Energy From Slow-Moving Rivers and Ocean Currents Could Power the Entire World

Alt Dot Energy
December 3, 2008

Existing hydropower technologies require an average current of five or six knots
to operate efficiently, while most of the earth’s currents are slower than three
knots. This new technology can generate electricity in water flowing at a rate
of less than one knot - about one mile an hour - meaning it could operate on
most waterways and sea beds around the globe.

Existing technologies which use water power, relying on the action of waves,
tides or faster currents created by dams, are far more limited in where they can
be used, and also cause greater obstructions when they are built in rivers or the sea.

This new device utilises a novel approach to extract energy from flowing water
currents. It is unlike any other ocean energy or low-head hydropower concept.
VIVACE is based on the extensively studied phenomenon of Vortex Induced
Vibrations (VIV), first observed 500 years ago by Leonardo DaVinci in the form
of “Aeolian Tones.” For decades, engineers have been trying to prevent VIV from
damaging offshore equipment and structures. By maximizing and exploiting VIV
rather than spoiling and preventing it, VIVACE takes this ‘problem’ and
transforms it into a valuable resource for mankind.

When water flows past, the cylinder creates vortices, which push and pull the
cylinder up and down. The mechanical energy in the vibrations is then converted
into electricity.

The Cylinders when arranged over a cubic metre of the sea or river bed in a flow
of three knots can produce 51 watts. This is more efficient than similar-sized
turbines or wave generators, and the amount of power produced can increase
sharply if the flow is faster or if more cylinders are added.

If a “field” of cylinders was built on the sea bed over a 1km by 1.5km area, at
the height of a two-storey house, a flow of just three knots, could generate
enough power for around 100,000 homes. Just a few of the cylinders, stacked in a
short ladder, could power an anchored ship or a lighthouse.

The systems could be sited on river beds or suspended in the ocean. The
scientists behind the technology, which has been developed in research funded by
the US government, say that generating power in this way would potentially cost
only around 3.5p per kilowatt hour, compared to about 4.5p for wind energy and
between 10p and 31p for solar power. They say the technology would require up to
50 times less ocean acreage than wave power generation.

The system, conceived by scientists at the University of Michigan, is called
Vivace, or “vortex-induced vibrations for aquatic clean energy”.

Michael Bernitsas, a professor of naval architecture at the university, said it
was based on the changes in water speed that are caused when a current flows
past an obstruction. Eddies or vortices, formed in the water flow, can move
objects up and down or left and right.

“This is a totally new method of extracting energy from water flow,” said Mr
Bernitsas. “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.”
Such vibrations can cause damage to structures built in water, like docks and
oil rigs. But Mr Bernitsas added: “We enhance the vibrations and harness this
powerful and destructive force in nature.

“If we could harness 0.1 per cent of the energy in the ocean, we could support
the energy needs of 15 billion people. In the English Channel, for example,
there is a very strong current, so you produce a lot of power.”

As the parts only oscillate slowly, the technology is likely to be less harmful
to aquatic wildlife than dams or water turbines. And as the installations can be
positioned far below the surface of the sea, there would be less interference
with shipping, recreational boat users, fishing and tourism.

Engineers are now deploying a prototype device in the Detroit River, which has a
flow of less than two knots. Their work, funded by the US Department of Energy
and the US Office of Naval Research, is published in the current issue of the
quarterly Journal of Offshore Mechanics and Arctic Engineering.


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