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Device Turns Stray Microwaves into Energy

This five-cell metamaterial array developed by Duke engineers converts stray microwave energy, as from a WiFi hub, into more than 7 volts of electricity with an efficiency of 36.8 percent—comparable to a solar cell. (Duke)
Scientists have built a device they say can capture stray microwave signals and convert them into electrical power capable of recharging a cell phone or other small electronic device.

Researchers at Duke University’s Pratt School of Engineering in North Carolina say the device is similar to a solar panel and has a similar efficiency, and while it harvests microwaves, the researchers think the harvester could be tuned to harvest the signal from other energy sources, including satellite signals, sound signals or Wi-Fi signals.

The harvester uses something called “metamaterials,” which are “engineered structures that can capture various forms of wave energy and tune them for useful applications.”

“Until now, a lot of work with metamaterials has been theoretical,” said graduate student Alexander Katko. “We are showing that with a little work, these materials can be useful for consumer applications.”

Researchers used a series of five fiberglass and copper energy conductors wired together on a circuit board to convert microwaves into 7.3V of electricity. By comparison, Universal Serial Bus (USB) chargers for small electronic devices provide about 5V.

“We were aiming for the highest energy efficiency we could achieve,” said engineering student Allen Hawkes. “We had been getting energy efficiency around 6 to 10 percent, but with this design we were able to dramatically improve energy conversion to 37 percent, which is comparable to what is achieved in solar cells.”

With additional modifications, the researchers said the power-harvesting metamaterial could potentially be built into a cell phone, allowing the phone to recharge wirelessly while not in use. This feature could, in principle, allow people living in locations without ready access to a conventional power outlet to harvest energy from a nearby cell phone tower instead.

“Our work demonstrates a simple and inexpensive approach to electromagnetic power harvesting,” said lead investigator Steven Cummer. “The beauty of the design is that the basic building blocks are self-contained and additive. One can simply assemble more blocks to increase the scavenged power.”

The Duke study appears in the journal Applied Physics Letters in December 2013.