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Is this buildable?

  1. Jan 23, 2009 #1
    Have you heard of those cars that are going to utilize a compressed air generator? Well, it seems to me that compressed air is a good and cost effective medium to store energy. Am I wrong? I’m wondering if you think whether a device that can be plugged into your home outlet that stores excess electric capacity at night as compressed air and redistributes it the next day into the home is one, feasible and two, would it be cost effective. En masse, this would effectively increase our capacity without building new power plants. Good idea? Bad idea?
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  3. Jan 23, 2009 #2


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    Feasible yes - efficent probably not.

    The difficulty with using compressed air is that as you compress it you heat the air up (if you double the pressure you also double the absolute temperature).
    If you just release this heat then you have wasted a lot of energy, when you expand the air again to generate power it will get extremely cold. In some applications this might not matter - such as in a car, where it is going to be released into the atmosphere.
    The alternative would be to keep the heat in the gas so when it is released it is returned to room temperature, but now you have the problem of building a tank that can hold very high pressures at very high temperatures.

    Finally compressed air doesn't store very much energy, a 12 litre tank (typical scuba cylinder) at 200bar is only about 1MJ (0.3KWhr) by comparison 12litres of gasoline would be 500MJ

    It's not a complete scam - compressed air engines are used in industry where you have a lot of compressors anyway and especially where a spark is dangerous such as in mining.
  4. Jan 23, 2009 #3
    I believe theres at least one company (from France IIRC) that has a car that runs on compressed air.
  5. Jan 23, 2009 #4


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    I don't know if they have actualy built/sold any - but it's not a completely silly idea.
    The energy density is lower than gasoline, but not far off batteries - they are using carbon fibre tanks so the energy/mass might be better than a lead acid battery.

    They (aircar) seem to be using a piston engine - they describe something about having twice the efficiency of expansion with quasi equal temperature which is either a dodgy translation or a scam.

    One issue you are going to have is heating - not only do you not have any energy to warm the car, but you must have quite a temperature drop when the 300bar air expands into the cylinder. So in England you would have a large block of ice attached to the car - perhaps in the south of France the refrigeration might be welcome.

    As I mentioned, compressed air trains have been used for >100years, it's not much of a modification for a steam train.
  6. Jan 23, 2009 #5
    I use compressed air in my car all the time. Compress, inject fuel, burn. Repeat ad naseum.
  7. Jan 24, 2009 #6
    My car uses compressed air in its tires.
  8. Jan 24, 2009 #7
    Last edited: Jan 24, 2009
  9. Jan 24, 2009 #8


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    Are you sure about your claim I have marked in red? We already do store excess electric capacity at night, en masse; why would your scheme be more efficient?
  10. Jan 24, 2009 #9
    I think the claim is in reference to using off peak electricity during the night. We still have a surplus at 2:00 AM. I really have never seen an electrical storage facility except for hydroelectric.

    Wiki's explanation of temporary electrical storage sounded a bit iffy except for hydroelectric.


    The Salt River project east of Phoenix buys off peak electricity off of the grid to pump water back up into Roosevelt lake during the night.
  11. Jan 24, 2009 #10


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    Some windfarms use compressed gas storage - they were built on an exhausted salt mine/natural gas well so there is a vast air tight underground tank ready built.
    Last edited: Jan 24, 2009
  12. Jan 24, 2009 #11
    Thats interesting, I'd like to find out more about that.
  13. Jan 24, 2009 #12


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    I think I saw it on daily planet / new scientist or similair

    There is www.generalcompression.com ,looks like it's more of an idea looking for money than an implementation.

    Nice if it worked - even without the storage I could see the advantage of having the windmill power an air/hydraulic pump and have all the generating plant at ground level - it would also solve the problem of matching turbine to constant generator speed.
  14. Jan 24, 2009 #13


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    Only if it actually does use 100% renewable and zero-emission energy. AFAIK, there is nowhere in the world that can claim that for a large population - France would be the closest, being that it is all nuclear.

    See, air isn't a fuel, these devices are basically just very inefficient batteries.
  15. Jan 24, 2009 #14
  16. Jan 26, 2009 #15
    ummm... you have to get the energy to compress the air from somewhere, ie a powerplant. therefore, the engine may be 100% emmission free and use a renewable resource, but the powerplant used to generate the energy to compress the air is probably not, unless it is run by a wind turbine, solar panel, etc. And even these are made from non renewable resources.
  17. Jan 26, 2009 #16
    Well, it looks like this technology exists. Russ Waters is right in that it is basically a battery. It would take what is essentially electric capacity that is lost at night and store it in an environmentally friendly way. While the original source of electricity may not be clean, the ability to add capacity to an electric grid without adding to the waste produced makes this zero-emission. I guess the question lies in whether this approach of power management offers any cost benefit. It does seem that while the US may have some capacity to store excess electricity, other countries where rolling blackouts continue to occur, this may be a very cost effective way to effectively increase the electricity available within the country. I seems the answer lies in the world of number crunching.
  18. Jan 26, 2009 #17


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    Still not sure it's viable as a storage scheme.
    Energy density is close to simple battery (lead acid) technology.
    Safety / inspection costs are similar, replacing batteries every few years vs removing and hydrostatic testing cylinders. Recycling gas cylinders might be better than lead acid batteries.

    The big loss is the inefficiency and cost of turning compressed air back into electricity via a motor and generator. Data centers looked at using massive flywheels instead of batteries as power backup, although the power density is higher and they last for ever the cost and complexity of generating the electricity was too high.
  19. Jan 26, 2009 #18
    The Fench vehicle obviously does work. But there are always drawbacks. It requires the air pressure to be very high.

  20. Jan 26, 2009 #19


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    For a vehicle it has big advantages over lead-acid, it's lighter if you use CF tanks and you can drive a piston engine directly (the previous comment was to the OP about using it for large scale grid power storage)

    The pressure is 'only' 290Bar - a bit higher than a scuba cylinder but about the same as an industrial gas bottle.
    Carbon-Fibre tanks are the break through. They should be cheaper to make and ship than steel / aluminium once you get the production volumes up, they don't corrode and if the did fail they should just rip open rather than throwing shrapnel everywhere.
    Not sure how easy it is to inspect them - metal cylinders get periodic induction/ultrasound crack checking.

    People have looked at CF before, but for scuba they have the disdvantage of being too light when empty so causing buoyancy control problems. For industrial gas cylinders they aren't as robust when being thrown around. I think some emergency services are using them for breathing apparatus.
    With no mass market there wasn't the scale economies and so they remained very expensive.
  21. Jan 26, 2009 #20
    Thanks for the informative post. I apologize for slipping in the air powered car bit. It wasn't meant to be a reply to the previous post. I was thinking that the car had been mentioned before.

    Come to think of it using off peak electricity to produce compressed air for air powered vehicles might be a feasible idea.
    OH OH there I go again bringing up that air powered vehicle.:smile:
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