Replacing wakes with streaks in wind turbine arrays

Wind turbine wakes negatively impact downwind turbines in wind farms reducing their global efficiency. The reduction of wake-turbine interactions by actuating control on yaw angles and induction factors is an active area of research. In this study, the capability of spanwise-periodic wind turbine arrays with tilted rotors to reduce negative turbine-wakes interaction is investigated by means of large-eddy simulations. It is shown that by means of rotor tilt it is possible to replace turbine far wakes with high-speed streaks where the streamwise velocity exceeds the freestream velocity at hub height. Considering three aligned arrays of wind turbines, it is found that the global power extracted from the wind can be increased by tilting rotors of upwind turbine arrays similarly to what already known for the case of a single row of aligned turbines. It is further shown that global tilt-induced power gains can be significantly increased by operating the tilted turbines at higher induction rates. Power gains can be further increased by increasing the ratio of the rotor diameters and turbine spacing to the boundary layer thickness. All these findings are consistent with those of previous studies where streamwise streaks were artificially forced by means of arrays of wall-mounted roughness elements in order to control canonical boundary layers for drag-reduction purposes.Comment: revised versio

Numerical investigation of the effect of tower dam and rotor misalignment on performance and loads of a large wind turbine in the atmospheric boundary layer

A modern horizontal axis wind turbine was simulated by means of computational fluid dynamics (CFD) simulations. The analyzed machine has a diameter of 100 m and is immersed in the atmospheric boundary layer (ABL). The velocity and turbulence stratification of the ABL is correctly preserved along the domain by the adoption of modified wall functions. An overset technique is employed to handle the rotation of the turbine rotor throughout the operation of the machine. The ABL induces periodically oscillating loads and generated torque on the rotor blades. Several configurations are analyzed. First, the results of a rotor-only simulation are compared to the ones obtained from the simulation of the full machine in order to evaluate the effect of the supporting structures on the produced torque and on the loads acting on the blades. Then, a tilt angle is introduced on the analyzed rotor and its effect on the oscillating loads of each blade is highlighted by comparing the results to the untilted configuration. Lastly, a yaw misalignment is also introduced and the results are compared to the unyawed configuration

Large-eddy simulations, wake models, and control: Power grid frequency support with wind farms

Improved integration of wind farms into frequency regulation services is vital for increasing renewable energy production while maintaining power system stability. In particular, wind farms of the future will need to be able to provide secondary frequency regulation by tracking a power reference signal controlled by the grid operator. Wind farm wake models, estimation methods, and control techniques are developed to improve wind farm secondary frequency regulation capabilities. Large-eddy simulations (LES), where the large scales are directly simulated, are combined with the actuator disk model, which represents a wind turbine as a drag disk, to simulate large wind farms. LES provides an ideal test bed for wake model validation and control algorithm development. A dynamic wind farm model is developed for time-varying changes in wind farm thrust and validated against LES of a wind farm at start up. A new yawed wind turbine theory is derived for the near-disk inviscid region of the flow and compared to numerical simulations. This model yields more accurate predictions of the initial transverse velocity and wake skewness angle than existing models. We use these predictions as initial conditions in an extended dynamic wake model for yawed turbines and compare predicted wake deflection with wind tunnel experiments. Sensing and estimation methods are developed to assimilate power measurements into the new dynamic wake model. Using LES, the dynamic wake model, and sensing and estimation methods, we propose the use of model-based receding horizon control to provide secondary frequency regulation for a power grid using thrust coefficient modulation. We implement the controller in high-fidelity numerical simulations of a wind farm with 84 turbines and then test the controlled farm's ability to track a power reference signal. The results demonstrate the ability of the control algorithm to track two types of power reference signals used by a US independent system operator. Furthermore, the controller achieves accurate power tracking and reduces loss of revenue in the bulk power market by requiring less setpoint reduction (derate) than the power level control range. The control design is subsequently extended to include generator torque, blade pitch actuation, and the rotational inertia of the rotor