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