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Topic :   paper:
Assessment of an Alternative Concept for a High-Altitude Wind-Power Generator
  • May 2018Journal of Physics Conference Series 1037(4):042023
    DOI: 10.1088/1742-6596/1037/4/042023
    LicenseCC BY 3.0
    Max Langbein                                 [attended AWEC2017]
    Maja Ruby
    Nicolas Gauger
  • Abstract. To  generate  power  from  high-altitude  winds,  concepts  using  kites  or  planeslinked to the ground with tether are in development.  The most popular high-altitude wind generation concept is one using a flying wing attached to a single tether whose movement generates power by turning a winch.  The usual trajectories for power generation consist of a  period  where  the  kite  distance  is  increased,  and  the  pulling  force  enlarged  by  figure-of-eight movements, interrupted by a pull-back phase where power is consumed.  We comparethat with a new concept we introduce here.  It uses a triplet of tethers whose length sum is kept more or less constant using differential gears, resulting in a trajectory surface.  It does not have a pull-back phase and allows to have similar power output in a closed trajectory. Moreover, starting and landing can be achieved without additional equipment when using a soft kite as wing, and keeping the wing flying without any wind is easier.  Also the control can be easier, as one has more degrees of freedom in the force direction and the movement of the kite. Its disadvantages are an increased effort for the ground stations and more restrictions on the location.  Also the tether’s air drag is increased.  Optimal power generation is compared using an example configuration and state with given wind speed under the assumption of an  optimal  steering  of  the  generators  and  the  kites.   This  is  done  for  state  snapshots,  for  example trajectories, wind speeds, and kites.

    (PDF) Assessment of an Alternative Concept for a High-Altitude Wind-Power Generator. Available from: HERE [accessed Feb 12 2020].

  • For convenience, I quote the paper for its use of the following symbols:
    1r one-tether, angle of attack optimized, distance is allowed to increase,
    3r. three tethers, angle of attack fixed, distance (tether length sum) fixed,
    3r three tethers, angle of attack optimized, distance (tether length sum) fixed.

    Quoted:
    “4. Discussion & Outlook
    It has been shown that:
    3r is comparable in terms of power output.
    3r makes it possible to have a kite with constant angle of attack.
    3r makes it possible to land and start the kites without additional equipment.
    3r makes it possible to keep the kite airborne when no wind is present.
    A key feature of 3r could be a constant power production which could be achieved by steering the distance motor-generator accordingly while keeping a closed trajectory. An optimization achieving this would proof the value of this new concept. To increase the precision of the evaluation, acceleration forces based on the trajectory should be included and whole trajectory optimizations should be done.”
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Feb. 13, 2020, post by Dave Santos
Re:  Assessment of an Alternative Concept for a High-Altitude Wind-Power Generator

The paper finds 3r is "comparable" in power to 1r (rather better, actually).
The authors provide a spectrum of "trajectory surfaces," each with its own geometric proportions.
Fig 5 patent drawing is schematic. Its a fallacy to see it as an angular specification.

The cosine-gain flatter triangles are best.

Fig 5 does apply to the tri-tether legs doing work; and the third tether is mostly for 360 deg capability.
June 29, 2018, post by Dave Santos
More notes ...

More notes, touching on points raised by your inspiring paper, summarizing related art and opinion-

- Two partial prior art tri-tether cases- vintage Zeppelin 3r anchoring patent and Tigner's AWE patent.

- Higher load velocity as key advantage of crosswind load trajectories like 3r, over 1r downwind reeling. Less transmission machinery is needed to gain high angular shaft velocity.

- Tether drag almost negligible with large slow power kites, after all 4 lines is the kitesport and PG standard. High L/D wings are far more tether-drag limited. 3r multi-lines can be thinner than 1r and still more resistant to runaway.

- Optimal to maintain max working load of kite (~2x safety factor), not, eg., rated human load (~8x safety factor). Elastic aft bridling dumps excess load like a shock absorber.

- UHMWPE and Nylon return most of their elastic energy when tensioned-untensioned. Elasticity is more of a dynamics issue than a source of loss, in normally proportioned rigs.

- No hills or towers really needed; turf-grass/hay is a fine surface that does not degrade kites, and the hay is an extra carbon-crop. A large dragged kite actually floats over grass, as a fluidized bed. An elevated 3r bridlepoint invites a looping foul of the kite around the downwind leg. The kite can be moved about and launched from a wheeled trolley. Good passive-control kites want to fly, and take off readily. "Kitekillers" are a means to avoid landing uncertainties.

- AWE is "sailing in the sky", still requiring skilled human labor to be productive. Full automation of large AWES is a distant hope, but a kPower looping foil under a pilot kite kite has run all-modes for two weeks, since the sled pilot kite always self relaunched after lulls. The outdoors is simply too dynamic and not structured enough for industrial automation. FAA requires Pilot-In-Command and Visual-Observer. Supervisory automation is the sweet-spot.

- Three lines form a rigger's triangle, a mechanical advantage transmission. A bowstring and arrow act similarly to what a tri-tether can do.

- Macroscopic application of QM methods is called "analog QM", to distinguish from microscopic QM. As such, there are fine similarity cases, like Yves Crouder et al, with walking-drop experimental model. De Broglie-Bohm Mechanics is now applied in economics, sociology, and other fields.

- Single-skin kites may ultimately dominate by both cost and power-to-mass, at slightly lower L/D. SS progress is revolutionary in PGs and power kites. The NASA Power Wing remains a potent wing, popular with DE kite-skateboarders. These kites must stay on a trajectory surface, since they do not glide back fast like a hot kiteplane.

- Control Theory is another useful analytical lens. Conventional AWE control theory presumes algorythmic digital control with sensing and actualtion. Classic kite passive dynamic stability of equivalent functionality is far more elegant classical cybernetics (Gauss saw toys as profound mathematical objects). Active digital control can be dedicated to supervisory override, exception-handling, mode transitions, active tuning, etc. Passive control is an embodied-logic analog computing case, even a quantum computing analog where DOF states embody qubits.

- Standing Lattice waves are Faraday Waves. Acoustics, heat, and mechanical action are all phonon waves under scale invariance. Lots of applicable wave science in in disciplinary silos. Classical harmonic analysis and resonance are key kite attributes. Synchrony emerges naturally from periodic order of spring-mass units.

- A cool AWE scicomp project would be a dynamic simulation environment hosting endless promising combinations of kite/tether/wind/load, assessed in real time. An ultimate AWE scicomp concept could consist of public global wind geodata overlaid with load demand-interface geodata, supporting global AWES infrastructure simulation, an engineering basis for solving the global clean energy gap with kites.

- KiteLab 3r demos played out the pulleys from the anchor points to match wind direction and kite angle to fixed center generator. Lowest part count and simplest cheapest parts drove transmission design.

Hope something here helps,

dave