Comparing Abnormalities in Onshore and Offshore Vertical Wind Profiles

Abstract

Understanding the vertical wind profile is paramount for design & operation of wind turbines. It is needed not only for extrapolation of the wind velocity to hub height but also for structural load calculations, to name the most obvious issues.As wind turbines grow in size and development transitions offshore, issues such as shallow surface layers, low-level jets and internal boundary layers are raising questions to the applicability of the commonly used Monin-Obukhov similarity theory to accurately describe the vertical wind development to modern wind turbine hub heights. In this study the 10-minute averaged 5 vertical wind profile up to a minimum elevation of 100m is analyzed through measurements collected from seven sites which represent a span of conditions. Three sites are located offshore in the North/Baltic Sea with varying fetch, two onshore by the Norwegian coast, one further onshore by the Danish coast, and one is an inland forested site in Sweden. Through analysis of data series ranging from 8 months to several years depending on the site, the wind profile has been quantitatively categorized according to the number of exhibited local maxima which are not possible within Monin-Obukhov similarity theory. The 10 results reveal that the occurrence of local maxima scales inversely to the roughness length, causing 65−75% abnormal profiles offshore which decreases as the location transitions from offshore to coastal to further inland, and is lowest at the forested site.The results indicate that issues in predicting the vertical wind profile are most prevalent offshore, where very stable inflections cause severe deviations which may be related to an offshore internal boundary layer. These findings suggest that there is evident need of an improved vertical wind profile description in order to improve the accuracy of power predictions and load 15 calculations, especially at offshore and coastal sites.

Offshore wind farm global blockage measured with scanning lidar

“…Schneemann et al (2020) did not find signatures of the wakes of single wind turbines in the inflow of GT I. Nygaard and Newcombe (2018) showed dual-Doppler radar measurements of a wind farm wake with the signatures of single wind turbines disappearing less than 10 km behind the farm. Generally, the data we present here could be influenced by cluster wakes.…”

“…(Theuer et al, 2020a;Peña et al, 2008), which might be related to the occurrence of e.g. kinks or low-level jets (Møller et al, 2020); leads to uncertainty in wind speed correction to lidar height Not corrected for, not considered in uncertainty estimation Laser beam deflection due to thermal gradients in the lower boundary layer Results in measurement height error Not corrected for, not considered in uncertainty estimation the total propagated uncertainty u TP are understood as the 1.96σ values of the corresponding error distributions; i.e. we expect them to include 95 % of all values.…”

“…van Dooren et al, 2016;Stawiarski et al, 2013). Another measurement system to assess global blockage could be the remote sensing method Doppler radar which was successfully deployed for wind turbine and wind farm wake measurements (Nygaard and Newcombe, 2018).…”

“…He found a good agreement of the model and the results of RANS simulations. Nygaard et al (2020) coupled an engineering model for global blockage with a wind turbine wake model modified to better represent the far wind farm or cluster wake. Their wind farm blockage model was able to predict the trend in the variation of power in the front row of turbines but underestimated its amplitude.…”

Offshore wind farm global blockage measured with scanning lidar

“…However, as in this set-up an extrapolation of wind speed u m at measuring height z m to lidar height z TP is required we estimated the uncertainty associated with it in more detail in the following. (Theuer et al, 2020a;Peña et al, 2008), which might be related to the occurrence of e. g. kinks or low level jets (Møller et al, 2019…”