Hello Brian,

You seem to have firm grasp of the physics involved. Are you able to suggest
how efficient an electric power system powered by LH2 would have to be to
offset the power density difference from gasoline/kerosene?

Larry


On Mon, 4 May 2015 22:27:45 +0000 (UTC), Skywise
wrote:

Larry Dighera wrote in
:

I'll agree that a liquid H2 tank will likely be more massive than
today's gasoline tanks, but couldn't liquid H2 be stored in a Styrofoam
containing vessel?

The problem is either temperature or pressure, or both.

Liquid H2 is cryogenic. It doesn't exert pressure any more than
water does in a tank. But it has to be kept at -423F or -253C.

Styorofoam would just take up space.

If the idea is to avoid the crygenic temperatures, you then
need to fight the pressure. If I did my math right, and read
the phase diagram for hydrogen right, then liquid H2 at room
temperature has a pressure of about 2.5 million atmospheres.
There's no tank in the world that can hold that back.

Pressurized hydrogen at room temperature is just compressed
gaseous hydrogen. So a vehicle with that is like carrying around
a bunch of scuba tanks, which IIRC are only 3000-4000 psi or
about 200 to 270 atmospheres pressure, and look at how heavy
those are!!

I have heard about efforts to store hydrogen in metallic foams
but don't know the state of that work.

The problem is, the energy is in the hydrogen atoms. The more
atoms you have, the more energy you have. So if you want a lot
of energy, you have to cram a bunch of hydrogen atoms together
in a small space.

Now here's the killer. The properties of hydrocarbon molecules
is such that gasoline has a higher density of hydrogen atoms
than even liquid hydrogen!!! There's more hydrogen atoms per
unit volume. That's why gasoline has a 3x higher energy/density
value than liquid hydrogen. There are simply more hydrogen atoms
and therefore more energy.



Aren't the relative efficiencies of electrical propulsion vs internal
combustion powerplants being overlooked here?

My thought on electrical propulsion is, how is the electricity
produced in the first place? One rule of reality is that every
time you convert one form of energy to another, there are losses,
eventually ending up as heat. Basic Laws of Thermodynamics stuff.

Internal combustion (or turbine) engines burn the fuel and directly
convert it to mechanical work. That's bascially only one stage of
conversion to have any conversion losses.

Or, burn the fuel to drive a generator (loss 1), which generates
electricity (loss 2), which is then stored in a battery (loss 3),
which then is drawn from the battery (loss 4) to power an electic
motor (loss 5).

All those conversion losses add up. That's why gasoline is so hard
to beat. Doesn't matter if you like fossil fuels or hate it, it's
a simple fact that right now and in the forseable future, it's the
most efficient energy storage mechanism around.

The only alternative I see is to use elctricity from batteries
but generate the electricity by some other means than fossil fuels.
After all, isn't the whole point of this? to stop burning oil and
polluting the atmosphere? Burning the fossil fuels to generate
electricity to run cars and busses and planes only changes the
location of where it's burned. All these people driving their
electric cars feeling smug about themselves are not realizing that
the electricity is most likely coming from a coal fired generating
plant. And due to conversion losses, there's a good chance they
are actually increasing their "carbon footprint" than decreasing it.

Brian