Motors generate interest in wave power

Edited by the Engineeringtalk editorial team May 12, 2004

Eight generators for the world's first commercial-scale floating wave energy convertor have been supplied by AEG Electric Motors of Crewe.

The UK coastline (and many others) could soon be the home for giant sea-serpent like configurations.

They are not strange creatures but floating, triple-sectioned alternative energy generators converting wave power into electricity.

The first preproduction unit of the Pelamis wave energy convertor (750kW), designed and built by Ocean Power Delivery of Edinburgh, produced its first electricity in April 2004 during sea trials conducted in the North Sea off the Firth of Forth.

The machine is soon to be transferred to the European Marine Energy Centre on the Orkneys, where it will be hooked up to a sub-sea cable feeding the electricity grid for final proving.

Eight generators to be used for this project, the world's first commercial-scale floating wave energy convertor, have been supplied by AEG Electric Motors of Crewe.

The contract for the generators was awarded to AEG following extensive testing in a full-scale simulator.

The 125kW generators are based upon AEG's standard AM 315 asynchronous electric motor.

They have special rotor laminations and windings to increase generator efficiency and are connected for 690V.

The latter permits the size of the power cables involved to be reduced which, in turn, improves power transmission efficiency.

In addition, the generators are sealed to IP68 which makes them reclaimable in the event of flooding.

A special terminal arrangement was designed in co-operation with the hydraulics vendor to secure the integrity of the IP rating in a limited space.

In addition, the generators are designed to be run up to operating speed as electric motors so avoiding the need for synchronisation.

Once near synchronous speed, positive torque is applied by the attached hydraulic motor to push the unit into generation mode.

Speed sensors are fitted to permit continuous monitoring.

Full heat-run testing was performed at AEG's plant at Terrassa, Spain where there are extensive in-house proving facilities.

Of the eight generators supplied, one was for the initial testing another is a spare and the remainder are installed in pairs in Ocean Power Delivery's preproduction Pelamis wave energy convertor.

The 120m long, 750kW Pelamis wave convertor is a semisubmerged, articulated structure consisting of cylindrical steel sections linked by hinged joints.

Moored at its nose, the Pelamis is free to swing about its mooring, pointing into the dominant wave direction.

Waves travel down the length of the wave generator causing each section to articulate about the hinged joints between the sections.

Hydraulic rams at each joint resist this movement and pump high-pressure fluid to hydraulic motors via smoothing accumulators.

The hydraulic motors drive the AEG generators to produce electricity.

The power produced is fed down an umbilical cable to a junction on the seabed connecting it and other wave energy convertors via a common sub-sea cable to shore.

The design of the Pelamis gives it excellent inherent load-shedding and power-limiting capabilities that allow the machine to survive even the most extreme wave conditions.

It is also designed maximise power capture in small seas through an adaptive control system that "tunes" the system by varying the stiffness of each joint.