A Methodology for the Design of Kite PowerControl Systems

Airborne Wind Energy (AWE) systems are a novel wayto harvest wind energy at higher altitudes without theneed for a heavy tower and foundation. AWE systemsare expected to work with a high capacity factor, be­cause the wind­speed at increasing altitudes is higherand steadier. In many applications they also promisea substantial decrease of costs per kWh.AWE systems could become a competitive alternativeto coal power plants even at locations that are notsuitable for conventional wind turbines.

Kite Power instead of Coal

Kite power systems combine one or more computer­controlled inflatable membrane wings with a mo­tor­generator unit on ground using a strong and lightweight cable.

Each pumping cycle consists of an energy ge­nerating reel­out phase, in which the kites areoperated in figure­eight flight manoeuvres tomaximize the pulling force, and a reel­in phasein which the kites are depowered and pulledback towards the ground station using a smallfraction of the generated energy.

To reach a high efficiency of the wind harvestingmechanism, a high reel­out and a low reel­in forceare required. A high reel­out force is achieved by fly­ing crosswind, a low reel­in force is realized by de­powering the kite. The current demonstrator achie­ves a pumping efficiency of about 80 %.

4­point Kite ModelFor the automated power production a simple flightpath planner is used: The kite is always steeredtowards one of three points: During reel­in andparking it is steered towards zenith (directly abovethe ground station). During reel­out it is steered toa point on either the right or left side of the windwindow.The orientation of the kite (the heading angle) iscontrolled. Great circle navigation is used todetermine the heading needed to steer towards the

target point. Thedifference betweenthe requiredheading and theactual heading isthe error signalthat is going into aPI controller that iscontrolling thesteering signal is ofthe kite controlunit. In addition

the KCU has an input id for the depower signal. Theset value id is low during reel­out and high duringreel­in (predefined, fixed values).The steering signal differentially changes the lengthof the left and right steering lines, the depowersignal changes the length of the steering linesrelative to the length of the depower lines. Theactuators are modelled such that they have amaximum speed (derivative of the output controlsignals us and ud). They use a P­controller tocontrol the output signal. In addition a delay of 150ms was implemented in the model. The delay ismainly caused by the motor controllers.

Automated Control

At the beginning of this project no method for theautomated control of kite power system was publishedthat was proven to work in practice.The major goal of this research is to develop amethodology for the systematic implementation ofrobust and optimal kite power control systems. Withthis methodology in place a major obstacle for thesuccess of AWE systems will have been removed.

Solving the Control Challenge

reel­out phase

reel­in phase

[1] Fechner, U., & Schmehl, R. (2013). Model­Based Efficiency Analysis of Wind Power Conversion by a Pumping KitePower System. In U. Ahrens, M. Diehl, & R. Schmehl (Eds.), Airborne Wind Energy (chap. 14, pp. 245–266). SpringerBerlin Heidelberg.

[2] Fechner, U., van der Vlugt, R., Schreuder, E., & Schmehl, R. (2014). Dynamic Model of a Pumping Kite Power System.(Submitted to) Renewable Energy.

[3] Fechner, U., & Schmehl, R. (2012). Design of a Distributed Kite Power Control System. In Proceedings of the IEEEMulti ­ Conference on Systems and Control. Dubrovnik, Croatia.

[4] Fechner, U., & Schmehl, R. (2014). Feed­Forward Control of Kite Power Systems. (Submitted to) The Science ofMaking Torque from Wind – 18­20 June, Copenhagen Denmark.

PhD Candidate: Uwe FechnerDepartment: AWEPSection: Wind energySupervisor: R. SchmehlPromotor: W. Ockels † , G. van BusselStart date: 01­07­2010Contact: u.fechner@tudelft.nlWeb: www.kitepower.euType: Engineering

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Research Methodology

Principle of Operation

1. A theory about the efficiency of kite powersystems in pumping mode of operation wasderived and major performance factorsidentified [1].

2. A dynamic model of a pumping kite powersystem was developed and verified with flighttest data [2].

3. A distributed kite power control system wasdeveloped, implemented and tested against themodel and an implemented 20 kW kite powersystem [3].

Progress and Results

Most Innovate Results

1. Even with soft kites a total efficiency of more than50 % can be achieved [1].

2. Simple PID controllers combined with model basedfeed­forward control are sufficient to achieve goodresults for normal operation [4].

3. Since 2012 the power output could be increasedby 40% by optimizing the control strategy. (Not yetpublished.)

4. In most cases a kite specific state estimator isneeded to achieve robust control. (Not yetpublished.)

The four point model takes the rotationalinertia of the wing into account anddiscretises the aerodynamics by a centresection and two tip sections.

The local angles of attack at the tipsections are assumed to be functions ofthe corresponding steering lineactuations.

Optimize Operational

Implement DynamicModel

Implement ControlSystem

ModelVerification

EstimatorVerification

VerifyRobustness

Control SystemVerification

Identify OptimizationParameters

Implement SystemState Estimator

Parameters