Mean (a) hub-height difference in wind speed (SW–NE), (b) difference in hub-height TI (SW–NE) and (c) mean potential temperature gradient for the SW location only, binned as an average of each two hours as a function of local hour of day (LST) for the unwaked (#1, black) SW farm-waked (#2, red), SW direct-waked (#3, blue), NE farm-waked (#4, green) and NE direct-waked (#5, magenta) wind direction bins

Figure 2. Mean (a) hub-height difference in wind speed (SW–NE), (b) difference in hub-height TI (SW–NE) and (c) mean potential temperature gradient for the SW location only, binned as an average of each two hours as a function of local hour of day (LST) for the unwaked (#1, black) SW farm-waked (#2, red), SW direct-waked (#3, blue), NE farm-waked (#4, green) and NE direct-waked (#5, magenta) wind direction bins. The error bars denote one standard deviation from the mean. Also plotted in (c) are events for which u < 3 ms⁻¹ for the SW farm-waked (#2, yellow) and SW direct-waked (#3, cyan) direction sectors.

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.