Environment

Mirror coating to cool buildings by pumping interior heat into space

Mirror coating to cool buildin...
The Stanford material cools buildings by both reflecting sunlight and radiating interior heat into space (Image: Nicolle R. Fuller, Sayo-Art LLC)
The Stanford material cools buildings by both reflecting sunlight and radiating interior heat into space (Image: Nicolle R. Fuller, Sayo-Art LLC)
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The Stanford material cools buildings by both reflecting sunlight and radiating interior heat into space (Image: Nicolle R. Fuller, Sayo-Art LLC)
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The Stanford material cools buildings by both reflecting sunlight and radiating interior heat into space (Image: Nicolle R. Fuller, Sayo-Art LLC)
Stanford Professor Shanhui Fan, (center), gazes into the pizza- size prototype with co-authors Linxiao Zhu, (left), and Aaswath Raman, (right) (Photo: Norbert von der Groeben)
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Stanford Professor Shanhui Fan, (center), gazes into the pizza- size prototype with co-authors Linxiao Zhu, (left), and Aaswath Raman, (right) (Photo: Norbert von der Groeben)

Keeping buildings cool isn't easy. In fact, conventional air conditioning methods are very energy intensive and account for up to 15 percent of the energy used in buildings in the United States alone. However, engineers at Stanford University have come up with a new ultrathin, multilayered, nanophotonic material that not only reflects heat away from buildings, but also directs heat from inside out into space, cooling both the building and the planet as well.

Heat moves by one of three methods. It can be conducted, which is what happens when you touch a hot stove; it can be convected, which is why you get a blast of hot air in the face when you open an oven and forget to stand back; and it can radiate in the form of infrared rays, which is how you know an oven is hot before you touch it. The Stanford team, led by electrical engineering Professor Shanhui Fan, has developed a coating that handles both infrared radiation and incoming sunlight in such a way that it not only reflects the Sun's rays away like a mirror, but also allows a building's interior heat to be radiated into space, cooling the building.

Stanford Professor Shanhui Fan, (center), gazes into the pizza- size prototype with co-authors Linxiao Zhu, (left), and Aaswath Raman, (right) (Photo: Norbert von der Groeben)
Stanford Professor Shanhui Fan, (center), gazes into the pizza- size prototype with co-authors Linxiao Zhu, (left), and Aaswath Raman, (right) (Photo: Norbert von der Groeben)

Dubbed photonic radiative cooling by the researchers, the process relies on a coating made of a combination of silicon dioxide (SiO2) and hafnium oxide (HfO2) set on a thin layer of silver. These are formed into seven layers only 1.8 microns thick, which the team says is thinner than the thinnest aluminum foil. Despite this thinness, the layers are engineered into a sort of metamaterial that can not only reflect sunlight, but also conduct infrared rays.

Essentially, this coating acts like a thermal balance sheet that's running on a deficit. Using a weather analogy, it's like, instead of a hot, sunny day in summer, it's a cold sunny day in winter with lots of snow on the ground. In the summer on a clear day, sunlight pumps a lot of heat into an area. Some of this is reflected back into space, but not enough, so the balance between input and output is a surplus, and ice lollies sound like a good idea. On the other hand, in the winter there isn't as much sunlight, and what there is gets reflected back into space by the snow on the ground. The result is that even though there's enough sun to give you a sunburn, the heat balance is in the red, so it gets colder.

The same thing happens with the Stanford coating. It reflects the sunlight away, so the building doesn't heat up, but it also lets out the infrared radiation given off by the building, so it cools. More important, the Stanford team has tweaked the coating so it lets out the infrared frequencies that pass through the air near the building without heating it and directs it on out of the atmosphere, which minutely cools the Earth by way of a "window into space."

According to Stanford, tests of the coating show that it reflects away 97 percent of incoming sunlight and this, combined with the photonic radiative cooling, results in the material being about 9° F (5° C) cooler than the surrounding air.

The team says that though the results are promising, there are still technical obstacles to overcome. Though it works in a laboratory setting, to make it practical some mechanism needs to be developed to deliver a building's heat from the interior to the outer coating. In addition, the coating has only been made to cover an area the size of a personal pizza, so a process for manufacturing large panels at a reasonable costs has to be developed. However, once perfected, the technology could not only help to reduce air conditioning costs, but also turn the cold of space into a new resource.

"Every object that produces heat has to dump that heat into a heat sink," Fan says. "What we've done is to create a way that should allow us to use the coldness of the universe as a heat sink during the day."

The team's results were published in the journal Nature.

Source: Stanford University

9 comments
BillGee
A cool car in summer... I want this for my car roof --- PLEASE!
hkmk23
Reflects into space......? I sort of read this as accelerating global warming due to the greenhouse effect......
Mel Tisdale
If it is possible to aim the infrared radiation, then it would be ideal for central heating radiators. Instead of relying on convection, as they do now, they could take the place of an old fireplace and we could all sit round it enjoying the radiated heat as they used to back in the day. It would not matter if the rest of the room was not up to temperature, and even if it did on occasion, there is no reason why a radiator could not be installed for such times.
Running costs would almost certainly be reduced, though psychologically we would need to run a flame effect artificial fire in order to complete the illusion. All we would need then is for this system to get hot enough to roast chestnuts and toast bread (I jest, of course) to really bring back the joys of those times, which I actually remember with fondness.
Robert Flieger
If it works as advertised it should be great for summer. How do they plan on stopping it from cooling the building in the winter?
Stephen N Russell
Must for NE & Midwest US alone, maybe No CA OR area. Radical, Must for Canada for winters
Windsor Wilder
More like warming the sky from below but since that's what conventional AC does what's the difference?
Gadgeteer
You guys worrying about "accelerating global warming" and "warming the sky from below" don't seem to understand atmospheric physics. The Earth radiates heat back out into space all the time. If infrared radiation couldn't escape, everything on the surface would have been broiled to ashes ages ago. It's why the temperature drops every night after the sun goes down and especially quickly on a clear, cloudless night. Google "nighttime radiative cooling."
Slowburn
The radiant heat spectrum was carefully chosen to not be absorbed by the atmosphere so the heat shines out into space.
divenpuke
It's called a "cold mirror" in optics and it's a commodity item. Not sure what's so special about this. Silver plating and hafnium plating a building? that does sound like a challenge.