ArticlePDF Available

Abstract and Figures

Kites can be used to harvest wind energy with substantially lower material and environmental footprints and a higher capacity factor than conventional wind turbines. In this paper, we present measurement data from seven individual tow tests with the kite system developed by Kyushu University. This system was designed for 7 kW traction power and comprises an inflatable wing of 6 m2 surface area with a suspended kite control unit that is towed on a relatively short tether of 0.4 m by a truck driving at constant speed along a straight runway. To produce a controlled relative flow environment, the experiment was conducted only when the background wind speed was negligible. We recorded the time-series of 11 different sensor values acquired on the kite, the control unit and the truck. The measured data can be used to assess the effects of the towing speed, the flight mode and the lengths of the control lines on the tether force.
Flight paths of the kite in tests 1 and 2 [19]. The results of tow tests 1-7 are illustrated in Figures 4-10. For each figure, sub-figure (a) represents the longitude versus latitude of the kite and the truck, in red and black, respectively. Towards the ends of the runway, the truck needed to perform U-turns. In order to do this, any flight maneuvers were stopped and the kite was positioned in a steady flight state towards the side of the turning truck, and in this way pulled around during the U-turns. As a consequence, the turning kite was tracing a larger radius than the turning truck. It is straightforward to distinguish figure-of-eight flight maneuvers from the fluctuation along the runway, as shown in tests 2 and 4. Sub-figure (b) represents the height of the kite, completing the information about the flight path for each test. It is notable that the height decreased during the turn as a result of the decrease in towing speed, which is illustrated in sub-figure (c). Height and towing speed are strongly coupled and the corresponding diagrams show, thus, similar time-series. Again, it is straighforward to distinguish steady flight (tests 1, 3, 5, 6 and 7) and flight in figure-of-eight maneuvers (tests 2 and 4) by the fluctuations in height. Sub-figures (d) and (e) represent the orientation/attitude of the kite. While sub-figure (d) shows the time-series of the roll and pitch angles, sub-figure (e) separately shows the yaw behavior, to illustrate the correlation with the height and towing speed shown in sub-figures (b) and (c), respectively. It is important to note that the body-fixed reference frame of the kite, in which we measured roll, pitch and yaw, was similar to the body-fixed reference frame of a conventional aircraft. The x-axis pointed
… 
Content may be subject to copyright.
A preview of the PDF is not available
... Due to these facts, some alternatives have been addressed so far. Among them, insect-inspired kites (Khaheshi et al., 2021), various subsystems for drag power kites (Bauer et al., 2019), power kites with inflatable wings (Rushdi et al., 2020), analysis of LIDAR (light direction and ranging) and mesoscale models (Sommerfeld, Crawford, Steinfeld, et al., 2019), filtration method of analyzing tethered kite wings (Schmidt et al., 2020), vortex-induced vibration (VIV)-based piezoelectric energy harvester (Shi et al., 2021), and vortex wind generation showed distinct outcomes (Ren et al., 2021). However, from an environmental perspective, some limitations have been observed in the current alternatives. ...
... In the case of turbulent wind, the flight path takes a figure-of-eight pattern, widening the occupied space; decreasing the kite population further (shown in Figure 8 (Vermillion et al., 2021). The volume occupied by typical AWES in flight (b)(c) (Rushdi et al., 2020). ...
... Eventually, the above-discussed mathematical models can be implemented in transducer operated control logics, mentioned in the study of Bauer et al.(Bauer et al., 2019) and Rushdi et al.(Rushdi et al., 2020). Moreover, the models can further be used in the recently developed simulators, like the one proposed by Kakavand et al.(Kakavand et al., 2021), which can measure power transmission with 98% accuracy. ...
Article
Full-text available
The world has witnessed an unprecedented growth of WF installation, driven by national and international energy policies. Considering the negative impacts of fossil fuel and associated climate changes,wind is an important form of renewable energy. Nevertheless, the conventional WFs also have some environmental effects. Besides, the conventional WTs lack in performance due to technical limitations. Upon comprehensively reviewing the impacts and the technicalities, this literature focused on the recent developments in the research community to predict the potential research pathways for technical optimization and modification of the relevant policies.
... It is not surprising then that most prototypes built for field tests are movable, installed either on a truck or on a trailer. Tow-test experiments, where a ground station is moved to artificially generate a controllable wind flow, have become a popular immediate method for testing AWE systems [86,156]. Testing of a stationary system in a relatively remote site, away from offices and labs, immediately increases cost, time and complexity of test procedures and thus requires some time and accumulated experience before proving effective. ...
... A 6 m 2 soft kite system with a suspended kite control unit was used for towing tests by Kyushu University, to explore the potential of machine learning [164]. The data set is available in open access form [156,165]. ...
Article
Full-text available
Airborne wind energy systems convert wind energy into electricity using tethered flying devices, typically flexible kites or aircraft. Replacing the tower and foundation of conventional wind turbines can substantially reduce the material use and, consequently, the cost of energy, while providing access to wind at higher altitudes. Because the flight operation of tethered devices can be adjusted to a varying wind resource , the energy availability increases in comparison to conventional wind turbines. Ultimately, this represents a rich topic for the study of real-time optimal control strategies that must function robustly in a spatiotemporally varying environment. With all of the opportunities that airborne wind energy systems bring, however, there are also a host of challenges, particularly those relating to robustness in extreme operating conditions and launching/landing the system (especially in the absence of wind). Thus, airborne wind energy systems can be viewed as a control system designer's paradise or nightmare, depending on one's perspective. This survey article explores insights from the development and experimental deployment of control systems for airborne wind energy platforms over approximately the past two decades, highlighting both the optimal control approaches that have been used to extract the maximal amount of power from tethered systems and the robust modal control approaches that have been used to achieve reliable launch, landing, and extreme wind operation. This survey will detail several of the many prototypes that have been deployed over the last decade and will discuss future directions of airborne wind energy technology as well as its nascent adoption in other domains, such as ocean energy.
Article
Full-text available
Kites can be used to harvest wind energy at higher altitudes while using only a fraction of the material required for conventional wind turbines. In this work, we present the kite system of Kyushu University and demonstrate how experimental data can be used to train machine learning regression models. The system is designed for 7 kW traction power and comprises an inflatable wing with suspended kite control unit that is either tethered to a fixed ground anchor or to a towing vehicle to produce a controlled relative flow environment. A measurement unit was attached to the kite for data acquisition. To predict the generated tether force, we collected input-output samples from a set of well-designed experimental runs to act as our labeled training data in a supervised machine learning setting. We then identified a set of key input parameters which were found to be consistent with our sensitivity analysis using Pearson input-output correlation metrics. Finally, we designed and tested the accuracy of a neural network, among other multivariate regression models. The quality metrics of our models show great promise in accurately predicting the tether force for new input/feature combinations and potentially guide new designs for optimal power generation.
Article
Full-text available
We compare the available wind resources for conventional wind turbines and for airborne wind energy systems. Accessing higher altitudes and continuously adjusting the harvesting operation to the wind resource substantially increases the potential energy yield. The study is based on the ERA5 reanalysis data which covers a period of 7 years with hourly estimates at a surface resolution of 31 31 km and a vertical resolution of 137 barometric altitude levels. We present detailed wind statistics for a location in the English Channel and then expand the analysis to a surface grid of Western and Central Europe with a resolution of 110 x 110 km. Over the land mass and coastal areas of Europe we find that compared to a fixed harvesting height at the approximate hub height of wind turbines, the wind power density which is available for 95% of the time increases by a factor of two.
Article
Full-text available
We have developed a tow test setup for the reproducible measurement of the dynamic properties of different types of tethered membrane wings. The test procedure is based on repeatable automated maneuvers with the entire kite system under realistic conditions. By measuring line forces and line angles, we determine the aerodynamic coefficients and lift-to-drag ratio as functions of the length ratio between power and steering lines. This nondimensional parameter characterizes the angle of attack of the wing and is varied automatically by the control unit on the towed test bench. During each towing run, several test cycles are executed such that mean values can be determined and errors can be minimized. We can conclude from this study that an objective measurement of specific dynamic properties of highly flexible membrane wings is feasible. The presented tow test method is suitable for quantitatively assessing and comparing different wing designs. The method represents an essential milestone for the development and characterization of tethered membrane wings as well as for the validation and improvement of simulation models. On the basis of this work, more complex maneuvers and a full degree of automation can be implemented in subsequent work. It can also be used for aerodynamic parameter identification.
Article
Full-text available
Wind tunnel testing of large deformable soft kites for wind energy conversion is expensive and in many cases practically not feasible. Computational simulation of the coupled fluid–structure interaction problem is scientifically challenging and of limited practical use for aerodynamic characterization. In this paper we present a novel experimental method for aerodynamic characterization of flexible membrane kites by in situ measurement of the relative flow, while performing complex flight maneuvers. We find that the measured aero- dynamic coefficients agree well with the values that are currently used for flight simulation of soft kites. For flight operation in crosswind maneuvers during which the traction force is kept constant, the angle of attack is inversely related to the relative flow velocity. For entire pumping cycles, the measurements show considerable variations in the aerodynamic coefficients, while the angle of attack of the kite varies only in a narrow range. This finding questions the commonly used representation of aerodynamic coefficients as sole functions of the angle of attack and stresses the importance of aeroelastic deformation for this type of wing. Considering the effect of the power setting (identical to the trim) solely as a rigid-body pitch rotation does not adequately describe the aero-structural behavior of the kite. We show that the aerodynamic coefficients vary as functions of the power setting (trim) of the kite, the steering commands and the flight direction.
Article
Full-text available
The traction force of a kite can be used to drive a cyclic motion for extracting wind energy from the atmosphere. This paper presents a novel quasi-steady modelling framework for predicting the power generated over a full pumping cycle. The cycle is divided into traction, retraction and transition phases, each described by an individual set of analytic equations. The effect of gravity on the airborne system components is included in the framework. A trade-off is made between modelling accuracy and computation speed such that the model is specifically useful for system optimisation and scaling in economic feasibility studies. Simulation results are compared to experimental measurements of a 20 kW kite power system operated up to a tether length of 720 m. Simulation and experiment agree reasonably well, both for moderate and for strong wind conditions, indicating that the effect of gravity has to be taken into account for a predictive performance simulation.
Book
Full-text available
This book provides in-depth coverage of the latest research and development activities concerning innovative wind energy technologies intended to replace fossil fuels on an economical basis. A characteristic feature of the various conversion concepts discussed is the use of tethered flying devices to substantially reduce the material consumption per installed unit and to access wind energy at higher altitudes, where the wind is more consistent. The introductory chapter describes the emergence and economic dimension of airborne wind energy. Focusing on “Fundamentals, Modeling & Simulation”, Part I includes six contributions that describe quasi-steady as well as dynamic models and simulations of airborne wind energy systems or individual components. Shifting the spotlight to “Control, Optimization & Flight State Measurement”, Part II combines one chapter on measurement techniques with five chapters on control of kite and ground stations, and two chapters on optimization. Part III on “Concept Design & Analysis” includes three chapters that present and analyze novel harvesting concepts as well as two chapters on system component design. Part IV, which centers on “Implemented Concepts”, presents five chapters on established system concepts and one chapter about a subsystem for automatic launching and landing of kites. In closing, Part V focuses with four chapters on “Technology Deployment” related to market and financing strategies, as well as on regulation and the environment. The book builds on the success of the first volume “Airborne Wind Energy” (Springer, 2013), and offers a self-contained reference guide for researchers, scientists, professionals and students. The respective chapters were contributed by a broad variety of authors: academics, practicing engineers and inventors, all of whom are experts in their respective fields.
Chapter
Full-text available
The efficient and economic operation of tethered kites for accessing highaltitude winds as a renewable source of energy requires fully automated setups. During the last decade the SkySails kite systems have been developed for applications in marine propulsion and energy generation. In this chapter we give a descriptive overview of the flight control of the tethered kite and of the control of the tether reeling speed leading to pumping cycles for energy generation. This chapter focuses on the discussion and justification of the overall design choices, functional dependencies and the presentation of the complete system in a self-contained way. For details of the mathematical modeling and control theoretical aspects references for further reading are provided. After an introduction, the dynamical model for the tethered dynamics is briefly summarized. Subsequently, the estimation and sensor system is presented. Then, the control approach is discussed and the parts of the control system are reviewed in detail. Finally the implemented system is briefly compared to other control approaches.
Chapter
This chapter presents an overview of the evolution of present wind technology, present trends and future technologies. It discusses airborne technology, energy storage and innovative concepts not based on rotors. Airborne turbine concepts had appeared in patent documents for many years, but now such concepts are generating increased interest, with much research being conducted and some prototypes being tested. Such systems continue in the spirit of moving from land into the other elements, water and air. The main types of energy storage presently being researched and developed comprise the following: battery storage, gas pressure storage, compressed air storage, flywheel energy storage and thermal energy storage. Electrostatic generators, also referred to as electro‐hydrodynamic on account of interaction with wind flow, have been considered for wind energy conversion since the 1970s. A recent example is the electrostatic wind energy convertor (EWICON) concept under development in the High Voltage Laboratory at TU Delft.