Microscopic device harvests power from heat

US scientists have designed the most efficient “optical rectenna” yet. This tiny device, too small to be seen with the naked eye, can turn excess heat from the environment into usable electricity – and might be a game-changer for renewable energy.

Rectennas (“rectifying antennas”) have been around for over 50 years – in 1964, they were used to power a small helicopter with microwaves. They’re composed of an antenna, which absorbs light in the form of electromagnetic waves, attached to rectifying diodes, which convert the received energy into DC power.

However, in order to capture optical wavelengths (as first demonstrated in 2015) rectennas need to be super small – much thinner than a human hair. This is a difficult feat, not least because the smaller an electrical device becomes, the higher its resistance and the lower its power output.

“You need this device to have very low resistance, but it also needs to be really responsive to light,” explains Amina Belkadi from the University of Colorado Boulder, lead author of the new paper published in Nature Communications.

“Anything you do to make the device better in one way would make the other worse.”

Left wc brown holding the rectenna helicopter platform right in fl ight 1
Left: W.C. Brown holding the rectenna helicopter platform. Right: In flight demonstration of the rectenna helicopter on October 1964. Credit: Evgeniy Donchev / Donchev et al. 2014.

But Belkadi and team have now sidestepped the problem entirely, seeking a solution in the quantum realm.

In traditional rectennas, the power-generation process involves electrons passing through an insulator, which adds resistance to a device and reduces the electricity output.

Their newer, more efficient rectenna works by exploiting an enigmatic property of electrons that allows them to pass through solid matter without using any energy.

“They go in like ghosts,” says Belkadi.

This process, called resonant tunnelling, hasn’t before been applied to rectennas.

Counter-intuitively, the researchers added two insulators to their device instead of one, creating a quantum “well”. If an electron hits it with the right energy, the particle can simply tunnel right through both insulators without any resistance, like a ghost drifting through walls.

“If you choose your materials right and get them at the right thickness, then it creates this sort of energy level where electrons see no resistance,” says Belkadi. “They just go zooming through.”

Researchers had previously suggested that this was possible in theoretical modelling, but this the first time it has been demonstrated in an energy-harvesting optical rectenna.

In theory, rectennas could harvest otherwise wasted heat from places like factory smokestacks or bakery ovens, turning it into power – although efficiency is an issue.

Belkadi and team tested a network of 250,000 rectennas on a hot plate in the lab, and found that they could capture less than 1% of the heat produced.

“Right now, the efficiency is really low, but it’s going to increase,” says co-author Garret Moddel, also from the University of Colorado Boulder. “This innovation makes a significant step toward making rectennas more practical.”

Modell foresees rectennas in wide use, installed on solar panels on the ground and on lighter-than-air vehicles in the atmosphere.

“If you can capture heat radiating into deep space, then you can get power anytime, anywhere.”

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