Cologne Anyone who drives out to sea from the municipality of Karmøy on the west coast of Norway will come across a lonely wind turbine after about ten kilometers. In itself, an offshore facility on the open sea is nothing special. But this differs from conventional variants. The bright yellow, tetrahedron-shaped tubular steel construction on which the wind turbine stands is new.
It makes a decisive contribution to the plant being able to safely produce electricity in the 200 meter deep water.
“Tetraspar” is the name of the prototype of a floating wind turbine – referred to in technical circles as “floating” systems. They were designed by the Danish technology specialist Stiesdal.
Unlike wind turbines at shallower depths, Tetraspar is not attached to what are known as monopiles – long pipes that are rammed directly into the ground. The floating model stands on a floating foundation that is anchored to the ground with long ropes.
In addition to the German energy supplier RWE, Siemens Gamesa is also involved in the test in Norway – the Siemens Energy subsidiary built the wind turbine, which stands on the yellow steel construction and is now feeding electricity into the Norwegian grid.
Endurance test for pilot plants
“Through such demonstration projects, we learn, for example, how our turbine behaves on floating foundations and which processes work during installation under real conditions,” says Stephan Buller, Head of Offshore Portfolio Management & Floating Offshore Wind at Siemens Gamesa.
The expectations are high. Because floating wind turbines are considered to be technology of the future. Around 80 percent of the sea areas that are suitable for wind power are currently still unused because conventional offshore systems cannot be erected here. This is the result of a study by the Wind Europe association, an international association in the industry.
The expansion could be accelerated with floating systems. So far, floating wind turbines with a manageable total output of 100 megawatts are in operation all over Europe. By 2030, this should increase to ten gigawatts, according to Wind Europe’s goal. For comparison: Classic offshore wind farms already have around 28 gigawatts.
The “Floating” concepts are inspired by the drilling platforms of the oil and gas industry. The wind turbine stands on a platform that floats on the sea and is anchored to the ground with ropes. So far there are three types of floating foundations being used in pilot projects. In the Scottish “Hywind Park”, cylindrical floating bodies keep the turbines afloat by providing buoyancy like buoys.
In other projects such as Tetraspar, so-called semi-submersible boats are used, which are designed in a similar way to ship hulls. The third common variant is a platform with tension legs. Unlike buoys and semi-submersibles, the foundation of the wind turbine in this so-called tension-leg construction is not fixed with ropes that still have slack. The ropes are stretched taut, so the system is fixed.
It remains to be seen whether one of the procedures will become the standard. It is possible that different foundations and anchorages make sense depending on the region, weather conditions and soil type. Christian Navid Nayeri, head of the wind energy working group at the TU Berlin, sees constructions with loose anchoring cables as more sensible in areas with particularly high waves. “The flexible structure follows the waves and compensates for them,” explains Nayeri.
This is not the case with tightly tensioned anchorages – they could be damaged more quickly there. In any case, the buoy constructions in Hywind Park in Scotland have already proven that they can withstand waves of 19 meters in height.
Floating wind turbines currently cost around twice as much on average as conventional variants. Nevertheless, experts are convinced that they will prevail. A report by insurer Allianz predicts that the global offshore wind industry will grow at an annual rate of 12.3 percent between 2021 and 2026, also driven by new floating structures.
And RWE expects floating foundations to be competitive by 2030. The manufacturer Siemens Gamesa is also confident. “The systems are now becoming more and more powerful, which means that costs are falling,” says Buller. “Then the market will also grow.”
According to experts, rotor blades of up to 200 meters in length are conceivable for offshore wind turbines in the future.
(Photo: picture alliance)
At the same time, the manufacturers are working on increasing the yield of the systems. One of the tasks is to find the optimal distance between the wind turbines and the orientation of the turbines and rotor blades to the wind on the open sea. Christian Navid Nayeri and his working group are coordinating the “Floatech” project that is being researched. “The goal is that we take into account the movement that is generated by the floating of the system,” says Nayeri.
Sensors on the wind turbine measure the wind and waves and supply the data to the control software. “The rotor blades can then be individually adjusted so that more can be obtained from the wind,” says the wind power expert. At the same time, this would prevent incorrect loading and overloading of the rotors – which in turn reduces the need for repair and maintenance work.
Because they are more complex with floating systems than with offshore systems that are firmly anchored in the ground. For example, the special maintenance ships can only dock with the floating wind turbines when there is little swell.
Plastics replace steel
There is still a lot to be done in terms of fastening technology for the floating foundations. Currently, the ropes are mostly made of steel. It has the advantage that it is very resilient and can survive the life of an offshore system – around 20 to 30 years. At the same time, thanks to their mass, steel cables help to dampen the movements of the wind turbines caused by the waves.
“However, it remains to be seen whether plastic fasteners are so much cheaper to produce that replacing them every few years is more worthwhile than investing in a steel cable,” says expert Nayeri.
In order to reduce costs, some developers are now also experimenting more with the shape of the wind turbines. In the EU-funded project “PivotBuoy”, for example, the vertical windmill tower was tilted. The turbine is additionally supported by two struts, so the construct looks like a floating tetrahedron. This should reduce the costs for the platform and anchoring. This could halve the cost of power generation.
The Norwegian start-up World Wide Wind is also delivering a completely new design. Its concept is reminiscent of a power pole sticking out of the water at an angle. The rotors should turn vertically directly around this mast – instead of horizontally as with conventional wind turbines.
In order to stabilize the whole thing, two rotors per wind turbine work in opposite directions – and thus also produce twice the amount of electricity. So far, the concept only exists on paper, the first prototype should be ready in 2026.
In this project, the vertical wind turbine tower was tilted.
(Photo: X1 Wind)
The designers of classic wind turbines are also considering dual use, as with World Wide Wind, in order to make the platforms more economical. “In the future we could see two turbines on one platform,” says Christian Navid Nayeri.
Here, too, there are already initial tests – for example with the “Nezzy2” project, in which the energy supplier EnBW is involved. After a 1:10 scale model had already been successfully tested in the Baltic Sea, the double wind turbine is set to prove itself in its original size off the coast of China this year.
Rotor blades are getting longer
Another trick for more efficiency: The rotor blades could be longer on the open sea. This also ensures higher energy generation at the same rotational speed. Today, an offshore rotor blade is a good 100 meters long – that’s roughly the height of the towers of Munich’s Frauenkirche.
According to Nayeri, they could even grow up to 170 meters long. “On the sea we have the advantage that we don’t have any obstacles in our way and the rotor blades can reach quite deep to the water surface,” explains the wind power researcher.
More: Space for wind power is also scarce on the high seas