Assessment of structural loads in wind farms under consideration of wake redirection control

Abstract

Wind farm control enables the operation of wind farms in a collective optimum that considers all turbines instead of operating the individual wind turbines in their local optima. The development of new wind farm control techniques requires knowledge in various disciplines that include wind turbine engineering (control design and implementation, structural and aerodynamic design, load assessment and validation), multidisciplinary optimisation, wind resource modelling and atmospheric boundary layer modelling connected with the modelling of wakes. This thesis covers many of the aforementioned disciplines in order to investigate the wake redirection control concept as one option for wind farm control in detail. For the numerical assessment, the aeroelastic simulation tool FAST.Farm is utilised. An adequate setup of the tool is developed to allow the investigation of various realistic operating conditions including different atmospheric stabilities. In addition, FAST.Farm is improved by implementing a model to include the wake-added small-scale turbulence. FAST.Farm is then calibrated against high-fidelity large eddy simulations and validated by using measurement data from the alpha ventus wind farm. Overall, good agreement between simulations and measurements is achieved for the structural loads in terms of statistical results and frequency response at the tower-base and blade-root. The inclusion of wake-added turbulence is crucial to avoid underestimation of the turbulence in the wake and consequently the loads. This is especially relevant in stable atmospheric conditions, where the ambient turbulence intensity is low and the meandering of the wake is weak. An extensive investigation of the wake redirection control concept and its consequences on the structural loads is performed in a simulation study with the validated tool FAST.Farm. For a turbine in free-stream conditions, the fatigue loads at different turbine components are analysed, with changing atmospheric conditions and yaw misalignment angles. The largest effects of yaw misalignment on the load variations are found for stable atmospheric conditions with strong vertical wind shear and low turbulence intensity. In contrast, the influence of yaw misalignment on the fatigue loads becomes less important in unstable atmospheric stability with low vertical wind shear and high turbulence intensity. The investigation is extended for a turbine that is subjected to waked inflow conditions. Especially in partial wake situations, the load distributions differ significantly from free-stream conditions. A directional dependency of the loads is found with respect to the lateral wake offset: The loads tend to be higher for negative lateral wake offsets compared to the loads from the same positive lateral wake offsets, because of higher load amplitudes over one rotor revolution. The gained knowledge is finally applied in the derivation of optimal operation strategies by using the wake redirection control approach for exemplary wind farm configurations and changing environmental conditions. The resulting optimal operation strategies are assessed by using aeroelastic simulations in FAST.Farm. The long-term evaluation suggests that the annual energy production (AEP) of the considered turbine array setups can be increased compared to the baseline scenario without wind farm control, when the main objective is set to power maximisation while the fatigue loads at the turbines are not or equally weighted. Consequently, the strategies that focus on minimising the fatigue loads result into less AEP compared to the baseline strategy. The consideration of fatigue loads in the optimisation of operation strategies is realised with an efficient surrogate model. With this approach, strategies are derived that are able to reduce the fatigue loads at specific components significantly.