gets more power from shakes
Technology Research News
Energy is all around, the trick is finding
ways to harvest it to do the work of your choosing.
Researchers at Pennsylvania State University have boosted the efficiency
of piezoelectric circuits, which transfer the mechanical energy of vibrations
into useful electric power. Piezoelectric circuits are commonly used to
convert a watch-wearer's motion into energy to power the watch.
The Penn State piezoelectric circuit harvests four times more power from
vibrational energy than the circuits currently in use, said George Lesieutre,
a professor of aerospace engineering at Penn State. Key to the increased
efficiency is an adaptive control technique that optimizes the actions
of a converter that transfers the harvested energy to a battery for storage.
The researchers prototype generates 50 milliwatts of energy, and should
be efficient enough to convert the motion of a runner into the power needed
to run an electronic music device, according to Lesieutre. Fifty milliwatts
is one twentieth of a watt. The circuitry could also be used to power
wireless networks of tiny sensors, and to power devices that dampen vibrations,
according to Lesieutre.
In principle, energy can be extracted from any place where two areas consistently
hold different amounts of energy, such as the temperature difference between
a warm body and the outside air or between sun and shadow. Similarly,
energy can be extracted from mechanical vibrational fields such as those
caused by people walking or tires running over a road. One working example
is tiny light-emitting diodes on downhill skis that are powered by the
motion of the skis.
Like all piezoelectric circuits, the researchers' device depends on a
key property of piezoelectric crystals like quartz. These crystals do
not conduct electricity, but when they are exposed to mechanical stress,
electrical charges appear on their surfaces.
Piezoelectric crystal also works in reverse. When exposed to an alternating
electric current, the crystal vibrates, generating high-frequency sound
waves. This effect is widely used in loudspeakers, microphones and devices
that control frequencies in radio transmitters.
The Penn State circuit works by capturing the alternating voltage formed
by piezoelectric crystal in response to vibrations. The device includes
a rectifier, which converts the alternating electrical input to a positive
electrical flow, a filter capacitor that smooths the electrical flow,
and a converter that allows a battery to store the harvested energy.
The key to making the device more efficient is the engineering involved
in the converter, said Lesieutre. The converter includes an adaptive control
technique that adjusts to find the optimal power transfer options for
the moment, including an efficient duty cycle. A duty cycle dictates when
and for how long a device is active. "The smarts is in the... converter,
which operates at [a specific] frequency and duty cycle," he said. The
adaptive technique allows the converter to transfer power more efficiently
in order to maximize the power stored by the battery. "The duty cycle
selection is especially critical," he added.
The researchers also sized the circuit with an eye toward maximum efficiency,
In the researchers' experiments, the adaptive converter increased power
transfer by more than 400 percent, he said.
Devices that require more than 50 milliwatts could be powered using an
array of several piezoelectric circuits, or they could be powered periodically,
For example, the device could power an environmental-control sensor that
periodically wakes up, processes data, then communicates it to a central
location via wireless transmitter, said Lesieutre. Even though it may
take more than 50 milliwatts to power the sensor and transmitter, they
can be operated intermittently. One can store a lot of energy in a battery,
even at low charging rates, if one can wait long enough," he said.
The circuit could also be used for control and guidance robotics in manufacturing,
patient monitoring and diagnostics, home security systems, and interactive
toys, according to Lesieutre. Researchers are also looking to use power-scavenging
techniques to power microelectrical mechanical systems (MEMS) and micro-robots.
The work is "another useful piece of analysis and engineering," said Andrew
Brown, an electronics professor and head of the electronic systems design
group at the University of Southampton in England. The amount of power
that can be scavenged by techniques like these is still extremely small,
However, recent technology has enabled the efficient and cheap extraction
of these small amounts of power, and the ability to use them. "Two aspects
of technology are converging to make this application workable: it's becoming
feasible to mine low levels of power, and it's becoming possible to do
useful things with that level of power," said Brown. "Very low power design
is an enormous and growing field in its own right," he said.
The researchers' next steps are to make the system even more efficient,
and to size it for various applications, including very small ones, Lesieutre
said. The possibilities include powering wearable electronics, he said.
The device could be used in products in less than two years, he said.
Ways to power sensors and wireless transmitters would be useful to pursue
first, he said.
Lesieutre's research colleagues were Geffrey K. Ottman, Heath Hoftmann
and Archin C. Bhatt. They published the research in the September, 2002
issue of IEEE Transactions on Power Electronics. The research was funded
by the Office of Naval Research (ONR).
Timeline: < 2 years
TRN Categories: Energy; Engineering
Story Type: News
Related Elements: Technical paper, "Adaptive Piezoelectric
Energy Harvesting Circuit for Wireless, Remote Power Supply," IEEE Transactions
on Power Electronics, September, 2002.
Coax goes nano
Webs within Web boost
Circuit gets more
power from shakes
Method measures quantum
Biochip sprouts DNA strands
Research News Roundup
Research Watch blog
View from the High Ground Q&A
How It Works
News | Blog
Buy an ad link