Airborne Wind Energy

Design and aerodynamics of kites

Roland Schmehl

6 December 2024

CC BY 4.0

Outline

Kitepower

Learning objectives

This lecture is about the design and working principles of kites, both soft wing as well as fixed wing kites. Some aspects of flight dynamics were already derived in previous lectures, and are here illustrated by experimental results (e.g. steering law, roll, pitch yaw, …)

Subtopics I

Subtopics II

  • Update Bangor Erris: dataset, map and video of flight 2023-11-23
  • Typology of kites for AWE (recap of lecture 1)
  • Structural design concepts: design tree?
  • How are soft-kites built (Roland V paper)

Kite design

Design requirements for AWE kites

  • Fly-gen AWES
    • High flight speed ⤳ high \(\CLmax\)


  • Ground-gen AWES
    • High pulling force during reel-out phase
    • Low pulling force during reel-in phase
    • Fast reel-in phase

Basic kite designs

Cherubini et al. (2015)

Boxwing flygen system

Kitekraft

Makani M600 vs Oktoberkite

Homsy (2020)

Makani M600 vs Oktoberkite

Homsy (2020)

Makani M600 vs Oktoberkite

Homsy (2020)

MegAWES kite

Eijkelhof and Schmehl (2022)

MegAWES kite

Eijkelhof and Schmehl (2022)

Scaling of kites for AWE applications

  • Design of Solar Powered Airplanesfor Continuous Flight (Fig. 6) https://ethz.ch/content/dam/ethz/special-interest/mavt/robotics-n-intelligent-systems/asl-dam/documents/projects/Design_Skysailor.pdf
  • The Simple Science of Flight - From Insects to Jumbo Jets (p. 14) http://home.kpn.nl/ronvans/forums/Simple_Science_of_Flight.pdf

Enerkite wing

Soft kites

Scaling tube kites

Static force equilibrium of finite membrane element

⤳ maintaining maximum tensile stress in membrane requires reducing tube pressure when scaling up

Soft kite aero-structural designs







TU Delft soft-kites (front views)

TU Delft soft-kites (side views)

Depowering

Heuvel (2010)

Relative flow

It can be shown that the angle of attack does not vary along the flight path of a massless kite if the angle between wing and tether is constant.

Vlugt et al. (2019)

Single skin kite

Courtesy of Bryan van Oostheim

Flying the single skin kite

Single skin kite

Courtesy of Bryan van Oostheim

Flying the single skin kite

Single skin kite

Courtesy of Bryan van Oostheim

Flying the single skin kite

Hybrid kites

Kitedynamics

Kite aerodynamics

Learning objectives

Introduction

We first qualitatively illustrate the aerodynamics of rigid kites and softkites using experimental flow visualization and computational fluid dynamics. Material from the MSc thesis of Aart de Wachter. Two- and three-dimensional flow phenomena are distinguished. This is purely about the fluid dynamics of the wings.

2D aerodynamics

  • Rigid kites
    • Conventional airfoil
    • High-lift airfoil
    • Multi-element airfoil
  • LEI tube kites
  • Ram air kites
  • Single-skin kites

Attached and separated flow

  • Low angle of attack
  • Laminar, attached flow
  • Viscous effects in boundary layers and wake flow

  • High angle of attack
  • Fully separated flow on suction side
  • Viscous effects also in separated flow

DLR

Boundary layer separation

Olivier Cleynen

LES of a pitching airfoil

http://doi.org/10.1115/1.4039235

Multi-element airfoil Makani Oktoberkite

Multi-element airfoil Ampyx AP3

Multi-element airfoil Ampyx AP3

Multi-element airfoil flow phenomena

http://doi.org/10.1016/S1270-9638(02)00002-0

Multi-element airfoil optimization

De Fezza and Barber (2022)

Leading-edge inflatable airfoil


Single skin airfoil



3D aerodynamics

  • All three types of kites are jointly discussed as the three-dimensional flow is similar.
  • Flow visualizations and interpretation by lifting line theory

Prandtl lifting line theory

Computed flow field around the Ampyx AP3

Computed flow field around a ram-air kite

Trailing vorticity



Folkersma et al. (2020)

Wachter (2008)

Wind tunnel analysis

Photo credits: Max Dereta (2008)

Experimental challenges

  • Scaling down large inflatable membrane wings
    • Ram air wing:
    • LEI wing: pressure in tube
  • Suspension in the wind tunnel

Ram air wing

Finished wing setup

Photogrammetry markers

Camera setup

Pulse2 in wind canal

Wing tip vortex

Max Dereta (2008)

Kiteplane

Kiteplane

Kiteplane in wind canal

In situ flow visualization

Tell tales

Heuvel (2010), https://en.wikipedia.org/wiki/Telltale

Flow tufts

Kasper Masure, Vantage

In situ measurement of aerodynamics

CL and CD identification

Schelbergen2021

Learning Through Simulation & Real World Testing

After the in-flight break up of the Makani M600 in RPX-09 in April, 2018, the team did a comprehensive root cause analysis and dedicated even more effort to improving Makani’s suite of simulation tools so that we could crash in simulation before we crashed in the real world whenever possible.

Learning how the kite responded to different test cases in simulation also meant Makani could continue to accelerate learning even while we were not flying in the real world. Makani tried to prioritize learning as much as possible in simulation so that each real world flight test could teach us things we could not learn from simulation alone.

This video was shot between the crash of RPx-09 and Makani’s first All Modes Crosswind Flight off of the GS02 base station.

[Filmed by Kate Stirr and Andrea Dunlap. Edited by Andrea Dunlap.

Tech Topic - 20181109

Internet Archive - Makani Power collection

References

Cherubini, A., Papini, A., Vertechy, R., Fontana, M.: Airborne wind energy systems: A review of the technologies. Renewable and Sustainable Energy Reviews. 51, 1461–1476 (2015). https://doi.org/10.1016/j.rser.2015.07.053
De Fezza, G., Barber, S.: Parameter analysis of a multi-element airfoil for application to airborne wind energy. Wind Energy Science. 7, 1627–1640 (2022). https://doi.org/10.5194/wes-7-1627-2022
Eijkelhof, D., Schmehl, R.: Six-degrees-of-freedom simulation model for future multi-megawatt airborne wind energy systems. Renewable Energy. 196, 137–150 (2022). https://doi.org/10.1016/j.renene.2022.06.094
Erhard, M., Strauch, H.: Control of towing kites for seagoing vessels. IEEE Transactions on Control Systems Technology. 21, 1629–1640 (2013). https://doi.org/10.1109/TCST.2012.2221093
Folkersma, M., Schmehl, R., Viré, A.: Steady-state aeroelasticity of a ram-air wing for airborne wind energy applications. Journal of Physics: Conference Series. 1618, 032018 (2020). https://doi.org/10.1088/1742-6596/1618/3/032018
Heuvel, J. van den: Kitesailing – improving system performance and safety, http://resolver.tudelft.nl/uuid:cee4dcf5-e2a8-49d0-b731-92db25b44a17, (2010)
Homsy, G.: Oktoberkite and the MX2: Toward best practices in energy kite design. In: Echeverri, P., Fricke, T., Homsy, G., and Tucker, N. (eds.) The energy kite: Selected results from the design, development, and testing of Makani’s airborne wind turbines, Part I of III. Makani Technologies LLC (2020)
Lebesque, G.H.M.: Steady-state RANS simulation of a leading edge inflatable wing with chordwise struts, http://resolver.tudelft.nl/uuid:f0bc8a1e-088d-49c5-9b77-ebf9e31cf58b, (2020)
Oehler, J., Schmehl, R.: Aerodynamic characterization of a soft kite by in situ flow measurement. Wind Energy Science. 4, 1–21 (2019). https://doi.org/10.5194/wes-4-1-2019
Roullier, A.: Experimental analysis of a kite system’s dynamics, (2020)
Schmidt, E., De Lellis Costa de Oliveira, M., Saraiva da Silva, R., Fagiano, L., Trofino Neto, A.: In-flight estimation of the aerodynamics of tethered wings for airborne wind energy. IEEE Transactions on Control Systems Technology. 28, 1309–1322 (2020). https://doi.org/10.1109/TCST.2019.2907663
Vermillion, C., Cobb, M., Fagiano, L., Leuthold, R., Diehl, M., Smith, R.S., Wood, T.A., Rapp, S., Schmehl, R., Olinger, D., Demetriou, M.: Electricity in the air: Insights from two decades of advanced control research and experimental flight testing of airborne wind energy systems. Annual Reviews in Control. (2021). https://doi.org/10.1016/j.arcontrol.2021.03.002
Vimalakanthan, K., Caboni, M., Schepers, J.G., Pechenik, E., Williams, P.: Aerodynamic analysis of ampyx’s airborne wind energy system. Journal of Physics: Conference Series. 1037, (2018). https://doi.org/10.1088/1742-6596/1037/6/062008
Vlugt, R. van der, Bley, A., Schmehl, R., Noom, M.: Quasi-steady model of a pumping kite power system. Renewable Energy. 131, 83–99 (2019). https://doi.org/10.1016/j.renene.2018.07.023
Wachter, A. de: Deformation and aerodynamic performance of a ram-air wing, http://resolver.tudelft.nl/uuid:786e3395-4590-4755-829f-51283a8df3d2, (2008)

Questions?





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