extra second smaller rotor on wind turbines increases their efficiency

Originally posted by DeepThought
I suspect that the main engineering reason to have them connected to the same shaft is to keep it simple.

But it simply wouldn't work. A smaller blade has to go at a different speed to be efficient.

Originally posted by twhitehead
But it simply wouldn't work. A smaller blade has to go at a different speed to be efficient.

The loss in efficiency has to be pretty drastic for it to not work at all, that's to say to act as a brake rather than a contributor to the torque on the drive shaft. Plausibly the larger front rotor takes so much energy out of the wind that the rear rotor is rotating at the most efficient speed. I'd hazard a guess that the size of the rear rotor is chosen so that it's rotating at the right speed given the amount of energy they expect the front rotor to take out of the wind.

Originally posted by twhitehead
But it simply wouldn't work. A smaller blade has to go at a different speed to be efficient.

To be as efficient as the large primary rotor...yes. I'm going to go with Deepthought, does it have to? They most likely talking about capturing a portion of 5% of the energy captured by the large rotor. The extra effort in design, maintenance, potential retrofitabillity most likely render the secondary turbines operation at maximum efficiency unviable.

Originally posted by twhitehead
But it simply wouldn't work. A smaller blade has to go at a different speed to be efficient.

Also after a second read I noticed this paragraph under the flow model figure

"This large eddy simulation shows air going through a dual-rotor wind turbine. (The three lines at the front are the blades of the main rotor; the secondary rotor is embedded in the ring near the center.) By tailoring the rotation and turbulence behind the turbine, Iowa State engineers say the dual rotors can boost the recharge of wind loads. And that can improve the energy harvest of wind farms. Credit: Anupam Sharma"

I take from this that the goal isn't for maximized efficiency of an individual turbine, but increased efficiency of the entire farm, by reduction of downstream turbulence. As for how "tailoring the rotation" will be achieved, your guess is as good as mine.