Duke team finds new calling for cellphone waves

CorrespondentDecember 22, 2013 

While he was a student at Apex High School, Allen Hawkes made a hovercraft with his dad’s leaf blower and created a snow machine to spread the white stuff across the family’s lawn in Cary.

By his freshman year at Duke University, Hawkes was ready for some new creative adventures, so he approached Steven Cummer, professor of electrical and computer engineering, to talk about the field of electromagnetics.

Hawkes had been intrigued by Cummer’s previous research – conducted with scientists in London and San Diego – which demonstrated that the long-sought “cloak of invisibility” was indeed possible. The team successfully concealed a length of copper tubing using microwaves and metamaterials, which are made by rearranging microscopic elements of traditional materials, such as plastics. The findings were published in the journal ScienceExpress in 2006.

In response, Cummer invited Hawkes to join him and graduate student Alexander Katko in another research project: to find out whether microwave signals – the kind created by cellphone towers and satellites – could be harvested and used to generate power.

“He suggested the idea, and I started working on it that summer, but most of the work was done last year, during my junior year,” said Hawkes, 21.

Once completed, the project was featured in the December issue of the science journal Applied Physics Letters. The lead author of that article was Hawkes, who will receive his undergraduate degree in mechanical engineering from Duke in June.

“For a research paper, whoever does most of the heavy lifting during the project is typically the first author,” Cummer said. “That was Allen. He’s a bright young man.”

A little power in all directions

The researchers – all affiliated with Duke’s Pratt School of Engineering – focused on microwaves, the low-level radio waves that are emitted by Wi-Fi transmitters, cellphone towers and other sources. Because this type of power output goes out generally, in all directions, much of it is never picked up by cellphones or computers, but just “bounces around,” Cummer said.

“In a typical office environment, where transmitters like Wi-Fi and others are around, a ball park estimate of available power from those transmitters is about a nanowatt – a billionth of a watt,” he said.

“So it’s not a lot of power, but not so little you can’t imagine doing useful things with it.”

The researchers created a circuit board using fiberglass and copper that is capable of harnessing microwaves and converting them to electrical energy.

“We built a small metal structure whose job it is to intercept the radio energy, and we built electric circuitry that converts the energy into a form that can be used,” Cummer said.

Microwave energy is AC, or alternating current, and must be converted to DC, or steady voltage, before it can be harnessed for most devices.

“What was new was the design of the structure,” Cummer said. “We created a structure that could be a little smaller and more efficient than what had been done previously.”

While other researchers have done similar work, the Duke team came up with a more efficient device, with an ability to soak up 37 percent of the ambient microwaves for use as electric power, compared to the less-than-10 percent of previous devices. The new device functions at an efficiency level similar to that of solar panels.

Compiling milliwatts

At one-inch square, the device is about one-sixth of the size of previously made microwave conductors, Cummer said.

While the amount of electricity generated from the microwaves may be small, it can be stored for later use. A cellphone would take nearly a year to charge on a nanowatt of power, but capturing and stockpiling that same amount of energy could make it more useful, Cummer said.

In a lab demonstration, the Duke researchers put their device in a confined area between two metal plates and bombarded it with microwaves. Under those conditions, it was able to soak up and transmit a milliwatt of power – enough to power a small light.

“So if you have a nanowatt, a billionth of a watt, and you store it up every day for a while, you suddenly have enough power to do something interesting for a short period of time, 10 seconds, let’s say,” the professor explained.

“It may not be enough to run a consumer device, but it becomes possible for things like sensors, something you might want to be very flexible about. You might place one in an office, to measure humidity or moisture. It can sit there and soak up power until it has enough to do what it does: measure and transmit.”

Cummer hopes to identify various applications for the device. Another challenge facing researchers is finding a way to harvest microwaves even when the level present is so low that existing energy harvesting devices do not pick up any signal.

“There tends to be a threshold of available power below which the conversion process does not occur,” he added. “It’s an abrupt change, where it suddenly drops to zero.”

Hawkes, too, is also considering ways to expand the converter’s potential.

“One idea I thought of would be something in a remote village of a developing country,” said Hawkes. “There may be a cellphone tower in that location whose signal could be used to charge a phone or help power other low-powered devices.”

The oldest of four children whose dad works as an electrical engineer, Hawkes said he has been interested in research and invention as long as he can remember.

“I was always building projects with my dad or my friends.”

In school, he enjoyed and excelled at math and science. But he also likes athletics. He ran track and cross country at Apex High and now plays basketball on an intramural team at Duke.

He believes the research project will help him as he applies to graduate schools next year, possibly in electrical engineering.

“I originally decided on mechanical because I thought what my dad did was really boring,” he said. “Now it turns out that electrical is really what I’m more interested in.”

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