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Does the wind speed up after a wind turbine blade?

  1. Aug 3, 2011 #1
    What does happen to the wind after it passes through a wind turbine blade?

    Does it increases the wind speed?
     
  2. jcsd
  3. Aug 4, 2011 #2

    boneh3ad

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    It would decrease the wind speed, actually.
     
  4. Aug 4, 2011 #3
    But doesn`t the wind turbine blade works like as an exhaustor?
     
  5. Aug 4, 2011 #4

    DaveC426913

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    What's an exhaustor? I can find no such English word, nor guess what you might have meant in this context.


    Perhaps what you should do is recite to us what you're reading, so we can get an idea of what you're thinking. What makes you think the air would speed up? That would not make sense, since a turbine is designed to extract energy from the wind.
     
  6. Aug 4, 2011 #5
    What I am trying to figure is what if we put several wind turbines just behind the others the rotation of the ones just after the other would be higher than the one on the front...

    With that situation we could generate more energy in a less area of a eolic park.
     
  7. Aug 4, 2011 #6

    DaveC426913

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    Stop. Let's first figure out what's wrong with your thinking. what makes you think the wind speeds up?
     
  8. Aug 4, 2011 #7
    I am thinking as a Fan model working in a reverse mode... it sucks the air and generate an air current... got it?

    Sorry about the English! I am Brazilian and my English is not so good.
     
  9. Aug 4, 2011 #8

    DaveC426913

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    A fan is not a turbine. They are exactly opposite.

    A fan uses energy which is converted into air flow (speeding it up).
    A turbine uses air flow (slowing it down) which is converted into energy.
     
  10. Aug 4, 2011 #9
    Alright!!! but my thinking is if you reverse the direction of a Fan it will suck the air due to the design of the blades... what I was guessing is if the wind turbine blades had the same wing design... so if a wind pass through a wind turbine blade and it rotate the blades the effect generated by that movement would produce more air flow behind the turbine, got it?
     
  11. Aug 4, 2011 #10

    boneh3ad

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    No it wouldn't. Put simply, the wind moves through the wind turbine blades, transferring some of its energy to the turbine which is converted to electricity. With less energy, the wind slows down.
     
  12. Aug 5, 2011 #11
    Right! it is transformed in eletricity because it rotates the blades, but that is the point the blades are still rotating, and so this rotation would cause and air flow... part of the wind passes throgh the blades between the empty space of the blades, the rotation would cause an air flow due to design of the blades, so the rotation plus the wind that went throgh those blade would generate more air flow?
     
  13. Aug 5, 2011 #12

    boneh3ad

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    No. No no no no no. I don't know what you aren't getting. The wind turbine takes energy out of the air. How would it speed the air up by taking energy out? That doesn't make any sense. If you remove energy from something, it either slows down or gets colder. The wind doesn't get colder, so it has to slow down.
     
  14. Aug 5, 2011 #13
    Or, the pressure drops. Remember the air obeys Bernoulli's equation, which has pressure (P/rho), kinetic energy (V^2/2), gravity (g*z), and sensible enthalpy (cp*T) terms. Basically, you add those terms to get the "total enthalpy" (though gravity for air is fairly negligible on human scales). The change in total enthalpy equals the power extracted by the turbine. I think for wind turbines what you mainly see is a pressure drop across the turbine because the Mach numbers are low.

    As the air moves downstream of the turbine, though, the streamlines spread out, the flow slows down, and the pressures recovers. So you could also say that the wind turbine slows the air down to extract the energy. How you describe it depends on where you draw the control volume.
     
  15. Aug 5, 2011 #14

    DaveC426913

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    No. You have it exactly backwards. It is the airflow that is causing the rotation, not the rotation causing the airflow.

    To generate electricity, the axle of the turbine provides resistance. It is the wind that overcomes this resistance. The flow of useful work is from wind to turbine to generator. You cannot then turn around and try to use the rotation of the blades to do other work, such that work flows from turbine to wind.
     
  16. Aug 5, 2011 #15

    Averagesupernova

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    This is the sort of thinking that led me to believe that I could insert another light bulb in series with the first bulb next to the negative terminal at no cost of brightness, etc. since the first one 'was done with the electricity'. I was probably 7 or 8 years old and assumed electricity flowed from + to -.
    -
    To the OP: Think about what would happen if the wind sped up. Simply keep installing wind turbines to capture the energy that has been 'gained' from the previous turbine. Of course it is not possible.
     
  17. Aug 5, 2011 #16
    Ok, thank you guys for the detailed explanation it is perfect clear for me now...

    Thank you very much!
     
  18. Aug 5, 2011 #17
    Removing power from the wind slows it down, but it must not stop completely. For a wind turbine, the incident wind power is the wind velocity cubed times the frontal area of the turbine (HAWT) times the air density times 1/2. The maximum power a wind turbine can remove from the air is about 59% (Betz limit), because the air cannot be stagnant behind the blades.
     
  19. Aug 6, 2011 #18
    Small correction: instead of "sensible enthalpy h=cp*T" I should have written "internal energy u=cv*T". Because, of course, u+P/rho = h so writing h double counts the pressure energy. And my writing "or, the pressure drops" is just another way to think about the cooling, because the pressure term in Bernoulli's equation is, by the ideal gas law, P/rho=R*T, so P/rho drops when T drops.
     
  20. Dec 1, 2011 #19
    I have a <2/12 pitch roof, with the ridge in a N/S axis. and generally the wind is from the east, at my Anchorage, Alaska location. My question is: better location for a wind generator to the East of the ridge line, or behind the ridgeline to the West. My understanding is the roof deflection helps increase windspeed...am I on the right thinking??
    any help/explanations welcome.
     
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