Structural Design of Intelligent Wind Turbine Rotor Blades

The aim of the present master thesis is to investigate concepts of structural designs of wind turbine rotor blades, which should result into a load reduction. This load reduction should lead to larger rotor diameters being feasible for wind turbines, or existing blades being produced more cost-effectively by reducing the weight of the blade. With the help of automated processes, the structural models are constructed using a reference rotor blade and then the associated loads are simulated. The resulting loads and masses of the different concepts are compared with those of the reference rotor blade. The results show that both a structural design with a c-beam and a structural design with a swept beam leads to a load reduction. Another concept based on the use of an active trailing edge flap can only be evaluated using reference loads. This shows a significant increase in weight, which must be absorbed by a load reduction through the flap

Aero-structural coupled optimization of a rotor blade for an upscaled 25 MW reference wind turbine

One major challenge of the wind turbine industry is the reduction of the levelized cost of energy (LCoE) while following the strong demand for a higher annual energy production (AEP). To meet these goals, larger wind turbine sizes are required. The common method of upscaling existing wind turbine designs comes along with the problem of faster growing blade masses and costs compared to the AEP. Investigations in new technologies to improve the structural efficiency of larger blades can be supported by aero-structural coupled optimizations. The present work introduces a two-step aero-structural coupled design process to capture the multi-disciplinary trade-offs between costs and AEP, aiming at minimizing LCoE for a 25 MW wind turbine. In a first step, a preliminary aero-structural optimisation is carried out using simplified and fast methods. The output is then refined with respect to additional design criteria with an advanced optimization process, including an aero-servo-elastic coupled loads analysis. The process is applied to a 25 MW blade, upscaled from the IEA 15 MW reference wind turbine. Based on the results of an utilization analysis, the structural design is adapted, and a stiffness optimization is performed. The optimum airfoil positions are identified to reduce the amount of material while limiting losses in the aerodynamic performance. The obtained blade designs facilitate a consistent AEP compared to the upscaled reference design. A mass reduction of 35% could be achieved, which results in a reduced LCoE of 1.7% compared to the purely upscaled blade design

Aero-structural coupled optimization of a rotor blade for an upscaled 25 MW reference wind turbine

One major challenge of the wind turbine industry is the reduction of the levelized cost of energy (LCoE) while following the strong demand for a higher annual energy production (AEP). To meet these goals, larger wind turbine sizes are required. The common method of upscaling existing wind turbine designs comes along with the problem of faster growing blade masses and costs compared to the AEP. Investigations in new technologies to improve the structural efficiency of larger blades can be supported by aero-structural coupled optimizations. The present work introduces a two-step aero-structural coupled design process to capture the multi-disciplinary trade-offs between costs and AEP, aiming at minimizing LCoE for a 25 MW wind turbine. In a first step, a preliminary aero-structural optimisation is carried out using simplified and fast methods. The output is then refined with respect to additional design criteria with an advanced optimization process, including an aero-servo-elastic coupled loads analysis. The process is applied to a 25 MW blade, upscaled from the IEA 15 MW reference wind tubine. Based on the results of an utilization analysis, the structural design is adapted, and a stiffness optimization is performed. The optimum airfoil positions are identified to reduce the amount of material while limiting losses in the aerodynamic performance. The obtained blade designs facilitate a consistent AEP compared to the upscaled reference design. A mass reduction of 35% could be achieved, which results in a reduced LCoE of 1.7% compared to the purely upscaled blade design

Lightworks, a scientific research framework for the design of stiffened composite-panel structures using gradient-based optimization

Efficient structural optimization remains integral in advancing lightweight structures, particularly concerning the mitigation of environmental impact in air transportation systems. Varying levels of detail prove useful for different applications and design phases. The lightworks framework presents a modular approach, for the consideration of individual design parameterizations and structural solvers for the numerical optimization of thin-walled structures. The framework provides the combination of lightweight fibre composite design and the incorporation of stiffeners for a gradient-based optimization process. Therefore, an analytical stiffener formulation is implemented in combination with different continuous composite material parameterizations. This approach allows the analysis of local buckling modes, as well as the consideration of load redistribution between stringer and skin. The flexibility achieved in this way allows a tailored configuration of the optimization problem to the required level of complexity. A verification of the framework's implementation is carried out using established literature results of a simplified unstiffened wing box structure, where a very good agreement is shown. The accessibility of solvers with different fidelity through a generic solver interface is demonstrated. Furthermore, the usage of the implemented continuous composite parameterizations as design variables is compared in terms of computational performance and mass, providing different advantages and disadvantages. Finally, introducing stringer into the wing box use case demonstrates a 38% mass reduction, showcasing the potential of the inline optimization of stiffeners

Design of a sustainable rotor blade for wind turbines

At the Institute of Lightweight Systems of the German Aerospace Center, research in the field of the further development of wind turbines is also being conducted. One research topic is the development of sustainable rotor blades. The constructive approach of facilitated material separation in later recycling processes in combination with a material replacement of the conventional materials is pursued here. Therefore, different material combinations based on wood veneer materials or natural fibre composites were first analysed with the NREL-preliminary design tool WISDEM and then optimised with the DLR-optimisation framework lightworks. In this thesis, the state of the art about rotor blades in wind turbines, concepts for a sustainable design of these and the rotor blade design process are first discussed. Then the definition of the investigated use cases and the reference turbine is described. Furthermore, the preliminary and the advanced optimisation procedures are documented. Finally, the generated rotor blades are analysed with respect to the trade-off between sustainability and performance. This shows, that the design concepts based on birch LVL with a recycled foam core and on flax fibre in combination with a balsa wood core are the most balanced concepts with regard to the parameters blade mass, Levelised Cost of Energy (LCoE) and Global Warming Potential (GWP)

Design- and Manufacturing Constraints within the Gradient Based Optimization of a Composite Aircraft Wingbox

The gradient-based structural optimization plays an important role in the multi- disciplinary optimization of composite wings. To obtain the necessary convex design space, lamination parameters are used to parametrize the composite laminates. In contrast to metallic structures, more criteria must be considered during the opti- mization to determine a realistic result. The intention of using such criteria is to avoid the occurrence of critical failure modes and simultaneously guarantee a manu- facturable design. Common examples of these rules, usually defined on ply-level, are the symmetry of stackings in thickness direction for a better stress distribution in the laminate or the continuity of plies between neighbouring structural regions to ensure the manufacturability of the design. Up to now the ply-based definition of the design rules facilitated constraint formulations for genetic optimization algorithms, because they directly use the laminate stacking sequences as discrete design variables. In the present work a new method is presented to correlate discrete design rules with the lamination parameter space for a selected set of design- and manufacturing rules and a pre-defined set of ply angles. The method is used to derive constraints for the con- tinuous lamination parameter space of a gradient-based optimization process. The constraints interrelate the design variables of adjacent structural regions. This causes a restriction of the feasible domain of the design space in a way that selected design rules are fulfilled. The influence of the constraints on the structural design is shown with the help of a gradient-based optimization process which is applied to a generic wing box

Structural Optimisation of an Aircraft Wing using rapid Analytical Methods

The investigation of new aircraft designs requires robust and rapid evaluation methods on a physical basis allowing to explore the unknown design space. The structural optimisation of aircraft wings is particularly challenging due to aeroelastic coupling efects. The minimum structural mass of an aircraft wing with a given outer shape can only be found in a multi disciplinary optimisation including structural and load calculations. In this thesis a structural optimisation framework for wing-like structures is developed. Material formulations based on lamination parameters allow the usage gradient based optimisation algorithms. The modular optimisation framework provides a general interface to structural solvers enabling a multi-fidelity optimisation process. In this thesis the structural solver PreDoCS calculates internal structural loads with an analytical cross-section theory in combination with one dimensional finite beam elements. Outer loads are provided by external tools and imported through the standardised CPACS interface, allowing multi disciplinary coupling. A comparison of the optimisation results of PreDoCS and a finite element based structural solver establishes confidence in the framework. The optimisation of a mid range aircraft wing shows the potential of lamination parameter optimisation with a gradient based, analytical optimisation framework