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Recovering pressure-volume energy from compressed hydrogen?

  1. Apr 25, 2005 #1
    The energy density usually given for hydrogen is 142 MJ/kg, by which comparison is made to how much hydrogen would have to be carried for it to be competitive with gasoline-fueled vehicles:

    Bottling the hydrogen genie.
    "One kilogram of hydrogen provides about the same chemical energy (142 MJ) as 1 gal of gasoline (131 MJ). Factoring in the greater efficiency of PEMs [fuel cells], we need to store about 1 kg of hydrogen for every 2 gal of gasoline on a similar internal-combustion-engine vehicle."
    http://www.tipmagazine.com/tip/INPHFA/vol-10/iss-1/p20.html

    But one of the storage methods is by high pressurization. Quantum Technologies has a hydrogen tank that can store hydrogen at 10,000psi.
    But the energy density quoted of 142 MJ/kg does not take into account how much extra energy is added by the high pressure. It seems to me you should get more energy out by using this high pressure.
    Let's say you have 8 kg of hydrogen stored at 10,000 psi = 6.8*10^7 Pa and 300K temperature. There is deviation from the ideal gas law at this pressure and the density is only about 32kg/m^3. See the hydrogen properties here:

    Hydrogen Properties Package.
    http://inspi.ufl.edu/data/h_prop_package.html

    Then the volume of this is 8/32 = .25 m^3. Let's say this was compressed using a constant pressure of 10,000 psi from the volume that obtains at 300K and standard pressure of 1 bar. Then the density then was .09 kg/m^3. So the volume at this T and P was 8/.09 = 88.9 kg/m^3. So the change in the (pressure*volume) energy is 6.8*10^7*(88.9 - .25) = 6.03*10^9 J.

    But the energy contained in 8 kg of hydrogen at 142 MJ/kg energy density is 8*142*10^6 = 1.13*10^9.
    So the energy added to the hydrogen just by compressing it to 10,000 psi is close to 6 times that already contained in it as chemical energy.
    Is there anyway of recovering this energy back?
    If so then hydrogen at least at 10,000 psi would not have just 3 times the enrgy content of gasoline by mass but 3*7 = 21 times as much.



    Bob Clark
     
  2. jcsd
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