Median values of daytime (11:00–17:00 LST) and nighttime (00:00–06:00 LST) turbulence intensity (TI) (%), shear exponent (α) (unitless), and vertical potential temperature gradient (Δθ/Δz) (°C m⁻¹) for each wind sector (see figure 1)

Table 1.  Median values of daytime (11:00–17:00 LST) and nighttime (00:00–06:00 LST) turbulence intensity (TI) (%), shear exponent (α) (unitless), and vertical potential temperature gradient (Δθ/Δz) (°C m⁻¹) for each wind sector (see figure 1).

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.