Alternative Fuel Sources - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 100

Metal hydrides can hold a large amount of hydrogen in a small vol-
ume. A metal hydride tank may be one third the volume of a 5,000 psi liq-
uid hydrogen tank. Hydride tanks can take on different shapes depending
on the vehicle design.
Many hydrides have a theoretical capacity to store a higher percent-
age of hydrogen by weight and are a major subject of ongoing research.
Research also continues in the area of suboptimal hydrogen release. This
is the release of only a part of the stored hydrogen.
Refueling can take more than five minutes since some hydrides are
slow to absorb hydrogen. Others are slow to release it during use. The
chemical process in irreversible hydrides can also be very energy-inten-
sive.
Hydride materials absorb hydrogen like a sponge and then release it
when heated. There are hundreds of hydride materials. The first hydride
systems used in automotive vehicles consisted of metal particles of iron
and titanium that were developed at Brookhaven National Laboratory.
These were tested by Daimler-Benz in Stuttgart, Germany. These early
hydride systems were shown to be safe for storing hydrogen in automo-
biles, but they are almost 5 times heavier than liquid hydrogen storage
systems.
Other hydride systems do not have such weight penalties and in-
clude magnesium nickel alloys, non-metallic polymers, or liquid hydride
systems that use engine heat to disassociate fuels like methanol into a mix-
ture of hydrogen and carbon monoxide.
An iron titanium hydride tank system, for a range of 300 miles (480
kilometers), could weigh about 5,600 pounds (2,520 kilograms). A liquid
hydrogen tank for this range would weigh about 300 pounds (136 kilo-
grams), a comparable gasoline tank would weigh about 140 pounds (63
kilograms).
An electric vehicle with a similar range and lead acid batteries would
have a battery weight of about 6,500 pounds (2,925 kilograms). More effi-
cient battery systems are becoming available but the most efficient electric
vehicles of the future may be energized by fuel cell systems that convert
hydrogen and oxygen directly into electricity. These systems would de-
pend on having hydrogen fuel more readily available.
There has also been work with hydrogen storage in buckyballs or
carbon nanotubes. These are microscopic structures fashioned out of car-
bon. This research indicates a potential storage technique using a combi-
nation of chemical and physical containment at very high temperature