Offshore wind turbines swirl water and air

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From ESKP (Earth System Knowledge Platform)

Offshore wind turbines stand in the constantly moving seawater and in the wind. There they take energy from the environment. But what does this do to the environment, to the air and water in their immediate and wider surroundings?

Fig. 1: View from the research vessel “Ludwig Prandtl” of an offshore wind farm in the North Sea. (Photo: Anna Ebeling/HZG)

Offshore wind turbines cause changes in their environment, both in the surrounding air and in the water. They form obstacles to the air and water that flow around them. And as befits their raison d’être, they take energy from the system. What are the consequences of these impacts for the environment, for air and water in their immediate and wider surroundings? How do they work beyond the physical effects, e.g. on biological processes? This is exactly what is currently being investigated at the Helmholtz-Zentrum Geesthacht (HZG) in cooperation with partners from science and industry.

Wind turbines act like large whisks

The air and water are often stratified. Warmer water is then above a colder, more nutrient-rich layer, for example. Wind turbines can act like large whisks and disrupt this stratification. Studying this influence on stratification and the development of turbulence is important – and a prerequisite for a better understanding of the influence of wind turbines on plants and animals, the food web and ecosystem functions. This will also make it possible to better assess the possible effects on ecosystems and their components.

And there is another interesting and important aspect of this great “

“: The individual wind turbines and also the wind farms as such also influence each other. So there are so-called shading effects. Over what distances do such effects play a role in the air (atmosphere) and in the water? What influence does this have on sedimentation processes, for example? On the ecosystems? Or on the possible energy yield? These are questions that are also important in the construction and maintenance of wind turbines and especially in the planned further expansion of offshore wind energy.

The offshore wind power situation and future expansion targets …

Offshore wind energy is an essential component of Germany’s energy and climate policy. The wind blows stronger and more constant over the sea than over the land. The rotors of offshore wind turbines therefore rotate (almost) every day and supply a large amount of electricity, reaching almost twice as many full-load hours as onshore wind turbines (Knorr et al., 2017).

The North Sea has a high wind potential, so that the intensive expansion of wind energy use is economically worthwhile. The relatively shallow water depth also plays a role here, which allows wind turbines to be installed on the seabed in many areas. Ambitious expansion targets have been formulated for Germany: Wind turbines with a capacity of 2020,7 MW have been installed in German waters by the end of 760 and 2030,20 MW of wind energy capacity are to be built by 000. In Germany, the majority of offshore wind farms are built in the exclusive economic zone (EEZ, coastal distance 12 to 200 nautical miles) and outside the 12 nautical mile zone. The Federal Maritime and Hydrographic Agency (BSH) is responsible for the approval of the installations in the EEZ and the respective coastal federal state is responsible for installations within the 12 nautical mile zone.

Since the usable area in the German Bight is limited, the wind farms are built in groups, so-called clusters, of up to several hundred wind turbines. With the increasing expansion of offshore wind energy in a certain area, the interactions through shading effects between different wind turbines and especially between different wind farms play an increasing role.

… bring interactions between the wind turbines and thus new questions for research

The large-scale expansion and operation of offshore wind turbines in coastal waters brings new challenges. As a result, there are always new questions about current and future environmental conditions – on the one hand in the context of planning, construction and operation of the plants and on the other hand about their direct and indirect influences on the environment and ecosystems.

Here, the focus will be on the physical aspects: Wind turbines and entire wind farms influence atmospheric and oceanic processes. Turbulence and wake vortices occur in the surrounding air and seawater. Questions here are, for example: What effects do wake vortices have on the environment and what role do possible interactions between individual wind turbines that stand together in a cluster play? How do wind farms influence each other and how do they possibly affect the local climate? And last but not least: How do atmospheric and oceanic processes interact with each other? Due to the intensive expansion, it is necessary, but also possible, to investigate these effects directly on site. The dynamics are extremely complex and are influenced by various factors in the atmosphere, but also by properties of the water surface.

The wind yield can be reduced by wake vortices in the lee of wind turbines and wind farms (“windward” = side facing the wind, “leeward” = side facing downwind). In order to estimate the economic potential of planned wind farms, the wind industry is therefore very interested in the analysis of wake vortices.

The Institute of Coastal Research at the Helmholtz-Zentrum Geesthacht (HZG) conducts research on a wide range of issues in the field of offshore wind energy use (Fig. 1). Questions about turbulence and wake vortices are investigated in an interdisciplinary approach. The turbulence researchers led by Jeff Carpenter investigate, for example, small-scale flowing, mixing and transport processes in the ocean by means of direct measurements with ocean gliders and theoretical models. Using measurement data and numerical models, the department led by Joanna Staneva and Johannes Schulz-Stellenfleth is investigating the wind fields behind large wind farms on a large scale. And together with the radar hydrographers (working group led by Jochen Horstmann), they will try to track down the interactions between wake vortices and swell in the future.

Wind turbines swirl the air in the lee

In the slipstream behind individual wind turbines and also behind entire wind farms, so-called wake vortices (wakes, turbulence drags, English wakes) occur. In these wake vortices, there is a lower wind speed, changed pressure conditions and increased turbulence. Such on-carriage can be many kilometers long. They can be observed from the aircraft and are also visible from space (Fig. 2a) and on images from ship-based radars (Fig. 2b) (Eschenbach & Horstmann, 2019).

Fig. 2a: Wake vortices of individual wind turbines in an offshore wind farm are clearly visible from the satellite. (Photo: ESA/HZG/Kerstin Heymann, Martin Hieronymi)

Fig. 2b: The wake vortices of individual wind turbines can also be clearly seen on radar. (Photo: HZG/Jochen Horstmann)

Measurement of (atmospheric) wake vortices

Under offshore conditions over the sea, wake vortices are generally longer than over land (due to the lower “roughness”). In addition to the wind, the size of the wind farm as well as the height and arrangement of the wind turbines play an essential role in the extent of the resulting wake vortices. But there is another important factor that influences the expansion: air stratification, or more precisely the conditions in the atmospheric boundary layer. The atmospheric boundary layer is the lower part of the Earth’s atmosphere (up to about 2000 m thick), which borders the Earth’s surface and is directly influenced by it, for example by temperature, humidity and friction on the ground. The wind turbines protrude into this atmospheric boundary layer and interact with it, the processes are particularly challenging in terms of numerical modelling and measurements, especially due to the strong influence of turbulence.

Better understanding of wake vortices in the air

Offshore wind turbines extract energy and momentum (“momentum”) from the wind field. In the lee of the turbines, a wake vortex is created with lower wind speed (wind speed deficits of up to 20% were measured) and higher turbulence. At some distance from the wind turbine, the wake vortex gradually dissipates and the conditions adapt to the ambient conditions. This approximation (“diffusion”) takes place more quickly when the ambient air is also already turbulent. Therefore, wake vortices are less pronounced in unstable air stratification and at higher wind speeds.

For the conditions in the German Bight, for example, the horizontal and vertical structure of the atmospheric boundary layer is strongly influenced by the coastline. When the wind blows seaward from the land and warmer layers of air come to lie over the colder sea surface, a stable stratification is created offshore. A situation that is often observed in spring and summer. Such stable stratification is a favorable condition for the formation of extensive wake vortices. For wake vortices of individual wind farms, up to 32 km in length were measured, with superposition of the effects of different wind farms, the length was up to 72 km or even more than 100 km (Djath et al., 2018; Djath & Stellenfleth, 2019). If, on the other hand, unstable stratification prevails – with cold air, often coming from the northwest, over a warmer sea surface – the wake vortices are weakened and shorter. In winter, wake vortices are therefore less pronounced. These regularities not only occur over the course of the year, but can even be observed during the course of the day: In the evening, stratification is often more stable than in the morning (due to the air heated during the day) and the probability of wake vortices is therefore somewhat higher (Djath & Stellenfleth, 2019).

Wind turbines also stir the water

Fig. 3: View from the aircraft of a wind farm, some of which is still under construction. On the surface of the water, an effect of the individual pillars is clearly visible. (Photo: HZG/Sabine Billerbeck)

In the vicinity of offshore wind farms, however, there are not only large-scale changes in the wind field, but also changes in the water body (Fig. 3). The wind farms also provide more turbulence in the water. The pillars of the wind turbines interact with the natural flow. The water in the North Sea is naturally always in motion due to wind and tidal forces: it typically circulates counterclockwise over a large area in the North Sea and the German Bight. Tides, wind and freshwater input from the rivers also play an essential role. During low tide, for example, the water now runs off between the pillars of the wind farms and rises again during high tide. The water sloshes back and forth between the pillars of the wind farms in the German Bight.

So far, little is known about how the wind farms influence hydrodynamics, i.e. the movement of the water and the forces at work. In any case, stratification and turbulence also play a decisive role here.

The reason for the increasing importance of this question is that wind turbines are now also built in areas of greater water depth, where the water is stratified in summer. During the warm season, for example, in the northwestern German Bight at water depths of more than 25-30 m, (thermal) stratification forms due to solar radiation: The upper areas are then 5-10 °C warmer. Flatter areas are usually mixed even in summer. “Natural” turbulence, which destroys the stratification, is caused by wind at the surface of the water and by shear forces at the bottom. Now, however, wind turbines are also getting involved. The tidal currents – the sloshing back and forth, so to speak – cause increased turbulence on the foundation structures of the wind turbines. This can cause changes in stratification, and thus change the vertical distribution of temperatures, nutrients, oxygen and suspended solids – and influence biology and ecology.

The stirring effect affects the water stratification …

Using measurement data and simulations with computer models, the turbulence researchers determined the impact of the structures of wind turbines (Carpenter et al., 2016). The range of mixing effects is large and depends, for example, on the depth of the water and the strength of the tides. As part of measurement campaigns, significant turbulence was detected in the lee of wind turbines in the wake vortex, while the water in the surrounding area was stratified. The turbulence caused by a wind farm is clearly recognizable compared to the natural state unaffected by wind turbines (significant, for the site studied, Schultze et al., 2017). Computer simulations confirm that intensive expansion of wind turbines in the German Bight could significantly influence the large-scale stratification regime and stratification dynamics over the course of the season (Schultze et al., 2020). The current expansion situation has so far had very little influence on a larger spatial scale.

… and sedimentation dynamics

The turbulent kinetic energy in the turbulences of the water triggered by the pillars also amplifies the mobilization of sediment from the seafloor (Grashorn & Stanev, 2016). Satellite images show such sedimentary wake vortices in the lee of wind turbines in coastal waters. They can also be observed with measuring instruments in situ (for example with ADCP).

Sedimentation is characteristic of coastal waters and the dynamics are strongly dependent on turbulence. In the Wadden Sea, sedimentation dynamics are also important because of its status as a UNESCO World Heritage Site.

Summary and outlook: Recognizing and understanding connections

The use of wind energy over the sea has great potential, but also brings many new challenges and questions. Here, it was highlighted that and how turbulence forms in the lee of individual wind turbines and entire wind farms, both in the air and in the water. By combining and integrating data measured in situ with satellite data and model simulations, it is possible to explore, quantify and visualize wake vortices over large areas with high spatial resolution.

In most weather situations, local wake vortices form in the air within wind farms. Atmospheric stability plays an essential role in the formation of wake vortices and can significantly increase their length by a factor of 3 or more. Based on the expanses of tens of kilometers found alone, it can be assumed that the wind yield can be reduced by wake vortices from wind turbines and wind farms. This is an important finding for the estimation of the wind harvest and the economic yield of (planned) wind farms as well as for the optimized planning of future offshore wind farms. The new findings can then also be taken into account in the engineering models (Cañadillas et al., 2020). New research enables the automated analysis of large data sets, for example to better compare the extensive measurement data from different studies.

Turbulence zones also form in the water behind wind turbines. It is important to better understand the evolution of stratification under the additional influence of wind farms. In this way, questions about the locally different influence of wind farms on natural processes can be addressed. In the future, the effects of different stratification and flow conditions as well as volatile tidal effects will be investigated in more detail with the help of extensive computer simulations. Then the effects on biological processes can also be better estimated.

In November 2019, the new research project “Interaction of the wakes of large offshore wind farms and wind farm clusters with the marine atmospheric boundary layer” (X-Wakes) was launched. By combining measured data (in situ, satellite, aircraft measurement campaigns) with modelling, it will be examined in more detail how the wind farm clusters influence each other and what effects a large-scale expansion of offshore wind farms will have on future wind conditions. The yields of the wind farms are to be predicted under realistic conditions for future expansion scenarios.

An exciting question and task of future research projects is the investigation of coupling, i.e. the interdependencies of processes in air and water. How exactly do the current, the swell and the wind field influence each other? How does the swell change in the turbulence drags in the slipstream of wind turbines and entire wind farms? Even without the influence of wind farms, coupling processes are already extremely complex and become a particularly exciting challenge for scientists from different disciplines due to the additional interactions of the turbines with the boundary layer. Research in the field of process coupling is of broad interest and applicable in many respects, for example for studies on climate change.

Text: Dr. Christiane Eschenbach (Helmholtz-Zentrum Geesthacht | HZG)

References

  Cañadillas, B., Foreman, R., Barth, V., Siedersleben, S., Lampert, A., Platis, A,. Djath, B., Schulz-Stellenfleth, J., Bange, J., Emeis, S. & Neumann, T. (2020). Offshore wind farm wake recovery: Airborne measurements and its representation in engineering models. Wind Energy23(5). 1249-1265. doi:10.1002/we.2484

  Carpenter, J. R., Merckelbach, L., Callies, U., Clark, S., Gaslikova, L. & Baschek, B. (2016). Potential Impacts of Offshore Wind Farms on North Sea Stratification. PLoS ONE11(8):e0160830. doi:10.1371/journal.pone.0160830

  Djath, B. & Schulz-Stellenfleth, J. (2019). Wind speed deficits downstream offshore wind farms – A new automised estimation technique based on satellite synthetic aperture radar data. Meteorological Journal, 28(6), 499-515. doi:10.1127/metz/2019/0992

  Djath, B., Schulz-Stellenfleth, J. & Cañadillas, B. (2018). Impact of atmospheric stability on X-band and C-band synthetic aperture radar imagery of offshore windpark wakes. Journal of Renewable and Sustainable Energy, 10(4):043301. doi:10.1063/1.5020437

  Eschenbach, C. & Horstmann, J. (2019, April 15). Predict wind gusts in the short term. Earth System Knowledge Platform [eskp.de], 6. doi:10.48440/eskp.056

  Grashorn, S. and E.V. Stanev (2016) Kármán vortex and turbulent wake generation by wind park piles. Ocean Dynamics, 6612, pp 1543–1557. doi:10.1007/s10236-016-0995-2

  Schultze, L., Merckelbach, L., Raasch, S., Christiansen, N., Daewel, U., Schrum, C. & Carpenter, J. (2020). Turbulence in the wake of offshore wind farm foundations and its potential effects on mixing of stratified tidal shelf seas [Presentation]. Presented at Ocean Sciences Meeting 2020, San Diego, CA, US.

  Schultze, L. K. P., Merckelbach, L. & Carpenter, J. R. (2017). Turbulence and mixing in a shallow stratified shelf sea from underwater gliders. Journal of Geophysical Research – Oceans, 122(11), 9092-9109, doi:10.1002/2017JC012872

  Witsch, K. (2019, October 25). Offshore wind farms will permanently change the energy world. Handelsblatt [www.handelsblatt.com]. Accessed on 08.01.2021.

DOI
https://doi.org/10.48440/eskp.059

Published: 15.01.2021, Volume 8

Citation: Eschenbach, C. (2021, January 15). Offshore wind turbines swirl water and air. Earth System Knowledge Platform [eskp.de], 8. doi:10.48440/eskp.059

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