| Hydropower generator
mimics fish
R. Colin Johnson
EE Times
(11/25/2008 11:12 AM EST)
PORTLAND, Ore. — Researchers claim to have
fashioned a fish-like hydrokinetic scheme that harnesses both fast and slow
underwater currents to generate electricity.
Called Vivace, for Vortex Induced Vibrations for Aquatic Clean Energy, the
technique has the potential to generate electricity, even from lazy river
currents, at a cost lower than other energy sources. Michael Bernitsas, a
professor at the University of Michigan (Ann Arbor), also has launched a
company, Vortex Hydro Energy, to commercialize the technology.
The technique mimics the ability of fish to use underwater vortices to swim
upstream.
Traditional methods of harnessing water power like turbines require currents
of at least 5 knots (5.8 miles per hour). The Vivace generator, it is
claimed, can use currents as slow as 1 knot (1.15 miles per hour). Bernitsas
estimated that an underwater array just a few stories high could supply
enough power for 100,000 homes near a river.
Hydokinetic energy generation, according to Bernitasas, could lower
electricity costs to just 5.5 cents per kilowatt/hour.
The technique works by taking advantage of the tendency of moored bodies to
bob up and down in a current. Called vortex-induced vibrations, they cause
an anchored boat to bob up and down in direct proportion to the swiftness of
the current. Bernitasas used cylinders suspended horizontally and transverse
to the current, allowing them bob at their natural resonant frequency.
"Vortex-induced vibrations are alternating, which creates alternating lift,"
said Bernitsas. "The body of our cylinder affects the current, and the
current [in turn] drives the body, resulting in alternating lift that is
transverse to the current. This drives the cylinder up and down, performing
work that can be transformed into electricity."
Bernitsas previously devised ways to suppress vortex-induced vibrations in
moored platforms such as oil derricks. "It dawned on me that if I could
enhance, instead of suppress, vortex-induced vibrations, then we could
harvest that energy," said Bernitsas.
Bernitsas has since developed methods for enhancing slow oscillations, which
he claims to have increased by 540 percent. Normally, an anchored underwater
object will bob up and down only along its own height. Bernitsas claims his
cylinders move up and down 2.7 times, or a total of 5.7 times, all the while
turning a generator to produce electricity.
Bernitsas' technique mimics the way fish surf underwater vortices by
utilizing their power to swim upstream through currents. By curving their
bodies, fish collect a vortex on the curved side which provides a push. Fish
then shed that vortex and curve their bodies in the opposite direction to
catch another vortex. By alternating between these vortices, fish are able
to swim upstream through currents stronger than their own muscle power.
However, vortex-induced vibrations can become violent in architectural
designs that do not compensate for them. Perhaps the most famous example was
the 1940 collapse of the Tacoma Narrows Bridge in Washington state. The
bridge oscillated in the wind until it tore itself apart. Water intensifies
the effect.
Bernitsas claims to have harnessed this phenomenon to generate electricity
by suspending a cylinder in a vertical slot with elastic springs. The scheme
enabled the cylinder to move up and down to drive the generator. Like fish,
the up-and-down motion alternatively collects and sheds vortices on each
side, thereby harnessing vibrations to produce electricity.
The U.S. Navy funded construction of a prototype that will be tested in
2009. The Energy Department, the Office of Naval Research, the National
Science Foundation also funded the research along with sources in Michigan.
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