Hydrogen Storage
Introduction:
Hydrogen is widely regarded as the most promising alternative to carbon-based fuels: it
can be produced from a variety of renewable resources (e.g. wind and solar), and - when
coupled with fuel cells - offers near-zero emissions of pollutants and greenhouse gases.
Developing hydrogen as a major energy carrier, will require solutions to many scientific
and technological challenges
Challenges:
Conventional storage solutions include liquefaction or compression; however there are
energy efficiency and major safety concerns associated with both these options. A more
promising alternative is solid-state hydrogen materials: however the conventional alloys
used, such a LaNi5, have very poor gravimetric hydrogen storage densities.
Another challenge, is delivering high purity hydrogen. Polymer Electrode Membrane
(PEM) fuel cells are sensitive to gas impurities and, for prolonged exposure, require a
very pure hydrogen feed.
For automotive applications, a dense thin-metal membrane purifier has a number of
advantages: it is compact, has a low capital cost, and offers a one-stage high-purity
hydrogen output. However, the thin-metal membrane alloys currently used (Pd-Cu and
Pd-Ag) are relatively thick (~25 microns) and need to be operated at high temperatures,
making them unacceptably expensive in material and operating costs
Storage Technologies:
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Metal Hydride tanks
Compressed Hydrogen
Liquid Hydrogen
Chemically Stored Hydrogen
Carbon Nanotubes
Glass Microsphere
Liquid Carrier Storage
1. Metal Hydride tanks
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Metal hydrides are specific combinations of metallic alloys that act similar to a
sponge soaking up water
Posses the unique ability to absorb hydrogen and release it later, either at room
temperature or through heating of the tank
The total amount of hydrogen absorbed is generally 1% - 2% of the total weight
of the tank
The percentage of gas absorbed to volume of the metal is still relatively low, but
hydrides offer a valuable solution to hydrogen storage
Advantages:
Metal hydrides offer the advantages of safely delivering hydrogen at a constant
pressure.
The alloys act as a sponge, which absorbs hydrogen, but it also absorbs any impurities
introduced into the tank by the hydrogen. The result is the hydrogen released from the
tank is extremely pure.
Disadvantages:
Tank's lifetime and ability to store hydrogen is reduced as the impurities are left
behind and fill the spaces in the metal that the hydrogen once occupied.
Applications:
Hydride tanks are already used in several prototypes. (DaimlerChrysler, ETA-ING,
Fraunhofer-ISE, Linde, Motor Zeitler-Speinshart, GfE)
Chemical Reactions:
2. Compressed Hydrogen
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Hydrogen can be compressed into high-pressure tanks. This process requires
energy to accomplish and the space that the compressed gas occupies is usually
quite large resulting in a lower energy density when compared to a traditional
gasoline tank
A hydrogen gas tank that contained a store of energy equivalent to a gasoline tank
would be more than 3,000 times bigger than the gasoline tank
Hydrogen can be compressed into high-pressure tanks. High-pressure tanks
achieve 6,000 psi, and therefore must be periodically tested and inspected to
ensure their safety
Disadvantages:
Compressing or liquefying the gas is expensive.
3. Liquid Hydrogen
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Hydrogen does exist in a liquid state, but only at extremely cold temperatures.
Liquid hydrogen typically has to be stored at 20o Kelvin or -2530 C.
The temperature requirements for liquid hydrogen storage necessitate expending
energy to compress and chill the hydrogen into its liquid state
The storage tanks are insulated, to preserve temperature, and reinforced to store
the liquid hydrogen under pressure
Applications:
 Linde Cryogenic Tank used in GM fuel cell car
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Robotic arm filling liquid hydrogen to a BMW 5 Series hydrogen car
Limitations:
 The cooling and compressing process requires energy, resulting in a net loss of
about 30% of the energy that the liquid hydrogen is storing
 The margin of safety concerning liquid hydrogen storage is a function of
maintaining tank integrity and preserving the Kelvin temperatures that liquid
hydrogen requires. Combine the energy required for the process to get hydrogen
into its liquid state and the tanks required to sustain the storage pressure and
temperature and liquid hydrogen storage becomes very expensive comparative to
other methods
4. Chemically Stored Hydrogen
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Hydrogen is often found in numerous chemical compounds. Many of these
compounds are utilized as a hydrogen storage method
The hydrogen is combined in a chemical reaction that creates a stable compound
containing the hydrogen. A second reaction occurs that releases the hydrogen,
which is collected and utilized by a fuel cell. The exact reaction employed varies
from storage compound to storage compound
5. Carbon Nanotubes
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Carbon nanotubes are microscopic tubes of carbon, two nanometers (billionths of
a meter) across, that store hydrogen in microscopic pores on the tubes and within
the tube structures
Similar to metal hydrides in their mechanism for storing and releasing hydrogen,
the advantage of carbon nanotubes is the amount of hydrogen they are able to
store
Carbon nanotubes are capable of storing anywhere from 4.2% - to 65% of their
own weight in hydrogen
The US Department of Energy has stated that carbon materials need to have a
storage capacity of 6.5% of their own body weight to be practical for
transportation uses
Carbon nanotubes and their hydrogen storage capacity are still in the research and
development stage. Research on this promising technology has focused on the
areas of improving manufacturing techniques and reducing costs as carbon
nanotubes move towards commercialization
6. Glass Microspheres
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Tiny hollow glass spheres can be used to safely store hydrogen. The glass spheres
are warmed, increasing the permeability of their walls, and filled by being
immersed in high-pressure hydrogen gas
 Spheres are then cooled, locking the hydrogen inside of the glass balls. A
subsequent increase in temperature will release the hydrogen trapped in the
spheres
Advantages:
Microspheres have the potential to be very safe, resist contamination, and contain
hydrogen at a low pressure increasing the margin of safety
7. Liquid Carrier Storage
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This is the technical term for the hydrogen being stored in the fossil fuels that are
common in today's society. Whenever gasoline, natural gas methanol, etc.. is
utilized as the source for hydrogen, the fossil fuel requires reforming
The reforming process removes the hydrogen from the original fossil fuel. The
reformed hydrogen is then cleaned of excess carbon monoxide, which can poison
certain types of fuel cells, and utilized by the fuel cell