TaaB [Tether as a Barrier]

Hi. I think we did not talk too much about using tether as a barrier.

The simple problem description would be; can we assume the tether will be connected to the kite and thus provide a hard limit to how big area that must me assesed for crash risk. OR do we need to assume the tether could be released, either by a weak link, a release mechanism or failure or kite or tether, and then what?

It is a broad and complex problem statement. I think there are no right and wrong answers, a lot is left to judgement.

I would be interested to hear if anyone has thought more about this though.

Preliminary thoughts on the crash zone were raised, especially with the third sketch, showing a dotted crash zone in addition to the in-flight location of the kite depending on its elevation angle.

But I am afraid that such (mine) estimations are still optimistic, because when the tether is broken, tether + kite can drag downwind over large distances (assuming the release tether mechanism is failing if it exists).

One way to remedy this would be to multiply the tethers for the same kite (see the SkySails nose line), at the risk of them breaking quickly one after the other. Then one way of dealing with this would be to automated kite depowering when a tether is broken.

Now the risk of crash without tether breaking would lead to other solutions.
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I don’t understand the title. Do you mean the tether as a limiting factor or something to the size/radius of the crash zone?

In reeling systems, or perhaps all systems, you would have automatic and constant monitoring of the state of the tether, so tether break should not happen accidentally. You could decide to slice the tether at the ground station or release or slice it at the kite, or have a weak link there.

If you find that crash risk is very low above some sectors, say above non dominant wind direction sectors or kite mostly reeled out sectors, you could agree with the insurance company to reduce the cost to insure against crashes in these sectors, like insurance against flood being less expensive in less flood prone regions.

Launch and landing is the most dangerous in aviation. Is it the same in AWE? Then assign the highest risk to those sectors. Maybe you don’t launch when the wind direction or speed is wrong.

The system would have automatic fault detection. You would automatically know the crash zone, so you could warn people in the area with sirens, and possibly integrate with the traffic signs in the area to stop or limit traffic in the area.

How does the tether behave when you release it from the kite and what dangers does it pose? Is it equally as dangerous as with the kite still attached or not? Can the kite recover if it is released from the tether and is above a minimum altitude? If yes, maybe it can glide to safety zones, or avoid roads and houses and so on.

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I think it should be possible to design a kite where the tether could be a safe barrier. It will however come at a cost.

For for soft-kites I imagine that the penalty will be much less than for rigid kites as the mass and kinetic energy is much lower.

Turbines on the ground can certainly through their wings quite far away from the tower. Perhaps TaaB is a useful concept (I don’t know), but reality is that it will be a probability distribution. There will be a “once every” 10, 100 and 1000 years zones.

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Yes this is a sure thing. Though that is also how any safety measures of an inherently dangerous activity must be handled. There is no “zero” risk, rather just a very low one that is acceptable or not to the surroundings.

Lets initially state that the AWE is very safe (eg. one crash in 1000 years). This still means that if you build 1000 AWE plants, you will crash once every year, which may or may not be acceptable, depending on the further mitigation actions and the surroundings.

The next thing would be to figure out what happens after you crash. If you are bound to the volume of a given tether that is tied in the end, you could only crash at a certain area. If that area is sparsely populated (eg just some roads with less traffic), the chance of hitting something in that crash may be so small that its an acceptable risk (comparable to roads in areas with possible rockslides).

Though if you can’t ensure that the tether is acting as a barrier, the area that you could crash in will surely be a lot bigger. You may get cities and such in the crash area. Hitting such an area would most likely not be acceptable in any circumstance.

So, if we decide to use tether as a barrier, we are also saying:

  • There is no weak link, everything must be made to hold loss of control of the kite. Luckily this is not too hard to ensure, as the wing can physically only generate so much lift, and the kite will make that lift in normal operations.(*)
  • If you have a deliberate tether release mechanism, that must be very safe from accidental release, also if the kite power fails.
  • If you are unable to release the tether, you will likely crash in a failure event. If the tether can be released, you could maybe implement a controlled safe landing
  • If you use TaaB, some other mitigation actions like deploying a parachute will not be viable anymore
  • You should still have a plan for tether or structural failure

(*) Still some extra work must be done to handle high wind scenarios

I believe this is hard to implement and also hard to guarantee 100% certainty of tether state. My guess is such monitoring will not be implemented or maybe just for a certain fraction of AWE [if AWE ever takes off].