Optimal Coordinated Control of Power Extraction in LES of a Wind Farm with Entrance Effects

We investigate the use of optimal coordinated control techniques in large eddy simulations of wind farm boundary layer interaction with the aim of increasing the total energy extraction in wind farms. The individual wind turbines are considered as flow actuators, and their energy extraction is dynamically regulated in time, so as to optimally influence the flow field. We extend earlier work on wind farm optimal control in the fully-developed regime (Goit and Meyers 2015, J. Fluid Mech. 768, 5–50) to a ‘finite’ wind farm case, in which entrance effects play an important role. For the optimal control, a receding horizon framework is employed in which turbine thrust coefficients are optimized in time and per turbine. Optimization is performed with a conjugate gradient method, where gradients of the cost functional are obtained using adjoint large eddy simulations. Overall, the energy extraction is increased 7% by the optimal control. This increase in energy extraction is related to faster wake recovery throughout the farm. For the first row of turbines, the optimal control increases turbulence levels and Reynolds stresses in the wake, leading to better wake mixing and an inflow velocity for the second row that is significantly higher than in the uncontrolled case. For downstream rows, the optimal control mainly enhances the sideways mean transport of momentum. This is different from earlier observations by Goit and Meyers (2015) in the fully-developed regime, where mainly vertical transport was enhanced

〈原著論文〉A method for the design of a scaled wind turbine for wind tunnel experiments

The current work introduces a blade element momentum theory-based tool called BEMTurbine for designing and evaluating the performance of wind turbines. BEMTurbine is a python-based open source tool that can be used to optimize blade parameters (chord length and twist angle) for the aerodynamic performance. This tool is then used to design a model wind turbine with a rotor diameter 0.25 m and the optimum tip speed ratio of 5. Rotor of this turbine is manufactured using a 3D printer. The BEM analysis shows that the maximum power coefficient of the model turbine is 0.47, and is attained at the design tip speed ratio of 5.Ⅲ.論文

Optimal control of wind farm power extraction in large eddy simulations

In the present work we couple flow simulations performed using Large Eddy Simulations (LES) with gradient based optimization to control individual turbine in a farm, so as to achieve an increase in the total power output. The controls in our optimization problem are the disk-based thrust coefficients C′T,n of individual turbines as function of time. We use a gradient-based algorithm for the optimization and the gradients are computed using the adjoint method; the adjoint equations are formulated directly from the LES equation and the cost functional. We employ a receding-horizon predictive control setting and solve the optimization problem iteratively at each time horizon based on the gradient information obtained from the evolution of the flow field and the adjoint computation. In this paper we further elaborate the optimization techniques, interpret the simulation of adjoint field and present results for the wind-farm boundary layer cases. We find that the extracted farm power increases by approximately 20%, during optimal control. However, the increased power output is also responsible for an increase in turbulent dissipation, and a deceleration of the boundary layer. These issues are further discussed.status: publishe

Optimal Control of Energy Extraction in Large-eddy Simulation of WindFarms

In large wind farms, the vertical interaction of the farm with the atmospheric boundary layer plays an important role, i.e. the total energy extraction is governed by the vertical transport of kinetic energy from higher regions in the boundary layer towards the turbine level. The current dissertation investigates the use of optimal control techniques in large-eddy simulations of wind farm boundary-layer interaction with the aim of increasing the total energy extraction in wind farms. The individual wind turbines are considered as flow actuators and their energy extraction is dynamically regulated in time so as to optimally influence the flow field and the vertical turbulent energy transport. The dissertation focuses on the development of a framework for a gradient-and adjoint-based scheme for wind-farm power optimization. To this end, a receding-horizon optimal control approach is employed in combination with the non-linear Polak-Ribière conjugate gradient method and the Brent line search algorithm. The gradient of the cost functional required by the conjugate-gradient method is determined using a continuous adjoint-based approach. The adjoint equations for the standard Navier-Stokes equations are extended to include the adjoints for the subgrid-scale model and wall-stress model, and the adjoint of the wind-turbine model. In the first optimization studies, the optimal control of an infinite wind farm is investigated. The first control case focuses on the direct maximization of the energy extraction. It is found that the energy extraction increases by 16% compared to the uncontrolled reference. This is directly related to an increase in the vertical fluxes of energy towards the wind turbines, and vertical shear stresses increase considerably. A further analysis, decomposing the total stresses into dispersive and Reynolds stresses, shows that the dispersive stresses increase drastically, and that the Reynolds stresses decrease on average, but increase in the wake region, leading to better wake recovery. It is further observed that turbulent dissipation levels in the boundary layer increase, and overall, the outer layer of the boundary layer enters into a transient decelerating regime, while the inner layer and the turbine region attain a new statistically steady equilibrium within approximately one wind-farm through-flow time. Two additional optimal control cases study the penalization of turbulent dissipation. For the current wind-farm geometry, it is found that the ratio between the wind-farm energy extraction and turbulent boundary-layer dissipation remains roughly around 70%, but can be slightly increased by a few percent by penalizing the dissipation in the optimization objective. For a pressure-driven boundary layer in equilibrium, it is estimated that such a shift can lead to an increase in wind-farm energy extraction of 6%. The second optimization study investigates the application of the optimal control to a finite-sized wind farm. A fringe region is employed to impose non-periodicity to the domain and the adjoint for the fringe forcing term is added to the original adjoint LES equations. It is found that the energy extraction increases by 7.3% compared to the uncontrolled case. The value is significantly lower when compared to the optimization of the infinite wind farm. One possible reason for this could be that the turbines in the front row - which contribute 16.5% of the whole farm power in the current case - are already operating close to the optimal condition, and hence, their performance cannot be improved much further by coordinated control. However, even the 7.3% gain achieved in this dissertation can be beneficial, especially for large wind farms.nrpages: 132status: publishe

沿岸近傍の洋上風力発電の流体解析用の高精度数値流体シミュレーションツールの開発

研究種目：奨励研究助成金; 課題番号：SR1

〈原著論文〉Measurement of flow fields in the wake of upwind and downwind wind turbines in wind tunnel experiments

The current study compares aerodynamic characteristics and performance of upwind and downwind wind turbines in wind tunnel experiments. The scaled model of wind turbine, named KDWT25, is designed and fabricated. Comparison of wake fields of the two configurations showed that the wind speed recovery in the wake of downwind turbine is faster than the upwind turbine for the uniform and laminar inflow conditions considered in this study. The tendency of turbulence profiles for both configurations is similar, with the maximum turbulence intensity observed around the rotor edge. Power coefficient at design tip speed ratio is 30% higher for the downwind turbine, even when operated in the same inflow conditions. It is assumed that the nacelle-induced blockage which deflects the flow toward more efficient outer region of the rotor, may be responsible for the better performance of the downwind turbine.Ⅲ.論文

A framework for optimization of turbulent wind-farm boundary layers and application to optimal control of wind-farm energy extraction

status: publishe