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## Sun, May. 31st, 2009, 10:13 pm Kite wind power

There are a number of different projects going on to harness wind power using kites. Most of them are utter lunacy, involving so many gizmos and unproven techniques that you might as well be trying to get power from lightning. That said, I have an approach which I think is plausible, but before even idly thinking about working on it I'd like to know if the whole concept of kite-based power even works on paper. Gathering the necessary information and running the numbers is a bit beyond my off the top of the head skills, so I'd greatly appreciated it if anyone would help me calculate the following:

For several points of interest on earth (particularly plausible/notable/typical places) calculate the following for both ground level and an altitude of half a kilometer:

What is the minimum, average, and max wind velocity? For the average wind velocity what would be the pull of a kite with an area of a hundred square meters? What would be the weight of the kevlar necessary to hold down something producing that much force at the maximum wind velocity? After factoring in holding up all that kevlar, could a kite stay up at minimum wind speed? How much power could be generated by its remaining upwards force? How much of that would be lost in the electrical generator? How does that amount of power output compare to the per capita max and average power consumption in the United States?

My suspicion is that flying kites at an altitude of more than a few hundred meters is simply not worth it, and that the optimal kite size in practice is around a thousand square meters, and that in an area where the wind never dies down a kite system compares quite favorably to a turbine system, once you get all the engineering problems worked out.

Also, I have a dumb question about controls - At what altitude do the sorts of control systems used for power kites simply stop working due to even very tensioned materials having a lot of slop at that length?

### Mon, Jun. 1st, 2009 07:30 am (UTC)ext_191161: Friendly Physics

Good questions for thermodynamics. The longer (less obstructed) the wind, the colder it blows. Our wind is between 3 and 35 m/s. In Europe there are most winds during the winter year when also demand for energy is highest. In any given year the wind energy resources may vary ± 20 %.
There's project I update about all forms of clean energy, techrotation.com
It computes with velocity for a vector and speed for a number. Flow and flux are 2 measures to distinguish and our basic Boyle's pV=nRT. Relative motion, Navier-Stokes unsolved equations and the Lorentz diffractor are open wind power related research I newly updated
best wishes
Niklasr (quantum version http://lh4.ggpht.com/_JY9JCk3uEtM/SFnw4hvYh3I/AAAAAAAAAFU/ctCEAnFGcoI/s576/Physics.jpg)
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### Mon, Jun. 1st, 2009 05:24 pm (UTC)wsc9tt

I had always assumed the kite was just a platform where you would mount turbines that would generate the actual power. Then the power is transmitted down the wire that is used to hold up the kite. So that says to me you are look at the weight of a long cable.

### Mon, Jun. 1st, 2009 06:02 pm (UTC)bramcohen

Having a generator mounted on the kite is in my lunatic ideas category.

### Mon, Jun. 1st, 2009 06:01 pm (UTC)bramcohen

That's the thing which I have an idea for. Here's a hint: Think about how funiculars work.

### Mon, Jun. 1st, 2009 06:13 pm (UTC)wsc9tt

So you have 2 (or N) kites on a large loop with the generator on the ground? Then the kites that are ascending are fully deployed for maximal lift and the descending kites are flown to have no lift and just stay stable as they are pulled down to the ground.

Neat picture, perhaps with the kites on short cables of the main loop so they never actually "land" and don't interfere with the pulleys on the generator.

### Mon, Jun. 1st, 2009 06:33 pm (UTC)bramcohen

Basically yeah, but having them go in a loop is dubious. Far better to just have two and alternate which is going up.

### Mon, Jun. 1st, 2009 06:39 pm (UTC)wsc9tt

Indeed, nice idea
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### Tue, Jun. 2nd, 2009 07:02 am (UTC)stellar678

http://en.wikipedia.org/wiki/MS_Beluga_Skysails

These people are, on some level, building exactly that. They claim 20-30% reduction in fuel use. Which is not cruising the oceans for free, but is pretty significant nonetheless.

### Tue, Jun. 30th, 2009 07:38 am (UTC)inpetto

Not the windiest place on Earth, but the windiest city is purported to be Wellington, New Zealand with an annual mean wind velocity of 22 kph. (FWIW. I know that's not everything you asked for, but it may be a useful data point.)

### Sun, Jan. 3rd, 2010 08:23 am (UTC)patrickfinger12: Kite Power

It is true that wind power from kites is tough to sustain. Makani has been attempting this for a while.

The generators and the kites themselves aren't that hard. And having the generator in the kite is not as bad as it sounds. The generator can also reverse into an engine to keep the kite in the air if the wind dies down a bit.

The control systems can fairly easily easily counter for any line slop and kite imperfections.

The bigger problem is that the materials just don't have the durability to stay flying under stress for too long (only a couple thousand hours).

PC