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Wind turbines with many small generators

  1. Sep 19, 2015 #1
    Hello, This is not a homework question. I'm not a student. In fact I'm retired.

    The other day, I was passing by a wind turbine. I thought "wouldn't it be better to have hundreds of small generators with small wings connected together instead of one huge generator with huge wings?"

    But I did not know how to compute if small generators would be better. How can this be computed? I practically know nothing about electricity.

    Thanks for any answer you may have
     
  2. jcsd
  3. Sep 19, 2015 #2

    mfb

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    It's not about electricity, it is about aerodynamics and mechanics. The theoretical optimal efficiency (power harvested divided by power from the wind) is 59.3%, this is called https://en.wikipedia.org/wiki/Betz's_law]Betz's[/PLAIN] [Broken] law. Modern wind turbines are not far away from this limit, and the fraction does not depend significantly on the size of the turbines.
    Smaller turbines are stronger (Square-cube law) so their efficiency might be a tiny bit better, but then you need more separate parts, more infrastructure to mount all of them, and so on.
     
    Last edited by a moderator: May 7, 2017
  4. Sep 21, 2015 #3

    russ_watters

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    And more of them. It's all about how much air they capture: if you cut the radius in half, you need four turbines instead of one.
     
  5. Sep 23, 2015 #4
    Thank you for the answers. This is very interesting for me. I will investigate further but can you also help with this question:

    I am imagining really small wings. The radius may be as small as 3 inches. The idea is to try to capture the smallest wind. The calculation I'd like to make is to find out how many such small turbines I need to get the same power as one big turbine. How should I approach such a calculation?
     
  6. Sep 23, 2015 #5

    mfb

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    What is the "smallest wind"? Turbulences? Those don't add much to the energy budget. And rotating every single turbine to be in the ideal direction looks like a huge mess. A support structure every 10 centimeters would reduce the efficiency of the setup significantly.
     
  7. Sep 23, 2015 #6
    No, I don't mean turbulence. I mean wind strength, like in Beaufort scale: https://en.wikipedia.org/wiki/Beaufort_scale

    İ'm trying to get some numbers. For instance what is the minimum wind strength that would turn a conventional turbine? What is the smallest wind strength that would turn a minuscule turbine?

    Why rotate the turbines? If they are free to turn like a weather wane they automatically be in the ideal position.
     
  8. Sep 23, 2015 #7

    mfb

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    Where is the main point in replacing 1 turbine with area A by 100 turbines with area A/100 each then? You don't increase the total area. You just add complexity.
    Doesn't work that well for wind turbines.
     
  9. Sep 24, 2015 #8
    One benefit maybe to get more power from light winds. This is what I would like to calculate. Smaller turbines may be producing power in light winds while the big turbine is idle. But I don't how to compute this relation.

    Another benefit would be in strong winds when there is too much power. Instead of turning off the entire turbine only a number of small turbines can be turned off to get as much power as needed.

    It may be possible to have two layers of turbines. The second set can be downwind catching the wind that passed through the front turbine.
     
  10. Sep 24, 2015 #9

    mfb

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    Why? You keep claiming something like this without any justification.
    Why?
    Also, power grows with speed to the third power, low wind speeds give a very small contribution.
    Why?
    That reduces the efficiency of the first one. More complexity, no gain. You cannot avoid Betz's law.
     
  11. Sep 25, 2015 #10
  12. Sep 27, 2015 #11
    I think the main problem is us tbh. We tend to try build the big picture and forget 2 basic principles. Peak demand vs average demand. We have different uses and should have our circuits isolated and separated.
    An average household uses 1000 W for 8 hrs a day, 2000 W for 4 hrs and 5000 W for less than 2 hours a day and on average 20kW a day. This shows we need the systems broken into 2 or 3 different supply sources. Solar creates power when we least need it. Wind is normally morning and evening ( when we need it most) storing the excess power will always be a problem due to current methods of storage
    Wind turbines are quite complicated as the rotational speed is uncontrollable and much of the output can not be used.
    We need to think out of the box as there is a solution, we have yet to find it.
     
  13. Sep 27, 2015 #12

    mfb

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    At least in Europe, electricity demand peaks during daytime, and it doesn't look so different for California, for example.
    I don't see how this is related to the topic, however, which compares different ways to harness wind.
     
  14. Sep 27, 2015 #13

    DaveC426913

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    A 1 knot wind over a 100m2 rotor is going to produce essentially the same power as 100 rotors of 1m2.

    I think you're envisioning a tiny rotor being more able to be moved by a light wind. I'm not sure that's a valid assumption. (However, challenging assumptions is how we invent new things!)
     
  15. Sep 27, 2015 #14

    DaveC426913

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    Perhaps that's the key.

    We have turbines for medium wind already; perhaps a complementary turbine - one attuned to light winds - could extract an additional fraction of energy on light wind days. Maybe we'll see wind turbines with one large rotor for strong wind days, and then an array of small rotors up and down the tower's trunk for light wind days.
     
  16. Sep 27, 2015 #15
    Agree, sorry I am off topic, im working on the power generation side atm. I am presuming the discussion is about private generation.
    During the day most households don't consume much electricity, it is normally 2 to 3 hours before work and after work, heating/cooling is not included as it is seasonal and area specific.
    All big wind turbines adjust the blade angle to control the speed of the propeller, these are all expensive and computer controlled. There is the slow and high speed where power generation is impossible to control.
    In the domestic market it is different however, and this is where my comments apply. Small users are the key to reduce our footprints, but we cant see the wood for the trees.
    I have a design which I think is brilliant but am struggling to get it into the market. There are a number of reasons and one of them is its to small, yet it out performs solar.
    It seems everyone wants a unit to produce 100kW all the time even if they are only consuming less on average.
     
  17. Sep 27, 2015 #16
    The problem here is not the wind or the blade, its the drag cause by the generator turbine. In idle mode the blades will turn, when the generator is switched on it causes drag on the propellers. So a smaller turbine with a lower output turbine will turn in the same wind that cant drive a big turbine. Its called the cutting in speed. A generator is sized to the RPM of the blades and wind conditions.
     
  18. Sep 27, 2015 #17

    russ_watters

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    That's true in most places: the sun heats buildings, requiring the most air conditioning during the late afternoon.
     
  19. Sep 27, 2015 #18

    russ_watters

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    In what sense? Solar and wind are so different from each other, it is tough to compare them except perhaps by $/kWh.
     
  20. Sep 28, 2015 #19
    each have negatives, solar daylight hours and consistency of the wind etc, solar is stable, however wind power is not. If both are in an optimal environment wind generation is the out-performer. I live near the ocean and am investigating and designing a different method of wind extraction for small power. It is a complicated subject and well debated. I however feel we cant see the wood for the trees in this situation.
     
  21. Oct 1, 2015 #20
    this thread will help you, https://en.wikipedia.org/wiki/Cost_of_electricity_by_source
     
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