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Why do Wind Turbines have such large blades

  1. Jan 21, 2014 #1
    This might sound like a ridiculous question and I am sure it is (they don't build them like that for fun) but why on earth do they? I read an article that said the new PMG style turbines only require the shaft to be rotating at 16rpm to generate up to 8.6MW.

    Is there so much resistance that a huge amount of torque is required to turn the shaft so they need massive blades?
     
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  3. Jan 21, 2014 #2
    the more power they put out the stronger the magnetic fields used to make the electricity so bigger blades to get the power from the wind. the strength of the wind is the limiting factor, it is the same for all turbines from small domestic ones to these huge megawatt ones.
     
  4. Jan 21, 2014 #3
    I guess I kind of get it, but.....

    There is only one shaft being rotated and it is moving at 16rpm. If I hand cranked it to 16rpm (they say they are almost frictionless so I am taking I to the extreme) I would not be exerting the same force on the shaft but the motion in the PMG would be the same. Why does more power in = more power out when the bits inside moving go the same speed? I was once told there were no stupid questions :)
     
  5. Jan 21, 2014 #4

    cjl

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    The only way to get 8.6MW of power out is to be putting slightly more than 8.6MW in (due to losses in the system). To input 8.6MW of shaft horsepower at 16rpm, assuming I'm doing the calculation correctly, you need a torque of 3.8 million foot pounds. If you're putting in less torque than that, you can't output as much power, since the power out must always be less than the power in.
     
  6. Jan 21, 2014 #5

    russ_watters

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    Frictionless perhaps, but that isn't saying anything about the electrical/magnetic resistance!
    Electrons don't want to be moved through wires. You move a magnet over a wire and it pushes the electrons and the electrons push back. It's exactly the opposite of an electric motor.

    Also, due to gearing the speed of the turbine doesn't imply the speed of the generator is also 16rpm.
     
    Last edited: Jan 21, 2014
  7. Jan 21, 2014 #6

    AlephZero

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    If you increase the area swept out by the blades as they rotate, you get more power from the same wind speed. Twice as much area gives twice as much the power.

    Big wind turbines seem to be moving "slowly," because you usually look at them from a long distance away. But the tips of the blades are moving at around 100 miles per hour or faster, even when the RPM is slow. That is much faster than the speed of the wind. To make an efficient turbine, you want to blades to be moving faster than the wind, but there are practical limits to how much faster.

    If you forget about the air resistance of blades, you could probably crank the generator by hand at 16 RPM, but only if it was not generating any electricity. But if you wired it up to generate some electrical power, you have to do the put in the same amount of power to turn it as the electrical power you get out. Since one human can only generate a few hundred watts of power for a short time, it would take literally thousands of people to turn it by hand when it was generating megawatts of electrical power.
     
  8. Jan 21, 2014 #7
    How is the kinetic energy from the blades transferred then. Is this in the gearing? I read the new direct drive PMGs have no gears. The shaft still turns at 16rpm which is just screwing with my mind.

    Blades - Shaft - Generator

    "Electrons don't want to be moved through wires. You move a magnet over a wire and it pushes the electrons and the electrons push back. It's exactly the opposite of an electric motor."

    Does this mean if the generator was wired to produce electricity and I tried to hand crank the shaft I wouldn't be able to turn it? It would require the 3.8million foot pounds to turn it 16 times in a minute?

    I am just realising how utterly stupid I sound but I am really interested to understand it.
     
  9. Jan 21, 2014 #8

    AlephZero

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    It is easier to design an efficient high power electrical frequency/voltage converter, than a high power mechanical gearbox.

    For example http://www05.abb.com/global/scot/scot232.nsf/veritydisplay/c4de28147e528b0cc1257a8b00595934/$file/PCS6000Wind_3BHS351272_E01_RevA.pdf [Broken] handles up to 9MVA and converts 8 to 100 Hz generator output to mains frequency, with 98% efficiency.
     
    Last edited by a moderator: May 6, 2017
  10. Jan 21, 2014 #9

    AlephZero

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    That's right. Since you can't "see" the electricity, it's not "obvious", and if you have played with small electric motors and generators the power levels were probably too low for you to notice the effect.

    If you took the alternator from a car, there is no problem spinning it by hand if there are no electrical connections to it, but if you wired it up to light two car headlight bulbs (about 100 watts total power) you would still be able to turn it, but you would definitely feel you were doing some work!
     
  11. Jan 21, 2014 #10
    Ah, I think I have something. So there is a sort of 'resistance' created by the electromagnets and in order to generate the generators capacity you have to hit the sweet spot of 16rpm but to do this you need to expend a lot of energy. This means the equivalent amount of kinetic energy coming in = the power out put of the generator and you get the 6MW or 8MW or whatever the turbine is.

    Having read everything you guys put and reading other sites I think it makes sense. Ironically the final piece came from a site explaining why wind turbines stop working if it is too windy. SERIOUSLY, TOO WINDY for wind power. But it makes sense with the exact ratio they work to.

    Thanks guys, I know that was a struggle for you all.
     
  12. Jan 21, 2014 #11

    russ_watters

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    I understand that, but I don't think that that means most turbines are gearless (though apparently some are - I didn't expect that).

    My point was simply to convey that the rpm doesn't necessarily have anything to do with the power output.
     
    Last edited by a moderator: May 6, 2017
  13. Jan 21, 2014 #12

    AlephZero

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    The only gearboxes that handle of the order of tens of MW of power that I have seen are aircraft engine designs (either for turboprops, or more exotic engine concepts). But those typically run 100 times faster than 16 RPM, so the torques on the wind turbine gears would be 100 times bigger for the same power handling.

    And the top of a wind turbine tower is not a "good" place to put a gearbox that would probably weigh several hundred pounds, plus cooling and lubrication systems etc, compared with an electrical converter that could be located more or less anywhere (e.g. on shore, for an offshore wind farm).
     
  14. Jan 21, 2014 #13

    cjl

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    The aircraft gearboxes also care much more about weight than the wind turbines do though - a wind turbine gearbox could weigh several thousand pounds and still not be much of a load on the tower compared to the wind loading and weight of the turbine itself (in fact, from what I can find, real wind turbine gearboxes weight tens of thousands of pounds - see http://gedrivetrain.com/assets/PDFs/GE_DrivetrainTechnologies_Gearboxes_broch.pdf [Broken] for examples). From what I can find, most wind turbines do indeed use a gearbox, since 1 revolution every 4 seconds is very slow for a generator (and it is difficult to design a generator that works efficiently at this speed). A faster spinning generator will also be smaller for a given power output, which helps offset the weight of the gearbox.

    You do still need a frequency converter, as linked before though, since a wind turbine generator is almost never going to be synchronous to the grid AC.
     
    Last edited by a moderator: May 6, 2017
  15. Jan 21, 2014 #14

    berkeman

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    That's not my understanding, or my experience living near large wind farms (Altamont Pass in Northern California east of Livermore). The windmill generators are frequency locked to the grid. They all turn at exactly the same speed (for a given windmill type) as they are delivering power to the grid, regardless of the local wind speed at each windmill. They sometimes feather the blades when the wind is too slow to generate power for the grid...
     
  16. Jan 21, 2014 #15

    russ_watters

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    Wind turbines aren't tens of megawatts and their hubs are much, much bigger than than turboprops so the forces (not including the weight...) would be much smaller.
    Fair enough, but it is done.
     
  17. Jan 22, 2014 #16

    cjl

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    After a bit of research, it looks like modern wind turbines do tend to use frequency-locked generators, but the wind turbine rotor itself is not frequency locked. Instead, they appear to use a variable transmission to keep the generator speed constant for a range of input speeds from the turbine. (from this document: https://www.cumminsgeneratortechnol...rid_Coupled_Wind_Generator_White_Paper_LR.pdf). As for a given windmill type all turning the same speed, that will tend to be the case for a given wind speed, since they are all trying to run at the rotational speed that generates maximum efficiency, and the wind speed is likely similar at all the turbines at a given location.
     
  18. Jan 22, 2014 #17

    russ_watters

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    What would lead one to the conclusion that they are locked to the grid is that they don't just tend to run ABOUT the same speed, but EXACTLY the same speed and aligned with each other. That wouldn't happen with independent speed controllers.
     
  19. Jan 22, 2014 #18

    cjl

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    They wouldn't necessarily need to be aligned with each other, so long as the poles of the generators were aligned (which could occur without the blades aligned). Also, as supported by the document in my previous post, modern ones could be running at different blade speeds and still be synchronous.

    (If it's an old wind farm though, they could be fixed-ratio, synchronous generators, since those were apparently commonly used in the 1990s)
     
  20. Jan 22, 2014 #19

    russ_watters

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    Again, as with Alpha, you are talking about what could be and I (and Berkeman) am talking about what is. I fully agree that it could be done the way you say.
     
  21. Jan 22, 2014 #20

    rcgldr

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    Getting back to the original question, the reason for long blades on the rotors is efficiency. Increasing the chord (leading edge to trailing edge) length of the rotor blades or having more than three rotor blades doesn't help as much as increasing the length of the rotor blades. Longer rotor blades on the turbines slow down a larger amount of air, so the result is more power extracted from the wind. The limit on size is mostly due to cost and the engineering difficulty in building large wind turbines.

    There's talk of adding chicken wire like shields to wind turbines to avoid bird strikes, and if this is done, this would add another constraint to the practical maximum size of a wind turbine. (Alternatives to this include some type of noise making devices to scare off birds, but they could get acclimated to the noise).
     
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