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Can One Make the atmospheric pressure to store energy

  1. Mar 14, 2010 #1
    If I Make two(many) tanks to fill water each one at different height levels.
    1. Fill the lowest tank with water, open it to atmosphere.
    2. close the second tank and create vaccum in it. make water flow from first tank to this till it gets filled.
    will this process allow me to store extra energy than needed to create vaccum?.
    please say if this is any reasonable?
     
  2. jcsd
  3. Mar 14, 2010 #2

    mgb_phys

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    First law of thermodynamics - you can't win
     
  4. Mar 15, 2010 #3
    Thanks,
    But the atmospheric pressure will be doing work on the water right?
     
  5. Mar 15, 2010 #4

    mgb_phys

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    Yes - but it's really no different from you pumping water into the top bucket and letting the water flow back to generate electricity.
    In either way you have to put in exactly the work you get out + a bit more to make up for any losses.

    You could imagine a system where on a high pressure (sunny day) you opened a valve to let atmopsheric pressure push water into the top tank, then released it on a lower pressure (stormy) day - then you are using the difference in pressure.
    This is a bit like how a hydroelectric dam uses the energy from the weather, it evaporates water from the sea and lifts it to the mountains, then it falls as rain in your dam and you let it flow back down hill to make electricity.

    But the changes in air pressure is very small so you would need a lot of water
     
  6. Mar 15, 2010 #5
    If we pump water to the higher tank, we assume both tanks are having the same pressure.
    But if we create vacuum in the top tank, then the water will flow from the lower tank to the upper tank right?
     
  7. Mar 15, 2010 #6

    russ_watters

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    Yes....which is exactly what a pump would do (create a pressure difference that causes water to flow uphill). You've just described a pump and are suggesting that if you pump water up and then let it flow down, it will generate more electricity than it uses (as already said). No, it won't.
     
  8. Mar 15, 2010 #7
    It is not we who pump the water up. we create a vacuum, the atmospheric pressure will pump the water up.
     
  9. Mar 15, 2010 #8

    russ_watters

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    Yeah. How do you create the vacuum? With a pump.

    A pump (any pump) creates a pressure differential in order to move a fluid. If you use a pump to remove the air to create the pressure differential in the air to move the water, you haven't done anything fundamentally different than if you had just used the same pump to create the same pressure differential in the water.

    Draw yourself a diagram and examine the following two cases:

    1. A piston pump (such as a syringe) with zero volume, being drawn to 1m in length with a cross sectional area of 1m, while remaining sealed (ie, pulling a complete vacuum of 1 cubic meter).
    2. Using the same pump to draw 1 cubic meter of water up into it.

    Calculate the energy required to do both of these. I think you'll actually find that evacuating the air first is worse because it requires a constant force equal to the atmospheric pressure times the area whereas the pump only has a force equal to the weight of the water column.
     
    Last edited: Mar 15, 2010
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