Why are wind turbines with less blades better?

Discussion in 'General Science & Technology' started by Edont Knoff, Apr 4, 2018.

  1. Edont Knoff Registered Senior Member

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    I remember that wind turbines with less blades work better, but I have a hard tiome to uinderstand why. Naively, I would think that more wind passes through between the blades, if there are fewer blades?

    I know of a few reasons why one- and two-blade designs were more problematic than the most frequently used three-blade design, but I don't understand why 5, 7, or any higher odd number of blades isn't even better.
     
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  3. Gawdzilla Sama Valued Senior Member

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    One reason would be "why add blades if it doesn't increase production significantly?" The weight of seven blades would be a drag on the system compared to three. (Just my SWAG.)
     
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  5. exchemist Valued Senior Member

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    There indeed seems to be a trade-off between performance and cost, but also other factors such as rotational speed (more blades means it runs more slowly, creating issues for gearing to generate electricity, I imagine) and blade thickness (more blades means they need to be thinner, which poses engineering challenges) seem to be functions of the number of blades employed.

    More about it here: http://www.learnengineering.org/2013/08/Wind-Turbine-Design.html

    But I know next to nothing about the subject. This is the sort of thing that billvon may be able to comment on, I should think.
     
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  7. Edont Knoff Registered Senior Member

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    From the article: "But as we move from 3 blades to 4 blades design, efficiency gain is marginal."

    It seems my basic idea was right, more blades are better, but the gains are so minimal that for commercial turbines the costs of additional blades are higher than the gain through energy production, so the 3-blade construction is a good compromise.

    Thanks for the link!
     
  8. exchemist Valued Senior Member

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    Yes.

    I seem to recall another issue regarding blade numbers is that 3 blades are preferred to 2 for aesthetic reasons. The human eye can resolve the rotation of 3 blades rationally but 2 blades rotating is rather disconcerting, the apparent direction of rotation seeming to reverse at random. So people get driven a bit nuts by looking at a 2 blade rotor!
     
  9. Edont Knoff Registered Senior Member

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    There is also the fact that a two-blade rotor will have one blade in the wind-shadow of the pylon while the other is in full wind, having maximum shearing forces on the axle. Three-bladed rotors have lower shear-power peaks, because not 1/2 but only 1/3rd of the rotor can be in the wind-shadow of the pylon.

    I felt driven nuts by the one-blade experimental GROWIAN turbine <.<

    http://www.usm-nord.de/fileadmin/bildmaterial/_processed_/csm_Growian-4-barbt_9a0d3bea6c.jpg

    There are also pictures with two and three blades, seems they tested several configurations. This one definitely still makes me wonder.

    Edit: Wikipedia says GROWIAN was designed as a two-blade turbine, and suffered from the above mentioned force peaks. The one-wing configuration that I saw on TV must have been something else.
     
    Last edited: Apr 4, 2018
  10. exchemist Valued Senior Member

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    Crikey! Reminds me of the WW 2 Blöhm & Voss reconaissance aircraft: https://en.wikipedia.org/wiki/Blohm_&_Voss_BV_141

    I remember an apocryphal story that Hitler had it cancelled because he hated the design - probably condemned it as "Jewish" or something!
     
  11. Edont Knoff Registered Senior Member

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    Human perception favors symmetry quite a lot. One reason given by biologists was, that people with good body symmetry likely grew up in good conditions and were perceived to be healthy and well-nutered. Seems to have to do with partner selection, healthy partneers gave a beter chance of having healthy offspring, so that eventually well grown symmetric people had better chances to reproduce and so the preference was reinforced over time.

    Not very convincing, maybe I got some things a little wrong there ... but it could explain a bit. Funnily that we are symmetric most on the outside, quite some organs in our belly (liver, spleen, stomach, heart, small and big intestine) are unsymmetric. But well hidden from sight.

    I must say, I like the design of the plane. Particularly because it explores something new. And at times, it is good to have two different sides, which are suitable for different tasks or features. But it violates our symmetry instinct, so one really must look at the technical details to see if it's good.
     
  12. exchemist Valued Senior Member

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    Something in that, I'm sure.

    And this is no more than an aviation version of the motorbike/sidecar combination, which was popular at that time.
     
  13. billvon Valued Senior Member

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    Because the primary determinant for wind energy harvest is swept area. In other words, if you make the blades longer, you will harvest geometrically more wind (because the area goes up by the square of blade length.) If you add a blade, you might see a slight increase, a slight decrease or no change, depending on the dynamics of the system. For most designs three blades provide a good tradeoff between rotor stability and wind harvesting. Without the yaw stability issues, two blade designs are the most efficient (wind harvested vs. material for rotor.)

    (BTW I have never seen a one blade design; they are very hard to balance.)
     
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  14. sculptor Valued Senior Member

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    And then, consider that wind turbines are usually clustered into wind farms.
    Then we must consider downwind turbulence and what blade configuration works best for that.

    If memory serves, this became an issue in/with a Scottish wind farm.
    accurate?
     
  15. billvon Valued Senior Member

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    It's certainly true that you want to space turbines out to prevent wind shadowing. The rule of thumb is 3 to 10 rotor diameters depending on circumstances (how variable the wind is, nearby obstructions etc.) However that is really a consideration for spacing and total power output, not number of blades.
     
  16. iceaura Valued Senior Member

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    Another factor:
    The faster the blades move the better, rule of thumb (up to a limit not often approached in practice).
    The faster the wind the better (again, up to a limit not often approached).
    The less turbulence in the airflow the better - the blades extract the most energy from laminar flow at the designed optimum angle of attack.
    So the blades need to be spaced widely enough to spin at high speed without running into each other's wakes and back pressure slowdowns.
    And that's much wider than intuition suggests - the blades are moving considerably faster than the wind is blowing through them.
     
  17. Gawdzilla Sama Valued Senior Member

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    I've never seen one that didn't face into the wind. Is this something new?
     
  18. Edont Knoff Registered Senior Member

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    It seems the wind flow is also reduced in front to the pylon (a zone of higher pressure and less wind speed). But you have a point there, I was wondering about that too.
     
  19. Gawdzilla Sama Valued Senior Member

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    How far in front would be the factor. I can't picture one with the blades behind the pylon, for the reason we're discussing. Either there are gears to rotate the vanes into the wind or there is a "weather vane" that does the same thing.

    I wish I had paid more attention when Mike Rowe was up inside one, but his near vertigo was a pure giggle farm.
     
  20. Edont Knoff Registered Senior Member

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    Maybe I am wrong on this. I think I have read it in an article a while ago that the alignemenntof one blade with the plylon while the other being in full wind is a problem and that there are tilting/shearing forces ... but maybe I got that wrong.

    I googled and found this:

    When the blades are vertical the forces required to yaw the rotor are low, but when the blades are horizontal the forces are much higher.

    The cyclic forces impose significant stresses on several parts of the structure and these forces are much lower when a three-blade machine is yawed, as the asymmetric forces encountered as the rotor rotates are much less.

    Source: https://www.windpowermonthly.com/article/1083653/three-blades-really-better-two

    Don't ask me for the details of the why and how's. Chances are that I got quite some things wrong there, but the three blade design comes with less force-related problems as it seems.

    Edit: Another point google just gave me:

    Two-blade wind turbine rotors offer fundamental advantages in light weight and low cost—if they teeter stably. The rotor assembly of a mainstream three-blade wind turbine moves with one mechanical degree of freedom: it rotates. As blades turn and bend unequally due to turbulence, wind shear, and tower shadow, the resulting bending moments are reacted by the bearings and structure supporting the main shaft.

    The rotor assembly of a teetering two-blade wind turbine moves with two mechanical degrees of freedom: as it rotates, it teeters on a hinge between its hub and main shaft. Within engineered limits, this teetering can be mechanically free, eliminating bending fatigue loads through the main shaft due to turbulence, wind shear, and tower shadow. Eliminating these fatigue loads enables reduced weight throughout the main bearings, gear case, mainframe, yaw bearing and tower. See the brief video below showing how a wind turbine rotor teeters.

    Source: https://www.windflow.co.nz/turbines/why-two-blades
     
  21. billvon Valued Senior Member

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    They work fine if they are fixed. When you try to yaw them to get them to face into the wind you run into problems with two bladed rotors.
     
  22. iceaura Valued Senior Member

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  23. iceaura Valued Senior Member

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    Side point re downwind vs upwind: It nags me that naively straightforward means of handling cumulative yaw in a downwind configuration via - say - introducing a vertical twisting cable junction (or even a rigid planetary gearing) between the powered shaft and the generator ( to handle the cumulative twist of yaw sequences without involving the power feed cables) are seemingly never mentioned even in dismissal. That would allow the automatic start, cheaper flex handling, and built-in yaw correction of the downwind configuration etc. I'm assuming there are objections, it's just that they aren't mentioned.

    Btw: can a kind soul correct the title to "fewer blades"? Just for the sake of a title in a science forum - -
     

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