Radical shade

A nano-engineered panel that reflects sunlight all the way back into freezing space could be the key to electricity-free cooling.

A team of researchers at Stanford University has designed a new form of panel that cools what’s underneath no matter how hard the sun beats down on it.

If produced, it could cool buildings and vehicles without the need for power-hungry air conditioning.

The panel, which works by reflecting sunlight super-efficiently back into the chilly vacuum of space, was described by researchers in a paper published recently in the journal, Nano Letters.

"We’ve developed a new type of structure that reflects the vast majority of sunlight, while at the same time it sends heat into that coldness, which cools manmade structures even in the day time," Shanhui Fan, professor of electrical engineering at Stanford University and the paper’s senior author, told science news site

The trick, from an engineering standpoint, is two-fold.

First, the surface must reflect as much sunlight as possible. Poor reflectors – such as your common mirror – absorb far too much sunlight and get hot.

Second, the surface must radiate the heat all the way back to where it came from, space, by tapping in to a specific wavelength range.

Outside that range, Earth’s dense atmosphere just bats it back down – a process that gives us the greenhouse effect.

The structure Prof Fan and his team have proposed pulls off both these tricks.

Professor Shanhui Fan, centre, with co-authors Aaswath Raman, left, and Eden Rephaeli. (Credit: Norbert von der Groeben)

It’s a highly effective mirror, and it emits thermal radiation within the crucial wavelength range needed to escape the atmosphere.

The concept, called radiative cooling, has been studied extensively as way of battling climate change.

But, said Eden Rephaeli, a doctoral candidate in Prof Fan’s lab and co-first-author of the paper, "no one had yet been able to surmount the challenges of daytime radiative cooling-of cooling when the sun is shining."

The researchers believe they have succeeded by using nanostructured photonic materials, which can be engineered to enhance or suppress light reflection in certain wavelengths.

"We combine the thermal emitter and solar reflector into one device, making it both higher performance and much more robust and practically relevant," said Aaswath Raman, also a doctoral candidate and the paper’s co-first-author.

Made of quartz and silicon carbide, both very weak absorbers of sunlight, the radiative cooling panel has a cooling power of more than 100 watts per square metre, the researchers believe.

They say a typical one-story house with just 10% of its roof covered by these panels could have 35% of its entire air conditioning needs met during the hottest hours of the summer.

Because it’s a passive cooling system that requires no energy and has no moving parts – you just put it on the roof and it starts chilling – the idea could be a big hit commercially, saving power for homes, offices and cars.

But Prof Fan and his collaborators believe it could have a social impact as well, by helping the majority of the Earth’s population who live in sweltering places where electricity is expensive and unreliable.

"We can foresee applications for radiative cooling in off-the-grid areas of the developing world where air conditioning is not even possible at this time," Prof Fan said. "There are large numbers of people who could benefit from such systems."

See abstract here.

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