Turbulence Patterns Around A Large Wind Farm

Abstract

A wind-tunnel investigation was performed at the St. Anthony Falls Laboratory atmospheric boundary layer wind tunnel to study fundamental properties of the turbulent flow around a perfectly staggered wind turbine array. The staggered wind farm consisted on a series of 10 rows by 2-3 columns of miniature wind turbines spaced 5 and 4 rotor diameters in the streamwise and spanwise directions, respectively. The wind turbine array was placed in a boundary layer flow developed over a smooth surface under neutrally stratified conditions. Cross-wire anemometry was used to obtain high resolution measurements of streamwise and vertical velocity components at locations within and above the wind farm. Results show that the staggered configuration is much more efficient in terms of momentum transfer from the background flow to the turbines compared to the case of an aligned wind turbine array under similar turbine separations in the streamwise and spanwise directions. This lead to improved power output of the overall wind farm. A simplified analysis suggests that the change of power in the two configurations is on the order of 10%. Maximum levels of turbulence intensity in the wind farm were found to be very similar to that of observed in the wake of a single wind turbine, differing substantially with that observed in an aligned configuration under similar spacings. The dramatic changes in momentum and turbulence characteristics in the two configurations shows the importance of turbine layout in engineering design. Lateral homogenization of the turbulence statistics above the wind farm allows the development of simple parameterizations for the adjustment of flow properties, similar to the case of a surface roughness transition. The development of an internal boundary layer was observed above the wind farm. In this layer, flow statistics are affected by the background flow and wind turbines. The adjustment of the flow in this layer is much slower in the staggered situation (respect to its aligned counterpart) which implies change in the momentum/power available at turbine locations.