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No Power Needed: High-Tech Mirrors Could Cool Buildings

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Linxiao Zhu, Shanhui Fan and Aaswath Raman with what they call a breakthrough energy-saving material. | Photo: Norbert von der Groeben, Stanford University

Okay, this made our eyes widen when we read it: A team of engineers at Stanford University has found a way to cool buildings by using interplanetary space as a heat sink. If the technique proves viable outside the lab, the breakthrough could mean it takes a lot less energy to keep buildings cool during the day. And the best part: The process uses no energy once installed.

In a letter published last week in the science journal Nature, a team led by Stanford electrical engineering professor Shanhui Fan and research associate Aaswath Raman describe field tests in which they placed what amount to high-tech mirrors on a small structure. Those mirrors reflected a very high percentage of incoming solar energy -- 97 percent or more -- away from the structure, keeping the structure cooler.

But that's not the real breakthrough. The mirrors were also radiators, designed on the nano-scale to turn heat from inside the structure into a type of infrared radiation that isn't blocked by the earth's atmosphere. That means the mirrors radiated heat from inside the structure directly into space without warming the atmosphere. The combined reflectors and radiators dropped temperatures inside the structure by 4.9°C, almost nine degrees Fahrenheit, compared to ambient air temperature -- without any external power source.

Given that up to 15 percent of the energy used in buildings in the United States is spent keeping their interiors cool, the idea that we might be able to cool buildings without using power suggests considerable energy conservation potential.

The key to the radiators' ability to beam heat energy directly into deep space is a laminated series of layers of hafnium oxide and silicon dioxide on a thin layer of silver. Those semiconductor layers capture heat energy and emit it as infrared radiation.

That's not unusual: The vast majority of objects, including you and me, turn heat into infrared radiation at common ambient temperatures. The difference between you and me and the Stanford team's mirrors is that by tinkering with the thicknesses of the hafnium oxide and silicon dioxide layers, the engineers were able to fine-tune the precise wavelengths of infrared radiation the mirrors emitted to between eight and 13 micrometers.

That's important because the earth's atmosphere does absorb infrared radiation of other wavelengths, which is much of the reason for the greenhouse effect that warms our planet. But the atmosphere is essentially transparent to infrared of between 8 and 13 micrometers. That means that infrared of that range of wavelengths passes through the atmosphere into space without heating the atmosphere. And that means that heat energy from a structure equipped with the Stanford team's mirrors gets radiated off-planet without warming the rest of the planet.

The technique is called photonic radiative cooling -- cooling a structure by radiating its heat away as infrared photons. The Stanford team is confident that the technique can be extended past the admittedly very small prototype described in the Nature paper, which they describe as about the size of a pizza box.

"Across the developing world, photonic radiative cooling makes off-grid cooling a possibility in rural regions, in addition to meeting skyrocketing demand for air conditioning in urban areas," said co-author Aaswath Raman in a press release.

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