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Confused about Vertical Windmills

  1. Dec 2, 2009 #1
    I'm thinking of building a small vertical-axis wind turbine as a project over school vacation. By small I mean maybe three feet across and three or four feet high. I'm trying to come up with the best design. This type of turbine is good at generating torque, not RPM's, so I want to try to maximize torque.

    This type of vertical turbine works on drag, not aerodynamic lift. So the "scoops" facing into the wind on one side catch the wind, swinging the turbine, while the scoops on the opposite side are aerodynamically shaped (enough) to advance against the wind. If you google this subject for pictures you will see that there are many different approaches: wide short ones, narrow tall ones, ones with many scoops, ones with a few.

    I'm trying to understand how the size, shape, and mass (rotational inertia) of the turbine influences the torque output. Now imagine two turbines that are exactly the same shape and size. One is made of balsa wood, the other is steel. In a good constant breeze the balsa wood one will whizz around easily but you'll be able to stop it easily = low angular momentum. The steel one will have much more angular momentum. It won't accelerate as fast, but once its up to speed it can do more work. I've read that an ideal turbine of this type can at best move as fast as the wind. So does that mean that over a long enough period in a constant breeze, the heavy steel turbine will end up moving as fast as the balsa wood? Or under the same conditions, will it always move slower? If I imagine two paddle-wheels, one of balsa wood and one of steel, in a moving stream of water, it seems reasonable that they will spin at the same rate. But is that just because water isn't compressible?

    This suggests I build a heavier one to get that extra torque. Is that right? But obviously I can build one that's too heavy, too. Since rotational inertia for a cylindrical shell goes up with the square of the distance from the axis, I should try to maximize axis distance even at the expense of total area of the scoops. So that would mean a short, wide turbine rather than a narrow tall one. Am I thinking at all straight here? Thanks.
  2. jcsd
  3. Dec 2, 2009 #2


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    This is the source of the confusion.
    When it's upto speed the heavier turbine stores more energy but it took longer to build up the energy.
    So you can extract more energy from the heavier turbine but once you slow it down it will take longer to get back upto speed - in simple terms both turbines will harvest the same wind energy.

    In practice it's trickier.
    It will be easier to connect the heavier one to a a generator because you have more torque available. But in a light wind it might never start moving if the force isn't enough to overcome friction.

    The light one will start spinning in any wind speed but it will be harder to couple the output to a motor.
  4. Dec 2, 2009 #3
    ...But the steel turbine will never reach the same RPM as the balsa wood turbine. Replace the steel turbine with a 120,000 Lbs. lead turbine with three inch-thick blades and you'll see what I mean.

    The lighter blade is always better than the heavier one from an engineering and design standpoint...
  5. Dec 2, 2009 #4


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    A higher angular momentum would smooth out variations in the wind. If there windmill + generator system includes a torque sensor combined with variable load, then the generator load could be modulated based on the torque sensor reading, compensating for a higher angular momentum windmill's reduce acceleration rate. With torque sensor feedback, I'm not sure what the optimum setup would be.
  6. Dec 2, 2009 #5
    Use whatever is cheapest and strong enough. The weight of the material is only confusing the matter. After the generator gets up to speed it has no effect whatsoever. At that point the only opposing forces are friction and the generator attached to the turbine. But the above post is a good point; when air speed is not constant the speed of the lighter material will adjust faster than the speed of the heavier material. Even so, if this is desirable you could always add a heavy flywheel (heavy wheel) to either setup.

    Air flow is a complicated problem, so you'll have to experiment to find the optimum turbine speed. Equal to the air speed would mean there is no resistance whatsoever, neither from friction nor the generator. And if the generator isn't pushing back then it's not drawing any power and thus not producing any either. Likewise if it the generator is completely stopped. The ideal is somewhere in between.

    Note that since the air is causing a force in one direction and the generator is causing an opposing force, whatever structure you use will have to handle that much torque without breaking.
  7. Dec 2, 2009 #6
    There are two basic designs for VAWTs (vertical axis wind turbines):

    1)Darrieus VAWT; eggbeater design, has airfoil blades.

    2) Savonius VAWT; air-drag design, slower and less efficient.

    Bob S
  8. Dec 3, 2009 #7
    Thanks for all your replies. The point (above) that the weight isn't really a factor definitely helps clarify things.

    Yes, I'm doing a Savonius design. I don't want to decapitate anybody on the lawn with long blades, and I want something compact I can move easily. So I'm thinking two thin plywood disks three feet across for the top and bottom. For the scoops I'm thinking cylindrical steel conduit used for heating ducts (six or eight inches in diameter), cut down its length, maybe three or four feet high. A 1/2 inch threaded steel rod would run down the center and drive the load. There's the issue of how many scoops: four seems too few, eight to many, so I thought six. I think this is a factor in evening out the torque. I keep trying to imagine what an ideal turbine would be like: would it have many closely spaced scoops? But if the wind is from a given direction, don't they then just block the wind from their neighbor?

    I've heard that you can use two toroidal Neodymium magnets as a bearing for this kind of turbine: the steel rod would go through them, the magnets would repel each other, and the turbine would be levitated.
  9. Dec 3, 2009 #8
    There seems to be a very nice feature of the two-scoop offset-cylinder Savonius design; air that enters one scoop crosses to the other scoop near the vertical axis, and exits the other scoop while transferring additional torque to it. But unfortunately, the two-scoop design is not self-starting (so I hear).
    Bob S
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