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Harvesting Energy in the Sky
withoutfeathers writes "The Economist magazine has an article on Flying wind farms. Mind you, we're not talking about ordinary, terrestrial windmills here. We're talking about actual airborne — up to 10km in the sky — wind farms intended to harvest the immense supply of energy in the jet stream. On the surface, the idea seems a little eccentric but, in fact, San Diego (California, US) based Sky WindPower has, apparently, thought their concept through pretty thoroughly and believes they can not only make this work, but do so profitably. The article discusses several other ideas for high-flying wind farming including a Dutch proposal to use pairs of kites to drive a generator."
"Is it even possible for us to tap enough power from the jet stream (or other high altitude winds) to cause problems?"
No. The total power we could possibly harvest with systems like those in the article is not worth mentioning in the scale of the total energy in the jet stream. Windmills take a few percent of the energy of the wind that actually passes over them, wich would only be a tiny fraction of the wind in the jet stream.
Update: Just for kicks, I Read The Fine Article. In it, it is estimated that 1% of the power in the jet stream would power the entirety of human civilization. Not that you'd ever get that much, but again, not a problem.
That situation exists today in the form of tethered high-altitude balloons used for border surveillance...I believe there are 15 of them along the US/Mexican border. They have to be charted carefully, but so far the aviation community has dealt with them.
rj
Less than 9.81 m/s^2 I would expect. They are in atmosphere and I would expect a large part of each to be pretty undense. (what is the antonym of dense?)
As the Americans learned so painfully in Earth's final century,free flow of information is the only safeguard against...
Windmills take a few percent of the energy of the wind that actually passes over them, wich would only be a tiny fraction of the wind in the jet stream.
Even of the wind that actually passes through the area "swept" by the blades, the max it can harvest is about 59.3%. This is the "Betz Limit", the aerodynamic counterpart of the Laugher Curve of government revenue versus tax rates:
- Extracting power slows and deflects the air.
- Slowing and deflecting the air reduces the amount of moving air you can extract power from.
- Don't slow/deflect it and you get no power, stop it completely and you get no air - and thus no power. Zero at both ends, non-zero between. Somewhere there's a maximum.
- The maximum (for compressible fluids in free space) is where you extract 16/27ths of the energy from the air you affect (which is essentially the stream of air that passes through the area swept by the blades).
Real turbines can get very close to that, and most of the shortfall is a bit of energy left as rotation and turbulence in the wake.
Bantam Dominique roosters crow a four-note song. Once you've heard it as "Happy BIRTHday" you can't NOT hear it that way
"Perhaps you are neglecting the Butterfly Effect."
I am not. I have a degree in Mathematics in which I made a particular study of chaotic dynamical systems. I've written papers about the Butterfly effect; I've constructed physical models that demonstrate it. Let me tell you about the Butterfly Effect, so that you can refrain from bringing it up in discussions such as this in the future:
In a system which exhibits sensitive dependence on initial conditions (such as the weather), you cannot predict the details of long-term behavior (will there be a tornado in Iowa exactly 1 year from today) because tiny variations, well below what your measurement of the system could possibly account for (such as the breeze generated by the flapping of butterfly wings) will cause reality to drift further and further out of synch with your model until there is no resemblance on the detail level.
So the butterfly effect makes it impossible to ever predict what day it will rain months in advance, for example. But it does not prevent predictions about the aggregate, macroscopic behaviour of the system as a whole. In Meterological terms, long term weather prediction is impossible, but short-to-mid term climate prediction is easy.
Lets be ridiculously generous, and say this system takes a thousanth of a percent of the wind energy in the jet stream out. Is it reasonable to suppose this might cause significant changes in the world climate that will make a huge difference in its suitability for humans? No; it is not remotely reasonable. It's just not enough energy to make much difference.
Would it mean sometime in the future there will be a thunderstorm one day and not another? Absolutely. Whether you exhale the next breath you take slowly or forcefully means exactly the same thing; the minute difference in the velocity of a few thousand molecules of air your breathing pattern makes will eventually mean the difference in what day you get a thunderstorm.
The relevance of the Butterfly Effect in deciding whether to build this wind farm is the same as its relevance in deciding how forcefully to exhale your next breath. It means that the exact effect of either cannot be predicted, and that's it. It's not a reason to not do anything. (Well, except things like attempting long term prediction of weather detail.)
Hope that helps.
You're thinking of the Laffer Curve, and it's been mostly discredited in the economics world AFAIK
It just so happens I have a bit of experience designing aluminum and graphite aircraft parts, and my brother is an EE so by osmosis I know enough electrical stuff to fake some calculations.
Thing is, for a constant-diameter cable of a given density and a given strength, the length that can hang under it's own weight is an intrinsic property. For example, I would guess electrical grade (fairly pure) aluminum has a strength of at most 10,000 psi, and a density of .1 lb/in^3. The maximum hanging length would be 1.6 miles. If you taper it you might double that.
That doesn't even include the pulling loads from the monster at the top, which would be large. So aluminum is out.
Carbon nonotubes are hocus-pocus for real-world stuff right now, so forget about that.
However, graphite fibers conduct electricity not too bad, they might work, and they have fabulous strength in tension.
You could probably load a carbon fiber cable up to at least 100,000 psi, and it's density is .06 lb/in^3. That gives a hanging length of 26 miles. That looks better.
Let's fake an electrical calculation to see if it has a chance to work as a transmission cable.
If we use 10KV then for 10MW we have 1000A. The resistivity of graphite fibers is about 4 micro ohms-in. So lets say we make a 1 in^2 section cable, the resistance would be 4 micro-ohms/inch. A 10 mile length would have a resistance of 2.5 ohms and the power dissipation would be 2,500 watts. Surely we could dissipate that over about 10 miles without it overheating the cable.
The weight of the cable would be 1*10*5280*12*.06 = 38,016 lbs (19 tons). Since you need two of them, the total cable-weight load on the monster would be 38 tons.
Add to that the air loads of the horizontal windmill action (which is the whole point of the stupid thing) and what I'll call the catenary multiplier effect for lack of a better term, and the actual load on the cables will probably be something on the order of 100 tons. Since the cables were only stressed to about half their capabilities by the hanging load, it might work.
But you see why I call it a monster. The rotors have to genterate 100 tons of (inclined) lift.
(I used a 10 mile cable length throughout because while the altitude is about 6 miles, the monster is blown sideways and the cable hangs in a catenary shape.)
Equine Mammals Are Considerably Smaller