Alternative Fuel Sources - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 98

Liquid hydrogen can be stored in newer vessels that are relatively
compact and lightweight. General Motors has designed a 90-kg cryogenic
tank that holds 4.6-kg (34 gallons) of liquid hydrogen.
Liquefying hydrogen requires special equipment and is very energy-
intensive. The refrigeration requires multiple stages of compression and
cooling and about 40% of the energy of the hydrogen is required to liquefy
it for storage. Smaller liquefaction plants tend to be more energy-intensive
which presents a problem for local fueling stations.
Another problem with liquefied hydrogen is evaporation since hy-
drogen in its liquid form can easily boil off and escape from the tank.
NASA loses almost 100,000 pounds of hydrogen when fueling the shut-
tle requiring 44% more to fill the main tank. In an automobile, this can be
important particularly when it remains idle for a few days. The GM tank
has a boil-off rate of up to 4% per day. There are techniques for bleeding
and using the evaporating hydrogen, but this adds system complexity.
Liquid hydrogen fuel on board a vehicle would also allow the use of a
small, efficient fuel cell Stirling engine cryocooler system to provide air
conditioning.
Liquid hydrogen requires extreme precautions in handling because
of the low temperature. Fueling is usually done mechanically with a ro-
bot arm. Even in large, centralized liquefaction units, the electric power
requirement is high with 12 to 15 kilowatt-hours (kWh) needed per kilo-
gram of hydrogen liquefied.
COMPRESSED HYDROGEN
Compressed hydrogen has been used in demonstration vehicles for
many years and most prototype hydrogen vehicles use this type of storage.
Hydrogen compression is a mature technology and low in cost compared
with liquefaction. The hydrogen is compressed to 3,600 to 10,000 pounds
per square inch (psi), but even at these high pressures, hydrogen has a much
lower energy per unit volume than gasoline. The higher compression al-
lows more fuel to be contained in a given volume and increases the energy
density but it also requires a greater energy input.
Compression to 5,000 or 10,000 psi takes several stages and requires
an energy input equal to 10 to 15% of the fuel's energy. Compressing 1-kg
of hydrogen into 10,000 psi tanks can take 5-kWh or more of energy.
Compressed hydrogen can be fueled relatively fast, and the tanks