Technology Status of Hydrogen Road Vehicles

hydrogen engines may have lower oil costs and very slightly lower maintenance costs than gasoline engines.
Generally, dual-fuel (H 2/gasoline) vehicles and engines are more efficient and have lower emissions than stock
gasoline ones.....
6. Safety Issues
Hydrogen has a largely undeserved reputation as a particularly dangerous fuel.
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Hydrogen is more hazardous than gasoline in several ways. Like methane, it is invisible and odorless, and
therefore an odorant must be added to enable detection. Hydrogen flames are very hot, yet radiate very little
heat and are invisible, which makes them harder to locate, and thus harder to extinguish or to avoid. A flame
colorant would make detection easier. (Odorants and colorants must not contaminate hydrides). Hydrogen
can ignite within a rather large range of hydrogen/air densities, from 4 to 74 % (by volume), and compared
to methane or gasoline needs very little energy to ignite. However, the lower volumetric ignition limits for
CH 4 and H 2 are close, and in a weak ignition source such as an electrostatic spark, there is already sufficient
energy to ignite CH 4. Thus in practice, H 2 may not be much more prone to ignition than CH 4.
Hydrogen has a much higher normal burning velocity than has methane or propane. This means that, given
a burning, detonable mixture of hydrogen, methane or propane in a confined space, the hydrogen conflagration
is more likely to detonate. However, if the mixture is not burning, is not capable of exploding or is not in a
confined space, this is no longer true. Thus hydrogen is more explosive than the other fuels only in certain
circumstances.
Contact with LH 2 destroys human tissue. Wearing gloves reduces the risk.
On a volume-of-gas basis, hydrogen has a lower explosion potential than either methane or propane; on a
mass-of-fuel basis, hydrogen has the most explosion potential; but volume considerations are more pertinent
to storage safety.
Hydrogen fires burn very rapidly, and radiate very little heat, and thus are relatively short lived.... a person
can be closer to a H 2 fire than a gasoline fire without being burned.
...in a test crash of an LH 2 vehicle the fuel system remained intact....similar outcomes of on-road crashes of
demonstration vehicles. LH 2 tanks seem less likely than jet fuel tanks to explode when struck by lightning
or punctured...this probably applies to LH 2 dewars vs gasoline tanks as well. If LH 2 does leak in a crash, it
will evaporate and disperse exceedingly fast, unlike gasoline which will puddle and remain a fire hazard for
much, much longer.
It has been shown that hydrides are safer than gasoline tanks. Unless a continuous supply of heat is available
to desorb the hydrogen, a leak from a hydride tank will be self-limiting.
LH 2 refueling is expected to be automatic, fast and safe. Hydride refueling also presents no unusual hazards....
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Current technology LH 2 vehicles vent at 3-5 bar which is reached after 2-5 days. If boiloff control proves to
be feasible and relatively inexpensive, then the market potential of LH 2 vehicles will not be limited by safety
concerns related to boiloff.
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Assuming that advanced technology tanks will have a lock-up time of 4 days, and a boiloff rate of 1.3% per
day, a vehicle that remained idle for a week would lose less than 5% of its fuel. This likely would not be
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