Compressed air battery/propulsion

Discussion in 'Propulsion' started by Paulc, Jun 15, 2014.

  1. Paulc
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    Paulc Junior Member

    Has anyone ever heard of a compressed air battery/motor used on a boat? They are claimed to have an energy density equal to that of a lead acid battery, but are much lighter.

    If the tanks could double as pontoons then they could be space efficient as well.

    I’m working on harnessing wave energy, this could be the best way to store it.

    It might not even need a propeller if the air jet could push against the water efficiently enough?

    What are your thoughts?

    Paul
     
  2. gonzo
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    gonzo Senior Member

    Do you have any references to those claims? The density of air is very low, like all gases, so the energy stored will be low unless to increase the pressure to the point of liquefaction. A diving tank only gets about 3000-3500 PSI and it barely floats. Pontoons would need to be way too thick. If you are talking about batteries that produce energy through chemical reaction to oxygen, that is totally different. They are used for equipment that needs very small energy source.
     
  3. Stumble
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    Stumble Senior Member

    I had to look them up, but apparently they operate on 215psi. So within reasonable. But are really designed to replace storage batteries in UPS systems. They are intended for industrial operations between when power goes out and generators can kick in, not to replace regular use.

    It would be a challenge to figure out how to recompress air. And dealing with the sound of an air compressor all the time would be terrible.
     
  4. CDK
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    CDK retired engineer

    Pneumatics are hopelessly inefficient because of thermal effects, both during compression and expansion.
    In fact the reasons pneumatic tools exist are only twofold: less theft and no danger of electrocution due to damaged cables. And the energy stored in a tank with compressed air is very disappointing, both in volume and weight.
     
  5. Stumble
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    Stumble Senior Member

    Ya, but if all you need is 30 seconds of instantaneous power it may work instead of using batteries that need constant maintenance and recharge to overcome self-discharge rates. And where the cost of the air is immaterial since hopefully it never runs.

    As I mentioned using this on a boat seems far fetched.
     
  6. Paulc
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    Paulc Junior Member

    But if the tank was submerged in sea water it would be water cooled and heated.

    And boats generally need a large volume ballasted hull.

    Look at this: http://en.wikipedia.org/wiki/Compressed_air_car
     
  7. CDK
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    CDK retired engineer

    It certainly would, but that doesn't alter the fact that the process is inefficient, except in case a "free" energy source is used for the compression phase.
     
  8. Paulc
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    Paulc Junior Member

    Well yes, free energy from waves, that is the idea.
     
  9. daiquiri
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    daiquiri Engineering and Design

    The cooling phase of the air compression is exactly the reason for the inefficiency of the compressed air as energy storage. During the compression phase, part of the compressor's work go into a pressure rise and part into a temperature rise. That's a fact which comes from the laws of thermodynamics and cannot be avoided.
    Once stored at high pressure and temperature, the gas starts to cool off at constant volume, thus dissipating the accumulated thermal energy. This energy is irreversibly lost and is the main culprit for the inherent inefficiency of the whole process. You can improve the compression efficiency by adopting the multi-stage compression with intercooling, and the storage efficiency by thermally insulating the tank. However, these can only decrease the heat dissipation and slow down the air cooling, but cannot eliminate them.

    Did you read the data in the link you have posted? It clearly states that Li-Ion batteries are much better performers, besides being safer.

    Cheers
     

  10. Stumble
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    Stumble Senior Member

    As a practical matter you would also need to find a compressor pump capable of running off the available power. I don't know what you are expecting in energy output, but high pressure compressors tend to take a lot of juice.
     
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