The problem with this is that the energy required to break the water into
hydrogen and oxygen is exactly equal to the energy you get from burning the
hydrogen, almost by definition. The reaction is
2H2 + O2 = 2 H2O + 572 joules
where all chemical quantities are measured in moles. As you can see, to
get the reaction to go backwards, you have to input 2 moles of water and
572 joules, so that requires an input of energy; when you burn that
hydrogen, you get exactly 572 joules back, and there's no energy left over
to power your car.
> OR why
>we don't have electric cars everywhere we turn.
Well, we're still getting over our phase of "I must have a big SUV to carry
groceries." I see this changing, though, as we once again become more
environmentally conscious. The next step will be widespread acceptance of
hybrid vehicles, such as Toyota's offering, supposed to be available this
summer:
http://www.toyota.com/afv/prius/intro_prius.html
In my view, the largest difficulty will be creating an infrastructure able
to support electric vehicles. Hybrid vehicles are attractive because they
are significantly more fuel efficient than regular cars (the Prius gets 66
mpg on city streets), and can be made lighter than regular cars, with less
metal, and can use the preexisting gasoline infrastructure. As computers
become more powerful, then we will get even more efficient cars that
implement ideas like regenerative breaking and solar panels. We may even
get a car smart enough to use the energy generated as it moves through the
earth's magnetic field!
When I buy my next car, it will be a hybrid.
>
>It strikes me that with water, its a matter of the volume of
>hydrogen production so that enough was produced to be able
>to sustain the engine operation AND that the hydrogen would
>not damage the engine via embrittlement OR rusting when the
>hydrogen mixes with oxygen to recombine as water in the
>cylinder.
Hydrogen, to me, seems to be a poor choice, because it is not very dense
and doesn't pack as much energy as a hydrocarbon. Even boring old methane
packs more than 3.1 times as much energy per liter as hydrogen.
>
>Not sure about how to accelerate the hydrogen production
>although both embrittlement AND risk of rust could be
>negated by either ceramic or aluminum or alloy engines, both
>which would be too expensive to retrofit the many existing
>vehicles.
>
>In the case of electric cars, its a matter of expense to
>convert existing vehicles to electric as well as sufficient
>battery banks to give a useful driving distance between
>recharges....possibly a faster way to recharge...
Why are you concerned with converting existing cars to electric? This
doesn't seem cost effective to me. Few cars last longer than 10-15 years.
If auto manufacturers made electric cars a compelling choice, they would
thus dominate the roads in 10-15 years. Electric cars are made to be very
light, have a great deal of space for the batteries, and are made to be
extremely aerodynamic. I don't see how you can convert a car for any less
money than a new electric car would cost.
>
>In either case, if it involves REPLACING the engine, it
>would be expensive to do the retrofit and most likely show
>up in only new vehicles which would eventually replace the
>internal combustion engines that are everywhere.
>
>Now, there is something really elegant about this idea of
>using compressed air to drive the pistons up and down.
I believe that this has been tried before on a smaller scale. For example,
brakes were thrust against the axles using compressed air. The largest
problem was that the air tank got extremely cold when it expelled its gas,
and this caused ugly things like water condensation, cracking, etc.
>
>Such a retrofit would require a compressor, a storage tank,
>a variable throttle (like the gas pedal) and a distributor
>to transfer high pressure air into each cylinder at the
>right time.
Let's do some math. If we had a tank that's 20 gallons, a typical size for
a mid-sized car's gas tank, and air compressed to 130 lbs/in^2 (a
reasonable value for an industrial compressor), we get about 68,000 joules.
Assuming a weight of 1360 kilograms, which is a reasonable weight for a
current mid-sized car, and assuming that all the energy is converted into
motion, we get just enough energy to accelerate our car to 22 miles per
hour, once. You can talk about various optimizations, etc., but the above
calculation should make it clear that, barring a huge increase in
technology, compressed air just doesn't pack enough energy to move a car
and keep it moving at a reasonable clip.
By the way, the energy increases linearly with tank size. If we increase
the size of the tank by 10, we increase the energy by 10, and therefore get
enough energy to accelerate our car to 70 miles per hour. Once.
>
>It sounds like a simply retrofit that would not be complex
>or expensive. The only problem is what runs the compressor
>to charge the tank? Perhaps solar cells so that it runs all
>the time to keep the tank charged. The compressor would
>thus be recharging the tank all the time and be able to
>'catch up' anytime the car was not running or otherwise in
>motion.
The average intensity of sunlight striking the earth is 250 watts/m^2.
Even generously doubling that to 500 to account for day/night and cars
being driven during the day, and allowing the surface of the car to be the
3 square meters and allowing a generous 10% efficiency for the conversion
of sunlight to pressure-energy (for lack of a better term), we get 150
watts. At this rate, it would take 220 days to get a tank of gasoline
worth of energy.
>
>This way, ANY internal combustion engine could be
>retrofitted and even new cars coming out could use the same
>engine but with the compressor and all built in.
>
>There would be times when the compressor might need to be
>driven from another source of power, whether it be
>electrical or even a small gas engine (back to the
>problem). An extension cord or inductive coil over which
>the car parks to induce sufficient current flow (by
>transformer action) to drive the compressor.
>
>Even a few batteries could be added so that they provide the
>power for the compressor, are recharged by the solar cells,
>the inductive tap or extension cord.
>
>I understand the tank only requires about 500-600 psi to
>keep an engine running just like with gasoline explosions
>but with NO pollution from emissions.
I don't think it's reasonable to demand that a compressor produce that
much. The largest fixed industrial compressors I could find produced 225
pounds per square inch.
>
>The beauty of it is that ANY internal combustion engine
>could be modified very easily and very economically to do
>this. A more advanced retrofit could add compressors to the
>drive train so that when the vehicle was braking or coasting
>down a hill or otherwise not requiring air pressure, the
>tank would be recharged from that recovered energy. This is
>more expensive but some inventors claim there is sufficient
>energy to allow this to further extend the tank charge
>between refills.
>
>With the compressor onboard and trickle charging the tank
>all the time, it could provide sufficient thrust to drive
>the vehicle for short distances...all dependent on how fast
>it takes to recharge...maybe a turbo charge to fill the tank
>up fast and the trickle mode when you know you'll have time
>for it to trickle charge...just a thought...
I just don't see how it can be a feasible way to store energy in the near
future.
-Peter
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