Airborne wind energy (AWE) is an emerging renewable energy technology that harnesses wind energy using tethered flying systems. The extra degrees of freedom allow these systems to harvest wind resources at altitudes currently unrealisable by conventional turbines. These flying devices, often resembling kites or drones, are typically divided into two classes. The first converts kinetic energy into electricity using onboard generators and transmits it to the ground via a conductive tether. The second class transfers aerodynamic forces via the tether to the ground, where the mechanical energy is converted into electrical energy using an electrical machine.As the tether’s length constrains the system, once the flying device reaches this tether length limit, some energy must be used to retract it back to its initial position. This cycle of traction and retraction is known as a pumping cycle. Therefore, AWE systems must be designed to maximise the harvesting or traction phase while minimising the retraction phase to ensure a net positive power output. From the AWE system landscape, this thesis is based on tethered aircraft-style fixed-wing systems. Typically, such systems utilise composite structures owing to their high stiffness-toweight ratios. Designing these composite structures demands special attention due to their anisotropic nature, which results in complex load-deflection couplings. Here, a multi-disciplinary simulation framework for tethered composite aircraft wings is developed. The research focuses on methods used during the iterative phases of initial (conceptual and preliminary) design that are commonly employed in a spiral system engineering approach. The proposed framework integrates computational methods for the design of the aerodynamic A, bridle B, and structural S domains. The bridle is a system of segments of tether and pulleys that distribute the tether forces into the wing structure. The aerodynamic and structural domains are divided into 2D and 1D models, which are then integrated to determine the 3D response of the wing. A nonlinear vortex-lattice method (VLM) is utilised for the aerodynamic domain. For the structural domain, an anisotropic 1D finite element (FE) model is developed that is coupled with a 2D FE sectional solver. In addition, methods are proposed that enable detailed topology optimisation. For tailless swept-wings, like those used by EnerKíte, the aero-structural-bridle interactions are crucial. The developed framework is used to investigate the impacts of different wing and bridle configurations to determine the sufficient level of fidelity required at the initial design phases. Typically, such aeroelastic phenomena are captured during detailed design stages wherein full 3D structural and aerodynamic simulations are employed. However, this mandates design knowledge typically unknown at the initial design stages. This motivates a multi-fidelity modelling approach to include these coupling effects while abstracting the composite ply level details during the design exploration. This is achieved by combining geometric discretisation approaches with lamination parameters. Thus, the framework aims to provide viable design options during the initial stages while considering aero-structural-bridle couplings.@en

In this work we explore the initial design space for composite kites, focusing on the configuration of the bridle line system and its effect on the aeroelastic behaviour of the wing. The computational model utilises a 2D cross sectional model in conjunction with a 1D beam model (2+1D structural model) that captures the complex composite coupling effects exhibited by slender, multi-layered composite structures, while still being computationally efficient for the use at the initial iterative design stage. This structural model is coupled with a non-linear vortex lattice method (VLM) to determine the aerodynamic loading on the wing. In conjunction with the aerodynamic model, a bridle model is utilised to determine the force transfer path between the wing and the bridles connected with the tethers leading to the ground station. The structural model is coupled to the aerodynamic and bridle models in order to obtain the equilibrium aero-structural-bridle state of the kite. This computational model is utilised to perform a design space exploration to assess the effects of varied load introduction to the structure and resulting effects on the kite.

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The EnerKíte team has been successful in automation of kite systems for more than a decade. Our first autonomous kite flights with figure of eight patterns took place in May 2008. Since then the technology of the ground-station, the wing and tether and the landing and launching systemhas been systematically developed, implemented and improved ś driven by cost, performance and safety targets derived and approved by customers. @en

The earlier in the design process the trade-offs between a system’s cost and its performance can be determined, the easier it is to narrowin on an optimal final design. In order to explore the initial design space for composite carbon kites, it is imperative to assess the load couplings effects and its impact on the aerodynamics of the wing, and ultimately the performance of the system’s yield. CFD and 3D finite element methods are currently too computationally expensive to efficiently explore the design space at such an early stage of the design process. This leads to the need for a toolchain that has sufficient modelling fidelity while being efficient enough to be used for conceptual design. An efficient aero-structural toolchain is the focus of this work. @en

For pumping cycle airborne wind energy systems, the airborne mass of the system plays a crucial factor in the performance of the system[1]. This is especially pronounced during low-wind conditions, where the additional force component to overcome gravity is more pronounced in comparison to the aerodynamic forces. Additionally, the airborne mass also affects the take-off speed, thus further influencing the site-specific Levelized Cost of Electricity (LCOE) of the system. For rigid as well as semi-rigid kites, it is essential to analyse and model the structure of the kite right from the initial design stage, especially given the load couplings commonly witnessed in composite structures. Complete 3D finite element analysis of such composite structures is computationally expensive, and thus uncommon in the initial design stage. However, oversimplified structural models, such as simple uniform and isotropic beams do not capture the intricacies of composite structures and either lead to too optimistic or too pessimistic results, depending on the material assumptions. An approach to capture the anisotropic coupling effects, which are important for an accurate estimate of composite structure deflections is described here. The main load-bearing member of the structure - the wing box, is modelled as a slender composite beam. The 3D composite shell problem is solved by determining the complete anisotropic 2D cross sectional stiffness, which is then utilised in a 1D beam analysis. This approach serves to reduce the 3D problem to a 2+1D finite elements problem which is computationally fast, while being sufficiently accurate for initial design. This structural model is then utilised to minimise the weight of the composite wing box, by optimising the internal geometrical shape and orientations of the composite ply fibre. @en