The system uses a special polymer film that's installed on the bottom of a specially designed solar “shelter.” The film absorbs heat from the air inside its solar shelter, before transmitting it through the Earth's atmosphere into outer space.
“The polymer stays cool as it dissipates heat through thermal radiation, and can then cool down the environment,” says co-first author Dr. Lyu Zhou, a Ph.D. candidate in electrical engineering at the University at Buffalo School of Engineering and Applied Sciences.
“This is called radiative or passive cooling, and it’s very interesting because it does not consume electricity – it won’t need a battery or other electricity source to realize cooling.”
Absorbing heat and beaming it into space
For their device, the engineers used a special polymer/metal film that can both absorb heat and reflect it back. This film is made from a sheet of aluminum coated with a clear polymer called polydimethylsiloxane. The aluminum in the film reflects sunlight, while the polymer absorbs and dissipates heat into the surrounding air.
The special film, however, can't handle the cooling on its own. Naturally, heat tends to dissipate in a uniform manner, which defeats the cooling effect that the heat absorption brings. To make sure the heat is redirected out into space, the engineers placed the film at the bottom of a foam box and built a solar “shelter” on top of the box. To do this, they used a solar absorbing material to construct four outward slanting walls and an inverted square cone inside those walls.
With this design, any heat that is being dissipated by the film is sent straight up into space and not into the area surrounding the device.
“One of the innovations of our system is the ability to purposefully direct thermal emissions toward the sky,” says lead researcher Dr. Qiaoqiang Gan, Ph.D., associate professor of electrical engineering at the University of Buffalo. “Normally, thermal emissions travel in all directions. We have found a way to beam the emissions in a narrow direction.”
Getting radiative cooling to work in the day in crowded environments
What makes the engineers' design innovative is how it addresses an important problem in the field of radiative cooling – how to get them to work during the day and in crowded urban areas.
“During the night, radiative cooling is easy because we don't have solar input, so thermal emissions just go out and we realize radiative cooling easily,” explains co-first author Dr. Haomin Song, Ph.D.
“But daytime cooling is a challenge because the sun is shining. In this situation, you need to find strategies to prevent rooftops from heating up. You also need to find emissive materials that don't absorb solar energy. Our system address these challenges.”
“This enables the system to be more effective in urban environments, where there are tall buildings on all sides,” Gan added.
In addition, Gan also points out that the system they designed uses low-cost, commercially available materials, which is important considering the box's size and the amount of space it can cool. (Related: Stay cool without electricity: New "radiative sky cooling" system may prove useful for preppers.)
The current design for the shelter-and-box system measures about 18 inches tall, 10 inches wide and 10 inches long. With this size, the team says that numerous units of the system would be needed to cover a roof.
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