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High-tech rooftop mirrors bounce sun's heat back into space

Stanford researchers suggest a unique place to store excess heat -- the coldness of outer space -- which could help keep buildings cooler and help save energy.

A new super-thin film from Stanford researchers can keep buildings cooler by bouncing 97 percent of the sun's rays away from them. Nicolle R. Fuller, Sayo-Art

You know how they say that in brainstorming, there are no bad ideas? Kudos, then, to the person at Stanford University who, during a discussion about how to cool buildings more efficiently, said, "I know, let's put big mirrors on top of them and bounce the heat away."

OK, I'm not sure it went exactly that way, but the idea at first glance does sound a bit outrageous -- and deceptively simple. But it also works. At least it does according to research recently presented in the journal Nature.

Of course, we're not talking about an ordinary mirror here. The reflective film the researchers built is thinner than even the slimmest piece of aluminum foil. "It is made of seven layers of silicon dioxide and hafnium oxide on top of a thin layer of silver," says a Stanford report about the research. "These layers are not a uniform thickness, but are instead engineered to create a new material. Its internal structure is tuned to radiate infrared rays at a frequency that lets them pass into space without warming the air near the building."

That last part is important. The film the researchers made fights heat in two ways. First, it reflects visible sunlight, just as an ordinary mirror or a white roof would. More importantly, it directs heat-causing infrared rays to bounce right back into the coldness of space so the air around the mirror doesn't heat up -- meaning the film won't contribute to global warming.

What it will do, however, is help keep buildings cooler using what the researchers term photonic radiative cooling. It can prevent 97 percent of the sun's rays from hitting a building, and it can cool the air around by 9 degrees Fahrenheit, all without needing any energy source of its own.

This could dramatically help cut air-conditioning use by buildings, which, the report says, is responsible for up to 15 percent of the energy used by US buildings. It could also help cool buildings in the developing world, where electricity is scarce.

The inventors reflected in the pizza-sized mirror they created. Norbert von der Groeben

"There are parts of the infrared spectrum where the atmosphere is transparent and allows the infrared radiation emitted by sky-facing surfaces to escape to space," lead paper author Aaswath P. Raman told Crave. "Beginning in the '60s and '70s, many scientists and engineers realized that instead of just letting this effect happen, one might actually be able to actively exploit it more directly and optimize surfaces to maximally take advantage of this effect."

"Every object that produces heat has to dump that heat into a heat sink," electrical engineering Professor Shanhui Fan, who worked on the project, said in the Stanford report. "What we've done is to create a way [to do this] that should allow us to use the coldness of the universe as a heat sink during the day."

Next, the researchers will tackle how to enlarge their current pizza-size prototype and how to bring heat from buildings to rooftop panels covered in their material so that it can be offloaded outside our atmosphere.