Boosting Wind Plant Power Output by 4%–5% through Coordinated Turbine Controls
July 30, 2014
Wind plant underperformance has plagued wind plant developers for years. To address this problem, researchers and wind turbine manufacturers have been focusing on increasing the performance of individual turbines. Although wind turbine performance has been greatly improved and the cost of wind energy has plunged over time, the issue of underperformance in wind plants with multiple turbines persists. The National Renewable Energy Laboratory’s (NREL’s) Simulator Of Wind Farm Applications (SOWFA) is shedding new light on the causes of wind plant underperformance and how, with the use of coordinated wind turbine control, power output can be increased by as much as 4% or 5%.
To find a solution to wind plant underperformance, NREL researchers believed that one first had to gain a better understanding of the complex environment in which wind turbines operate. Next, they would have to combine that knowledge with how the environment impacts the different wind turbine components. To do this, they developed SOWFA—a software platform that couples the National Center for Atmospheric Research’s Weather Researching and Forecasting Model (WRF) and OpenFOAM, a computational fluid dynamics model that simulates the atmospheric boundary layer and complex flows within a wind plant, with NREL’s FAST model that simulates the impacts of the environment on wind turbine components. Simulations conducted with SOWFA showed researchers that wind turbine wakes, the turbulent air patterns created downstream of an operating wind turbine, have a negative impact on the performance of wind turbines downstream.
NREL researchers then worked with the Delft University of Technology in the Netherlands to develop a super controller for SOWFA that would allow them to investigate whether or not they could redirect the damaging wakes created by wind turbines and increase wind plant performance by implementing coordinated plant-wide control. The control systems in modern commercial wind turbines actively control blade pitch, generator torque, and nacelle yaw angle to maximize power capture while minimizing turbine loads. However, this optimization of power and loads is done only at the individual turbine level and without regard for the ways in which multiple turbines interact through their wakes.
Two studies recently published by NREL describe how researchers used SOWFA to investigate the potential of several wake mitigation strategies, including independent yaw, tilt, and pitch controls as well as siting strategies to redirect turbine wakes and increase power production. In the best case for yaw-based control, power production was increased by 4.6% in the turbine downstream. However, the downstream turbine also experienced a slight increase in blade bending, drivetrain torsion, and yaw bearing loads, which was probably due to the movement from full to partial wake overlap. In the best case tilt control option, a maximum power gain of 7.1% was observed in downstream turbines, but again, there was an increase in blade bending and yaw bearing loads.
To mitigate the wake overlap effect, researchers applied independent pitch control and found that blade loads, tower loads, and yaw bearing loads were consistently reduced when compared to the baseline case that did not use pitch control. Siting strategies also produced dramatic effects. The performance of a downstream turbine improved 41% when it was moved a full rotor diameter out of the rotor axis of the upstream turbine.
Read more about NREL’s SOWFA simulation of control strategies in:
- Evaluating Techniques for Redirecting Turbine Wakes using SOWFA
- High-fidelity Simulation Comparison of Wake Mitigation Control Strategies for a Two-Turbine Case