Airborne wind energy (AWE) is a novel concept aiming to substantially reduce the material demand and environmental impact of wind energy generation. AWE systems can also access steadier and stronger wind at higher altitudes, which is inaccessible to conventional wind turbines. The lower material effort, the increased capacity factor, and the access to so-far unused wind resources render AWE a potential cornerstone in a future low-carbon energy economy. The new conversion concept is currently being demonstrated by several start-up companies using different technical implementations at different maturity levels. The unifying challenge for commercialization is the operational robustness of the technology. For a successful market acceptance, the introduced aviation and ground-related safety risks need to be mitigated.

The present work aims to bring AWE closer to commercial success through two main contributions. As a first contribution, well-established practices of reliability engineering are used to measure and then systematically improve the safety and reliability of AWES systems. Experience from other safety-critical domains such as aviation, space, automotive, and medical are used to achieve this objective. A fault tree analysis (FTA) and failure mode and effects analysis (FMEA) are applied to an existing demonstrator system. A common practice in the safety-critical domain is automatically monitoring the system's health and taking action in case of faults. In this regard, a systematic fault detection isolation and recovery (FDIR) model is proposed for AWES. This architecture is generally applicable and flexible and can be applied to different AWE systems.

After reaching the required reliability and safety levels, formalization by the certification authorities is required. As a second contribution, the current regulatory framework is reviewed, the relevant authorities identified and a roadmap for aviation certification is presented. The ``Specific Operations Risk Assessment'' (SORA) by the Joint Authorities for Rulemaking on Unmanned Systems (JARUS) is a comprehensive and well-structured framework. Therefore, following the SORA is considered the best way forward to get the flying permit for AWES, claiming the ``specific'' category from the European Union Aviation Safety Agency (EASA) regulation. This permit is applicable for commercial operations in Europe. Other civil aviation authorities may also recognize the EASA's flying permit. In this respect, the SORA is applied to a hypothetical commercial operation scenario, and requirements for the flying permit are discussed.@en

Reducing the cost of constructing wind turbines, as well as extending their design life, are two ways of reducing the cost of energy from wind. To achieve this, this project tested two innovative tuned-mass damper systems, which reduce resonance of the tower, and do so minimizing additional mass needed on the nacelle. These TMD systems allow the generator and full drive-train to oscillate with respect to the tower top. MATLAB was used to model these turbine-damper assemblies as simple 2-degree-of- freedom systems excited by thrust signals extracted from the FAST aeroelastic simulator. Furthermore, it was found that both tuned mass damper configurations have a net positive effect on the tower behaviour, both achieving double digit fatigue percentage reductions. It was also found that neither of these systems put significant burdens on acceleration-sensitive components, such as the generator and gear box. Two issues to constructing these devices may reside in their need for additional space for oscillations, as well as aggravated tip deflections in the case of the oscillating rotor-drive-train.

The work presented in this thesis explores a new way of generation, collection and transmission of wind energy inside a wind farm, in which the electrical conversion does not occur during any intermediate conversion step before the energy has reached the offshore central platform. A centralized approach for electricity generation is considered through the use of fluid power technology. In the proposed concept the conventional geared or direct-drive power drivetrain is replaced by a positive displacement pump. In this manner the rotor nacelle assemblies are dedicated to pressurize water into a hydraulic network. The high pressure water is then collected from the wind turbines of the farm and redirected into a central offshore platform where electricity is generated through a Pelton turbine. A numerical model is developed to describe the energy conversion process as well as the main dynamic behaviour of the proposed hydraulic wind power plant. The model is able to capture the relevant physics from the dynamic interaction between different turbines coupled to a common hydraulic network and controller. Two case studies are considered in the time-domain simulations for a hypothetical hydraulic wind farm subject to turbulent wind conditions. The performance and operational parameters of individual turbines are compared with those of a reference wind farm with conventional technology turbines, using the same wind farm layout and environmental conditions. For the presented case study, results indicate that the individual wind turbines are able to operate within the operational limits with the current pressure control concept. Despite the stochastic turbulent wind conditions and wake effects, the hydraulic wind farm is able to produce electricity with reasonable performance in both below and above rated conditions.
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A wind turbine is designed to produce a maximum amount of energy at minimum cost while it withstands any possible wind condition. In the wind conditions with the highest wind speeds the wind turbine has to cope with the most extreme loads. In these most extreme cases the wind turbine is parked, the blades do not rotate, and the blades deform elastically under these wind loads. An increase in the flexibility of the blades results in higher elastic blade deformations. The main objective of this project is to design blades in a way that the extreme loading on the blades reduces with more than 10 % without compromising the energy yield by making use of flexible materials and the corresponding increased deformations. The reason to focus on load reduction is that a wind turbine can be made cheaper if these loads are reduced. The research methodology is a three-step approach: Firstly a modelling tool is created to design and evaluate blades with new flexible materials at different locations in the blade. Secondly a verification procedure is performed to check the accuracy of the modelling tool. Thirdly this tool is used to design blades with highly flexible materials and to perform an iteration procedure to design the best flexible blade design. The modelling tool is based on the cross sectional software BECAS to design new flexible blades and on the aeroelastic software HAWC2 to analyse the behaviour. This BECAS-HAWC2 modelling tool is based on the existing XANT M-21 wind turbine of which only the blade materials are variable parameters, the rest of the wind turbine remains as it is. A verification procedure compares the modelling tool with two other models of the same blade. The minor differences between several modelled parameters increase the confidence in the BECASHAWC2 model. A material with unidirectional fibres and a highly flexible matrix material is stacked in different orientations to design different blade materials. These flexible materials are introduced in specific locations of the blade. The design exploration approach makes it possible to design and evaluate many different blades using different flexible materials at different locations. The current results show that the best option is to use the flexible material with fibres only in longitudinal and transverse direction in the skin of the blade. Not replacing the full skin but only the part of the root up to the middle of the blade results in the best flexible design. This best design has a reduction in maximum thrust force and maximum root bending moment of respectively 23 % and 26 % compared with the original blade, easily exceeding the predefined goal of 10 %. This significant load reduction is due to a significant blade twist rotation thereby reducing the area exposed to the wind. The annual energy yield is not compromised, it even shows a considerable 11 % increase due to a stall delay effect in the higher wind regimes which is also caused by an increase in blade twist. The best flexible design is a preliminary design that shows promising results. These results show that by using flexible materials in the blade skin a significant load reduction is combined with an increase in energy. This indicates an untapped potential for future wind energy which makes further research on this topic recommended.

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.

The continued grow of the wind industry has resulted in an increase in turbine cluster density in wind farms. This has made the topic of wind turbine and wind farm wake aerodynamics more relevant than ever. Wind turbine wake models are used to predict the deficit and recovery of the wind velocities in the wake. The currently used wake models in the industry contain a relatively simple approach to model the near-wake of the turbine, assuming an initial hat or Gaussian deficit profile at a fixed location in the wake. This thesis presents an alternative near-wake model based on discrete, inviscid, perfectly circular vortex rings for use in conjuncture with existing far-wake models.

As building insulation and airtightness are constantly improving, ventilation plays a dominant role in fresh air supply, thermal comfort and managing heat losses and cooling needs. By designing an integrated ventilation system, it is possible to improve air quality and thermal comfort, at the same time enhancing energy efficiency. This thesis focuses on the development of such a ventilation system. It is done by employing the best features of existing systems and including several new features.
Throughout the thesis, ventilation, night cooling, solar shading, heat balance, heat recovery unit and renewable energy sources are studied. In this study, the demanded yearly ventilation air flow rates are found, the optimal heat exchanger model is selected and optimized, yearly energy consumption is calculated, while at the end, a wind cowl is selected and saved yearly energy using the cowl is calculated. The Excel Macro tool is developed and used to establish the needed yearly air flow rates, applying night cooling and solar shading. The Matlab tool
is used for the selection and optimization of the heat exchanger as well as yearly energy consumption, with and without the wind cowl. The main specifications of the new system are found. Since a resize of the heat exchanger passage is recommended, an experiment is carried out in order to verify the heat transfer coefficient of the air traveling in the heat exchanger.
The developed ventilation system provides the dwelling with a ‘good’ indoor air quality at any time of the year, using minimum required air flow rates per room. Thus, combined with the heat recovery device the heating demand for ventilation air is minimized. The system also provides thermal comfort at most times of the year using natural cooling and optimal shading. Thus, the cooling need of the dwelling is minimized. However, due to the heat recovery unit and high night cooling flows, the newly developed ventilation system has an enhanced electricity use. Therefore it is recommended to also redesign the air distribution ductwork system within the dwelling for low pressure use The wind cowl contributes to electricity savings even more if the ventilation system has a well-designed, low pressure air distribution system.

In order to be more competitive with conventional, polluting energy resources, the levelised cost of energy (LCoE) of wind turbines has to decrease significantly [1]. A method to achieve this goal is to reduce the amount of parts in a wind turbine and more specifically avoid the use of expensive and failure-prone pitch systems. Using blades which passively deform under high aerodynamic loading and effectively reduce the peak, design-driving loads on the turbine structure might offer an interesting alternative to the pitch system. However, the conventional material for construction of blades, glass-fibre reinforced plastics,
does not allow for such large deformations during normal operating conditions. Prior to this project, a study to incorporate more flexible materials has been carried out in the design of the current XANT-21 wind turbine in order to increase the flexibility of the blades. However, introducing a high degree of flexibility in the wind turbine blades might increase the risk of aeroelastic instabilities. These aeroelastic instabilities often result, in combination with a non-linearity in the aerodynamics or structure, in limit cycle oscillations (LCOs). The need to identify the key structural parameters which play a role in the initiation of the aeroelastic instabilities has led to the objective of this thesis: Investigate the influence of the critical structural parameters on the onset velocity and the behaviour of high-amplitude limit cycle oscillations of a wind turbine airfoil by means of numerical simulations.

Energy demand from wind greatly increases, as such more remote sites need to be explored in order to find good wind resources. These remote sites are driving the industry further offshore and therefore into extreme wind and sea conditions. This push towards extreme conditions requires technological advancements concerning the wind turbine loads, power production and installation. The levelised cost of wind energy is strongly dependent on the capital expenditures and thus on the installation and logistics of erecting a wind turbine offshore. Improving the robustness of the installation to higher wind velocity and turbulence will increase the weather window and therefore drastically decrease the levelised cost of energy (LCoE). This thesis will focus on one particular technique of wind turbine installation: Horizontal Single Blade Mounting (HSBM).
HSBM is a wind turbine blade lifting technique performed by Siemens Wind Power (SWP). This technique is currently limited to low wind speeds, which contradicts the fact that the industry is targeting high wind speed offshore sites. This technical problem is the basis of the thesis study. This thesis study will answer three research questions in order to improve future lifting
methods. In a nutshell these questions are: one, how to model the aerodynamic forces and dynamic behaviour of a hoisted wind turbine blade?, two, what are the critical parameters affecting the blade response? and three, how can single blade mounting be improved for higher wind speeds?.

The objective of this research is to determine the impact of design variables on power output of the Makani Power Wing7 airborne wind turbine. This system consists of a rigid wing connected to the ground using a using a number of bridles and a tether. The tethered wing flies crosswind trajectories and generates power using on-board turbines. Sensitivities of power output to design variables are required for various design trade-offs. A simulation and optimization approach is presented which allows analysis of a fairly complex model with realistic controls and constraints. Using trajectory optimization removes the assumption of a predefined trajectory and allows analysis of the impact of the flightpath itself. The wing is modeled as a rigid body with six degrees of freedom. Aerodynamic forces are determined using a response surface on results of a vortex lattice code. A time-varying position of the bridle attachment point couples the wing to the tether. This tether is implemented as a straight elastic element with analytic approximations for the effects of mass and aerodynamic drag. The power system model consists of a turbine response surface and DC-equivalent generator model. Optimal trajectories are determined using the direct collocation method resulting in a capacity factor of 48.1% for a site with uniform wind field and annual mean wind speed of 7.5 m/s. The relative impact of aerodynamic lift and drag on long term averaged power output are found to be an order magnitude more important than wing mass. Topology and circularity of the flight path are found to have a low impact on power.

In an effort to achieve economic sustainability, the major challenge for offshore wind remains to bring down costs by approximately 35 % before 2020. With wind farm sites going into deeper waters, further from shore, with more difficult bottom conditions and rougher wave climate, costs are rising faster than the improvements in the technology are able to drive them down. Despite these obstacles, wind farms further offshore offer better wind resource, less socio-economic restrictions and diminished environmental impacts. To satiate this need, floating wind turbines provide a feasible promise. In the wake of current Horizontal Axis Wind Turbine (Hawt) floating projects such as HyWind, IDEOL etc, the perceived positive impact of the floating system, especially for larger scale machines, is reduced due to large overturning moments and difficult accessibility of the \moving" nacelle. A possible solution to these obstacles is presented by the DeepWind project which focuses on implementing a Floating Offshore Vertical Axis Wind Turbine (Fo-Vawt) on a spar structure. In this report, the economic feasibility of a Fo-Vawt concept in far offshore wind farm is assessed - from current to multi-megawatt rated power. The possible benefits of a Fo-Vawt are translated into cost indicators for the main aspects of an offshore wind farm project. The difference in turbine capex, perceived reductions in balance of plant costs and the impact on opex, is modelled into an engineering framework to determine the Levelised Cost of Energy. Considering mass and swept area as the base factors, the mass of the baseline DeepWind turbine design [66] is approximated and broken down into material and manufacturing costs with adjustments for inflation and other financial variables over the lifetime of the wind farm. Using ECN's omce package, a detailed operations and maintenance (O&M) strategy is devised and implemented with respect to Vawt systems and a floating wind farm. The grid infrastructure, installation and project development costs are estimated using TU Delft's Offshore Wind Farm Design Emulator tool developed by Zaaijer [75]. All these components are combined to deduce the cost of energy for the DeepWind and the turbine capital costs for similar turbine designs. To study the design space of the cost model, a sensitivity study of the main presumptions was done along with an evaluation of LCoE impact from rotor, floater and generator design. Using geometric up-scaling methods, the baseline Fo-Vawt design was sized for higher rated power capacities and corresponding loads simulated using HAWC2. Considering a 245 MW wind farm, a workspace was created to assess where the greatest potential in cost reductions exist and provide impact trigger's for future research . It was seen that the turbine capital costs for a Vawt are not significantly higher than for a comparable state-of-the-art 5MW Hawt. The reliability of a Vawt is better and subsequently the wind farm has a higher availability, assuming the same technology maturity level as a present day Hawt. Although, possible higher fixed capital costs originate for the specialised supply chain to maintain floating turbines and the DeepWind subsystems. This on average pushes the annual opex beyond e120/kW. The capex/kW extends between e2520 to e2780/ kW for the 5 to 15 MW Fo-Vawt's respectively, leading to a Levelised Cost of Energy frome113.56 to 117.1 per MWh.

The objective of this research is the simulation of aero-elastic behaviour of Makani’s large airborne wind turbine. This tethered wing operates in crosswind motion, and is equipped with on-board wind turbines. The tether-bridle system attaches the energy generating system to the ground station. It is likely that the structure of this, 28m span, carbon fibre, high aspect ratio wing, will deform considerable under aerodynamic loads. In the worst case scenario, static and/or dynamic aero-elastic e_ects cause destructive failure. The aero-elastic simulation program ASWING is used for the analysis. This program uses a fully nonlinear Bernoulli-Euler beam representation for structural modelling in combination with a lifting linerepresentation for aerodynamic modelling. Linearised unsteady analyses are derived for the Eigenmode analysis. Since the tether-bridle system cannot be modelled in the current program version, an additional, ASWING compatible, module is written. The tether is modelled as a spring with user defined characteristics for the spring sti_ness, mass and aerodynamic drag area. The bridle lines are assumed massless and perfectly rigid. The tether and bridle forces are dependent on the wing flexibility, and wing position and orientation. The tether-bridle module is verified against analytical expressions and by using MATLAB. A wind tunnel test validates the dynamic aero-elastic responses. For the Makani wing, divergence, aileron e_ectiveness and reversal, and flutter behaviour is analysed. Divergence and aileron reversal are no critical modes. However, aileron e_ectiveness is critical. The requirements state a minimum aileron e_ectiveness of 75% at 95m=s flight speed. The program calculated this minimum aileron e_ectiveness at 92m=s flight speed. These problems can be resolved by a 10% increase in the wing’s torsional sti_ness or a 10% increase of lift force increment with aileron deflection. The Eigenmode results showed a critical flutter mode at flight speeds higher than 90m=s, whereas the design flutter speed is equal to 120m=s. This susceptibility to flutter can be resolved by (1) a 50% increase in torsional sti_ness, (2) a 50% increase in in-plane-bending sti_ness or (3) a 10cm upstream shift in center of gravity. A 50cm upstream shift of bridle-wing attachment location increases the flutter speed to 110m=s. It was found that the e_ects, of tether aerodynamic drag and tether weight, are negligible for the aero-elastic behaviour. Also, in the analysis for the Makani wing, the tether spring constant does not contribute to the static and dynamic aero-elastic e_ects. The position of the bridle-wing attachments influences the twist angles and tip deflections of the wing. These results are useful in case maximum twist angles and/or wing tip deflections are critical. For the dynamic aero-elastic behaviour the wing-bridle attachment positions can be adjusted to decrease the susceptibility to flutter.