Presentations

Abstract

To meet Europe's demand for sustainable and independent energy, an increasing number of wind farms are being erected in close proximity in the North Sea. The cumulative extraction of momentum by large wind farms generates wakes that can persist over tens to hundreds of kilometers, giving rise to the phenomenon commonly referred to as “wind theft”. When impinging on a downstream farm, such a wake can cause substantial power losses. In recent years, “wind theft” has sparked the first legal disputes among operators over compensation payments for the estimated $778 million in losses resulting from the wake of a neighboring planned wind farm. This reduces the revenue of existing farms, prevents operators from participating in new tenders, and consequently threatens the European Union's sustainability goal of climate neutrality by 2050. Active Cluster Wake Control (ACWC) represents the first technical solution to mitigate “wind theft” by reducing the severity of the cluster wake. For this purpose, it perceives the entirety of the turbines in a wind farm or wind farm cluster as a distributed actuator. Each turbine subordinates its individual thrust to a coordinated farm-wide control scheme. This enables the creation of farm-wide thrust perturbations that translate into uneven flow decelerations throughout the farm. In principle, this allows for an infinite number of thrust perturbation patterns. However, only a few of them are expected to trigger physical principles capable of mitigating the cluster wake. To elucidate those patterns, we investigate five different patterns that increase in complexity. The simplest is a static deration of all turbines in the farm, followed by dynamic oscillations around a mean thrust value. Phase-shifting the oscillations between the turbines enables patterns that vary in both streamwise and lateral directions. Additionally, the patterns can propagate downstream through the farm such that one parcel of air experiences a constant deceleration through the farm. Using Large Eddy Simulations (LES) of a 120-turbine wind farm, operating in a shallow conventionally neutral atmospheric boundary layer, we demonstrate that appropriately tuned ACWC patterns create periodic large-scale deformations of the wake. Depending on the actuation strategy, the farm wake exhibits either periodic oscillations of the wake width or lateral displacement. These responses are characterized by streamwise wavelengths of approximately 10 km, representing one of the largest-scale flow-control phenomena reported to date. The induced dynamics are accompanied by localized perturbations of the inversion layer thickness and temperature in the free atmosphere above, indicating the excitation of dynamic atmospheric gravity waves. Furthermore, we demonstrate that certain ACWC patterns increase the power generated in the upstream wind farm by 1.5 % and in a waked downstream wind farm by up to 25 %. This corresponds to a collective power gain of 3.4%, benefiting all stakeholders involved. As such, ACWC introduces a fundamentally new perspective on the operation of large wind farms where operators have the control authority to contribute to and benefit from a fair distribution of wind as a shared resource.