The devices, called optical rectennas, combine micron-scale antennas (which absorb radiation) and sub-micron diodes (which convert that energy into DC currents) to provide a promising way to efficiently harvest low-grade waste heat. Too small to see with the naked eye, their devices, say the scientists, are roughly 100 times more efficient than similar tools used for energy harvesting.
They achieve that feat through a process called “resonant tunneling,” in which electrons pass through solid matter without spending any energy. Such rectifying antennas – or rectennas – work similarly to radio antennas, but instead of picking up radio waves and turning them into sound, optical rectennas absorb light and heat and convert it into power.
“It’s like a radio receiver that picks up light in the form of electromagnetic waves,” says Garret Moddel, professor of the Department of Electrical, Computer and Energy Engineering (ECEE) and coauthor of a paper on the study.
Such rectennas could, say the scientists, theoretically, harvest the heat coming from factory smokestacks or bakery ovens that would otherwise go to waste. Some scientists have even proposed mounting these devices on airships that would fly high above the planet’s surface to capture the energy radiating from Earth to outer space.
Up to now, rectennas haven’t been able to reach the efficiencies needed to meet such goals. In the new study, however, the researchers say they have have designed the first-ever rectennas that are capable of generating power.
“We demonstrate for the first time electrons undergoing resonant tunneling in an energy-harvesting optical rectenna,” says Amina Belkadi, lead author of the paper on the research, who recently earned her Ph.D. from ECEE. “Until now, it was only a theoretical possibility.”
Study coauthor Garret Moddel adds, “This innovation makes a significant step toward making rectennas more practical. Right now, the efficiency is really low, but it’s going to increase.”
However, in order to capture thermal radiation and not just microwaves, rectennas need to be incredibly small – many times thinner than a human hair. And that can cause a range of problems, say the researchers. The smaller an electrical device is, for example, the higher its resistance becomes, which can shrink the power output of a rectenna.
“You need this device to have very low resistance, but it also needs to be really responsive to light,” says Belkadi. “Anything you do to make the device better in one way would make the other worse.”
To overcome this, the researchers’ approach relies on a strange property of the quantum realm. In a traditional rectenna, electrons must pass through an insulator in order to generate power. These insulators add a lot of resistance to the devices, reducing the amount of electricity that engineers can get out.
However, in the latest study, say the researchers, they decided to add two insulators to their devices, not just one. That addition had the counterintuitive effect of creating an energetic phenomenon called a quantum “well.” If electrons hit this well with just the right energy, they can use it to tunnel through the two insulators – experiencing no resistance in the process.
It’s not unlike a ghost drifting through a wall unperturbed, say the researchers. A graduate student in Moddel’s research group had previously theorized that such spectral behavior could be possible in optical rectennas, but, until now, no one had been able to prove it.
“If you choose your materials right and get them at the right thickness,” says Belkadi, “then it creates this sort of energy level where electrons see no resistance. They just go zooming through.”
And that, say the researchers, means more power. To test the effect, the researchers arrayed a network of about 250,000 rectennas, which are shaped like tiny bowties, onto a hot plate in the lab, and then applied the heat.
While the devices were able to capture less than 1% of the heat produced by the hot plate, the researchers say those numbers are only going to go up.
“If we use different materials or change our insulators, then we may be able to make that well deeper,” says Belkadi. “The deeper the well is, the more electrons can pass all the way through.”
The researchers say they are looking forward to the day when rectennas sit on top of everything from solar panels on the ground to lighter-than-air vehicles in the air.
“If you can capture heat radiating into deep space,” says Moddel, “then you can get power anytime, anywhere.”