Technology Status of Hydrogen Road Vehicles

During the past 5 years, a carbon sorption technique in the pressure vessel has allowed both the mass and
volume densities to be almost doubled, depending on the bulk hydrogen pressure. Unfortunately, the effect
seems to be most marked at pressures lower than 70 bar (Hynek et al. 1994).
Zeolites (aluminosilicates) may be a promising way of allowing more hydrogen to be held within a given
volume. Querubin et al. (1994) states that zeolites, whose bulk density is about 10 times that of activated
carbon, are expected to be about four times better for hydrogen storage on a unit volume basis. A similar line
is represented by micro-encapsulation of hydrogen in glass. Duret and Saudin (1992) in France and Akunets
et al. (1992) in Russia have investigated this. Both achieved improved weight density.
Griesinger et al. (1996) treat ongoing basic work on zeolites, and mention the possibility of new synthetic
zeolites with improved storage capability.
No figures are available that predict the results of these various lines of research. The research will continue,
however, because, at high pressures and with improved sorption techniques, GH 2 systems (at liquid nitrogen
temperatures) may approach those for LH 2 on a mass and volume density basis.
Ewald (1996) states, however, that there is little point in using pressures higher than 300 bar because the
supply infrastructure becomes quite difficult, and the filling factor decreases because of non-ideal gas
behavior.
Compared to LH 2 or MH, refilling simple GH 2 vehicles is rather straightforward if a bulk storage vessel with
the necessary pressure difference is available. Open-air, well-ventilated stations are required, and the level
of personnel expertise is not too high. A vehicle can probably be refilled in several minutes, but little
information is available.
3.2 Metal Hydrides
Metal hydrides are formed when hydrogen is added under modest pressure to suitable alloys in an exothermic
reaction that seems simple to control:
e.g. FeTi + H 2 FeTiH 2 + heat + 25% volume increase
The GH 2 is recovered by adding heat, in this case at a modest temperature of 70 o C, which could easily be
supplied by the exhaust heat or cooling water of the prime mover. Startup requires supplementary measures.
The metals of interest (the most common after Fe, Ti being Mg, Ni, Mn, etc.), although not as rare as some
catalysts they resemble, can probably experience decomposition and pulverization over a limited life, and rapid
loss of their storage capabilities. This may be the only way of storing hydrogen so it cannot escape in bulk
during an accident or operational leaks and vents. Heat must be deliberately added to release the hydrogen,
and the system never has more free hydrogen than required for immediate needs.
So this great advantage decreases as the other storage systems reach excellent standards of containment. Still,
it explains the favor bestowed on it for early, startup research and development and for some prototypes. It
is, however, almost the only advantage. Its major drawback is poor energy density on a weight basis, which
is about half that of 300 bar GH 2. Even though on a volume basis it can outperform GH 2 (though not LH 2)
the weight penalty so far is severe in mobile applications. However, interest remains high, and much work
is underway to unearth the metal combinations that could reduce the weight penalty to manageable proportions
and maintain structural integrity under repeated cycling over many years. Related fields of powder metallurgy
and catalysts may provide unexpected inputs.
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