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Piston moving 1m per hr, how to power a generator?

  1. Jul 4, 2011 #1
    hi, thank you for taking the time to read my post,

    im looking at how to convert the force from a piston moving at approximately 1 meter per hour into electrical power via generator, any ideas appreciated

    the piston will be slow moving but generate an extremely large force, its a question of how to translate this slow moving force into a fast moving mechanical spinning to power up a generator, a question of gearing


  2. jcsd
  3. Jul 4, 2011 #2
    I, myself, have always had a hard time with Energy, Work, and Power...but maybe it would help to look at the units...I mean, for example:

    J = N m = W s

    A joule is the same as a newton-meter and the same as a watt-second...

    So, if your piston, for example, exerts a force of 3600 newtons along a path of 1 meter, you have done work equivalent to 3600 Nm or 3600 J ...but it took you a whole hour , so let's divide by 3600 seconds....so you have 1 J/s or 1 watt ...1 W, that's starting to sound electric...

    of course, not all available mechanical energy will be converted into electricity...you will have losses along the way...but at least you know what's available if your system was 100% efficient.

    Hope this helps
  4. Jul 4, 2011 #3


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    I have a feeling that some hydraulic 'lever' could be a good way of achieving what would be a huge and inefficient mechanical gearing up ratio. In fact, what is the fluid that is pushing this piston? Why not just direct the same volume of fluid into a smaller diameter piston and cut out the middle hydraulic system?
    Could this be something to do with tidal energy? The timescale sounds about right.
  5. Jul 4, 2011 #4
  6. Jul 4, 2011 #5
    thanks for all replies

    @gsal - if the force is lets say 1 metric tonne or 1000kg , how much watts would that produce?
  7. Jul 4, 2011 #6


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    Can you plug that into the equation yourself...?
  8. Jul 4, 2011 #7
    probably if i search for long enough im sure i could calculate it, but its not a calculation im familiar with so i hoped some kind soul would speed things up for me..
  9. Jul 4, 2011 #8
    Well...that's another point of confusion...I mean, we always talk kilograms and pounds and never bother to distinguish between kilogram-mass and kilogram-force or pound-mass and pound-force...

    A kilogram is a unit of mass.
    A kilogram-force is a unit of force...not an official unit, more like a convenience unit...this is the force that results due to the weight of 1 kilogram when gravity pulls on it...

    ...remember F=ma? F = 1kg X 9.8 m/s2 = 9.8 kg-m/s2 = 9.8 N

    So, the force of 1 kilogram under gravity is 9.8N, but for convenience we also said is 1 kilogram-force or 1kgf

    So, what exactly do you have? 1000 kgf? If that is the case, you need to multiply by 9.8 (as a conversion factor) to get newtons...

    1000kgf*(9.8N/kgf)*1m/3600s = 2.72 W

    If this force of 9800 newtons acts for an entire meter per hour, looks like it can provide an instantaneous (and continuous) power of 2.72 watts
  10. Jul 4, 2011 #9
    thanks gsal,

    2.72 watts seems to be a very small amount for the 1000kg weight? i expected higher, but im not familiar with the equations so will need to investigate further
  11. Jul 4, 2011 #10
    As I was driving back from the store, the same thought occurred to me...can't even turn a 15W light bulb? Seems to little power, then again, I have no practical experience on these matters.

    Sure, you have a 1000 kilograms, but they are hardly moving! Think about it, you could have 100,000 kilograms, but if they are not moving in anyway, you cannot get any energy out of them...

    I would be curious to know what kind of system we are talking about here...what is the main purpose of the system? is the system loaded with energy for some other purpose and then you are trying to recuperate such energy by converting into electricity? Or is the main purpose of the system to actually produce electricity? Why in the world does it only move at 1 meter per hour?...I mean, a freaking garden snail moves an order of magnitude faster than that!
  12. Jul 4, 2011 #11


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    We get questions very similar to this every now and then. People just don't realize how much energy things use.
  13. Jul 5, 2011 #12
    Hi all,

    the project is lloking at harnessing tidal power through using floating barges that have an upforce when tide is rising, at the top off the tide the barges can be flooded with water, causing a large and approx equal downforce. Barge or floatation devices can run up a suspended pylon set into the water, so you have an upforce over 6 hour periods of approx 5 metres followed by a subsequent downforce. Force is dictated by size and displacement of tanks, eg a 10m square tank full of water is approx 1000 kg.

    Looking at how to harness this force via a piston or other method to produce electricity, seeing if its viable, and how it could be done,

    one method would be to use the upforce & downforce to compress air or oil in a hydraulic arrangement and use that hydraulics to turn a generator, otherwise its trying to gear a very slow moving force
  14. Jul 5, 2011 #13
    10 m square tank? As in the cross-sectional area at the bottom of the tank? how high is this tank? ... 1 cubic meter alone weighs 1000 kg, if you now say you have 10 square meters...if this tank is at least 1 m high, you are talking about 10,000 kg, then....just checking back with your initial 1000 kg and seeing if it still applies...
  15. Jul 6, 2011 #14
    many thanks gsal

    the tanks can be any size in theory, however if they are too large for the amount of electricity they generate it will not be commercialy viable,

    for simplicity was looking at a tank 10m x 10m at base and 1 meter high, how much kg force would that produce? and how much electricity?

  16. Jul 6, 2011 #15
    just tried the calcs from your previous post and im getting 272 watts for a 10m x 10m x 1m tank, does that tally with your figures?
  17. Jul 6, 2011 #16
    That's 100 tonnes, now, not just one; so, yes, you should be getting 2.72 x 100
  18. Jul 6, 2011 #17


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    I haven't done the sums on this but I seriously suggest that you all
    stick to SI units. Kg Force is not one of them. Newtons are SI force and using them will reduce the possibility of getting any answers WRONG by a factor of about ten, in either direction.
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