Generating energy underwater from good vibrations

An engineering professor from the University of Michigan has developed a marine energy system that simulates fish movements to convert hydrokinetic energy into electricity.

The project is known as the vortex induced vibration for aquatic clean energy converter, VIVACE for short.

The system could be used to generate power for coastal businesses such as lighthouses or desalination plants. A portable application could involve a smaller unit used to recharge electrical devices on military excursions.

Michael Bernitsas, a professor at the University of Michigan’s Department of Naval Architecture and Marine Engineering, was granted a patent for the system. He’s also founded a company called Vortex Hydro Energy, serving as its chief technology officer to develop commercial applications for VIVACE.

See the VIVACE laboratory model here >>>

The system generates power from "vortex induced vibrations" that occur when a rounded object disrupts the smooth flow of liquid or air. Eddies or vortices form on opposite sides of the object, making it move perpendicular to the flow.

Engineers usually work to prevent such vibrations because they can damage structures such as oil rigs and docks. In 1940, the Tacoma Narrows bridge in Washington collapsed due to the vortices created by the wind. Bernitsas told the Cleantech Group that controlling the vibrations is the key to making them beneficial rather than harmful.

VIVACE emulates the movements in a school of fish, where each fish curves its body to avoid the vortices left by those moving ahead of them and being propelled forward as they ride the wake. At his university lab, Bernitsas and his students have built several VIVACE prototypes, including one consisting of a single 25-centimeter (9.8 inches) diameter hollow cylinder of coated aluminum suspended by springs and lying horizontally in a tank of water.

When the simulated water current flows between 1 to 3 knots, the cylinder’s presence creates vortices at the top and bottom of the cylinder, causing it to move up and down, perpendicular to the current. The resulting mechanical energy generated by the movement is converted into electricity.

A recent study from Frost & Sullivan shows marine energy from sources such as wave energy and tidal projects could provide 10 percent of the world’s energy needs (see UK holds half of Europe’s wave energy potential). Last year California’s Electric Power Research Institute predicted that ocean-based energy could someday meet as much as 10 percent of the total U.S. demand (see Riding the marine power wave with Roger Bedard).

But marine energy systems such as turbines and water mills need water currents of 6-7 knots per hour in order to be commercially viable, Bernitsas said, citing an EPRI study. Most water currents, however, run much slower at rates of 3 knots per hour or less.

Bernitsas' calculations show VIVACE can generate 51 Watts per cubic meter of water assuming a constant flow rate of 3 knots per hour. If the system were installed in a 10,000 square meter area at two meters depth, he said in an example, the system would generate 1 megawatt, enough to power 1,000 homes.

The engineering professor said that the company doesn't expect to have a commercially-viable system ready until mid-2010 at the earliest, but he’s already receiving calls from interested investors and potential clients.

VIVACE is being designed in a scalable modular form that resembles ladders with movable rungs attached to a base floating several inches off the seabed to avoid disrupting sediments. He said the final design would be one easily installed or removed from the ocean by a crane.

"I'm leaving lots of space because I'm not as smart as fish," Bernitsas said. "Fish use vortices much more intelligently than we can at this point."

Bernitsas said he's done studies on VIVACE's effects on the ocean surface and seafloor based on the laboratory model, but hasn’t looked at the effects of the system on marine life yet.

He estimated that a commercially viable version of the system with a 10 MW capacity could cost $0.055 per kilowatt hour including underwater installation below the navigation line and, at least in the beginning, maintenance charges.

Vortex Hydro Energy has raised $2 million USD, primarily from government agencies, for research and development purposes and to construct two prototypes that are expected to be ready for testing by the end of 2009.

According to Bernitsas, one of the prototypes will be used by the company’s first "beta customer," the university’s neighboring Detroit/Wayne County Port Authority which is building a port for cruise ships coming to the Great Lakes. The second prototype is expected to be tested in an as-yet undetermined ocean site.

Bernitsas is still working on additions to the VIVACE system such as a fish tail and scales. The scales would provide friction to make the vortices around the cylinders stronger, so they generate more mechanical energy to be converted into electricity. The fish tail on the other hand would smooth the flow of the current and separate the vortices on each end of cylinder.

Further details of the studies done on the VIVACE lab model are outlined in the current issue of Journal of Offshore Mechanics and Arctic Engineering.

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