This article discusses the performance optimization of wind turbine rotors with active flow control. An extensive multi-parameter investigation with a thorough matrix-grading system was performed to identify the most suitable solution for industrial quality, short/mid-term implementation on actual utility scale wind turbines. A very wide selection of aerodynamic flow control solutions was analyzed based on extensive multi-disciplinary literature review and through aerodynamic and aeroelastic simulations. It is suggested that the trailing edge devices have the most favorable performance in the field of system integration and mechanical design performance. Compliant structures like the flexible flap keep the number of moving parts to a minimum while maintaining high performance and manufacturing simplicity. The use of flexible and elastic materials based on polymers or rubber material improves the lightning strike resistance of these solutions and allows for low-cost large-scale production. The actuator principle, sensitivity, and reliability are decisive parameters, and pneumatic actuators seem to strike a good balance between performance, cost, and reliability.
The Research Motivation
All wind turbine manufacturers struggle to keep the aeroelastic loads of their turbines low since loads come in direct connection to wind turbine cost. Advanced blade pitch controller algorithms are being developed in order to reduce the extreme and fatigue loads in combination with optimized blade designs which are tuned in a way that they combine high energy capture with low aeroelastic loads. The latest generation of extremely large blades span more then 60m and under extreme wind conditions their tips deflect more than 5m thus increasing the danger of tower impact. Furthermore the large blade chords at the inner blade region in combination with high aerodynamic and aeroelastic loads increase the danger of local buckling effects. This combination of fatigue and extreme load reduction as well as deflection control is currently a headache for wind turbine blade designers. The design optimization however is reaching its limits and new technical solutions are required for further improvement.
All the loads mentioned above have to be handled by the blade structure and the rest of the wind turbine structure for the entire 20-year design life-time of the system.This means that there is a unique combination of heavily loaded components which have to withstand more than 107 cycles with very scarce maintenance cycles (usually twice per year). Furthermore, modern wind turbines are expected to achieve very competitive cost of energy levels. The most promising solution for the extreme and fatigue aeroelastic load issues of wind turbines seems to be the introduction of cost effective active flow control solutions. In the 1st part of this article, the performance of a pneumatically actuated flexible trailing edge flap and an electromechanically actuated micro flap was presented. These were some of the best performing aerodynamic flow control solutions that were selected through a rigorous selection process, which is presented below.
The Selection Process
There is a very wide range of aerodynamic flow control devices that were and still are under investigation in various fields of engineering. Many of these are very promising and perform exceptionally in certain industrial and commercial applications, while others still remain in the prototype phase. In order to identify the most suitable of these solutions for industrial-quality, short/mid term implementation on actual utility scale wind turbines, the authors have performed an extensive multi-parameter investigation with a thorough matrixgrading system. A very wide selection of aerodynamic flow control solutions was analyzed (Fig. 1) based on extensive multi-disciplinary literature review and through aerodynamic and aeroelastic simulations. The aerodynamic performance was compared with performance results in several other operational fields and a selection matrix was generated. Through this rational selection process the most promising solutions were selected for further experimental and numerical investigations.
The development of the flow control solution selection matrix was based on engineering performance results with respect to performance parameters but also involved subjective, yet justified, performance and cost estimations. This was necessary in order to evaluate the performance of each solution under the diverse and adverse wind turbine operational conditions. Figure 2 shows the various grading parameters which were included in the flow control selection methodology. It is obvious that the pure aerodynamic performance of each solution is only a part of the whole equation, while other issues such as integration complexity and reliability are equally important.
The selection process was constantly correlated with other similar efforts of a few other research groups around the world that are active in this field. The exact results and solution selection of each group are slightly different mostly due to the fact that there are different parameters and weight factors involved but the overall tendency is similar. Most of the research efforts focus on solutions that are able to provide significant lift control authority (a Cl variation of ±0.3 to ±0.4). At the same time the trailing edge devices have the most favorable performance in the field of system integration and mechanical design performance. Compliant structures like the flexible flap keep the number of moving parts to a minimum while maintaining high performance and manufacturing simplicity. The use of flexible and elastic materials based on polymers or rubber material improves the lightning strike resistance of these solutions and allows for low cost large scale production.The actuator principle, sensitivity and reliability are decisive parameters and there pneumatic actuators seem to strike a good balance between performance, cost and reliability.
The development of reliable and cost effective active flow control devices for wind turbine rotorblades is an extremely demanding task. The unprecedented amount operational cycles and the extremely harsh operational environment are some of the most critical success/failure factors. The need however of load alleviating elements for the ever-larger rotorblades of the future is evident; therefore it is the aim of the authors to contribute to the development of such solutions for the next generation of “smart” wind turbine rotorblades. The results presented by the authors cover the 1st research phase of a large active flow control research effort at the Technical University of Berlin. Results from more detailed follow-up investigations on the aeroelastic performance and the controller development of such “Smart Blade” solutions are presented by the authors at the ASME IGTI Turbo Expo 2012.