With regard to energy technology, hydrogen has become relevant again today due to environmental problems. The extensive combustion of fossil fuels such as coal, petroleum and natural gas is a large-scale intervention by mankind into the natural balance of the earth. The global warming provoked by the increasing concentration of CO2 in our atmosphere will have threatening effects if a further rise in the average temperature cannot be kept within narrow limits. The EU has set an ambitious goal of 2° C above the preindustrial level for this rise [4]. Today, 0.8° of this EU goal has already occurred [4], so that many countries have established climate-protection programs. They support technical alternatives to an energy supply which releases large quantities of CO2. The climate program of the German Federal government stipulates a broad-based energy supply, depending primarily on renewable energy sources and including very ambitious CO2 reduction goals [4].
Hydrogen as an energy carrier plays a role today in two areas, owing to the increasing proportion of sustainable energies in use and the more stringent requirements for energy efficiency. It thus represents the bridge between stationary electric power production and the transport sector:
1. For transportation, automobiles and buses can be powered very efficiently and with no CO2 emissions using hydrogen and fuel cells;
2. Many renewable energy sources, especially wind energy, but also solar energy, whose technical and economic feasibility are at present guaranteed, generate electric power in a fluctuating mode and therefore require energy storage systems. This storage could be accomplished by using part of the wind (and solar) power generated to produce hydrogen electrolytically, and then storing it.
These two elements can be coupled in a rational way by using so-called surplus power, e. g. wind power which is not needed by the power grid at the time when it is generated, for the production of hydrogen. This hydrogen can then substitute mineral fuels directly.
For heavy machinery such as trucks, construction equipment, locomotives, ships and aircraft, liquid fuels cannot be readily substituted, and this will continue to be true in the foreseeable future. For these uses, the energy storage density of hydrogen at the current and foreseeable state of the technology is insufficient in comparison to liquid fuels. For stationary applications also, gaseous hydrogen will not play a role in the foreseeable future; here, a mature infrastructure for utilizing natural gas is already in place.
Hydrogen could, however, be added at up to several percent to natural gas in the pipeline networks, as has been discussed under the buzzword “wind hydrogen”. As an alternative, methane production from hydrogen has been suggested, where hydrogen and carbon dioxide are reacted to give methane with an energy input (thermodynamically more precisely: an uptake of enthalpy) of 206 kJ/mole:
CO2 + 4 H2 о CH4 + 2 H2O.
The carbon dioxide could be provided by a future carbon capture and sequestration (CCS) scheme. This alternative indeed makes use of an established infrastructure, but it is not energy efficient and requires the availability of concentrated and purified CO2, e. g. through CO2 capture from exhaust gases.
Methane production is in addition economically questionable, since pure electrolysis hydrogen used as a fuel would be in competition with it and would have at least the value of gasoline today, 65-70 N-ct./liter of gasoline equivalent before taxes. Even twice this price would be justified and therefore probably acceptable to the market, since hydrogen in fuel-cell powered vehicles is twice as energy-efficient as gasoline burned in an internal-combustion engine. Thus, one could expect that a price of 1.3-14 N per liter of gasoline equivalent would be achievable for hydrogen (gasoline has an energy content of 32 MJ/l; MJ = megajoules; l = liter). Hydrogen used for methane production and added to natural gas would be sold at the same price as the natural gas, i. e. around 1 N-ct./MJ or 32 N-ct./liter gasoline equivalent. Thus, the value of a pure electrolysis product would be reduced by half or even to only a quarter by a process technology which involves additional energy losses and added costs.
