Influence of blade structural deformation on wind turbine rotor performance

In this study, the behavior of the wind turbine blade working under various loads (aerodynamic, centrifugal, gravitational etc.) have been investigated. Considering the interaction between the aerodynamic and structural disciplines a methodology was developed to be used during blade design stages starting from the preliminary design to the performance calculations, and parametric studies have been conducted on a sample blade. Blade Element Theory and Classic Lamination Theory are the principal theories that were utilized during the study. To conduct these methods and the interaction between them software has been developed in MATLAB environment. The outputs of the developed software have been compared with the studies performed by NREL and it was seen that the results were close to each other. The thesis was concluded by carrying out the necessary inter-disciplinary studies, to understand the structural and aerodynamic behavior of the blade and the interaction between them, required for the design of a blade with the desired properties. The thesis was aimed to be a guide for the designers and researchers in the design of a high-performance blade by using the disciplines mentioned and investigating the relation between them. During the preliminary design stage a base structure is presented that includes the parametric studies such as material selection, layup angle, profile selection and dimensioning and the calculation of the effect of these parameters on performance swiftly. The evaluation of the aerodynamic – structural interaction is one of the critical points of the thesis. The energy production of the wind turbine is dependent on the interaction between the rotor and the wind. The wind speed can be assumed as the combination of an average speed and speed deviations that oscillate around the average speed, including turbulence. It was seen that the main parameters of the wind turbine performance (average power production and average loads) are determined by aerodynamic forces created by average speed. Wind shear, side wind, periodic aerodynamic forces due to the rotation of the rotor, turbulence and forces that show random oscillation due to dynamic effects are the source of fatigue loads and used for determining the highest load value that a wind turbine could be subjected to. In the chapter about the aerodynamic studies, primarily the analysis of an ideal wind turbine is featured. The general behavior of the rotor, the surrounding flow and important concepts are covered. Analyses are used for the calculation of theoretical performance limits of wind turbines. General aerodynamic concepts and the working principles of profiles (airfoil) are expressed. The first classical aerodynamic analysis of the wind turbine was performed by Betz and Glauert in the 1930s. The theory was later expanded so that it could be used incomputers. In all these methods, the momentum theory and blade element theory were combined and transformed into a band theory that enabled the calculation of performance characteristics of a circular section of rotor. The performance of the whole is calculated by integrating all values found for the circular sections. Primarily, the momentum theory and the blade element theory are expressed and used for the design of an optimum blade design under ideal operational conditions. Concepts that determine the outline of the modern wind turbine blades. Then, the combination of the two methods, band theory, or in other words blade element momentum theory was used to create a procedure for the aerodynamic design of a wind turbine rotor and its performance analysis. Moreover, taking into account the aerodynamic losses and considering a realistic flow space, a starting point for an optimum blade design has been reached. During the blade design, the designer should take into account the inducted torsion angle resulting from structural deformation due to the aerodynamic, gravitational, inertia and control-based forces that occurs during the operation. Thus, in order to design the blades that would show the best performance in targeted operational conditions, the structural behavior of the composite wind turbine blades should be well understood. This type of analysis requires establishing an interaction between composite mechanics and aerodynamics. The first option that comes to mind here is establishing this interaction by finite elements and computational fluid dynamics which are 3D solutions, but these methods are time consuming are not flexible enough for quick studies during the preliminary design stage. The alternative is providing the designer a systematic approach and information about blade behavior by using reduced models both on the aerodynamic and structural side. In the aforementioned chapter the aerodynamic side was studied by blade element momentum theory. In the latter chapter the structural side was focused on, and applicable and quick method was developed by using Classical Lamination and Thin Walled Structures theories. This method forms a basis of a tool that enables parametric studies, selection of material and lamination parameters during the preliminary design stage. Turbine blades are long and slender structures that are suitable for modelling like beams, thus it is a good approach to perform a modelling that calculates structural properties like profile torsion and bending stiffness. Two theories are used for this approach. The first one is the Classical Lamination Theory (CLT) required to calculate the main characteristics of the composite structure, and the second one is Thin Walled Structures Theory which establishes the relation between the CLT and the profile geometry. In the relevant chapter of the thesis the theories above are given in detail and the mechanical properties of composite blade geometries based on NACA profiles are calculated. In the thesis chapter named The Establishment of the Aerodynamic – Structural Interaction the required theoretical basis is created for the aerodynamic-structural interaction for both disciplines. In this section, the change in aerodynamic loads due to the deformation in the blade and the relation between the loads and deformation are expressed by using an iterative interaction. This is achieved by firstly calculating the aerodynamic loads and the deformation due to these loads, and then this deformation was used to calculate new aerodynamic loads. This approach was continued until reaching an acceptable error level between two consecutive iterations, and thus a final load and deformation state was reached. In order to apply the studies conducted in this thesis parametric calculations were done for a sample 12 m blade considering different operational conditions. In the light of the result of these calculations, the effect of assuming the blade as rigid or elastic on the energy production performance was presented. Important structural parameters for the elastic blade was calculated with the help of the developed software. Differences below were principally observed after the studies made on the sample blade. • Difference between loads occurring at blade tip stations are about 2,5% • The difference in the power curve of the turbine is changing between 0,1% – 12%, proportional to the increase in wind speed. As expected, higher speeds results in more deformation and the performance decreases. • In a study using wind measurement data for three different provinces of Turkey, energy production differences between 1,3% - 1,5% were observed between the assumptions of rigid and elastic blades. • The possibility of the bending-torsion interaction having a positive effect on the performance was also investigated with the developed methods. The aim is finding the optimum torsion angle that would increase the aerodynamic performance of blade to the highest level with the help of interaction and determining parameters like the layup angle, chord length and the wall thickness in this direction. In the last part of the thesis a study was made on the sample blade in this context. Although it has small effects, increasing the performance with the bending-torsion effect is potential field of study.