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Mesoscale modelling for an offshore wind farm

Mesoscale modelling for an offshore wind farm. J. Badger, R. Barthelmie, S. Frandsen, M. B. Christiansen Risø National Laboratory, Denmark. EWEC, Athens 2006. Outline. Motivation Introduction Offshore impact of onshore orography Internal boundary layer growth

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Mesoscale modelling for an offshore wind farm

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  1. Mesoscale modelling for an offshore wind farm J. Badger, R. Barthelmie, S. Frandsen, M. B. Christiansen Risø National Laboratory, Denmark EWEC, Athens 2006

  2. Outline • Motivation • Introduction • Offshore impact of onshore orography • Internal boundary layer growth • KAMM simulations for Nysted wind farm • Conclusions

  3. Motivation Offshore wind farms are large, consider variation of: • wind conditions over farm area • wind speed at hub height • wind profile • turbulence Offshore in situ wind measurements: • accurate • limited point measurements SAR images: • spatial fields • can be limited in number

  4. Motivation Mesoscale modelling provides: • spatial fields • vertical profiles and can be verified against and complement: • point measurements • SAR images

  5. Introduction: KAMM Karlsruhe Atmospheric Mesoscale Model non-hydrostatic, regular horizontal grid, stretched vertical coordinate (terrain following) Typical resolution Δx = 2km size 200 x 200 x 5.5 km

  6. Introduction: gap wind SAR image from Radarsat. Colour shows wind speed. Wind vectors from NOGAPS model, operated by U.S. Navy. http://fermi.jhuapl.edu/sar/stormwatch/user_guide/ Source: Young and Winstead, section II of Beal et al: see web link.

  7. Introduction: barrier wind SAR image from Radarsat. Colour shows wind speed. Wind vectors from NOGAPS model, operated by U.S. Navy. Source: Young and Winstead, section II of Beal et al: see web link.

  8. Introduction: offshore effect of onshore orography Mesoscale modelled climatology (KAMM/WAsP) for Morocco. Shows gap flow and barrier wind features

  9. Introduction: offshore effect of onshore orography The flow around/over an obstacle is dependent on thermal stratification and wind speed. Froude Number = U / (h * N) U = velocity scale h = height scale of obstacle N = Brunt-Väisälä frequency N2 = g/θ0(dθ/dz) Froude Number > 1 Froude Number <1

  10. Introduction: offshore effect of onshore orography For higher stability condition, higher N • upstream influence of obstacle can increase • lower obstacles can block flow • higher wind speed is blocked Possible influence of terrain ~100 km, could play large role in offshore wind resource.

  11. Introduction: effect of change of surface type surface roughness change  balance of forces disturbed. u u

  12. Introduction: effect of change of surface type Internal boundary layer • roughness change (coastal jets) • surface fluxes changes (nocturnal jets, maritime jets) • inflow stratification • influence of shape of coastline • detachment and interaction of coastal jets (Orr et al, 2004) • Scales LR = (N * h0) / f ~ 100 km

  13. Nysted flow modelling: set-up • Domain • centred on Nysted • dx = 2 km • size 200 x 200 km • Orography for modelling domain contour interval 25 m

  14. Nysted flow modelling: set-up KAMM has been integrated with the • wind speed profile • temperature profile • 12 direction sectors, 30 degree interval ~mean NCEP/NCAR 1965-1998

  15. Nysted flow modelling: set-up Land Offset Sea Offset Set A 0K 0K Set B -5K +5K Set C +5K -5K What impact does the surface temperature offset have on wind characteristics at the site? control warm sea cold land cold sea warm land

  16. Nysted flow modelling: results, example • Example wind field • 70 m winds • geostrophic forcing 240 degrees

  17. Nysted flow modelling: results, comparison Johns Hopkins University SAR image

  18. Nysted flow modelling: results, surface temperature impact warm sea cold land cold sea warm land control Set A Set B Set C wind speed at 70 m for 240 degrees geostrophic forcing

  19. Nysted flow: direction, speed characteristics across farm warm sea cold land cold sea warm land Set A Set B Set C wind vectors at 70 m for 12 forcing direction sectors

  20. Nysted flow: speed profile characteristics across farm

  21. Summary from Nysted Set B warm sea cold land • smallest wind speed shear from 25 m to 70 m above sea level • largest wind speed gradients across the wind farm, when there is a very short sea fetch Set C cold sea warm land • largest wind speed gradients across the wind farm, when there is a medium length or complex sea fetch

  22. Conclusions • Offshore wind resource inhomogeneous due to: • topographical effects • internal boundary layer development effects (jets) • influences can be felt far from coastlines ~ 100 km • wind conditions vary within a wind farm • Mesoscale modelling offers a useful tool to: • understand and predict these flow features • estimate resource Acknowledgements: Danish PSO project Large Wind Farms Shadow Effects (PSO F&U 4103)

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