Hi DaveS,

Thanks for your comments. I’ll add a few more based just on the physics.

“ The faster a rope is driven, less tension and smaller diameter line is needed, under basic physics (f=ma).. “

That is true in general, but not when the rope drive is a loop belt tether. The loop belt must still be strong enough to act as the tether, which usually requires more strength than a fast moving loop belt that is only transferring energy.

“  Over the years, various AWE theorist-inventors have proposed, designed, and flown rope-drive AWES prototypes. KiteLab Portland's KiteMotor1 was a notable example flown at large public events in 2007. Lately, KiteWinder of France has developed a rope-drive AWES product ( video link below). There is no doubt rope-drive AWES work as expected, reaching altitudes higher than torque or electrical conductors have been able to. “

Since a loop belt is only a single line as it passes around the pulleys, it would be roughly twice the weight of a static tether to handle the same tether tension force.

“  It still remains to prove in testing and market acceptance just what manual AWES transmission method will dominate, but they have all been shown to work in prototypes.

By “manual” do you mean “mechanical”?  As opposed to electrical?  “Manual” implies that a person is part of operating the transmission, and I assume that is not what you mean.

“  Torque drives output power smoothly, but require rigid spars or tubes, which compound flying mass and safety risk, and limit practical altitude. “

Torque drives also require a lot more parts and assembly time, and some types can become easily tangled during launch or retrieval. Some types have a lot of drag. Stacked rings seem best with respect to resisting tangling, and they might be easier to assemble, but they are heavier.

Another issue is reeling in a kite, and most torque drives make that very difficult to do.

“  Pumping lines reach higher, as reeling AWES show, but require a recoil motion by either tandem kites or recoil spring-action or winching. Rope-drives also reach higher and output power smoothly, at a fraction of torque drive mass, but have a slack return-side.  “

There need not be much slack in the return side if the loop speed is high because kite drag will tension the return side. The tension due to drag will be much higher than the tension do to energy transfer from the kite to the ground.

The viability of torque tethers is in doubt if they must compete with loop belt tethers and ram air rotors. A ram air rotor moving at 6 times the wind speed can produce as much power as over 200 stacked HAWT rotors that are part of a rotary tether. Plus, ram air rotors connected to a fast moving loop belt tether can eliminate the need for a transmission (or eliminate the need for a large diameter, direct-drive generator), which is a major expense for larger scale kites.

PeterS

Hi DaveS,

"  Ten years ago, a physics mystery perplexed me: How does a standard rope hawser towing a ship at ~10MW of sustained power stay cool to the touch? Its transmitting all that power at effectively 100% efficiency, which is the criteria for super conductance! On the other hand, a conventional electrical conductor would heat up and be heavier to transmit so much power. What's going on?  "

The answer is that the rope is not doing any work. Check out the physics definition of “work” and notice that in the frame of reference of the moving system itself, the rope is not doing work. It merely connects the kite to the ship, as if they are all part of the same object. The kite, rope, and ship form a static system within that frame of reference, so there is no movement within that frame of reference.

“  Work is done when a force that is applied to an object moves that object. The work is calculated by multiplying the force by the amount of movement of an object (W = F * d).  “ – Physics4Kids.com

The rope tether is like a supporting cable of a suspension bridge, which is under great tension, but it is not doing work. Would you expect the supporting cable to get hot? If so, then where would the energy come from? If the supporting cable got hot, it would solve our energy problems, and we wouldn’t need kites.

The rope tether does not transmit power, as defined in physics. Power is defined in units of work per second. But the rope is not doing work, so it can’t transmit power.

The tension in the rope transmits a static tension force from the kite to the ship. In mechanics, a static force does not do work and so cannot generate power.

In practice, the towing force of the kite will vary, which causes periodic stretching and contraction of the rope, and that does generate heat and absorb heat, respectively, because stretching and retracting is doing work, as defined in physics. The kite does some work on the rope if the kite stretches the rope. If the rope were highly elastic, which would increase the stretching and retracting, you might be able to feel the rope become warm, as is the case when you rapidly stretch and retract a rubber band.

Although I am an amateur, I hope that my explanation helps to clarify.

I urge you to learn at least elementary physics before attempting grand unifying analogies in physics. I’ve seen that even your minor analogies, when based on your many misconceptions about elementary mechanics, are seldom sensible and edifying.

But I sincerely applaud you for asking a question. That’s a good beginning.

In my experience, elementary mechanics can be quite difficult when it comes to understanding some aspects of WECS. I’ve often seen even PhD engineers make mistakes due to over-confidence in, and false assumptions about, elementary mechanics. An example is the assertion that: It is not possible to sail directly downwind faster than the wind because one would run out of wind.

PeterS