Offshore wind farm Barth II in the North Sea. -image: M. Krüger/HZG-
Large-Scale Offshore Wind Energy Development –
A Case for Coastal Research
During the last decade, the North Sea has experienced a considerable increase in offshore wind farm (OWF) development. At the end of 2016, the capacity in the North Sea has reached a total capacity of 9.1 GW with 44 wind farms and over 2700 individual piles. By 2020, the total offshore wind capacity is expected to be 24.6 GW. This rapid transformation of the southern North Sea into an energy seascape may compromise environmental development goals stipulated by national and international legislation and bears conflict potential with other natural resources, such as fisheries.
The Helmholtz-Zentrum Geesthacht, therefore examines physical and biogeochmical effects of OWFs on the North Sea as well as social and planning aspects - in particular in the German Exclusive Economic Zone of the North Sea.
- Wind farm planning
- Wind Gusts
- Sediment Wakes
- Ocean Turbulence
- Wind Wakes
- Biogeochemical Perturbations
- Carbon Stock
- Sociocultural Effects
Offshore wind farms (red dots) planned with coastDat data in the North Sea and Baltic Sea; e.g. in the exclusive economic zone (EEZ) of Germany (blue).
There is considerable interaction between offshore wind farms and the environment, providing challenges and limitations for their operability.
The latter has been addressed in several HZG studies using the coastDat database. For example, a climatology of wind energy over the North Sea was developed also considering potential synergies between different offshore wind farm arrays. In particular, met-ocean data from coastDat is used by nearly all planned and operated offshore wind farms in the German exclusive economic zone for optimization of design and logistics.
For futher information download the coastDat booklet.
B. Geyer, R. Weisse, P. Bisling, and J. Winterfeldt: Climatology of North Sea Wind Energy Derived from a Model Hindcast for 1958-2012, Journal of Wind Engineering and Industrial Aerodynamics, 147, S. 18–29. doi:10.1016/j.jweia.2015.09.005, 2015.
R. Weisse, P. Bisling, L. Gaslikova, B. Geyer, N. Groll, M. Hortamani, et al.: Climate Services for Marine Applications in Europe, Earth Perspectives, 2(3), doi: 0.1186/s40322-015-0029-0, 2015.
Surface radar retrieved wind fields (120 s apart in time) of a wind gust propagating towards the offshore platform Fino-3 in the German Bight. -image: HZG-
For an improved predictive control of offshore wind farms, HZG has developed a short-term (30 to 60 s) wind speed and gust prediction system. Wind fields are retrieved from marine radar image sequences to identify wind gust and estimate their size and propagation speed and direction.
For more information see project Wind vor Dan Tysk.
Snapshots for surface sediment concentration of suspended particle matter for a model basin with a depth of 21.6 m. -image: HZG-
To study vortices generated by the piles of offshore wind farms, an unstructured grid ocean model (SCHISM) was applied. It was shown that the model is able to reproduce suspended matter concentration features, which were previously observed in optical satellite imagery.
By introducing turbulence in the water offshore wind farm piles can have a significant influence on the sediment dynamics. Higher turbulent energy behind the piles causes increased vertical mixing and hence can lead to higher concentrations of sediment near the sea surface. This effect was successfully simulated using the unstructured grid model SCHISM run at HZG. The model is well suited for this application, because it can resolve small scale processes in the vicinity of the piles using a finer computational mesh in that region. The vortices of increased surface sediment concentrations shown in the image are driven by tidal currents typical for the German Bight.
S. Grashorn, and E. V. Stanev: Kármán Vortex and Turbulent Wake Generation by Wind Park Piles, Ocean Dynamics, 66:1543–1557, 2016, doi:10.1007/s10236-016-0995-2
Turbulent velocity field in the wake of a cylindrical offshore wind farm structure. This is snapshot from a Large Eddy Simulation looking from above on the water surface, with the current moving from left to right past the circular structure. Blue indicates a strong current, and red a weak current. -image: J. Carpenter, HZG-
Ocean turbulence is caused by the interaction of tidal currents and the OWF structures. It provides an additional mixing potential for the seasonal stratification that forms throughout large areas of the German EEZ. In this way, it is possible for large-scale OWF constructions to have a significant impact on North Sea stratification.
Current work is focused on understanding and quantifying the localised mixing process of a single OWF foundation using Large Eddy Simulations and in situ measurements. Initial results suggest that the low thermocline turbulence is expected to be enhanced by OWFs.
L.K.P. Schultze, L. Merckelbach, and J.R. Carpenter: Turbulence and Mixing in a Shallow Stratified Shelf Sea from Underwater Gliders, Journal of Geophysical Research -- Oceans, 122, doi:10.1002/2017JC012872, 2017.
J. Floeter, J.E.E. van Beusekom, D. Auch, U. Callies, J.R. Carpenter et al.: Pelagic Effects of Offshore Wind Farm Foundations in the Stratified North Sea, Progress in Oceanography, 156, 154–173, doi:10.1016/j.pocean.2017.07.003, 2017.
J.R. Carpenter, L. Merckelbach, U. Callies, S. Clark, L. Gaslikova, and B. Baschek: Potential Impacts of Offshore Wind Farms on North Sea Stratification, PLOS ONE, 11(8), e0160830, doi:10.1371/journal.pone.0160830, 2016.
Sentinel-1 SAR image showing the large-scale wind wake of OWF DanTysk (left) and a wind field retrieved from HZG’s marine radar in the area marked by the circle on the SAR image. -image: HZG-
Wind turbines extract momentum from the wind field and add turbulence at the same time. The resulting wakes are visible in radar remote sensing data and show the dependencies of the wakes on the wind mill's size and height as well as farm size; The wakes scale up to hundred kilometers and more.
By comparison of synthetic aperture radar (SAR) data, acquired by the SENTINEL-1 and TerraSAR-X satellites, with profile measurements taken at the FINO-1 research platform, it was shown that the wake length increases with atmospheric stability. Additional studies were performed to analyse the interaction of wakes originating from different wind parks in the German Bight and effects of single wind turbines on wind and friction velocity are observed with high spatial and temporal resolution using HZG marine radars showing strong wind speed shear zones across the individual wakes.
B. Djath, J. Schulz-Stellenfleth, and B. Canadillas, Impact of Atmospheric Stability on X-Band and C-Band Synthetic Aperture Radar Imagery of Offshore Windpark Wakes, Journal of Sustainable and Renewable Energy, under submission 2017.
R. Vicen-Bueno, J. Horstmann, E. Terril, T. de Paolo, and J. Dannenberg: Real-Time Ocean Wind Vector Retrieval from Marine Radar Image Sequences Acquired at Grazing Angle, J. Atmos. Oceanic Technol., Vol. 30, p. 127–139, doi: 10.1175/JTECH-D-12-00027.1, 2013.
Emeis, S., S. Siedersleben, A. Lampert, A. Platis, J. Bange, B. Djath, J. Schulz-Stellenfleth, and T. Neumann: Exploring the Wakes of Large Offshore Wind Farms. In /Journal of Physics: Conference Series/, 753:092014. IOP Publishing, 2016. doi:10.1088/1742-6596/753/9/092014
Initial results of mapping heavy metal sediment concentrations in the vicinity of wind farms and background reference areas in the German Bight. The stable isotope signature of heavy metals will clarify if wind farms are a source of contamination. -image: HZG-
The large-scale construction of offshore wind farms may have effects on the marine environment through introduction of new pollution sources. Examples are steel-reinforced foundations, scour prevention by geotextile sand containers, anti-corrosion measures (coatings, anodes) for pile protect, and activities related to regular maintenance.
Geotextiles used for scour protection around piles are permeable fabrics and mostly based on polyethylene or polypropylene. These fabrics typically contain plastic additives, such as plasticizers, UV-stabilizers, and flame retardants. Since additives are not chemically bound to the plastic material, they are leached by seawater and/or are accumulated by biota settling on the fabric after deployment.
Current research carried out in collaboration with the Federal Maritime and Hydrographic Agency also concentrates on possible pollution by sacrificial anodes and surface coatings used to protect offshore wind turbines against corrosion. Such investigations provide important information relevant for design specifications and the approval process for new wind farms by the Federal Maritime Agency.
Simulated changes of phytoplankton biomass expected from epistructural filtration by the blue mussel Mytilus edulis on newly created habitats on offshore wind farm piles. -image: HZG-
The build-up of offshore wind farms (OWF) provides new hard surfaces in the water column that is a preferred habitat of the blue mussel (Mytilus edulis). These blue mussels filter phytoplankton (algae) out of the water thus make the water clearer.
Computer simulations of the southern North Sea indicate how much clearer the water could be if all planned OWF piles were settled by blue mussels. An estimated 10% of the algae are removed by the filtration from these epistructural blue mussels in or near the wind farms, and up to 10% more algae are expected due to secondary effects, such as more nutrient availability leeward of the OWF areas.
K. Slavik, C. Lemmen, W. Zhang, O. Kerimoglu, K. Klingbeil, and K.W. Wirtz: The Large Scale Impact of Offshore Windfarm Structures on Pelagic Primary Production in the Southern North Sea., Hydrobiologia, submited 2017
Arguments mentioned by local residents in support of offshore wind farms along the Wadden Sea coast in Schleswig-Holstein.-image: HZG-
Surveys at the Wadden Sea coast of the Netherlands and Schleswig-Holstein, Germany, highlight arguments from local residents to argue in favor or against offshore wind farms. A comparison of these arguments with the respondent's personal values, beliefs, and perception of the area, reveal different patterns of argumentation requiring various communication strategies by planners and investors within planning and approval processes. Analyses of sociocultural perspectives of offshore wind farms thus support a better understanding of resistance, acceptance and support by residents and particular actor groups. Results are transposed by HZG researchers into methodological proposals to improve marine planning and management, such as by outlining a concept for recognizing culturally significant areas, and into international training in Maritime Spatial Planning.
K. Gee: Trade-offs Between Seascape and Offshore Wind Farming Values: An Analysis of Local Opinions Based on a Cognitive Belief Framework. Phd thesis University of Goettingen, p. 245, 2013.
K. Gee, A. Kannen, R. Adlam, C. Brooks, M. Chapman, R. Cormier, C. Fischer, S. Fletcher, M. Gubbins, R. Shucksmith, and R. Shellock: Identifying Culturally Significant Areas for Marine Spatial Planning, Ocean and Coastal Management, 136, pp. 139-147, doi:10.1016/j.ocecoaman.2016.11.026, 2017