Model‐based analysis of wake‐flow data in the Nysted offshore wind farm

Abstract

Wake-flow data of the Nysted offshore wind farm is analysed with the engineering wake model of Jensen. Two-parameter fits referring to wake-decay parameter and wind direction are made to each individual 10 min-averaged recording of turbine powers. The fitted wind direction neither agrees with the measurements at the meteorological masts nor the turbine orientations, but is in agreement with the power deficit ratios between second-and first-row turbines. The fitted wake-decay parameter varies roughly between k = 0.02 and 0.04, depending on the wind direction with respect to the wind farm geometry. Its average <k> ≈ 0.03 is smaller than the current standard k = 0.04, indicating that offshore wakes might be more narrow. No significant dependence on flow stability is found. Compared to two-parameter fits, one-parameter fits with pre-assigned wind direction perform worse; they lead to <k> ≈ 0.04 and tend to overpredict wind farm power.

The influence of humidity fluxes on offshore wind speed profiles

“…(1), the change of wind speed with height in the surface layer depends not only on the prevailing atmospheric stability but also the surface roughness. The latter varies with wind speed offshore (Charnock, 1955), but due to the very low values for the roughness length even under high wind conditions, variations in z 0 have a negligible impact on the average vertical profile of wind speeds (Barthelmie, 2001;Frank et al, 2000). Many other physical mechanisms influence wind speed profiles, particularly in the near-coastal zone.…”

“…The generalizability of inferences drawn from the Anholt experiment are evaluated using meteorological data collected from Nysted offshore wind farm (Cleve et al, 2009). The data were analyzed to assess how frequently the conditions likely to be associated with substantial impact of humidity fluxes on the wind speed profile are observed, and to assess the degree to which inclusion of L q in computing L improves the fit between the predicted and observed wind speed profiles.…”

“…Europe is currently the global leader in offshore wind installations and installed capacity in European waters passed 1.5 GW in 2008, with a further 430-760 MW planned for installation in 2009, and 1100-1180 MW proposed for installation in 2010 (de Vries, 2009). While installed capacity offshore has not grown as fast as anticipated, the European Wind Energy Association…”

“…This move towards increased reliance on offshore deployment of wind turbines provides both new challenges and opportunities (Barthelmie et al, 2009;de Vries, 2008;Musial, 2007). One of the challenges is that turbine deployment offshore coupled with the increase in turbine hub-heights means that wind energy is extending into a historically undersampled region of the atmosphere -the marine boundary layer between 50-200 m above the surface.…”

“…The average commercial wind turbine erected in 2007 was close to 1.5-2.0 MW in installed capacity with a hub-height of 70-90 m and a rotor diameter of 70-90 m (IEA, 2008). The rotor plane thus extends from about 30 m to 140 m. The trend is to use even larger turbines in offshore environments and thus extend even higher from the surface (de Vries, 2008).…”