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Multiple rotors on a wind turbine

  1. Aug 25, 2017 #1
    Could multiple, smaller rotors on a wind turbine (on the same shaft) be more effective than a single, larger rotor? Why do most wind turbines have only one big rotor? I was thinking about doing a science fair project on this topic, but I have been unable to find a conclusive answer to this question online.
    ^AB355758EFFE2B61D070B204BC0257AF8CFF9EC77199633E78^pimgpsh_fullsize_distr.jpg
     
    Last edited: Aug 25, 2017
  2. jcsd
  3. Aug 25, 2017 #2

    anorlunda

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    What do you mean by multiple rotors? Can you make a sketch and post it here using the UPLOAD button?
     
  4. Aug 25, 2017 #3
    Done. I've uploaded the sketch. Sorry if it's a little ugly though, I was just trying to show what I meant.
     
  5. Aug 25, 2017 #4

    anorlunda

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    Try again, I see no sketch. After uploading you must press the FULL IMAGE button.
     
  6. Aug 25, 2017 #5

    berkeman

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    I can think of at least 2 reasons that they are not configured that way. Can you maybe figure out what those two issues would be? :smile:
     
  7. Aug 25, 2017 #6

    berkeman

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    Weird, I saw the image and now it's gone. It shows 3 rotors mounted coaxially.
     
  8. Aug 25, 2017 #7
    Ok, now it should be there
     
  9. Aug 25, 2017 #8
    Not really... Perhaps the rotation of each rotor would slow the others down?
     
  10. Aug 25, 2017 #9

    anorlunda

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    Think about it. Is is likely that the world's best wind turbine designers have not already considered all possible variations on design and selected the best. It would be very hard for me or you to think of something they have not already considered and rejected.

    Here's some advice for your project. Do a project on all the designs that have been considered and rejected. What are the criteria for "best" design? Use this wikipedia article as a source. https://en.wikipedia.org/wiki/Wind_turbine_design

    You can also be scientific and explain Betz's Law as part of your project. https://en.wikipedia.org/wiki/Betz's_law as part of your project. That's the real reason why multiple rotors aren't good. The designers are already close to Betz's Law performance without multiple rotors.
     
  11. Aug 25, 2017 #10

    berkeman

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    @anorlunda said it pretty well. The "shadowing" effect was my first issue with the coaxial propeller design proposal. My 2nd issue involves contrasting a single larger propeller/turbine versus multiple smaller ones (ignoring shadowing). Either in the Wikipedia link or in your other reading, have you seen an issue with smaller faster turbines versus larger slower turbines? It's a bit subtle, but very important in the real world.

    Another approach for your science fair project might be to prototype several different turbine designs and test them with a simple fan-driven wind tunnel. There are some pretty weird turbine designs that could make it a fun and visual presentation in your Science Fair presentation...

    https://i.pinimg.com/originals/41/76/b0/4176b0726d042beba72f40db51a1ab43.jpg
    4176b0726d042beba72f40db51a1ab43.jpg
     
  12. Aug 25, 2017 #11
    These types of ideas have been tried and any small gains they deliver are not worth the engineering challenges; better to put that second and third rotor on their own towers, sited to avoid anything upwind or downwind that affect wind flow. I'm not familiar with the specific issues, but I suspect air turbulence immediately behind a wind rotor would be very significant. The overall wind speed would be reduced as well as being more turbulent. Any rotor sited directly behind another is going to have reduced output - optimal spacings for towers are meant to ensure they spend the least time in line with each other. Wind speeds are affected ahead of an obstruction as well as behind it so the first rotor's output will probably be reduced just by putting a second rotor behind it. Close spaced and they would effectively be like a single rotor with more blades; more blades have been tried but don't deliver better performance; two or three blade rotors work the best, but whilst you don't see them with 10 or 20 blades, you can find single blade wind turbines and they have good efficiency.

    Having a long axial shaft that extends a long way past the bearing housings would have greater stresses - huge stresses I would think if 3 were mounted like the model pictured. One ahead and one behind the gearbox, paired on a fixed shaft would be the simplest high strength arrangement but independent rotation would probably be needed. But would two deliver more power than a single rotor? As I understand it a wind turbine's maximum wind energy to rotation efficiency is near 60%, but closer to 40% in practice. 40% of the less than 60% of wind energy left, when some of that initial 40% will be lost and the second rotor is in turbulent air, reducing the efficiency... I would be surprised if there is much overall improvement. Conversely I would not be surprised if there was no improvement or even a reduction in efficiency.
     
  13. Aug 25, 2017 #12
    With a bit more searching I did find attempts at twin rotor wind turbines - counter-rotating ones - with claims of a 40% gain in output from the same swept area over single rotors, so my final remarks look incorrect. However they have not found any commercial success. It's always tricky to equate commercial success with technical superiority - there are so many other factors, including management and financing that can see better technologies outcompeted - but it does look like the efficiency gains from twin rotors aren't enough to displace the simpler single rotor designs.
     
  14. Aug 25, 2017 #13

    Astronuc

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    Counter rotating rotors would be a possibility because it reduces the minimum wind speed nescessary for a given rpm, but then one has two rotors instead of a rotor and stator. Think about induction generators and getting current from a rotor vs a stator.

    Wind turbines are unducted fans, so there are losses at the tips that increase with speed.

    Two fans, either vertical or horizontal are potentially practical, but one must understand the stage losses and interactions (e.g., potential instabilities, tip losses, flutter, . . . ) to ensure stability over the range of flow.
     
    Last edited: Aug 26, 2017
  15. Aug 27, 2017 #14

    CWatters

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    There is only so much energy available to be captured. The larger the swept area the better.

    Remember that extracting energy causes the air to slow down. You can try and use multiple rotors but the air can't come to a complete stop - It has to get out of the way for the next lot.
     
  16. Aug 29, 2017 #15
    For what it's worth, I did find a lot of wind turbines that are ducted fans.
     
  17. Aug 29, 2017 #16

    anorlunda

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    I believe that is the dominant factor in wind turbine design. It is cheaper to put up two simple turbines and get 2x power than spending more money and getting less reliability for a more complex design that might only give 1.5x more power.

    The KISS principle in engineering is frequently underestimated.
     
  18. Aug 30, 2017 #17
    By using a large rotor and a small rotor, a dual-rotor design can be superior to a single rotor.The blades of most modern turbines are very large and as a result the inner section of the blade is optimized for structure, not aerodynamics. There is energy in the wind that is not being extracted due to this compromise. A much smaller rotor, that is optimized for aerodynamics and placed co-axially with the larger rotor can help to recover some of this energy. In addition, this design can also help to mitigate the wake losses in turbine farms because it promotes mixing in the wake.
     
  19. Aug 30, 2017 #18
    well my 2 c - "outside" of the issues with the blades, the gearbox / construction would be a nightmare.... $ & $ ... as it is the gearbok is a significant cost and reliability point.
     
  20. Aug 30, 2017 #19

    anorlunda

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    The question comes down to
    1. Designing a more efficient wind turbine.
    2. Designing a more power efficient wind power generator.
    3. Designing a more cost efficient wind power generator.
    They can lead to drastically different answers.
     
  21. Sep 8, 2017 #20

    Baluncore

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    Two turbines rotating in the same sense is equivalent to one turbine with twice as many blades. The rpm of an efficient turbine or propeller is inversely proportional to the number of blades. Less blades are needed at higher rpm to “slice” the air sufficiently to extract optimum energy. The minimum stable system has three blades, which also minimises gearbox weight. The limiting speed is then determined by the blade tip velocity becoming supersonic.

    There is already a problem of blade noise or pressure waves reflecting from the support tower and back to the same or another blade. When two counter rotating turbines are used, the two helical “dirty air” disturbances of opposite sense cross at a regular rate. That produces an oscillation in the blades that then flex to the point of collision and destruction.
     
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