In sci.physics, Don Lancaster
wrote
on Sat, 19 Aug 2006 10:56:45 -0700
:
The Ghost In The Machine wrote:
In sci.physics, Tony Wesley
wrote
on 18 Aug 2006 16:22:55 -0700
.com:
Robert Clark wrote:
I meant using cryogenic liquid hydrogen would make it easy to liquify
the water.
As noted by Cowan, 4 bar might be too high for a lightweight system. I
got this number from high performance fuel cells. They would work at 1
bar just not as efficiently.
Er, Bob, just how big is the H2 tank going to be?
The fuel tank can be highly pressurized and the hydrogen fed through a
regular, presumably. I for one don't see that as a problem although the
regulator might get rather cold. :-) (Same issue as with air
conditioners.) Of course that might be advantageous, as one can then
route the exhaust past it.
By "highly pressurized", I assume you mean a contained energy density of
LESS THAN ONE PERCENT of gasoline.
http://www.tinaja.com/glib/energfun.pdf
Well, my assumptions are as follows -- and yes, the energy
density is pretty bad. This is all pretty basic chemistry
stuff, but if I make any obvious errors, please let me
know. :-)
First, the typical form of hydrogen is diatomic hydrogen,
H2. (Monatomic hydrogen is theoretically possible but
would probably be highly unstable at room temperature.)
The bond energy of H2 is about 436 kJ/mol. The bond
energy of O2 is about 498 kJ/mol, and an H-O bond about
464 kJ/mol. Ergo, a reaction of 2 moles H2 and 1 mole O2
proceeds as follows:
H2 + 1/2 O2 = H2O + E
where E is the breakage of 2 H2 bonds, 1 O2 bond, and the formation
of 2 H-O bonds.
H2 bond = -436
1/2 O2 bond = -498/2
2 HO bonds = 2*464
Net: 243 kJ/mol
Darned good if one counts by *weight*; if one burns 1 kg
of gasoline one gets 45 MJ or so, but if one burns 1 kg
of diatomic hydrogen one gets 500 * 243 kJ = 121.5 MJ.
Therefore 1 kg of diatomic hydrogen can replace 1 gallon
(about 2.65 kg, since gasoline is about 0.70 kg/l) of gasoline.
However, hydrogen is a gas, making storage of large
quantities rather difficult. As you probably already
know, PV=nRT is a variant of the ideal gas law. T =
293 K on a spring day (68F); P = 101325 (1 atm), and V is
what we're trying to calculate. R = 8.314472 J/(mol K),
the gas constant.
If I want 1 kg of hydrogen, that's 500 moles; therefore
V = nRT/P = 500 * (8.314472) * (293) / (101325) = 12.02 cu m.
at 1 atm pressure. That's 12,020 liters or 3175 gallons.
Horrid.
If one assumes 4 atm for a fuel cell one still gets 793 3/4 gallons -- an
energy density by volume of 0.34%, when compared to gasoline,
if my computations are correct.
I don't consider 4 atm all that high a pressure though; my bicycle tires
take a higher pressure than that (about 90 psi, or 6 1/8 atm).
The best I can do pressurewise (there might be specialists
out there who can do higher) is SCBA/SCUBA gear. There
are probably a fair number of issues regarding seals and
regulators (though it's obviously doable since 3300psi is
not the pressure fed to one's lungs while diving!) but
if one assumes the pressure, instead of 101325 Pascal,
is 3300 psi = 22.75 MPa as specified in various tanks at
www.scuba.com, then one gets
V = nRT/P = 500 * (8.314472) * (293) / (22750000) = 53.54 liters,
or 14 gallons.
In other words, if I fill up my, say, 14 gallon "gas
tank" with standard liquid gasoline/petrol, I might get
420 miles, assuming 30 mpg. (Diesel is even slightly
better, though there are a number of issues since engine
compression for diesels is higher, among other things.)
If I fill up my "gas tank" with highly pressurized
hydrogen gas, I might get
420 / 14 * 121.5 / 45 = 81 miles.
(This is assuming, of course, that everything else is the
same: car size, car weight, car shape, driving habits,
engine power, and engine efficiency. There is a minute
possibility of extracting some of the energy from the
actual pressurization but that's not all that much.)
A prior poster mentioned the possibility of liquid
hydrogen; this is indeed possible but very problematic.
For starters, the boiling point of H2 is 20.28 K or
-252.87 C. Storage of liquid hydrogen would therefore
have to be in Dewar flasks or equivalent, and even then
some of the ambient heat would eventually leak in, which
would cause at least the following effects.
[1] Condensation on the tank, and at some point ice on
the tank. Since ice expands things could get nasty,
especially around the valve area.
[2] Loss of the hydrogen and displacement of the air,
if one's parked in an enclosed space. The best one can
hope for if a car's been sitting sufficiently long is
suffocation, but a H2/O2 mixture is highly explosive.
The good news: liquid H2 has a molar volume of 11.42 cm^3,
or 0.663 kg/gallon. The bad news: 14 gallons would have
1.128 GJ, or about 600 pounds of TNT, especially when it
leaks into the garage (that 14 gallons will fully displace
about 112 m^3 of space when evaporated, and will form a
dangerously explosive mixture for many times that amount).
Will insurance cover such explosions? I'd prefer walking
in that case... :-)
One final note: despite what one sees in the movies, according
to
http://www.intuitor.com/moviephysics/
(
http://www.intuitor.com/moviephysics...tml#cigarettes)
it is very difficult to ignite gasoline with a
glowing cigarette, glowing (but not flame-lit) taper,
or metal spark. (If one does wish to test this, let me
suggest they know what they are doing first, and take all
appropriate safety precautions! Also, stay well downwind
of me... :-) ).
I'd be interested in whether someone has tested an H2/O2
mixture using similar methods -- with a very long pole,
of course. I do know that at least one experiment uses a
"bomb" of presumably rather thick glass with one or two
inlets and electrodes (though a better method of burning
hydrogen would be to use a fuel cell or a platinum (?)
catalyst). The "bomb" in this case is small and
protected, so it doesn't fly apart with the force of
the explosion within, and one gets droplets of water,
presumably, on the inside, after cooling. But scaling
upward in, say, one's garage, would get a little messier.
All in all, I'd be more in favor of biodiesel than a pure
hydrogen economy; it looks easier to handle.
--
#191,
Windows Vista. Because it's time to refresh your hardware. Trust us.