The relevance of energy storages for an autarky of electricity supply ...

generation, but novel materials are needed to reduce the temperature of operation.
Stationary fuel cells are suitable for distributed power generation sited close to the
customer load. Furthermore, costly investments in transmission and distribution
system upgrades can be eliminated. (Barbir, 2007; Ellison, 2011)
Storage
Hydrogen has an enormous volume of 11 m 3 per kg at ambient temperature. To
increase density, it has to be either compressed or the temperature has to be
decreased below the critical temperature or repulsion is reduced by interaction with
another material. Hydrogen has a good energy density by weight (33.3 – 38 Wh/kg)
but poor energy density by volume (2.5 Wh/l). Therefore, under normal conditions it
requires about 3000 times more space than gasoline for an equivalent amount of
energy. Hydrogen can be stored in several different ways: as gaseous hydrogen in
high-pressure tanks, as liquid hydrogen in insulated tanks or as protons in solids.
Traditionally high pressure storage tanks have been made out of steel but recently
new fibre reinforced composite materials have been developed that can withstand
internal pressures up to 5,000 – 10,000 PSI. Even at such high pressures hydrogen
has a relatively large volume, about twice that of liquid hydrogen. Hydrogen can be
stored liquid in thermally insulated containers at a temperature below 20.3 K. Liquid
hydrogen has the huge disadvantage that the process of liquefaction requires about
30 – 40 % of the final energy content of the hydrogen. Furthermore, liquid hydrogen
has to be consumed in rather a short time, as it continuously boils-off. Hydrogen can,
furthermore, be stored in solids, which are called metal hydrids. The hydrogen is
stored as protons (H + ) ions in this case. As the electrical charge in solids must always
be balanced, an equal number of extra electrons exist. The capacity depends upon the
amount of hydrogen the material can absorb in their crystal structures. Normally heat
arises during loading, which has to be removed to avoid a reduction of the storage
capacity. Adding heat or lowering the pressure can afterwards extract hydrogen from
the solid. Table 4.4 summarizes the main figures of different storage media.
(Huggins, 2010; Rummich, 2009; Ellison, 2011; Züttel, 2008)
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