Alternative Fuel Sources - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 99

can be reused many times. The main technical issues are the weight of the
storage tank and the volume needed. Tank weight can be improved with
the use of stronger and lightweight materials. Tank volume is improved
by increasing the pressure. Until recently, a 5,000 psi tank was considered
to be the maximum allowable, but now 20,000 psi tanks are being built.
GM has made a successful vehicle test of a 20,000 PSI (700 bar) hydrogen
storage system. The newer 20,000 PSI tank technology extends the range
of the HydroGen3 fuel cell vehicle by 60-70 percent compared to an equiv-
alent-sized 5,000 PSI system.
The higher pressures also increases costs and complexity requiring
special materials, seals and valves. Pressure tanks are usually cylindrical
in order to provide integrity under the pressure. This reduces some flex-
ibility in vehicle design. Liquid fuel tanks can be shaped according to the
needs of the vehicle. The cost of storage increases with the pressure. An
8,000 psi storage vessel may cost several thousand dollars per kilogram
of capacity. This can be almost 100 times the cost of a gasoline tank, but
advances in material science and economies of scale could greatly reduce
this cost.
STORAGE IN METALS
Metal hydrides may also be used for hydrogen storage where the
hydrogen is chemically bonded to one or more metals and released with
a catalyzed reaction or heating. The hydrides can be used for storage in a
solid form or in a water-based solution. When a hydride has released its
hydrogen, a byproduct remains in the fuel tank to be either replenished or
disposed of.
Hydrides may be reversible or irreversible. Reversible hydrides act
like sponges, soaking up the hydrogen. They are usually solids. These al-
loys or intermetallic compounds release hydrogen at specific pressures
and temperatures. They may be replenished by adding pure hydrogen.
Irreversible hydrides are compounds that go through reactions with other
reagents, including water, and produce a byproduct. This byproduct may
have to be processed at a chemical plant.
Some hydrides are heavy and their storage capacity may be less than
2% by weight. So each 1-kg of hydrogen can require 50-kg or more of tank.
A tank with 5-kg of hydrogen could weigh more than 250-kg. This weight
reduces fuel efficiency.