Wake Identification and Characterization of a Full Scale Wind Energy Converter in Complex Terrain with Scanning Doppler Wind Lidar Systems

Kurzfassung

As the resource wind is increasingly exploited to produce electricity, wind energy converter (WEC) deployment expands to more complex terrain such as hilltops or mountain ridges. In that context it is crucial to understand the interaction between the atmospheric boundary layer (ABL) flow and the WEC in order to predict downstream flow characteristics. In the context of the Perdigão 2017 experiment, the German Aerospace Center performed full-scale wake measurements on a single WEC of type Enercon E82 with three Leosphere Windcube 200S longrange scanning Lidar systems. The experimental setup covers two parallel ridges 1.4km apart, separated by a 200m deep valley. The ridges are oriented in NW-SE direction, perpendicular to the main wind direction, which is SW. Two of the three scanning Lidar systems are positioned downstream of the WEC in line with the main wind direction to span a vertical plane, perpendicular to the ridges, with range-height indicator (RHI) scans. This allows to investigate wake events with single or dual-doppler Lidar techniques. The third Lidar system, which is positioned along the WEC ridge, is used to measure the wake position outside the above mentioned measurement plane. Wake events in three different ABL regimes (neutral, stable and convective) are evaluated with respect to wake position, dispersion, propagation and the wind-speed deficit. It is found that wake position and propagation are strongly influenced by the atmospheric stability, forcing the wake to deviate from hub height, migrating to higher levels for convective regimes. For stable ABL conditions the wake descends into the valley and is clearly detectable up to at least nine rotor diameters downstream of the WEC, in some cases. Furthermore, the coplanar scanning strategy allows to calculate the two-dimensional wind vector in the vertical scanning plane, indicating that vertical wind components with up to 2 m/s play an important role in the interaction between ABL flow and WEC. With the help of the third Lidar system on the WEC ridge, wake meandering can be quantified. This work will provide a thorough analysis of three exemplary measurement days.