Microwaves Wirelessly Converted to Direct Current

A new device designed using manmade, inexpensive materials operates as a power harvester with efficiency similar to solar panels but can wirelessly convert microwave signals into direct current voltage capable of recharging a cell phone battery or other small electronic device.

The metamaterial device operates on a similar principle to solar panels, which convert light energy into electrical current, but is more versatile in that it can be tuned to harvest the signal from other energy sources, including satellite signals, sound signals or Wi-Fi signals, say researchers at Duke University’s Pratt School of Engineering.

This five-cell metamaterial array developed by Duke engineers converts stray microwave energy, as from a Wi-Fi hub, into more than 7 V of power with an efficiency of 36.8 percent — comparable to a solar cell. Images courtesy of Duke University.

The microwave-harvesting electrical circuit was designed by undergraduate engineering student Allen Hawkes, working with graduate student Alexander Katko and lead investigator Steven Cummer, professor of electrical and computer engineering.

They used a series of five fiberglass and copper energy conductors wired together on a circuit board to convert microwaves into 7.3 V of electrical energy. By comparison, USB chargers for small electronic devices provide about 5 V of power.

“We were aiming for the highest energy efficiency we could achieve,” Hawkes said. “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.”

“It’s possible to use this design for a lot of different frequencies and types of energy, including vibration and sound energy harvesting,” Katko said. “Until now, a lot of work with metamaterials has been theoretical. We are showing that with a little work, these materials can be useful for consumer applications.”

A metamaterial coating could be applied to the ceiling of a room to redirect and recover a Wi-Fi signal that would otherwise be lost, Katko said. Another application could be to improve the energy efficiency of appliances by wirelessly recovering power that is now lost during use.

Duke engineering students Alexander Katko (left) and Allen Hawkes show a waveguide containing a single power-harvesting metamaterial cell, which provides enough energy to power the attached green LED.

“The properties of metamaterials allow for design flexibility not possible with ordinary devices like antennas,” Katko said. “When traditional antennas are close to each other in space they talk to each other and interfere with each other’s operation. The design process used to create our metamaterial array takes these effects into account, allowing the cells to work together.”

With additional modifications, the metamaterial could potentially be built into a cell phone, they said, allowing it 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,” Cummer said. “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.”

For example, a series of the blocks could capture the signal from a known set of satellites passing overhead. The small amount of energy generated from these signals might power a sensor network in a remote location such as a mountaintop or desert, allowing data collection for a long-term study that takes infrequent measurements.

The work will appear in Applied Physics Letters in December and is now available online. http://dx.doi.org/10.1063/1.4824473

For more information, visit: www.pratt.duke.edu