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 the microwave signal to direct current
voltage capable of recharging a cell phone battery or other small
electronic device, according to a report appearing in the journal
Applied Physics Letters in December 2013. (It is now available online.)
It operates on a similar principle to 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 satellite signals, sound signals or Wi-Fi 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 on a circuit board to convert microwaves into 7.3V of
electrical energy. By comparison, Universal Serial Bus (USB) chargers
for small electronic devices provide about 5V of power.
"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 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."
For instance, 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.
"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."
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 (Contract
No. W911NF-09-1-0539). "A microwave metamaterial with integrated power
harvesting functionality," Allen M. Hawkes, Alexander R. Katko, and
Steven A. Cummer. Applied Physics Letters 103, 163901 (2013); doi:
10.1063/1.4824473
