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Offshore wind turbines

Offshore wind farms could deliver on renewable energy targets - but require investment in technology

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Scramble to design supersized turbines to maximise wind power potential

David Strahan

1st March, 2011

Offshore wind power is crucial if the UK is to meet its renewable energy targets - but a lack of suitably powerful and reliable technology could hamper efforts, reports David Strahan

When Thanet wind farm off the Kent coast opened to great fanfare last September, it was no surprise that Energy Secretary Chris Huhne was there to cut the ribbon. At 300 megawatts (MW), 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 generate around a third of UK electricity from renewables by 2020 – a stretching six-fold increase from today.

Building wind farms offshore makes perfect 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 from the Offshore Valuation Group, a government-industry body, 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 over a billion tonnes of CO2, 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 them more likely to break down and harder to fix when they do. They are much more expensive to install and maintain – costs have risen over the past decade, rather than fall 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 produce more energy from each turbine, meaning they must become both more powerful and more reliable than their land-based predecessors. 

Stalled growth

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. While the industry talks of building a 10MW machine, the biggest operating offshore today is just 5MW. 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 generate depends on the size of the area swept by the blades, and the area of a circle 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 2. But longer blades need bigger gearboxes, generators, towers and foundations, which expand in three dimensions, so weight and material costs increase to the power of 3. 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.

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 production and adds cost. The gearbox, with many moving parts working under great stress, is particularly prone to fail; the Thanet project was delayed 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 15RPM – and because early turbine developers used off-the-shelf industrial generators that need to spin at around 1500RPM. You can do away with the gearbox if you replace the relatively small high speed generator with a low speed one, but these ‘direct drive’ machines need up to twenty times more electromagnets to deliver the same power, and are therefore bigger and heavier. The largest direct drive generator – also the world’s biggest onshore turbine – is a 7.5MW machine made by Enercon of Germany that measures 12 metres in diameter.  The company refused to answer questions about whether its design is too heavy to grow any further.


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 permanent 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 underground, but production is now concentrated in China, which controls 97 per cent of the world’s supply and has a policy of restricting 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 mines will take many years to develop. The British Geological Survey and other forecasters 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 turbines.

One recent idea has been to adapt the superconductor technology used in MRI scanners and specialized electricity transmission cables. Superconductors are materials that offer zero electrical resistance when cooled to very low temperatures, which means 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.

Cutting investment

American Superconductor has designed a 10MW direct drive turbine called the SeaTitan based on superconducting magnets cooled to around minus 240C that it claims will transform offshore wind market. The company says the machine will weigh the same as a 5MW direct drive turbine with conventional electromagnets, yet produce twice as much power. This could cut overall investment costs by 20 per cent or more, and allow turbines to grow much larger. 'Conventional turbines have a real hurdle at around 5-6MW', says company spokesman Jason Fredette, 'but superconductors open the way to 20MW machines'.

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 36MW superconducting ship motor it built for the US navy (an electric motor is just a generator working in reverse).  American Superconductor says it expects to build the first SeaTitan ‘in the next few years’ and enter volume production by ‘mid decade’.

Another superconductor manufacturer, Zenergy Power of Germany, has also designed 10MW machine, and has begun to build a 500kW prototype with a British company, Converteam, which it expects to finish next year. Chief executive Jens Müller is confident of securing the funding to build the full-sized machine and claims the technology will be fully established by the end of the decade. 'The challenge now is to industrialize the supply chain', he says.

Superconducting turbines are intended to slash the weight and increase the power of large, low speed generators, but that’s only necessary if you have got rid of the gearbox. An alternative approach would be to stick with the traditional high-speed generator – which is small and well proven – and try to replace the gearbox with something lighter and more reliable.

Digital displacement

One obvious contender is hydraulic transmission, which is widely used where a fast, steady source of power like a diesel engine has to drive slow, heavy and irregular work – in diggers or rock crushing machinery for instance - the problem of wind turbines in reverse. But the drawback is that hydraulics are typically very inefficient: even in the best systems, scarcely 80 per cent of the energy pumped in does useful work, and the rest is lost to the process. But now a small British company, Artemis Intelligent Power, thinks it has cracked this with a computer controlled hydraulic transmission technology it calls Digital Displacement.

With Digital Displacement the wind turbine blades are connected to a cam shaft that drives a series of hydraulic piston-cylinders arranged around it. These pump fluid through a pipe to drive the cylinders of two hydraulic motors, which in turn drive a pair of standard high speed generators. A computer controls how many of the cylinders are used from one millisecond to the next, using only as many as needed at any given moment, creating an infinitely variable transmission ratio.

This dramatically raises the efficiency to well over 90 per cent, and the company hopes to reach 95 per cent eventually. It also means the system can respond instantly to changes in wind speed and keep the generators running at exactly the right, steady speed to deliver grid-quality power - however wild the gusts. That in turn means a Digital Displacement machine can dispense with the expensive power electronics that almost all other turbines need to do that job. Because the system contains far less steel than a gearbox it weighs about half as much, and that should help produce more powerful turbines.  'We see our technology getting more competitive as machines get bigger', says managing director Win Rampen.

Mustafa Kayikci, of Manchester-based energy consultants TNEI, whose PhD thesis was on how to incorporate wind power into electricity grids, is impressed with the Artemis approach. 'It’s not proven yet', he says, 'but it could be the ideal solution'.

The Japanese engineering giant Mitsubishi Heavy Industries seems to agree. Last December it bought Artemis as part of a £100 million deal, and plans to incorporate Digital Displacement in North Sea turbines by the middle of the decade. 'This is a massive vote of confidence in our technology', says Rampen.

However much weight newer turbine designs can shed, they would still sit atop a 100 metre tower. And some say that’s the real problem: not so much the weight per se, but where you put it. Structurally conventional wind turbines are 'trying to hold up a sledgehammer in the wind' says entrepreneur Theo Bird, the founder of Wind Power Ltd, a British company developing a turbine that turns conventional thinking if not on its head, then at least on its side.

Today’s turbines make life difficult for themselves by putting all their weight at the top of a long lever that the wind is constantly trying to knock down. That creates huge forces that will ultimately limit the size and power of all conventional machines, says Bird.

Reduced costs

His answer is the Aerogenerator X, a 10MW vertical axis design featuring two diagonally outstretched arms with rigid sails at the end, which is shorter and wider than a normal turbine. There is no tower, and the arms rotate horizontally around a base that houses the generating equipment. Because the weight is concentrated at the bottom of the structure it is not so critical, and this means the design could work with any type of generator or gearbox. It also makes the structure more stable, meaning the foundations can be smaller. The company claims its machine would be half the cost of a conventional turbine, but refuses to disclose detailed figures.

The Aerogenerator would have a number of practical benefits. The generating equipment should be much easier to access and maintain, cutting downtime and cost, and the stability of the design makes it well suited to floating platforms that could operate in deeper water where the wind is stronger. And critically the design also lends itself to much larger machines.  'Vertical axis machines are not limited by the strength of existing materials in the same way as conventional turbines', says Professor Feargal Brennan of Cranfield University Offshore Engineering Department.

Cranfield was part of a group of British universities and specialist engineering companies that helped Wind Power develop the Aerogenerator, with funding from the Energy Technology Institute (ETI), whose remit is to bring the cost of offshore wind down to the level of onshore. Wind Power is waiting to hear if the ETI will fund the next phase of detailed design work, but either way Bird plans to build a 1MW machine in 2012, and a wind farm of 5MW machines by 2015. 'It’s a pretty whacky concept at first sight, but we’ve really kicked the tyres and we’re very convinced it’s plausible', says Professor Brennan. 'It could be a game changer'.

Maybe so, but there could be stiff competition for that title from superconductors and hydraulic transmission - and a rash of other developments. Clipper is developing the Britannia, a prototype 10MW machine with an improved gearbox that reduces loads by a factor of four; EDP of Portugal will test turbines on floating platforms this year; and Gamesa is leading a Spanish industry study to research the technologies needed to build a 15MW machine. While the challenges of offshore wind remain formidable, the industry is not short of ideas.

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