November 25, 2008 Hydro-power systems are by far the most widely used form of renewable energy
on the planet (in 2005 they accounted for an estimated 63% of all
electricity produced via renewable sources), but despite their
eco-friendly appeal the implementation of large scale facilities –
particularly where dam building is involved – has some serious
environmental and economic drawbacks. One solution to this impasse that
is growing in support is to use leaner, less destructive systems like Bourne Energy’s RiverStar to harness current flow and provide energy at a local level. This is the thinking behind VIVACE, a machine developed at the University of Michigan
which applies the same principles fish use to swim efficiently in order
to generate power from currents much slower than those required to
drive designs based on turbines and water mills.
The majority of the Earth’s ocean
and river currents which are less than 3 knots. Most turbines and water
mill systems need an average of 5 or 6 knots to operate efficiently,
but VIVACE (which stands for Vortex Induced Vibrations for Aquatic Clean Energy) can make energy with less than 2 knots, so the potential is huge.
The system achieves this by
harnessing what are known as “vortex induced vibrations”. These are
caused by the flow of liquid or air over a rounded or cylinder-shaped
object. In this case a cylinder underwater is subjected to current and,
without delving too far into fluid dynamics,
as the liquid in contact with its surface “sticks” and slows down
around it to create vortices or eddies on opposite sides of the
cylinder. Alternating vortices form above and below the cylinder,
exerting force perpendicular to the current. Attach the cylinder to
springs so that it oscillates and you have a renewable source of
mechanical energy which can be used to produce electricity.
This phenomena is actually very
destructive when it comes to building structures underwater or into the
air, causing fatigue damage of offshore oil rigs and even toppling the
Tacoma Narrows bridge in Washington in 1940.
“For the past 25 years,
engineers—myself included—have been trying to suppress vortex induced
vibrations. But now at Michigan we’re doing the opposite. We enhance
the vibrations and harness this powerful and destructive force in
nature,” said VIVACE developer Michael Bernitsas, a professor in the
U-M Department of Naval Architecture and Marine Engineering.
Fish on the other hand, use vortex induced vibrations to their advantage.
“VIVACE copies aspects of fish
technology,” Bernitsas said. “Fish curve their bodies to glide between
the vortices shed by the bodies of the fish in front of them. Their
muscle power alone could not propel them through the water at the speed
they go, so they ride in each other’s wake.”
The prototype machine uses a
cylinder hanging horizontally across the flow of water in large tank
tank in which water in the tank flows at 1.5 knots and at this stage,
it looks nothing like a fish, but Bernitsas’ says future versions will
have the equivalent of a tail and surface roughness a kin to scales.
Just a few cylinders might be enough to power an anchored ship, or a lighthouse
according to Bernitsas and an array of VIVACE converters the size of a
running track and about two stories high could power about 100,000 houses and produce energy at 5.5 cents per kilowatt hour.
“There won’t be one solution for
the world’s energy needs,” Bernitsas said. “But if we could harness 0.1
percent of the energy in the ocean, we could support the energy needs
of 15 billion people.”
The system would also be kind to
the creatures that helped inspire it as the slowly oscillating VIVACE
would in theory, pose no threat to marine life.
A feasibility study in the
Detroit River was recently completed and researchers are working to
deploy a pilot project within the 18 months.
The technology is being
commercialized through Bernitsas’ company, Vortex Hydro Energy and a
paper on VIVACE is published in the current issue of the quarterly
Journal of Offshore Mechanics and Arctic Engineering.
Source: University of Michigan