wrote in :

For aviation use, the energy density by weight and volume are both
important.

Quite right.


Try he

http://en.wikipedia.org/wiki/Energy_density

Using information from above...

source MJ/kg MJ/l MJ/kgl
H2 liquid 141.86 8.491 1204.53
H2 690 bar 141.86 4.5 638.37
gasoline 46.4 34.2 1586.88
100LL 44.0 31.59 1389.96
Jet A 42.8 33 1412.4
Li-ion .875 2.63 2.30 (best values for range)

I calculated the MJ/kgl by multiplying the two other values.
This gives an efficiency factor by which to compare fuels.
As can be seen, liquid hydrocarbons outperform even liquid
hydrogen.

The energy is in the hydrogen atoms. The reason hydrocarbons
outperform pure hydrogen is that gasoline simply has more
hydrogen atoms in it that even pure liquified hydrogen due
to the hydrocarbon molecular structure. That's why there is
more energy per unit volume, which more than makes up for
it's much lower energy per unit mass.

And this doesn't even take into account the storage container.
I'm sure a wing tank in a Cessna 172 weighs a lot less than
a compressed H2 bottle for an equivalent amount of total
energy. I doubt LH2 would ever fly (pun intended) as it
requires cryogenic storage design which adds yet more weight.

Another factor not considered is energy conversion. The total
mass of the engine (ICE or fuelcell/electric motor) and it's
conversion efficiency. Is a hydrogen fuel cell motor system
light enough to offset the extra weight of the storage
container? Is the system more efficient at converting the
theoretical energy values listed above into usable work?

Li-ion sucks. Though I love 'em for RC airplanes. No mess.

Brian
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