Photo credit: Chloe Doiron

From Popular Mechanics

Perhaps one of the greatest challenges facing solar technology is wasted heat. One crucial measure of solar panels is how efficient they are in converting the sun's rays into electricity. So it's potentially huge news that a team from Rice University has produced a method they say could radically improve solar panels work.

It's not that the scientists have created significantly more efficient solar panels. Instead, they're working to capture the thermal photons—the heat—those panels are releasing.

“Thermal photons are just photons emitted from a hot body,” says Junichiro Kono of Rice’s Brown School of Engineering in a press statement. “If you look at something hot with an infrared camera, you see it glow. The camera is capturing these thermally excited photons.”

When the sun transmits heat and light, it also sends an invisible energy known as infrared radiation. Humans have been able to utilize infrared radiation in a variety of ways, ranging from personal saunas to weather satellites. But it doesn't take up much space in the electromagnetic spectrum.

“Any hot surface emits light as thermal radiation,” says Gururaj Naik, who coauthored the resulting paper with Kono and others, published in ACS Photonics. “The problem is that thermal radiation is broadband, while the conversion of light to electricity is efficient only if the emission is in a narrow band," Naik says in the press statement.

Photo credit: Naik Lab

Those emissions are being wasted, the team realized.

When Rice graduated student Chloe Doiron found that about 20 percent of our industrial energy consumption is waste heat—nearly three years of electricity just for the state of Texas—Naik and Kono were motivated to figure out a solution.

“The most efficient way to turn heat into electricity now is to use turbines, and steam or some other liquid to drive them,” Naik says. “They can give you nearly 50 percent conversion efficiency. Nothing else gets us close to that, but those systems are not easy to implement.

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So the pair, along with Rice grad students including Doiron, worked to capture those emissions with wafer-scale films of closely packed carbon nanotubes. Nanotubes had the advantage of simplicity with no moving parts. They were also conduits that could absorb waste heat and, in turn, convert it into narrow-bandwidth photons.

Nanotubes are also sturdy. The team's proof-of-concept devices operated at up to 1,292 degrees Fahrenheit.

Once they've absorbed the heat waste photons, the nanotubes gain a level of control over them. Photons can enter the tube in any number of ways, but as soon as they're inside, the tube directs them on where to go.

“Instead of going from heat directly to electricity, we go from heat to light to electricity,” Naik says. “It seems like two stages would be more efficient than three, but here, that’s not the case.”

Efficiency is the watchword in solar energy. Current solar panels can successfully convert around 20 percent of their collected energy. That might sound low, but scientists believe that with current technology, solar panels will hit a peak efficiency of around 29 percent. While that would be tremendous in terms of real-world capabilities, 29 percent still seems somewhat low.

“By squeezing all the wasted thermal energy into a small spectral region, we can turn it into electricity very efficiently,” he said. “The theoretical prediction is that we can get 80 percent efficiency.”

Obviously, that sort of efficiency rating is unheard of in the world of solar panels. But even though a proof of concept is a long way from being used in the real world, any further developments in the nanotubes could bolster solar panels in ways we haven't seen yet.

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