# Rope-drive transmission

Some elements of the topic are on:

https://www.resilience.org/stories/2013-03-06/the-mechanical-transmission-of-power-3-endless-rope-drives/ : “As we will see, this makes rope transmission more efficient than any other alternative up to a distance of a few kilometres.” and "Transmitting this energy to Earth is most advantageously done by mechanical power transmission, says researcher Dave Santos from KiteLab Group in an interview:

“Electric cables would be too heavy. With kites, power-to-mass-plus-aerodrag is critical, and the mechanical case wins by a large factor. Wire rope is not quite so amazing as our new materials, but good enough for a critical advantage over electrical. The main challenge is to learn how to drive ropes at speeds of hundreds-of-miles-an-hour.”"

A high tension due to the power or/and lifting kite(s) would favor a better transmission.

Heres my take on the rope drive: You need two tethers. The energy is transferred due to motion and a difference in tension in the two ropes. The low tension return rope must still have some tension in order to not have too much of a curvature. This tension must be generated twofold in the kite, once for the return ripe and once for the power rope. So this is a source of loss.

The second source of loss is the fact that you are only utilizing one half of the tether at all times (the return tether is not creating energy). So you get twice the tether drag essentially. This is an issue for high altitude AWE where tether drag can be one of the biggest losses.

Third, the length of the tether is not easily variable, putting severe constraints on the design overall, in particularily wrt deployment and retrieval. But also in terms of varying production height according to the current winds.

So on the extremely positive side, there is no real limit on the speed of the rope drive. For eg. yoyo the speed of the tether is typically 1/3 of the wind speed. This makes it necessary to have largest possible tension (that you anyways need for crosswind power generation). Still, the optimum speed of a rope drive is probably quite a lot faster than for yoyo. Faster speed means more power throughput without using a thicker tether. This could possibly offset all the bad things described above.

I guess the big question is: how much tether pull is required to generate a given amount of power. Then dimension a rope drive and a yoyo, and see how big speed the rope drive should have. If that speed is really high, then the rope drive is a winner. Otherwise yoyo is still the best option IMHO.

There are interesting observations. Perhaps the second source of loss is not too important as the circular rope (rope-drive) does not undergo the crosswind move.

Please @tallakt what do you think about the rope-drive transmission like @Kitewinder’s kiweeone in regard to torque transfer like @Rodread’s Daisy or @someAWE_cb’s OTS? Potential height? efficiency with the same height?

I agree the fact that the rope drive is not moving with the wing will reduce the tether drag immensely. That is if we assume that the wings are rotating with the gearbox for the rope drive placed in the middle of rotation. (second source)

+1 for the rope drive

I would have liked to do some initial calculations, but it seems to me that rope drive has more potentian for high altitude AWE compared to torque transfer.

I also did not mention any weight og rope drive mechanical equipment. It would have to be factored in, also the fact that the most obvious tether UHWMPE is very slippery and not initially well suited to transfer torque to a drum by friction.

Kiwee proved it possible at smaller scales, so thats a really good start. Im sure they have more intimate knowledge about the current design limitations.

I think a structure with a torque transfer closest to the wings (eg similar to daisy or your design) and then a gearbox to get the speed up could word.

I think we would probably also be looking at two counterrotating, controlled, rings for the wings, because in the air it is difficult to find a reference to convert torque into rope pull

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Such a weight can be a main concern as the system scales up, or variants should be studied in order to limit said weight.

So I did some quick calculations.

I am comparing a Y tethered yoyo rig with a rotating rig with rope drive.

None of these are severely affected by tether drag as it is mostly not moving much, and the complexity is similar (painting in very broad strokes here).

Next, with the same wing area, glide number and lift coefficient, I am estimating that the numbers for tether pull and power generated is almost equivalent. The reasoning is that the yoyo has wind speed reduced by 1/3 due to reeling out and the rotating rig has wing speed reduced by 1/3 relative to maximum speed to generate max torque. So if this holds, tether tension and generated power are the same.

This means, that disregarding any additional losses in the rope drive, the rope speed required to transfer the power to the ground must be … (suspense) … 1/3 wind speed.

So there seems to be no gain in the rope drive. Just additional complexity and losses.

Having thought some more, the tension on the return rope is not all bad. You could have a lot of tension on the return tether and then increase the rope speed on the rope drive. But keeping under breaking strength of the tether is a bit more involved for the rope drive.

The first challenge is to make small proofs of concepts. Later challenges would include trying to reduce the wear on the rope. I guess you would want to find ways to drive the rope as slowly as possible to try to reduce wear.

Nice analysis Tallak.

I guess the tension you need to prevent slippage on the pulleys is already higher than the tension you need for this, so this becomes irrelevant. Also, unless you are increasing the windings on the pulleys, the ratio between tension on the slack and tight side of the loop will not I think become very large. And because you don’t want too small a line angle you have to have a higher line tension.

I am fairly sure [1] is a solvable problem. You’d have to look at the specific architecture to decide if [2] might be solvable.

Higher speed and more tension = more wear I think.

Do you have a specific architecture in mind where that is needed? Do you think a scaled-up version of Kiweeone would need it?

What’s that?

Y tethered yoyo rig = dancing kites (?) as stated in previous discussions, so a sort of rotating system.

Decreasing the torque and increasing the rope speed with a gear for the rope-drive?

I dont understand this…

The gist is at optimum power of the given wing, either yoyo (with a dancing pair of kites, like Kite-X) and rotating torque based with a conversion to rope drive, must have essentially the same downwind pull and generate the same amount of power. Thus if one side of the rope drive is fully tensioned and one completely loose, the speed of the tether is essentially the same for either architecture.

Once you balance the force on the two tethers of the rope drive, the rope drive must run faster to transmit the power. But this is no big deal.

@Windy_Skies There are many ways to get enough friction on the rope drive without having any tension on the return rope. At least in theory, Im not sure if they are feasible

OK, but another concern: the yoyo kite is going downwind, losing 1/3 wind speed and corresponding force and power, while the rotating kite based with a conversion to rope-drive is stationary, with full wind speed. I can miss something.

A “Grandma’s clothesline” drive really “wants” to be a laddermill.
And a laddermill really “wants” to be a SuperTurbine™.
No matter what direction you approach AWE from, every groundgen system ends up “wishing” it was a SuperTurbine™. Today I went up to our local mountains skiing. On the way up I noticed the 3-phase powerlines along the road to the resort. I was thinking what a miracle that 3 thin lines of copper on wooden poles could be stretched over many miles and power that whole ski resort including all the lifts, snowmaking machines, lighting on the slopes for night-skiing, offices, lodges, bar, first-aid, etc., without any mechanical strain on the wires. Seemed like a miracle. Magic almost.
Yes the generator has weight, but a grandma’s clothesline drive can only transmit power in proportion to how hard it tries to pull itself downward out of the sky…

I think the 3 wires power the whole town actually…

The yoyo (Lift mode) is travelling downwind, experiencing 2/3 wind speed because of this.

The torque rig (drag mode) must be slowed down to 2/3 of the speed it would have if not producing any power.

This is why they are equivalent, except of course the return phase for lift mode and the Betz limit concerns (both important)

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You still need tensile strengt to withstand the downwind pull. My reasoning (unless flawed) is that whichever method you use, the tether cannot be much thinner for any architecture. So there doesnt seem to be a lot gained in terms of power transfer, as for lift mode the rope speed is already quite small. No method has a large advantage over the other.

I am also in the opinion that electric power transmission is a very good looking option. Increase the voltage and you dont need a lot of metal.

Actually, lift mode does have a huge advantage which is no airborne equipment and dual use of the tether for tensile force and power transmission.

@tallakt your explains make sense.

True. The kite rotation for rope-drive transmission is slowed down to 2/3 in a similar way as Makani’s wing is slowed down to 2/3 due to the turbines aloft drag, in a comparable way as yoyo is slowed down to 2/3, if I am not wrong.

On a previous message on Rope-drive transmission I evoked some planning to make the rope-drive to go faster than wind speed, without making any change about efficiency.

The no market Hypothesis :“We won’t loose efficiency with height as our belt is static in the sky, only friction with air is slightly increased ( 1.2 mm belt travelling at around 40 km/h )…”, so about 11 m/s with a wind speed probably lower, perhaps about 7 m/s wind speed for 100 kW https://www.facebook.com/pg/kitewinder/posts/. So it looks possible a gear increases the rope speed.

If it is implementable a faster rope-drive leads to a lesser torque and a faster and smaller ground generator.

The rope-drive transmission has a main advantage: a continuous power, so a management of only one flight mode instead of two for yoyo.

Yoyo has a main advantage: the simplicity, saving material.

The ones I am aware of are increasing the coefficient of friction of the pulley, changing the geometry of the pulley to let it better grip the rope, and various ways to increase the number of windings on the (single grooved/multiple grooved/multiple) pulleys. Testing and theory should inform what the drawbacks of those with respect to wear on the rope are.

If you by “torque rig” mean airborne HAWT lifted by a kite or balloon, the slowing of the turbine is already accounted for by the Betz Limit, or they’re saying the same thing?

I think (you are correct, but) it is probably better to just have a slow moving tether and put the gearbox on the ground rather than producing with a fast moving rope. The exception is that if your rig is already rotating at a high angular speed (RPM) then no extra gearing is required for fast rope transmission. For the larger diameter looping rigs, high RPM is difficult to achieve without an airborne gearbox.

But does seem true, that if you have a rope drive, then you will probably want to run it faster than 1/3 windspeed to make better use of tether strength (more equal tension in production and return tether).

To be honest I think an airborne generator is a simpler solution, then transfer electrical power like Makani does. Because the gearbox is probably similar size to a generator, and then you dobt need to deal with double tether, friction, extra ground equipment and such.

I also mentioned that to transition from torque based transmission in air to a rope drive, you need counterrotating rings, of an air anchor sort of like a helicopter tail or connecting rings in a mesh.

It sounds, but if I am not wrong for kiwee the (low ratio) unavoidable gearbox is settled between the hub and the right angle pulley (please @Kitewinder correct me) with the rope-drive. As the device scales, we can expect a (higher ratio) gearbox on the ground is better as heavy parts are not aloft.

Kiwee doesn’t seem to contain counterrotating rings thanks to the right angle pulley for rope-drive, or the rope-drive (or the turbine) is itself a right angle counterrotating part, in a similar way as the tail rotor in regard to the main rotor of an helicopter whose respective axes are at right angles.

Indeed.

The grip of a rope on a reel seems better than that on a pulley, doesn’t it?

Half a turn around a drum is not much for a UHWMPE tether (pulley). I think for serious power transmission more turns (eg serpent like running through wheels) or pinching wheels could be used. For the airborne part this is a challenge to put it mildly.

A reel or rather more turns on a single wheel has problems related to twisting the rope and the fact that the rope will be moving sideways on the reel. One could perhaps use this if the airborne and ground reels were twisting in sync, netting zero twist. This is more of a mechanical design question that I am not too competent to look at, but no doubt challenging