Alternative Fuel Sources - Hydrogen and Fuel Cells: Advances in Transportation and Power - page 96

the obstacles in using hydrogen as an automotive fuel is storing it safely
and efficiently on board vehicles. Although it is possible to store hydrogen
as a high pressure gas in steel containers, disadvantages exist because of
the weight of the storage containers and the safety hazard in the event of
an accident. Other methods of storage for hydrogen include solid or liquid
hybrids, low temperature cryogenic liquids, or a combination of the two.
Liquid hydrogen as a method for storing and transporting hydrogen
can have several advantages over gases. The liquid form has a higher en-
ergy density and is easier to transport and handle. At atmospheric pres-
sures, hydrogen is a liquid at -253°C (-423°F), which is only a few degrees
above absolute zero. It must be stored in highly insulated tanks. Liquid
hydrogen is a cryogenic fuel. Cryogenics is the study of low temperature
physics. A beaker of liquid hydrogen at room temperature will boil as if
it was on a hot stove. If the beaker of liquid hydrogen is spilled on the
floor, it is vaporized and dissipates in a few seconds. If liquid hydrogen is
poured on the hand, it would feel cool to the touch as it slides through the
fingers. This is due to the thermal barrier that is provided by the skin. But,
place a finger in a vessel containing liquid hydrogen and severe injury will
occur in seconds because of the extremely cold temperature. In most ac-
cidents, the most serious concern would be a fuel fed fire or explosion. In
this case, liquid hydrogen is generally considered to be a preferred fuel.
Liquid hydrogen as a fuel option could be utilized on a large scale
since it most resembles gasoline in terms of space and weight. Although
a liquid hydrogen storage tank for a vehicle could weigh about five times
heavier in dry weight than a 30 pound gasoline tank, in vehicles that carry
greater volumes of fuel, such as trucks or trains or aircraft, the difference
in tank weight could be more than offset by the difference in fuel weight.
Studies by Lockheed Aircraft have shown that a large commercial aircraft
could have its overall takeoff weight reduced by as much as 40% if liq-
uid hydrogen were used instead of aviation fuel. Liquid hydrogen has the
lowest weight per unit of energy, with relatively simple supply logistics
with normal refuel times and is generally safer than gasoline in accidents.
However, cryogenic fuels like liquid hydrogen are more difficult to handle
and substantially more difficult to store compared to hydrocarbon fuels
like gasoline or aviation kerosene.
Even with highly-insulated double-walled, vacuum-jacketed stor-
age tanks liquid hydrogen can evaporate. This evaporation increases the
pressure on the tank wall and the gaseous hydrogen must be vented to the
atmosphere to keep the tank from rupturing. Stationary liquid hydrogen