The wind turbines of the future swings on the sea waves instead of standing firmly on the sea floor. Because only a tiny fraction of the world's oceans is flat enough with less than 60 meters of water depth to anchor conventional offshore systems in the sea floor. Floating wind turbines, on the other hand, produce energy hundreds of miles beyond the flat coast room and thus open up inaccessible locations on the open sea. The potential is huge. Companies have already secured rights for systems with a total of ten gigawatts. Technically, however, this is more complicated than it appears at first glance. The interplay of wind, waves and wind turbine is complex - and different types of plant are competing for the best solutions. "This is a very young technology and not yet mature," says Christian Navid Nayeri from the TU Berlin.
However, Frank Adam, Head of Wind Engineering at GICON in Dresden, is already talking about a trend, because many countries have many – previously unbuildable – deep-water areas off their coasts. His company is currently developing a four-megawatt pilot plant for Asia. Above the water surface, the systems with names such as Nezzy, Hywind, FLOATGEN or WindFloat often do not have their special structure. The crucial thing happens underwater. Floating wind turbines need a counterweight for stability as well as anchors that fix them to the seabed. The latter theoretically works at sea depths of up to 1000 meters. For the time being, however, Adam only considers depths of 300 meters to be economically sensible. The more than arm-thick ropes that anchor a wind turbine to the seabed can weigh more than 100 tons. And the longer they are, the more expensive they make the wind turbines.
The first swimmers off the Scottish coast
The first floating wind farm called "Hywind Scotland" went online in October 2017 around 24 kilometers before the Scottish city of Aberdeen. Five wind turbines from the Danish company Vestas with a total output of 30 megawatts provide electric electricity into the network. Since in front of Scotland's coastal regions the sea floor is mostly very steep, the sea is otherwise rather unsuitable to ram the so -called monopiles into the ground on which normal wind turbines are located. That is why in December 2021 another floating wind farm off Scotland's coasts started operating. With 50 megawatts, the Kincardine wind farm is now one of the world's largest floating energy collectors at sea.
However, at the end of 2021, such plants were still leading a niche existence due to a lack of international interest. Just around 100 megawatts of power are installed on the oceans worldwide - a fraction of the fixed offshore wind turbines. However, since the beginning of 2022, the expansion of floating wind power seems to be accelerating. Numerous energy suppliers and mineral oil companies applied for the best places in the sea in the gigawatt range for floating wind turbines.
In January 2022, Shell, Iberdrola, BP, Vattenfall, Ørsted and Baywa secured the rights for the expansion of »Floating Offshore« systems with around 14.5 gigawatt nominal output alone off the Scottish coast. Before the Brittany in France, energy suppliers are planning as well as before Spain and Norway's coast floating wind farms. There is also potential on the coasts and the Great Lakes in North America, in the Mediterranean and in the North Sea around the Faroe Islands and Ireland. Here, too, it goes down very quickly beyond the coast.
At the end of 2021, South Korea also announced that it would build a floating wind farm with a capacity of six gigawatts in the Sea of Japan off the city of Ulsan over the next ten years. The seabed here is also very deep off the coast. The mega swimming park will then supply almost six million households with renewable electricity and also produce green hydrogen.
In Germany, simulation and modeling is carried out
In Germany, the activities are more limited to participation in European research projects. Only once in the Baltic Sea, a reduced version of a floating wind turbine was left to water. In September 2020, the ENBW energy group, together with the wind power maker Aerodyyn, tested a prototype in scale 1: 10 of a 15-megawatt wind system called »Nezzy²« in the Greifswalder Bodden-within sight of the port.
There are understandable reasons for this reluctance: The German part of the North Sea is flat, so the classic pile dwelling is sufficient. Professor Jan Wenske from the Fraunhofer Institute for Wind Energy Systems (IWES) in Bremerhaven regrets this, because he also believes that "the potential is gigantic", also to export the technology once.
Walt Musial is driving the floating offshore activities of the USA. He heads the offshore wind research platform of the National Renewable Energy Laboratory (NREL) and estimates that four times as much wind as has been used so far can be used in marine areas with water depths of more than 60 meters. Here usually stronger and steady wind blows. But there are also other advantages. Not every country wants an "asparagus salad" in front of its most touristically beautiful coasts, says Nayeri, "but we also have much more degrees of freedom to harvest more energy". The floating wind farms can be built where there is a lot of wind, or they can be relocated depending on the season.
How to stop the swimmers in their place
Nayeri is coordinating the FLOATECH project, which the EU is funding with four million euros. The aim is to develop an industrial-grade design software in order to optimize the material selection and the design of the plants and to increase the efficiency of the turbines. The program is intended to investigate and simulate the complex interactions between the seabed, the tether and the tower with wind and waves – even at high sea swell of 20 meters and very strong wind.
Much is still unclear. So far, no design has prevailed for the structure that keeps such platforms stable. Sometimes it is a spar, as long as the tower itself, which serves as a counterweight under water, sometimes it is more the anchor ropes, sometimes a floating platform made of concrete - or sometimes two wind turbines that turn on a system and like this Stabilize each other. The GICON company has also developed its own version with preloaded anchor cables, which is kept stable by ballast weights on the ground and therefore fluctuates less in the waves.
So far, there is also no optimal solution for the material of such anchor cables. Walt Musial still presented ropes made of blue nylon at the Offshore Technology Conference in Houston 2013. But steel ropes are also tested that keep the connection to the sea floor. "We are still looking for the best material," says Christian Nayeri.
When the anchor forces dwindle
The anchor ropes not only have to endure a lot in the maritime environment, they also have an effect on the seabed. Professor Nils Goseberg explains that when real breakers hit the floating platform in high seas, it's like a bell being rung, the vibrations propagate in waves. These vibrations and the wave movements themselves put a strain on the anchors. If the anchor rope is very long, it can begin to swing like a guitar string, thereby pulling cyclically at the anchorage in the ground. In the NuLimas project, Professor Goseberg and project coordinator Christian Windt from the Leichtweiß Institute for Hydraulic Engineering (LWI) at the TU Braunschweig are investigating how the interaction of waves and the tower affects the seabed. With computer simulations and tests in the Large Wave Channel in Hanover, your consortium will answer the questions of whether and how to adapt the design of the anchors and the system to the respective type of soil.
The waves themselves also affect the anchor attachment. When they run along the water surface, Wellenberg and Valley are characterized as cyclical pressure fluctuations to the sea floor. If the soil is permeable, the water pushed into it flows out again. But if the pores are very fine and many waves work, this compensation no longer works. The pressure in the pores increases and destabilizes the sea floor. "Some floors will then become like dough, liquefy and the anchor forces can no longer be transferred," says Goseberg, head of the Hydromechanics, coastal engineering and Seebau Department at the LWI.
The coastal engineer is also familiar with the phenomenon of soil liquefaction of the seabed in other components in the sea, »even large structures such as breakwaters have sunk into the ground during storms«. In the worst case, the entire system can slip away. So the offshore wind power plant must be as stable as possible by means of a suitable design or with expansion joints, or the seabed at the anchor points must be reinforced with more stable material, says Goseberg. His goal is to have to shut down the wind turbine as rarely as possible due to bad weather, high seas or strong winds.
The technology is still very expensive
But waves, wind or doughy sea floors are not the main problem. The biggest hurdle is economy. "The things are just too expensive." According to Nayeri, it takes 10 to 15 years to make a profit. Walt Musial from the NREL estimates that the systems will only become profitable from 2028 to 2030.
The technology is still expensive: fitters who may have to be dropped off far in the sea by helicopter, the monitoring, the long tethers and power cables cost money. But there is a lot of potential to reduce costs. This is also one of the goals of the project in which Nayeri is conducting research, because "there is just the right amount of movement in there. The research is massively supported by the EU." This allows the wind turbines to be much closer together if they are controlled intelligently without taking the wind away from each other, and a radar can detect the incoming waves and optimize the operating parameters of the rotors. "A lot is being tried out right now," says Nayeri.
Wenske from the Iwes thinks even further and introduces itself to floating systems that drive in the west wind belt and do not produce any electrical stream, but equally green hydrogen. The proportion of floating wind turbines is still below 0.3 percent of the installed offshore capacities. The fact that the prices for natural gas are increasing due to the Ukraine War and thus the production of gray hydrogen made of methane is becoming increasingly expensive, the expansion and development of floating systems could give additional tailwind far outside at sea.
