Technology Status of Hydrogen Road Vehicles

The design and layout of conventional vehicles never gave much priority to fuel storage. The tank was always
comparatively simple, operated at ambient conditions, and contained gasoline or diesel fuel for a range of
about 500 km.
On-board storage of hydrogen is quite a different matter. The tank is not simple, and conditions are never
ambient. Some combination of high pressure, cryogenic temperature, and thermal processing is necessary,
depending on the storage form. Refueling such tanks is correspondingly complicated.
Ewald (1996) states that a successful hydrogen vehicle must be tailored to the storage system. Three
fundamentally different ways have been developed for on-board hydrogen storage, and even though much
research and development is still ongoing, working versions are currently available:
! Compressed GH 2 in high-pressure reinforced vessels, ambient temperature
! Metal hydrides (MH) in medium-pressure vessels, ambient temperature when passive, heat input for active
flow
! Liquid LH 2 in low-pressure vessels, cryogenic temperatures of -253 o C.
Other hydrogen storage methods have been attempted, such as iron pre-reduced by H 2 released again in a
steam-iron reaction, or as a hydrogen-rich reformable liquid under ambient conditions. These efforts are at
least partially related to a more general problem of stationary bulk storage and shipping of hydrogen, whereas
a methylcyclohexane/toluene cycle is foreseen for seasonal storage of hydropower energy (Schucan and Taube
1986). Methanol can perform a similar function for biomass (Frolov et al. 1994). These and other efforts will
no doubt contribute to the separate question of optimized bulk stationary storage of hydrogen. Attempts to
use methanol on-board are meeting with some success; in fact, Daimler-Benz has operated an A-class vehicle
on a reformed methanol FC. Due to the limited number of successes to date of these storage methods, they
will not be considered further in this report, but future developments may warrant their inclusion in later
studies. In fact, several papers presented at the 11th WHEC illustrated continued interest in these topics:
! Ekdunge and Råberg (1996) consider on-board reforming of methanol
! Ewald (1996) prefers only direct use of methanol, Cleghorn et al. (1996) quote record performance of
direct methanol fuel cells, and Schmidt and Stimming (1996) mention the latter possibility
! Pizak et al. (1996) conclude that on-board steam-iron production of hydrogen could be practicable, but
is best started in heavy transportation systems such as locomotives.
3.1 Compressed Gas
For many years GH 2 was supplied to various industries in steel cylinders of reduced diameter and pressures
as high as 300 bar. These cylinders had poor energy densities on a mass and volume basis, but the quantities
involved were usually small, and stationary storage the norm. The advent of fiber-wrapped aluminum vessels,
and then composite material in the pressure vessel, has alleviated the problem, and prototype pressures are
reaching higher than 600 bar. Energy density has had a strong effect on a mass basis, but only a modest effect
on a volume basis.
Substituting the steel pressure vessel with much lighter material has the major effect and is compounded by
the pressure increase. These advanced vessels seem to have received regulatory approval in several countries,
since there are references to their use on a prototype basis. But actual H 2 vehicles (presented in Table 1) do
not exceed 300 bar, and little information is available for the next generation, presented in Table 2.
As Hynek (1995) pointed out, lowering the temperature to that of liquid nitrogen (-195 o C) at 250 bar gives
GH 2 almost the same density as LH 2.
4