Re: End of airliner evolution?

Date:         23 Nov 96 03:36:25 
From:         Bob Standaert <standaert@chemvx.tamu.edu>
Organization: Texas A&M University
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trussell@magmacom.com wrote:
>
> Larger vehicles ie: aircraft, ships, trucks have fewer problems that a
> passenger car for using H2 because it's bulky.
>
> Planes and ships can use it with ease because the size of the fuel
> tank is not as important but smaller vehicle such as a car currently
> wouldn't have a trunk, it would be all tank and then some.
>
Bulk is a big problem (in addition to to the problems with cost, safety,
and practicality of use).  Hydrogen has a significant intrinsic
limitation as a fuel:  low energy to volume ratio.  Hydrogen has more
energy than jet fuel on a pound-for-pound basis, but not on a
gallon-for-gallon basis.

Here are some rough calculations; I have assumed that heat of combustion
is the relevant point of comparison, that thermal efficiencies are
equal, and that jet fuel can be approximated as a simple alkane
hydrocarbon (I took the data for n-decane, but the exact compound
doesn't matter much).  The heat of combustion of decane (kerosene) is
about 10,500 kcal/kg (18,900 btu/lb), and the heat of combustion of
hydrogen is 28,700 kcal/kg (55,700 btu/lb), meaning the heat content per
unit weight is 2.7 times larger for hydrogen.  However, hydrogen is not
very dense.  The liquid (bp -253 C) has a density of 70 g/L, vs about
800 g/L for jet fuel; therefore, you would need 4.2 times as much liquid
hydrogen by volume.  The real ratio might need to be even higher, since
combustion of hydrogen leads to a gas volume decrease (H2 + 1/2 O2 -->
H2O) whereas combustion of a hydrocarbon leads to a substantial gas
volume increase, which I assume translates into added thrust.

Now, consider a 747-400.  Its fuel capacity of 215,000 L (57,000 gal)
corresponds to about 172,000 kg (378,000 lbs) of jet fuel; to get the
same net heat of combustion (i.e., range) it would need only 63,000 kg
of H2, but it would need an Olympic size, 900,000-liter (cryogenic) fuel
tank to hold it all.  A fuel cylinder the diameter of the cabin (20 ft)
would need to be 77 ft long to carry the added volume.  Alternatively,
you could add a 200-ft long by 12-ft diameter hump.  A spare 757
fuselage strapped on top as a fuel tank would hold less than 3/4 of the
necessary extra capacity!

In terms of airliner evolution, this one sounds like a dinosaur destined
for quick extinction.

Regards,

Bob