Wind turbines Turbine revolution


Wind energy targets require larger turbine designs, or at least more energy produced by each machine. David Strahan rounds up some of the concepts being developed, including gearbox replacement, superconducting materials, hydraulic transmission and a vertical axis machine.


W


hen Thanet wind farm off the Kent coast opened to great fan- fare last September (see Energy


World November 2010), it was no surprise that Energy Secretary Chris Huhne was there to cut the ribbon. At 300MW, Thanet is the world’s largest offshore wind farm, and offshore wind is central to Britain’s energy policy. The government is counting on it to deliver the bulk of its target to gen- erate around a third of UK electricity from renewables by 2020 – a stretching six-fold increase from today. Building wind farms offshore makes per-


fect sense: that’s where the wind blows hardest, and where turbines are least likely to raise ‘NIMBY’ hackles. And the potential is vast. According to a report fromthe gov- ernment-industry body, Offshore Valuation Group, if Britain exploited just a third of the practical offshore resource by 2050, it could produce the energy equivalent of a billion barrels of oil, avoid emitting over a billion tonnes of carbon dioxide, become a net electricity exporter and create 145,000 new jobs. The prize is huge, but offshore wind is


also fraught with difficulties. At sea, wind turbines are constantly battered by the stronger wind and the waves, which makes themmore likely to break down and hard- er to fix when they do. They are much more expensive to install and maintain – costs have risen over the past decade, rather than falling as you might expect, according to a report from the UK Energy Research Centre – and that makes projects difficult to finance. Although the rate of construction is forecast to rise strongly over the next few years, it still falls short of the pace needed to hit the government’s renewables targets. All of this creates an urgent need to pro-


duce more energy from each turbine, meaning they must become both more powerful and more reliable than their land-based predecessors.


Turbines need to lose weight But some experts doubt whether turbines can grow much further using existing materials and technologies. Growth in the power rating of new models has stalled over the past five years, according to Peter Jamieson, Principal Engineer at Garrad Hassan, the renewable energy consultancy, after rising exponentially for decades.


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While the industry talks of building a 10 MW machine, the biggest operating off- shore today is just 5 MW. He suspects the plateau is due to a curious property of wind turbines: the bigger they get, the less economic they become. That’s because the energy turbines gen-


erate depends on the size of the area swept by the blades, and the area of a cir- cle is proportional to the square of the radius. So as you increase the length of the turbine blades, the electricity and income generated rise to the power of two. But longer blades need bigger gearboxes, gen- erators, towers and foundations, which expand in three dimensions, so weight and material costs increase to the power of three. ‘If turbines are going to be more powerful,’ says Jamieson, ‘they need to lose weight.’ This is forcing the industry to rethink


almost every element of turbine design. ‘In ten years’ time offshore wind turbines could look very different from the ones we have today,’ says Professor Feargal Brennan of Cranfield University Offshore Engineering Department, who has helped develop a new vertical axis turbine design.


Gearboxes and permanent


magnets At the same time, offshore turbines must become much more reliable. Repairing a turbine on top of a 100 metre tower is far harder at sea than on land, and can be delayed by bad weather and a scarcity of floating cranes –which all cuts energy pro- duction and adds cost. The gearbox, with many moving parts working under great stress, is particularly prone to fail; the Thanet project was delayed for two years because of gearbox problems with the developers’ preferred model at another offshore wind farm. The industry has spent a lot of effort developing alternative approaches, but until now they have all come with major drawbacks including extra weight and expense. Gearboxes were necessary only because


turbine blades move slowly – at about 15 RPM – and because early turbine develop- ers used off-the-shelf industrial generators that need to spin at around 1,500 RPM. You can do away with the gearbox if you replace the relatively small high speed gen- erator with a low speed one, but these ‘direct drive’ machines need up to 20 times


more electromagnets to deliver the same power, and are therefore bigger and heav- ier. The largest direct drive generator – also the world’s biggest onshore turbine – is a 7.5 MW machine made by Enercon of Germany that measures 12 metres in diam- eter. To reduce the weight of direct drive


machines, other manufacturers such as Siemens, and Goldwind of China, have replaced the heavy electromagnets made of wound copper wire with powerful per- manent magnets made from neodymium, a rare earth metal that is naturally highly magnetic. This makes the generator more efficient, smaller and lighter, but the downside is that rare earth metals are, well, rare. It’s not that there is any shortage under- ground, but production is now concentrat- ed in China, which controls 97% of the world’s supply and has a policy of restrict- ing exports. There are large deposits in the US, Russia, Australia, Greenland, and Tanzania, and a number of supply deals have been announced recently, but new mineswill take many years to develop. The British Geological Survey and other fore- casters predict shortages within the next few years, and these look likely to worsen as green technologies proliferate. But, if the industry seems confounded at


every turn, several solutions are now on the horizon, which each claim to unblock the way for double-digit megawatt tur- bines.


Superconducting turbines One recent idea has been to adapt the superconductor technology used in MRI scanners and specialised electricity trans- mission cables. Superconductors are mate- rials that offer zero electrical resistance when cooled to very low temperatures, which means that a superconducting wire can carry 100 times more current than a copper wire of the same diameter. And that means superconductors can be used to make electromagnets with an even higher power-to-weight ratio than neodymium permanent magnets. American Superconductor has designed


a 10 MW direct drive turbine called the SeaTitan based on superconducting mag- nets cooled to around minus 240°C that it claims will transform the offshore wind market. The company says the machine will weigh the same as a 5MWdirect drive tur- bine with conventional electromagnets, yet produce twice as much power. This could cut overall investment costs by 20% or more, and allow turbines to grow much larger. ‘Conventional turbines have a real hurdle at around 5–6 MW,’ says company spokesman Jason Fredette, ‘but supercon- ductors open theway to 20MWmachines.’ So far the SeaTitan exists only on paper.


The company has not yet built a prototype, although it has successfully demonstrated the technology in a 36 MW superconduct- ing ship motor it built for the US navy (an electric motor is just a generator working


Energy World April 2011


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