The device absorbs the energy in vortices formed around an object placed in a water current. The innovator, Michael Bernitsas, had previously spent his time trying to mitigate the damage caused by these vortices as they swirled around submerged pillars, pylons and cables and induced harmonics and vibrations. He realised that there was energy in them thar vibrations and set about finding out how to harness them. The natural world already does - salmon use the energy from such vortices to assist their swimming upstream.
The prototypes have cylinders laid horizontally across the current flow. How simple is that? An added advantage is that the cylinders just oscillate up and down so are far less harmful to fish than great big turbines or propellers. The ability to operate in very slow currents means it would be very flexible. They expect 5.5cents/kilowat and 90cu ft per megawatt.
Can we stop building those ghastly wind turbines now?