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B Internal energy of compressed gas

  1. Jul 18, 2017 #1
    Hi,

    I've been reading about compressed air energy storage and keep coming across that in 300 bar containers the achievable energy is 0.1MJ/L. Is this 0.1MJ/L of the volume of the air it is compressed to or of the total L of air that was initially used? (E.g If 1500L is compressed to 300 bar into a 5L cylinder will the total energy be 0.5MJ or 150MJ)

    I have been trying to work this out but cannot seem to come up with the the 0.1MJ/L value.

    Using the internal energy calculation U = 5/2PV I get 0.375MJ which is 0.00025MJ/L . (5L volume at 300 bar = 1500L total air. If I compress 1500L of air at 300 bar it can compress to 5L with total internal energy of 0.375MJ)

    Could someone explain how they worked out the 0.1MJ/L value and where I am going wrong?

    Thanks.
     
  2. jcsd
  3. Jul 18, 2017 #2

    scottdave

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    Would you be able to give a link to the article that you read?
     
  4. Jul 18, 2017 #3
    Look at the Wiki of compressed energy storage.
    https://en.wikipedia.org/wiki/Compressed_air_energy_storage
    Isothermal storage or adiabatic can theoretically give a 100% return, but most likely there will be losses for the shaft work.

    Yes, a link to what you are reading would be wonderful.
     
  5. Jul 18, 2017 #4
  6. Jul 18, 2017 #5

    scottdave

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    I read the IJRMET article. They mention about 0.1 MJ/L, so 0.075 MJ/L I guess is in the ballpark (25% off). It also tosses out 0.1 MJ/kg, which at 300 bar, air has a density of 0.3 kg/L, so that figure is more achievable. It was really showing as a comparison to other ways of storing energy such as batteries or fuel, so being 25% off, doesn't seem too bad for those comparisons.
     
  7. Jul 18, 2017 #6
    OK thanks but where did you get the 0.075 MJ/L figure from?

    I calculated the total being 0.7MJ so works out as either 0.14MJ/L if only considering the 5L (which is similar) or 0.00047 MJ/L if considering the total gas used of 1500L.

    If it is the 0.14MJ/L then I am assuming it is per L of compressed gas.
     
  8. Jul 18, 2017 #7

    scottdave

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    You had a figure of 0.375MJ for 5 liters, which I divided. From the context of the article, it looks like it is the compressed volume they are talking about. I remember reading an article in Popular Science, probably 30 years ago, where they had tested something to drive a small vehicle around a warehouse or something, then there would be stations at different locations to swap out the air tank (which could refill). The "engine" was all plastic parts, if I remember. I'm going from memory.
     
  9. Jul 18, 2017 #8
    OK so they only consider the 5L final volume rather than the initial volume of air used at the start (1500L).

    Thanks for your help.
     
  10. Jul 18, 2017 #9

    scottdave

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    Yes. Suppose we want to compare compressed air and liquid propane (LP) and maybe compressed natural gas as possible energy sources for running a golf cart. You would look at these, as they are going to be installed at the equipment, not in the uncompressed state.
     
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