# New metamaterial enhances natural cooling without power input

13 Feb 2017 Sarah Tesh

### Midday Sun

Field tests in Boulder, Colorado and Cave Creek, Arizona, revealed that the film’s average cooling power was more than 110 W/m2 over 72 hours. Even in the midday Sun, its average was 93 W/m2. This is roughly equivalent to the electricity generated by a typical solar panel of the same area.

The new material is similar to one produced by a group at Stanford University in 2014, which emitted roughly 40 W/m2 in direct sunlight. The previous design was fabricated using electron evaporation and consisted of seven alternate layers of silicon dioxide and hafnium dioxide. Not only does the new material appear to outperform the old (although testing methods are not directly comparable), it is also easier to make in large quantities.

The glass-polymer sheet has many potential cooling applications. By applying it to a solar-panel’s surface, the film could not only cool the panel but also recover an additional one or two per cent of solar efficiency, because overheating hampers the ability to convert solar energy. “That makes a big difference at scale,” says Xiaobo Yin, another researcher on the project.

But applying the film to buildings is not as simple as putting a layer on the roof. “You cannot just use our material to wrap a building,” explains Yang. This would cool the building in hot summers, but it would also continue cooling on cold nights and during the winter. The team therefore needs to create a thermal system design whereby water is cooled then circulated around the building. This is similar to a hot-water heating system and means the temperature can be controlled.

“The key advantage of this technology is that it works 24/7 with no electricity or water usage,” says Yang. “We’re excited about the opportunity to explore potential uses in the power industry, aerospace, agriculture and more.”

The next step for the researchers is to create a 200 m2 “cooling-farm” prototype in Boulder. They have also applied for a patent and are working with CU-Boulder’s Technology Transfer Office to explore commercial applications.