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> Would you still sit on top of the tank if it was a fiber glass composite?
Well it wouldn't produce metal shrapnel when it fails. I think it would be practical to engineer a housing that would be adequately safe. Whether this would in fact be done in a 3rd world environment with aggressive price targets is another question.
The more important point is that this is a very small vehicle with limited range and probably very limited applicability.
A 60 gal. tank @ 3600 psi??? About the same threat as driving around with a box of plastic explosives. Also air liquifies at about 3000psi (room temp), to achieve that we are talking cryogenic processes combined with sophisticated mechanical pumping equipment. This is not like an Air Hogs toy.
A 60 gal. tank @ 3600 psi??? About the same threat as driving around with a box of plastic explosives.
I really don't get what all this whining is about safety when cars/trucks all use a gasoline, ethanol, or diesel stored in tanks on the vehicle that all equal boxes and boxes of dynamite in explosive power!!
But some of you folks worry about exploding air cylinders!
OK some real numbers, based on Wikipedia (of course, a somewhat suspect reference). Correct me if they got the numbers wrong.
The energy stored in 60 gal for an ideal gas at 3600 psi is 1.3E5 Joules.
Gasoline (always according to Wikipedia) has an energy density of 132 MJ/ gal. So a 12 gal tank has stored energy of 1.6E9 J. FAR more than the compressed air. This, of course, is why the air car needs to be so light and why the range is so low.
(Fuels have an incredibly large energy density compared to most other ways of storing energy. Not all this energy can be converted to mechanical work but even with the inefficiencies liquid fuels are VERY attractive for portable things).
Air does NOT liquefy at 3000 psi, although I don't know the exact pressure where it does. If it's liquid at room temperature then, umm, you don't need to use cryogenics 'cause its, umm, at room temperature. If it's liquid because it's cold, as it is in a cryostat, then you don't need to keep it under pressure to keep it cold, you only have to limit the heat leakage. Any heat leakage boils some of the liquid air. Liquid air is typically kept under pressure but not much pressure and the boiloff is vented.
OK some real numbers, based on Wikipedia (of course, a somewhat suspect reference). Correct me if they got the numbers wrong.
The energy stored in 60 gal for an ideal gas at 3600 psi is 1.3E5 Joules.
Gasoline (always according to Wikipedia) has an energy density of 132 MJ/ gal. So a 12 gal tank has stored energy of 1.6E9 J. FAR more than the compressed air. This, of course, is why the air car needs to be so light and why the range is so low.
When it comes to immediate danger there is a difference between stored energy and the volatility of said stored energy… you’re comparing chemical energy to mechanical energy. Why not throw nuclear energy in there too? I'll tell you what, we will both stand 10 ft. from a 60 gallon container. Yours can be full of air @3600 psi and mine will be full of either gasoline or diesel. Then we can both unload a pistol into each and see what happens... or rather whats left.
Quote:
Originally Posted by rational1
Air does NOT liquefy at 3000 psi, although I don't know the exact pressure where it does. If it's liquid at room temperature then, umm, you don't need to use cryogenics 'cause its, umm, at room temperature. If it's liquid because it's cold, as it is in a cryostat, then you don't need to keep it under pressure to keep it cold, you only have to limit the heat leakage. Any heat leakage boils some of the liquid air. Liquid air is typically kept under pressure but not much pressure and the boiloff is vented.
Cryogenic processes take place under vacuum. The substance is condensed on a “cryogenic” surface, collected and then stored. I have inserted a phase diagram of nitrogen for your edification.
Nitrogen
Last edited by Lux Hauler; 08-21-2012 at 08:39 AM..
> When it comes to immediate danger there is a difference between stored energy and the volatility of said stored energy…
True, it is a difficult comparison to make. In an accident gas could end up spilled all over the roadway and ignited. Not an explosion but a major fire. It's also hard to know what would happen to a compressed gas container in an accident. The gas could be released with nothing worse than a hissing sound.
> (your figure)
I don't know who is responsible for this (hand-drawn) figure. However the people who sell compressed gas (airliquide) say that the critical point for nitrogen is at -147 degrees C and 40 bar (approximately 40 atmospheres). This is very far different from your figure.
> When it comes to immediate danger there is a difference between stored energy and the volatility of said stored energy…
True, it is a difficult comparison to make. In an accident gas could end up spilled all over the roadway and ignited. Not an explosion but a major fire. It's also hard to know what would happen to a compressed gas container in an accident. The gas could be released with nothing worse than a hissing sound.
> (your figure)
I don't know who is responsible for this (hand-drawn) figure. However the people who sell compressed gas (airliquide) say that the critical point for nitrogen is at -147 degrees C and 40 bar (approximately 40 atmospheres). This is very far different from your figure.
It is technically a supercritical fluid (it is "liquified" yet shares vapor properties as well, in other words its both... really cool stuff) however the phase is irrelevant. The tank has to be lightweight so what is a likely conclusion about it's construction? If you breach a 3600PSI tank, it will continue to rupture... violently is my guess. A serious car crash could cause the tank to severely rupture and unlike petroleum fuel, all of that energy is released instantly.
With cryogenics you can cool the substance to a point where it condenses and exerts no pressure (look at the phase diagram). Vacuum is used to isolate the substance from heat.
Last edited by Lux Hauler; 08-21-2012 at 09:34 AM..
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