07 Oct 2022
07 Oct 2022
Status: this preprint is currently under review for the journal WES.

Modelling the impact of trapped lee waves on offshore wind farm power output

Sarah J. Ollier1 and Simon J. Watson2 Sarah J. Ollier and Simon J. Watson
  • 1CREST – Loughborough University, Holywell Park, Loughborough LE11 3TU, UK
  • 2Wind Energy Section, Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, Netherlands

Abstract. Mesoscale meteorological phenomena, including Atmospheric Gravity Waves, or Trapped Lee Waves (TLWs) can result from flow over topography or coastal transition in the presence of stable atmospheric stratification, particularly with strong capping inversions. Satellite images show that 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. This parametric study investigates the potential impact of TLWs on a UK near-coastal offshore wind farm, Westermost Rough (WMR) resulting from westerly – south-westerly flow over topography in the Southeast of England.

Computational fluid dynamics (CFD) modelling (using ANSYS-CFX) of TLW situations based on real atmospheric conditions at WMR was used to better understand turbine level and whole wind farm performance in this parametric study based on real inflow conditions. These simulations indicated that TLWs have the potential to significantly alter the windspeeds experienced by and the resultant power output of individual turbines and the whole wind farm. The location of the wind farm in the TLW wave cycle was an important factor in determining the magnitude of TLW impacts, given the expected wavelength of the TLW. Where the TLW trough was coincident with the wind farm, the turbine windspeeds and power outputs were more substantially reduced compared with when the TLW peak was coincident with the location of 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 under the different stability conditions include differing: hill and coastal transition recovery, windfarm blockage effects and wake recovery. 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.

Sarah J. Ollier and Simon J. Watson

Status: open (until 23 Dec 2022)

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Sarah J. Ollier and Simon J. Watson

Sarah J. Ollier and Simon J. Watson


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Short summary
This modelling study shows that topographic trapped lee waves (TLWs) for modifying the flow behaviour and power output in offshore windfarms. We demonstrate that TLWs can substantially alter the windspeeds at individual wind turbines and effect the power output of the turbine and whole windfarm. The impact on windspeeds and power is dependent on which part of the TLW wave cycle interacts with the wind turbines and windfarm. Positive and negative impacts of TLWs on power output are observed.