Unique hydro energy system harnesses slow water currents

http://www.gizmag.com/unique-hydro-energy-system-harnesses-slow-water-currents/10439/

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.

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instead of being blended by it.