It was a sensation 25 years ago when the world’s first big offshore wind farm was built off the coast of the Danish island of Lolland. Eleven wind turbines were arranged next to one another in water several meters deep. There had never been anything like it before in the same magnitude. Since then, offshore wind energy has become established worldwide. In 2014, according to the European Wind Energy Association, there were several thousand offshore wind turbines installed around the world, particularly off the coast of Great Britain, and their total rated capacity was 8,795 megawatts. Of course, compared with the capacity installed on land, that is still a reasonably small amount. For instance, the German state of Lower Saxony currently has approximately 8,500 megawatts of capacity. But if you consider how much offshore wind energy has grown recently, it becomes clear that it is on a trajectory to becoming a dominant source of power. In 2011, there were no more than 4,117 megawatts installed at sea worldwide. Offshore capacity has thus more than doubled between 2011 and 2014. The reason is that the wind is usually stronger and more consistent at sea than on land.
Sustainable Power Generation
Wind without End
Floating wind farms will one day supply power wherever the sea is too deep for farms built on piles — places such as the coasts of France, Portugal, Spain, Japan, and California. In collaboration with engineers from Norway’s Statoil, Siemens is studying the technical challenges involved in creating such systems. The companies are now building the world’s first floating wind farm off the coast of Scotland. Beginning in late 2017, five 6-megawatt wind turbines will generate enough power to supply some 20,000 households.
However, it is also true that costs for offshore wind farms rise in proportion to the depth at which their foundations must be anchored – particularly once a depth of 50 meters has been exceeded. Therefore, in order to exploit maritime regions with deep waters, planners intend to use floating wind turbines that are held in place with steel cables, chains and anchors on the sea floor.
On the whole, these floating offshore wind turbines have a number of points in their favor. Wind power expert Jochen Bard from the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES) in Kassel, Germany estimates that floating platforms could increase the potential of offshore power generation by a factor of 2 to 2.5 in Europe if a depth of just 100 meters were practical. If the floating offshore sector were expanded, the systems could be built in large numbers and thus at lower cost. One major advantage of floating wind farms is that they can be completely pre-assembled on quayside and then towed to an operating site at sea. That saves time and money. By contrast, anchoring the foundations for stationary offshore wind farms is comparatively expensive beyond a depth of 50 to 60 meters. Floating farms also have the advantage that in many cases they can be placed at sites with better wind conditions than those used for conventional farms.
In order to test this technology, Siemens and Statoil Wind Limited (SWL), a subsidiary of Norwegian energy company Statoil, erected the world’s first floating wind turbine off the coast of Norway in 2009. Since then, the turbine has been feeding its power into the Norwegian grid via an undersea cable. Statoil supplied the turbine’s anchoring system. The tower and turbine came from Siemens. The project was called “Hywind Demo” and originally had a design life of only five years. But despite the rough Norwegian sea, the wind turbine continues to work reliably, and so it is still in operation.
Inspired by this success, SWL and Siemens will this year begin building the world’s first floating wind farm. By the end of 2017, five floating wind turbines will be installed approximately 25 kilometers off the coast of the city of Petershead at the northeast tip of Scotland, and are expected to generate 135 gigawatt-hours of electricity per year – enough to supply 20,000 households. With this “Hywind Scotland” project, SWL wants to determine to what extent the costs of floating wind power stations can be reduced by manufacturing a larger number of units, producing and anchoring them in series, and achieving a high rate of utilization of associated transport and supply ships. In addition, there are technical issues to clarify, such as the degree to which floating wind towers influence or interfere with one another as a result of the wake produced by nearby turbines.
It is crucial that the turbines move as little as possible despite the waves, wind, and currents. The reason for this is that any tilting or swaying leads to severe stress on the nacelle, rotor, and generator. As in the Hywind prototype, SWL is using what it calls a “spar buoy” design in Scotland: an undersea float of steel and concrete with ballast tanks, which is moored to heavy anchors on the sea floor with steel cables.
Whereas the original system off the coast of Norway was equipped with a 2.3 MW generator, Siemens is now supplying larger, 6 MW turbines to Scotland. “The 6 MW system is more than just a scaled-up version of the small turbines,” says Jesper Møller, head of the Offshore Concepts & Solutions unit at Siemens Wind Power in the Danish town of Brande. “We’ve optimized the design and material of the turbines and the tower, so that this large system isn’t too heavy and doesn’t put too much stress on the structure.” Engineers have also equipped the turbines with many sophisticated systems designed to automatically reduce loads during operation. All of this is expected to have a positive effect on the service life of the new windmills’ components.
SWL experts expect that floating wind power stations have the potential to reduce construction costs in comparison with bottom-fixed wind turbines, especially at water depths of several hundred meters. Møller believes that there are promising regions along the Pacific coast of the U.S. and off the coast of Japan, which has hardly any shallow marine areas. There are also deep areas off the Spanish, Portuguese and French coasts where floating offshore wind facilities are likely to be feasible. There is currently no other project comparable to Hywind Scotland apart from two pilot wind turbine installations in Japan. “Our technology, on the other hand, can already be viewed as being at the pre-commercial stage,” says Møller. “No one else has come this far.”