Structural Design and Optimization of Sub-scale and Extreme-scale Wind Turbine Rotors

The main objectives of this dissertation are to develop some new design and optimization solutions for large/extreme-scale (up to 50 MW) wind turbine rotors, as well as a structural design method for a sub-scale wind turbine blade for manufacturing and field testing. To accomplish these objectives, a series of 13.2 MW downwind rotors is designed and optimized. A key question to enable large rotor designs is how to configure and optimize structural designs to constrain blade mass and cost while satisfying a growing set of challenging structural design requirements. In this dissertation, the performance of a series of three two-bladed downwind rotors with different blade lengths (104.3 meters, 122.9 meters, and 143.4 meters) all rated at 13.2 MW is investigated. The primary goals are to achieve 25% rotor mass and 25% LCOE (levelized cost of energy) reduction. A comparative analysis of the structural performance and economics of this family rotors is presented. To further explore optimization opportunities for large rotors, the new results in a root optimization and a spar cap design study are presented. The structural design solutions that achieve 25% rotor mass reduction in a SUMR13i design (104.3 meters) and 25% LCOE reduction in a SUMR13C design (143.4 meters) are provided. A new sub-scale field-prototype design solution is also developed to realize the dynamics, structural response, and distributed loads (gravitational, aerodynamic, centrifugal) that are characteristics of a full-scale large, modern wind turbine rotor. The challenge lies in producing a structural design meeting two competing objectives: novel scaling objectives that prescribe the sub-scale blade to have low mass and stiffness; and traditional structural safety objectives that drive the design to have high stiffness and mass. A 20% gravo-aeroelastically scaled wind turbine blade is developed successfully that satisfies these competing objectives. First, it achieves close agreements for non-dimensional tip deflection and flap-wise blade frequency (both within 2.1% error) with a blade mass distribution constrained to produce target gravitational and centrifugal loads. Second, the entire blade structure is optimized to ensure a safe, manufacturable solution meeting strict strength requirements for a testing site that can experience up to 45 m/s wind gusts. Next, 50 MW wind turbine rotors with blades’ length over 250 meters are designed and optimized. Key questions in this work include: what is the structural limit for the size of a wind turbine rotor to be feasible or cost-effective, and what are the technologies and approaches needed to achieve large rotors. The largest wind turbine design in prior work is a 25 MW rotor, and here a 50 MW rotor design is considered, the largest ever design with blades’ length over 250 meters, which is 2.5 times the length of a football field. This dissertation shows that a 50 MW design is indeed possible from a detailed engineering perspective and presents a series of aero-structural blade designs for 50 MW wind turbine rotors, and a critical assessment of technology pathways and challenges for such extreme-scale rotors. The rotor design for the 50 MW rotor begins with Monte Carlo simulations focused on optimizing the carbon spar cap design, which is found to be a major cost driver in the blade design. Further, a study of blade root fatigue performance is performed, which is found to be the key limitation for the extreme-scale machine at 50 MW scale. A baseline, initial design results in a 250-meter blade with a mass of 500 metric tons. This initial study indicates a significant opportunity for improvement through the aero-structural design; thus, an aero-structural design and optimization study is performed to reduce the blade mass/cost and achieves more mass/cost-effective 50 MW rotors that result in more than 25% mass reduction and over 30% cost reduction by determining the optimal blade chord and the optimal airfoil thickness for the best aerodynamic and structural performance. Wind turbine blade reliability is critical once blades go into service in operation to avoid costly repairs and lost revenue due to turbine downtime resulting from blade damage. With increased size of the blade, especially for the extreme-scale blades, the blade can experience a more complicated loading. A new method utilizing the panel behavior for structural health monitoring and nondestructive damage detection is examined in this dissertation. The localized panel resonance (panel mode) is identified by the experimental modal test of the BSDS blades at Sandia National Laboratories and numerical analysis of an open-source BSDS design model. Then the study is extended to a larger, novel concept SUMR-D blade design, which was presented in Chapter 3. Some classical wind turbine damage modes are simulated based on the SUMR-D ANSYS model, including shear webs disbonding simulations and trailing edge disbonding simulations. A relation is established to correlate the panel mode with the damage size, which can be applied to structural health monitoring applications. For the shear webs damage cases, a relation is established to correlate the panel mode with panel buckling performance as a function of damage size based on numerical results and analytical formulae, which has potential applications in the nondestructive buckling capacity evaluation.