Study of Coastal Effects Relevant for Offshore Wind Energy Using Spaceborne Synthetic Aperture Radar (SAR)

Abstract

Coastal wind speed gradients relevant for offshore windfarming are analysed based on synthetic aperture radar (SAR) data. The study concentrates on situations with offshore wind directions in the German Bight using SAR scenes from the European satellites Sentinel-1A and Sentinel-1B. High resolution wind fields at 10 m height are derived from the satellite data set and respective horizontal wind speed gradients are investigated up to about 170 km offshore. The wind speed gradients are classified according to their general shape with about 60% of the cases showing an overall increase of wind speeds with growing distance from the coast. About half of the remaining cases show an overall wind speed decrease and the other half a decrease with a subsequent increase at larger distances from the coast. An empirical model is fitted to the horizontal wind speed gradients, which has three main parameters, namely, the wind speed over land, the equilibrium wind speed over sea far offshore, and a characteristic adjustment length scale. For the cases with overall wind speed increase, a mean absolute difference of about 2.6 m/s is found between wind speeds over land and wind speeds far offshore. The mean normalised wind speed increase with respect to the land conditions is estimated as 40%. In terms of wind power density at 10 m height this corresponds to an absolute average growth by 0.3 kW/m2 and a normalised increase by 160%. The distance over which the wind speed grows to 95% of the maximum wind speed shows large variations with maximum above 170 km and a mean of 67 km. The impact of the atmospheric boundary layer stability on horizontal wind speed gradients is investigated using additional information on air and sea temperature differences. The absolute SAR-derived wind speed increase offshore is usually higher in unstable situations and the respective adjustment distance is shorter. Furthermore, we have found atypical cases with a wind speed decrease offshore to be often connected to stable atmospheric conditions. A particular low-level jet (LLJ) situation is analysed in more detail using vertical wind speed profiles from a wind LIDAR system.

The Atmospheric Stability Dependence of Far Wakes on the Power Output of Downstream Wind Farms

“…The ASM results (dashed lines) seem to have very tentatively captured the wake effect as evident from the z/L = 1.5 line (yellow) but this is quite speculative at this stage. The ASM energy ratios are lower than the SCADA energy ratios because we know there is wind speed from the coast of up to 35% as documented previously [36,48]. Hence, when modelling such large-scale coastal areas, long-range wakes are probably of lesser significance with accounting for the correct flow inhomogeneity [31], which is why these speculative results are included here in the discussion.…”

“…Much modelling work has been invested in demonstrating the importance of long wakes from offshore wind farms [32,33], particularly in stable conditions [34] which probably extend many tens of kilometres downstream. Measurements of hub wind speed using a number of different techniques such as with lidar [10], aircraft [35], and satellite observations [36] have demonstrated the stability dependence of far wakes on offshore flow [37], but to what extent can we see these effects in the wind power measurements of turbines? From the perspective of an operator, the satellite images and modelling studies of long wakes may be merely interesting until hard and persistent evidence of an effect on turbine power can be demonstrated.…”

Fit-for-Purpose Information for Offshore Wind Farming Applications—Part-II: Gap Analysis and Recommendations

“…The atmosphere and ocean dynamics around OWFs is characterized by a strong coupling of these different spatial scales. For example, the presence of the adjacent land has an impact on the land/sea wind speed gradients [126,127]. The length of atmospheric wakes can extend up to 100 km downstream and the impacts on waves can reach even farther.…”

Wind Lidar and Radiosonde Measurements of Low-Level Jets in Coastal Areas of the German Bight

“…The data sets for this study were recorded using ground-based pulsed Doppler wind lidar systems of the types WindCube WLS8-8, WindCube v1, and WindCube v2, developed and manufactured by the former French company Leosphere (now Vaisala). The pulsed laser infers the horizontal wind speed from so-called line-of-sight (LOS) or radial wind speeds V r using a four-Doppler-beam-swing (DBS) scan [54,55]. An internal optical switch steers the laser beam at four azimuth angles in successive intervals of 90 • (i.e., 0 • , 90 • , 180 • , and 270 • ) to form a conical scan with a fixed elevation angle with respect to the horizontal.…”