Methods of hydrogen storage for subsequent use span many approaches,
including high pressures, cryogenics, and chemical compounds that reversibly
release H2 upon heating. Underground hydrogen storage is useful to provide
grid energy storage for intermittent energy sources, like wind power, as well
as providing fuel for transportation, particularly for ships and airplanes.
Most research into hydrogen storage is focused on storing hydrogen as a
lightweight, compact energy carrier for mobile applications.
Liquid hydrogen or slush hydrogen may be used, as in the Space Shuttle. However
liquid hydrogen requires cryogenic storage and boils around 20.268 K.
Hence, its liquefaction imposes a large energy loss. The tanks must also be well
insulated to prevent boil off but adding insulation increases cost. Liquid
hydrogen has less energy density by volume than hydrocarbon fuels such as
gasoline by approximately a factor of four. This highlights the density
problem for pure hydrogen: there is actually about 64% more hydrogen in a
liter of gasoline than there is in a liter of pure liquid hydrogen. The
carbon in the gasoline also contributes to the energy of combustion.
Compressed hydrogen, by comparison, is stored quite differently. Hydrogen gas
has good energy density by weight, but poor energy density by volume versus
hydrocarbons, hence it requires a larger tank to store. A large hydrogen tank
will be heavier than the small hydrocarbon tank used to store the same
amount of energy, all other factors remaining equal. Increasing gas pressure
would improve the energy density by volume, making for smaller, but not
lighter container tanks. Compressed hydrogen costs 2.1% of the energy
content to power the compressor. Higher compression without energy recovery will
mean more energy lost to the compression step. Compressed hydrogen storage can
exhibit very low permeation. Automotive Onboard hydrogen storage
Targets were set by the FreedomCAR Partnership in January 2002 between the
United States Council for Automotive Research and U.S. DOE. The 2005 targets
were not reached in 2005. The targets were revised in 2009 to reflect new data
on system efficiencies obtained from fleets of test cars. The ultimate goal
for volumetric storage is still above the theoretical density of liquid
hydrogen. It is important to note that these
targets are for the hydrogen storage system, not the hydrogen storage
material. System densities are often around half those of the working
material, thus while a material may store 6 wt% H2, a working system using
that material may only achieve 3 wt% when the weight of tanks, temperature
and pressure control equipment, etc., is considered.
In 2010, only two storage technologies were identified as being susceptible to
meet DOE targets: MOF-177 exceeds 2010 target for volumetric capacity, while
cryo-compressed H2 exceeds more restrictive 2015 targets for both
gravimetric and volumetric capacity. = Established technologies =
Compressed hydrogen Compressed hydrogen is the gaseous state
of the element hydrogen which is kept under pressure. Compressed hydrogen in
hydrogen tanks at 350 bar and 700 bar is used for hydrogen tank systems in
vehicles, based on type IV carbon-composite technology. Car
manufacturers have been developing this solution, such as Honda or Nissan.
Liquid hydrogen BMW has been working on liquid tank for
cars, producing for example the BMW Hydrogen 7.
= Proposals and research = Hydrogen storage technologies can be
divided into physical storage, where hydrogen molecules are stored, and
chemical storage, where hydrides are stored.
Chemical storage Chemical storage could offer high
storage performance due to the strong interaction. However, the regeneration
of storage material is still an issue. A large number of chemical storage systems
are under investigation, which involve hydrolysis reactions,
hydrogenation/dehydrogenation reactions, ammonia borane and other boron hydrides,
ammonia, and alane etc. = Metal hydrides =
Metal hydrides, such as MgH2, NaAlH4, LiAlH4, LiH, LaNi5H6, TiFeH2 and
palladium hydride, with varying degrees of efficiency, can be used as a storage
medium for hydrogen, often reversibly. Some are easy-to-fuel liquids at ambient
temperature and pressure, others are solids which could be turned into
pellets. These materials have good energy density by volume, although their
energy density by weight is often worse than the leading hydrocarbon fuels.
Most metal hydrides bind with hydrogen very strongly. As a result, high
temperatures around 120 °C – 200 °C are required to release their hydrogen
content. This energy cost can be reduced by using alloys which consists of a
strong hydride former and a weak one such as in LiNH2, LiBH4 and NaBH4. These
are able to form weaker bonds, thereby requiring less input to release stored
hydrogen. However, if the interaction is too weak, the pressure needed for
rehydriding is high, thereby eliminating any energy savings. The target for
onboard hydrogen fuel systems is roughly