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WIND POWER

BY RALF BUTSCHER

WITH A FAINT RATTLING SOUND, the ele-vator makes its way up the steel shaft inside the giant tower and comes to a gentle halt. Climb a few more meters up a ladder and through a hatchway, and you fi nd yourself in a large room which looks like a carefully tidied workshop. Yet a glance out the window is enough to remind you that you are actually inside the nacelle of a huge wind turbine, some 90 meters above the ground. This enormous tower is just one of the ma-ny winged giants that loom over the grassland and drainage ditches directly behind the dike in Høvsøre, a small mu-nicipality in the north-west of Denmark.Since May 2011, engineers from Sie-mens Wind Power have been working here, within sight of the Danish North

Sea coast, to test the prototype of a new wind turbine – the SWT-6.0-120. The name indicates a power rating of 6 mega-watts and a rotor diameter of 120 meters, a category of turbine which is currently the pinnacle of technology for converting wind into electricity.

The test site in this rural idyll illus-trates how researchers and engineers are preparing for the future of energy supply. These huge rotating blades are ultimately destined for offshore wind power plants, out on the open sea where the condi-tions are excellent for harvesting wind energy. Offshore winds blow more con-sistently and, on average, more strongly than on land, which means they can be used to generate considerably more electricity each year from the power of

the wind. Wind turbines in near-offshore areas are capable of supplying some 40 to 50 percent more electricity than tur-bines in good land-based coastal areas.

THE BOOM BEGAN 10 YEARS AGO The wind energy boom got underway in Germany around a decade ago. Fuelled by the German Renewable Energy Act, which guaranteed a high minimum feed-in tariff for renewable electricity sources when it came into force in April 2000, the number of wind turbines skyrocket-ed, especially in windy areas such as north and east Germany and on high-land peaks. Germany gradually took on a pioneering role in generating energy through wind power, and in 2002 it overtook hydroelectric power to become

Dynamic growth: The use of wind power has picked up huge momentum world-wide. Giant blades such as those on this six-megawatt turbine will be spinning above offshore waters in the future.

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In the future, more and more electricity will be produced by offshore wind power plants. This will call for wind turbines that are particularly robust and effi cient.

the most important renewable energy source in Germany’s energy supply network. Today, some six percent of the country’s electricity is generated by wind power. Between 1999 and 2011, the total installed capacity of Germany’s wind turbines climbed from fi ve to 30 gigawatts – and the German Wind Ener-gy Association predicts that this latter fi gure will double again by 2020.

This growth in the wind turbine sector will increasingly take place at sea. By 2030, the German federal government plans to have offshore wind farms in the North and Baltic seas supplying up to 25 gigawatts of power, and the Hamburg-based Federal Maritime and Hydrogra-phic Agency has already set aside large areas out at sea for this purpose. More than two dozen planning applications for offshore wind power plants – a total of some 8 gigawatts of installed capacity – have already been approved, and se-veral wind power plants are under con-struction. In April 2010, the fi rst German offshore wind power plant came online some 45 kilometers off the North Sea island of Borkum. Known as ‘alpha ven-tus’, it is primarily intended as a test facility. The fi rst commercial offshore wind power plant went into operation in the Baltic Sea in May 2012. The 21

Winged

Trial run in the factory yard: Testing out tur-bines at Siemens Wind Power in the Danish town of Brande.

Room with a view: On top of the nacelle of a six-megawatt prototype, some 90 meters above the ground.

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WIND POWER

Danish waters in 1991. Since then, how-ever, it is the UK that has become the world’s leading market for offshore wind development thanks to the frequent low-pressure fronts that offer such good wind production off its coasts. It already has more than a dozen offshore wind pow-er plant hooked up to its national grid, including ‘Walney’, the world’s biggest offshore wind farm, which was put into operation in the Irish Sea in February 2012. The farm consists of 102 Siemens-made turbines capable of producing up to 370 megawatts of power.

British plans for developing offshore wind power are signifi cantly more am-bitious than those of their German coun-terparts. Some wind farms off the UK’s

wind turbines supplied by Siemens for the ‘EnBW Baltic 1’ wind farm generate up to 185 gigawatt-hours of electricity a year, enough to supply some 50,000 households.

Denmark took to the seas considerably earlier than Germany. Bonus Energy, a company that has been part of Siemens since 2004, built the world’s fi rst off-shore wind power plant, ‘Vindeby’, in

A service technician checks external equipment on a giant wind turbine at a test site on Denmark’s North Sea coast.

Landing site for maintenance personnel: A spacious platform on top of the nacelle allows service technicians to be lowered onto the wind turbine from a helicopter. This makes it easier to access offshore wind farms far out at sea.


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coast will be built on a truly extraordi-nary scale. One example is the ‘London Array’ – already under construction in the outer reaches of the Thames estuary – which will have an installed capacity of one gigawatt (1000 megawatts) when the fi nal stage is completed. The initial stage of 175 wind turbines is scheduled to come on line by the end of 2012.

A wind farm with an energy output of up to 4.2 gigawatts is already being planned for the Irish Sea, while an even bigger wind farm of 9 gigawatts has been given the go-ahead on the Dogger Bank in the North Sea. The next round of offshore projects, Round 3, has seen approval granted for an additional 32 gigawatts of new capacity. The aim is to cover a quarter of the UK’s total elec-tricity needs using offshore wind power by 2020. In Germany, progress has been markedly slower.

One important caveat is that harnes-sing wind power is far more challenging out at sea than on land. Swells and salt water gnaw away at the towers unless

specifi c steps have been taken to protect them. Constructing the wind turbines and anchoring them to the ocean fl oor require extraordinary technical and lo-gistical skills, especially if the founda-tions are more than 100 kilometers away from the coast and are submerged in 40 meters of water – the situation facing most of the offshore wind farms being planned in Germany. Maintaining the turbines is a costly and sometimes dan-gerous process, with wind and waves making access diffi cult. Storms and heavy rain often make it impossible for the service engineers to reach the tur-bines at all.

DEMAND FOR NEW CONCEPTS Researchers and developers – for example the team led by Henrik Stiesdal, Chief Technology Offi cer (CTO) of Siemens Wind Power, in the Danish town of Brande (see p.24. “The whirlwind”) – are there-fore seeking new concepts and feats of engineering to create wind turbines that

are highly robust and reliable and easy to install. Above all, they are aiming to boost performance based on the funda-mental precept that the more power a wind turbine can produce, the greater its effi ciency – in other words, the greater its ability to pull energy out of the wind. Maximizing energy output is a crucial consideration, especially when building offshore wind power plants.

However, greater capacity inevitably means larger dimensions, and the cur-rent fl agships of the wind turbine sector are already enormous. The six-megawatt giant off Høvsøre, for example, features a tower more than 90 meters high which is equipped with three blades covering a diameter of some 120 meters. The tips of these extraordinarily long blades can reach speeds of up to 300 kilometers an hour in strong winds. By mid of 2012, the six-megawatt class will include a second machine featuring a new rotor with a diameter of 154 meters.

Over the last 30 years, there has been a tremendous increase in both the installed capacity and size of wind turbines. In the future, offshore wind farms could feature wind turbines with 200-meter rotor diameters that are capable of generating 20 me-gawatts of power.

TREMENDOUS GROWTH


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WIND POWER

Wind strength varies across Europe, but it blows especially hard over the North Sea and off the coast of Scotland. The wind power densities are mean values and do not take topo-graphy into account.

WIND IS CAPRICIOUS

In the past, Siemens has primarily used turbines from the 2.3 and 3.6-mega-watt categories for offshore wind farms, but, in the future, the plan is to use power-houses such as the six-megawatt co-lossus to capture energy from offshore wind. Prototype testing at the Danish test site on the North Sea coast is going well, and the new wind turbines have also demonstrated their effi ciency in an accelerated life testing program carried out at the Siemens Wind Power site in Brande. These tests apply a dynamic load to individual components such as the blades in order to simulate the forces they will be exposed to during 20 years of operation. By the end of 2013, Siemens hopes to have installed a pre-series version of the six-megawatt wind turbine at various sites in Denmark, Germany, Great Britain and the Nether-lands. Series production is scheduled to begin in 2014. The giant machines will be assembled at a port site and then transported by ship to their offshore destination.

In the future, wind farm operators will be able to choose between different variants of this Herculean construction of steel and concrete. The largest version will have a rotor diameter of 154 meters, making it bigger than any wind turbine ever built before. Yet despite their enor-mous size, these wind turbines are as-tonishingly lightweight. The combination of the tower head and rotor blades of the Høvsøre prototype weighs in at just 350 tons. The nacelle weighs 200 tons – only marginally more than the weight of a nacelle in a wind turbine offering half as much energy output.

SAME WEIGHT, GREATER CAPACITYSignifi cantly more power with a minimal increase in weight: That’s the solution Siemens Wind Power CTO Stiesdal is

focusing on to take the technology to the next level. The ingenious Dane often refers to the cubic law of wind power in this context: Doubling the rotor size of a wind turbine quadruples the energy yield – and results in an eightfold incre-ase in weight. “Our job is to overcome that law,” says Stiesdal. The six-mega-watt wind turbine marks the fi rst occa-sion that Siemens developers have suc-ceeded in breaking this rule – primarily thanks to the use of ‘direct drive’ tech-nology. Instead of using complex, heavy gearboxes to convert the rotation of the blades into faster revolutions to drive an


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electrical generator – the method em-ployed in most current wind turbines – the Siemens direct drive technology uses a magnet generator to generate electrical power directly from the mechanical rota-tion of the blades. This gearless solution reduces the number of moving parts in a turbine by almost 50 percent, resulting in considerable weight savings and lower construction costs.

One handy knock-on effect of gearless wind turbines is that they require less frequent maintenance thanks to the smaller number of wear parts – just one example of a trend towards making wind turbine technology simpler.

The machine room of the SWT-6.0 is considerably more spacious without the bulky gearbox that would normally

take up so much room, so it feels very different to the oppressively cramped interior of the nacelles used in older wind turbines. On the rare occasions that maintenance or repairs are re-quired, the service technicians can use this extra space to work and sleep in. The turbine also features a platform on top of the nacelle which provides helicop-ter access for maintenance personnel, thereby avoiding long and arduous boat journeys and the risky climb from the boat to the base of the tower.

The direct drive concept paves the way for even bigger and more powerful wind turbines. In fact, Siemens engineers are already working on wind turbines that will be capable of producing up to 10 megawatts of electricity. “We expect them to be commercially viable sometime after 2015,” says Stiesdal. Experts even consider 20-megawatt wind turbines to be feasible. As part of the European project UpWind, researchers from vari-ous companies, universities and public institutions from all over Europe devel-oped the technological basis for very large wind turbines of the future. These could have a rotor diameter of 200 meters or more.

However, sheer size is not the only consideration. Developers also have a whole series of technical innovations up their sleeves which are designed to increase the energy output of wind turbines. For example, the blades on more recent models are equipped with a device known as a ‘winglet’, a kind of mini-spoiler which is positioned on the wing tips to avoid the problem of tip vortices.

A similar purpose is served by small slits in the blades and saw-shaped grooves on the edges of the blades. “In 2013, we also plan to test initial proto-types of wind turbines with scimitar blades,” says Stiesdal. Modeled on the oriental weapon of the same name, these blades are aerodynamically opti-mized to exhibit the least possible air resistance when they rotate.

In the future, new materials will help to reduce weight and enhance turbine effi ciency. For example, lightweight yet extremely durable carbon fi ber com-posites will replace the fi berglass mats

that are typically used to produce rotor blades, while aluminum and plastic will replace the steel in the nacelles. At the same time, extensive automation of turbine operation and maintenance could help reduce costs. All new wind turbines produced by Siemens over the last ten years come with a Condition Monitoring System which monitors the functions and status of the turbine and sounds the alarm as soon as there is any risk of something going wrong. In the future, an interactive controller should make it possible to control the wind tur-bine systems from land.

PLUNGING ELECTRICITY COSTSUltimately, all these developments are aimed at reducing the cost of genera-ting energy from wind power to a level comparable to conventional gas and coal-fi red power plants. Currently, one kilowatt-hour of electricity from a coal-fi red or nuclear power station costs between four and fi ve eurocents on the European Energy Exchange. Michael Weinhold, Chief Technology Offi cer at Siemens Energy in Erlangen, reckons that

A door at the base of the steel tower provides access to an elevator which can take service technicians up to the nacelle in a matter of minutes.

A majestic sight: A giant wind turbine towers over fields and grassland in north-west Denmark on the North Sea coast.


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WIND POWER

The countries that border the North Sea and the Baltic Sea have big plans to generate electricity in offshore wind farms. The first offshore wind farm went into operation off the south-east coast of Denmark in 1991. Today, Germany and Great Britain are leading the way in the development of offshore wind turbines. Sweden, Spain, Norway and France are also planning numerous wind farms out at sea.

WIND POWER HEADS OFFSHORE

electricity generated by wind turbines at good land-based sites could be available at approximately the same price in just a few years time. In the case of offshore wind farms, it could take slightly longer to achieve a price level that can compete with electricity from conventional power

plants. Yet Weinhold is confi dent that by 2020 even offshore wind power should be capable of producing electricity at a competitive cost.

In the meantime – largely thanks to government subsidies – the use of wind power is expanding in leaps and bounds,

not only in Europe, but also in Asia and North America. Over the last few years, China has become the world’s biggest market for wind power, representing al-most a quarter of total global installed capacity in 2010 with its more than 42 gigawatts of wind turbine installations.

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FLOATING TURBINES

Many parts of the world have long coastlines with lots of wind, but are still not suitable for building offshore wind power plants. In many such areas, the sea fl oor drops off so steeply that the water gets too deep just a few kilometers out – for example, off Norway, Japan, and the West Coast of the USA. One solution is fl oating wind turbines, whose towers are not set fi rmly on the sea fl oor in the usual way, but anchored to it by long steel cables. One technical concept for such fl oating systems has been developed by the Norwegian oil and gas conglomerate Statoil, working jointly with Siemens.

A fi rst full scale prototype of the “Hywind” has been undergoing a trial run since the fall of 2009, about 20 kilometers offshore from the coast near the southern Norwegian city of Stavanger. The unit comprises a tower that rises 65 meters to the hub, supporting a Siemens wind turbine with a capacity of 2.3 megawatts. As a counter-weight to the gondola, the tower, and the three rotor blades, a steel cylinder fi lles with ballast of water and extends almost 100 meters below the water’s surface. Three steel mooring lines hold the turbine fi rmly in place in 200 meters of water. To keep the system stable in the swell, the engineers included a stabilizer system: sensors measure the water movement and an electronic control calibrates the fl oating turbine so that it always remains stable even in high seas. “Stabilizing the unit is the biggest technical challenge in Hywind,” says Kristin Aamondt, a Project Manager at Statoil Wind Energy in Stavanger.

The trial run of more than two years was almost trouble-free, and quite promising. The turbine ran almost without downtime, and supplied signifi cantly more electricity than the Statoil experts had expected. In 2011, Hywind produced over 10 GW hours of electricity energy, or the equivalent to power over 600 Norwegian homes. Statoil is assessing locations for developing a small pilot park of 3-5 turbines which would test the next phase of the concept, .equipped with higher-power turbines. “From depths of about 30 meters on out, it’s most likely going to be cheaper to build a fl oating turbine than one standing on the sea fl oor,” Aamondt says. Floating wind turbines are technically feasible out to depths of about 700 meters – which will make it possible to tap vast additional potential from the power of the wind over the sea.

The USA is the second biggest market, with Germany in third place. In 2010, almost every second gigawatt of new capacity was installed in Chinese wind farms.

Currently, most new wind turbines are still being built on land, but offshore wind farms are also gradually picking up momentum in many parts of the world, including China. Current plans envisage the installation of wind turbines with a total capacity of 30 gigawatts in China’s coastal waters by 2020.

In contrast, Germany’s wind farms in the North and Baltic Seas are progres-sing slower than hoped. This is partly due to the cost, which is higher than in countries such as Denmark and Great Britain because the offshore locations are further from the coast. A further obstacle is the sluggish pace at which transmission lines are being installed to bring the electricity to land and transport it to the main centers of con-sumption (see p. 35, “Arteries for green power”).

Offshore wind farms can only be con-nected to the grid by installing current collectors at sea and laying cables, some of which may be several hundred kilo-meters long. Wind turbine manufactur-ers such as Siemens are braving harsh environments to carry out pioneering work in this fi eld. They plan to install high-tech wind turbines far out at sea – in some cases more than 100 kilometers from the coast – which are designed to generate power for decades under the most severe weather conditions. ■

20 turbines, 40 megawatts of installed capacity: When the Middelgrunden wind farm went into operation off the coast of Copenhagen in the year 2000, it was the largest offshore wind farm in the world. More recent projects are on a far bigger scale.

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