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Question about cooling.

  1. Jun 27, 2009 #1
    How much electricity does it take to liquify oxygen? I am just curious as if it only took small amounts one could create a power plant that liquified oxygen, put it in a small chamber outside to return it to a gas and used the pressure to turn a turbine with the gas returning for cooling. This of coarse would only work if it put out more energy then it took in.
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  3. Jun 28, 2009 #2


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    Welcome to PF.

    If the oxygen is to return to its original state, it is impossible for it to put out more energy than is put in. This is known as the first law of thermodynamics, which essentially says that energy is conserved.
  4. Jun 28, 2009 #3
    Actually it gets the energy to return to gas from the sun. That is why the tank for the liquid oxygen would be outside, to take the heat from the outside air, which anywhere in the world is warm enough.
  5. Jun 28, 2009 #4


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    It doesn't matter where it gets the energy to return to the gas phase. The energy into the system will always be less than the energy you can get back out of it--less because of losses. If if you could magically remove all sources of loss, the energy out still cannot be more than the energy in.
  6. Jun 28, 2009 #5
    It sounds like you are saying that out of habit. If you just liquified oxygen in a chamber, let it return to a gas and had it go through a turbine, I would agree, you would lose energy. However if you put that liquid oxygen in a smaller chamber, one only as big as the liquid, it would have greater pressure when it returned to a gas. I am not saying that your are not right, one may get less electricity out of this system if the cooling takes too much, but if it only takes small amounts to get it going, you could make a lot of energy.
  7. Jun 28, 2009 #6
    I see what you are saying.

    Your idea is that you use energy to liquefy a gas (O2 in your example), and then use ambient heat to expand it, doing work. Then, remove heat from expanded gas, and continue.

    This really isn't anything new though; it is essentially a steam cycle except it uses a working fluid much more volatile than water, and uses ambient heat/solar power as a heating source instead of a more conventional fuel source (nuclear, coal, etc.).

    It will NEVER be able to produce more work than is put in as energy, but there is no rule that says it can't produce more energy than YOU put in as work (in this case, the liquefaction process and pump required to keep the liquid flowing).

    This is similar to how, from the point of view of work/energy put in, an oil rig produces huge quantities of energy. All you have to do is pump the crude oil up; the biochemical processes that guided its formation already happened, free of charge. This is similar to using the sun; an extant free energy source.

    Your process MAY work, but the amount of work produced will not exceed the difference between the gas in its highest enthalphy state minus its lowest. (I think...correct me if I am wrong, anyone, but I believe this is technically accurate.)
  8. Jun 29, 2009 #7
    That process is basically exactly how an air conditioner works. A gas is compressed and liquefied, then pumped into an area that you want to cool, a fan is there to blow ambient air over the liquid and transfer heat into it and then the gas is pumped back to an area where it can be compressed again. They of course are quite costly to run, and definitely don't produce energy, even though the heat taken in by the refrigerant would cause it do to work, it is nowhere near enough to regain anything more than a fraction of the energy put into producing the liquid phase.

    I don't want to discourage from thinking of stuff like this. If you had come up with this idea for unlimited energy before refrigerators and air conditioners were invented, and tested it out, you would have failed to produce unlimited energy, but you would be sitting on a gold mine of an invention anyway.
  9. Jul 10, 2009 #8
    dont u need a sink
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