A key challenge in the Airborne Wind Energy industry is the minimisation of the time and energy required for the transfer and return phases inherent to the power cycles of ground-generator-based kite power systems. This thesis contributes to solving this problem by exploring the use of bleed air spoilers during the return phase in terms of their potential to improve the average cycle performance. To this end, a mathematical model is first introduced to evaluate the individual phases of the characteristic power pumping cycles in a general sense. This model is further used to demonstrate that a given kite configuration with the capacity to selectively lower its glide slope and/or resultant aerodynamic loading could improve the net power output of the system by around 30%. A CFD-based simulation environment for investigations of the effects induced by bleed air spoilers is developed using Simcenter STAR-CCM+. To establish the reliability of the setup, a first study revolves around its validation with experimental data from independent wind tunnel tests on bleed air spoilers effects on ram-air parachutes. The second CFD study is based on a standard kite geometry employed by SkySails Power GmbH. Two parameter studies testing the influence of spoiler size and chordwise position are carried out to investigate how bleed air spoilers are best used to reach the desired aerodynamic effects. The results show that bleed-air spoilers should ideally be applied in the vicinity of the suction peak and that relatively small spoiler geometries can already entail a major effect on the resultant flow field. It is predicted that the evaluated configurations would improve the system's overall performance by 9 to 18.3%, mostly due to substantial reductions in the glide ratio.

In recent years, the research on Vertical Axis Wind Turbines (VAWTs) have been paving the way for the very large-scale (10-20 MW) floating offshore applications. For a cost effective utilization, VAWTs need to deliver superior aerodynamic and structural performance. Nowadays, the complex ow phenomenon of VAWTs have been better understood and the computational tools have reached to a mature level. It is time to explore the capabilities of this concept and improve its aerodynamic features by innovative design ideas. The main goal of this thesis is to enhance the performance of the lift-driven VAWTs by customized airfoil design and the active ow control with trailing edge aps. To achieve the research goals, four main lines of work are carried out: 1) performance exploration of an actuator cylinder, 2) design of the azimuthal flap control sequences, 3) comparison between the Actuator Cylinder Model and Panel Model in the presence of the active flap control and 4) airfoil design that is specified for effective flap operation on a VAWT blade. The flap control sequences are optimized with the use of the Actuator Cylinder Model for three aerodynamic objectives. These objectives are aiming to improve the power efficiency by maximizing the CP , provide a rated power control by minimizing the CP and decrease the cyclic load ranges by minimizing the CT . Whereas, RFOIL and NSGA-II genetic algorithm are coupled for the airfoil design, which aims for objectives such as superior single airfoil performance, extended flap sensitivity and high flap-wise bending stiffness. For a rotor solidity of 0.1 operating at tip speed ratio of 4, a 10% active flap is able to increase the CP by 7% , decrease the CP by 10% and alleviate 12% of the CT by sacrificing 3% of the CP . It is shown that depending on the solidity, tip speed ratio and the flap authority these figures could be increased. It is possible to brake the rotor with a relatively larger flap authority. The airfoils designed in this work are slightly cambered and span from 27% to 35% thickness, deliver higher maximum CP than a NACA 0018, show good performance in the dirty conditions and have high flap effectivity. Overall, the performance gains acquired by the new airfoils and flap control promises extensive improvements in multiple features of VAWTs such as power effciency, power control, load control that lead to reduced weight and decreased Cost of Energy.

Using renewable energy to power a Mars habitat is a technological challenge because resources such as solar and wind are significantly weaker than on Earth. This work investigates the feasibility of using airborne wind energy (AWE) systems in combination with solar photovoltaic (PV) modules to power a Mars habitat. The Luchsinger model and the higher fidelity QSM are used to simulate the performance of the AWE system and compared. This thesis builds upon two earlier design synthesis exercise (DSE) projects by implementing a version of the quasi-steady model (QSM) that accounts for the transition phase, models a realistic retraction trajectory, and accounts for the mass of the airborne components. Additionally, the results of the first DSE indicate that the Luchsinger model used did not appear to consume any energy during the reel-in phase, which is not realistic and led to an over-prediction of mean cycle power. However, this thesis aims to implement the Luchsinger model correctly. Creating a road map for sizing AWE kite systems on Mars is the main objective of this thesis. Since the performance of the AWE system is varying in time and space, the Mars Climate Database (MCD) is used to retrieve atmospheric and surface solar flux data including wind Weibull probability distribution functions (PDF). The second DSE used the MCD and validated the results against wind data from various Mars landers. The MCD is based on numerical simulations of the Martian atmosphere using a general circulation model and validated with available observational data. Seasonal vertical wind profiles are generated from the meteorological data to characterise the boundary layer over time. A scaling study assesses how AWE on Mars differs from that on Earth, performing dimensional analysis. In the system characteristics chapter, the initial sizing of the kite area and mass is computed using the scaling study. The performance models create the power curves, which together with the wind PDFs and surface solar flux data are used in the habitat energy model to verify whether the power requirements are met. Due to an insufficient amount of quantitative information on the energy consumption of the robotic construction of the habitat, the design of the microgrid is covering only the use of the habitat, which is 10 kW of continuous power. This is similar to remote off-grid solutions on Earth, with the additional challenge of having lower resource availability, both for wind and solar. This thesis concludes that various configurations of a hybrid power plant can continuously provide 10 kW of power throughout the entire Martian year. Moreover, the results indicate that using kites alone could generate sufficient power for the habitat without using solar PV. Rhizome project: http://www.roboticbuilding.eu/project/rhizome-development-of-an-autarkic-design-to-robotic-production-and-operation-system-for-building-off-earth-habitats/ Published poster on thesis for Airborne Wind Energy Conference in Milano, Italy,: https://repository.tudelft.nl/islandora/object/uuid%3A4013ca0b-5508-4413-b8cb-5251ca1e7781 Video made to illustrate how Mars flight would look like.: https://www.youtube.com/watch?v=YS20vHhqgKk&ab_channel=AirborneWindEnergyOnline

When designing an airborne wind energy system, it is necessary to be able to estimate the traction force that the kite produces as a function of its flight trajectory. Being a flexible structure, the geometry of a soft kite depends on its aerodynamic loading and vice versa, which forms a complex fluid-structure interaction (FSI) problem. Currently, kite design is usually done on an experimental basis since no model meets the requirements of being both accurate and fast. In this project, an FSI methodology is developed to study the steady-state aerodynamic performance of leading-edge inflatable (LEI) kites by coupling two fast and simple models. On the structural part, the deformations are calculated with a particle system model, based on the assumption that the shape of the kite can be modelled using a wireframe wing model represented by the bridle line attachment points, whose coordinate changes are modelled using a bridle line system model and canopy billowing relations. On the aerodynamic side, the load distribution is calculated with a 3D nonlinear vortex step method, coupled with 2D polars obtained with a correlation model derived from Reynolds averaged Navier-Stokes (RANS) analysis, to account for viscous effects and flow separation. Furthermore, with the 2D correlation model it is possible to consider changes in the thickness and the camber of each section. Based on 2D thin airfoil theory, the three-quarter chord point is used to determine the magnitude of the forces, and the one-quarter chord point is used to determine the direction of these forces. Moreover, the model developed for LEI kites can consider canopy billowing and variations in kite and airfoil geometry while proving robust and inexpensive. This model has been validated with several geometries and a RANS analysis of the LEI kite, showing great accuracy for pre-stall angles of attack. The coupling of these two models results in a fast aeroelastic model of LEI kites capable of predicting the steady-state deformations and aerodynamic forces on the kite for the range of actuation settings and inflow conditions expected during a normal pumping cycle. Furthermore, the results show that the deformations follow the same trends as the results from the photogrammetry analysis and that, by taking into account the deformations that the kite undergoes, the aerodynamic forces more closely resemble experimental data.

In order to reduce the levelised cost of energy, the rotors of wind turbines are increasing in size. To increase the energy yield, wind turbine rotors need to have an innovative tip design; such as winglets. Winglets are used widely in aircraft design; however, they remain mostly absent in state-of-the-art wind turbine design. The low-fidelity wind turbine design models used by industry, such as BEM, are insufficient to capture the full 3D flow physics in such an innovative design. Therefore, high-fidelity methods, such as vortex methods, are becoming more and more important in such a design phase in wind energy research. Single-objective optimisation has been applied in earlier works to maximise power production. Winglets have shown the potential of increasing power production while simultaneously increasing the design-driving loads (DDLs), for example, the thrust or flapwise bending moment. This work focuses on optimisation using machine learning of a winglet that increases power production without increasing DDLs. A parameterised design for a winglet on a wind turbine is created. Different design constraints, such as DDLs or a diameter constraint, are explored to determine under which constraints and conditions a winglet can have an added value to the wind turbine blade design. Multi-objective Bayesian optimisation is used to maximise the rotor's power production and minimise DDLs. Surrogate models, created using machine learning techniques such as Gaussian Processes and Bayesian Neural Networks, are used in combination with an acquisition function, to determine what designs should be evaluated by the lifting line model AWSM. This has the goal to obtain designs that lie on the Pareto front of two or more objectives. The recent Bayesian Neural Networks were able to find the Pareto front most effectively in this work. Furthermore, the results show that different DDL constraints led to different winglet designs, with noticeable differences between upwind and downwind winglet designs obtained by the optimiser. Both a downwind and upwind winglet were found to be able to increase power without increasing the thrust, root flapwise bending moment and flapwise bending moment at 80% of the rotor radius.

Aeroelastic codes are fundamental for the design of wind turbines and the prediction of instabilities. Such codes rely on engineering models which limit their accuracy. With wind turbine blades becoming more slender to lower the costs, it is important to reduce these uncertainties to maintain a safe and stable turbine design. The dynamic stall model is one of these engineering models which simplifies the physics involved and so an improvement in accuracy might be possible. Various dynamic stall models have been published over the last 50 years. However, dynamic stall has proven to be a complex phenomenon to model accurately over a wide range of conditions and is an ongoing topic of research. Here four semi-empirical dynamic stall models, the Øye, Risø, Snel and ONERA models, are compared first in 2D against wind tunnel data to understand their accuracy and limitations. The experimental data for this comparison uses the NACA0015, NACA0030 and NACA4415 airfoils over a variety of cases relevant to wind turbines. Hereafter the models are compared within an aeroelastic code as a part of complete horizontal axis wind turbine simulations for an extreme load case (IEC design load case 1.4), standstill instabilities (IEC design load case 6.2) and the flutter speed to understand the effect of the different dynamic stall models. For the standstill cases the aerodynamic damping provided by each model is compared by reducing the structural damping of the blades to find the point where the blades become unstable. Next the Øye and Risø models are tuned to the wind tunnel data. The results from the 2D comparison are inconclusive with each model showing different strengths and weaknesses. In general the attached flow physics in the Risø and ONERA models improves the fit for the 15% thick airfoils. The ONERA model captures the lift peak the best, although it usually has the largest least squares error to the data due to the drop in lift after this peak being too early and too sharp. Once in the aeroelastic code the ONERA model started to show unphysical behaviour by reducing the deflection in the extreme load case and adding negative damping to the standstill cases. Snel’s model, on the other hand, adds so much damping that the blades remains stable even when the blades have only a small amount of structural damping, which is likely not physical. The Øye and Risø models show similar damping levels and reduce the required structural damping to prevent the standstill instabilities by at least a factor two with respect to no dynamic stall model being used. For the classical flutter analysis the Risø model shows an increase in flutter speed as expected from the implementation of Theodorsen’s theory, while the ONERA model decreases the flutter limit. Overall the Risø model is seen as the best for in an aeroelastic code due to showing better behaviour in the full turbine cases than the ONERA and Snel models. Furthermore, it is superior to the Øye model due to correctly modeling the attached flow physics and improving the drag and moment coefficients.

The renewable energy need facilitates the development of offshore wind power. The abundant wind resource has pushed the wind farms into deep water sites. The floating VAWT concept is emerges as the times require. This research is part of Deepwind concept development. The project focuses on verifying the feasibility of using flap control to increase the power conversion of a VAWT through the numerical investigation. As a preliminary implementation, only a 2-D section is taken out from the Darrieus rotor as the modeled object. The report concentrates on the purely aerodynamic modeling of a VAWT. All the blades are assumed to be fully rigid in order to exempt the structural response. VAWTs present a distinct aerodynamic pattern from HAWT. To start up, a historical review on the aerodynamic models of VAWTs is performed to recap the critical issues in the aerodynamic modeling. The study mainly emphasizes on the momentum models, covers SST, MST, DMST and Actuator Cylinder (AC) models. AC mainly advantages on including the lateral inductions, and the wake interaction. Its precise prediction was confirmed in the simulation results of the aerodynamic model comparison study [10]. Hence, AC has been chosen as the numerical tool for VAWT modeling, both for the self-developed routine and HAWC2. A promising solution to determine the load-form for optimal power is constructing a uniform distribution so that the power coefficient could reach Betz limit. The normal load distribution on the blade path is the link between the power conversion and the blade force. At the moment of the determination of the load distribution that leads to the optimal power, the required blade force at the different azimuth positions are also settled. The simulations are performed with TSR=2, 3, 4, and 5 to verify the possibility on achieving the ideal power production in order to draw up an optimized control option. The result of the study is presented as a form of different flap angles at discretized azimuth positions. Even though the load-form is universal, the resultant flap variation only works for the specific rotor because of the introduction of airfoil data in the computation. An H-rotor VAWT with aps is set up in HAWC2 to validate the control strategy. As the supplement to AC model, a dynamic stall model called ATEFlap is integrated in HAWC2 to calculate the dynamic stall impact, and the influences from flap detection for the given airfoil. The model is supposed to return the aerodynamic loading of the blades under the dynamic stall. Hence, the deviations between the target load distribution obtained from the quasi-steady environment and the semi-dynamic one is observed as expected. In the end, the aps running with the given function could almost give the desired loading, but the deviations are observed in downwind part. It is mainly the hysteresis of the lift coefficient causes the difference. It is proven to be partly made up by increasing the flap deflections in the downwind side.

In the present world of conventional wind energy, the National Renewable Energy Laboratory (NREL) 5MW reference wind turbine for offshore system development has become an important piece of the puzzle towards a renewable future based on wind energy. DTU showed the benefit of such a reference model with their 10 MW reference wind turbine and continued the evolution in multi-megawatt wind power systems. Even though these two systems exist in the conventional wind turbine industry, a publicly available reference system does not exist in airborne wind energy. Currently, Airborne Wind Energy Systems (AWES) are still in the prototype testing phase as no commercial utility-scale product has been released to the market yet. A reference model like the NREL 5MW turbine can therefore significantly increase the speed of AWES development and open a door towards more publicly available research. This thesis solves the following main research question: "How does a multi-megawatt utility scale airborne wind energy reference system look like, focusing on the main wing parameters?". This is done by setting up a relatively computationally efficient optimisation framework based on a Fluid Structure Interaction (FSI) model combined with a flight dynamics simulation model which can be used in early design optimisations, for example. The FSI model consists of a 3D linear structural Finite Element model coupled with a potential-flow based 3D panel method. The aircraft structure is parametrised and parameters are found for an aircraft with a wing area of approximately 150 m2. The wing mesh and other structural components are created in Matlab while Nastran is exploited to obtain the stiffness, mass and inertia matrices. A model order reduction technique is applied to the structural model, relying on the mode superposition method to decrease the computation effort by several orders. The wing's aerodynamic behaviour is calculated by the 3D panel method. A model order reduction technique is also applied here, based on a Taylor expansion of the aerodynamic influence coefficient matrices. A modified fixed wing aircraft flight controller is used to fly a circular flight path where the ground station periodically allows the tether to be reeled out and in. The navigation of the aircraft is split up in two components, namely the lateral controller (based on a modification to the L1-control logic) and radial dynamics controller (depending on the elevator and tether reel-out behaviour). A Covariance Matrix Adaptation Evolution Strategy (CMAES) optimisation method is applied with a specific objective function to find the system design parameters. This work then presents a detailed representation of the aircraft design, consisting of the planform parameters, material choices and composite layup. It demonstrates the ability of the framework to obtain a wing that can sustain high wing loadings. Also the system performance in a full power cycle illustrates the full potential of a 150 m2 wing which is able to generate multiple megawatts of power. This thesis serves as a foundation of reference systems in airborne wind energy, which other researchers can use and adapt to contribute further to a benchmark network in Airborne Wind Energy.