for the new material has been submitted and the researchers are working with the Technology Transfer Office at CU Boulder to explore possible commercial opportunities. This year, they are also planning on creating a prototype for a "cooling farm."

The new material is the result of a 2015 grant awarded to Yin, Tan, and Ronggui Yang, also one of the study's authors, worth $3 million by the Advanced Research Projects Agency-Energy of the Department of Energy.

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