It has been estimated that their is sufficient energy in the world’s ocean currents to meet 3,000 times the current power needs of the the entire world’s population. The difficulty is in harnessing that energy.

Turbines and watermills need water flowing at 5 to 6 knots to operate effectively. But most of the ocean currents flow at less than 3 knots.

Michael Bernitsas, a professor in the University of Michigan, has now developed a device that works in water flowing at just 2 knots.

The machine, called a VIVACE, relies on "vortex induced vibrations". These are undulations that a rounded or 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, or vortices, 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 powerful vibrations regularly damage docks, oil rigs and coastal buildings.

"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 Professor Bernitsas. "VIVACE copies aspects of fish technology. 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 VIVACE device is a cylinder. The presence of the cylinder in a current causes alternating vortices to form above and below the cylinder. The vortices push and pull the cylinder up and down on its springs, creating mechanical energy. Then, the machine converts the mechanical energy into electricity.

Bernitsas says VIVACE energy would cost about 5.5 cents per kilowatt hour. The current US price for electricity from coal is between 3 and 5 cents per kilowatt hour; from nuclear it averages 4.6 cents, from wind it averages 6.9 cents and between 12 and 18 cents from solar thermal sources.