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  #21  
Old 11-23-2007
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Quote:
Originally Posted by Idiens View Post
There is an impressive line of turbines east of Copenhagen. See Google Earth

55 41 08.14 N 12 40 13.95 E

They show up nicely from above
Thanks for those co-ordinates Idiens, very impressive when viewed in Google Earth as a fly-over - the detail is very good when zooming in. There are also several photos linked, this haunting image being one:

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  #22  
Old 11-23-2007
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Quote:
Originally Posted by Diva27 View Post
Otherwise the outer part of the blade would be stalling like crazy. Maybe this reduces noise as well by limiting turbulence.
That's not the reason the blades are twisted.

They are twisted to ensure that lift at the tip is equal to lift at the root.

This is achived by twisting as you say, thus reducing angle of attack and aspect ratio, guaranteeing that lift os proportaional and the same along the lenght of the blade.

If this was not so, the blade would colapse and fold, due to the greater lift present at the tips.
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  #23  
Old 11-23-2007
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I'm opposed to anything powered by the wind.
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  #24  
Old 11-23-2007
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I think wind power is generally a great idea.

There are some concerns with the fact that the best wind patterns tend to also be the most favoured bird migration pathways, a big concern on Lake Ontario, where two of the four North American "skyways" for birds converge.

But I can live with a few bird strikes if it keeps a few fossil fuel plants from being built, because these plants are obviously the source (if not the only one) of particulates, sulphur and C02 emissions. The "brown band" around the Toronto skyline in summer is largely due to American coal plants southwest of here, although there is a large coal plant on Lake Erie that contributes a portion.

One of the criticisms of wind turbine power is its intermittant nature. If used to feed directly into the electrical grid (after transformation to AC, one assumes), this criticism is correct. What needs to be built is a storage facility that includes co-generation in concert with the wind power.

This could take the form of electrolysis of water and the storage of hydrogen in fuel cells (static fuel cells are far more practical at the moment than the ones being developed for cars), or for use to boil water for steam turbines. Another use would be to power the pumps involved in geo-thermal extraction of heat energy, which would also power traditional turbine generators. A less desirable method would be to use the wind power to reform natural gas.

In other words, when the wind blows, you convert the power gathered into some medium that is able to be stored and used when the wind isn't blowing.

Of course, this is precisely how it works on a boat with a large battery bank.
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  #25  
Old 11-23-2007
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The twist does incidentally reduce noise caused by stalling at the tips -- on many smaller turbines, the blade tips actually have a negative angle of attack, because their apparent wind is essentially in the plane of the blades, the tips are moving so fast. Some of those machines have tip-speed ratios of eleven or higher (tips moving eleven times actual wind speed).



About 35 miles north of my house in Wyoming is Foote Creek Rim, home to one of the largest wind farms in America. At last count, 183 big Mitsubishis on a mesa near Arlington. We also have large farms in Medicine Bow and on the Terry Bison Ranch south of Cheyenne. I fancy them. They look quite splendid along the skyline, like kinetic sculpture. But I'm biased -- wind power drives my entire house and business.

Designers could do a better job of blending the turbines into the landscape without making them significantly more a navigation hazard; visual impact is the number one complaint against them. Noise is the second, but that's not too honest: I've stood beneath the Arlington beasts in full flutter, and they are not loud. Had trouble hearing them over truck noise from the highway, which no one seems to notice. Third gripe is some vague notion of bird mortality. This perception, that wind machines puree birds by the thousands is provably false, and nearly all the websites devoted to hand-wringing over the issue are, provably, financed by the fossil fuels industry. The raptor mortality rates at Arlington are 0.03 birds per tower per year; more birds probably die on our front picture windows.

Some sailors use sea-based turbines as lighthouses and racing buoys. I love the idea of giving each tower a distinct code -- maybe the nav lights flashing colors? Blue-white-white means the twelfth to shoreward, style of thing?

Here's a funny Casper Star-Trib article on the Arlington installation: note the date (late October 2007), and the 105-mph wind speeds. Yeah, I recall that couple of days. It was pretty windy. (Now y'all see why I wanted a boat with some ballast?)

As for the "not in my back yard crowd": clean up your back yard first, then you'll have some standing in the matter. Anyone burning thirty-five kWh per day of electricity is in no position to pick and choose its provenance.
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  #26  
Old 11-23-2007
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Quote:
Originally Posted by Giulietta View Post
That's not the reason the blades are twisted.

They are twisted to ensure that lift at the tip is equal to lift at the root.

This is achived by twisting as you say, thus reducing angle of attack and aspect ratio, guaranteeing that lift os proportaional and the same along the lenght of the blade.

If this was not so, the blade would colapse and fold, due to the greater lift present at the tips.
Much thanks for the explain. But I did think apparent wind was also at play. In multielement wing sails for C-cats, they introduce twist in the top to prevent stallage that would otherwise occur. Apparent wind is higher at the top of the mast (as in all sails) and from a different direction because of wind gradient.But with rotating blades maybe this is less important than the lift loading issues.
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  #27  
Old 11-23-2007
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Quote:
Originally Posted by Valiente View Post
I think wind power is generally a great idea.


One of the criticisms of wind turbine power is its intermittant nature. If used to feed directly into the electrical grid (after transformation to AC, one assumes), this criticism is correct. What needs to be built is a storage facility that includes co-generation in concert with the wind power.
One of the solutions wind power proponents have is that you put enough installations across a wide enough geographic area so that there never is a serious or persistent wind shortage. "It's always windy somewhere." The key is to feed a shared grid, not to rely on one installation to provide power only to its immediate vicinity. And if solar is part of the mix, then you hopefully have sunshine somewhere when the wind's not blowing, and conversely wind when the weather is cloudy.
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  #28  
Old 11-23-2007
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[QUOTE=bobmcgov;227081]The twist does incidentally reduce noise caused by stalling at the tips -- on many smaller turbines, the blade tips actually have a negative angle of attack, because their apparent wind is essentially in the plane of the blades, the tips are moving so fast. Some of those machines have tip-speed ratios of eleven or higher (tips moving eleven times actual wind speed).

I was at a sailing club meeting last night and we somehow afterwards got on the subject of onboard wind generators. A friend told me about being up in the North Channel and visiting aboard with a guy who had a wind generator mounted at the stern. The guy was proudly telling him about how many amps the thing could put back into his battery bank, and my friend could barely make himself heard over the noise of what sounded like a small Cessna bolted to the boat.
I too have heard that the sound from the big generators is fairly benign, more like gentle ocean surf. Setting standards for acceptable noise levels and residential setbacks is a really sticky issue. The wind farm industry in Ontario argues that there's no one-size solution, that each installation has to be judged individually for setbacks. But they also argue that background levels of 40-50 dB should be expected at the setback perimeter. I guess the objections up on Georgian Bay from the cottagers (above and beyond their NIMBY predilections) is that when you live somewhere that you can normally hear a pin drop, suddenly introducing a steady 40-50 dB of whooshing noise is going to be noticeable. (That would also apply to cruising anchorages.) But I bet the wind in the pines cranks out something at that level. It always sounds windier along the bay's coast than it usually is because of the way the pines generate sound.
I'm with you on the aesthetics. I generally like the look of them.
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Last edited by Diva27; 11-23-2007 at 03:48 PM. Reason: typo
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  #29  
Old 11-23-2007
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Diva-

I think that the larger wind gens tend to be less noise since the blades don't have to move as quickly to produce electricity. That is why many computers have gone from an 80mm to a 120mm cooling fan... since they move the same volume of air but spin slower and are less noisy as a result.
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  #30  
Old 11-23-2007
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Quote:
Originally Posted by Diva27 View Post
Much thanks for the explain. But I did think apparent wind was also at play. In multielement wing sails for C-cats, they introduce twist in the top to prevent stallage that would otherwise occur. Apparent wind is higher at the top of the mast (as in all sails) and from a different direction because of wind gradient.But with rotating blades maybe this is less important than the lift loading issues.
Always learnin.'
My vector math was wrong. With the C-cat, increased wind velocity due to gradient or sheer at the masthead moves the apparent wind direction aft, thus requiring twist to avoid stalling. With the rotating blade, true wind speed is constant but forward motion changes from root to tip. That means with the blade moving faster at the tip than the root, the apparent wind direction moves forward, as has been noted.
I would think they manage loading by changing the lift area along the length of the foil. Lift force varies in direct proportion to the square of velocity, so you need very small blade area as you move toward the tip to produce the same amounts of lift that you do near the root.
That's enough aerodynamic theory of out me for a while.
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