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Wind turbine efficiency question

  1. Jul 31, 2012 #1
    If we look at wind turbines used for power generation, they typically have 3 blades, and the blades are very slender compared to the blades of a house fan. I don't understand how such a slender blade can capture a significant amount of the wind's energy. The energy available in the wind is proportional to the swept area under the blades. But it seems to me that only a small fraction of the circular area is actually occupied by a blade, so most of the air will just pass through the circle without hitting a blade.

    So, how are these turbines able to extract a significant fraction of the available energy in the wind?
     
  2. jcsd
  3. Jul 31, 2012 #2
    The rotational inertia has to consume some of that wind energy, so lighter is necessary
     
  4. Jul 31, 2012 #3
    The blades move much faster than the wind. All the air that goes through will get close to a blade.
     
  5. Jul 31, 2012 #4
    please explain how the rotational inertia consumes wind energy.
     
  6. Aug 1, 2012 #5
    You can find the detailed answer here. The short answer though is that it's the same reason that glider planes have so long and slender wings.

    If I understand it right, You'll find it explained there that the more blade surface is in the way, the more the air pressure accumulates in front of the blades, this will tend the air to flow outwards around the blades, which means loss of efficiency. That wind is simply lost. You want to minimize wing tip vortices, one way is to have the least possible amount of wing tips and that can be achieved by long slender wings.

    Another effect, not there, is that the higher the wings can reach the stronger the windspeed. But that's maybe only a side effect.
     
    Last edited: Aug 1, 2012
  7. Aug 1, 2012 #6

    NascentOxygen

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    Not on the big ones I see arrayed along hilltops. Lots of wind can get through the swept area after one blade passes and before the next comes by. If the bladetips swept closer to earth, even I could jump through safely (I think :wink: ).
     
  8. Aug 1, 2012 #7

    sophiecentaur

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    The blades have a large volume of disturbed air trailing after them. You have to avoid the following blade getting involved with this disturbed air. So their effective size is a lot bigger than their apparent geometrical size.

    The problem with all forms of turbine is that, once the energy has been transferred from the moving fluid, you still have to get rid of the fluid, downstream, to make room for the next lot. That means it must still be moving, representing unused Kinetic Energy.
     
  9. Aug 7, 2012 #8

    A.T.

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    This guy flies his RC-helicopter between the spinning blades (at 4:15), despite the turbulence.

    https://www.youtube.com/watch?v=pz2zU5bNcaQ
     
  10. Aug 7, 2012 #9

    sophiecentaur

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    This is a bit counter-intuitive, I guess but the sort of turbulence that would affect the efficiency wouldn't necessarily be high speed, wind-tunnel stuff, it would involve air speeds around the same as the normal wind speed. The helicopter should be able to handle that easily. The pilot still needs to have his wits about him though! You would feel a bit of a plonker if you flew into a blade.
     
  11. Aug 7, 2012 #10
    Why is a pinwheel toy any different? It uses a lot of surface area to get hit by a lot of wind.

    http://en.wikipedia.org/wiki/File:Green_pinwheel.jpg
     
  12. Aug 8, 2012 #11

    haruspex

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    Whatever the aerodynamic considerations, you certainly couldn't scale up a pinwheel toy design. The blades would be far too heavy. So one answer might be that you want a large catchment area but a light structure.
     
  13. Aug 8, 2012 #12

    cjl

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    Pinwheel toys aren't really designed for optimum efficiency - they have other considerations. A maximally efficient pinwheel toy would look somewhat like a wind turbine, but the long, narrow blades would be quite a bit more fragile than the stubby, fat blades that they actually use. The narrow blades would probably make it more prone to having a small child poke him/herself in the eye too.

    As stated above, the blades actually affect a fairly large region around them, so the end result is that a much larger fraction of the energy is extracted than you might think. They can never extract all of the energy, since as mentioned before, the air needs some residual velocity in order to allow for new air to flow in, but they can extract a pretty significant (and close to theoretically optimal) amount.
     
  14. Aug 10, 2012 #13
    Does anyone know of experiments conducted to confirm the theory cited above?
     
  15. Aug 10, 2012 #14

    AlephZero

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    The force on the blade doesn't come from it "being hit by the wind". It comes from changing the direction of the wind, (i.e. changing the momentum of the air flowing through the turbine.

    The amount of air affected by the blade extends a long way out from the actual blade. in the same way that an airraft wing affects the airflow for a large distance above and below the wing (typcally many times the front-to-back length of the wing itself).

    Apart from the increased weight, a turbine where the blades "block off" a significant amount of the area is less efficient because the wind doesn't go through the turbine at all - it just blows around the sides, the same as it would if there was a complete "blockage" like a building.

    A pinwheel toy isn't designed to be efficient, but cheap to make. It doesn't have to produce any power except to overcome friction. For small wind turbines (e.g. less than 1 meter diameter) the total amount of power available is low anway, so if you can get say half the maximum theoretical efficiency for one tenth of the cost, that's a very good tradeoff. But for large turbines it pays to design them as efficiently as possible which is why they have a small number of carefully designed blades.

    The blade tip speeds on big turbines is faster than you might think, looking at them from a distance. For a 40 meter long blade rotating at 10 rev/minute, the tip speed is more than 80 mph, which is a lot faster than the average wind speed.
     
  16. Aug 10, 2012 #15

    NascentOxygen

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    Windmills used on farms to lift underground water have a swept area almost filled with vanes. http://www.thebackshed.com/windmill/articles/SouthernCross.asp Perhaps this reflects markedly different design criteria, with farm windmills being required to operate with almost daily regularity to ensure water tanks are kept filled. This means the mills are expected to turn during even slight breezes, since in inland areas brisker winds may be absent for months on end. Plunger-type water pumps have no minimum speed of operation, so even a slow-turning turbine is effective in raising water, albeit at a slow rate.
     
  17. Aug 11, 2012 #16
    Let us consider a 10 m long board standing up in the wind. Near the board there is stale air. If this stale air is sucked or blown away from the board, large amounts of slowly moving air can be produced.

    This is how a 10 meters tall and 0.1 meters wide board can slow down an air mass that is 10 meters wide and 10 meters tall.

    One board moving at speed 10 m/s transverse to the wind causes the air to lose kinetic energy about the same amount as ten similar boards moving at speed 1 m/s transverse to the wind.

    With fast moving narrow board compressed air can be produced from the wind. The small board exerts a large force on a large air mass, so there is a large air pressure near the board.
     
    Last edited: Aug 11, 2012
  18. Aug 11, 2012 #17

    sophiecentaur

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    I agree. It is unlikely that the two solutions to the two different problems would be the same. Both styles of design are well tried and tested so it is not likely that either of them is fundamentally wrong. There must be a factor (perhaps just the size and consequent tip-speed) in the design of massive generators that dictates the use of a different approach.
     
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