Vertical profiles of (a) wind speed (u) (ms⁻¹), (b) turbulence intensity (TI) (%) from the ZephIR lidar at SW mast on 04/20/2012 at 09:05 (red), 10:45 (blue) and 14:25 (green)

Figure 5. Vertical profiles of (a) wind speed (u) (ms⁻¹), (b) turbulence intensity (TI) (%) from the ZephIR lidar at SW mast on 04/20/2012 at 09:05 (red), 10:45 (blue) and 14:25 (green). Profile times are indicated in figure 4(a).

Abstract

Observations of wakes from individual wind turbines and a multi-megawatt wind energy installation in the Midwestern US indicate that directly downstream of a turbine (at a distance of 190 m, or 2.4 rotor diameters (D)), there is a clear impact on wind speed and turbulence intensity (TI) throughout the rotor swept area. However, at a downwind distance of 2.1 km (26 D downstream of the closest wind turbine) the wake of the whole wind farm is not evident. There is no significant reduction of hub-height wind speed or increase in TI especially during daytime. Thus, in high turbulence regimes even very large wind installations may have only a modest impact on downstream flow fields. No impact is observable in daytime vertical potential temperature gradients at downwind distances of >2 km, but at night the presence of the wind farm does significantly decrease the vertical gradients of potential temperature (though the profile remains stably stratified), largely by increasing the temperature at 2 m.