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It's well known that average wind velocity increases with altitude. There have been ideas to put floating wind turbines way up in the atmosphere to harness this. But I have yet to read anywhere of people just building bigger towers and putting traditional (although highly up-scaled) turbines on them.
Efficiencies (both the cost and energetic types) are supposed to go up with scale too. Not only that, but I've seen calculations that show that towers upwards of 1km are well within the reach current technology and materials. I propose a 2 km tower, with a hefty but very low density footprint, probably recursive truss construction and a significant trailing edge support to prevent blowing over. The rotor might be 800-1200 m in diameter, three-bladed as in current large turbines, also built of carbon fiber for weight considerations. Cost would be staggering, perhaps upwards of 3 billion dollars, but just look at the power involved. Say we have a 1 km rotor diameter, thats about 785000 m^2 swept area, about 100 times the largest units in the world, and we're looking at average winds of perhaps 3 to 5 times greater than what's available at 100 m altitudes in certain areas. That gives a total of 100 multiplied by, say 4^2 = 1600 times the kinetic energy available from traditional systems. I believe in Denmark or Germany they built a 5 MW turbine with a 100 m diameter, so were looking at 5 MW * 1600 = 8 GW power. Even if we have only 3 GW, and a 2 billion dollar price tag, I'd say were comparable in price with some of the giant nuclear plants, and certainly more cost efficient than Three Gorges damn, for example. Best of all, there's zero pollution, zero cost of fuel, and could be designed for very long life. Construction, however, might be difficult.
Sierpinski Pyramid
http://www.langara....004/pic_pyramid.jpg
lots and lots of tiny tetrahedra [Freefall, Jul 12 2005]
Wired: Windmills in the Sky
http://www.wired.co...,2782,67121,00.html
This might be what [elhigh] remembers reading. [Laughs Last, Jul 13 2005]
Tip speed ratios
http://www.itdg.org...nergy_from_wind.pdf
and other interesting stuff [Ling, Jul 15 2005]
MHD wind tunnel
http://www.mecheng....hd-wind-tunnel-view
Wind could be converted at high altitude from ions in the atmosphere. [travbm, Oct 29 2015]
[link]
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Staggering size, staggering cost of failure. Fishbone. |
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The failure prone part would be the giant generator on the rotor. The tower itself would probably not catastrophically fail - it could be repaired before it came down. Instead of one huge 1 km blade, the tower could be used to mount a large array of conventional windmills. If one went down, the operation would not be silent. |
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But this is a fine halfbaked scheme - the kind I like. Correct spelling, no whores, no war - good stuff. |
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Interesting. It sounds a bit like the Kiritimaticentrifugomobile on it's side. |
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I wonder if you could build a circular track around it, with a little car attached with a cable going up to the center of the turbine. Have the car automatically drive clockwise and counter-clockwise to face the wind. This will remove the need for the trailing edge support, and materials are stronger in tension than compression so you'd need less steel. |
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Perhaps the tower bit should be left out, and the giant windmill contructed on an existing mountain with a sheer 1 km face. Devils Tower and some of the pillars in Monument Valley are > 1km. The Matterhorn might also do. Haleakala on Maui does not have the drop, but certainly has the wind - the mill could be built semirecumbent. This would save a lot of money in tower construction and serve as proof of principle. The Haleakala mill would generate enough power that there would need to be some way to store it. Perhaps it could be used to reduce agricultural carbohydrates to straight chain alkanes, which could be shipped out by tanker. Hawaii would become an oil exporter. |
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That would reduce the cost, but I'm pretty sure that the turbine must be supported on a tower of negligible wind resistance in order to maintain an unobstructed path for wind to both strike and leave the turbine area. I've heard that even small 50 ft trees within 5 to 10 tower heights of some of the commercial models (which are over 200 feet high) can dramatically reduce efficiency. |
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I like Worldgineer's idea. Not sure if the tower would be designed to swivel or not, perhaps at high altitude the wind flow is relatively stable directionally, removing the need for a moving anchor for the cable. I assume the cable would mount to a swiveling joint at the nose cose tip. |
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A recursive truss? As in a "Sierpinski pyramid" (see link)? |
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You may be up above the trees, but with one very large rotor, you'd be getting into layers of varying wind speed and direction. Stick with a lot of smaller rotors. |
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I like the bungston method. |
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Yeah, baconbrain is right. The big problem with the large rotors is that they bend and also stress the bearing. Wind shadow is a particular problem (caused by the supporting structure). |
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<plug> My kite pump idea uses high speed winds to get power, but with minimal structural requirements (but my presentation style failed). |
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What I like is these concepts all get the rotor up well above birdblender altitudes. At the levels under discussion, the only birds these things might hit would probably be mythical. |
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I saw a concept that used a "kite generator," where the generator acted as a motor to get a sort of tethered autogyro into the sky up to jet stream, then the jet stream itself provided enough power to keep the rotor turning and generating power, and the excess power was sent down the tether. |
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I like freaky big ideas. Bun! |
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This is a nice idea in principle. I would be interested to know which parts of the structure would be hard to scale up. The only limit on building height these days is budget - but I don't know about tall buildings under this much stress. As [Ling] points out, the bearing would be hard to scale up, [bung] - what would be the problem with a big generator? Overall this must be a bun. |
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/ [bung] - what would be the problem with a big generator? / |
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Nothing intrinsic. Small ones could be bought "off the shelf", with ready expertise and support to fix and maintain them. The big one would be unique, and require custom fixes and parts. Also if you have to stop the big one to fix it, you lose money. If you have to stop a little one to fix it, all the rest can keep working for you. |
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That logic can be applied to all parts of this 'big' idea, except for the basic principle that it needs to be high up. |
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Rotor tip speed/wind speed ratio is normally max 4. It's not much different in concept from a glider. |
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[Ling], "Rotor tip speed/wind speed ratio is normally max 4"? Say what? Where did that number come from? |
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I have had an avid interest in alternative power ever since I knew how to spell it, and I usually see a tip/wind ratio on the order of 10 or so. The relatively new TVA installation in TN is a good example, showing tip speeds around 125 mph in a prevailing wind of around 14 mph. These are enormous German or Dutch mills, so you know the engineering behind them is anally sound. |
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Anally sound engineering???!! |
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The engineering will be terrific. A standard 5MW machine has a THM (Top Head Mass) i.e. all the stuff sitting on top of the pylon, of 400 tons. 1600 times this, or even 1000 times, accounting for scale efficiency, you still get a supertanker sitting half a mile up. Perhaps a gigantic helium balloon in the stratosphere thethered to the top to alleviate the problem? Anyway, a colossal croissant for thinking big. + |
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I'm an anal sound engineer. |
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Could we possibly have a small head mass with just a simple gear and a really long drive shaft down to the base which then drives a generator? Also, I doubt that the THM scales linearly with power capacity. Not an ME, so I have no idea how beefy the shaft would need to be. (anything thicker than a couple feet would weight too much i'd imagine) |
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So far the general consensus is that one would need to break down into many smaller turbines, and I must say, they are probably right as far as practicality goes. |
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However, from an aesthetic point of view they are wrong. |
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[elhigh], Ah, yes, two blades go up to about 7 TSR. Perhaps technology has improved the aerofoil design so it is even higher than that. My bad memory gave me the ratio of 4 from my favourite - the Darrieus, where I made plans for my father. This was based on an H shape to make it easy to design the aerofoil. |
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Those vertical axis machines never did have the TSR of a horizontal axis mill. Don't forget the TSR of a Savonius: never higher than 1, usually significantly less. |
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I did see a hybrid Sav./Darrius, where the Sav. spun the D up to its dynamic threshhold, then the D took off. |
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I once saw a site where they had cross connected the D. to a binomial multiplexer proving I/R response!! We did all giggle when we saw the D outputs they were getting! And their Sav rating made Darius look like a lame pop star. What a weekend that was. |
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I worry about anything that will ice up in winter when it's really needed. |
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