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Wind power is a fast-growing source of non-polluting, renewable energy with vast potential. However, current wind technology must be improved before the potential of wind power can be fully realized. One of the key components in improving wind turbines is their blades. Blade failure is very costly because blade failure can damage other blades, the wind turbine itself, and possibly other wind turbines. A successful structural health monitoring (SHM) system incorporated into wind turbines could extend blade life and allow for less conservative designs. Impedance-based SHM is a method which has shown promise on a wide variety of structures. The technique utilizes small piezoceramic (PZT) patches attached to a structure as self-sensing actuators to both excite the structure with high-frequency excitations, and monitor any changes in structural mechanical impedance. By monitoring the electrical impedance of the PZT, assessments can be made about the integrity of the mechanical structure. Recent advances in hardware systems with onboard computing, including actuation and sensing, computational algorithms, and wireless telemetry, have improved the accessibility of the impedance method for in-field measurements. The feasibility of implementing impedance-based SHM on wind turbine blades is investigated in this work. Experimentation was performed to determine the capability of the method to detect damage on blades. First, tests were run to detect both indirect and actual forms of damage on a section of an actual wind turbine blade provided by Sandia National Laboratories. Additional tests were run on the same blade section using a high-frequency response function method of SHM for comparison. Finally, based on the results of the initial testing, the impedance method was utilized in an attempt to detect damage during a fatigue test of an experimental wind turbine blade at the National Renewable Energy Laboratory's National Wind Technology Center. / Master of Science

Wind turbine blades experience tremendous stresses while in operation. Failure of a blade can damage other components or other wind turbines. This research focuses on developing an electromagnetic energy harvester for powering structural health monitoring (SHM) equipment inside a turbine blade. The harvester consists of a magnet inside a tube with coils outside the tube. The changing orientation of the blade causes the magnet to slide along the tube, inducing a voltage in the coils which in turn powers the SHM system. This thesis begins with a brief history of electromagnetic energy harvesting and energy harvesters in rotating environments. Next a model of the harvester is developed encompassing the motion of the magnet, the current in the electrical circuit, and the coupling between the mechanical and electrical domains. The nonlinear coupling factor is derived from Faraday's law of induction and from modeling the magnet as a magnetic dipole moment. Three experiments are performed to validate the model: a free fall test to verify the coupling factor expression, a rotating test to study the model with a load resistor circuit, and a capacitor charging test to examine the model with an energy storage circuit. The validated model is then examined under varying tube lengths and positions, varying coil sizes and positions, and variations in other parameters. Finally a sample harvester is presented that can power an SHM system inside a large scale wind turbine blade spinning up to 20 RPM and can produce up to 14.1 mW at 19 RPM. / Master of Science

Conventional three dimensional structural analysis methods prove to be expensive for the preliminary design of wind turbine blades. However, wind turbine blades are large slender members with complex cross sections. They can be accurately modeled using beam models. The accuracy in the predictions of the structural behavior using beam models depends on the accuracy in the prediction of their effective section properties. Several techniques were proposed in the literature for predicting the effective section properties. Most of these existing techniques have limitations because of the assumptions made in their approaches. Two generalized beam theories, Generalized Timoshenko and Generalized Euler-Bernoulli, for the static analysis based on the principles of the simple 1D-theories are developed here. Homogenization based on the strain energy equivalence principle is employed to predict the effective properties for these generalized beam theories. Two efficient methods, Quasi-3D and Unit Cell, are developed which can accurately predict the 3D deformations in beams under the six fundamental deformation modes: extension, two shears, torsion and two flexures. These methods help in predicting the effective properties using the homogenization technique. Also they can recover the detailed 3D deformations from the predictions of 1D beam analysis. The developed tools can analyze two types of slender members 1) slender members with invariant geometric features along the length and 2) slender members with periodically varying geometric features along the length. Several configurations were analyzed for the effective section properties and the predictions were validated using the expensive 3D analysis, strength of materials and Variational Asymptotic Beam Section Analysis (VABS). The predictions from the new tools showed excellent agreement with full 3D analysis. The predictions from the strength of materials showed disagreement in shear and torsional properties. Explanations for the same are provided recalling the assumptions made in the strength of materials approach.

Offshore wind power production is an attractive clean energy option, but the difficulty of access can lead to expensive and rare opportunities for maintenance. Smart loads management (controls) are investigated for their potential to increase the fatigue life of damaged offshore wind turbine rotor blades. This study will consider two commonly encountered damage types for wind turbine blades, the trailing edge disbond (bond line failure) and shear web disbond, and show how 3D finite element modeling can be used to quantify the effect of operations and control strategies designed to extend the fatigue life of damaged blades. Modern wind turbine blades are advanced composite structures, and blade optimization problems can be complex with many structural design variables and a wide variety of aeroelastic design requirements. The multi-level design method is an aeroelastic structural design technique for beam-like structures in which the general design problem is divided into a 1D beam optimization and a 2D section optimization. As a demonstration of aeroelastic design, the multi-level design method is demonstrated for the internal structural design of a modern composite rotor blade. Aeroelastic design involves optimization of system geometry features as well as internal features, and this is demonstrated in the design of a flying wing aircraft. Control methods such as feedback control also have the capability alleviate aeroelastic design requirements and this is also demonstrated in the flying wing aircraft example. In the case of damaged wind turbine blades, load mitigation control strategies have the potential to mitigate the effects of damage, and allow partial operation to avoid shutdown. The load mitigation strategies will be demonstrated for a representative state-of-the-art wind turbine (126m rotor diameter). An economic incentive will be provided for the proposed operations strategies, in terms of weighing the cost and risk of implementation against the benefits of increased revenue due to operation of damaged turbines. The industry trend in wind turbine design is moving towards very large blades, causing the basic design criterion to change as aeroelastic effects become more important. An ongoing 100 m blade (205 m rotor diameter) design effort intends to investigate these design challenges. As a part of that effort, this thesis will investigate damage tolerant design strategies to ensure next-generation blades are more reliable.

Designing horizontal-axis wind turbine (HAWT) blades to achieve satisfactory levels of performance starts with knowledge of the aerodynamic forces acting on the blades. In this thesis, HAWT blade design is studied from the aspect of aerodynamic view and the basic principles of the aerodynamic behaviors of HAWTs are investigated. Blade-element momentum theory (BEM) known as also strip theory, which is the current mainstay of aerodynamic design and analysis of HAWT blades, is used for HAWT blade design in this thesis. Firstly, blade design procedure for an optimum rotor according to BEM theory is performed. Then designed blade shape is modified such that modified blade will be lightly loaded regarding the highly loaded of the designed blade and power prediction of modified blade is analyzed. When the designed blade shape is modified, it is seen that the power extracted from the wind is reduced about 10% and the length of modified blade is increased about 5% for the same required power. BLADESIGN which is a user-interface computer program for HAWT blade design is written. It gives blade geometry parameters (chord-length and twist distributions) and design conditions (design tip-speed ratio, design power coefficient and rotor diameter) for the following inputs / power required from a turbine, number of blades, design wind velocity and blade profile type (airfoil type). The program can be used by anyone who may not be intimately concerned with the concepts of blade design procedure and the results taken from the program can be used for further studies.

Wind energy, one of the most promising sources of clean energy, has developed rapidly over the last two decades. Wind turbines (WT) are arguably clean during operation, offering minimal pollution and zero CO2 emissions, but significant amounts of energy are used and CO2 emitted during their manufacture, and, furthermore, the turbines are environmentally problematic at end-of-life (EoL), especially the blades. WT blades are mainly made with composite materials comprising thermosetting resin and glass fibre. They are lightweight and strong but problematic to recycle. Large volumes of waste will be generated when these WT blades are decommissioned and environmental concerns have been raised. The main aim of this study is to understand the environmental impact of wind turbine blades and to find solutions to reduce it. A quantitative method is adopted, first evaluating the WT blade waste inventory then calculating its environmental impact, and finally analysing the differences between all possible EoL options in terms of environmental and financial performance. The results firstly identify the global wind turbine blade waste inventory with detailed generation time and location which could help policy makers to gain an understanding of the size and severity of this problem. Secondly, the outputs indicate where most impact is generated and identify what to prioritise to reduce waste and reduce environmental impact, which is of value to blade manufacturers and other stakeholders. Moreover, this work highlights previous incorrect assumptions and provides findings to build on for future work. Thirdly, ‘optimal’ EoL options for the WT blade waste have been characterized: the current ‘optimal’ EoL option is life extension; mechanical recycling is the current ‘optimal’ recycling option; chemical recycling will be the ‘optimal’ option for the future. Future research is suggested as aiming to improve the performance of recycled fibre or to reduce the energy consumption of recycling processes.

The Wind Turbine industry continues to drive towards high market penetrationand profitability. In order to keep Wind Turbines in field for as long as possiblecomputational analysis tools are required. The open source tool QBlade[38] softwarewas extended to now contain routines to analyse the structural properties of WindTurbine blades. This was achieved using 2D integration methods and a Tapered Euler-Bernoulli beam element in order to find the mode shapes and 2D sectional properties.This was a key step towards integrating the National Renewable Energy LaboratoriesFAST package[32] which has the ability to analyse Aeroelastic Responses. The QFEMmodule performed well for the test cases including: hollow isotropic blade, rotatingbeam and tapered beam. Some improvements can be made to the torsion estimationof the 2D sections but this has no effect on the mode shapes required for the FASTsimulations.

Large wind turbine blades, manufactured from fibre reinforced laminated composite materials, are key structural components of wind turbine systems. The demands for efficient and accurate modelling techniques of these composite blades have significantly increased. Over past decades, although complex computational models have been widely developed, more analytically based models are still very much desired to drive the design and optimization of these composite blades forward to be lighter, stronger, efficient and durable. The research work in this thesis aims to develop such more analytically based aerodynamic, structural and aero-elasticity models for large wind turbine blades manufactured from fibre reinforced laminated composite materials. Firstly, an improved blade element momentum (BEM) model has been developed by collectively integrating the individual corrections with the classic BEM model. Compared to other existing models, present BEM model accounts for blade tip and root losses more accurately. For laminar flow, the 3-D cross-flow is negligibly small. In this case, present BEM model with statically measured 2-D aerodynamic coefficients agrees closely to experimental measurements. However, stall delay correction is required for a 3-D rotating blade in stall. A new stall delay model is developed based on Snel s stall delay model. Verifications are performed and discussed for the extensively studied NREL UAE phase-VI test. The predictions of distributive and collective factors, e.g. normalised force coefficients, shaft torque and etc. have been compared to experimental measurements. The present BEM model and stall delay model are original and more accurate than existing models. Secondly, significant deficiency is discovered in the analytical thin-walled closed-section composite beam (TWCSCB) model proposed by Librescu and Vo, which is widely used by others for structural modelling of wind turbine blades. To correct such deficiency, an improved TWCSCB model is developed in a novel manner that is applicable to both single-cell and multi-cell closed sections made of arbitrary composite laminates. The present TWCSCB model has been validated for a variety of geometries and arbitrary laminate layups. The numerical verifications are also performed on a realistic wind turbine blade (NPS-100) for structural analysis. Consistently accurate correlations are found between present TWCSCB model and the ABAQUS finite element (FE) shell model. Finally, the static aero-elasticity model is developed by combining the developed BEM model and TWCSCB model. The interactions are accounted through an iterative process. The numerical applications are carried out on NPS-100 wind turbine. The numerical results show some significant corrections by modelling wind turbine blades with elastic coupling.

Bakgrunden till projektet är problem med kanterosion av turbinblad inom vindkraftverks- industrin. Det är ett fenomen som uppstår när turbinblad roterar i höga hastigheter och träffar partiklar, främst vattendroppar i regn. Denna erosion skadar turbinbladen, vilket både minskar vindkraftverkens effektivitet och sprider partiklar i den lokala miljön. För att både undersöka detta fenomen och ge möjlighet att utvärdera olika materials motståndskraft mot erosion ska en testrigg tas fram. Utöver detta ska testriggen även möjliggöra uppsamling av partiklar för vidare forskning kring deras effekt på miljön. Som utgångspunkt används en tribometer med rotationsmekanism från ett föregående maskinkonstruktionsprojekt på KTH. Ombyggnationen av denna avgränsas till att endast genomföras digitalt med hjälp av CAD för att hålla mängden arbete till en rimlig nivå. Förutom CAD har arbetet även inkluderat kravspecifikationer, beräkningar i MATLAB, FEA-analyser och kostnadskalkyler. Resultatet är en digitalt styrd testrigg med tillhörande komponentlista och instruktioner för tillverkning och genomförande. Inköpskostnad för ombyggnationen uppskattas till 28 000 kr. Riggen för en cylindrisk provbit genom ett artificiellt regn i hög hastighet, vilket resulterar i en accelererad nötningsprocess. Under testets gång dokumenteras erosionen visuellt med hjälp av en kamera och efter testet kan mängden förlorat material mätas i vikt och partiklar samlas upp. Flera andra parametrar dokumenteras automatiskt under testets gång för att ge en mer detaljerad bild av processen och data för undersökningar av repeterbarhet. Slutligen konstateras att testriggen uppnår alla krav ställda på den förutom att den inte har en nödbroms. Avsaknaden av nödbroms diskuteras och det leder till slutsatsen att testriggen, även utan nödbroms, är fullt fungerande, enkel och säker att använda. / The background for this project is a problem in the wind turbine industry, namely leading edge erosion of turbine blades. This occurs when wind turbine blades rotate at high speeds and collide with particles, mainly water drops in rain. This erosion damages the turbine blades, reducing the efficiency of the wind turbines and releasing particles into the local environment. To investigate this phenomenon and evaluate the durability of different materials, a test rig is to be developed. In addition, the test rig will enable the collection of particles to facilitate further research into their environmental impact. A tribometer with a rotation mechanism from a previous project at KTH serves as the starting point. The reconstruction of this tribometer is limited to a digital implementation using CAD to keep the amount of work at an appropriate level. In addition to CAD, the work has also included requirements specifications, calculations in MATLAB, FEA, and cost estimates. The result is a digitally controlled test rig with an accompanying component list and instructions for manufacturing and implementation. The estimated purchase cost for the reconstruction is 28,000 SEK. The rig tests a cylindrical sample by propelling it at high speed through artificial rain, resulting in an accelerated wear process. The erosion is visually documented using a camera during the test. Afterwards the amount of lost material can be measured by weight and the particles collected. Several other parameters are automatically recorded during the test to provide a more detailed picture of the process and data for investigations into repeatability. Finally, it is concluded that the test rig meets all its requirements except for the absence of an emergency brake. The absence of an emergency brake is discussed, leading to the conclusion that despite missing an emergency brake, the test rig is fully functional, easy to use, and safe.