Trapped lee wave interactions with an offshore wind farm

Offshore wind energy is an important contributor to world energy production. Whilst new wind farms will be built for net zero carbon targets, improving the output efficiency of new and existing wind farms will be important. Thus, an understanding of the meteorological conditions in the marine atmospheric boundary layer (MABL), where offshore wind farms are located, is essential. Mesoscale meteorological phenomena, including Trapped Lee Waves (TLWs) result from flow over topography and the coast-sea transition in the presence of stable atmospheric stratification, particularly capping inversions. Such phenomena make winds in the MABL deviate from traditional wind theory. Topographically forced TLWs frequently occur around near coastal offshore wind farms. Yet current understanding of how they interact with individual turbines and whole farm energy output is limited to four publications, only one of which is based offshore.

This research investigates TLW impacts at a UK near-coastal offshore wind farm, Westermost Rough (WMR). Topographically forced TLWs at WMR result from westerly – south-westerly flow over topography in the Southeast of England. TLWs were frequently observed in the region of WMR with 45 TLW events detected in Synthetic Aperture Radar (SAR) images at WMR offshore wind farm over a two-year period (01.01.2016 - 31.12.17). Evidence of TLW impacts at WMR is investigated using preliminary SAR investigations, statistical analysis of turbine SCADA (Supervisory Control And Data Acquisition) data from WMR and Reanalysis Data (ERA5) for the same two-year period. This shows that row-wise variability in windspeeds and power output is greater during TLW events. However, data resolution and differences in initial base states for TLW and non-TLW events meant it was not possible to conclusively decouple TLW atmospheric influences.

Computational fluid dynamics (CFD) modelling (ANSYS-CFX) of TLW situations at based on real atmospheric conditions at WMR was used to better understand turbine level and whole wind farm effects. These simulations indicated that TLW have the potential to significantly alter the windspeeds experienced by individual turbines and across the whole wind farm and the resultant power output. The location of the wind farm in the TLW wave cycle was an important factor in determining the magnitude of TLW impacts. Where the TLW trough was coincident with the wind farm, the turbine windspeeds and power outputs were more substantially reduced than when the TLW peak was coincident with the wind farm. These reductions were mediated by turbine windspeeds and wake losses being superimposed on the TLW. However, the same initial flow conditions interacting with topography under different atmospheric stability settings produce differing near wind farm flow. Factors influencing the flow within the wind farm are summarised in Figure 0-1. Determining how much of the differences in windspeed and power output in the wind farm resulted from the TLW is an area for future development.