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Hydrogen as a Fuel for Automobiles
By Carl Johnson, from Index of Public Service Pages.
On first glance, hydrogen
seems to be the ideal fuel for automobiles and other vehicles. It doesn't seem
like one could get any cleaner burning, since hydrogen burns (oxidizes) to form
simply water vapor. No pollution! What a seeming advancement over our current
internal combustion engines that put thousands of tons of pollutants into the
Earth's atmosphere, as well as giving off massive amounts of heat that
contribute to global warming, and many other environmental problems.
Hydrogen (H2) plus Oxygen (O) makes H2O, water, or
actually, water vapor, at higher temperatures. And Hydrogen is actually capable
of nearly meeting those high expectations.
Environmental Impact
There are a couple minor environmental issues. Our
Earth's atmosphere is not pure Oxygen, but it is a mixture of gases, with around
4/5 of it being Nitrogen and around 1/5 being Oxygen, and a lot of other gases
in small amounts. When Hydrogen (or any other fuel) burns in our atmosphere, a
lot of heat is generated (which is sort of the whole point!) When the Nitrogen
in the air, it also can oxidize. It can combine with the nearby Oxygen atoms in
a variety of ways, such as NO2, NO3,
N2O5, and many others. These new compounds are
collectively referred to as NOx, and they generally are considered to
cause an assortment of health problems in people and other living things.
In addition to NOx production, if the device in which the burning
occurs has any lubricants, like oil, there are also oxidation products of the
Carbon in them, which can contain CO, carbon monoxide. When Hydrogen is burned
in a decently designed device, these environmental problems are fairly minor and
they are rarely considered to be any great danger.
Logistics
Hydrogen does have some more significant drawbacks. One of the
most difficult to deal with is that it is such a light gas! A pound of Hydrogen
contains around 61,000 Btus of latent energy in it, which seems like a lot! For
comparison, a pound of regular gasoline only contains around 20,500 Btus in it!
Sounds good!
However, a pound of Hydrogen is HUGE! At standard atmospheric pressure and
temperature, it takes up around 190 cubic feet of space. In contrast, that pound
of gasoline only takes up about 1/50 of a cubic foot.
We can say this same thing in terms of "gallons". A gallon of gasoline
contains around 6 pounds, or 125,000 Btus of energy in it. A gallon of hydrogen
(gas) only contains around 40 Btus in it. Quite a difference! Instead of a two
cubic foot gasoline tank (15 gallons) in your car, you would need a tank more
than 3,000 times bigger, over 6,000 cubic feet, for the equivalent Hydrogen!
That's a little more than TWO standard semi trailers (8'wide x 8'high x 45' long
or 2900 cubic feet each). Pretty big gas tank!
Well, that is obviously not going to happen! So, the many ongoing
explorations into using Hydrogen as a fuel always involve carrying COMPRESSED
Hydrogen in very thick, heavy tanks. If you have ever seen the kinds of tanks
used for the Oxygen for a worker's oxyacetylene cutting torch, that's the kind.
Such tanks can hold Hydrogen at around 100 times atmospheric pressure, or 1500
PSI, an extremely high pressure.
Well, at 100 times atmospheric pressure, the Ideal Gas Law tells us that the
Hydrogen would now only take up 2900/100 or 29 cubic feet. That works out to
around 60 of those high pressure storage tanks (to match the effective capacity
of the 15 gallon gasoline tank.). Each tank is very massive to withstand the
very high pressure, and each weighs nearly 100 pounds empty. (And around 1/4
pound more when filled with Hydrogen!) So the normal American car which
presently weighs around 2800 pounds would have around an extra 6,000 pounds
added, so the vehicle would now weigh more than three times as much as current
cars! (This tremendously affects acceleration and other performance, and it
would be like that car pulling a huge 6,000 pound trailer behind it.
Safety Considerations
There are obvious safety considerations in trying
to drive a 9,000 pound vehicle down the road. Handling and stopping would be
very seriously affected. But there is a bigger concern.
Those 60 very high pressure tanks present another complication. If industrial
workers ignore proper safety rules when working with a high pressure Oxygen
tank, it could fall over. As the hundred pound tank falls over, it quickly
develops a lot of momentum. If there should happen to be something in the way on
the floor, where the neck and valve of the tank hit it, the neck and/or valve
tends to just snap off. Suddenly, 1500 PSI of compressed gas has an easy way
out, and it all goes out almost immediately. Isaac Newton told us about the Law
of Action and equal Reaction. The hundred pound body of the tank then zooms off
at extremely high speed in the other direction. There have been many industrial
accidents where such Oxygen tanks flew many hundreds of feet through the air and
passed completely through many concrete walls.
Most suppliers of industrial Oxygen display photographs of vehicles where ONE
such Oxygen tank had not been strapped down properly and the neck wound up
snapping off. Usually, the vehicles shown in those pictures are hard to tell as
being vehicles, except for maybe a tire somewhere in the picture.
Get the point? Imagine having 60 such tanks in a car. Either one vibrates
loose from its clamps, or the guy who last replaced them didn't strap them all
down properly, or an accident occurs where you hit another vehicle or a tree. If
even one of those tanks ruptures, bad things would result. And have you ever
even seen what happens to any car when a semi hits it?
Notice that this issue is not actually related to any hazard of Hydrogen
itself, but rather the fact that it would have to be stored at extremely high
pressures due to its very low density. Whether it was a high-pressure Oxygen
tank or a high-pressure Hydrogen tank, this danger is virtually the same, and is
entirely due to the pressure that the gas is compressed to.
Because of this extraordinary safety hazard, which is only due to the very
high pressures involved and really has nothing to do with the Hydrogen itself,
there is no imaginable way that the US Government would ever allow such vehicles
to be licensed. It would conceivably be safer to drive a dynamite truck!
Cost Considerations
It would be wonderful if massive amounts of
compressed Hydrogen were easily available. In that case, except for the safety
and size considerations just discussed, Hydrogen would be a nearly ideal fuel
for vehicles. However, no compressed gas of any kind exists naturally and so
mechanical compression is required. An air compressor that can commonly be
bought for $300 can compress air to around 100 PSI, around seven times natural
atmospheric pressure. However, compressors that are capable of 1500 psi or 100
times atmospheric pressure are very large, very complex, and VERY expensive. In
addition, every pipe and every fitting used must also be able to safely
withstand such pressures. (Normal pipes would just burst.) In addition, whoever
operated such a compressor would have to be very extensively trained, to keep
all of its parts from bursting from the pressure and killing someone. The point:
People are not ever likely to have their own Hydrogen compressors, and so they
would certainly always have to buy the Hydrogen from some large corporation.
Logically, it figures that corporation will be the very same ones that now
own all the oil and gasoline companies!
However, even if there was some way to do all that compression, it takes a
good amount of electricity for the compressor motor to drive the compressor. A
significant cost would be involved for that compression, even if you somehow had
your own compressor.
In addition, free Hydrogen does not exist. All of the Hydrogen that might be
collected is now in various compounds. The simplest to deal with is water. If
you had Chemistry in High School, then you hooked up some electricity to an
apparatus that contained water, and you saw little bubbles of Hydrogen form in
one upside down test tube and Oxygen form in the other. That is called
Electrolysis, or the Dissociation of water. It is obviously pretty easy to do.
But those are just little bubbles of Hydrogen that you collect. Remember that
you are going to need an amount of Hydrogen that would more than fill two semi
trailers, to just equal one tank of gasoline! It is possible to calculate the
amount of electricity needed for that, but you must get the idea that it is a
LOT of electricity! So, you get to pay your electric company for that, too.
So, you would wind up paying for the electricity to Dissociate the water in
the first place, plus the cost of the electricity needed for the extreme
compression. Of course, all of this would be after you bought the necessary
equipment!
An alternative, of course, would be to buy (rent actually) tanks of
industrial Hydrogen that is already compressed. Current prices for Industrial
Hydrogen (the lowest purity available) are around $42 for a large, very high
pressure tank which contains 197 standard cubic feet of Hydrogen, plus a monthly
rental fee for the tank. The 2900 cubic feet that we had earlier determined were
equal to one 15 gallon tank of gasoline, would therefore be around 15 of these
tanks, which would cost around $630 for the compressed Hydrogen plus the monthly
rental of around $150 for the tanks themselves.
We complain today at paying $2 per gallon for gasoline, which would be $30
for our 15 gallon tank. How many people would be willing to pay $630 and more
for the same trip?
Flame Speed
Even if all the other hurdles are overcome regarding using
Hydrogen as a fuel, it seems to have yet another disadvantage, one that it
shares with most other gaseous fuels: the speed at which a flame front travels
is rather slow for the purposes of conventional engines. With an ideal
Hydrogen-air mixture, a flame front can travel at around 8 feet/second. For
comparison, a gasoline-air mixture creates a flame front speed that ranges from
around 70 feet/second up to around 170 feet/second in normal engines.
Consider the inside of an engine cylinder in a normal car engine traveling
down the highway. The engine may be rotating at 2,000 rpm, or 33 revolutions per
second. The piston must therefore move upward and downward 33 times every
second, and its speed in the middle of its stroke is around 45 feet/second. If a
fuel burning in the cylinder is to actually push down on the piston, in order to
do actual work in propelling the vehicle, the fuel-air mixture needs to burn at
a speed faster than the piston is moving! Otherwise, the slow-burning mixture
would actually act to SLOW DOWN the piston! It would not only not do productive
work, but it would require work FROM the piston.
The fact that a Hydrogen-air mixture has a flame-front speed of around 1/10
that of a gasoline-air mixture seems to indicate that only a very slowly moving
mechanism could be used. That might be possible, but it suggests that yet
another hurdle might lie in front of Hydrogen ever becoming a common motor fuel.
Conclusion
Yes, fuel cells, which are effective mechanisms for
converting Hydrogen and Oxygen into water vapor and releasing a lot of energy,
certainly seem to be fascinating potential sources of energy for vehicles.
However, it certainly seems that sufficient Hydrogen cannot be stored in a car
for any length of trip without compressing it to extremely high pressures. THAT
fact causes both cost and safety considerations which seem to make practical use
of Hydrogen remain a fascinating dream which will probably never become reality.
Yes, Hydrogen can be demonstrated in experimental vehicles, and they can have
impressive acceleration and speed. But that's with a rather small Hydrogen tank
aboard. If you ever see an impressive demonstration like that of a Hydrogen
powered vehicle, make sure to ask how long that vehicle could continue to
perform like that. The answer is certain to be no more than a few minutes at
most. So, as a demonstration, Hydrogen can seem quite impressive, because it is!
But in actual practical applications, the details probably make it never to be
usable in our vehicles.
Carl Johnson, from Index of Public Service Pages.
© Copyright energysavingnow.com 2000.
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