November 19, 2009

Wave turbine takes wing

Make the blades of a paddle-wheel-style wave turbine like airplane wings and you can boost the amount of electricity that can be extracted from the ocean.

Previous paddle wheel, or cycloidal, turbines work by drag -- the flow of water pushing against the blades -- or by a combination of drag and lift. A new turbine, designed by U.S. Air Force Academy aeronautical engineers, works by lift alone, which is more efficient. Drag produces more turbulence, which reduces the amount of energy transferred from waves.

Lift is the way airplane wings and helicopter blades work; a specially-shaped blade produces a perpendicular force (up in the case of aircraft) by reducing the pressure on one side of the blade when it moves edge-wise through air or water.

The turbine is designed to work in the open ocean where wave energy is strongest. The turbine can remain in place without mooring because a cluster of lifting blades can be angled to cancel the wave energy. Mooring is difficult and expensive in the deep ocean.

Computer models and small-scale prototypes show that under optimal conditions the turbine would convert 97 percent of wave energy to power that turns a generator shaft. Other high-efficiency wave energy devices are limited to low-wave shallow waters.

A lift-based cycloidal turbine in the North Atlantic, where wave energy is about twice the global average, would be able to transfer up to 100 kilowatts of power per meter of wave crest to a generator. Assuming a generator efficiency of 90 percent, a turbine spanning 40 meters would produce about 3.6 megawatts of electricity, which is the same as a 120-meter-diameter state-of-the-art offshore wind turbine.

Research paper:
Deep Ocean Wave Cancellation Using a Cycloidal Turbine
62nd Annual Meeting of the APS Division of Fluid Dynamics, Minneapolis, November 24, 2009

Researchers' contact:
NSF press release -- see Stefan Siegel's contact information

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