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Turbine-powered subway train (3 parts)

  1. May 15, 2012 #1
    Hi,

    For my Writing for Engineers class I have decided to take on a feasibility inquiry: "Is it possible to power a NYC subway train using wind turbines atop that same train?"
    (provided that a NYC subway train requires 2.1 MW of power to run at 55 mph)
    Now, I have come to the conclusion that this is impossible because of things like air resistance and weight but I am not sure of the fine details.

    1- If somone could give me a thorough explanation I would very much appreciate it!

    I have emailed a professor about this and received the answer:
    "Any electrical power that you take out of the turbine will add to the 2.1MW that the train draws from the power lines just to move forward at 55mph. Thus, if you connected up some electrical device that consumed 1MW of electricity from the turbine, the train would then draw 3.1MW of power from the power lines. As you draw more power from the turbine, the blades become harder to turn and the wind resistance goes up, causing the train's motor to work harder."

    2 - Everything here is understandable in a general sense except why do the turbine blades become harder to turn? If a 55 mph wind is still hitting the turbine, what is making it harder to turn?

    Now, what if the power lines were not used to power the train with the turbines helping but instead a system of batteries (with the turbines recharging the batteries) were used to power the train? I know that this still would not work. But again, only in a general sense.

    3 - A clear explanation as to why this would still not be feasible would be much appreciated!

    Thank you all in advance!
     
  2. jcsd
  3. May 15, 2012 #2
    How can you come to the conclusion it's impossible without having done some working first?

    You are right it wont work, but for wrong/incomplete reasons.

    Turbines take energy from the moving air. This is fine when it's the turbine that's stationary and the air is moving past. When you are providing the energy to move it through the air, to then take energy from it...

    So you see the problem?

    http://en.wikipedia.org/wiki/Perpetual_motion
    http://en.wikipedia.org/wiki/Drag_(physics [Broken])
    http://en.wikipedia.org/wiki/Efficiency
     
    Last edited by a moderator: May 6, 2017
  4. May 15, 2012 #3
    Yes, that makes sense. But again, it seems like a vague explanation. As you can probably tell, I don't have much knowledge in this area. That's why I asked for a thorough and clear explanation. Also, how about part 2's more specific question, why would it become harder for the turbine blades to turn?

    Thank you for the speedy reply!
     
  5. May 16, 2012 #4
    You've already said it's a peice of work. Which means you need to do some reading rather than someone giving you the answer.

    The blades becoming harder to turn seems to be a slip of the keyboard. The faster you go, the more drag is created.
     
  6. May 16, 2012 #5
    The turbine blades are connected to a generator. As soon as you allow that generator to create electricity, that same electricity will create a magnetic field which will act AGAINST the generator, making it harder to turn. The more electricity it makes, the stronger the magnetic field and the harder it is to turn. Whether that electricity goes directly to the train motors or a battery or into the city grid is irrelevant.

    This is a key aspect of generators that a lot of people don't realize. You can test it yourself by turning a simple electric motor by hand. It is easier to turn if it's not connected to anything. The difference between an easy to turn and hard to turn generator is really a flick of a switch away.
     
  7. May 16, 2012 #6

    russ_watters

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    Staff: Mentor

    An electric generator is an energy conversion device: power in = power out. It works by electromagnetism, so the more energy it generates, the stronger the magnetic force opposing the rotation. It is exactly the opposite of an electric motor.

    Frankly, though, while it is good that you are angling toward the right conclusions, I hope the technical content of this paper is not critical (given that it is a writing class) because it is iffy whether it is worthy of a college level engineering level thought process. Conservation of energy is such a basic concept that the issue of a train powered by its own wind turbine doesn't require a complex analysis. At face value, it is a clear-cut violation of conservation of energy, since the turbine is both a source of power and drag. If a wind turbine provided more power than drag, that would be Pout > Pin. Conservation of energy isn't something we try to prove by analysis of systems, it is something assumed and used to aid the analysis of systems.
     
    Last edited: May 16, 2012
  8. May 16, 2012 #7
    Great! Thank you! I did a little digging around. You are referring to Lenz's Law (about CEMF), correct?

    This does a better job of explaining it, thank you. The English professor who assigned this paper asked only that we have an inquiry, inquire honestly, and write about it. Both he and I had no idea what I was getting into with this particular inquiry. This did not have to be college level "engineering," only college level writing. I'm not sure that my professor will even understand the technical stuff, so I guess I'll have to explain in terms of physical laws like conservation of energy/perpetual motion/drag. Thank you for your honest concern and your helpful answer. I rarely get such direct answers with quesitons like these, even with specifically used words in the post, like "clear" and "thorough."
     
  9. May 16, 2012 #8

    russ_watters

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    Staff: Mentor

    Then you're fine.
    You're welcome - and thank you for being open to constructive criticism. It speaks well for your openness to learning and is distressingly uncommon.
     
  10. May 21, 2012 #9
    Yeah. Although admittedly I didn't know this phenomenon had it's own name or even "law". I just kind of figured it out by gluing together random bits of info with my own curiosity. You learn something new every day :)
     
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