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| Titel |
Observation of high-resolution wind fields and offshore wind turbine wakes using TerraSAR-X imagery |
| VerfasserIn |
Tobias Gies, Sven Jacobsen, Susanne Lehner, Andrey Pleskachevsky |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250098359
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| Publikation (Nr.) |
 EGU/EGU2014-14029.pdf |
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| Zusammenfassung |
| 1. Introduction
Numerous large-scale offshore wind farms have been built in European waters and play
an important role in providing renewable energy. Therefore, knowledge of behavior of wakes,
induced by large wind turbines and their impact on wind power output is important. The
spatial variation of offshore wind turbine wake is very complex, depending on wind speed,
wind direction, ambient atmospheric turbulence and atmospheric stability. In this
study we demonstrate the application of X-band TerraSAR-X (TS-X) data with high
spatial resolution for studies on wind turbine wakes in the near and far field of the
offshore wind farm Alpha Ventus, located in the North Sea. Two cases which different
weather conditions and different wake pattern as observed in the TS-X image are
presented.
2. Methods
The space-borne synthetic aperture radar (SAR) is a unique sensor that provides
two-dimensional information on the ocean surface. Due to their high resolution, daylight and
weather independency and global coverage, SARs are particularly suitable for many ocean
and coastal applications. SAR images reveal wind variations on small scales and thus
represent a valuable means in detailed wind-field analysis. The general principle of imaging
turbine wakes is that the reduced wind speed downstream of offshore wind farms modulates
the sea surface roughness, which in turn changes the Normalized Radar Cross Section
(NRCS, denoted by Ïă0) in the SAR image and makes the wake visible. In this
study we present two cases at the offshore wind farm Alpha Ventus to investigate
turbine-induced wakes and the retrieved sea surface wind field. Using the wind streaks,
visible in the TS-X image and the shadow behind the offshore wind farm, induced by
turbine wake, the sea surface wind direction is derived and subsequently the sea
surface wind speed is calculated using the latest generation of wind field algorithm
XMOD2.
3. Case study alpha ventus
Alpha Ventus is located approximately 45 km from the coast of Borkum, Germany, and
consists of twelve 5-Megawatt wind power turbines. The retrieved results are validated
by comparing with QuikSCAT measurements, the results of the German Weather
Service (DWD) atmospheric model and in-situ measurements of wind speed and wind
direction, obtained from the research platform FiNO1, installed 400 m west of Alpha
Ventus.
4. Conclusion
In the presented case study we quantify the wake characteristics of wake length, wake
width, maximum velocity de?cit, wake merging and wake meandering. We show that SAR
has the capability to map the sea surface two-dimensionally in high spatial resolution which
provides a unique opportunity to observe spatial characteristics of offshore wind turbine
wakes. The SAR derived information can support offshore wind farming with respect to
optimal siting and design and help to estimate their effects on the environment. |
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