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Exactly how do i control the fall of a falling object? (for power generation)

  1. Oct 21, 2014 #1
    Hi!

    So i have been having this discussion with a friend. we have an old shaft going down about 100meters and we want to tie a 100kg sand bag and use it to generate electricity as it falls. (we will simply pull it back up)

    if my understanding is right, then:

    i can either generate 980kw for 1 second OR aprox 200watts for 1 hour. am i correct?

    if i am correct, then how exactly do i make the weight fall over 1 hour? also, i know that the figure of 980kw does not take into account efficency loss so can i get plz get some advice on this as well? what will be the true output once friction and all that is taken into account?
     
  2. jcsd
  3. Oct 21, 2014 #2

    russ_watters

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    Welcome to PF!
    Er...I'm getting 98 kW for 1 second or 27 watts for an hour....could you post your math?
    Well, it is connected to a generator, so the generator may regulate the fall itself depending on the type of load you connect to it. Otherwise, you'd need some control circuitry.
    Friction and other inefficiencies don't have to be a very big factor for such a simple system. You could probably do 75-90% total efficiency.
     
  4. Oct 21, 2014 #3

    Simon Bridge

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    The power calcs you have done are averages only, you will get more power out at the end of the fall.
    I don't think you can get it to fall in less than about 4.5s without throwing it... but the reasoning (not the arithmetic) is fine ... there is a maximum energy that you can get out of such a set-up. Not all this energy will get turned into electricity - how much should depend more on the generator you use than the rest of the setup.... but I haven't seen your materials.


    In practice - you convert kinetic energy into electricity - not gravitational PE ... this is important.
    If you converted all the kinetic energy into electricity - the mass would end with zero kinetic energy and stop.
    A stationary mass cannot turn your generator...

    If you want the mass to fall slowly, then you need a rocking ratchet type mechanism like in a clock... there's a special name for these that keep forgetting. You can also release the mass a little at a time of course, and, whatever you do, the generator itself will slow the mass.
     
  5. Oct 21, 2014 #4
    Thank you for the welcom! you are correct russ, the figures are 98 and 27 respectively. i hit an extra 0.

    can you please give me a name of a generator that would regulate the fall? will this mean that if i have a 27watt bulb plugged in, the weight would fall over an hour and if i had a 13watt bulb plugged in, the weight would fall over 2 hours? or did i misunderstand?

    Simon, at the moment, the only materials i have is a sandbag in which i can fill a 100kg of sand. i think the shaft used to be a well because it has a pully with bucket tied to it, something like this:

    water-well-bucket-9242274.jpg

    the plan is to replace the bucket with the 100kg sand bag and allow it to fall over a period of hour or 2 hour. the hole is about 6-8 meters in diameter so i am sure i can add some more sand bags to get more watts out. but i want to try it with one bag first.
     
    Last edited by a moderator: Oct 22, 2014
  6. Oct 21, 2014 #5

    russ_watters

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    Well, sorta. Falling at constant speed yields constant power output -- at least for the realistic slow falls. Trying to get the energy quickly is problematic because you have to let the weight accelerate and then absorb the energy at the end of the fall (as you were saying) and there is a minimum time...
    Yes, that's a good point: when trying to get the energy in a short time, you have to deal with the acceleration due to gravity issue. At the minimum time of 4.5 seconds, that's 100% acceleration, which means you get no energy output, just an impact with the bottom of the well. As you lengthen the time, you hyperbolically approach the theoretical maximum energy extraction.
    No, that's mostly wrong. Except in the very fast fall case, you are harnessing potential energy, not kinetic energy. Harnessing kinetic energy means you are slowing something down whereas harnessing potential energy means applying a constant force over a distance. A lot of hydroelectric dams work each way.

    [edit: softened the last part] Examples: Hoover dam is a Pelton (kinetic energy) turbine, while the Three Gorges Dam is a potential energy type Francis turbine. Most dams use Francis turbines. Pelton turbines work best with high head (Hoover Dam: 590 feet, Three Gorges Dam: 264 feet).
     
    Last edited: Oct 21, 2014
  7. Oct 21, 2014 #6

    russ_watters

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    I'm a little thin on the electrical part, so others might do better, but have a look here for a start:
    http://www.wikihow.com/Produce-Electricity-from-a-DC-Motor [Broken]

    Basically, if you have a purely resistive load (an incandescent bulb?) and the generator is matched to it well, essentially what happens is the voltage increases as the generator spins faster. Then, the higher the voltage, the brighter the light bulb gets and more energy it uses. As it does this, it increases the torque on the generator, which stops the acceleration, resulting in a constant-speed fall of your weight.
     
    Last edited by a moderator: May 7, 2017
  8. Oct 22, 2014 #7

    Simon Bridge

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    The generator does slow something... but that's mostly splitting hairs. I mean maybe there is some conventional terminology that makes that definition. What I mean is that an electric generator that is likely to be useful here needs to move in order to work.

    Since the weight gets hauled up by hand each time, it seems this is something that should be experimented with.
     
  9. Oct 22, 2014 #8

    DrClaude

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    Escapement
     
  10. Oct 22, 2014 #9

    jack action

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    The only sure thing is that you will produce no more than 98 kJ of energy per cycle (the weight going up once and down once).

    Then, the time it takes to complete the cycle will dictate the power you can get out of the system in the long run.

    The best thing you could do is to charge a battery, no matter how much time it takes to do it. How fast you discharge the battery is up to you. If your power demand is greater than the power of your generator, you will simply have some intermittent down time.

    The problem with your concept is that your using an unnecessary energy transfer: watching the weight fall. No matter how you will get the weight up, you could use that energy to directly turn your generator. Storing it in a mass under potential energy just to release it somewhere else is a waste of time and a source for losses.
     
  11. Oct 22, 2014 #10

    Danger

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    I believe that the term you're looking for is "escapement".
     
  12. Oct 22, 2014 #11

    Danger

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    By the bye, Shipton... that picture of yours is the most vivid "Hitchiker's Guide" flash that I've had without tequila being involved.

    edit: Sorry Dr. Claude. I just now saw your response.
     
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