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Overtopping devices

The working principle of overtopping devices is very different compared with OWCs and oscillating bodies. Overtopping or run-up is a non-linear phenomenon that cannot be modelled by linear wave theory, and so requires different modelling tools.

Author: António F. de O. Falcão IDMEC, Instituto Superior Técnico, Technical University of Lisbon.

An overtopping device acts basically as a pump that converts wave energy into potential energy in a water reservoir whose main function is to provide a stable supply to a conventional low-hear hydraulic turbine (or a set of turbines).

In the Tapchan (mentioned above), the run-up effect is produced by a gradually narrowing channel with wall heights equal to the filling level of the reservoir (about 3 m in the Norwegian prototype) such that as the waves propagate down the channel their height is amplified until the wave crests spill over the walls and fill the water reservoir.

In other devices, the run-up effect takes place along a sloping wall or ramp, as is the case of the Wave Dragon, an offshore floating system developed mostly in Denmark. The Wave Dragon consists of a floating slack-moored platform with two long arms acting as wave reflectors to focus the waves towards a ramp. A 1:4.5-scale model, 57m-wide, equipped with seven turbines, was deployed in 2003 off the Danish coast in the North Sea and tested for a couple of years. Plans to construct of a multi-MW full-sized device have been announced.

The Seawave Slot-cone Generator (SSG) is a Norwegian breakwater-version of the
run-up concept that utilizes multiple reservoirs placed on top of each other, into which the water overspills through slots spaced at different levels on the sloping sea-facing side of the breakwater. The device is equipped with a special multi-stage vertical-axis turbine.

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