Inverters drive Aquamarine’s Oyster marine energy generator13 July 2010

The world's largest working hydroelectric wave energy device – Aquamarine Power's Oyster – has entered a new phase, having been installed at the European Marine Energy Centre's test site in Orkney.

The original test unit was installed just off shore, in Orkney, comprising a large buoyant steel flap hinged to a base that sits on the sea bed. As waves roll over the device, the flap drives two hydraulic pistons attached to each side.

High pressure water from these two pumping cylinders is pumped ashore, driving a Pelton wheel with spoon-shaped buckets that then harvest the energy via a flywheel connected to an induction generator. Control Techniques 315kW high-efficiency inverter then provide an interface with the grid.

Paul Smith, lead electrical engineer at Aquamarine Power, explains that, in the latest installation, two Control Techniques inverters now provide a clean supply to the grid, with low harmonic distortion. The ac output from the generator is normally equally split via sharing chokes, to produce a clean output to the grid – with big waves producing 200kW per metre of flap.

Aquamarine has fitted many strain gauges, pressure monitoring and other sensors at key points on the system to give a complete picture of performance. All signals are fed back to the PLC.

The units that make up the system are from the Unidrive SPM range which can be used to implement high efficiency grid-tie inverters for renewable energy systems including wave, tidal, photovoltaic and wind.

"The next generation of Oyster drives, which will have three operating flaps, is already in development, and we are looking at an output of 2.4MW per unit," says Smith.

Brian Tinham

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Nidec Control Techniques Ltd

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