Research into offshore wind power: Where the wind blows
Using satellite radar images, scientists at the Helmholtz-Zentrum Geesthacht (HZG) are investigating how wind turbines in the German Bight are modifying the wind conditions in the surrounding area. Radar technology has shown these effects up to 70 kilometres behind the installations. Results that may be significant for the optimal siting and operation of future plants.
Photo: Fabian Schmidt/Fotolia
More than 1,000 wind turbines are already in operation in the German Bight and the Baltic Sea. According to plans set out by the German government, the capacity of offshore parks is to be expanded to a total of 15 gigawatt by 2030. HZG coastal researchers are therefore evaluating satellite radar images of the German North Sea in the framework of the "Wind Park Fernfeld – WIPAFF” project to collect data that may be used to support the planning of new facilities in the future.
Dr Johannes Schulz-Stellenfleth, coastal researcher at the Helmholtz-Zentrum Geesthacht and one of the authors of the study published in the Journal of Renewable and Sustainable Energy, says on the subject: “More than 2,500 wind turbines are to be installed in German waters by 2030 due to further expansion. To help minimising shadowing effects, we are analysing satellite images, which can contribute to improved model simulations of the changed wind conditions."
Wind conditions in the North Sea
Radar image taken by the Sentinel-1 satellite (ESA) above the Alpha Ventus wind farm from 2015. The bright dots visible in the magnification are the individual wind turbines in the wind farm. Dark areas indicate a smooth sea surface. Areas with higher wind speeds and resulting rougher sea show up as as brighter areas. Source: B. Djath & J. Schulz-Stellenfleth/HZG
In their work, the Geesthacht coastal researchers use radar images provided by the Earth observation satellites Sentinel-1 and TerraSar-X. The radar measures the small scale roughness (ripples of a few centimetre length) of the North Sea’s surface. Since waves in the North Sea are mainly caused by wind, the images allow conclusions to be drawn about wind conditions. Even up to as much as 70 kilometres behind the turbines of the offshore wind farms, the scientists have been able to detect large-scale wakes. The wind speeds in these areas are lower than in undisturbed areas and the energy yield is reduced as a result. This is because the upstream wind turbines act as an obstacle to the wind and slow it down.
Optimise positions for wind farms
Comparisons with meteorological measurements at the research platform FINO-1, which is located 50 kilometres off the island of Borkum in direct proximity to the Alpha Ventus wind farm, have also shown that the length of the wakes depends on the stability in the atmosphere. For example, in a situation where the water is warm and the air above it is cold, this creates a situation like in a pot with boiling water. The wakes are then “mixed away” by the turbulence relatively quickly and, as a result, are shorter.
However, wind turbines do not always lead to a reduction of the water surface roughness. Within the first ten kilometres behind the wind turbines, strong radar signals even indicated increased roughness.
“We think this amplifying effect is caused by mechanically generated turbulence directly behind the turbines. These new findings can help us to optimise positions for wind farms in future planning," explains Dr Johannes Schulz-Stellenfleth.
The research project "Wind Park Fernfeld – WIPAFF" is funded by the Federal Ministry of Economics and Energy. In addition to the HZG, the network partners include the Karlsruhe Institute of Technology (KIT), the Technical University Braunschweig, UL International GmbH, and the Eberhard Karls University Tübingen.
“Impact of atmospheric stability on X-band and C-band Synthetic Aperture Radar imagery of offshore windpark wakes”, 10, 043301 (2018); doi: 10.1063/1.5020437
Further information on the research area offshore wind farms at the Institute for Coastal Research: Offshore Windparks
Helmholtz-Zentrum Geesthacht, Institute of Coastal Research, Hydrodynamics and Data Assimilation Unit
Phone: +49 (0)4152 87-1511E-mail contact
Helmholtz-Zentrum Geesthacht, Public Relations
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