Wind Veer and Wind Shear Affect Wind Turbine Performance

Numerous studies have shown that atmospheric conditions affect wind turbine performance. However, some

findings have exposed conflicting results for different locations and diverse analysis methodologies. For example, Wharton and Lundquist (2012) found that stably stratified flow enhanced wind turbine performance, whereas in a different location, Vanderwende and Lundquist (2012) found that stably stratified flow undermined turbine performance. These locations differed in their experience of wind direction shear and wind speed shear, but detailed measurements of wind profiles were not available.

Here, we use wind turbine power production data coupled with profiling lidar data to explore how the change in wind direction with height (directional wind shear or veer) and speed shear affect wind turbine performance. We use lidar and turbine data collected from the 2013 Crop Wind Energy eXperiment (CWEX) project between June and September in a wind farm in north-central Iowa. Wind direction and speed shear follow a diurnal cycle (Figure 1). Using a combination of speed and direction shear metrics, we find that large directional shear coupled with small speed shear results in underperformance. For wind speeds in the middle of the power curve, we find power losses on the order of 10%.

Several interesting features emerge from these data. First, as expected for the Northern Hemisphere, more clockwise direction shear (wind veering) cases occur compared to counterclockwise (wind backing). Secondly, large veer conditions occur preferentially in the early morning (Figure 1), coincident with a time period of ramping electricity demand (0600 – 0900 LT). During this time, veer-induced power losses exceed 10% (Figure 2). Moreover, large veering exerts greater detrimental effects on turbine performance compared to small backing values. Given recent observations of large veer in offshore situations, these onshore results suggest that careful planning for grid integration studies will be required to effectively incorporate wind-generated electricity into power grids, especially during ramping periods.