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Green Cars Part 4 - Compressed Air

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Posted 02-03-2009 at 01:15 PM by mattW

Could the answer to our transport needs have been right in front of our faces all this time?Conventional cars work by exploding fuel and air in a chamber to exert force on the pistons, but what if rather than using an explosion to provide that pressure on the piston, you just stored your Ďfuelí at a constant high pressure. Thatís the concept behind the air car, and it looks like a pretty promising technology. Like hydrogen the air is not actually the source of energy, it is the storage medium, but unlike hydrogen the production of an air powered car is surprisingly cheap. According to Wikipedia, without the need for a cooling, ignition or starter system these vehicles can be produced for approximately 20% cheaper than conventional car engines. That factor should not be underestimated since up until now, greener options have always seemed to come at a premium. Unlike battery electric vehicles a full tank of air will stay more or less full for a long time, they have very low discharge rates. Again compared to battery electrics the storage medium (tank) is not limited by a shelf life or number of cycles and therefore reduces waste. As the air expands in the engine it actually cools it, which can be used for passenger comfort. The car itself has no emissions since it just uses normal air (the air might actually end up cleaner because of filtration) and the compression can be done using renewable energy to make the entire system emission free. It can even run underwater if need be! This technology is honestly quite promising, but like the other Ďgreení options will this fail to stack up when it is looked at more closely?

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Well the answer is yes and no. The cars certainly have their limitations. Range and power are obvious hurdles since the air-tank stores the equivalent of about a gallon of gasoline, the advertised range for an MDI air car is 125km (78 miles, possibly optimistic?), though the addition of conventional fuels may change this dramatically to over nearly 1000 miles (I'd be very surprised if that were true but time will tell) and apparently boosting the top speed from 35mph (55km/h) to 100mph (160km/h). According to this interview it uses the liquid fuel to heat the air 'increasing the volume' to extend the range. Having air stored at 300 Bar (4350psi) under your seat is a little disconcerting but the experts assure us that carbon fibre will do the job, air isn't flammable on its own so there is no fire risk. Compressed air cars have the same chicken and egg infrastructure problem as hydrogen cars, since standard compressors at service stations don't go anywhere near the required psi and the on-board compressor takes approximately 4 hours to charge. When the air is compressed at the compressing station the process also heats the air as a byproduct. This can effect range since when the air cools in the tank the pressure will also drop. More energy can either be used to cool the air before it goes into the car or the heat can be recycled eg. for domestic hot water. The cooling effect of the engine can cause ice formation especially at lower ambient temperatures and requires addition fuel for heating the engine. But efficiency wise compressed air cars perform better than any of the alternatives we have seen in this series. According to the efcf, the cars can have a theoretical compression efficiency of 46% and a tank to wheel efficiency of up to 84%, the report concludes that given 100MJ of energy a hydrogen FCEV could travel 42km, an air car 46 and a Li-ion BEV an impressive 133km.

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Compared to the other alternatives we have looked at, the compressed air car certainly has some impressive advantages. It is obviously superior to liquid fossil fuels since it can be cheaper to build and has the potential to be renewable and emission free. They don't have the wider environmental and humanitarian consequences of biofuel vehicles. Provided the range and power issues are resolved, it betters hydrogen on both efficiency and production cost. Its only rival in the alternative fuel game is the battery electric car, which trumps it on efficiency but not on production costs and battery life. The widespread adoption of compressed air cars I think will depend a great deal on public preference rather than the technology itself (unless it manages to exceed battery electric efficiency with clever thermodynamic tricks, or battery cost and life can be drastically reduced). Will consumers choose lower vehicle production and maintenance costs despite the increase in infrastructure and energy usage? Or perhaps will they decide that the higher vehicle costs of battery electrics are worth the price for the higher efficiency? Personally I think there is probably room for both as the technologies develop, but we will perhaps be able to make a more informed decision after we look at battery electric cars next week.
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  1. Old Comment
    I think the fact that an air storage tank does not degrade over time is an incredible advantage. I am building a BEV right now and the idea that I only have 500 cycles before the batteries must be replaced kills some of the fun, even if it does work out to be cheaper in the long run. If an air tank's life expectancy was 20+ years you would essentially have a vehicle that needed no upkeep other than tires.

    Regenerative braking could be more useful with such a system than with an electric; your "engine" simply acts as an air compressor when needed forcing air back into the tank. Regen braking on an electric is limited by how quickly that batteries can accept a charge, there would be no such limitation with air.

    If only there were a way to compress air more efficiently.

    PS.. Anyone know where I could get one of those carbon fiber air tanks cheap? I've got a few 6hp air motors I'd like to create a similar experimental vehicle around.
    Posted 03-28-2008 at 09:52 AM by AmpEater AmpEater is offline
  2. Old Comment
    JRP3's Avatar
    Actually 500 cycles doesn't necessarily mean you need to replace a pack, it usually just means the pack has degraded to only 80% of original capacity. If sized properly you should still have years of usable capacity left. Also newer technologies substantially increase cycle life.
    It's very important to remember that all these air car numbers are pure speculation, based on very small and light vehicles. BEV's are well known and proven technologies. Take those same vehicles and make them BEV's and I'd bet the BEV's would outperform in every way.
    The air car engine and compressor are basically 2 ICE's with the associated inefficiencies and maintenance issues, oil changes, internal wear, etc. Electric motors avoid these.
    Also the two versions of the "Air" car are very different animals. Running on compressed air only verses an ICE with air assist.
    I doubt you can find cheap high pressure carbon fiber air tanks. Carbon fiber is not cheap, and as far as I know these tanks are not mass produced.
    Posted 04-06-2008 at 08:42 PM by JRP3 JRP3 is offline
  3. Old Comment
    Cornelius's Avatar
    Compressed air powered vehicles were used over 100 years ago, commonly as coal mine engines. They were largely replaced by electric propulsion, as a problem with compressed air power is the engine efficiency. It is for this same reason I suspect that we will see EVs rather than compressed air powered cars in the future.

    Here's a photo from the late 1800s of a compressed air powered tram refilling. Interesting stuff...

    More information and photos at:
    Posted 04-18-2008 at 03:08 PM by Cornelius Cornelius is offline
  4. Old Comment
    JRP3's Avatar
    I meant to bring this up before:
    Unlike battery electric vehicles a full tank of air will stay more or less full for a long time, they have very low discharge rates.
    Is this really an issue for an EV? Most will be hooked up to a charger when not in use, most will be used every day anyway, and a healthy battery discharges very slowly. I don't see this as a real problem.
    Posted 04-20-2008 at 08:41 AM by JRP3 JRP3 is offline
  5. Old Comment
    Are there any realistic avenues for obtaining a CompAir motor? It seems that scuba and paintball provides tank and delivery infrastructure.

    The best that I've seen so far is trying to get a old Mazda Wankel motor and adapting it for compressed air.

    Any other suggestions?

    Posted 04-28-2008 at 10:56 AM by ga2500ev ga2500ev is offline
  6. Old Comment

    I regret that the MDI air car is pretty much a fraud and their past is littered with casualties. It's actaully been around for quite a while now, and I used to be on their forum which slowly over time became more and more annoyed by the misstatements that issued from MDI. There was an Israeli guy who actually went to some effort to spell out the nonsense.

    The key point is that to hold the amount of air required by the aircar to propel it for the range that is claimed, they would need to have air tanks running at huge amounts of pressure made of indestructible material. There would also be a significent issue around the release of cold air. The actaul range of the aircar, it was revealed, was about 5k.
    Posted 05-30-2008 at 12:36 AM by Weka Weka is offline
  7. Old Comment
    At the end of the day I think that compressed air probably doesn't have the energy density required to be effective. A good compressed air system would require very high pressure tanks, and a system to recapture and reuse the heat generated by the compression process.

    A better idea seems to be using the large expansion of phase change by carrying liquid nitrogen or liquid air. Simply boiling liquid nitrogen by exposing it to ambient temperature causes it to expand to many times its original volume. So a heat engine that uses heated liquid nitrogen could be effective.

    One such example can be found here:

    The University of North Texas and Washington have both build LN2 powered vehicles. It seems that a hybrid may be a better configuration though.

    Posted 05-30-2008 at 08:41 AM by ga2500ev ga2500ev is offline
  8. Old Comment
    I have been very interested in an alternative fuel power for a long time, but I don't have any intricate knowledge (as my name implies) so feel free to riddle this idea full of holes.

    My idea is, instead of using batteries to power an EV, would it be possible or feasible to use a generator being turned by a beefed up pneumatic drill type device powered with compressed air?

    it sounds good in my head, but without the technical knowledge, I don't know, which is why I'm offering it up here to be picked appart by the pros
    Posted 06-17-2010 at 10:35 AM by ignorantideaman ignorantideaman is offline
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