Offshore wind turbines will float like a buoy and tilt with the wind

13. september 2022 kl. 16:41
Offshore wind turbines will float like a buoy and tilt with the wind
The Norwegian company World Wide Wind will develop floating offshore vertical axis wind turbines. Windmills will be up to 400 meters high and produce 40 MW. Illustration: World Wide Wind.
Floating 400-meter-high vertical axis turbines will bring the costs down and increase power production.
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The development of offshore wind turbines has, right from the early start with the Vindeby Offshore Wind Farm in 1991, been a matter of making the turbines as large as possible.

Apart from that, the wind turbines have resembled the ones we use on land; i.e. they had three blades and a horizontal shaft at the top.

The Norwegian company World Wide Wind (WWW) wants to change that with its new concept. The starting point is that modern wind turbines that are installed on floating foundations are heavy, rigid, and expensive to install.

So instead of using a rigid tower that needs to support the blades, shaft, and generator, the turbine will float like a buoy and tilt with the wind. The generator will be placed at the bottom and serve as a centre of gravity around which the turbine can rotate.

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On top of that, the tower will be equipped with two rotors that turn in different directions and function as two turbines. One will be mounted on the generator’s stator and the other on the rotor so that they practically double the effect.

According to WWW, the principle must be able to be scaled up to build turbines with a capacity of 40 MW and a height of 400 metres.

Behind WWW are a number of Norwegian energy professionals as well as Hans Bernhoff, a Swedish professor from Uppsala who has been developing vertical axis wind turbines for past 20 years.

A bearing at the bottom ensures that the turbines can tilt with the wind. Illustration: World Wide Wind.

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In an interview with the American media site New Atlas, Hans Bernhoff highlights several of the advantages of the principle. He acknowledges that there is little experience with vertical axis wind turbines and that they can rarely measure up to traditional wind turbines when it comes to efficiency. But he also says that there is great potential for development of the new type of turbines. He estimates that the material consumption for a 40 MW turbine will be approximately the same as for a traditional turbine, which is four times smaller.

Several different principles in play

Researchers that Ingeniøren has spoken to agree that floating wind turbines are interesting, as they will be able to utilize greater water depths than turbines that are firmly fixed to the seabed. Therefore, a number of different principles are being explored, of which no clear winner has yet emerged.

In order to make the best possible use of the wind, the turbine is equipped with two counter-rotating rotors. One is mounted on the stator of the generator, while the other is mounted on the rotor of the generator. Illustration: World Wide Wind.

Professor Christian Bak, head of DTU’s Wind Turbine Design Division, does not deny that the design from WWW may work:

“But I can also see a lot of challenges. Among other things, it’s about the many bearings that must be built into the turbine and how the bearings can handle the forces,” he says, pointing both to the bearings that must keep the forces from the two rotors separate and to the fixation to the seabed.

“Based on the visualisation, I would also assess that it's probably not the world’s most efficient rotor for absorbing the wind’s forces and producing energy. But perhaps one can compromise on energy output if other parts of the construction can be made cheaper and thus lower the total cost.”

Danish researchers have tested a similar project

A floating vertical axis wind turbine is nothing new, says Flemming Rasmussen, former head of division at DTU Wind with more than 40 years of experience in wind turbine design. He does not want to write off the Norwegian concept from the start, but he points to a number of problems that need to be solved.

This is due to, among other things, experience acquired from a previous DTU project, where the researchers looked at a floating turbine based on the vertical axis principle.

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The DTU project, called Deep Wind, which Flemming Rasmussen and his colleagues worked on, was a so-called Darrieus wind turbine concept and a smaller (1 kW) prototype was tested in Roskilde Fjord:

“One of the challenges is that vertical axis turbines generally have a fairly low energy output. I think that the WWW concept faces that challenge too. Another question is how they regulate the energy: should it be pitch- or stall-regulated, for example? In general, rather advanced aerodynamic calculations must be used to be able to analyse such a concept,” Flemming Rasmussen says.

A basic principle in wind turbine design is that the total energy output is proportional to the swept area of the blades. When it comes to the WWW design, both Christian Bak and Flemming Rasmussen estimate that it is probably quite a bit smaller than in traditional wind turbines. WWW also admits that the turbines will produce less “wake effect” than traditional turbines.

“This probably also confirms that the rotor is not as efficient as a traditional turbine. But in principle, the Norwegian design is probably feasible, but the question is whether the total costs, for example due to reduced material consumption, will be lower than those we see in floating wind turbines of similar size today,” Flemming Rasmussen says.

First prototype and investments

The Danish researchers generally point out that if the Norwegian concept is to be successful with a low power production price, then it must be achieved through less material consumption and cheaper installation than the wind turbines we have today in order to compensate for a less efficient turbine.

The people behind WWW have previously claimed that a fully developed turbine would be able to produce at a total cost of $50 per MWh, which is about half the expected price for offshore wind projects that will be ready in 2027, according to calculations by the U.S. Energy Information Administration (EIA).

The first step for WWW was testing a 400 W prototype. It is two meters high and was used for the first experiments with the counter-rotating generator. The next goal is to have a 3 MW turbine ready in 2026 and to be able to present a 40 MW turbine already in 2029.

For the previous experiments, WWW received investments worth approximately DKK 7.5 million. Subsequently, approximately DKK 37 million has been raised. But if the company is to continue with the development, an additional DKK 220-300 million will be required, CEO Trond Lutdal said to New Atlas.

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