Advances in actuation techniques for wind farm flow control

Offshore wind farms suffer substantial energy losses due to the interference of wind turbine wakes, with estimates indicating losses of approximately 10%. This thesis aims to advance the state of the art of wind farm flow control techniques for maximizing wind farm power performance. Wind farm flow control can be divided into three categories: static induction control (SIC), wake steering control (WSC), and wake mixing control (WMC). With SIC, upstream turbines are derated to create higher velocity wakes. The losses that are incurred by derating are subsequently compensated by downstream turbines. WSC redirects the wake away from downstream turbines by misaligning a turbine with respect to the incoming wind direction. Using WMC, the wake mixing process is enhanced by continuously adjusting the operating conditions of the wind turbine. Generally, this is achieved through a periodic pitching motion of the blades.

This thesis covers all three categories of wind farm flow control. First, field experiments on an onshore wind farm were carried out to examine the effectiveness of SIC. Measurements indicated a 3.3% increase in power production, as well as a significant decrease in experienced turbulence intensity during favorable ambient conditions. Second, a framework was developed for improving the estimated energy for WSC with analytical steady-state wake models. Using Gaussian process regression, the framework combines the results from an analytical wake model and large eddy simulations with varying ambient conditions, resulting in a 76% increase in estimated annual energy production with respect to the analytical wake model. Finally, a set of wind tunnel experiments were carried to study the wake of a scaled wind turbine model operating with WMC using Particle Image Velocimetry (PIV). The PIV measurements showed enhanced levels of wake recovery with WMC compared to normal operation. Furthermore, a recent TU Delft innovation called ‘the helix approach’, which induces a helical velocity profile in the turbine wake, was shown to be capable of increasing the power of a two turbine array by as much as 15%.