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
    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.