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Special Issue "Airborne Wind Energy Systems"

A special issue of Energies (ISSN 1996-1073). This special issue belongs to the section "A3: Wind, Wave and Tidal Energy".

Deadline for manuscript submissions: 31 January 2023 | Viewed by 17166

Special Issue Editors

Prof. Dr. Christoph M. Hackl
E-Mail Website1 Website2
Guest Editor
Department of Electrical Engineering and Information Technology, Munich University of Applied Sciences (MUAS), 80335 Munich, Germany
Interests: modeling; control; efficiency enhancements; fault detection and condition monitoring of mechatronic and renewable energy systems
Special Issues, Collections and Topics in MDPI journals
Dr. Roland Schmehl
E-Mail Website
Guest Editor
Faculty of Aerospace Engineering, Delft University of Technology (TU Delft), 2629 HS Delft, The Netherlands
Interests: airborne wind energy; fluid–structure interaction; two-phase flows; liquid droplet; modeling; renewable energy technologies; numerical analysis; power generation; engineering thermodynamics; computational fluid dynamics; fluid mechanics; numerical simulation; thermal engineering; numerical modeling; aerodynamics

Special Issue Information

Dear colleagues,

Airborne wind energy (AWE) systems convert wind energy into electrical energy using autonomous tethered flying devices. Deemed a potentially game-changing solution to clean and sustainable energy generation, AWE is increasingly attracting the attention of governments, policymakers and industry worldwide. AWE technology can significantly reduce the levelized cost of energy (LCoE) by eliminating (i) the drive-train installation, (ii) a large part of the rotor blades, (iii) the tower, and (iv) the foundation, which make up for about 50% of the conventional turbine costs. On the other hand, the development of this technology is also facing substantial technical challenges. Important aspects are, for example, autonomous, efficient, reliable, safe, and uninterrupted operation of AWE systems and their interconnection with the future power grid.

We cordially invite original manuscripts presenting recent advances in this important and interdisciplinary research field with particular focus on but not limited to the following:

  1. Aerodynamics, aeroelasticity, and structural dynamics;
  2. Flight dynamic modeling;
  3. Flightpath planning and control of AWE systems;
  4. AWE design optimization;
  5. Higher altitude wind resources;
  6. Experimental testing of prototypes;
  7. Efficiency enhancements (by, e.g., intelligent design and/or control);
  8. Nonlinear, optimal, and fault-tolerant control strategies;
  9. Fault detection methods and condition monitoring approaches;
  10. Robust, fault-tolerant and flexible grid connection/integration (e.g., grid-supporting, -feeding, -forming including black start capability);
  11. Economic and market analysis;
  12. AWE policy-making, environmental impact, and societal acceptance.

The Special Issue will present cutting-edge research results connected to AWE technology as a basis for enduring operation. All research results must be presented in a mathematically thorough (e.g., in state space) but understandable manner to encourage insights and re-implementation by the readers so that the knowledge basis is shared, spread, and extended to promote AWE systems to the market. All results should be validated by simulation and measurement results (if possible).

Prof. Dr. Christoph M. Hackl
Dr. Roland Schmehl
Guest Editors

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Energies is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2200 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • airborne wind energy system
  • aerodynamics
  • aeroelasticity
  • dynamic modeling
  • performance modeling
  • fault-tolerance
  • reliable operation
  • safety engineering
  • flexible grid connection

Published Papers (11 papers)

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Research

Jump to: Review

Article
Wing Deformation of an Airborne Wind Energy System in Crosswind Flight Using High-Fidelity Fluid–Structure Interaction
Energies 2023, 16(2), 602; https://doi.org/10.3390/en16020602 - 04 Jan 2023
Viewed by 347
Abstract
Airborne wind energy (AWE) is an emerging technology for the conversion of wind energy into electricity. There are many types of AWE systems, and one of them flies crosswind patterns with a tethered aircraft connected to a generator. The objective is to gain [...] Read more.
Airborne wind energy (AWE) is an emerging technology for the conversion of wind energy into electricity. There are many types of AWE systems, and one of them flies crosswind patterns with a tethered aircraft connected to a generator. The objective is to gain a proper understanding of the unsteady interaction of air and this flexible and dynamic system during operation, which is key to developing viable, large AWE systems. In this work, the effect of wing deformation on an AWE system performing a crosswind flight maneuver was assessed using high-fidelity time-varying fluid–structure interaction simulations. This was performed using a partitioned and explicit approach. A computational structural mechanics (CSM) model of the wing structure was coupled with a computational fluid dynamics (CFD) model of the wing aerodynamics. The Chimera/overset technique combined with an arbitrary Lagrangian–Eulerian (ALE) formulation for mesh deformation has been proven to be a robust approach to simulating the motion and deformation of an airborne wind energy system in CFD simulations. The main finding is that wing deformation in crosswind flights increases the symmetry of the spanwise loading. This property could be used in future designs to increase the efficiency of airborne wind energy systems. Full article
(This article belongs to the Special Issue Airborne Wind Energy Systems)
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Article
Dominant Designs for Wings of Airborne Wind Energy Systems
Energies 2022, 15(19), 7291; https://doi.org/10.3390/en15197291 - 04 Oct 2022
Cited by 1 | Viewed by 583
Abstract
This paper focuses on the design of the wings used in airborne wind energy systems. At the moment, two different designs are being developed: soft wings and rigid wings. This paper aimed to establish which of the two alternative design choices has the [...] Read more.
This paper focuses on the design of the wings used in airborne wind energy systems. At the moment, two different designs are being developed: soft wings and rigid wings. This paper aimed to establish which of the two alternative design choices has the highest chance of dominance and which factors affect that. We treated this problem as a battle for a dominant design, of which the outcome can be explained by factors for technology dominance. The objective was to find weights for the factors for technology dominance for this specific case. This was accomplished by applying the best worst method (BWM). The results are based on literature research and interviews with experts from different backgrounds. It was found that the factors of technological superiority, learning orientation and flexibility are the most important for this case. In addition, it appeared that both designs still have a chance to win the battle. Full article
(This article belongs to the Special Issue Airborne Wind Energy Systems)
Article
Electrical Launch Catapult and Landing Decelerator for Fixed-Wing Airborne Wind Energy Systems
Energies 2022, 15(7), 2502; https://doi.org/10.3390/en15072502 - 29 Mar 2022
Cited by 1 | Viewed by 1194
Abstract
This paper presents a (pre)feasibility study of the rail-based ultra-short launch and landing system ElektRail for fixed-wing airborne wind energy systems, such as Ampyx Power. The ElektRail concept promises airborne mass reductions through the elimination of landing gear as well as decreased landing [...] Read more.
This paper presents a (pre)feasibility study of the rail-based ultra-short launch and landing system ElektRail for fixed-wing airborne wind energy systems, such as Ampyx Power. The ElektRail concept promises airborne mass reductions through the elimination of landing gear as well as decreased landing stresses and ground stability requirements, opening possibilities for improved aerodynamics through a single fuselage configuration. Initially designed for operating fixed-wing drones from open fields, the ElektRail concept had to be significantly shortened for application in an airborne wind energy (AWE) context. This shorter size is required due to the much more limited space available at AWE sites, especially on offshore platforms. Hence, a performance enhancement using the integration of a bungee launching and landing system (BLLS) was designed and a system dynamics model for the launch and landing was derived. The results demonstrated the possibility for the ElektRail to be shortened from 140 m to just 19.3 m for use with an optimised tethered aircraft with a mass of 317 kg. A system length below 20 m indicates that an enhanced ElektRail launch and landing concept could be viable for airborne wind energy operations, even with relatively low-tech bungee cord boosters. Linear motor drives with a long stator linear motor actuator could potentially shorten the system length further to just 15 m, as well as provide better control dynamics. An investigation into improved AWE net power outputs due to reduced airborne mass and aerodynamic improvements remains to be conducted. Full article
(This article belongs to the Special Issue Airborne Wind Energy Systems)
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Article
Aerodynamic Performance and Wake Flow of Crosswind Kite Power Systems
Energies 2022, 15(7), 2449; https://doi.org/10.3390/en15072449 - 26 Mar 2022
Cited by 2 | Viewed by 1606
Abstract
This paper presents some results from a computational fluid dynamics (CFD) model of a multi-megawatt crosswind kite spinning on a circular path in a straight downwind configuration. The unsteady Reynolds averaged Navier-Stokes equations closed by the kω SST turbulence model are [...] Read more.
This paper presents some results from a computational fluid dynamics (CFD) model of a multi-megawatt crosswind kite spinning on a circular path in a straight downwind configuration. The unsteady Reynolds averaged Navier-Stokes equations closed by the kω SST turbulence model are solved in the three-dimensional space using ANSYS Fluent. The flow behaviour is examined at the rotation plane, and the overall (or global) induction factor is obtained by getting the weighted average of induction factors on multiple annuli over the swept area. The wake flow behaviour is also discussed in some details using velocity and pressure contour plots. In addition to the CFD model, an analytical model for calculating the average flow velocity and radii of the annular wake downstream of the kite is developed. The model is formulated based on the widely-used Jensen’s model which was developed for conventional wind turbines, and thus has a simple form. Expressions for the dimensionless wake flow velocity and wake radii are obtained by assuming self-similarity of flow velocity and linear wake expansion. Comparisons are made between numerical results from the analytical model and those from the CFD simulation. The level of agreement was found to be reasonably good. Such computational and analytical models are indispensable for kite farm layout design and optimization, where aerodynamic interactions between kites should be considered. Full article
(This article belongs to the Special Issue Airborne Wind Energy Systems)
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Article
Effect of Chordwise Struts and Misaligned Flow on the Aerodynamic Performance of a Leading-Edge Inflatable Wing
Energies 2022, 15(4), 1450; https://doi.org/10.3390/en15041450 - 16 Feb 2022
Cited by 1 | Viewed by 1575
Abstract
Leading-edge inflatable (LEI) kites use a pressurized tubular frame to structurally support a single skin membrane canopy. The presence of the tubes on the pressure side of the wing leads to characteristic flow phenomena for this type of kite. In this paper, we [...] Read more.
Leading-edge inflatable (LEI) kites use a pressurized tubular frame to structurally support a single skin membrane canopy. The presence of the tubes on the pressure side of the wing leads to characteristic flow phenomena for this type of kite. In this paper, we present steady-state Reynolds-Averaged Navier-Stokes (RANS) simulations for a LEI wing for airborne wind energy applications. Expanding on previous work where only the leading-edge tube was considered, eight additional strut tubes that support the wing canopy are now included. The shape of the wing is considered to be constant. The influence of the strut tubes on the aerodynamic performance of the wing and the local flow field is assessed, considering flow configurations with and without side-slip. The simulations show that the aerodynamic performance of the wing decreases with increasing side-slip component of the inflow. On the other hand, the chordwise struts have little influence on the integral lift and drag of the wing, irrespective of the side-slip component. The overall flow characteristics are in good agreement with previous studies. In particular, it is confirmed that at a low Reynolds number of Re=105, a laminar separation bubble exists on the suction side of this hypothetical rigid wing shape with perfectly smooth surface. The destruction of this bubble at low angles of attack impacts negatively on the aerodynamic performance. Full article
(This article belongs to the Special Issue Airborne Wind Energy Systems)
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Article
L0 and L1 Guidance and Path-Following Control for Airborne Wind Energy Systems
Energies 2022, 15(4), 1390; https://doi.org/10.3390/en15041390 - 14 Feb 2022
Cited by 1 | Viewed by 748
Abstract
For an efficient and reliable operation of an Airborne Wind Energy System, it is widely accepted that the kite should follow a pre-defined optimized path. In this article, we address the problem of designing a trajectory controller so that such path is closely [...] Read more.
For an efficient and reliable operation of an Airborne Wind Energy System, it is widely accepted that the kite should follow a pre-defined optimized path. In this article, we address the problem of designing a trajectory controller so that such path is closely followed. The path-following controllers investigated are based on a well-known nonlinear guidance logic termed L1 and on a proposed modification of it, which we termed L0. We have developed and implemented both L0 and L1 controllers for an AWES. The two controllers have an easy implementation with an explicit expression for the control law based on the cross-track error, on the heading angle relative to the path, and on a single parameter L (L0 or L1, depending on each controller) that we are able to tune. The L0 controller has an even easier implementation since the explicit control law can be used without the need to switch controllers. Since the switching of controllers might jeopardize stability, the L0 controller has an important theoretical advantage in being able to guarantee stability on a larger domain of attraction.The simulation study shows that both nonlinear guidance logic controllers exhibit appropriate performance when the L parameter is adequately tuned, with the L0 controller showing a better performance when measured in terms of the average cross-track error. Full article
(This article belongs to the Special Issue Airborne Wind Energy Systems)
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Article
Cascade Control of the Ground Station Module of an Airborne Wind Energy System
Energies 2021, 14(24), 8337; https://doi.org/10.3390/en14248337 - 10 Dec 2021
Cited by 2 | Viewed by 1790
Abstract
An airborne wind energy system (AWES) can harvest stronger wind streams at higher altitudes which are not accessible to conventional wind turbines. The operation of AWES requires a controller for the tethered aircraft/kite module (KM), as well as a controller for the ground [...] Read more.
An airborne wind energy system (AWES) can harvest stronger wind streams at higher altitudes which are not accessible to conventional wind turbines. The operation of AWES requires a controller for the tethered aircraft/kite module (KM), as well as a controller for the ground station module (GSM). The literature regarding the control of AWES mostly focuses on the trajectory tracking of the KM. However, an advanced control of the GSM is also key to the successful operation of an AWES. In this paper we propose a cascaded control strategy for the GSM of an AWES during the traction or power generation phase. The GSM comprises a winch and a three-phase induction machine (IM), which acts as a generator. In the outer control-loop, an integral sliding mode control (SMC) algorithm is designed to keep the winch velocity at the prescribed level. A detailed stability analysis is also presented for the existence of the SMC for the perturbed winch system. The rotor flux-based field oriented control (RFOC) of the IM constitutes the inner control-loop. Due to the sophisticated RFOC, the decoupled and instantaneous control of torque and rotor flux is made possible using decentralized proportional integral (PI) controllers. The unknown states required to design RFOC are estimated using a discrete time Kalman filter (DKF), which is based on the quasi-linear model of the IM. The designed GSM controller is integrated with an already developed KM, and the AWES is simulated using MATLAB and Simulink. The simulation study shows that the GSM control system exhibits appropriate performance even in the presence of the wind gusts, which account for the external disturbance. Full article
(This article belongs to the Special Issue Airborne Wind Energy Systems)
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Article
Three-Dimensional Unsteady Aerodynamic Analysis of a Rigid-Framed Delta Kite Applied to Airborne Wind Energy
Energies 2021, 14(23), 8080; https://doi.org/10.3390/en14238080 - 02 Dec 2021
Cited by 1 | Viewed by 1360
Abstract
The validity of using a low-computational-cost model for the aerodynamic characterization of Airborne Wind Energy Systems was studied by benchmarking a three-dimensional Unsteady Panel Method (UnPaM) with experimental data from a flight test campaign of a two-line Rigid-Framed Delta kite. The latter, and [...] Read more.
The validity of using a low-computational-cost model for the aerodynamic characterization of Airborne Wind Energy Systems was studied by benchmarking a three-dimensional Unsteady Panel Method (UnPaM) with experimental data from a flight test campaign of a two-line Rigid-Framed Delta kite. The latter, and a subsequent analysis of the experimental data, provided the evolution of the tether tensions, the full kinematic state of the kite (aerodynamic velocity and angular velocity vectors, among others), and its aerodynamic coefficients. The history of the kinematic state was used as input for UnPaM that provided a set of theoretical aerodynamic coefficients. Disparate conclusions were found when comparing the experimental and theoretical aerodynamic coefficients. For a wide range of angles of attack and sideslip angles, the agreement in the lift and lateral force coefficients was good and moderate, respectively, considering UnPaM is a potential flow tool. As expected, UnPaM predicts a much lower drag because it ignores viscous effects. The comparison of the aerodynamic torque coefficients is more delicate due to uncertainties on the experimental data. Besides fully non-stationary simulations, the lift coefficient was also studied with UnPaM by assuming quasi-steady and steady conditions. It was found that for a typical figure-of-eight trajectory there are no significant differences between unsteady and quasi-steady approaches allowing for fast simulations. Full article
(This article belongs to the Special Issue Airborne Wind Energy Systems)
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Article
Flight Stability of Rigid Wing Airborne Wind Energy Systems
Energies 2021, 14(22), 7704; https://doi.org/10.3390/en14227704 - 17 Nov 2021
Cited by 2 | Viewed by 1416
Abstract
The flight mechanics of rigid wing Airborne Wind Energy Systems (AWESs) is fundamentally different from the one of conventional aircrafts. The presence of the tether largely impacts the system dynamics, making the flying craft to experience forces which can be an order of [...] Read more.
The flight mechanics of rigid wing Airborne Wind Energy Systems (AWESs) is fundamentally different from the one of conventional aircrafts. The presence of the tether largely impacts the system dynamics, making the flying craft to experience forces which can be an order of magnitude larger than those experienced by conventional aircrafts. Moreover, an AWES needs to deal with a sustained yet unpredictable wind, and the ensuing requirements for flight maneuvers in order to achieve prescribed control and power production goals. A way to maximize energy capture while facing disturbances without requiring an excessive contribution from active control is that of suitably designing the AWES craft to feature good flight dynamics characteristics. In this study, a baseline circular flight path is considered, and a steady state condition is defined by modeling all fluctuating dynamic terms over the flight loop as disturbances. In-flight stability is studied by linearizing the equations of motion on this baseline trajectory. In populating a linearized dynamic model, analytical derivatives of external forces are computed by applying well-known aerodynamic theories, allowing for a fast formulation of the linearized problem and for a quantitative understanding of how design parameters influence stability. A complete eigenanalysis of an example tethered system is carried out, showing that a stable-by-design AWES can be obtained and how. With the help of the example, it is shown how conventional aircraft eigenmodes are modified for an AWES and new eigenmodes, typical of AWESs, are introduced and explained. The modeling approach presented in the paper sets the basis for a holistic design of AWES that will follow this work. Full article
(This article belongs to the Special Issue Airborne Wind Energy Systems)
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Review

Jump to: Research

Review
A Review on Crosswind Airborne Wind Energy Systems: Key Factors for a Design Choice
Energies 2023, 16(1), 351; https://doi.org/10.3390/en16010351 - 28 Dec 2022
Viewed by 753
Abstract
Airborne wind energy (AWE) has received increasing attention during the last decade, with the goal of achieving electricity generation solutions that may be used as a complement or even an alternative to conventional wind turbines. Despite that several concepts have already been proposed [...] Read more.
Airborne wind energy (AWE) has received increasing attention during the last decade, with the goal of achieving electricity generation solutions that may be used as a complement or even an alternative to conventional wind turbines. Despite that several concepts have already been proposed and investigated by several companies and research institutions, no mature technology exists as yet. The mode of energy generation, the type of wing, the take-off and landing approaches, and the control mechanisms, to name a few, may vary among AWE crosswind systems. Given the diversity of possibilities, it is necessary to determine the most relevant factors that drive AWE exploration. This paper presents a review on the characteristics of currently existing AWE technological solutions, focusing on the hardware architecture of crosswind systems, with the purpose of providing the information required to identify and assess key factors to be considered in the choice of such systems. The identified factors are categorized into four distinct classes: technical design factors (aerodynamic performance, mass-to-area ratio, durability, survivability); operational factors (continuity of power production, controllability, take-off and landing feasibility); fabrication and logistical factors (manufacturability, logistics); and social acceptability factors (visual impact, noise impact, ecological impact, safety). Full article
(This article belongs to the Special Issue Airborne Wind Energy Systems)
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Review
The Social Acceptance of Airborne Wind Energy: A Literature Review
Energies 2022, 15(4), 1384; https://doi.org/10.3390/en15041384 - 14 Feb 2022
Cited by 3 | Viewed by 3227
Abstract
Airborne wind energy (AWE) systems use tethered flying devices to harvest higher-altitude winds to produce electricity. For the success of the technology, it is crucial to understand how people perceive and respond to it. If concerns about the technology are not taken seriously, [...] Read more.
Airborne wind energy (AWE) systems use tethered flying devices to harvest higher-altitude winds to produce electricity. For the success of the technology, it is crucial to understand how people perceive and respond to it. If concerns about the technology are not taken seriously, it could delay or prevent implementation, resulting in increased costs for project developers and a lower contribution to renewable energy targets. This literature review assessed the current state of knowledge on the social acceptance of AWE. A systematic literature search led to the identification of 40 relevant publications that were reviewed. The literature expected that the safety, visibility, acoustic emissions, ecological impacts, and the siting of AWE systems impact to which extent the technology will be accepted. The reviewed literature viewed the social acceptance of AWE optimistically but lacked scientific evidence to back up its claims. It seemed to overlook the fact that the impact of AWE’s characteristics (e.g., visibility) on people’s responses will also depend on a range of situational and psychological factors (e.g., the planning process, the community’s trust in project developers). Therefore, empirical social science research is needed to increase the field’s understanding of the acceptance of AWE and thereby facilitate development and deployment. Full article
(This article belongs to the Special Issue Airborne Wind Energy Systems)
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Planned Papers

The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.

Title: Swinging Motion of a Flexible Membrane Kite with Suspended Control Unit During Turning Manoeuvres
Authors: Mark Schelbergen and Roland Schmehl
Affiliation: Delft University of Technology
Abstract: xxx

Title: Life Cycle Assessment of Multi-Megawatt Airborne Wind Energy
Authors: Luuk Van Hagen; Kristian Petrick; Stefan Wilhelm; Roland Schmehl
Affiliation: Delft University of Technology
Abstract: A key motivation for airborne wind energy is its potential to reduce the amount of material required for the generation of renewable energy. On the other hand, the materials that are used for the airborne system components are generally linked to higher environmental impacts. This study presents comparative life cycle analyses for future multi-megawatt airborne wind energy systems and conventional wind turbines, with both technologies operating in the same farm configuration and under matching environmental conditions. The analyses quantify the global warming potential and cumulative energy demand of the emerging and established wind energy technologies. The cumulative energy demand is subsequently also used to determine the energy payback time and the energy return of investment. The selected airborne wind energy system is based on the design of Ampyx Power, using a fixed-wing aircraft that is tethered to a generator on the ground. The conventional wind turbine is primarily based on the NREL 5 MW reference turbine. The results confirm that an airborne wind energy system uses significantly fewer materials and generates electricity at notably lower impacts than the conventional wind turbine. Furthermore, the impacts of the wind turbine depend strongly on the local environmental conditions, while the impacts of the airborne wind energy system show only a minimal dependency. Airborne wind energy is found to be most advantageous for operation at unfavourable environmental conditions for the conventional systems, where the turbines require a large hub height.

Title: High fidelity fluid-structure interaction simulation of a multi-megawatt airborne wind energy reference system in cross-wind flight
Authors: N Pynaert 1,2; J Wauters 1,2; G Crevecoeur 1,2 and J Degroote 1,2
Affiliation: 1 Department of Electromechanical, Systems and Metal Engineering, Ghent University, Sint-Pietersnieuwstraat 41, 9000 Gent, Belgium; 2 Core lab EEDT, Flanders Make, Belgium
Abstract: xxx

Title: Modelling Aeroelastic Deformation of Flexible Membrane Kites
Authors: Jelle A.W. Poland; Roland Schmehl
Affiliation: Delft University of Technology
Abstract: Airborne wind energy systems using flexible membrane wings have the advantages of low weight, small packing volume, high mobility and rapid deployment. In this paper, we propose a particle system approach for simulating the deformation of a leading-edge inflatable kite. In this approach, each wing segment is represented by four point masses whose relative position along the tubular frame is kept constant by linear spring-damper elements. The billowing of the trailing edge of the wing is taken into account by an empirical correlation. Line segments of the bridle line system are modelled by two point masses connected by a spring-damper element. A pulley model is provided to connect three line segments. The generated aerodynamic force balances the spring- and damper forces and is determined using the lift equation with a linear lift polar for each wing segment separately. The particle system model can be used to describe the symmetric deformation of the wing in response to a symmetric actuation of the bridle lines used for depowering the kite, changing the pitch angle. The model also reproduces the typical twist deformation of the wing in response to an asymmetric line actuation used for steering the kite. Simulated wing shapes are compared with photogrammetric information taken from the video camera on the kite control unit that looks at the kite during flight. The results demonstrate how a particle system model can accurately predict the shape of a soft wing for low computational cost, making it an ideal structural building block towards the next generation of soft wing kite models.

Title: Low and high fidelity aerodynamic simulations of box wing kites for airborne wind energy applications
Authors: Dylan Eijkelhof; Gabriel Buendía; Roland Schmehl
Affiliation: Delft University of Technology
Abstract: Airborne wind energy systems (AWES) convert the kinetic energy of the wind into electricity, from which the power is directly proportional to the aerodynamic efficiency. The need of a high aerodynamic coefficient implies high aerodynamic loads and thus, for conventional AWE wings, a high thickness-to-chord ratio. The box wing concept opens the possibility of exploring thinner aerofoil profiles as the load distribution can be changed with a lower wing span and reinforcements between the wings. This study focuses on creating a tool for reliable aerodynamic simulations that check the superior performance of box wings to conventional wings and automating the design and meshing processes involved. The aerodynamic tools used, include a steady panel method solver (APAME) and CFD simulations based on a RANS model using a k-\texorpdfstring{$\omega$}{Omega} SST turbulence model (OpenFOAM). The CFD mesh is created automatically using Pointwise commands from a set of eight physical design parameters, 5 aerofoil profiles and mesh refinement specifications. Fast simulations and accurate values were obtained using the panel method for the linear region of the lift coefficient, but CFD simulations with high to medium quality meshes were required to obtain good agreement with experiments in the lift and drag coefficient. The CFD simulations showed delayed stall of the upper wing with respect to the lower one, increasing the stall angle of attack. Also, the wing tip boundary layer separation is delayed compared to the wing root for the straight rectangular box wing. Choosing the design point and operational envelope smartly could enhance the aerodynamic performance of AWE kites, which are likely to operate at high angles of attack to increase the aerodynamic forces, which maximises the tether force, leading to a higher power output.

Title: Hybrid Power Systems Using Airborne Wind Energy
Authors: Sweder Reuchlin; Rishikesh Joshi; Roland Schmehl
Affiliation: Delft University of Technology
Abstract: A majority of the remote area locations which are not connected to the main electricity grid rely on diesel generators for meeting their electricity requirement. High transportation costs for fuel along with significant carbon emissions has motivated the development and installation of hybrid renewable energy systems in these locations. Hybrid power systems combining wind and solar are of interest due to the their negative correlation on a daily as well as seasonal timescales. Since these resources are intermittent by nature, they are usually combined with a storage solution for a more secure power supply. This paper presents a modelling and sizing framework for hybrid power systems using airborne wind energy as one of its components. The framework uses hourly timeseries data of wind and solar combining with the load data to estimate the optimal sizing of the hybrid power system configuration based on the objective of minimzing the levelized cost of electricity of the system. Case-study results from a location in Marseille, France show that the cost of electricity could reduce by around 60\% by shifting from purely diesel based electricity generation to a hybrid power system comprising of airborne wind energy, solar PV, batteries and diesel.

Title: Optimization of a Multi-Element Airfoil for a Rigid Airborne Wind Energy Kite
Authors: Agustí Porta Ko; , Sture Smidt; Roland Schmehl 1 and Manoj Mandru
Affiliation: 1 Delft University of Technology, 2 Kitemill AS
Abstract: Airborne wind energy (AWE) systems benefit from a high lift airfoil to increase the power output, to that end, multi-element airfoils are investigated for AWE applications. This paper aims to design through optimization a multi-element airfoil for a rigid AWE kite that operates in a pumping cycle or ground-gen. Since the kite has distinct operational phases, i.e., reel-out and reel-in phase, the airfoil design must take into account the different design objectives for each phase. The airfoil is first optimized for reel-out or production phase and then it is adapted for the reel-in or return phase requirements by modifying its flap setting. The optimization is performed through a multi-objective genetic algorithm coupled to MSES as the aerodynamic solver. Once the multi-element airfoil has been optimized, its aerodynamic performance is verified through CFD RANS simulations, computed with OpenFOAM. The resulting airfoil offers a satisfactory performance for production and return phase; and the CFD verification shows a fairly good agreement in terms of lift coefficient although the drag has been significantly overpredicted by MSES.

Title: Fast aeroelastic model of a leading-edge inflatable kite
Authors: Oriol Cayon Domingo; , Mac Gaunaa; Roland Schmehl
Affiliation: 1. Technical University of Denmark, Department of Wind Energy, 4000 Roskilde, Denmark 2. Delft University of Technology, Faculty of Aerospace Engineering, 2629 HS Delft, The Netherlands
Abstract: Membrane kites used in airborne wind energy (AWE) function as a morphing structure whose shape depends on its aerodynamic loading and vice versa, posing a complex fluid-structure interaction (FSI) problem. Due to this complexity, kite design is usually done on an experimental basis since no model meets the requirements of being both accurate and fast. This paper presents a fast aeroelastic model of leading-edge inflatable (LEI) kites for the design phase of airborne wind energy systems. The FSI methodology couples two fast and modular models; a particle system model of the wing and the bridle line system \cite{Poland} and a 3D nonlinear vortex step method (VSM) coupled with viscous 2D polars. The aerodynamic model has been validated with several geometries proving to be accurate and inexpensive for pre-stall angles of attack. The coupled model has been validated using experimental data showing good agreement in the deformations and aerodynamic forces. Therefore, the speed and accuracy of this model make it an excellent foundation for a kite design tool.

Title: Ring Kite Turbine Systems with Tensile Rotary Power Transmission – Modelling, Analysis and Improved Design
Authors: Oliver Tulloch; Hong Yue; Abbas Mehrad Kazemi Amiri; Rod Read
Affiliation: University of Strathclyde
Abstract: A rotary kite turbine designed to use tensile rotary power transmission (TRPT) has a stable lifting structure, less tether drag and is easier to automate for larger deployments. In this work, a modular framework model is developed for this airborne wind energy (AWE) system. The ring kite turbine model includes power extraction, power transmission and the ground station, among which the TRPT system is the core component in power transmission. Models with different levels of complexity are proposed for TRPT, one steady state model and two dynamic models with spring-disc and multi-spring representations, respectively. To assess the torque loss on TRPT, a simple drag model is written for the steady state TRPT, followed by an improved model for the dynamic TRPT. The modular framework allows for multiple versions of the rotor, tether aerodynamics and TRPT representations. The developed models are validated by experimental data, then comparisons are made over a range of modelling options. Impacts of TRPT design, rotor design and tether drag on the kite turbine operations are analysed using a steady state analysis. Improved designs are discussed through multi-parameter optimisation based on steady state performance.

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