Hydrogen is a gas at
ambient conditions, so the volumetric energy density is very poor compared
to liquid fuels like petrol and diesel. There are, however, several options
to store hydrogen:
- pressurised gas
- liquid - metal hydrid - chemically bonded -
can be compressed to higher pressures in order to increase the energy
density. By doubling the tank pressure, one can roughly say that the stored
amount of energy is doubled as well. Today, storage at 200-350 bar is
available technology. However, the volumetric energy density is still almost
ten times lower than for petrol. For most mobile applications, this gives a
very limited operating range. An exception is city buses which can refuel
frequently. Prototype hydrogen tanks with up to 700 bar pressure have been
presented by GM/Opel, and several car manufacturers apply pressurised
hydrogen tanks in their fuel-cell cars. The 700 bar system was developed in
collaboration with GM's strategic fuel-cell partner, Quantum Fuel Systems
Technology Worldwide, Inc.
from Daimler Chrysler.
H2 pressurised storage tank.
hydrogen has to be kept at approximately -250°C to avoid boiling. The
energy density is much higher than the gaseous form, but a lot of energy is
required to liquefy and keep the hydrogen at such a low temperature. Despite
the fact that the high complexity of the storage and refilling systems makes
it more suitable for large amounts and/or long distances, automobile
producers like BMW and GM are pursuing liquid storage technology in their
hydrogen cars, internal combustion engines and fuel cells, respectively. A
large part of the liquid hydrogen research has been done in conjunction with
space rocket activities, where hydrogen is used as a fuel.
700 series with a hydrogen internal combustion engine.
HydroGen3 fuel-cell powered car with a liquid hydrogen tank.
stored in metal hydrides
(MH) offers an even higher volumetric energy density than liquid hydrogen.
Metal alloys (based on e.g. magnesium, aluminium or rare earth metals)
absorb hydrogen in the molecular structure, so the gas molecules are closely
packed together. Heat is released during charging and required during
discharging. The major disadvantages are the high mass and costs of the
materials. In addition, the slow refilling is also a problem. The
gravimetric hydrogen density is less than 5 wt%, and MH is therefore more
suitable where volume rather than weight is important. For instance in some
portable electronic devices, and special applications like forklifts and
fuel-cell submarine U-214 with metal hydride storage.
metal hydride storage unit at SRNL, USA.
somewhat similar to metal hydrides. As the name says, hydrogen is chemically
bonded in(to) a compound. By mixing the chemical hydrides with water, they
will produce hydrogen. For instance sodium borohydride, NaBH4:
+ 2H2O 4H2 + NaBO2
reaction is reversible and the produced sodium borooxide (NaBO2)
can be “recharged” back to sodium borohydride. NaH and LiBH4 are
other examples of compounds which are commonly used. Although the energy
density of the chemical hydride is high, there are still problems with
complete systems, both technically and concerning size.
Carbon structures may also be used to store hydrogen. Some extremely promising results
were presented in the 1990’s on carbon nano-structures. Unfortunately,
they could not be reproduced and were most probably incorrect. Another
possible option is the absorption of hydrogen in coal powders.
final choice of storage form is dependent on the type of application. Large
pressurised tanks generally cause no problem for stationary applications. In
mobile or portable applications however, size is one of the most crucial
issues. The operating range and time is limited by the amount of fuel and
this should be as large as possible. Additionally, the suitability and
overall costs influence which storage solution is chosen.