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S. Rapp

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9 records found

Electricity in the Air: Insights From Two Decades of Advanced Control Research and Experimental Flight Testing of Airborne Wind Energy Systems

Review - Chris Vermillion, Mitchell Cobb, Lorenzo Fagiano, Rachel Leuthold, Moritz Diehl, Roy S. Smith, Tony A. Wood, S. Rapp, R. Schmehl, None More Authors

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, ...

Robust Automatic Pumping Cycle Operation of Airborne Wind Energy Systems

Doctoral thesis - S. Rapp

Airborne wind energy (AWE) is a novel technology that aims at accessing wind resources at higher altitudes which cannot be reached with conventional wind turbines. This technological challenge is accomplished using tethered aircraft or kites in combination with either onboard or ground-based generators. In the former case, the kinetic energy of the air flow is transformed into electricity and transmitted via a conductive cable to the ground. In the latter case, the aerodynamic force of the aircraft or kite is translated into tether tension. The pulling force uncoils the tether from a drum which turns a generator and hence transforms the mechanical torque into electrical power on the ground. In this case two operational modes are required: In the first mode, the tether is reeled out until the maximum length is reached. It follows a reeling in phase where the aircraft or kite glides back towards the ground station and a fraction of the generated power is used to wind the tether again onto the drum of the winch. The cycle restarts as soon as the minimum tether length is reached. These two modes combined constitute a so-called pumping cycle. Reliability is a key system property that will decide over the success of AWE as a commercially feasible technology. To reach this goal, a well designed control system is required that can achieve the nominal control objectives as well as handle disturbances such as atmospheric turbulence and mismatches between the model used for the controller derivation and the real plant. In light of these challenges, the present work tries to make a contribution to bring AWE closer to commercial success. More specifically, a workflow to design a modular control architecture for a rigid wing AWE system operated in pumping cycle mode is presented. The thesis introduces models of different fidelity that are either directly used for the controller synthesis or in order to verify if the designed controller is able to meet its objectives. A quasi-stationary analysis is performed to describe the operational flight envelope and to derive linear state space models for the longitudinal and lateral flight controller synthesis. A generic outer loop controller, independent of the specific aircraft actuation, is designed which guides the system along the traction and retraction phase reference flight paths. A ground based winch controller is used to track the tether tension and hence the radial motion of the aircraft. To track the outer loop guidance commands several linear and nonlinear inner loop flight controllers are proposed. All controller designs are verified in detail using Monte Carlo simulations. The resulting distributions of critical metrics are used to quantify performance as well as robustness of the controllers in the presence of stochastic variations in the wind field and model uncertainties. In the last part of this thesis a methodology is proposed that can be used to systematically generate conditions in which the AWE control system is failing. The generated knowledge can be leveraged to create an analytic model that is able to predict during operation a critical flight state. Ultimately, this allows to trigger a mitigation maneuver to avoid the failure. Different prediction strategies are presented and eventually the methodology is specifically applied to the case of tether rupture condition generation, prediction and avoidance. @en

Enhancing resilience of airborne wind energy systems through upset condition avoidance

Journal article - S. Rapp, R. Schmehl

Airborne wind energy (AWE) systems are tethered flying devices that harvest wind resources at higher altitudes, which are not accessible to conventional wind turbines. To become a viable alternative to other renewable energy technologies, AWE systems are required to fly reliably ...

Vertical Takeoff and Landing of Flexible Wing Kite Power Systems

Journal article - S. Rapp, R. Schmehl

In this work, a novel vertical takeoff and landing methodology for flexible wing kite power systems is presented. Starting from a basic mast-based launching and landing concept, the operational envelope will be enlarged using the external assistance of a multicopter. The multicop ...

A Modular Control Architecture for Airborne Wind Energy Systems

Conference paper - S. Rapp, R. Schmehl, Espen Oland, Sture Schmidt, Thomas Haas, Johan Meyers

Airborne wind energy is an emerging technology that uses tethered unmanned aerial vehicles for harvesting wind energy at altitudes higher than conventional towered wind turbines. To make the technology competitive to other renewable energy technologies an automatic control system ...

Reference Design and Simulation Framework of a Multi-Megawatt Airborne Wind Energy System

Journal article - D. Eijkelhof, S. Rapp, Urban Fasel, Mac Gaunaa, R. Schmehl

In this paper, we present the design and computational model of a representative multi-megawatt airborne wind energy (AWE) system, together with a simulation framework that accounts for the flight dynamics of the fixed-wing aircraft and the sagging of the tether, combining this w ...

Rare Event Prediction for Enhanced Control System Reliability of AWE Systems

Abstract - S. Rapp, R. Schmehl

Reliable autonomous operation of Airborne Wind Energy (AWE) systems requires control algorithms that are able to attenuate the effect of stochastic disturbances on the control performance in continuously changing wind conditions. Assessing the stability and robustness of the cont ...

Cascaded Pumping Cycle Control for Rigid Wing Airborne Wind Energy Systems

Journal article - S. Rapp, R. Schmehl, Espen Oland, Thomas Haas

Towards Robust Automatic Operation of Rigid Wing Kite Power Systems

Abstract - S. Rapp, R. Schmehl

A key enabler of success in the field of airborne wind energy (AWE) is the ability to operate the AWE system in a reliable manner throughout every phase of flight. In this contribution a modular path following flight control architecture will be presented that allows to control a rigid wing AWE systemduring all phases of the flight, including vertical takeoff, power generation and vertical landing. It is a first step towards a flight control architecture with increased reliability and robustness in nominal aswell as in adverse conditions compared to existing approaches. The cascaded control structure consists of two separate outer loop controllers connected with a switching logic which is controlled by a high-level state machine. The flight path controller for vertical takeoff and landing (VTOL) guides the system along a predefined flight path up to the operational altitude or lands the system. To guide the kite along the path during VTOL mode common thrust vectoring is used. For improved control effectiveness the control surfaces of the wing are used along with the thrusters using a pseudo-inverse control allocation method to generate the required moments. This allows to exploit the complementary control effectiveness of thrusters aswell as control surfaces with respect to the current flight condition. For the power generation mode the second outer loop controller is activated. A novel path following controller for AWE systems has been developed that can be applied to soft was well as rigid wing kites. Different flight patterns can be generated consisting of circle and great circle segments. This allows to solve the path following control problem on a sphere similarly to straight line and orbit following control problems of untethered aerial vehicles. Since during power generation mode the thrusters are not in operation, only the lift vector is controlled using the orientation of the kite with respect to a tether reference frame. This allows to generate the required centripetal force to guide the kite along the curved paths on the tangential plane. In this way the model dependency of the controller is reduced to a minimum, while at the same time the control method is intuitive from a flight physical point of view. To enhance the disturbance compensation capabilities the baseline controller is enhanced with an adaptive part, which allows to recover a defined reference behavior in case of large disturbances or failures. So far simulation results using a generic rigid wing kite model demonstrate the feasibility of the proposed control approach. Due to the modularity of the control architecture, the low computational demand and the reduced model dependency the proposed flight control architecture can be easily tested on different platforms in the future. @en

Contributed

2 records found

A methodology for design, analysis and optimization of a vertical take-off system for rigid-wing airborne wind energy systems

Master thesis - N.W.O. Pynaert, S. Rapp, R. Schmehl, G. la Rocca, E. van Kampen

Airborne wind energy (AWE) is a new technology aiming at converting wind energy by flying crosswind patterns with a tethered aircraft. One of the main challenges of this technology is the launching and retrieving of the aircraft. A promising solution is the use of a vertical take-off and landing (VTOL) system in the form of multiple electrically driven rotors. Contrary to other approaches, such as a rotational and catapult assisted take-off, no detailed studies on a VTOL system for AWE have been conducted so far. The goal of this research is to investigate the opportunities and limitations of a VTOL system as a take-off system for a rigid-wing AWE system (AWES), by designing and simulating a VTOL system for the AP2, an AWES prototype of Ampyx Power B.V. The main research question to be answered by this research is how such a VTOL system can be designed. After having conducted a thorough state-of-the-art analysis (aka. literature study) the main research questions remain and are refined as: 1. What VTOL concept is most suitable for AWE applications? and 2. How do the aerodynamic forces of the AWES effect the VTOL system design? These are addressed and thoroughly analyzed in this report. The proposed and applied design methodology is based on the mass estimation of the VTOL system and its components: motors, battery and rotors. The sizing of the motors and battery is done by calculating the power and energy demand during take-off. Rotor mass is estimated using a statistical model. To calculate the required power, a blade element momentum model (BEM) combined with generalized momentum theory (MT) is used considering both inflow velocity and angle. A flight mechanics model is derived to calculate the required thrust and aircraft attitude. This model considers propulsion, gravity, and aerodynamic forces. A multi-disciplinary optimization (MDO) framework is developed to ensure consistency between the above-mentioned models and to calculate the minimum VTOL system mass for certain wind and operating conditions. A Simulink model of the VTOL system is developed using a multicopter flight path controller and a 2D dynamic model. This simulation model serves as a dynamic feasibility check on the VTOL system design and is used to iterate on assumptions taken in the design model. One of the main findings of this research is that by tilting the rotor depending on wind and flight conditions the minimum VTOL system mass is obtained. The VTOL system mass is estimated at 4.3 kg at wind speeds below 4 m/s and decreases to a mass of 2.1 kg at 10 m/s wind speed. However, for a fixed rotor tilt of -30 degrees (knee-sitter concept, newly introduced in this report), the VTOL system mass is only 100 grams higher for speeds between 5 and 10 m/s. Because a fixed rotor does not require any tilting mechanism, it can be concluded that this concept is most optimal in terms of mass for a direct take-off approach. For the tail-sitter and quad-plane concepts, the VTOL system mass increases with wind speed. A dynamic simulation has proved the feasibility of the sizing results for the tail-sitter, quad-plane and the new knee-sitter concept for a wind speed of 5 m/s and 10 m/s and a target elevation angle of 41.8 degrees. These simulations have shown that, under the assumptions of the simulation, the maximum motor power (from sizing model) is sufficient to successfully fulfill the take-off phase. It is found that the maximum motor power is only used to accelerate the rotor initially. It is not required in the transient flight phases. Furthermore, it is found that the additional power and energy, required for acceleration and transient phases is sufficiently low compared to the safety factor of 1.5 which has been taken into account in the preliminary sizing model.

The effect of kite control unit mass and drag on the flight behavior of an airborne wind energy system kite

Master thesis - X.G. Gerrmann, J.W. van Wingerden, S. Rapp, S. Baldi, R. Schmehl

Airborne wind energy (AWE) is an emerging field that aims to revolutionize the wind energy sector.This work focuses on the aspects of modeling AWE kite flight behavior. Research on such modelsis important since new or improved models can potentially reduce development time and costs andprovide insight in the role of key system parameters.This research investigates the effects of the mass and drag of the kite control unit (KCU) on the kite’sflight behavior since it is hypothesized that the mass of the KCU causes an outward swing of the KCUduring turns and that KCU drag reduces the angle of attack during the retraction phase.For this investigation, the simulation of a kite and tether system is required. Existing simulation environ-ments could not be used due to legacy code, limited validity or unsuitable model complexity. Therefore,a suitable kite and tether model need to be selected for implementation in a simulation environment(SE). Two kite models have been found suitable: Fechner’s and Ruppert’s model.In the early stages of this research the aim was to use a model that has been fitted to experimental flightdata. Due to the unavailability of experimental flight data, the parameters of both models are identifiedusing simulated flight data. This approach provides Kitepower with a method to identify the modelparameters with experimental flight data in the future. Kitepower is an AWE company that facilitatedon this research.The groundstation controller and flight controller signals of Kitepower have been integrated in the SE.Two experiments have been performed in the SE. For the first experiment, flights are performed withdifferent and increasing values for the drag coefficient of the KCU. In the second experiment the massof the KCU and kite have been varied independently. Both experiments have been performed in a lowand high wind condition.The high wind condition experiment showed that and increasing drag results in a reduction of 7% ofthe average cycle power. The KCU drag force can reach up to to 5% of the kite drag force for a KCUdrag coefficient of 1.5.Increasing the combined mass of the kite and KCU results in an increase of >150% in average cyclepower output for the low wind conditions and an increase of >40% for the high wind condition. Theincrease in power is caused by a reduction in elevation angle in combination with the assumption of aflat wind profile. Additionally an outward swing of the KCU of 4.5 ∘ has been observed.It is recommended to compare Fechner’s and Ruppert’s rigid body kite models through using experi-mental flight data in order to investigate which model reproduces actual kite flight the best. Furthermoreit is recommended to investigate the optimal altitude or elevation angle at which to operate the AWEsystem for maximal power output considering a non-flat wind profile, tether drag and kite and KCUdynamics. It is suggested that these findings should then be incorporated in the flight controller ofKitepower in order to increase the system power output.