Wavepower generator relies on low-friction bearing material

www.drives.co.uk
February 2010

Novel bearing and seal technologies are playing a vital role in a new UK-developed wave energy generator due to start testing off the Orkney Islands later this year. Engineers at Edinburgh-based Pelamis Wave Power have been developing wave power devices for more than a decade and have overcome the challenge of continuous operation in the punishing marine environment. Building on their experience of producing the world's first offshore wave energy converter, the team has developed a second-generation design which is more efficient and cost-effective.

Crucial to the new design are the bearings and seals. The P2 generator consists of a series of floating, linked tubes with four hydraulic rams at each main joint pivoting on precision bearings to drive hydraulic motors coupled to electric generators. "Our biggest challenge has always been how we manage the loads and motions from such an active and constantly variable environment, whilst at the same time extracting as much power as possible," explains Mike Woods, Pelamis Wave Power' senior engineer and bearings group leader. "The working forces generated across each joint can be several hundred tonnes, which can present huge problems for the bearings as they have to take up the reactive forces coming back through the joints."

Pelamis Wave Power' first wave energy converter (P1) was a simpler design with separated hinged joints. Although this arrangement allowed useful working space between the axes, it had to carry high transferred loads and was unable to manage the combined motions needed for the P2 configuration. Also, the bearings themselves were relatively high friction, affecting efficiency. "Our engineering team had been trying to work out a way of overcoming this problem and eventually came up with the idea of bringing the axes, or joints, together," Woods continues. "However, this meant a completely new bearing solution which was able to manage combined angles in a single package."

Pelamis Wave Power turned to Schaeffler for help and support. Key to the new joint is a low-friction material developed by Schaeffler. The modified PTFE fabric liner effectively eliminates the "stick-slip" effect, allowing the machine to perform better than standard bearing materials would have done. "Now that we have been able to put all the bearings in one place, we have taken a major technological step forward," says Woods. "As well as being a much more efficient bearing mechanism, the new design is a self-contained, modular unit. It's a bit like being able to take an engine out of a car in one go; it allows us to improve our inspection procedures and reduce our exposure to technological risk."

The first P2 wave generator, ordered by E.ON, is now nearing completion. Its first 190-tonne tubes were launched in Leith Docks, near Edinburgh, last month. These will be linked to form a 180m-long wave power station capable of generating 750kW which is due to be deployed at the European Marine Energy Centre, Orkney, later this year. Pelamis Wave Power and E.ON were last month awarded £4.8m of funding from the UK government's Marine Renewable Proving Fund, which will allow them to increase the scope and pace of their trials. Pelamis Wave Power has also formed a joint venture with the Swedish utility Vattenfall with the aim of installing an array of up to 26 P2 generators off the coast of Shetland, to generate up to 20MW.