Motors drive wave energy converter

The UK coastline could soon be dotted with giant, floating triple sectioned wave energy converters designed and built by a Scottish company. Dean Palmer reports

The first pre-production unit of a giant wave energy converter produced its first electricity during sea trials conducted in the North Sea in April this year. Designed and built by Edinburgh-based firm Ocean Power Delivery, the 'Pelamis' 750kW wave energy converter was put to the test just off the Firth of Forth and came out with flying colours. The converter, which is the world's first commercial-scale floating unit, is soon to be transferred to the European Marine Energy Centre on the Orkney Islands, where it will be hooked up to a sub-sea cable feeding the electricity grid for final proving. Behind the energy converter are eight specially-designed generators, supplied by AEG Electric Motors. The 125kW generators are based on the company's standard AM 315 asynchronous electric motor but 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. The generators are sealed to IP68 which makes them reclaimable in the event of flooding. A special terminal arrangement was designed in cooperation with the hydraulic motor supplier, in order to secure the integrity of the IP rating in a limited space. The generators are designed to be run up to operating speed as electric motors, which avoids 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 of the generators. The 750kW Pelamis is 120m long and is a semi-submerged, articulated structure consisting of cylindrical steel sections linked by hinged joints. Moored at its nose, the converter is free to swing about its mooring, pointing into the dominant wave direction. The energy conversion principle is straightforward. 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. This power is then fed down an umbilical cable to a junction on the seabed connecting it and other wave energy converters, via a common sub-sea cable, to the shore. The design of the Pelamis gives it excellent inherent load-shedding and power-limiting capabilities that enable the machine to survive even the most extreme wave conditions. It is also designed to maximise power capture in small seas through an adaptive control system that 'tunes' the system by varying the stiffness of each joint. Of the eight generators supplied by AEG, one was for initial testing, another is a spare and the remainder are installed in pairs on Ocean Power's pre-production Pelamis.