This is my first post here, though I've been visiting fieldlines and otherpower for several years. I see you received mixed responses on the details of the setup mentioned. I have not worked hands on with counter rotating wind turbines, but I have investigated and worked with counter rotation on propellers for rc toys, small UAVs and aircraft. There are indeed pro's and con's to such a setup, especially on aircraft where weight is critical. But when used properly, counter rotating props exhibit substantial efficiency and controlability benefits over a single propeller in aircraft.
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Contra-rotating (or Counter-rotating) propellers use a second propeller rotating in the opposite direction immediately 'downstream' of the main propeller so as to recover energy lost in the swirling motion of the air in the propeller slipstream. Contra-rotation also increases power without increasing propeller diameter and provides a counter to the torque effect of high-power piston engine as well as the gyroscopic precession effects, and of the slipstream swirl. However on small aircraft the added cost, complexity, weight and noise of the system rarely make it worthwhile.
- http://en.wikipedia.org/wiki/Propellor
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As my interests in wind energy have increased, I've been realizing all the similarities between aircraft and wind turbines...and through my many late night searches across the internet, I've came across some interesting related information.
Take a look at this PDF:
http://www.jstage.jst.go.jp/article/jsmeb/49/2/450/_pdf
Like everything, you have to be careful what you take from someone analyzing their own creation, and there's some hype in there. But it seems to offer some benefits. They use a co-axial style, counter rotating setup. One drive shaft runs inside the other, and each powers it's own generator. The second prop is rotating in the opposite direction as the first one, and it's cashing in on the swirling motion of the air as it interacts with the first prop.
Someone responded to your post proposing you try 1000 props in a row. This is not what is being suggested here. I am just looking at the advantages of a second prop turning the opposite direction of the first. Nor did anybody say they wanted to get more power out of the air than is possible with a given swept area. We are just looking to increase the % of power that we pull from that given swept area.
Here's some more information regarding counter rotating wind turbines:
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Counter-rotating horizontal-axis turbines
Counter rotating turbines can be used to increase the rotation speed of the electrical generator. As of 2005, no large practical counter-rotating HAWTs are commercially sold. When the counter rotating turbines are on the same side of the tower, the blades in front are angled forwards slightly so as to avoid hitting the rear ones. If the turbine blades are on opposite sides of the tower, it is best that the blades at the back be smaller than the blades at the front and set to stall at a higher wind speed. This allows the generator to function at a wider wind speed range than a single-turbine generator for a given tower. To reduce sympathetic vibrations, the two turbines should turn at speeds with few common multiples, for example 7:3 speed ratio. Overall, this is a more complicated design than the single-turbine wind generator, but it taps more of the wind's energy at a wider range of wind speeds.
Appa designed and demonstrated a contra rotor wind turbine in FY 2000-2002 funded by California Energy Commission. This study showed 30 to 40% more power extraction than a comparable single rotor system. Further it was observed that the slower the rotor speed better the performance. Consequently Megawatt machines benefit most.
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There are also some other dual rotor setups I'm sure some of you are aware of...these are two roters on a single shaft, sparated by a good distance, and the shaft is angled. This effectively increases swept area, and the makers claim that the disturbed air coming from the top of one prop gives a similar boost effect to the bottom of the next prop, since the those parts of the props are travelling in opposite directions. Here's some more info:
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Co-axial, multi-rotor horizontal-axis turbines
Two or more rotors may be mounted to the same driveshaft, with their combined co-rotation together turning the same generator -- fresh wind is brought to each rotor by sufficient spacing between rotors combined with an offset angle alpha from the wind direction. Wake vorticity is recovered as the top of a wake hits the bottom of the next rotor. Power has been multiplied several times using co-axial, multiple rotors in testing conducted by inventor and researcher Douglas Selsam, for the California Energy Commission in 2004. The first commercially available co-axial multi-rotor turbine is the patented dual-rotor American Twin Superturbine from Selsam Innovations in California, with 2 propellers separated by 12 feet. It is the most powerful 7-foot diameter turbine available, due to this extra rotor.
http://dualrotor.com/
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Alright, that's enough info for tonight. I haven't had time to do any testing myself yet, but it seems like a niche that could use some more exploring. I hope this sheds some light in that direction.
As is said a lot around here...Have fun!
Chad