Mirror coating to cool buildings by pumping interior heat into space

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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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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.

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.

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