Tidal generator goes online

Within the next few months, the world’s first megawatt tidal flow turbine is to be installed and commissioned in Northern Ireland. Producing more than four times more energy per square metre of rotor than a wind turbine, and designed for easy maintenance, it relies on a combination of breakthrough technologies – including the British-designed orbital gearboxes and a combination of leading edge materials. The 1.2MW, twin rotor Seagen tidal flow generator, which will be installed in Strangford Narrows, was originally conceived by alternative energy guru Peter Fraenkel. It has since been developed by a design team managed by Angela Robotham, engineering director of Bristol-based Marine Current Turbines. Tidal current flow is up to 8 knots – 4m/s while the site is relatively sheltered. It builds on experience obtained with a 300kW single turbine demonstrator installed in 2003 off the coast of Devon, which has been a technical success. Speaking at a meeting at the Thames Barrier organised by IOM3, Fraenkel – who is technical director of MCT – revealed some of the details of its construction and explained the reasoning behind its configuration. It has two side-by-side 16m-diameter twin bladed turbines, mounted each side of a central pile. “We have patented being able to lift them out of the water to work on them,” said Fraenkel. He added that using two blades allowed them to be turned horizontal, so that a barge could be brought underneath the raised turbine, to receive the complete units to be taken away for maintenance, much like, “It’s much like changing the engines on an airliner,” he said. He said that the reason that wind turbines tend to have three blades is to improve aesthetics and reduce noise. A structure with two blades can take the same amount of ‘bite’ as they rotate in the fluid going past, by making them “50% more chunky – whereupon the savings in cost offset any losses in efficiency”. Fraenkel claimed a rotor efficiency of 45%, against a theoretical maximum of 59.3%. The 7.5m long blades have carbon fibre main spars and composite skins and ribs. The blades were designed and manufactured under the direction of Angus Fleming of Aviation Enterprises in Lambourn, Berkshire. The load on the main spars is expected to range up to 25-30 tonnes force. Fraenkel said the shapes had been “optimised to the nth degree”. The rotors, which contain the variable pitch mechanism, allowing them to extract maximum energy and reverse when the tide reverses, are flooded with water. The servo motors are from remotely operated marine vehicles and designed to run in a marine environment. Right gear The gearboxes were designed by Frank Cunliffe of Orbital 2 in Powys and manufactured by Wikov in the Czech Republic. They use a patented method of compliantly mounting their eight planet gears in each of the back-to-back gearboxes. Most successful high power epicyclic gearboxes rely on some kind of load sharing to counteract the effects of manufacturing errors and tolerance build-up. If any planet gear is subjected to more load than the others, its mounting deflects by a greater amount and the remaining planets then take up the load. The gearboxes and generator units are sealed but rely on natural water cooling. As the tidal stream increases in speed, it enhances cooling at the same time as increased heat is produced. The cross arms are made of steel and with GRP farings for streamlining. The superstructure housing the hydraulic and pneumatic power packs is also made of composite. The supporting column is made of rolled steel, welded to form cylindrical monopile cans, and is 3m in diameter, tapering to 4m diameter lower down. The necessary environmental impact study has led to mandatory studies of seal behaviour, including fitting live seals with transponders and tracking them by satellite. Once the system starts working, there are to be active sonar tests using a synthetic “fake porpoise” and active sonar studies to observe what happens to any real animals. The blades turn fairly slowly and are expected to make a fair amount of underwater noise doing so. Fraenkel considers that large ship propellers are much more hazardous to wild life. He said that on disk, the environmental impact study takes up 62MB, and in paper form “takes a wheelbarrow to move it around”. The present phase in the development is expected to cost about £8.5 million at the end of the day, including grid connection, and is financially supported by the operating partners and the UK DTI who have awarded a grant of £4.27 million. Energy return on energy investment is thought to be about a year. Studies are under way into the next larger and more commercial stage, likely to be installed off Anglesey. The team believes that eventual large-scale systems should produce electricity at a cost of from 3 to 9p/kWh and that the production potential round the UK is in excess of 20 TWh/year. Design Pointers * The technology has already been demonstrated as technically viable * Two blades, lifting out of the water and lowering them onto a receiving barge greatly eases maintenance. These ideas should be applicable to wind turbines * Epicyclic gearboxes can be made much more compact by using more planet gears and mounting them compliantly