Wireless device converts ‘lost’ microwave energy into electric power

November 8, 2013

Prototype power harvester resonant at 900MHz (a GSM cell-phone frequency) (credit: Allen M. Hawkes at al./Applied Physics Letters)

Using inexpensive materials configured and tuned to capture microwave signals, researchers at Duke University’s Pratt School of Engineering have designed a power-harvesting device with efficiency similar to that of modern solar panels.

The device wirelessly converts a microwave signal to direct current voltage that is capable of recharging a cell phone battery or other small electronic device.

It operates on a principle similar to that of solar panels, which convert light energy into electrical current. But this versatile energy harvester could be tuned to harvest the signal from other energy sources, including Wi-Fi signals, satellite signals, or even sound signals, the researchers say.

The key to the power harvester lies in its application of metamaterials, engineered structures that can capture various forms of wave energy and tune them for useful applications.

Undergraduate engineering student Allen Hawkes, working with graduate student Alexander Katko and lead investigator Steven Cummer, professor of electrical and computer engineering, designed an electrical circuit capable of harvesting microwaves.

They used a series of five fiberglass and copper energy conductors wired together as an array on a circuit board to convert microwaves with an RF-to-DC conversion efficiency of 37 percent.

The efficiency of solar cells

Schematic of power harvester (credit: Allen M. Hawkes at al./Applied Physics Letters)

That efficiency is comparable to what is achieved in solar cells, according to Hawkes.

“It’s possible to use this design for a lot of different frequencies and types of energy, including vibration and sound energy harvesting. 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.”

For instance, a 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.

“The properties of metamaterials allow for design flexibility not possible with ordinary devices like antennas,” said Katko. “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.”

Wireless recharging for cell phones

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 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.”

For example, a series of power-harvesting blocks could be assembled to capture the signal from a known set of satellites passing overhead, the researchers explained. 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 research was supported by a Multidisciplinary University Research Initiative from the Army Research Office.

Abstract of Applied Physics Letters paper

We present the design and experimental implementation of a power harvesting metamaterial. A maximum of 36.8% of the incident power from a 900 MHz signal is experimentally rectified by an array of metamaterial unit cells. We demonstrate that the maximum harvested power occurs for a resistive load close to 70 Ω in both simulation and experiment. The power harvesting metamaterial is an example of a functional metamaterial that may be suitable for a wide variety of applications that require power delivery to any active components integrated into the metamaterial.