Flight Path Planning in a Turbulent Wind Environment

Fechner, Uwe; Schmehl, Roland

doi:10.1007/978-981-10-1947-0_15

Cited by 17 publications

(33 citation statements)

References 24 publications

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“…One option is to use the inflated center strut as a reference component to mount the flow measurement equipment (Borobia et al, 2018;van Reijen, 2018). Mounting the equipment directly on the suspended KCU is not considered to be an option because this relatively heavy component is deflected substantially when flying sharp turns (Fechner and Schmehl, 2018) and can also exhibit transverse vibrations. Another option is to mount the measurement equipment on the two power lines.…”

Section: System Description and Data Acquisitionmentioning

confidence: 99%

“…The z t axis is tangential to the tether, while the x t axis is located in the plane spanned by the z t axis and the normal vector of the plane spanned by the two tensioned power lines. This definition is identical to the "kite reference frame" used by Fechner et al (2015) as a basis for a point mass model. The measurement reference frame (x m , y m , z m ) is attached to the rigid frame on which the relative flow sensors are mounted.…”

Section: Geometry and Reference Framesmentioning

confidence: 99%

“…These structural requirements are generally best met if the front bridle lines, which transmit most of the forces, connect perpendicularly to the wing. It is in principle straightforward to account for a constant offset angle α 0 (Fechner et al, 2015); however, for the investigated kite design this offset angle is rather small. For this reason we set α 0 = 0.…”

Section: Geometry and Reference Framesmentioning

confidence: 99%

“…Instead of assuming a linear correlation between the relative power setting u p and the depower angle α d , as proposed by Fechner et al (2015), we use the geometric depower model illustrated in Fig. 5b to calculate an analytic equation for the depower angle α d by applying the law of cosines:…”

Section: Kinematics Of Depoweringmentioning

confidence: 99%

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Aerodynamic characterization of a soft kite by in situ flow measurement

Oehler

Schmehl

2019

Wind Energ. Sci.

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Abstract. 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 aerodynamic 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.

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Section: System Description and Data Acquisitionmentioning

confidence: 99%

Section: Geometry and Reference Framesmentioning

confidence: 99%

Section: Geometry and Reference Framesmentioning

confidence: 99%

Section: Kinematics Of Depoweringmentioning

confidence: 99%

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Aerodynamic characterization of a soft kite by in situ flow measurement

Oehler

Schmehl

2019

Wind Energ. Sci.

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“…A typical large-scale phenomenon is the spanwise bending and twisting of the entire wing due to steering line actuation. The ability of the membrane wing to deform asymmetrically and by that generate a substantially increased turning moment makes it particularly suitable for AWE applications which require excellent maneuverability (Breukels et al, 2013;Bosch et al, 2014; van Reijen, 2018;Fechner and Schmehl, 2018). Typical small-scale phenomena are the local flutter of the wing canopy or wrinkling, which is caused by local compression loads that can not be supported by the woven fabric material.…”

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confidence: 99%

Automatic Measurement and Characterization of the Dynamic Properties of Tethered Membrane Wings

Hummel¹,

Göhlich²,

Schmehl³

2018

Preprint

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The performance of an airborne wind energy system crucially depends on the aerodynamic, structural dynamic and flight dynamic properties of the tethered wing. In the scope of this paper, flight dynamic properties are determined against reproducible steering inputs. Current design methods for highly flexible membrane wings have achieved a mature product level by combining iterative testing with empirical and intuitive variation of wing parameters. However, for significant further improvements, experimental data of high quality is indispensable. We have developed a tow test setup for reproducible mea-5 surement 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 the lift-to-drag ratio as functions of the length ratio between power and steering lines.This non-dimensional 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 10 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 15 parameter identification.Wind Energ. Sci. Discuss., https://doi.org/10.5194/wes-2018-56 Manuscript under review for journal Wind Energ. Sci. Discussion started: 13 August 2018 c Author(s) 2018. CC BY 4.0 License. design method will allow only limited further improvements and that it is indispensable to develop a systematic understanding of how wing performance parameters such as aerodynamic coefficients, lift-to-drag ratio, steering forces and moments depend on the wing design.The empirical design method is used because compared to rigid wings the physics of flexible membrane wings is complex and the existing knowledge is limited, due to deforming under aerodynamic load and steering line actuation. This holds par-5 ticularly for Leading Edge Inflatable (LEI) tube kites and other single-skin kite types, since ram air wings have already been investigated systematically for several decades (Dunker, 2013;Johari et al., 2014;Dunker, 2018). Because of the high degree of flexibility and the low weight of the membrane structure, the flow around the wing and its shape are strongly coupled. A change in the flow field alters the aerodynamic load distribution to which the structure rapidly adjusts by deform...

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Improving reliability and safety of airborne wind energy systems

Salma

Friedl²,

Schmehl

2019

Wind Energy

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Airborne wind energy systems use tethered flying devices to harvest wind energy beyond the height range accessible to tower-based wind turbines. Current commercial prototypes have reached power ratings of up to several hundred kilowatts, and companies are aiming at long-term operation in relevant environments. As consequence, system reliability, operational robustness, and safety have become crucially important aspects of system development. In this study, we analyze the reliability and safety of a 100-kW technology development platform with the objective of achieving continuous automatic operation. We first outline the different components of the kite power system and its operational modes. In the next step, we identify failure modes, their causes, and effects by means of failure mode and effects analysis (FMEA) and fault tree analysis (FTA). Potentially hazardous situations and mechanisms which can render the system nonoperational are identified, and mitigation measures are proposed. We find that the majority of these measures can be performed by a failure detection, isolation, and recovery (FDIR) system for which we present a hierarchical architecture adapted from space industry. KEYWORDS airborne wind energy, fault detection, fault isolation, fault recovery, FDIR, FMEA, FTA, health monitoring, kite power, reliability, safety INTRODUCTIONThe increasing need for renewable energy has led to a widespread deployment of wind turbines: initially, only on-shore, but for more than a decade, also off-shore. 1 The trend goes to ever larger turbines with increasing capacity factors because the wind power density generally increases with the distance from the ground, as a result of the wind shear. 2 On the other hand, the cost of larger structures scales unfavorably with a square-cube law and modern wind turbines are approaching an economically feasible size limit. 3 Airborne wind energy (AWE) systems, on the other hand, use tethered flying devices to harvest wind energy beyond the height range accessible to tower-based turbines. 4,5 The use of a tether allows the harvesting height to be adjusted continuously to optimize the availability of the wind resource. Compared with harvesting at the fixed hub height of wind turbines, the wind power that is available 95% of the time increases roughly by a factor of two. 6 Of particular interest are deep-sea applications because a tower is in principle not needed for the operation of the system. The tether attaches to the ground station at sea level, which substantially reduces the structural loads and thus also the required material. 7,8 The lower material effort, the increased capacity factor, and the access to a so far unused wind resource render AWE a potential cornerstone in a future low-carbon energy economy.However, the technology is operationally more complex than conventional wind turbines. Most implemented concepts rely on aerodynamic lift, and the tethered flying devices can thus not be stopped immediately when unexpected wind conditions or system failures occur. Exact...

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Flight Path Planning in a Turbulent Wind Environment

Cited by 17 publications

References 24 publications

Aerodynamic characterization of a soft kite by in situ flow measurement

Aerodynamic characterization of a soft kite by in situ flow measurement

Automatic Measurement and Characterization of the Dynamic Properties of Tethered Membrane Wings

Improving reliability and safety of airborne wind energy systems

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