BY Detlef Stolten
In times when nuclear power in some countries is being phased out and CO2 emissions must be reduced, the path towards renewable energy sources is clearly marked out. However, due to their strong fluctuations, renewable sources can not be very readily integrated into existing energy infrastructures. Can hydrogen serve as an energy-storage medium and provide a breakthrough here?
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ydrogen as an energy carrier was intensively discussed and investigated as early as the 1970’s, due to the two oil price crises then. However, it soon became apparent that the technology was not mature enough for widespread applications and that it would not be cost competitive. The last hydrogen projects were discontinued in the early 1990’s.
In the meantime, the situation has again changed. One aspect is the increasing proportion of fluctuating power generation from renewable energy sources such as wind and solar power. It is not sufficient, though, that merely the technological scenario for the use of hydrogen has changed; successful widespread application will also require a new socio-economic scenario. This chapter discusses both of these aspects.
Properties of Hydrogen
Hydrogen is the lightest element. Down to -253° C, it occurs in gaseous form as the H2 molecule, while at still lower temperatures it is a liquid. The gas is completely nontoxic; this distinguishes it from e. g. gasoline, which is carcinogenic, damaging to genetic material, and a danger for ground water. Hydrogen has a broad flammability range of 4 – 75 volume percent in air; however it also has a very low density, so that it rises rapidly and is readily diluted. Hydrogen-fueled vehicles can thus be constructed to be generally safe and are certified for use in countries all over the world. They are already driving on the streets as licensed demonstration vehicles.
In nature on earth, hydrogen is not found in free form in technologically relevant quantities, since it reacts quick – 118 | ly to give water in our oxygen-rich atmosphere. Most of the
Renewable Energy. Edited by R. Wengenmayr, Th. Buhrke. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
further purified via membranes. In electrolysis, the energy required to decompose water molecules into hydrogen and oxygen is provided as electrical power. Hydrogen from electrolysis is pure; however, it is more expensive.
In the year 2002, worldwide 5 ■ 107 million tons of hydrogen as the pure substance and in hydrogen-containing gas mixtures were produced [2]. The major portion was used by refineries and for the synthesis of ammonia and methanol. Hydrogen production in 1991 was derived from natural gas (77 %) with a small proportion of oil; from coal (18 %); and from electrolysis (4 %) [3]. These fractions are still relevant today.
Aside from its use as a chemical reagent, hydrogen also has great potential as a chemical energy carrier and thus as a storage medium for fluctuating energy technologies. It must in any case be produced artificially. It is therefore not a primary energy source, but rather a secondary source. The main role of such sources is to serve as storage media for energy.
