The Hydrogen Economy?
At the moment the economies of the industrial world are primarily fueled by
fossil fuels. This is especially true of automobiles which use gasoline.
There is a good reason for this. The amount of energy stored in gasoline
is enormous and this fact makes it very difficult to move away from
gasoline as automobile fuel.
Consider an average small car with a 10 gallon (about 40 liters) fuel
tank. When the tank is filled with gasoline it stores
enough energy to power an average home for about two weeks.
That should already give pause. There is a huge amount of power
stored in a small volume and low weight, about 65 pounds (30 kg), of gasoline.
There is a great deal of energy in hydrogen, but it takes up a huge
volume. The same amount of energy stored in gasoline would be
3000 times larger as gaseous hydrogen.
To make it usable on a vehicle such as a small car
it needs to be stored as something
other than ordinary gas. Five options are under study:
- Compressed Gas: This is the obvious solution. The problem is
that the gas must be contained in some sort of pressure vessel
with heavy, thick walls. The best option is to use carbon fiber
walls which would require a 100 gallon tank which would weigh
about 300 pounds to store the same amount of energy as the
standard gas tank. Not a disaster for a car, but hardly ideal
since the tank is holding gas at 100's of times atmospheric
pressure. A breached tank would release a huge amount of energy
without even worrying about ignition of the fuel.
The best energy storage results when the tank is cooled
down to a few hundred degrees below zero.
- Liquid Hydrogen: As compared to compressed gas, liquid hydrogen
reduces the weight by a factor of two and the volume by 20%.
The problem is that hydrogen has to be at very cold temperatures.
Thus the tanks have to be heavily insulated and the
hydrogen leaks away as it warms up if the car is not used and
the cold is not renewed by the running car at least once in two weeks.
There is also the problem of consumers refueling such a cold fluid,
but we seem to do fine fueling cars with the very volatile gasoline.
- Bound to a Metal: Some metals or alloys of more than one metal will
form what is called a hydride with hydrogen. Heat the hydride and
the hydrogen comes free. Only a small fraction of the total
weight of the tank plus hydride is hydrogen, 1-2% although some hydrides
release up to 8% hydrogen if heated to 1000's of degrees. The major
problem is that the hydrogen that is stored in the metal has to be
very pure. Impurities become stuck in the metal and after repeated
cycles there is no room left for hydrogen. Thought has been given
to making hydride "batteries" that are replaced when the hydrogen
charge is exhausted and then recharged in controlled conditions.
- Carbon Nanotubes: Hydrogen can get trapped in pores in the lattice
of carbon atoms that make up carbon nanotubes. It is released
by heating like a hydrides. There
is a great deal of research on the amount of hydrogen that can stored
in in carbon nanotubes. Initial optimistic results of 50% or more
hydrogen by weight are proving to be wrong. More likely is in
the range of 4-5%. A more speculative variant is to store
the hydrogen in tiny glass beads as glass become more porous to
hydrogen when it is hot. Both of these are in the realm of pure
research at the moment.
- Hydrocarbons: Just as the name implies there is hydrogen bound
up in hydrocarbons such as gasoline. To get it out a device
called a reformer is needed. This is of course a very attractive
option as a bridge between now when gasoline is widely available
and hydrogen as fuel is impossible to find, and sometime in the
future when hydrogen would be available at "gas" stations.
Of course it is a drawback to have to carry the reformer around
and the left over carbon and oxygen are vented as carbon dioxide
and carbon monoxide both greenhouse gases. Most hydrogen produced
today comes from the reformer process so there are challenges associated
with the transformation from gasoline to hydrogen as vehicle fuel.
Today the best hydrogen vehicles, using compressed very cold hydrogen gas,
are more than a factor of two less efficient at energy storage
than a standard gasoline fueled vehicle. Given that the production of
hydrogen today is derived from hydrocarbons such as gasoline it is not
a surprise that gasoline fueled vehicles are not yet ready to give way
to hydrogen fueled vehicles. Fuel cells, which turn the energy in
hydrogen into mechanical energy to move the car and only emit water
vapor, are more efficient than internal combustion engines
and help narrow the gap and the major effort now is going on to improve
fuel cells and commercialize their production.
There are schemes to divorce hydrogen production from hydrocarbons.
Water is an abundant source of hydrogen, but getting the hydrogen out
of it costs more energy than can be extracted by using the hydrogen as
fuel. Thus the idea is to use reusable energy such as solar power to
extract hydrogen from water. That would be a large transition in energy
usage and require an enormous investment in infrastructure. Ideas
are also being pursued to produce hydrogen from biomass, but there
seems to be a great many hurdles to overcome to achieve commercial
There is a great deal of information on hydrogen available. Any search
engine query on "hydrogen storage", "hydrogen fuel", "hydrogen fuel cells",
etc. I used the fuelcellstore.com
which has good overviews and a very nice
paper summarizing hydrogen storage in automobiles from two Volvo engineers.
Beware of claims of very high hydrogen storage capacity for carbon nanotubes.
The initial results showing very high capacities, above 20%, are probably
in error. This article summarizes current understanding.
Rollout Disaster: An unknown company has announced that they have solved the problem
of storing compressed hydrogen gas and they have a prototype car. Despite much skepticism
many dignitaries show up at a rollout event for the prototype. As they cluster around
the gleaming prototype the car explodes in a horrible disaster killing many of them.
Investigation reveals that the car was a fake and the company but a shell. Who did
this and why?
Just Add Water: A notably unstable old friend drives up in a car. He or she fills the
tank with water from a garden hose ranting about infinite energy from a water to hydrogen
reformer powered by sunlight and drives off. The next day the friend shows up and says
that the car has been stolen. Can the characters help recover it? What is really going
Bugs of the Future: An abandoned, termite infested house is consumed in an
explosion. Investigation reveals that the explosion was caused by a build up of
hydrogen. Amazingly the cause seems to have been mutated termites that are turning
wood into hydrogen at an unheard of rate. Was it a natural mutation? Or did someone
genetically alter the termites? If so why are they in an abandoned house rather than