Land-based wind turbines with flexible rail-transportable blades – Part 2: 3D finite element design optimization of the rotor blades

Abstract

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.

Accurate Modeling of Material Nonlinearities in a Wind Turbine Spar Cap

“…Table 1: Mechanical properties used in the pultruded carbon fiber composite model. Material property values for the biaxial glass core were taken from Camarena [7] with properties for the carbon fiber USERMAT from Table 1. Nodes corresponding to the major span of the beam had their Y and Z translational degrees of freedom fixed while the X degrees of freedom corresponding to the span direction were allowed to move.…”

“…The data and FE results are compared with a linear beam model where the strain ratio is equal to one, for reference. transportable 100 meter blade with carbon fiber spar caps was chosen for the comparison [7].…”

“…Both models are meshed using linear SHELL 181 elements with a mesh size of 0.1 meters. Model creation was automated using NuMAD 3.0 [7]. BAR-DRC was designed using pultruded Zoltek carbon fiber in it's spar caps with a volume fraction of 68%.…”

“…BAR-DRC was designed using pultruded Zoltek carbon fiber in it's spar caps with a volume fraction of 68%. The material tested in this paper had a fiber volume fraction of 62%, so elastic properties for the 68% volume fraction spar cap were used from Camarena [7]. The scale factor in the nonlinear model is dependent on strain and not the Young's modulus, and it is assumed that the calibrated values for K would not change.…”

“…This work presents component-level test data from carbon fiber sandwich beams and the effect that the carbon fiber elastic nonlinearity has on the structural response in bending. Additionally, Finite Element (FE) modeling is performed to accurately model the carbon fiber nonlinearity and implemented into a FE model of a modern wind blade design [7]. The FE wind blade model with carbon spar cap nonlinearity is then analyzed using a critical set of static design load cases representing the extreme loads, and compared to predictions using a standard linear elastic wind blade model.…”