Wind turbine and sodar observations of wakes in a large wind farm

Measurements of wind turbine wakes have been carried out most commonly in offshore wind farms or over small wind farms on land, but to date, few observations have been made in large, land-based wind farms. The scarcity of such wake measurements may be due, in a large part, to large heat flux in the convective boundary layer over land, and large values of surface roughness, both of which are associated with larger turbulence velocity scales than would be present over water. In the present study, we use a combination of supervisory control and data acquisition (SCADA) data from wind turbines and sodar data taken from an operating wind farm in southeastern Minnesota. Two sodars were deployed in the farm from August to December 2009, and corresponding SCADA data were retrieved from all 43 wind turbines. Taken together, these two data types can be used within a large farm to explore characteristics of individual turbine wakes, accumulation of velocity deficits for wind flow parallel to turbine rows, and recovery of wind speeds downstream from turbines. Composite analyses of SCADA data revealed wind turbine wakes in all stability categories, but the wakes were most coherent during episodes of strong potential temperature stratification when the wind turbine hub may have been above the turbulent boundary layer. Raw SCADA plots in more unstable atmospheric temperature profiles show a lot of scatter, and the degree of scatter is roughly consistent with the relevant boundary layer turbulence velocity scales. Composite cross sections taken from sodar data reveal distortion of the wake due to low-level shear in stable boundary layer conditions, and rotation of the wake due to rotor torque effects can be observed. These characteristics, seen in sodar cross sections, agree qualitatively with wind tunnel observations and large eddy simulations.