[Oeva-list] Compressed air vs. Batteries?

Lawrence Winiarski lawrence_winiarski at yahoo.com
Sun Jul 1 16:04:00 PDT 2012


I agree with nearly everything said so far.    Compressing gas creates heat and this is a practical problem of energy storage
with compressed air.   The corollary is that when the gas is expanded against a piston, it does work and it cools the gas.   This
cooling reduces the pressure so you don't get as much work out of it as you might think.   So for a
practical problem of storing air in scuba cylinders, it has a lot of losses.

But, at the risk of complicating things, I'm gonna add some theoretical points that might contradict the above, but
I AM IN NO WAY saying compressed air cars are good.   I'm just striving to edjumacate.


*)  IF you compress the air and insulate it, you do get the energy back.     So it you just want to store it for a short time, then it's
not so bad.   That's not as easy as it seems, because compressing air 10:1 will actually heat it up to about 800 F.   But if you
were going to use it for something like "brake" regen, then it's not necessarily a bad idea.


*)  As strange as it seems, I think there is a "hole" in the logic about heating up the air.   If you compress the gas "slowly" at room temperature
(i.e. keeping the gas cool), then you can compress the gas much "further".   Heat will be lost to the environment, but it won't be heat at
800F, but rather 70F.   You can think about it like this:   If you compress the gas "quickly" 10|:1 and it heats up to 800F, then the pressure
might go to 30 Atmospheres, but as it cools, it goes back down to 10 atmospheres...then you can keep pushing the piston even farther all the
way to 30 atmospheres.  Now imagine that you never let the temperature build up, you can see that it will take less work to end up in the same place.
ie. 30 atmospheres and 1/30 the volume.    So it's far more "efficient" to compress the gas isothermally.   YOU ARE still losing energy to the
environment however...but we will get back to that later.

Likewise, as the gas expands, the air will cool and the pressure will drop dramatically.   But if you allow the super-cold gas to absorb

heat from the environment, then it will warm up again and the pressure will rise again.     Imagine that you expand the gas through a cylinder
and it drops to 1 atmospheric pressure but at -200F.   Now if you take that exhaust put in in a jar and let it warm back up to 70F, it will actually more than DOUBLE
in pressure (i.e. 2 atm) and you can get more energy back out of it.    So now you ARE recovering the energy you lost to the environment in the compression
state.

So paradoxically if you compress the gas isothermally and expand it isothermally, then you can still get 100% efficiency (in theory).   You
can approach this easier on the compressor side, than on the expansion side of things because the compressor can work more slowly.  

So the "holy grail" of compressed air engines gonna be to approach isothermal compression and isothermal expansion.    

It's still bad, but the reasons are more practical, than thermodynamical.

As far as safety goes.  I highly recommend this extremely entertaining video.............

http://video.google.com/videoplay?docid=5245624839807682996 


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If we don't halt population growth with justice and compassion, it will be done for us by nature, brutally and without pity - and will leave a ravaged world.
Nobel Laureate Dr. Henry W. Kendall 023934


________________________________
 From: Greg Long <greglongoregon at gmail.com>
To: Oeva-list at oeva.org 
Sent: Sunday, July 1, 2012 1:22 PM
Subject: Re: [Oeva-list] Compressed air vs. Batteries?
 
Interesting. What pressures are the tanks run at, and past the
regulator, what pressure is fed to the pistons? Obviously below the
piston-fed point performance would decline.

Are they REALLY only 20% efficient (roughly) the way ICE's are?
There's little "exhaust" and probably not much heat, maybe even a net
loss, since uncompressing air cools.

The heat generated at the point of compression, could, at least in
theory, be utilized to heat water or homes.

Are there any compressed-air car dealerships in the Metro area?

On Sun, Jul 1, 2012 at 9:55 AM, Mark Murphy <markmurphy1951 at aol.com> wrote:
> Air compressors require more energy than battery chargers since they are
> mechanical with losses, plus they still use electricity to do the
> compressing which takes a lot of energy. net loss. Plus, compressed air,
> like batteries can only release a portion of their stored energy since
> pressure must be released at a constant rate (not maximum rate) and soon
> drops to a useless pressure point. Pressure tanks must be very strong($$) to
> survive repeated pressurization and safety concerns. (natural gas powered
> vehicles have similar issues.) An air powered engine has all the mechanical
> losses and much of the complexity of a IC engine and so is very inefficient
> compared to an electric motor ( one moving part) A suitability powerful on
> board air compressor, pressure tank and air engine have to be located in the
> vehicle. An air engine system is useful in specialty industrial applications
> where a natural gas system is not applicable such as closed buildings or
> underground.
>
> (IMHO)
>
> mark
> BlueSkydsn.com
>
>
> -----Original Message-----
> From: Greg Long <greglongoregon at gmail.com>
> To: Oeva-list <Oeva-list at oeva.org>
> Sent: Sun, Jul 1, 2012 12:20 am
> Subject: [Oeva-list] Compressed air vs. Batteries?
>
> Hello OVEA enthusiasts,
>
> Most of you have a lot more experience with EV's than I do, what do
> you think of compressed air cars?
>
> Seems like a viable method for storing and transferring potential
> energy. In a sense they are electric cars, in that electricity powers
> air compressors and lke "conventional" Battery EV's, they're
> flexible-fuel in that the electricity can be generated by a wide
> variety of sources.  I'd welcome input and suggestions, I may do a
> story on them.
>
> Best regards,
>
> Greg Long
> Social Media Editor / Network Administrator
> ExploreMars.org
> Twitter: @exploremarsgreg
> Facebook: http://facebook.com/exploremarsgreg
>
> _______________________________________________
> Oeva-list mailing list
> Oeva-list at oeva.org
> http://www.rdrop.com/mailman/listinfo/oeva-list

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