In recent years, there has been an increasing interest in vertical axis wind turbines (VA WTs); as they show a number of benefits when installed in the built environment. Nevertheless, VAWTs still suffer from: complex aerodynamics; dynamic stall and hence lower efficiency. Additionally, when they are integrated in the built environment the challenges are low/zero starting torque, noise and vibrations, visual impact and blade safety. In response to these needs, a vertical axis augmented wind turbine (A WT) has been developed, which is composed of a stator and a rotor. In this thesis, the aerodynamics of the vertical axis augmented wind turbine (A WT) was examined by employing a Computational Fluid Dynamics (CFD) tool. The main focus was on the 2D and 3D simulations of the vertical axis augmented wind turbine (A WT). In the 2D investigation, an open rotor was analysed in order to select the most appropriate mesh, turbulence model, and timestep. Successively, the rotor was combined with a stator and the results compared to the open rotor. The development of dynamic stall in both open and augmented rotors was also examined. The CFD results of the open rotor were validated using experimental data. In the 3D stator, the focus was to understand how the stator blades, the diameter and the conical surfaces could affect the flow. In the 3D open and augmented rotors, the results were compared to the 2D cases in order to evaluate similarities and dissimilarities. The 2D investigation has shown that the forces generated on the blades of an open rotor were dependent on the mesh resolution and turbulence model selected, while the timestep had small impact. The introduction of an augmenter has the potential to increase the power coefficient by 1.35 times when compared to the open rotor. However, the stator blade and conical surface orientation were found to affect the performance of the A WT, while changing the rotor blade orientation had small impact. The generation of dynamic stall was detected at low tip speed ratios (TSRs), but the turbulence model could affect its formation. The CFD results of the open rotor were found to be in good agreement with experimental data. In the 3D investigation, the stator was found to guide and accelerate the flow, but the orientation of both conical surface and stator blade play an important role. The 3D simulations, related to open and augmented rotor, were found to have similarities and dissimilarities when compared to the 2D. This research has developed a modelling technique, which could help with the further development of the AWT.

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In this research control algorithms and implementation for variable speed stall regulated wind turbines are presented. This type of wind turbine has a simpler and more robust construction and can have lower requirements for maintenance than the existing pitch regulated wind turbines. Due to these features these wind turbines can have reduced cost, which is a crucial parameter especially for large scale wind turbines. However, this type is not commercially available yet due to existing challenges in its control. In this research a complete control scheme for variable speed stall regulated wind turbines has been developed and implemented in a fully dynamic hardware-in-loop simulator for variable speed wind turbines. The simulator was developed as part of the project in order to validate the designed control algorithms. The developed control system uses novel adaptive methods in order to maximize the energy production of the wind turbines at below rated wind speeds as well as to control the power of the wind turbine at above rated wind speeds. In addition, several types of controllers including robust controllers have been used and tested, which resulted to novel control solutions for stall regulated wind turbines. The main advantage of the proposed control method is that it uses existing hardware without requiring additional sensors, so it more effectively exploits information coming from measurements available in existing wind turbine converters. Through software and hardware simulations the proposed control algorithms seem to be quite promising and give confidence for the future development of variable speed stall regulated wind turbines.

Manufacturing defects are one of the main reasons for premature failure of wind turbine blades. This thesis investigates the effects of manufacturing defects on the fatigue performance of composite wind turbine blades. A strategic defect risk management protocol for wind turbine blade manufacture is proposed, in which the classification of defect characteristics, evaluation of the effects of defects, their criticality and implementation of manufacturing quality control strategies form the key phases. Based on published data, ply waviness, voids, delamination, adhesive failure and thickness difference are identified as the key manufacturing defects in composite wind turbine blades. These defects are attributed to poor quality raw materials and inadequate production process parameter control. Wet-layup, Prepreg, SPRINT-Prepreg and Vacuum Assisted Resin Transfer Moulding (VARTM) processes were considered for the analysis. Three small scale wind turbine blades were manufactured and a case study was conducted to determine the influence of VARTM process parameter variations to initiate manufacturing defects. It was identified that the resin flow of the VARTM process should be closely controlled to avoid defects.

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This thesis deals with the compliance of fixed speed and variable speed wind turbines based on doubly fed induction generator (DFIG) with the new UK grid code in terms of reactive power capability and fault ride through capability. Moreover, a new technique which enables the converter rating reduction of the DFIG based on variable frequency operation is proposed and evaluated in this thesis.

Recently, morphing structures have found increased interest due to their potential for combining conflicting requirements of strength, flexibility and low mass. Many concepts depend on continuously powered actuators to deform the structure. Others possess multiple stable states, i.e. multi-stability and power is only needed to change the shape, not to hold it. This dissertation explores the design space of a helical morphing, composite, twisting structure capable of large deformations along its axis of twist with non-linear stiffness properties. Multi-stability is achieved by a combination of pre-stress, geometry of the structure and material properties. Multi-stability is fully exploited by demonstrating the capability of the helix to be bi-stable or, depending on the design parameters, to hold any twisted configuration; hence presenting the remarkable property of zero torsional stiffness. Three proof-of-concept case studies are detailed: simple helices, a wind turbine twisting tip and a bi-stable I-beam. In the first instance, the helical structure is investigated and analytically modelled. Prototypes are developed to verify the non-linear stiffness properties of the structure and experimental results are correlated against analytical and finite element model data. Three different stability characteristics are explored and detailed. Multi-stability is analysed using a simple analytical model, predicting the positions of stable and unstable states for different design parameters and material properties. Actuation using piezoelectric material is also explored in a separate analytical study. Then, the negative stiffness property of the pre-stressed structure is incorporated into a half-scale twisting wind turbine blade. Overall, the manufactured blade achieves zero stiffness in torsion for angles of twist between _5° to +5° due to the added helical structure. Finally, a common structural component, the I-beam, is redesigned to show bi-stability.

This thesis focuses on the development of a computational model of the LR TM process. To date, no such solver exists either commercially or as an open source product. The purpose of this work was to construct a solver in order to optimise the production of a composite blade skin for a 6kW wind turbine being built as part of a larger research project. It was also used in conjunction with a technical cost model to compare the applicability of three potential blade manufacturing methods (RTM, VI and LRTM), allowing selection of the most appropriate process. A review of composite process modeling and the physical aspects of LRTM identified key areas for model consideration. Three distinct filling stages provided a basis for a coupled finite element analysis and infusion simulator, with data being interpreted and passed by a custom built Python script. LRTM specific permeability and compaction test procedures were developed, enabling fabric characterisation and the derivation of empirical material models. Empirical verification showed high levels of model agreement (>96%) based on two different metrics, and the wind turbine blade case study suggested a potential 95% reduction in fill time was achievable by careful selection of the infusion strategy. LRTM was demonstrated to be an economic option for low- 1 cost composite parts, giving high thiclmess stability and quick infusion times at low fibre volume fraction. 2

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