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Wind Turbine Blade Size

  1. Mar 22, 2014 #1
    Hi - I know someone must have thought of this already, so maybe you can help me understand.

    When I look at some of these modern wind turbines, the blades seem massively long. I see this lumbering set of blades moving slowly and I imagine that the bulk of the wind does not even touch the blades but rather goes through the large gaps between them (if you look at the blades straight-on, the blades take-up only a small percentage of the circular swept area).

    Would it not be more efficient to have a set of blades that took-up almost all of the circular swept area so that they extract as much of the wind energy as possible? Either more blades or different shaped blades (a crude example is a pinwheel).

    Of course I would not keep the diameter the same (if this idea actually works, then it would generate way too much power). Rather make the diameter smaller so as to produce the same power. The goal is to get the same amount of power using less cumbersome blades. I'm thinking that installing smaller blades would be easier, even though there would be more of them.

    Well i ust have missed something, so please let me know where I went wrong.
     
  2. jcsd
  3. Mar 22, 2014 #2

    SteamKing

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    Generally, More blades = lower efficiency. Usually, what drives increasing the number of blades beyond 2 is to reduce the average load on each blade. This is especially important for ship propellers, where heavily loaded blades tend to produce cavitation, which in turn leads to damage to the blades, increased vibration, loss of thrust, etc.

    You also have to remember that blades for wind turbines are trying to extract the relatively small amount of energy available in the wind. If you add blades, you also add weight, and this means the supporting structure for the wind turbine must get larger as well.

    http://en.wikipedia.org/wiki/Wind_turbine

    http://en.wikipedia.org/wiki/Betz'_law
     
  4. Mar 22, 2014 #3

    AlephZero

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    The blades in large wind turbines are not "lunbering". The blade tips are moving at about 100 mph. The motion only looks slow because you are usually a large distance from the turbine and you don't have anything to compare the speed with.

    The blades don't just extract the energy from the small amount of air that "hits" each blade. They change the direction of the air flow through all of the "gaps" between the blades as well.

    If you add more blades, you block the air from flowing through the turbine and instead it just flows past it, as if the turbine was more like a solid circular disk.

    Turbine designers do work out the optimum number of blades for a given sized turbine. That's why small turbines (less than a meter diameter) usually have more blades. Having only 2 blades causes some extra problems with the stability of the turbine (e.g. what happens if it is not pointing exactly into the wind) which is why large turbines usually have 3 blades, but some "medium sized" ones only have 2.

    The "old style" wind turbines like the one below were not very efficient, but they didn't need to be efficient to do what they were used for.
    400px-Water_Pumping_Windmill.jpg
     
  5. Mar 22, 2014 #4
    Yeah, now that's what I'm talkin bout.

    Ok, more blades are less efficient. When I look at turbofan jet engines, the bypass fan, compressor stages, and turbine stages look like the above (well a little more sophisticated than "green acres"). Should they be reduced to 3 blades as well?
     
  6. Mar 22, 2014 #5

    Baluncore

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    The blades effectively slice through the air. It is not the solidity of the turbine that counts, it is the distance between the blade “slices” that spiral through the clean airflow that gives the turbine maximum access to the volume of air that passes.

    If the disturbance created by the previous blade's passage has not been flushed down stream before the next blade passes, there will be an instability or a “dead spot” in the blade's path at some speed. For that reason, having many blades requires slow rotation and explains why the rotational speed of a wind turbine is inversely proportional to the number of blades. One blade needs an unproductive counterweight, two blades balance but are not inherently stable in rotation when changing direction, three blades are both balanced and stable. Three blades is therefore the minimum number of blades and that give the highest speed reliable performance.

    A turbine with many blades rotates slowly, but with a high torque, that is why they are used on farms to reliably pump small quantities of water.
     
  7. Mar 22, 2014 #6

    AlephZero

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    No, for several reasons. First the blades in a jet engine are inside a casing. That makes a big difference to the flow pattern, because all the air is forced to go through the blades. Second, a jet engine turbine is working under very different conditions from a wind turbine. The inlet gas is at high pressure and expands as it flows through the turbine. In a wind turbine, there is no pressure change between the air in front and behind the turbine. And finally, jet engine compressors and turbines are not just "mirror images" of each other, the blade designs in compressors and turbines look very different.

    The reason why all the fans on large diameter jet engines have about 24 blades (plus or minus about 2) is because that is the optimum number for what they are designed to do. It's not just a coincidence that three different engine manufacturers all use fans with similar numbers of blades.

    If you look the shapes of a jet engine fan blade and assorted turbine blades, they are obviously completely different from modern wind turbine blades, and the jet engine fan and turbine blades are very different from each other.

    Wind turbine blades look very much like like aircraft wings - which is not surprising, considering they operate in the same air flow conditions!

    DSC_0864_323x600.jpg
    1256811421.jpg rolls1.jpg
     
  8. Mar 23, 2014 #7

    OCR

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    In my case, that's usually right... lol

    http://www.cleantechrepublic.net/2012/03/13/flodesign-wind-turbine/ [Broken]

    https://www.google.com/#q=flodesign+wind+turbine+


    I also "fixed" your quote... :wink:




    OCR
     
    Last edited by a moderator: May 6, 2017
  9. Mar 23, 2014 #8

    AlephZero

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    The flodesign idea will only work for fairly small turbines, parrtly because of the weight and cost of the duct, stator blades, mixer, etc, and also because the effectiveness of the mixing would be less for a bigger turbine, because the circumference of the duct (where the mixing occurs) increases slower than the area inside as the diameter increases

    I suspect their "beating the Betz limit" claim is involves a bit of "creative accounting", because the mixer means they are taking energy from a bigger cross section of air than the circle swept out by the rotor blades.

    If you can build a complcated expensive 1m diameter turbine the flodesign way, and it only generates the same amount of power as a simple cheap 1.5m diameter turbine, what's the benefit to the end user? Of course there are benefits to the company in getting government grants to develop it, being able to advertise something that looks high tech, etc, but that's a different issue from actually using it to generate power.

    You can't escape the basic fact that if you want more power, you need more area to interact with more wind. Whether it is cheaper to build one huge turbine or many small ones is just a practical detail.
     
  10. Mar 23, 2014 #9

    Baluncore

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    A good farm wind pump has an offset head that folds sideways against the tail in strong winds. That regulates the speed and reduces stress on the tower. During a wind storm, flodesign cannot be turned out of the wind to reduce stress on the tower, the flodesign tower will therefore cost more.
     
  11. Mar 23, 2014 #10
    When I look at those blades spinning, I think of knives cutting through butter. This cutting causes a disturbance (think entropy / heat generation and loss of efficiency). Mathematically, to make the disturbance vanish, you would have to make the blades infinitely thin and infinitely long (which is obviously to be taken with a grain of salt). The engineering design can be thought of as a compromise between cost and this mathematical idealization. How meaningful / useful this perspective is, I don't know... I just thought it was cool.
     
  12. Mar 23, 2014 #11

    Baluncore

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    Wind turbines are a parallel to, but are different from aircraft propellers. They both cut a spiral path so as to gain efficient access to a volume of air. There are significant differences though that prevent an efficient propeller being used as an efficient turbine. A turbine blade is twisted along it's length in the opposite sense to a propeller. That is because the higher air pressure is on the other side relative to the leading edge of the aerofoil section.

    The turbine aerofoil profile is also different since it must be optimised as a turbine rather than a fan.

    The air stream behind a propeller is travelling faster than in front as kinetic energy has been added. The initial volumetric difference is displaced by the engine. With a wind turbine, the air is slowed as kinetic energy has been harvested. But in both situations the blades are traditionally placed in the clean air ahead of the wing, mast or nacelle. A wind turbine would be more efficient with the nacelle ahead of the blades, but the wind shadow generated by the tower then sets up a destructive cyclic oscillation in the blades at some rotational speeds.
     
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