Jed Rothwell wrote:
I do not think those gigantic kites would require ultra-strong tethers. They would not pull on the tether much. Most of the energy of the wind is dissipated either in lifting the kite or turning the electric generators.
Whoa. You are confusing force with energy. The tether provides a force, which can't be "dissipated"; it provides no energy (from the frame of reference of the ground).
The _force_ exerted upward on the kite is presumably balanced by the weight of the kite and the cable. The _force_ dragging the turbine downwind, on the other hand, is balanced only by the cable. If we ignore drag and consider only the useful part of the wind's force, we can get a vague idea of how much force must be required of the ground station.
The work done by a packet of air on the turbine (which is where you're getting the energy from) must go as force*distance, but the "force" in question is going to be perpendicular to the direction of the wind, which means that in order to figure out the precise longitudinal force one must know a lot more about aerodynamics than I do :-) However, there's a simple reality check we can apply: Suppose the air is stationary, and the turbine is being dragged along by the cable. Then it's obvious where the energy is coming from: It's coming from the cable, and the generated power must be no larger than F*V where F is the force on the cable and V is the velocity at which the generator is being dragged. Unless you've got some way of cooling the breeze passing over the fans and turning the extracted heat into electricity, there's just no other energy source in the picture.
So, to turn that around, the force on the cable must presumably be, at a minimum, the power generated, divided by the wind speed. Note that this minimum value is independent of turbine design or shape of the kite! (Somebody with more time and more facility with the various units involved should be able to come up with some actual figures here.)
If you can get the kite up into the jet stream, the force on the cable required to generate a given amount of power should, therefore, be much smaller than the force required as long as it's in the lower altitude, slower breezes!
Finally note that the tension in the cable is going to be at a maximum at the kite, where the weight of the cable is added to the force exerted by the ground station.
Note also that autogyro analogies are likely to be misleading because the vertically oriented turbine in an autogyro does no work on the plane -- a craft in level flight needs no energy to stay up. All it needs energy for is to overcome drag.
The kites would work like autogyros. (See: http://www.jefflewis.net/autogyros.html)
The tether has to overcome drag. If a high-altitude balloon were tethered to the earth, and it had on-board wind turbines, I think it would require a very strong and heavy tether. A balloon or airship has enormous drag. With the kite, drag is low. In other words, the tether supplies as much force as the motor in the autogyro, and if the wind produces much more lift than the kite needs to stay in place at a fixed altitude, the extra energy is converted to electricity and sent down to the ground. When the wind drops off too much and the kite begins to fall, power is sent back up to the kite to turn the rotors and keep it up.
The New Scientist recently described a scheme to use gigantic kites to pull freight ships, to augment the diesel engines. Look up SkySails for more information. 19th-century freight ships carried sails long after they also had onboard steam engines. Some people nowadays imagine this was foolish and redundant, but actually it was economical. Sails and slow steam engines together worked well. Also, the early marine steam engines were not reliable. If the engine, paddle wheel or the propeller shaft broke in the middle of the ocean they needed the sails to get back home. Paddle wheels were too fragile for ocean-going ships. They were often damaged in storms.
- Jed
