Harvesting energy from nature’s motions

DURHAM, N.C. — By taking advantage of the vagaries of the natural world,
Duke University engineers have developed a novel approach that they believe
can more efficiently harvest electricity from the motions of everyday life.

Energy harvesting is the process of converting one form of energy, such as
motion, into another form of energy, in this case electricity. Strategies
range from the development of massive wind farms to produce large amounts
of electricity to using the vibrations of walking to power small electronic
devices.

Although motion is an abundant source of energy, only limited success has
been achieved because the devices used only perform well over a narrow band
of frequencies. These so-called “linear” devices can work well, for
example, if the character of the motion is fairly constant, such as the
cadence of a person walking. However, as researchers point out, the pace
of someone walking, as with all environmental sources, changes over time
and can vary widely.

“The ideal device would be one that could convert a range of vibrations
instead of just a narrow band,” said Samuel Stanton, graduate student in
Duke’s Pratt School of Engineering, working in the laboratory of Brian
Mann, assistant professor of mechanical engineering and materials sciences.
The team, which included undergraduate Clark McGehee, published the results
of their latest experiments early online in Applied Physics Letters.

“Nature doesn’t work in a single frequency, so we wanted to come up with a
device that would work over a broad range of frequencies,” Stanton said.
“By using magnets to ‘tune’ the bandwidth of the experimental device, we
were able verify in the lab that this new non-linear approach can
outperform conventional linear devices.”

Although the device they constructed looks deceptively simple, it was able
to prove the team’s theories on a small scale. It is basically a small
cantilever, several inches long and a quarter inch wide, with an end magnet
that interacts with nearby magnets. The cantilever base itself is made of a
piezoelectric material, which has the unique property of releasing
electrical voltage when it is strained.

The key to the new approach involved placing moveable magnets of opposing
poles on either side of the magnet at the end of the cantilever arm. By
changing the distance of the moveable magnets, the researchers were able to
“tune” the interactions of the system with its environment, and thus
produce electricity over a broader spectrum of frequencies.

“These results suggest to us that this non-linear approach could harvest
more of the frequencies from the same ambient vibrations,” Mann said. “More
importantly, being able to capture more of the bandwidth makes it more
likely that these types of devices could someday rival batteries as a
portable power source.”

The range of applications for non-linear energy harvesters varies widely.
For example, Mann is working on a project that would use the motion of
ocean waves to power an array of sensors that would be carried inside ocean
buoys.

“These non-linear systems are self-sustaining, so they are ideal for any
electrical device that needs batteries and is in a location that is
difficult to access,” Mann said.

For example, the motion of walking could provide enough electricity to
power an implanted device, such as a pacemaker or cardiac defibrillator. On
a larger scale, sensors in the environment or spacecraft could be powered
by the everyday natural vibrations around them, Mann said.

Mann’s research is supported by the Office of Naval Research.

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