A device that could store unwanted energy and convert it into electric power

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

A group of researchers at Duke University's Pratt School of Engineering have designed a power-harvesting device which can wirelessy convert microwave signals to direct current voltage. 

The device operates on a similar principal to solar panels, which convert light into electrical current. But this device is so special since it could be tuned to harvest the signal from other energy sources, including satellite signals, sound signals or Wi-Fi signals. The key to the power harvester lies in its application of meta materials, engineered structures that can capture various forms of energy. 

The main ingredient of this device are metamaterials. They are composed of sub-wavelength particles that exhibit bulk properties that are different from their individual components. There are other group of metamaterials known as electromagnetic metamaterials that are engineered, which can achieve parameters not possible within naturally occurring materials. These metamaterials are well-suited for harvesting power. They also provide flexibility in design due to their electrically small, low-profile nature. The nature of these metamaterials have been utilized by the researchers of Duke for their development. 

Harvesting array

Thanks to all specially to Allen Hawkes, an undergraduate engineering student working with graduate student Alexander Katko and lead investigator Steven Cummer, professor of electrical and computer engineering. They designed this electrical circuit capable of harvesting microwaves. 

They 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, USB (Universal Serial Bus) chargers for small electronic devices provide about 5V. 

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

"It is 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." 

For instance, a meta material 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. 

These are just some of the applications of this device. It can be even built into a cell phone with some modifications, allowing the phone to recharge wirelessly while not in use. This feature could, in principle, allow people living in locations without ready to 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."

The research was supported by a Multidisciplinary University Research Initiative from the Army research Office.