Abstract. This work investigates the conceptual design and the aeroservoelastic performance of land-based wind turbines whose blades can be transported on rail via controlled bending. The turbines have a nameplate power of 5 MW and a rotor diameter of 206 m, and they aim to represent the next generation of land-based machines. Three upwind designs and two downwind designs are presented, combining different design goals together with conventional glass and pultruded carbon fiber laminates in the spar caps. One of the five blade designs is segmented and serves as a benchmark to the state of the art in industry. The results show that controlled flexing requires a reduction in the flapwise stiffness of the blades, but it represents a promising pathway for increasing the size of land-based wind turbine rotors. Given the required stiffness, the rotor can be designed either downwind with standard rotor preconing and nacelle uptilt angles or upwind with higher-than-usual angles. A downwind-specific controller is also presented, featuring a cut-out wind speed reduced to 19 m s−1 and a pitch-to-stall shutdown strategy to minimize blade tip deflections toward the tower. The flexible upwind and downwind rotor designs equipped with pultruded carbon fiber spar caps are found to generate the lowest levelized cost of energy, 2.9 % and 1.3 %, respectively, less than the segmented design. The paper concludes with several recommendations for future work in the area of large flexible wind turbine rotors.

Abstract. Increasing growth in land-based wind turbine blades to enable higher machine capacities and capacity factors is creating challenges in design, manufacturing, logistics, and operation. Enabling further blade growth will require technology innovation. An emerging solution to overcome logistics constraints is to segment the blades spanwise and chordwise, which is effective, but the additional field-assembled joints result in added mass and loads, as well as increased reliability concerns in operation. An alternative to this methodology is to design slender flexible blades that can be shipped on rail lines by flexing during transport. However, the increased flexibility is challenging to accommodate with a typical glass-fiber, upwind design. In a two-part paper series, several design options are evaluated to enable slender flexible blades: downwind machines, optimized carbon fiber, and active aerodynamic controls. Part 1 presents the system-level optimization of the rotor variants as compared to conventional and segmented baselines, with a low-fidelity representation of the blades. The present work, Part 2, supplements the system-level optimization in Part 1 with high-fidelity blade structural optimization to ensure that the designs are at feasible optima with respect to material strength and fatigue limits, as well as global stability and structural dynamics constraints. To accommodate the requirements of the design process, a new version of the Numerical Manufacturing And Design (NuMAD) code has been developed and released. The code now supports laminate-level blade optimization and an interface to the International Energy Agency Wind Task 37 blade ontology. Transporting long, flexible blades via controlled flapwise bending is found to be a viable approach for blades of up to 100 m. The results confirm that blade mass can be substantially reduced by going either to a downwind design or to a highly coned and tilted upwind design. A discussion of active and inactive constraints consisting of material rupture, fatigue damage, buckling, deflection, and resonant frequencies is presented. An analysis of driving load cases revealed that the downwind designs are dominated by loads from sudden, abrupt events like gusts rather than fatigue. Finally, an analysis of carbon fiber spar caps for downwind machines finds that, compared to typical carbon fibers, the use of a new heavy-tow carbon fiber in the spar caps is found to yield between 9 % and 13 % cost savings.

A work in progress is described that so far includes a finite element (FE) micromechanics modeling capability that generates random distributions of fibers in a periodic unit cell to predict fatigue damage in the matrix. The FE micromechanics model has automated features that facilitate parametric studies. This includes the ability to simulate any three-dimensional macroscopically uniform state of stress and to generate new fiber distributions within random-periodic unit cells that can have variable numbers of fibers. A damage evolution variable driven by the kinetic theory of fracture was implemented numerically in the FE computations. The spatial evolution of damage in the micromechanics model causes the deformation at some point to become unstable and define macroscopic failure of the composite. The effects of element size, time increment size, failed element residual properties, and fiber distribution were explored. Nomenclatureh = Planck's constant k = Boltzmann constant K b = molecular bond rupture rate T = absolute temperature U = activation energy γ = activation volume σ = effective stress σ 1 , σ 2 , σ 3 = ordered principal stresses ξ = hydrostatic stress η = second deviatoric stress invariant ξ 0 , η ∞ = constants in isotropic failure criterion ξ p , η p = stress point coordinates ξ f , η f = stress point coordinates on failure surface θ = orientation in the pi-plane

The development of a tightly coupled aeroelastic simulation capability for analysis and design is described in this paper. The method makes use of a well established unstructured mesh computational fluid dynamics solver, combined with a recently developed structural dynamics code. These two disciplinary codes are coupled through a fluid-structure interface and a mesh deformation capability. The discrete adjoint for all disciplinary software components has also been implemented with the goal of enabling time-dependent aeroelastic optimization. The individual disciplinary components are validated both in analysis and adjoint mode. Subsequently, the coupled aeroelastic analysis capability is demonstrated for both static and dynamic problems. Based on the validation and performance of these components, the future development of a time dependent coupled aeroelastic adjoint optimization capability is described.

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This report is available at no cost from the National Renewable Energy Laboratory at www.nrel.gov/publications.3. Completed research-and-development opportunity screening 4. Performed model improvements and detailed design studies 5. Assessed low-cost carbon fiber.The remainder of this report focuses on the findings from each task and includes excerpts and citations from previously published work.