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Vehicles fueled by compressed air....

Discussion in 'Cars and Transportation' started by danny, Mar 8, 2007.

  1. danny

    danny Administrator

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    http://jerrychacon.com/?p=159
    How do you guys think this compares to electric cars.
    I'm not very familiar with this type of fuel so I was hoping
    someone could shed some light on this.
    Thanks
     
  2. WarpedOne

    WarpedOne Mainecoon Butler

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    In short: energy density

    Compressed air is a form of energy storage and there are limits how much you can compress it. The more it is compressed, more energy it containes. On Tesla's blog their is a mention of 5 to 10 times LOWER energy density of compressed air than current LiIon batteries. It is an interesting thing for short show-ups, but you would never get any reasonable range out of it.

    Yet another thing that looks cool to a jurnalist, but an engineer would just wave his hand.
     
  3. tonybelding

    tonybelding Active Member

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    I have a special interest in compressed air, since I have a collection of air rifles. To give you an idea about energy density, there are very few air rifles powerful enough to challenge the least powerful (22 rimfire) firearms, and the airguns that can do this tend to be quite expensive.

    Here's my favorite air rifle, check it out: http://users.htcomp.net/tbelding/FX/tarantula.html

    The energy density of air is limited by the strength of your storage bottle. The stronger it is, the higher pressure it can contain. My FX Tarantula tops out at 200 bar, which is 3,000 PSI. However, the pressure drops steadily as you use air out of it, so it's not a constant power source. It requires complicated valving to keep the power from fluctuating too much.

    By way of comparison, firearms develop internal pressures as high as 50,000 PSI, or a bit higher. There really is no contest. Firearms have been called the world's simplest internal combustion engines.

    The other thing to remember is that there's always a certain degree of inefficiency when compressing air, due to the diesel effect. Compressed air becomes hot, and the heat then diffuses away and is lost. There's no way around it.

    Getting back to cars. . . The new carbon-fiber storage tanks allow higher pressure air to be stored with less weight, so maybe these things could have competed with electric cars -- if we were still stuck with lead-acid batteries. But battery technology has moved on, so I don't really see the point of fooling with compressed air now.

    There's also a safety concern, as a ruptured air tank would go up like a bomb. It's much more dangerous than gasoline. This is why I cringed when GM showed off the Sequel with its 10,000 PSI hydrogen storage tanks! All it would take is one "road rage" or drive-by shooting incident, with a bullet striking the tank, and that sucker could be headed for orbit. "Houston, we have a problem!"
     
  4. chacon

    chacon New Member

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    ** Raised Hand **

    May I ask another couple of questions? Since my post that started all this was just one big question to begin with...

    But first, Tony, thanks for the thoughtful comment on my blog. The substance of the following is over there as well.

    On the diesel effect, there is always going to be some energy loss — people haven’t found a way around entropy yet. I am curious however how mileage would compare. In particular, I don’t know what the mileage per dollar of electricity to recharge the Tesla batteries is, but the “journalist” (in the video) asserted that the first of the two compressed air cars got about 100km per $1 (and of course, he did not say whether that was US dollars, Canadian dollars or what-have-you) of electricity in refilling the tank at home. Thoughts?

    On the safety thing, yes, that’s a great deal of pressure, but (i) the video did make reference to the containers being designed to split rather than explode (and splitting doesn’t sound too bad) and (ii) air just isn’t hydrogen — so your last comparison is a little off. Or, am I misssing your point?

    The problem for me with the compressed air car video is the fact that mileage seemed to be based on the liberal use of Aluminum. If I were to be honest in my comparison of mileage from the two technologies, it would have to be with similar weight vehicles.

    Please don’t get me wrong, I am not interested in this compressed air car as a replacement for the less-astronomically-priced Tesla I want to buy, but rather as an alternative for those that need only in-city driving.

    Tesla seems to be trying to develop a real car. This seems to be uber golf cart technology — but if it works, I am all for it. Thoughts?
     
  5. tonybelding

    tonybelding Active Member

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    I seem to recall (but can't find it just now) that Tesla estimated about 2-3 cents per mile on grid electricity. If you have variable-rate metering set up and charge your car at night during off-peak hours, it would be about one cent per mile.

    I'm not sure that splitting is a lot better, because the energy is still being released, and for every action there is an equal and opposite reaction. It means something is going to move violently. And it doesn't really matter whether the tanks contain hydrogen or air, because 10,000 PSI is still 10,000 PSI either way.

    There's nothing wrong with aluminum. I'm all for it, as long as it gets the job done.

    Well. . . For an urban vehicle of limited cost and range, it might be practical. (Though keep in mind, carbon fiber air tanks aren't cheap!) Then you are still competing with all the low-end electric cars, NEVs and such, which have been well developed for over 100 years.

    It reminds me of flywheel power. Yes, you can power a car from a flywheel, but. . . What do you gain by it? Flywheel technology seemed promising a few years back, but then electric storage batteries advanced more quickly and left flywheels behind -- and flywheels probably still have more potential for improvement than compressed air systems. Put another way. . . Even if compressed air could compete with the battery systems of today, what about five, ten, fifteen years from now? Batteries are only getting better, but there doesn't seem to be much room for improvement in compressed air.
     
  6. Tesla2Go

    Tesla2Go Member

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    But how much energy is required to compress that air? How efficient is the compressor? Perhaps at most 60%? Then what is the efficiency of the engine running on compressed air? So instead of using energy to compress the air with a 40% loss, then run it in an compressed air engine with further losses, isn't it more efficient to run an electric motor? If you're solar panels, wind, hydro, nuclear to produce electricy, then you can't beat an electric motor at efficiency. The less energy conversions the better.
     
  7. chacon

    chacon New Member

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    @Tesla2Go: I posted your comment on my site under an entry entitled "A Blow for the Compressed Air Car" and attracted a pretty heated comment from someone that made some pretty good points. You are invited over to the post (http://jerrychacon.com/?p=165) to respond, or if you prefer that I ask the commenter to come over here, that's up to you.

    @Tony: Same goes for you too.
    -- Jerry
     
  8. Tesla2Go

    Tesla2Go Member

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    Well, I'm not sure who I would be responding to, but no matter if you drive a compressor directly from a wind turbine, or from electricity, you still have to ask yourself: What is the efficiency of the compressor? How much energy do I put in, and how much is left in the compressed air?

    I think you will find compressors have about 50% efficiency. That is, only half the energy used to compress the air is found in the air tanks. Compare that with the efficiency of batteries/electric motor.

    Compare also the simplicity and reliability of an electric motor vs a motor running on compressed air. Which has less moving parts and needs more maintenance?

    Has anyone shown any calculations how much compressed air you need to drive say 300 miles for a 1 ton car?
     
  9. electrified

    electrified Member

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    Remember that compressed air tools are used because they are stonger, lighter, cheaper, and longer lasting than other types of power tools. Granted, none of them actually carry around the air source with them, but it seems to me that the air car could fill a nitch market and do so cheaply. Some of the posts on the Tesla Motors site suggest that battery life is currently a major part of the cost of driving a pure electric vehicle, even several times the cost of electricity over the life of the battery system. Obviously, that cost is not a factor with compressed air, so for now, even with the inefficiencies of air compression, air is probably cheaper than battery and will remain so for some time to come.
     
  10. Tesla2Go

    Tesla2Go Member

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    Well, all that is irrelevant if it turns out you need a trailer behind your car just to carry the necessary air tanks along. Again I have to ask:

    Has anyone shown any calculations how much compressed air you need to drive say 300 miles for a 1 ton car?

    What did they mention on the Discovery Channel segment? I figured this would be the first thing you'd like to know.
     
  11. electrified

    electrified Member

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    If you want to go 300 miles, you need a series hybrid. Most large equipment including trains are powered by series hybrids because an ICE is more efficient when it is designed to run at a constant speed. It is just a matter of wanting to store energy in a battery which will wear out or a compressed air tank which will last longer. This was discussed in the video segment.
     
  12. vfx

    vfx Well-Known Member

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    http://iopscience.iop.org/1748-9326/4/4/044011/fulltext
     

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