Salter’s Nodding Duck

Image of a nodding duck (Thorpe, 1999)

It was 1974 when Stephen Salter, a professor at Edinburgh University, first introduced a unique wave-energy conversion concept with efficiencies approaching 90% in two-dimensional sinusoidal waves. The device is designed with a paunch shaped such that the dynamic pressure caused by the wave-induced water particle motions efficiently forces the duck to rotate about its axis of rotation. The changing in hydrostatic pressure also contributes to the rotation by causing the buoyant fore body near the beak to rise and fall. Since both of this pressure-induced motion is in phase, the nodding duck can convert not only the kinetic energy but also the potential energy of the wave into mechanical rotational energy. The rotational motion then is transformed into electricity by a hydraulic-electricity subsystem.  The system has never been tested in the real ocean environment because funding to the project was cut by the government; however detailed mathematical model seems to suggest that the promised efficiency cannot be met in calm water condition.

A set of nodding-duck shaped bodies, performing pitch oscillation, with different phases. Notice the calm water behind the ducks (due to loss energy to the ducks)

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