Adapted motors will harvest energy from the sea

Electric motors, adapted to operate as generators, are being harnessed to gather energy in the world`s first commercial-scale floating system for converting wave energy into electricity. The 125kW generators, based on AEG Electric Motors` standard AM315 asynchronous motors, have special laminations and windings to increase their efficiency, and will operate at 690V, allowing smaller cables to be used to transmit power from the generator to shore.

Eight of the motor-generators have been supplied to Edinburgh-based Ocean Power Delivery (OPD), which is developing the snake-like wave energy converter called Pelamis. The 120m-long system consists of three floating cylindrical steel sections, linked by hinged joints.

The converter is free to swing around a mooring at one end so that it points into the dominant wave direction. As waves travel along its length, the sections bend about the hinged joints, activating hydraulic rams which pump high-pressure fluid to hydraulic motors via smoothing accumulators. The hydraulic motors, in turn, drive pairs of the electric machines (shown above) to generate up to 750kW of power per converter.

To avoid the need for synchronisation, the generators also operate as motors to run up to their operating speed. Once they are near synchronous speed, the hydraulic motor applies torque to push them into the generating mode. A special terminal arrangement ensures that the motor-generators maintain an IP68 protection rating.

The Pelamis system first produced electricity during sea trials off the Firth of Forth earlier this year. It is now being transferred to the European Marine Energy Centre in the Orkneys where it will be connected to a subsea cable that will feed its output into the local electricity grid.

The system is designed to survive the most extreme wave conditions. In smaller seas, it "tunes" itself to maximise power capture by varying the stiffness of its joints.

OPD has recently completed a first-round investment of more than £7.5m, including more than £1.5m from the Carbon Trust. Other investors include 3i and Norsk Hydro. Over the coming two years, it plans to install pilot plants at sites which could include Scotland, South-West England and Portugal. A commercial "wave farm" (shown in an artist`s impression above) would consist of many interlinked converters, with a 30MW scheme occupying about a square kilometre of sea.