Applications of satellite winds for the offshore wind farm site Anholt

Abstract

Abstract. Rapid growth in the offshore wind energy sector means more offshore wind farms are placed closer to each other and in the lee of large land masses. Synthetic aperture radar (SAR) offers maps of the wind speed offshore with high resolution over large areas. These can be used to detect horizontal wind speed gradients close to shore and wind farm wake effects. SAR observations have become much more available with the free and open-access data from European satellite missions through Copernicus. Examples of applications and tools for using large archives of SAR wind maps to aid offshore site assessment are few. The Anholt wind farm operated by the utility company Ørsted is located in coastal waters and experiences strong spatial variations in the mean wind speed. Wind speeds derived from the Supervisory Control And Data Acquisition (SCADA) system are available at the turbine locations for comparison with winds retrieved from SAR. The correlation is good, both for free-stream and waked conditions. Spatial wind speed variations along the rows of wind turbines derived from SAR wind maps prior to the wind farm construction agree well with information gathered by the SCADA system and a numerical weather prediction model. Wind farm wakes are detected by comparisons between images before and after the wind farm construction. SAR wind maps clearly show wakes for long and constant fetches but the wake effect is less pronounced for short and varying fetches. Our results suggest that SAR wind maps can support offshore wind energy site assessment by introducing observations in the early phases of wind farm projects.

Offshore wind farm wake recovery: Airborne measurements and its representation in engineering models

“…In recent years, however, a number of analytical and experimental studies, as well as numerical investigations, have revealed that offshore wakes may extend to several dozens of kilometers downstream, in particular when the wind farm is large and when the atmospheric stratification is stable. Particularly in coastal environments, such as the North Sea, airflow from the land on warm days brings warmer air over the cooler sea surface.…”

“…For example, meteorological masts, dual‐Doppler radar, and scanning‐Doppler lidar provide high‐resolution data but are limited by their fixed locations, restricting the measurement of wakes to certain wind directions and/or limited distances from the wind farm. Satellite‐based synthetic aperture radar (SAR) observations enable the measurement of wakes over larger areas, but the spatial resolution of the raw images limits the level of detail that can be observed. In contrast, direct aircraft observations combine the advantages of in situ measurements directly within the wake with a large spatial coverage, but at the cost of a relatively long sampling time.…”

Europe's offshore winds assessed with synthetic aperture radar, ASCAT and WRF

“…Methods to deal with few satellite samples include the hybrid method (Badger et al, 2010) and the gap-filling method during periods with a lack of data due to sea ice (Doubrawa et al, 2015). The adjustment for few samples and for uneven diurnal or seasonal sampling only makes sense to perform for local sites or regions (Ahsbahs et al, 2019) rather than for all the European seas. In the case meteorological observations are accessible, these can be useful for comparison and adjustment.…”

“…Satellite winds retrieved at 10 m height are averaged into wind resource statistics using the software for SAR-based wind resource assessment (Hasager et al, 2008(Hasager et al, , 2011Ahsbahs et al, 2019) and for ASCAT using the methodology presented in Karagali et al (2018b). Wind turbines offshore have hub heights at around 100 m height.…”

Wind Energy Instrumentation Atlas

“…The spatial resolution is around 1 km or less, and although it is a better resolution than what can be obtained using microwave scatterometers, it is still relatively coarse. A summary of the use of synthetic aperture radars for wind measurements can be found in Dagestad et al (2013) and a recent application at the Anholt wind farm is discussed by Ahsbahs et al (2018).…”