Fuel Storage
Hydrogen
gas is not an energy-dense fuel, so it can be difficult to store enough
hydrogen gas into a fuel-cell powered car to power it over a practical
driving range. Storing hydrogen in liquid form increases its energy density,
but the extremely low temperatures and high pressures necessary for liquid
storage are unrealistic for vehicular applications.
If hydrogen
fuel is to be stored on-board a vehicle, a high-volume, low weight, safe
storage system must be devised. Some possibilities for storage tanks include:
Steel Tanks
Steel tanks provide a safe a reliable way to store hydrogen, since similar tanks are now used to stock hydrogen gas to laboratories and universities. Tanks made from steel can hold hydrogen gas at a pressure of approximately 5,000 psi.
However, steel tanks do have several disadvantages. They are subject to hydrogen embrittlement and if its structure is ruptured (for example, in a car accident), steel projectiles can cause injuries. In addition, the tanks are heavy, so hydrogen storage would only constitute 0.5-1% by weight. The tanks would be difficult to refill, since hydrogen gas expands upon heating.
Composite Tanks
Composite tanks, comprised of polyethylene, have a high storage capacity (about 7% by weight) and weigh less than steel tanks. Composite tanks can store hydrogen at a pressure of about 10,000 psi, and the tanks can be shaped to fit in the chassis of a car. Composite tanks have the added safety feature of powdering upon impact, absorbing the majority of the energy of the collision before it reaches the fuel, and allowing the hydrogen gas to escape into the atmosphere.
In addition to pressurized tank storage, higher volumes of hydrogen can be stored with the aid of several chemical reactions:
Sodium Borohydride
Millenium Cell, Inc., has trademarked the phrase "Hydrogen on Demand" to describe a safe way to store hydrogen with sodium borohydride using the following reaction:
When hydrogen fuel is needed, the sodium borohydride solution is pumped over a catalyst, producing hydrogen and sodium borate, a non-toxic compound found in detergents. Both products and reactants of this simple system are stabile to air.
The "Hydrogen on Demand" system has a high effective hydrogen pressure (about 7,000 psi) and can store more hydrogen by volume than a polyethylene composite tank alone. Cars with a sodium borohydride storage system get about 300-400 miles per tank.
One disadvantage of the "Hydrogen on Demand" system is its cost. Sodium borohydride, not currently a popular chemical reagent, is expensive to purchase, and it is unknown whether the price would decrease enough to be cost efficient if used in fuel cell hydrogen storage.
Millenium Cell, Inc.'s "Hydrogen on Demand" video
Micropore Storage
The principle of hydrogen micropore storage is that molecules of hydrogen, under high temperatures and pressures, can enter the microscopic holes in media such as carbon nanotubes, graphite nanofibers and zeolites. Hydrogen remains trapped in the cavities of the material, but can be released again when the temperature is re-elevated. Buckminsterfullerenes, also known as buckyballs, have a theoretical hydrogen storage ability of 7 percent by weight. Using this theory, hydrogen could be stored in solid materials at room temperature, taking up much less space than when stored in pure solid or liquid form. In the car, the materials could be heated and hydrogen fuel could be extracted.
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Carbon nanotube and zeolites.
Hydrogen Safety
People
tend to think of hydrogen as an extremely dangerous gas to store around
people. Images of the Hindenburg disaster and the hydrogen bomb make up
a large part of the public's conception of hydrogen's behavior. However,
these images do not accurately represent the facts about hydrogen storage
safety.
There is, of course, no chance that hydrogen stored in a tank could accidentally
start an uncontrolled fusion reaction. A fusion bomb requires special
isotopes of hydrogen and such extreme conditions that a fission bomb is
used as the detonator. The hydrogen on board the Hindenburg did burn,
but most of the fire's severity and fatal effects can be attributed to
the highly flammable fabric that encased the zeppelin and the diesel fuel
on board. The coatings applied to the fabric to make it reflective and
gas-tight are actually very similar to solid rocket fuel.
While hydrogen is
a flammable and explosive gas, there are several reasons why it is often
safer than gasoline. Since hydrogen is such a lightweight gas, it diffuses
very quickly in air. If a tank develops a leak, the hydrogen is likely
to diffuse out of the area so quickly that it cannot build up sufficient
concentration to ignite in air. The risk of ignition is further reduced
by the fact that the concentration of hydrogen in air must be fairly high
to sustain a flame, four times greater than the concentration required
for gasoline, in fact.
In the event of a
fire, hydrogen has the advantage that it tends to float up and away from
the source of the leak as it burns, rather than form a burning pool on
the ground like gasoline.