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24 pages, 9860 KiB

Open AccessArticle

Modelling Aero-Structural Deformation of Flexible Membrane Kites

by Jelle A. W. Poland and Roland Schmehl

Energies 2023, 16(14), 5264; https://doi.org/10.3390/en16145264 - 9 Jul 2023

Cited by 3 | Viewed by 3275

Abstract

Airborne wind energy systems using flexible membrane wings have the advantages of a low weight, small packing volume, high mobility and rapid deployability. This paper investigates the aero-structural deformation of a leading edge inflatable kite for airborne wind energy harvesting. In the first [...] Read more.

Airborne wind energy systems using flexible membrane wings have the advantages of a low weight, small packing volume, high mobility and rapid deployability. This paper investigates the aero-structural deformation of a leading edge inflatable kite for airborne wind energy harvesting. In the first step, a triangular two-plate representation of the wing is introduced, leading to an analytical description of the wing geometry depending on the symmetric actuation state. In the second step, this geometric constraint-based model is refined to a multi-segment wing representation using a particle system approach. Each wing segment consists of four point masses kept at a constant distance along the tubular frame by linear spring-damper elements. An empirical correlation is used to model the billowing of the wing’s trailing edge. The linear spring-damper elements also the model line segments of the bridle line system, with each connecting two point masses. Three line segments can also be connected by a pulley model. The aerodynamic force acting on each wing segment is determined individually using the lift equation with a constant lift coefficient. The particle system model can predict the symmetric deformation of the wing in response to a symmetric actuation of the bridle lines used for depowering the kite (i.e., 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. The simulated wing geometries are compared with photogrammetric information taken by the onboard video camera of the kite control unit, focusing on the wing during flight. The results demonstrate that a particle system model can accurately predict the geometry of a soft wing at a low computational cost, making it an ideal structural building block for the next generation of soft wing kite models. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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15 pages, 674 KiB

Open AccessArticle

Sizing of Hybrid Power Systems for Off-Grid Applications Using Airborne Wind Energy

by Sweder Reuchlin, Rishikesh Joshi and Roland Schmehl

Energies 2023, 16(10), 4036; https://doi.org/10.3390/en16104036 - 11 May 2023

Cited by 6 | Viewed by 2684

Abstract

The majority of remote locations not connected to the main electricity grid rely on diesel generators to provide electrical power. High fuel transportation costs and significant carbon emissions have motivated the development and installation of hybrid power systems using renewable energy such these [...] Read more.

The majority of remote locations not connected to the main electricity grid rely on diesel generators to provide electrical power. High fuel transportation costs and significant carbon emissions have motivated the development and installation of hybrid power systems using renewable energy such these locations. Because wind and solar energy is intermittent, such sources are usually combined with energy storage for a more stable power supply. This paper presents a modelling and sizing framework for off-grid hybrid power systems using airborne wind energy, solar PV, batteries and diesel generators. The framework is based on hourly time-series data of wind resources from the ERA5 reanalysis dataset and solar resources from the National Solar Radiation Database maintained by NREL. The load data also include hourly time series generated using a combination of modelled and real-life data from the ENTSO-E platform maintained by the European Network of Transmission System Operators for Electricity. The backbone of the framework is a strategy for the sizing of hybrid power system components, which aims to minimise the levelised cost of electricity. A soft-wing ground-generation-based AWE system was modelled based on the specifications provided by Kitepower B.V. The power curve was computed by optimising the operation of the system using a quasi-steady model. The solar PV modules, battery systems and diesel generator models were based on the specifications from publicly available off-the-shelf solutions. The source code of the framework in the MATLAB environment was made available through a GitHub repository. For the representation of results, a hypothetical case study of an off-grid military training camp located in Marseille, France, was described. The results show that significant reductions in the cost of electricity were possible by shifting from purely diesel-based electricity generation to an hybrid power system comprising airborne wind energy, solar PV, batteries and diesel. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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18 pages, 2459 KiB

Open AccessArticle

Optimisation of a Multi-Element Airfoil for a Fixed-Wing Airborne Wind Energy System

by Agustí Porta Ko, Sture Smidt, Roland Schmehl and Manoj Mandru

Energies 2023, 16(8), 3521; https://doi.org/10.3390/en16083521 - 18 Apr 2023

Cited by 3 | Viewed by 3495

Abstract

Airborne wind energy systems benefit from high-lift airfoils to increase power output. This paper proposes an optimisation approach for a multi-element airfoil of a fixed-wing system operated in pumping cycles to drive a drum-generator module on the ground. The approach accounts for the [...] Read more.

Airborne wind energy systems benefit from high-lift airfoils to increase power output. This paper proposes an optimisation approach for a multi-element airfoil of a fixed-wing system operated in pumping cycles to drive a drum-generator module on the ground. The approach accounts for the different design objectives of the tethered kite’s alternating production and return phases. The airfoil shape is first optimised for the production phase and then adapted for the requirements of the return phase by modifying the flap setting. The optimisation uses the multi-objective genetic algorithm NSGA-II in combination with the fast aerodynamic solver MSES. Once the optimal shape is determined, the aerodynamic performance is verified through CFD RANS simulations with OpenFOAM. The resulting airfoil achieves satisfactory performance for the production and return phases of the pumping cycles, and the CFD verification shows a fairly good agreement in terms of the lift coefficient. However, MSES significantly underpredicts the airfoil drag. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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23 pages, 1369 KiB

Open AccessArticle

Operation Approval for Commercial Airborne Wind Energy Systems

by Volkan Salma and Roland Schmehl

Energies 2023, 16(7), 3264; https://doi.org/10.3390/en16073264 - 5 Apr 2023

Cited by 1 | Viewed by 2599

Abstract

Integrating the operation of airborne wind energy systems safely into the airspace requires a systematic qualification process. It seems likely that the European Union Aviation Safety Agency will approve commercial systems as unmanned aircraft systems within the “specific” category, requiring risk-based operational authorization. [...] Read more.

Integrating the operation of airborne wind energy systems safely into the airspace requires a systematic qualification process. It seems likely that the European Union Aviation Safety Agency will approve commercial systems as unmanned aircraft systems within the “specific” category, requiring risk-based operational authorization. In this paper, we interpret the risk assessment methodology for airborne wind energy systems, going through the ten required steps of the recommended procedure and discussing the particularities of tethered energy-harvesting systems. Although the described process applies to the entire field of airborne wind energy, we detail it for a commercial flexible-wing airborne wind energy system. We find that the air risk mitigations improve the consolidated specific assurance and integrity level by a factor of two. It is expected that the framework will increase the safety level of commercial airborne wind energy systems and ultimately lead to operation approval. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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19 pages, 4381 KiB

Open AccessArticle

Fast Aero-Structural Model of a Leading-Edge Inflatable Kite

by Oriol Cayon, Mac Gaunaa and Roland Schmehl

Energies 2023, 16(7), 3061; https://doi.org/10.3390/en16073061 - 27 Mar 2023

Cited by 3 | Viewed by 4747

Abstract

Soft-wing kites for airborne wind-energy harvesting function as flying tensile membrane structures, each of whose shape depends on the aerodynamic load distribution and vice versa. The strong two-way coupling between shape and loading poses a complex fluid-structure interaction problem. Since computational models for [...] Read more.

Soft-wing kites for airborne wind-energy harvesting function as flying tensile membrane structures, each of whose shape depends on the aerodynamic load distribution and vice versa. The strong two-way coupling between shape and loading poses a complex fluid-structure interaction problem. Since computational models for such problems do not yet meet the requirements of being accurate and at the same time fast, kite designers usually work on the basis of intuition and experience, combined with extensive iterative flight testing. This paper presents a fast aero-structural model of leading-edge inflatable kites for the design phase of airborne wind-energy systems. The fluid-structure interaction solver couples two fast and modular models: a particle system model to capture the deformation of the wing and bridle-line system and a 3D nonlinear vortex step method coupled with viscous 2D airfoil polars to describe the aerodynamics. The flow solver was validated with several wing geometries and proved to be accurate and computationally inexpensive for pre-stall angles of attack. The coupled aero-structural model was 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. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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19 pages, 18981 KiB

Open AccessArticle

Low- and High-Fidelity Aerodynamic Simulations of Box Wing Kites for Airborne Wind Energy Applications

by Dylan Eijkelhof, Gabriel Buendía and Roland Schmehl

Energies 2023, 16(7), 3008; https://doi.org/10.3390/en16073008 - 25 Mar 2023

Cited by 1 | Viewed by 2313

Abstract

High aerodynamic efficiency is a key design driver for airborne wind energy systems as it strongly affects the achievable energy output. Conventional fixed-wing systems generally use aerofoils with a high thickness-to-chord ratio to achieve high efficiency and wing loading. The box wing concept [...] Read more.

High aerodynamic efficiency is a key design driver for airborne wind energy systems as it strongly affects the achievable energy output. Conventional fixed-wing systems generally use aerofoils with a high thickness-to-chord ratio to achieve high efficiency and wing loading. The box wing concept suits thinner aerofoils as the load distribution can be changed with a lower wing span and structural reinforcements between the upper and lower wings. This paper presents an open-source toolchain for reliable aerodynamic simulations of parameterized box wing configurations, automating the design, meshing, and simulation setup processes. The aerodynamic tools include the steady 3D panel method solver APAME and the CFD-solver OpenFOAM, which use a steady Reynolds-Averaged Navier–Stokes approach with k-

ω

SST turbulence model. The finite-volume mesh for the CFD-solver is generated automatically with Pointwise using eight physical design parameters, five aerofoil profiles and mesh refinement specifications. The panel method provided accurate and fast results in the linear lift region. For higher angles of attack, CFD simulations with high- to medium-quality meshes were required to obtain good agreement with measured lift and drag coefficients. The CFD simulations showed that the upper wing stall lagged behind the lower wing, increasing the stall angle of attack compared to conventional fixed-wing kites. In addition, the wing tip boundary layer separation was delayed compared to the wing root for the straight rectangular box wing. Choosing the design point and operational envelope wisely can enhance the aerodynamic performance of airborne wind energy kites, which are generally operated at a large angle of attack to maximise the wing loading and tether force, and through that, the power output of the system. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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18 pages, 5833 KiB

Open AccessArticle

Conformable Inflatable Wings Woven Using a Jacquard Technique

by Joep Breuer, Rolf Luchsinger and Roland Schmehl

Energies 2023, 16(7), 2952; https://doi.org/10.3390/en16072952 - 23 Mar 2023

Viewed by 2831

Abstract

Inflatable wings are of interest for applications where low weight, compact transport volume, and easy set-up are important. Examples are unmanned aerial vehicles with inflatable wings, paragliders and softkites for sport or airborne wind-energy applications. In this paper, a new method of designing [...] Read more.

Inflatable wings are of interest for applications where low weight, compact transport volume, and easy set-up are important. Examples are unmanned aerial vehicles with inflatable wings, paragliders and softkites for sport or airborne wind-energy applications. In this paper, a new method of designing and fabricating conformable inflatable wings by Jacquard three-dimensional weaving is presented. Depending on the weaving pattern, plane-parallel, tapered, or even curved structures can be produced. An analytical framework was developed to determine the shapes of pressurized structures produced by Jacquard weaving. Based on this theory, several design patterns suitable for inflatable wings are proposed. It is shown that the structural efficiency of the woven structure is identical to the structural efficiency of a cylinder. To validate the concept, different wing prototypes were built with the methods used for the mass production of airbags. The new method allows for the cost-efficient fabrication of inflatable structures, pressure vessels, and liquid containers with applications in the automotive, aerospace, and leisure industries. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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Graphical abstract

42 pages, 8008 KiB

Open AccessArticle

A Tensile Rotary Airborne Wind Energy System—Modelling, Analysis and Improved Design

by Oliver Tulloch, Hong Yue, Abbas Mehrad Kazemi Amiri and Roderick Read

Energies 2023, 16(6), 2610; https://doi.org/10.3390/en16062610 - 9 Mar 2023

Cited by 3 | Viewed by 2919

Abstract

A unique rotary kite turbine designed with tensile rotary power transmission (TRPT) is introduced in this work. Power extraction, power transmission and the ground station are modelled in a modular framework. The TRPT system is the key component of power transmission, for which [...] Read more.

A unique rotary kite turbine designed with tensile rotary power transmission (TRPT) is introduced in this work. Power extraction, power transmission and the ground station are modelled in a modular framework. The TRPT system is the key component of power transmission, for which three models with different levels of complexity are proposed. The first representation is based on the stationary state of the system, in which the external and internal torques of a TRPT section are in equilibrium, referred to as the steady-state TRPT model. The second representation is a simplified spring-disc model for dynamic TRPT, and the third one is a multi-spring model with higher degrees of freedom and more flexibility in describing TRPT dynamics. To assess the torque loss on TRPT, a simple tether drag model is written for the steady-state TRPT, followed by an improved tether drag model for the dynamic TRPT. This modular framework allows for multiple versions of the rotor, tether aerodynamics and TRPT representations. The developed models are validated by laboratory and field-testing experimental data, simulated over a range of modelling options. Model-based analysis are performed on TRPT design, rotor design and tether drag to understand any limitations and crucial design drivers. Improved designs are explored through multi-parameter optimisation based on steady-state analysis. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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18 pages, 4083 KiB

Open AccessFeature PaperArticle

An Aero-Structural Model for Ram-Air Kite Simulations

by Paul Thedens and Roland Schmehl

Energies 2023, 16(6), 2603; https://doi.org/10.3390/en16062603 - 9 Mar 2023

Cited by 7 | Viewed by 3844

Abstract

Similar to parafoils, ram-air kites are flexible membrane wings inflated by the apparent wind and supported by a bridle line system. A major challenge in estimating the performance of these wings using a computer model is the strong coupling between the airflow around [...] Read more.

Similar to parafoils, ram-air kites are flexible membrane wings inflated by the apparent wind and supported by a bridle line system. A major challenge in estimating the performance of these wings using a computer model is the strong coupling between the airflow around the wing and the deformation of the membrane structure. In this paper, we introduce a staggered coupling scheme combining a structural finite element solver using a dynamic relaxation technique with a potential flow solver. The developed method proved numerically stable for determining the equilibrium shape of the wing under aerodynamic load and is thus suitable for performance measurement and load estimation. The method was validated with flight data provided by SkySails Power. Measured forces on the tether and steering belt of the robotic kite control pod showed good resemblance with the simulation results. As expected for a potential flow solver, the kite’s glide ratio was overestimated by 10–15%, and the measured tether elevation angle in a neutral flight scenario matched the simulations within 2 degrees. Based on the obtained results, it can be concluded that the proposed aero-structural model can be used for initial designs of ram-air kites with application to airborne wind energy. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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19 pages, 3490 KiB

Open AccessArticle

Value-Driven System Design of Utility-Scale Airborne Wind Energy

by Rishikesh Joshi, Michiel Kruijff and Roland Schmehl

Energies 2023, 16(4), 2075; https://doi.org/10.3390/en16042075 - 20 Feb 2023

Cited by 4 | Viewed by 2811

Abstract

In the current auction-based electricity market, the design of utility-scale renewable energy systems has traditionally been driven by the levelised cost of energy (LCoE). However, the market is gradually moving towards a subsidy-free era, which will expose the power plant owners to the [...] Read more.

In the current auction-based electricity market, the design of utility-scale renewable energy systems has traditionally been driven by the levelised cost of energy (LCoE). However, the market is gradually moving towards a subsidy-free era, which will expose the power plant owners to the fluctuating prices of electricity. This paper presents a computational approach to account for the influence of time-varying electricity prices on the design of airborne wind energy (AWE) systems. The framework combines an analytical performance model, providing the power curve of the system, with a wind resource characterisation based on ERA5 reanalysis data. The resulting annual energy production (AEP) model is coupled with a parametric cost model based on reference prototype data from Ampyx Power B.V. extended by scaling laws. Ultimately, an energy price model using real-life data from the ENTSO-E platform maintained by the association of EU transmission system operators was used to estimate the revenue profile. This framework was then used to compare the performance of systems based on multiple economic metrics within a chosen design space. The simulation results confirmed the expected behaviour that the electricity produced at lower wind speeds has a higher value than that produced at higher wind speeds. To account for this electricity price dependency on wind speeds in the design process, we propose an economic metric defined as the levelised profit of energy (LPoE). This approach determines the trade-offs between designing a system that minimises cost and designing a system that maximises value. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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32 pages, 5121 KiB

Open AccessArticle

AWEbox: An Optimal Control Framework for Single- and Multi-Aircraft Airborne Wind Energy Systems

by Jochem De Schutter, Rachel Leuthold, Thilo Bronnenmeyer, Elena Malz, Sebastien Gros and Moritz Diehl

Energies 2023, 16(4), 1900; https://doi.org/10.3390/en16041900 - 14 Feb 2023

Cited by 6 | Viewed by 2767

Abstract

In this paper, we present AWEbox, a Python toolbox for modeling and optimal control of multi-aircraft systems for airborne wind energy (AWE). AWEbox provides an implementation of optimization-friendly multi-aircraft AWE dynamics for a wide range of system architectures and modeling options. It automatically [...] Read more.

In this paper, we present AWEbox, a Python toolbox for modeling and optimal control of multi-aircraft systems for airborne wind energy (AWE). AWEbox provides an implementation of optimization-friendly multi-aircraft AWE dynamics for a wide range of system architectures and modeling options. It automatically formulates typical AWE optimal control problems based on these models, and finds a numerical solution in a reliable and efficient fashion. To obtain a high level of reliability and efficiency, the toolbox implements different homotopy methods for initial guess refinement. The first type of method produces a feasible initial guess from an analytic initial guess based on user-provided parameters. The second type implements a warm-start procedure for parametric sweeps. We investigate the software performance in two different case studies. In the first case study, we solve a single-aircraft reference problem for a large number of different initial guesses. The homotopy methods reduce the expected computation time by a factor of 1.7 and the peak computation time by a factor of eight, compared to when no homotopy is applied. Overall, the CPU timings are competitive with the timings reported in the literature. When the user initialization draws on expert a priori knowledge, homotopies do not increase expected performance, but the peak CPU time is still reduced by a factor of 5.5. In the second case study, a power curve for a dual-aircraft lift-mode AWE system is computed using the two different homotopy types for initial guess refinement. On average, the second homotopy type, which is tailored for parametric sweeps, outperforms the first type in terms of CPU time by a factor of three. In conclusion, AWEbox provides an open-source implementation of efficient and reliable optimal control methods that both control experts and non-expert AWE developers can benefit from. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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23 pages, 2005 KiB

Open AccessFeature PaperArticle

Life-Cycle Assessment of a Multi-Megawatt Airborne Wind Energy System

by Luuk van Hagen, Kristian Petrick, Stefan Wilhelm and Roland Schmehl

Energies 2023, 16(4), 1750; https://doi.org/10.3390/en16041750 - 9 Feb 2023

Cited by 16 | Viewed by 6554

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 used for airborne systems’ components are generally linked to higher environmental impacts. This study [...] Read more.

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 used for airborne systems’ 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 on 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 less material 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 most advantageous for operation at unfavourable environmental conditions for conventional systems, where the turbines require a large hub height. Full article

(This article belongs to the Special Issue Airborne Wind Energy Systems)

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16 pages, 4153 KiB

Open AccessArticle

Wing Deformation of an Airborne Wind Energy System in Crosswind Flight Using High-Fidelity Fluid–Structure Interaction

by Niels Pynaert, Thomas Haas, Jolan Wauters, Guillaume Crevecoeur and Joris Degroote

Energies 2023, 16(2), 602; https://doi.org/10.3390/en16020602 - 4 Jan 2023

Cited by 5 | Viewed by 2445

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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11 pages, 243 KiB

Open AccessArticle

Dominant Designs for Wings of Airborne Wind Energy Systems

by Silke van der Burg, Maarten F. M. Jurg, Flore M. Tadema, Linda M. Kamp and Geerten van de Kaa

Energies 2022, 15(19), 7291; https://doi.org/10.3390/en15197291 - 4 Oct 2022

Cited by 3 | Viewed by 2402

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)

19 pages, 5242 KiB

Open AccessArticle

Electrical Launch Catapult and Landing Decelerator for Fixed-Wing Airborne Wind Energy Systems

by Johannes Alexander Müller, Mostafa Yasser Mostafa Khalil Elhashash and Volker Gollnick

Energies 2022, 15(7), 2502; https://doi.org/10.3390/en15072502 - 29 Mar 2022

Cited by 6 | Viewed by 3726

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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25 pages, 22561 KiB

Open AccessArticle

Aerodynamic Performance and Wake Flow of Crosswind Kite Power Systems

by Mojtaba Kheiri, Samson Victor, Sina Rangriz, Mher M. Karakouzian and Frederic Bourgault

Energies 2022, 15(7), 2449; https://doi.org/10.3390/en15072449 - 26 Mar 2022

Cited by 9 | Viewed by 4006

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

k - ω

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

k - ω

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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15 pages, 5184 KiB

Open AccessArticle

Effect of Chordwise Struts and Misaligned Flow on the Aerodynamic Performance of a Leading-Edge Inflatable Wing

by Axelle Viré, Geert Lebesque, Mikko Folkersma and Roland Schmehl

Energies 2022, 15(4), 1450; https://doi.org/10.3390/en15041450 - 16 Feb 2022

Cited by 3 | Viewed by 3734

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 = 10^{5}

, 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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16 pages, 1478 KiB

Open AccessArticle

L0 and L1 Guidance and Path-Following Control for Airborne Wind Energy Systems

by Manuel C. R. M. Fernandes, Sérgio Vinha, Luís Tiago Paiva and Fernando A. C. C. Fontes

Energies 2022, 15(4), 1390; https://doi.org/10.3390/en15041390 - 14 Feb 2022

Cited by 5 | Viewed by 3205

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 (

L_{0}

or

L_{1}

, 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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25 pages, 1285 KiB

Open AccessArticle

Cascade Control of the Ground Station Module of an Airborne Wind Energy System

by Ali Arshad Uppal, Manuel C. R. M. Fernandes, Sérgio Vinha and Fernando A. C. C. Fontes

Energies 2021, 14(24), 8337; https://doi.org/10.3390/en14248337 - 10 Dec 2021

Cited by 3 | Viewed by 4410

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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17 pages, 25296 KiB

Open AccessArticle

Three-Dimensional Unsteady Aerodynamic Analysis of a Rigid-Framed Delta Kite Applied to Airborne Wind Energy

by Iván Castro-Fernández, Ricardo Borobia-Moreno, Rauno Cavallaro and Gonzalo Sánchez-Arriaga

Energies 2021, 14(23), 8080; https://doi.org/10.3390/en14238080 - 2 Dec 2021

Cited by 5 | Viewed by 3390

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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41 pages, 914 KiB

Open AccessArticle

Flight Stability of Rigid Wing Airborne Wind Energy Systems

by Filippo Trevisi, Alessandro Croce and Carlo E. D. Riboldi

Energies 2021, 14(22), 7704; https://doi.org/10.3390/en14227704 - 17 Nov 2021

Cited by 6 | Viewed by 3660

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

40 pages, 29533 KiB

Open AccessReview

A Review on Crosswind Airborne Wind Energy Systems: Key Factors for a Design Choice

by André F. C. Pereira and João M. M. Sousa

Energies 2023, 16(1), 351; https://doi.org/10.3390/en16010351 - 28 Dec 2022

Cited by 13 | Viewed by 5182

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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24 pages, 2590 KiB

Open AccessReview

The Social Acceptance of Airborne Wind Energy: A Literature Review

by Helena Schmidt, Gerdien de Vries, Reint Jan Renes and Roland Schmehl

Energies 2022, 15(4), 1384; https://doi.org/10.3390/en15041384 - 14 Feb 2022

Cited by 15 | Viewed by 8540

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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