# Preprint: Towards a gigantic Magnus balloon with motorized belts

Some elements and considerations are in several topics. I gather them for a synthesis of what I think to be a suitable way for Magnus effect-based balloons. Then generally speaking, the ability to scaling considerably is a key to AWES since the length of the tethers is one of the inalienable dimensions of any AWES and determines the space used and therefore the energy density.

Description

This is about the implementation of external motorized belts in order to hold the balloon on its whole length, and allowing scaling a lot. The balloon is held both by the periphery and on its entire length with as many belts and tethers as necessary. The tethers are connected to the ground station which is parallel to the balloon, and can orient according to the wind direction.

In first this paper is based on the results of Omnideaâ€™s experiments with a flexible balloon (references in the paper), which can differ from theory or from experiments with a rigid cylinder.

Key points: the spin ratio (tangential speed/wind speed) cannot be too high because of the cubic increase in energy consumption as the tangential speed of rotation increases, and I retain the value of the tested spin ratio (to begin with and perhaps even to end with) to extrapolate with a higher wind speed of 10 m/s ; then a vertical trajectory (references in the document) leads to higher power (crosswind component) than a basic (oblique and downwind) trajectory, and allows optimum maximization of the space used due to the absence of turning; transmission by external motorized belts.

This document is a draft and may be subject to discussion.

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The biggest Â«wowÂ» for me is the scale you envision.

I think maybe you did not mention in the preprint, how to deal with different wind directions? The angle of the magnus cylider seems locked relative to the ground station[s].

Another thing that would be interesting to dive deeper into is how well the Magnus effect scales to these dimensions, both aerodynamically, relative to the motor power used and also mechanically. I guess for the mechanical part your solution is belts, but how many belts would be required at this scale?

Otherwise, I will need some more time to digest this properly before giving further opinions

Hi Tallak:
From what Pierre has shared with me, he envisions a possible large turntable or similar structure at ground level for aiming. Obviously that would tend to conflict with the very long inflated sausage.
There is at least some new thinking here, could lead to somethingâ€¦

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The description of the paper I reproduced above mentions: â€śThe tethers are connected to the ground station which is parallel to the balloon, and can orient according to the wind direction.â€ť

And the sketch page 4 mentions the â€śpivoting ground station (6)â€ť . This sketch is reproduced below:

In other words the ground station (6) rotates (by a motor) with the balloon when the wind direction is changing. In addition to be parallel each other, both are the same length, facilitating (at least on the paper) synchronous enough operation. If the sausage is a little twisted during the wind changes, is that so serious?

For the rest, aerodynamic behaviour is effectively unknown for such dimensions.

There is one point: the tangential speed cannot be high, and so the power per mÂ˛ will remain limited but more than sufficient.

Yeah. Maybe 10 100 m turntables could work for a total of 1 km length. The tube could be aligned with the most common wind direction. Still, on the wrong wind direction, that solution would not be effectively 1 km long.

Maybe also just work on the most common wind directions (eg the wind here would normally follow the coastline), and accept generously wind that is not exactly perpendicular to the tube.

In both cases, one would need to asses the market size of such somewhat limited wind power generation. Such an assessment may also have more generic value, as this seems to be a recurring problem with AWE

A balloon of 1 km long. A ground station of 1 km long. This station is retained by a circular track (7) to undergo the cantilever effect of the balloon towing on said station.

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An alternate possibility with a smaller ground station:

But I donâ€™t like it for gigantic dimensions, The control would be too difficult, and the space use is taken by the balloon, so adding a longer ground station does not modify it. Moreover a long station facilitates the maneuvers of a long balloon.

About the previous comment: concentric circular tracks (7) can be added to undergo the cantilever effect on every places of the ground station. These circular tracks are slightly elevated, allowing the passage of tractors for farming or maintenance, provided there is not too sudden a change in wind direction .

Having the tethers/belts non perpendicular to the cylinder would make it so much harder to implement the mechanical support.

For scalability of the magnus cylinder, you may have some good sources, but also this could be done in OpenFOAM to verify your sources (verify the openfoam model) and then verify scaling effects. I found some code for this by doing an internet search, Im sure there is more if you search more

If you dont know how to use OpenFOAM yourself, this could be an interesting student project eg at Delft or Polytechnico Milano

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From an aerodynamic point of view, I have to admit that I havenâ€™t looked into the matter. That said, planes can scale up to limits that we donâ€™t perhaps know about, and in any case beyond a wingspan of 100 m, and wind turbine rotors up to a diameter of 260 m and more.

But perhaps I didnâ€™t quite understand the question.

From a structural point of view, this remains to be verified, but transmission by external motorized belts on an inflatable flexible cylinder should help considerably.

Now there are some other points to verify, whose some concerning the vertical trajectory which was theorized but not tested (unlike almost elements of Omnideaâ€™s balloon which used basic oblique and downwind trajectory). The chapter 13 (second AWE book), pages 313-315, mentions a lot of elements to consider for a more accurate calculation of the expected efficiency, and summarize the advantages of the vertical trajectory:

The results of the optimization, providing average power of 89.18 kW, are significantly better than the basic control strategy results, which produced average power of 40.15 kW. This means that an improvement of 122% has been achieved through optimal control. Note that the basic control strategy causes radial lift, in contrast with the vertical lift associated with the crosswind motion of the optimally controlled system.

The advantage of the vertical trajectory seems obvious, but the apparent wind could also deviate a bit the â€śvertical liftâ€ť.

Other points: a balloon of 1 km long and 200 m diameter has a volume of 31 000 000 mÂł, leading to an air mass of more than 30 000 tons. I donâ€™t think it is a problem when using the pumping (yo-yo) mode: the rotation is stopped far before reaching the top of the trajectory, and is starting again far before reaching the bottom of the trajectory. Perhaps, during descent, a reversed rotation could mitigate the required force for the recovery reel-in phase. But such a mass can lead to a safety issue: valves are required in order to quickly deflate the balloon if it escapes or if the traction is too high.

Now if such dimensions are not practically feasible, a farm of â€śsmallâ€ť unities of 200 m long and 40 m diameter would be a possibility. The trajectories would then be staggered. The density would remain high because possible collisions between unities would not lead to serious consequences, as for AWES farm in bumper car mode. And testing such a balloon in plastique film would be possible during a good day, and not too expensive: taking a max of information (power consumption, wind speed, average powerâ€¦, then dispose of or recycle the balloon if it is out of order after tests. It can be easier than deeper simulations.

On the paper, I have favored experimental measurements wherever possible.

Balloon 1 km long. Ground station 1 km long. Both are parallel. What conflict?
As the paper indicates (sketch page 4) a circular track (7) retains the ground station (6) in order to mitigate the cantilever effect. Some concentric circular tracks could be added.

An example of a small farm of 4 unities:

Of course a farm can include more unities.

That said, even for gigantic dimensions, at least two unities are required when the pumping (yo-yo) mode is used.

I dont know really, but what I am thinking; the boundary layer has kind of fixed height, so maybe if you scale the magnus cylinder, the Magnus effect may not scale to match the new dimensions.

I really should calculate though before concluding anything, but this is really not my expertise, besides, its summer holidays tight now.

Still, this question does seem essential for the that be answered before any further expensive work is started.

Hi Pierre:
I did not intend to shoot holes in the idea, just pointing out what seems like a conflict between size and having a turntable or circular track, mainly due to the large uninterrupted ground space required, but also the idea of how fast it could actually change direction to keep up aim with the wind direction. A very large sausage might require, say, a half-hour to change aim 180 degreees, whereas the wind can reverse faster than that. Still, it is an interesting ideaâ€¦

One factor to keep in mind is, we tend to design for ideal conditions that we hope for/, or imagine, a -perfect day, with smooth, unidirectional, productive winds, just strong enough to make decent power, not so strong as to challenge the structure, but the wind is not always in such a cooperative mood! We see this with the giant blimps, the few times they are actually built and flown, there are always issues with the wind being unpredictable and causing bad behavior or slow-motion â€ścrashesâ€ť.

Hi Doug,
The ground station is a narrow and long turntable without any motorization. The pivot is settled in the center. The rest is mounted on wheels, not using rails, but being under the circular track (7) which retains it from the pull of the balloon. It would be a sort of all-terrain turntable. So to a certain extent the unevenness of the terrain would not be a problem. During wind changes, even rapid ones, the balloon will drive the turntable ground station which remains free. As there are several tether-belts, you can play with their respective lengths during the rotation. In any way the turntable will end to be aligned.

The ends of a 1 km balloon turning at 180 degrees will travel 1.57 km. Add to this the movement of the whole balloon, perhaps 2 km from one edge to the other (the tethers are just over 1 km long). This can last a few minutes, even for the turntable if it is correctly driven by the balloon.

At a far lower scale (16 m span) Omnidea noted the ability of the balloon to place naturally in the flow, even when wind changing.

In the worst case, you could have opposite winds at either end of the 1 km balloon. Youâ€™ll end up with a sausage thatâ€™s momentarily twisted but still edible. This is flexible material supported by a multitude of tethers: problems are solved just like a cat balances itself when it falls. At least on the paperâ€¦And for the moment I canâ€™t see anything satisfactory even on paper.

Correction: a motorization may be required to ensure that the ground station pivots to follow the balloon. Indeed the balloon pushes on the station without exerting any torque, excepted by specific maneuvers.

OK, Iâ€™m trying to wrap my feeble brain around this concept. Are we talking about a sausage-reeling power-producing apparatus? Belt-driven spin of the sausage from the ground, but also able to travel large vertical distances to produce power by reeling in and out? So are the same drive belts causing rotation, also changing length reeling in and out for producing the power, a portion of which power drives the sausage spin? So the reel-in/reel-out somehow drives the rotation too? I probably missed some major details - sorry, maybe you could re-explain it to me.

The answers and the references are on the preprint and the sketch page 4, and also the first comment.
I will try to summarize again: in the main option, this is a yo-yo (pumping) mode AWES. So, as for Omnidea, the Magnus effect is used to produce lift, not torque. So it is what you generally donâ€™t like .The belts are acted by the respective motors located just below the balloon (see the sketch). The motors are connected to the ground station with tethers. The vertical trajectory is explained in the chapter 13, above all pages 313-315. The balloon is located far from the winches (which act the generators) to allow the vertical trajectory. As the balloon rotates by the motorized belts, the resulting Magnus effect generates lift, thus the balloon raises. When the balloon rotation is stopped, there is no lift, only drag, so less force to facilitate the recovery phase. If the balloon rotates in the reverse direction during descent, the traction would perhaps be still lesser, but I am not sure if it is a good solution. Omnidea stopped the balloon rotation during the descent (recovery phase). Note: the trajectory (in both directions) is quite vertical; as a result the balloon does not go downwind while the tether is alternately unwound (reel-out power phase) and wound up (reel-in recovery phase).

In the end of the preprint I mention also another option: the static Magnus balloon as lifter, carrying wind turbines.

OK, thanks for explaining it again for me.
Airborne motors - almost like a flygen system. Yeah yeah yeah, wind turbines on every belt - sure.
Sounds a bit crazy at that point, but who knows?
Itâ€™s nice to have a place on the internet where we can discuss giant motorized airborne spinning helium-filled sausages generating reeling power from the wind. I donâ€™t think Elon or Zuck are this advanced.
Iâ€™d say itâ€™s time to build a similar prototype, driven by multiple belts, at a smaller scale, just to get things started.
One thing though, isnâ€™t there an issue with a shortage of helium again lately?
Helium is well-known to leak a lot.
How widely could this be deployed before causing a helium shortage? With or without leaks?
And isnâ€™t there some problem with hydrogen? Like it causes warming or something? (Besides leaking to outer space?)
Or are we talking about air-filled sausages?