Slow moving currents new source of renewable energy

November 22nd, 2008 - 2:12 pm ICT by IANS  

New York, Nov 22 (IANS) A US engineer has developed the prototype of a device capable of tapping slow-moving ocean and river currents for a new, reliable and affordable alternative energy source. Called the Vivace, the machine works like a fish and turn vibrations in fluid flows into clean, renewable power. It has been featured in the latest issue of the Journal of Offshore Mechanics and Arctic Engineering.

Developed by Michael Bernitsas of the University of Michigan, the device harnesses energy from water currents moving slower than three km per hour.

In comparison, turbines and water mills need an average flow speed of 7 to 9 km to operate efficiently.

Vivace, short for Vortex Induced Vibrations for Aquatic Clean Energy, depends on a unique hydrokinetic energy system that relies on “vortex induced vibrations” - or undulations that a cylinder-shaped object makes in a flow of fluid.

The presence of the object puts kinks in the current’s speed as it skims by. This causes eddies to form in a pattern on opposite sides of the object. The vortices push and pull the object up and down or left and right, perpendicular to the current.

These vibrations in wind toppled the Tacoma Narrows bridge in Washington in 1940 and the Ferrybridge power station cooling towers in England in 1965. In water, the vibrations regularly damage docks, oil rigs and coastal buildings.

“For the past 25 years, engineers 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 Bernitsas.

Fish have long known how to put the vortices that cause these vibrations to good use.

“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 energy produced is expected to cost about 5.5 cents per kilowatt hour. Wind energy costs 6.9 cents a kilowatt hour. Nuclear costs 4.6, and solar power costs between 16 and 48 cents per kilowatt hour, depending on the location.

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