Design of Large Wind Turbine Rotors Through Passive and Active Load Mitigation Strategies
Wind energy over the years has positioned itself to become a primary source of renewable energy and this is attributed to the reduction in the Levelized Cost of Energy (LCOE). Historically, this is accomplished by an increase in tower heights which allow access to higher wind speeds, and also by increasing rotor diameters which allow for more power capture. However, there are significant challenges that come with these large turbines like aeroelastic instabilities of the blades due to their long and slender nature, and the need for more robust turbine components that can withstand the larger loads associated with large turbines. This has motivated the development of design strategies that incorporate different methods of load alleviation to achieve optimized wind turbine designs that can result in lower LCOE. This dissertation presents various methods of designing wind turbine rotors that take advantage of passive and active load reduction strategies. First, classical flutter is addressed for the design of large blades. Flutter is an aeroelastic instability that contributes to fatigue damage, or in the worst case can least to sudden catastrophic turbine failure. A comprehensive evaluation of flutter behavior including classical flutter, edgewise vibration, and flutter mode characteristics for two- and three-bladed wind turbine blade designs is carried out. Further, a study is performed to evaluate mitigation of flutter in the design process via structural redesign by evaluating the effect of leadingvi edge and trailing edge reinforcement on flutter speed and hence demonstrates the ability to increase the flutter speed and satisfy structural design requirements (such as fatigue) while maintaining or even reducing blade mass. This flutter structural mitigation study is conducted for two wind turbine designs, one a two-bladed rotor and the other a three-bladed rotor. Second, a new rotor design methodology is developed to integrate active load control in the form of controllable gurney flaps. A comprehensive sequential iterative design procedure is developed that integrates aerodynamics, structural, and baseline turbine control system design with advanced active load control into a design process. This procedure also takes into account the contribution of loads on all major components of the turbine. To realize the best LCOE reduction solution, new methods to evaluate blade structural properties are developed wherein, the reductions in damage equivalent loads (i.e.; fatigue loads) due to a generic active load control system are mapped to structural design improvements in terms of blade mass reduction, cost reduction in other major turbine components, and LCOE reductions that result from integration and redesign of the turbine with the active load control system. Third, using the design methodologies established, newer rotor designs with a larger rotor radius are explored to examine the impacts of the active load control system. These rotors take advantage of the fatigue load reductions due to the controllable gurney flaps integrated into the design. The effect of the controllable gurney flaps is evaluated for various blade and non-blade component loads on the turbine. This methodology results in larger rotors that have increased energy capture and reduced LCOE’s. For this study, two different turbine operating strategies are followed, one limits the turbine power to that of the baseline while the other allows the turbine to extract more power at higher wind speeds. Finally, a method is introduced to support the realization of new passive and active load mitigation strategies by improving prototype wind turbine development. A novel method of developing a multi-fidelity digital twin structural model of a wind turbine blade is presented. The digital twin model development methodology, presented herein, involves a novel calibration process to integrate a wide range of information including design specifications, manufacturing information, and structural testing data (modal and static) to produce a multi-fidelity digital twin structural model: a detailed high-fidelity model (i.e., 3D FEA) and consistent beam-type models for aeroelastic simulation. Digital twin models are useful to cost-effectively evaluate the performance of new technologies in the field like novel downwind rotors and controllable gurney flaps. Finally, the new methodology is demonstrated for an as-built two-bladed downwind prototype rotor resulting in a multi-fidelity digital twin model which has a 1% match in mass properties, 3.2% in blade frequencies, and 6% in deflection to the as-built blade. The rotor examined is the SUMR – Demonstrator (SUMR-D), which was installed on the Controls Advanced Research Testbed (CART-2) wind turbine at the National Wind Technology Center. The digital twin model developed here was utilized to design controllers to safely operate SUMR-D in field tests, which are providing additional data for further evaluation and development of the multi-fidelity digital twin structural model.