Airborne Wind Energy Generators Research Articles

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

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.

Distribution parameter-determining method comparison for airborne wind energy potential assessment in the eastern coastal area of China

In recent years, airborne wind energy generation (AWEG) has increasingly been discussed as a better alternative to traditional wind turbine generation owing to less expensive raw materials, higher electricity capacity, and being more friendly to the surrounding environment. Thus, the planning of AWEG-based wind farms is necessary. To control the risk and ensure the future investment revenue to guarantee the ultimate success of this planning, airborne wind energy potential assessment (AWEPA) is essential; however, it has rarely been done so far. In this study, an AWEPA for the eastern coastal area of China was implemented. As wind speed distribution is an essential factor in AWEPA, to guarantee a reliable assessment, selecting a suitable distribution parameter-determining method is critical. In this paper, two conventional numerical parameter-determining methods and three intelligent parameter-determining methods were compared, and the intelligent parameter-determining method embedding the whale optimization algorithm that showed outstanding performance in the comparison was finally selected to be used in the AWEPA. The assessment results show that developing AWEG in the study area is highly likely feasible, and the results also provide some other valuable information for planning.

Structural analysis and optimization of a tethered swept wing for airborne wind energy generation

In Candade et al,1 the authors would like to correct the affiliations of Maximilian Ranneberg2 and Roland Schmehl1.

Structural analysis and optimization of a tethered swept wing for airborne wind energy generation

AbstractIn this paper, we present an aero‐structural model of a tethered swept wing for airborne wind energy generation. The carbon composite wing has neither fuselage nor actuated aerodynamic control surfaces and is controlled entirely from the ground using three separate tethers. The computational model is efficient enough to be used for weight optimisation at the initial design stage. The main load‐bearing wing component is a nontypical “D”‐shaped wing‐box, which is represented as a slender carbon composite shell and further idealised as a stack of two‐dimensional cross section models arranged along an anisotropic one‐dimensional beam model. This reduced 2+1D finite element model is then combined with a nonlinear vortex step method that determines the aerodynamic load. A bridle model is utilised to calculate the individual forces as a function of the aerodynamic load in the bridle lines that connect the main tether to the wing. The entire computational model is used to explore the influence of the bride on the D‐box structure. Considering a reference D‐box design along with a reference aerodynamic load case, the structural response is analysed for typical bridle configurations. Subsequently, an optimisation of the internal geometry and laminate fibre orientations is carried out using the structural computation models, for a fixed aerodynamic and bridle configuration. Aiming at a minimal weight of the wing structure, we find that for the typical load case of the system, an overall weight savings of approximately 20% can be achieved compared with the initial reference design.

Boundary layer transition modeling on leading edge inflatable kite airfoils.

We present a computational fluid dynamic analysis of boundary layer transition on leading edge inflatable kite airfoils used for airborne wind energy generation. Because of the operation in pumping cycles, the airfoil is generally subject to a wide range of Reynolds numbers. The analysis is based on the combination of the shear stress transport turbulence model with the γ−R˜eθt transition model, which can handle the laminar boundary layer and its transition to turbulence. The implementation of both models in OpenFOAM is described. We show a validation of the method for a sailwing (ie, a wing with a membrane) airfoil and an application to a leading edge inflatable kite airfoil. For the sailwing airfoil, the results computed with transition model agree well with the existing low Reynolds number experiment over the whole range of angles of attack. For the leading edge inflatable kite airfoil, the transition modeling has both favorable and unfavorable effects on the aerodynamics. On the one hand, the aerodynamics suffer from the laminar separation. But, on the other hand, the laminar boundary layer thickens slower than the turbulent counterpart, which, in combination with transition, delays the separation. The results also indicate that the aerodynamics of the kite airfoil could be improved by delaying the boundary layer transition during the traction phase and tripping the transition in the retraction phase.

Ram‐air kite airfoil and reinforcements optimization for airborne wind energy applications

AbstractWe present a multidisciplinary design optimization method for the profile and structural reinforcement layout of a ram‐air kite rib. The aim is to minimize the structural elastic energy and to maximize the traction power of a ram‐air kite used for airborne wind energy generation. Because of the large deformations occurring during flight, a fluid‐structure interaction (FSI) routine is included in the optimization, which determines the actual deformed rib geometry and its corresponding aerodynamic characteristics. A qualitative comparison between FSI inclusion and exclusion in the optimization is given. Discrepancies in airfoil profile and structural layout are observed.

Study on multirotor attached variable wing kite for airborne wind energy generation

Study on multirotor attached variable wing kite for airborne wind energy generation

Analysis of direct interconnection technique for offshore airborne wind energy systems under normal and fault conditions

Abstract Direct interconnection is a novel technique for interconnecting offshore airborne wind energy (AWE) generators which facilitates the removal of power converters from the offshore generation site. In this technique, unlike the conventional approach, all generators are interconnected directly and after dispatching the generated power to shore, a back to back converter or several paralleled back to back converters change the generated power to grid-compliant power. Considering that the high expenses of offshore operations for back to back converter repair and maintenance and the higher accessibility of shore-side back to back converters, this technique can improve the reliability and economy of the energy generation system. This research aims to implement and study the practicality and reliability of the direct interconnection approach for offshore non-reversing pumping mode airborne wind energy generator systems. The interaction of direct interconnected AWEs in normal and fault conditions is investigated, and synchronisation, frequency control and load sharing control of the AWE farm are examined and discussed.

A constraint-free flight simulator package for airborne wind energy systems

The LAgrangian Kite SimulAtor (LAKSA) is a freely available software for the dynamic analysis of tethered flying vehicles, such as kites and fixed-wing drones, applied to airborne wind energy generation. This software comprises four simulators. The one, two and four-line simulators, which consider flexible but inelastic tethers, are based on minimal coordinate Lagragian formulations and can be used for the analysis of fly and ground generation systems, kite-based traction systems, and kitesurfing applications, respectively. The configuration of the mechanical system in the fourth simulator can be defined by the user, who can select the number of flying vehicles and the properties of the elastic and flexible tethers linking them. In all the software tools, the kites or tethered fixed-wing drones are represented as rigid bodies and the dynamic equations of the tether-bridle-vehicle systems, together with the user-defined and time-dependent control variables, are solved self-consistently. Academic and research analysis can take advantage of the modularity of the simulators and their inputs and outputs interfaces, which follow a common and user-friendly architecture.

Warping NMPC for Online Generation and Tracking of Optimal Trajectories

Generation of feasible and optimal reference trajectories is crucial in tracking Nonlinear Model Predictive Control. Especially, for stability and optimality in presence of a time varying parameter, adaptation of the tracking trajectory has to be implemented. General approaches are real-time generation of trajectories or switching between a discrete set of precomputed trajectories. In order to circumvent the operational efforts of these methods for a special type of dynamical systems, we propose time warping as an alternative approach. This algorithm implements online generation of tracking trajectories by warping a single precomputed reference. In detail, warpable systems, feasibility and optimality of trajectories and the controller implementation are discussed. Finally, as an application example, simulation results of a tethered kite system for airborne wind energy generation are presented.

Towards robust sensor fusion for state estimation in airborne applications using GNSS and IMU

Precise estimation of position, velocity, and orientation is crucial for robust control in airborne applications such as the fast maneuvering power kites for airborne wind energy generators. In this paper we present a sensor fusion approach for the measurements of a global navigation satellite system receiver and an inertial measurement unit, using methods from direct optimal control. The resulting optimization problem is based on the minimization of the weighted squared residuals between model predictions and measurements and solved using a direct collocation discretization strategy. The framework allows the formulation of a batch and filter estimator which include beside the estimation of the navigational states the identification of sensor parameters such as biases of the inertial measurement unit. The results of the algorithms are evaluated against a reference trajectory of a maneuvering single propeller aircraft and achieve root mean squared errors below 1 m in position, 0.4 ms-1 in velocity, and 0.5 deg in orientation for the batch estimator. The contribution in this paper is a first step towards the required robustness of state estimation for airborne applications.

On the take-off of airborne wind energy systems based on rigid wings

The problem of launching a tethered rigid aircraft for airborne wind energy generation is investigated. Exploiting well-assessed physical principles, an analysis of four different take-off approaches is carried out. The approaches are then compared on the basis of quantitative and qualitative criteria introduced to assess their technical and economic viability. In particular, the additional power required by the take-off functionality is computed and related to the peak mechanical power generated by the system. Moreover, the additionally required on-board mass is estimated, which impacts the cut-in wind speed of the generator. Finally, the approximate ground area required for take-off is also determined. After the theoretical comparison, a deeper study of the concept that is deemed the most viable one, i.e. a linear take-off maneuver combined with on-board propellers, is performed by means of numerical simulations. The simulation results are used to refine the initial analysis and further confirm the viability of the approach.

Feasibility Study on a symmetric airfoil kitoon for the Airborne Wind Energy Generation Platform

Feasibility Study on a symmetric airfoil kitoon for the Airborne Wind Energy Generation Platform

Study on the direction of rotation of a vertical axis wind turbine for the Airborne WindEnergy Generation

Study on the direction of rotation of a vertical axis wind turbine for the Airborne WindEnergy Generation

Automatic Retraction and Full-Cycle Operation for a Class of Airborne Wind Energy Generators

Airborne wind energy systems aim to harvest the power of winds blowing at altitudes higher than what conventional wind turbines reach. They employ a tethered flying structure, usually a wing, and exploit the aerodynamic lift to produce electrical power. In the case of ground-based systems, where the traction force on the tether is used to drive a generator on the ground, a two phase power cycle is carried out: one phase to produce power, where the tether is reeled out under high traction force, and a second phase where the tether is recoiled under minimal load. The problem of controlling a tethered wing in this second phase, the retraction phase, is addressed here, by proposing two possible control strategies. Theoretical analyses, numerical simulations, and experimental results are presented to show the performance of the two approaches. Finally, the experimental results of complete autonomous power generation cycles are reported and compared with first-principle models.

On Sensor Fusion for Airborne Wind Energy Systems

A study on filtering aspects of airborne wind energy generators is presented. This class of renewable energy systems aims to convert the aerodynamic forces generated by tethered wings, flying in closed paths transverse to the wind flow, into electricity. The accurate reconstruction of the wing's position, velocity and heading is of fundamental importance for the automatic control of these kinds of systems. The difficulty of the estimation problem arises from the nonlinear dynamics, wide speed range, large accelerations and fast changes of direction that the wing experiences during operation. It is shown that the overall nonlinear system has a specific structure allowing its partitioning into sub-systems, hence leading to a series of simpler filtering problems. Different sensor setups are then considered, and the related sensor fusion algorithms are presented. The results of experimental tests carried out with a small-scale prototype and wings of different sizes are discussed. The designed filtering algorithms rely purely on kinematic laws, hence they are independent from features like wing area, aerodynamic efficiency, mass, etc. Therefore, the presented results are representative also of systems with larger size and different wing design, different number of tethers and/or rigid wings.

Direct feedback control design for nonlinear systems

We propose an approach for the direct design from data of controllers finalized at solving tracking problems for nonlinear systems. This approach, called Direct FeedbacK (DFK) design, overcomes relevant problems typical of the standard design methods, such as modeling errors, non-trivial parameter identification, non-convex optimization, and difficulty in nonlinear control design. Considering a Set Membership (SM) approach, we provide three main contributions. The first one is a theoretical framework for the stability analysis of nonlinear feedback control systems, in which the controller f̂ is an approximation identified from data of an ideal inverse model fo. In this framework, we derive sufficient conditions under which f̂ stabilizes the closed-loop system. The second contribution is a technique for the direct design of an approximate controller f∗ from data, having suitable optimality, stability, and sparsity properties. In particular, we show that f∗ is an almost-optimal controller (in a worst-case sense), and we derive a guaranteed accuracy bound, which can be used to quantify the performance level of the DFK control system. We also show that, when the number of data used for control design tends to infinity and these data are dense in the controller domain, the closed-loop stability is guaranteed for a set of trajectories of interest. The technique is based on convex optimization and sparse identification methods, and thus avoids the problem of local minima and allows an efficient online controller implementation in real-world applications. The third contribution is a simulation study, regarding the application of DFK to the challenging problem of control design for a class of airborne wind energy generators.

Wing/kite-based wind energy generation: An overview

Several technologies, aimed at converting high-altitude wind into electricity, are actually being investigated by companies, research centers and universities worldwide, and the community of people working in this field has coined the term airborne wind energy (AWE) as a common umbrella for these concepts. Indeed, many basic ideas that are now being developed in the context of AWE were already present in patents and publications since the ’70s. Then, these ideas remained somehow silent, until more recent years, when several research groups and companies started to carry out theoretical, numerical and experimental analyses, made possible by important advances in diverse fields like materials, aerodynamics, sensors, computation and control. In this lecture, the basic AWE concepts and results that have been up to date accomplished are overviewed, with a focus on a particular class of AWE generators, namely with flexible wings and ground level generators, and emphasis on optimization and control aspects. Finally, we delineate what challenges are still to be faced, in order to fully demonstrate the viability of airborne wind energy.

Optimization of airborne wind energy generators

SUMMARYThis paper presents novel results related to an innovative airborne wind energy technology, named Kitenergy, for the conversion of high‐altitude wind energy into electricity. The research activities carried out in the last five years, including theoretical analyses, numerical simulations, and experimental tests, indicate that Kitenergy could bring forth a revolution in wind energy generation, providing renewable energy in large quantities at a lower cost than fossil energy. This work investigates three important theoretical aspects: the evaluation of the performance achieved by the employed control law, the optimization of the generator operating cycle, and the possibility to generate continuously a constant and maximal power output. These issues are tackled through the combined use of modeling, control, and optimization methods that result to be key technologies for a significant breakthrough in renewable energy generation. Copyright © 2011 John Wiley & Sons, Ltd.