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Extra power from 40 mph exhaust fan

  1. May 6, 2007 #1
    Say I have a commercial building that has a 24 inch diam. exhaust fan that is blowing approx 40 mph wind out of the building. I wonrder if I can make good use of this wind by mounting a 24-26 inch pipe that may be 6 to 10 ft long on the outside of the building. I imagine cutting out sections in the "tunnel" and setting up electric generators on the outside. The shaft to the generator would be extended and a fan blade such as a paddle wheel type would be attached. the paddle wheel blade could be set into the new wind tunnel so that half of it is inside and half outside. The wind would spin and generate useful power.

    If anyone has any thoughts on this, I would like to hear them as well as possible details such as types of generators to use and what the output might be. Could enough generators be added to generate a significant amount of power?


  2. jcsd
  3. May 6, 2007 #2
    While you're at it, put a wind turbine on your car to turn a generator that charges your battery. This should remove the load on the alternator on the engine and improve fuel efficiency. :rofl:
    Ok, yes that was a bit sarcastic, but if you can see how the car/wind turbine can't work then you should be able to see how the exhaust fan in a building proposal will fail.
  4. May 6, 2007 #3
    The exhaust fan already exists on the building. Why not harness that wind exiting the building?
  5. May 6, 2007 #4
    The same way that the wind is blowing by the vehicle? Why don't we harness that? It's the same thing.
  6. May 6, 2007 #5
    I think that your too rigid in your thinking. With a 6" x 4" paddle wheel design fan blade 50% of which is inside the tunnel, the resistance would be negligable vs. the potential power production. We all understand the prior concept noted but this is an existing exhaust fan with a harnessable resource, at least I think it is, up to a point where the resistance is too great and impedes the functionality of the exhaust fan.
  7. May 6, 2007 #6
    Nope. This is a basic law of physics. They are rigid and to think they can be bent is foolish.

    So how far are you willing to take this? Negligible input with power output? You do realize what you are saying here right? Now I can understand that there may be more air flowing than necessary so a paddle wheel slowing things down wouldn't matter. If that's the case then it would be more efficient to change the fan or slow it down so there is less energy used by the fan which leaves more net energy to be used for something else which in your proposed generator would be produced but with a loss due to inefficiencies between the motor/fan and paddlewheel/generator.

    There is no difference. Existing fan... Existing moving vehicle...
  8. May 6, 2007 #7


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    Just in case Averagesupernova wasn't clear enough, the only way to have a negligible effect on the exhaust fan is to generate a negligible amount of power from it's airflow. If, for example, you want to generate 100w of power without decreasing the airflow of the fan, you'll need to run the fan faster, consuming an extra 100w of power (not including efficiency losses, of course).

    Conservation of energy!
    Last edited: May 6, 2007
  9. May 7, 2007 #8
    I fail to see your point unless you are trying to extract free energy. If you try and harness the exhaust fan power with a pipe you will obstruct the discharge air flow of the fan and decrease the efficiency of said fan. If the fan is discharging too much air then the thing to do is change the shiv (also known as sheaves or pullies) ratio and not try harnessing the excess air. Any thought of gain would be lost to friction. A good idea if you are not paying for the power, then you could steal the power with a chance of detection.
    Last edited: May 7, 2007
  10. Dec 9, 2007 #9
    Instead of trying to harness the wind, harness the heat loss off the fan motor. Use thermals engines like sterling motors to turn tiny generators and trickle charge batteries. You don't get perpetual motion but atleast your system efficiency goes up.
  11. Dec 9, 2007 #10


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    2016 Award

    Silly goose. A wind turbine would cause drag.
    Ooops. I see you're being sarcastic.
    Anyways, check out my solution to my serpentine belt idler arm going out one day, 12 miles from home:

    I'm glad the sun was shining that day. Barely made it home.

    Btw, I was wondering if there was a depository of novel ideas that should not be tried because someone has done the math.

    I calculated one day that the energy of the rainwater collected on my roof, run through tiny little water turbine electric generators, would be enough to cook about 2 packets of top ramen every year.
    Last edited: Dec 10, 2007
  12. Dec 9, 2007 #11


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    Staff: Mentor

    I've seen forums with a "trash bin" of crappy posts/ideas, but we don't have one.
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