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Harnessing Energy (Trains)

  1. Nov 20, 2014 #1
    Dear all,
    A group of students and I were designing a project aimed to harness a train's motion in order to produce electricity that can be used to run the train itself. Our idea was to connect a protruding wheel on a side of the train which consists of a dynamo located in the wheel. This dynamo will then convert the wheels motion into electric energy which will be discharged through capacitors. Our issue, however, was that some individuals say that this idea runs contrary to the law of conservation of energy. In other words, if we want to get more energy out of the train system (in the form of electricity) we have to put more energy in. And during the process of putting more energy in and taking more energy out, you will inevitably lose energy due to the Coefficient of Efficiency. What we're thinking is slightly different. Before the implementation of our project, some of the main power supply's energy is naturally wasted as sound and heat, but what we are trying to do is convert a fraction of this kinetic energy into an electrical form to instead of an unusable form. Yes this would require more input energy but only to the extent of the wheel's friction. However, our main target is to use a dynamo strong enough to generate an amount of energy larger than the wheels friction. For example, 1000 J Kinetic Energy BECOMES 700 J of heat and 300 J of sound before our idea is implemented. After the implementation though, 1000 J Kinetic Energy BECOMES 300 J of Heat, 100 J of sound and 600 J of energy goes to the wheel to rotate it. Does our point of view make sense or are these individuals correct about the impracticality of this idea? Please provide explanations for your response. Thank you!
     
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  3. Nov 20, 2014 #2

    Danger

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    Sorry, but that would be an "over-unity" device; ie: perpetual motion machine. Not possible.
     
  4. Nov 20, 2014 #3

    Bystander

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    Incorrect. The energy required is whatever energy you desire to derive from your dynamo.
     
  5. Nov 20, 2014 #4
    @Danger What exactly is an over-unity device?
     
  6. Nov 20, 2014 #5
    Thank you for the response!
     
  7. Nov 20, 2014 #6

    Danger

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    As I said, a perpetual motion machine. It does indeed violate Thermodynamics. Basically, it's the same idea as using a motor to drive a generator that then powers the motor, and on and on. You can never break even, let alone get ahead.
     
  8. Nov 20, 2014 #7

    Simon Bridge

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    From what read in post #1, it is not quite perpetual motion, more an attempt to improve the efficiency by recovering some of the "exhaust" energy.
    Look up "Carnot efficiency" ... you cannot avoid some heat and sound.
    i.e. the heat-recovery device would, itself, generate some heat, the sound recovery device will make noise, etc.

    The best approach is to change to a more efficient engine.
     
  9. Nov 20, 2014 #8
    @Simon Bridge I googled up Carnot Efficiency. So basically it is possible to generate an electric current from heat even though some heat will escape. But is it as simple as installing a Carnot Engine and generating electricity or does the installation also require energy resulting in a net loss of usable energy. Thank you!
     
  10. Nov 20, 2014 #9

    Simon Bridge

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    You didn't understand - the carnot efficiency is the most efficient you can get an engine.
    What you are considering is trying to make an existing engine more efficient by adding energy recovery devices to it.
    No combination of contraptions and machines can be more efficient than a carnot engine running between the same energy source and sink.
    So there will be a point where adding more energy recovery devices will do nothing to increase performance even if the devices were perfect in their construction and operation. More likely adding energy recovery devices to the engine will decrease its overall efficiency.
     
  11. Nov 20, 2014 #10
    @Simon Bridge So I'm guessing this series of energy recovery devices required to make this project efficient will cost more than $2500 for only a minimal energy gain.
     
  12. Nov 20, 2014 #11

    Simon Bridge

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    The devices will cost whatever you want to spend, and the gain, for a commercial engine, will, indeed, be very little.
    It is usually more cost effective to just buy a different engine that is engineered from the ground up to be more efficient.

    If you are stuck with a very inefficient engine, there may be some benefit in recovering some of the wasted energy.
    You could put thermoelectric cells in the stack of a steam train and use the electricity to run cabin lights for eg.
    It's also why it is worthwhile to insulate hot-water pipes and so on.
     
  13. Nov 21, 2014 #12

    Danger

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    Also investigate regenerative braking. That's recovering energy that is otherwise wasted.
     
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