Airborne Wind Energy

Tethered wings that fly fast in a crosswind direction have the ability to highly concentrate the abundant wind power resource in medium and high altitudes, and promise to make this resource available to human needs with low material investment. This chapter introduces the main ideas behind airborne wind energy, attempts a classification of the basic concepts that are currently pursued, and discusses its physical foundations and fundamental limitations.

A tethered wing can be used in two different ways, to lift payload or to provide traction power. The latter is the basis of several innovative technical applications, such as kite-assisted ship propulsion and pumping-kite wind energy conversion. This chapter presents a theoretical analysis of traction power generation by a tethered wing, with the objective to establish the fundamental relationships between system and operational parameters on the one hand, and achievable mechanical power output on the other hand. In a first step, it is assumed that the instantaneous flight state of the wing can be approximated by the steady equilibrium of aerodynamic and tether forces. The analysis considers controlled flight along an arbitrary predefined trajectory, distinguishing the cases of varying tether length with fixed point anchoring and constant tether length with anchoring at a point moving in the ground plane. Theoretical results are compared with literature. In a second step, the analysis includes the effect of weight and centrifugal acceleration of the wing.

Simple analytical models for a pumping cycle kite power system are presented. The theory of crosswind kite power is extended to include both the traction and retraction phase of a pumping cycle kite power system. Dimensionless force factors for the reel out and reel in phase are introduced which describe the efficiency of the system. The optimal reel out and reel in speed of the winch is derived where the cycle power becomes maximal. These optimal speeds are solely determined by the ratio of the force factors. Scenarios for wind speeds higher than the nominal wind speed are considered and power curves for the pumping cycle kite power system derived. The average annual power for a given wind distribution function allows to estimate the annual energy production of the pumping cycle kite power system. The role of the elevation angle of the tether is highlighted and a simple model to demonstrate the influence of the kite mass on the power output is discussed.

This review paper is devoted to analytical modeling of the so-called kite wind generator (KWG) whose power conversion operation uses a tethered kite to mechanically drive a groundbased electric generator. An important aspect of the KWG operating principle is the controlled crosswind motion of the kite, which is used to increase the kite traction force. A simple mathematical model for steady crosswind motion of the tethered kite is formulated on the basis of the refined crosswind motion law. An analytical approximation for the mean mechanical power output is presented in terms of the performance coefficient of the pumping kite wind generator. Optimal control of the tether length rate is considered for the open-loop and closed-loop figure-of-eight trajectories. The influence of the kite control and of the tether sag on the kite traction power output is discussed.

Airborne wind energy systems (AWES) are devices that effectively extract energy from the air flow, more specifically kinetic energy, and convert it to electricity. Wind is the manifestation of the kinetic energy present in the atmosphere. Understanding wind, its properties and power, as well as other atmospheric properties that can affect AWES, is the goal of this chapter.

Kites were essential platforms for professional exploration of the atmosphere for more than two centuries, from 1749 until 1954. This chapter details the chronology of kite-based atmospheric research and presents a brief examination of the well-documented scientific kiting based at the Royal Prussian Aeronautical Observatory. Parallels are drawn between scientific kiting from then and contemporary power-generation kiting. Basic kite types of the time are presented and the design evolution from those towards advanced payload carriers is discussed. These include the Lindenberg S-and R-Kites, the latter of which featuring an effective passive depower mechanism. The practices of launch and retrieval of kites and the components developed for this purpose are outlined, in particular those in use at the Meteorological Observatory Lindenberg. Their methods and techniques represented the state of the art after WWI and lay the groundwork for modern efforts at atmospheric energy extraction using kites.

The development and large-scale application of new technology will be a central element to meet the current challenges of the global energy system, such as accelerating climate change, concerns about future energy security, limited global energy access or deteriorating balances of payments. At the same time, the restructuring of the energy system has to happen at reasonable cost. Airborne wind energy (AWE) can play an important role in contributing to meet this challenge. Yet, despite the large potential of AWE, further financing will be required to establish commercial viability of the technology and enable its large-scale deployment. Drawing on the most recent literature as well as on a range of qualitative interviews among both CEOs of AWE companies and risk capital investors the article characterizes AWE from a financing perspective and sheds light on the potential barriers for attaining substantial risk capital. An understanding and the active management of the identified investment barriers offer AWE companies important toeholds to develop their financing strategies. Potential implications and current strategies in the industry are discussed in the article.

We present a simple model for the dynamics and aerodynamics of a tethered kite system and validate it by experimental flight data. After introduction of system setup and model assumptions, the equations of motion for the kinematics are derived and discussed. Then the turn rate law for the kite response to a steering deflection is introduced. The tutorial introduction of the model is finalized by an extension for varying tether lengths, which is the regular operation mode of certain classes of airborne wind energy setups. The second part starts with a summary of the sensor setup. Then, the turn rate law, as distinguishing feature of the model, is illustrated and validated by experimental data. Subsequently, we discuss the kinematics of the kite by comparing model based prediction to experiment. Conclusively, we briefly summarize controller design considerations and discuss the flight controller performance, which further proves the validity of the model as it is based on a feed forward term which in turn, is build on the presented model.

An overview of recent results on the control engineering aspects of airborne wind energy is given, for the particular problem of automatic crosswind flight of tethered flexible wings. Mathematical modeling, sensor fusion and control design are presented in a unified framework, hence providing a complete description of an approach to achieve autonomous figure-eight flight patterns. Differently from other existing techniques, the described approach involves few parameters, which can be tuned intuitively, it does not require a measurement of the wind speed, either at ground level or at the wing’s location, but just of the wind direction, and it does not rely on pre-computed flying paths or on complex on-line optimization. The presentation of the methodology is supported by the experimental results obtained with a small-scale prototype.

This paper presents a modeling approach for AWE systems that allows for developing models of low symbolic complexity and low nonlinearity. The approach is based on multi-body modeling, using natural coordinates and algebraic constraints as a representation of the system evolution. This paper shows how to build such models for AWE systems in the Lagrangian framework and how to efficiently incorporate a non-singular representation of the pose (i.e. 3D orientation) of the wing. The proposed modeling technique is illustrated on a single-wing AWE system for power generation and rotating start-up, and for a dual-wing AWE system.

In order to study design tradeoffs in the development of an AWE system, it is useful to develop a code to optimize a trajectory for arbitrary objective function and constraints. We present a procedure for using direct collocation to optimize such a trajectory where a model is specified as a set of differential–algebraic equations. The six degree of freedom single-kite, pumping-mode AWE model developed in

Chap. 10

is summarized, and two typical periodic optimal control problems are formulated and solved: maximum power and number of cycles per retraction. Finally, a procedure for optimally transitioning between two fixed trajectories is presented.

In order to allow for a reliable and lasting operation of Airborne Wind Energy systems, several problems need to be addressed. One of the most important challenges regards the control of the tethered airfoil during power generation. Tethered flight of rigid airfoils is a fast, strongly nonlinear, unstable and constrained process, and one promising way to address the control challenge is the use of Nonlinear Model Predictive Control (NMPC) together with online parameter and state estimation based on Moving Horizon Estimation (MHE). In this paper, these techniques are introduced and their performance demonstrated in simulations of a 30 m wingspan tethered airplane with power generation in pumping mode.

A novel Airborne Wind Energy Conversion System with a ground-based electric generator is proposed. The construction uses two interacting tethered wings with a single motion transfer cable, separate from the tethers. The speed of tangential motion of the cable exceeds the speed of the wings, flying cross wind, and is further increased by a block and tackle mechanism, thus ensuring high rotational speed and low torque on the receiving shaft of the ground-based drivetrain. The drivetrain does not require a gearbox. This device is estimated to be more than 10 times less expensive than a conventional wind turbine with the same average power output.

Airborne Wind Energy is gaining increasing attention. Compared to conventional wind turbines, this class of innovative technologies can potentially generate more energy at a lower price by accessing wind at higher altitudes which is stronger and steadier. In this chapter, first a theoretical system model of a kite power system in pumping mode of operation is presented. Then it is validated with electrical and mechanical measurement results. The model is used to predict the electrical power output and the size of the major components. The terms pumping efficiency, cycle efficiency and total efficiency are introduced. It is shown that the kite power demonstrator of Delft University of Technology currently achieves a maximum total efficiency of 20 %. The analysis indicates that it will be possible to design small to medium sized kite power systems with a total efficiency of 50 to 60 %. The terms nominal power of a ground station and system power of a kite power system are introduced, noting their particular difference: the nominal power is the installed electrical generator power whereas the system power is defined as the average net electrical power output at nominal wind velocity.

This chapter gives an introduction to the economic assessment of Pumping Kite Generator Systems on the basis of established methods for conventional Wind Energy Conversion Systems. Site and system characteristics as well as market factors that have an impact on the economic viability of a wind energy project are described and levelized cost of energy is introduced as an indicator. A specific Pumping Kite Generator concept is used as an example throughout the chapter to illustrate all steps of the analysis. This example is finally used to show how system and site parameters can affect the economics of a project. It is shown that the considered Pumping Kite Generator can be competitive under European market conditions.

The chapter describes a simulation framework for flexible membrane wings based on multibody system dynamics. It is intended for applications employing kites, parachutes or parasails with an inflated tubular support structure. The tube structure is discretized by an assembly of rigid bodies connected by universal joints and torsion springs. The canopy of the wing is partitioned into spanwise sections, each represented by a central chordline which is discretized by hinged rigid line elements. The canopy is modeled by a crosswise arrangement of spring-damper elements connecting these joints. The distributed loading of the wing structure is defined in terms of discrete aerodynamic forces. Acting on the joints, these forces are formulated per wing section as functions of local angle of attack, airfoil thickness and camber. The presented load model is the result of a comprehensive computational fluid dynamic analysis, covering the complete operational spectrum of the wing. The approach captures the two-way coupling of structural dynamics and aerodynamics. It is implemented as a toolbox within the commercial software package MSC ADAMS. For validation, the model is compared to existing wind tunnel data of a similar sail wing.

This chapter presents a computational method to describe the flight dynamics and deformation of inflatable flexible wings for traction power generation. A nonlinear Finite Element approach is used to discretize the pressurized tubular support structure and canopy of the wing. The quasi-steady aerodynamic loading of the wing sections is determined by empirical correlations accounting for the effect of local angle of attack and shape deformation. The forces in the bridle lines resulting from the aerodynamic loading are imposed as external forces on a dynamic system model to describe the flight dynamics of the kite. Considering the complexity of the coupled aeroelastic flight dynamics problem and the Matlab® implementation, simulation times are generally low. Spanwise bending and torsion of the wing are important deformation modes as clearly indicated by the simulation results. Asymmetric actuation of the steering lines induces the torsional deformation mode that is essential for the mechanism of steering. It can be concluded that the proposed method is a promising tool for detailed engineering analysis. The aerodynamic wing loading model is currently the limiting factor and should be replaced to achieve future accuracy improvements.

A framework for simulating tethered wings for kite power is presented. The simulation tool contains a detailed aerodynamic model and a realistic tether model. With the aerodynamic tool, two different wings are analyzed regarding their efficiency. The aerodynamic efficiency of kites is determined with a parameter study showing the trends of the most important geometrical parameters. Those wings are manually flown in the simulator and the flight behavior is discussed. Finally, power cycles of a pumping system are simulated and controlled automatically and results are compared.

The use of kites for auxiliary propulsion reduces oil consumption for vessels. But the complexity of the kite numerical simulation induces the development of computationally efficient models based on lifting line theory to evaluate the aerodynamic characteristics of the kite. The presented 3D lifting line model takes into account the three-dimensional shape of the kite and the viscosity of the fluid. The proposed model was applied to a F-one Revolt Leading Edge Inflatable kite to predict its lift-to-drag ratio. Finally, this method is in very good agreement with CFD simulations in the case of a paragliding wing, but needs a much smaller computational effort.

SkySails develops and markets large automated towing kite systems for the propulsion of ships and for energy generation. Since 2008 pilot customer vessels have been operating propulsion kites in order to reduce fuel costs and emissions. In this contribution the SkySails towing kite technology is introduced and an overview over its core components kite, control pod, towing rope, and launch and retrieval system is provided. Subsequently the principles of force generation and propulsion are summarized. In the following part the system’s application to airborne wind energy generation is presented, where the kite forces are used to pull the towing rope off a drum, powering a generator in the process. When the maximum tether length is reached, the kite is reeled back to the starting point using the generator as a motor. A functional model was constructed and successfully tested to prove the positive energy balance of this so-called pumping mode energy generation experimentally. An evaluation of the technology’s market potential, particularly for offshore wind farms, concludes the contribution.

The advances in the design and testing of a 60 kW Yo-Yo AWE generator are presented. The generator uses power kites, linked to the ground by two tethers, reeled on two drums that are connected to two electric drives. The flight of the wings is tracked using on-board wireless instrumentation and it is suitably driven by a ground control unit, through differentially pulling of the tethers. Electricity is generated at ground level obtained by continuously performing a two-phase cycle: a traction one, where the kite unreel the tethers, inducing energy generation through rotation of electric drives. When the maximum tether length is reached, the drives act as motors, to reel back the tethers to start another traction phase. The main components (electro-mechanical structure, sensors and data communication, energy management system, hardware and software for real-time control) are described. Results are presented from some of tests until now performed and the experimental energy and power values are compared with the theoretical optimal value based on the simplified analysis in Loyd’s seminal paper as well with computer simulations based on the model and control strategy developed by Kitenergy research group.

This chapter summarizes recent work at Worcester Polytechnic Institute to model, design, fabricate, and test a low-cost kite-powered water pump. The WPI system is to be used in developing nations to alleviate water shortages. It uses a kite and tether that transmits the generated aerodynamic forces to a rocking arm, and through a mechanical linkage to a displacement (or lift) pump on the ground. Dynamic equations were developed for the kite, a flexible tether with applied lift, drag, and weight forces, the rocking arm, mechanical linkage and pump. A steady-state analysis of the kite aerodynamics was incorporated into the dynamic equations of the kite-power system. The governing equations were solved numerically to assess how performance parameters of the system such as water pumping rate, tether profile and tension, and kite motion varied with tether length and diameter and wind speed. The results showed that for a kite area of 8 m

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and wind speeds of 6 m per second, the operation of a kite powered water pump is feasible with a maximum water pumping rate of 8,000 l/day. The kite-powered pump can provide water for about 400 people in a developing nation. Ongoing efforts to build and test a working kite-powered water pump are also reported.

The pumping kite concept provides a simple yet effective solution for wind energy conversion at potentially low cost. This chapter describes a technology demonstrator which uses an inflatable membrane wing with 20 kW nominal traction power on a single-line tether. The focus is on the innovative and scientifically challenging development aspects, especially also the supervisory control and data acquisition system designed for automatic operation. The airborne hardware includes a Kite Control Unit, which essentially is a remote-controlled cable manipulator, and the inflatable wing with its bridle system allowing for maximum de-powering during the retraction phase. On the ground, the drum/generator module is responsible for traction power conversion while constantly monitoring and adapting the force in the tether. The control software includes two alternating autopilots, one for the lying figure eight maneuvers during tether reel-out and one for the reel-in phase. As a result of monthly test-operation since January 2010, large quantities of measurement data have been harvested. The data acquisition and post-processing is presented and discussed for representative conditions. The power curve of the system and other characteristic operational parameters are determined by a statistical analysis of available data and compared to the results of a theoretical performance analysis.

EnerKite GmbH designed, built and demonstrated a three line AWE system. This article presents history of the enterprise and the decisions involved. The built ground station is described in detail, and flight data obtained during the course of a year in development is presented.

The AWE concept presented in this paper is put into practice by the NTS-GmbH in Berlin. An AWE plant based on this principle—an X-Wind plant—utilizes automatically steered kites at altitudes between 100 and 500 m to pull rolling carts continually along an oval railway track. Each cart is equipped with a generator to convert its kinetic energy into electricity. The mechanism applied is comparable to regenerative braking systems in modern trains and trams. Hence, the NTS concept merges well known technologies to a unique and flexible AWE plant: kites and rail technology. In this paper, a short introduction into the concept is given and the current status of the NTS-project is presented.

Ampyx Power develops a pumping kite system with a rigid aircraft that is attached to the tether by a single attachment point. This unbridled configuration allows three degrees of freedom (DOF) for the aircraft attitude and three for the position. The total system can be described by these 6 DOF. Operating the pumping kite system requires a novel view on conventional flight control. A tether based reference frame is introduced that in effect decouples the longitudinal and lateral motion which can then be designed independently allowing the highly dynamic motion of the glider to be controlled through simple control schemes. Furthermore the longitudinal motion is constrained through the tether of which the tangential velocity is controlled by the generator providing an additional control input besides the elevator to control longitudinal motion. Flight tests demonstrate that using the tether based flight control system reasonably simple and commonly used control methods provide satisfactory flight performance. This paper gives an overview of the system components and control strategy and gives a brief overview of preliminary flight tests and performance.

This chapter gives a detailed description of a test setup developed at KU Leuven for the launch and recovery of unpropelled tethered airplanes. The airplanes are launched by bringing them up to flying speed while attached by a tether to the end of a rotating arm. In the development of the setup, particular care was taken to allow experimental validation of advanced estimation and control techniques such as moving horizon estimation and model predictive control. A detailed overview of the hardware, sensors and software used on this setup is given in this chapter. The applied estimation and control techniques are outlined in this chapter as well, and an analysis of the closed loop performance is given.

Makani Power has developed an autonomous airborne wind turbine prototype incorporating a rigid wing with onboard generators. An overview of the design is given, and a simple characteristic power curve of the system is derived analytically. The performance of the system in flight tests as well as rigid body, finiteelement tether simulations is compared to that derived by the analytic formulation.

This contribution describes a technology for harnessing energy from high altitude wind through a pumping cycle, in a two-dimensional vertical trajectory, executed by a hybrid lighter-than-air tethered rotating cylinder, which generates dynamic lift through the Magnus effect. The historical development of the concept leading to an operational cycle is described. Specifications of the current system are given and are used to extrapolate a multi-stack configuration of four cylinders yielding an average cycle power of 80 kw in a pre-commercial unit.

Several wind energy concepts utilize airborne systems that contain lighterthan-air gas, which supplements aerodynamic lift and expands these systems’ available operating regimes. While lighter-than-air systems can incorporate the traction and crosswind flight motions of their heavier-than-air counterparts, several lighterthan-air concepts have also been designed to deliver large amounts of power under completely stationary operation and remain aloft during periods of intermittent wind. This chapter provides an overview of the history of LTA airborne wind energy concepts, including the design drivers and principal design constraints. The focus then turns to the structural and aerodynamic design principles behind lighterthan air systems, along with fundamental flight dynamic principles that must be addressed. A prototype design developed by Altaeros Energies is examined as an example of the application of these principles. The chapter closes with suggestions for future research to enable commercially-viable LTA systems.

This chapter provides useful reference information for applications using a ram-air wing for wind energy production, from the perspective of a Ram-air parachute background. A limited set of design considerations, as relevant to AWE, are discussed, including wing design guidelines, wing control and handling, scaling, and life of the system. The material herein serves as a reference to an AWE developer or user to educate and inform of additional possibilities using Ram-air wings or to prevent costly and time consuming experiments.

In this paper the authors present basic considerations on conceptual kite design in terms of overall system performance of an airborne wind energy system. This kite design process has been developed at SkySails GmbH for the design of large scale traction kites for sea-going vessels. All aspects are first presented in a brief discussion and then applied to the SkySails kite system. Further examples are provided where applicable. This chapter starts by introducing theoretical approaches for determining maximum system performance and certain other aspects of kite aerodynamics with respect to the SkySails kite system. An overview of the limitations considered during the kite design process is also presented. In the following sections, the influence of kite steering, launch and landing is discussed. Further, structural weight aspects are addressed. The last sections deal with the implications of ground handling on kites.

Airborne Wind Energy tethers are a critical component in many AWE systems. There are many diverse systems that are currently under development, this chapter focusses on tethers for the so called pumping Yo–Yo system. In these systems the tether is the critical component for transfer of kinetic energy from kite to ground station. Given the desired hardware and performance expectations, this chapter provides a first estimation of the tether dimensions for a tether made of HMPE fibers. Especially creep and bending fatigue considerations are described for long term performance checks. Other conditions that may influence the longevity of the tether are briefly mentioned, but since firm testing data is lacking, it is recommended to perform these checks on case by case basis.

The design of a pumping mode airborne wind energy (AWE) farm is presented in this chapter. This design centres on the use synchronous generators on a local frequency wild bus, with full scale power converter located at the point of grid connection. The design is well suited to a remotely located or offshore farm. The focus of the chapter is on the modelling of a ground based electromechanical system which provides a continuous electrical power output from multiple AWE devices whose individual operation delivers periodic mechanical power. The generators are not reversed during operation as the system presented separates the power and recovery tasks of the winch. Furthermore the use of permanent magnet synchronous generators (PMSG) enables the direct interconnection of multiple devices to a local bus. The AWE farm design philosophy is detailed and encouraging simulation results are discussed.

In order to exploit high altitude wind energy, automatic computer control of tethered kites is a key to success. In this contribution, we report on the automation experiences and development issues on the software architecture gained by the development and operation of our ship propulsion kites during the last eight years. The first part puts focus on the requirements, the architecture and the signal flows of the distributed computer control system. The second part presents control system components in detail, introduces the respective challenges and explains how these are tackled by means of software engineering techniques. We conclude with a brief description on our hardware-in-the-loop simulation and test setup.