Energy question of the week: Can wind turbines also float on the open sea?

23. August 2010, 10.00, Jan Oliver Löfken, 2 Comment/s
The twelve offshore wind turbines that make up the 'alpha ventus' wind farm, 45 kilometres to the north of the North Sea island of Borkum, can generate sufficient electricity for 50,000 households. Like all other wind farms in the North Sea, these turbines are installed on firm foundations at water depths of between 30 and 50 metres. However, not all coastal countries have such a flat and shallow seabed immediately off their coastline. Might it not be a great deal simpler and less expensive to install wind turbines on floating platforms?

Despite the fact that the technical challenges faced by wind power plants installed on the seabed are enormous, wind turbine manufacturers and power companies are already hard at work on designs for floating wind turbine platforms. The Norwegian energy provider StatoilHydro and Siemens are currently obtaining important experience 12 kilometres southeast of the island of Karmøy, in Norway. In 2009, they installed a 2.3-megawatt facility with a rotor diameter of 82 metres on a floating steel platform filled with ballast. This platform extends 100 metres below the waterline and is fixed to the seabed at a depth of 220 metres with three steel anchor cables. Over the next few years, this practical test will demonstrate just how well this facility is able to withstand storms, and the extent to which fluctuations in wind speed affect the operation of this wind turbine.

floating wind turbine - Pilotprojekt von Siemens-Division Renewable Energy und des norwegischen Energiekonzerns StatoilHydro (Grafik), alle Bilder: Siemens.

Floating wind turbine – a pilot project by the Renewable Energy Division of Siemens and the Norwegian power company StatoilHydro (graphic). Credit for all images: Siemens.

Floating wind turbines with a generating capacity of five megawatts

The benefits of floating wind farms of this kind are obvious; they could be anchored off virtually every coastline in windy areas, located well away from shipping routes. Steel anchor cables should be able to hold them in place at water depths of up to about 700 metres. A feasibility study published recently by American researchers in the magazine Journal of Renewable and Sustainable Energy confirms that floating wind turbines on the open sea can indeed be sufficiently stable. With simulations and a model in a water tank, the team – headed by marine architect Dominique Roddier of the Californian company Marine Innovation & Technology – was able to demonstrate that tripod-based turbine towers on a floating platform are capable of providing sufficient stability for powerful five-megawatt generating facilities.

Based on these simulations, Roddier is currently working on plans for a full-size prototype. By autumn 2012, a floating wind turbine will be constructed off the Atlantic coast of Portugal in collaboration with the power company Energías de Portugal. Despite the fact that the costs, based on current estimates, are believed to be no higher than for offshore wind farms built directly onto the seabed, these floating power stations are unlikely to be installed further than 100 kilometres from the nearest coastline. This is because every additional kilometre adds to the cost of the undersea cable through which the electricity has to flow, at up to 110 kilovolts, to reach the power grid on land. Furthermore, even the most rugged of wind turbines require regular maintenance. For smaller assignments, technical staff can fly in by helicopter, but for any more extensive work, long nautical journeys are required.

Massive potential for all coastal countries

The effort could well be worthwhile; the potential for generating electricity using offshore wind farms is enormous. For example, the European Environment Agency, the EEA, estimates that by 2030, and only off the coastline of the EU member countries, wind power could be generating about 3400 terawatt-hours, representing about 80 percent of estimated requirements. The actual potential amounts to 30,000 terawatt hours, way in excess of the above figure. China, the countries of South America and the USA could also source a high proportion of their electricity from the open sea, by expanding their offshore wind farm capacities, using floating platforms where appropriate.

Further information:

Study on floating wind turbines: "WindFloat: A floating foundation for offshore wind turbines", Dominique Roddier, Christian Cermelli, Alexia Aubault & Alla Weinstein; J. Renewable Sustainable Energy, doi:10.1063/1.3435339

The DLR Energy question of the week in 'The future of energy' Year of Science

The Federal Ministry of Education and Research (BMBF) has given the Year of Science 2010 the motto 'The future of energy'. For this reason the science journalist Jan Oliver Löfken will this year answer a question on the subject of energy in his blog each week. Do you have a question about how our energy supply might look in the future? Or do you want to know, for example, how a wave power plant works and how it can efficiently generate electricity? Then send us your question by email. Science journalist Jan Oliver Löfken will investigate the answers and publish them each week in this blog.

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