The planned increase of solar and aeolic within the german energy mix is confronted with large fluctuations in electricity production due to fluctuating saisonal and metereological conditions. A boost of electricity networks' capacity, as well as of the technology for conversion and storage of excess electricity is therefore extremely important. Given the current german energy mix, which is mainly sustained by electrification of fossil fuels and controlled nuclear fission, the storage of large amounts of energy has not been a relevant issue.

Besides the direct storage of electric energy (i.e., in condensers or batteries) and the conversion in mechanical energy (i.e., in pumps or potential energy storages) or thermal energy (i.e., teleheating), the conversion in chemical energy (Hydrogen or Methane) is expected to reach a sufficiently higher efficiency for economic and environmentally sustainable use.

A joint development initiative under the name of "Energy Storage" has therefore been started by the Federal Ministry of Education and Research, for Environment, Natural Conservation and Reactor Security, and for Economy and Technology aimed at realising the targeted energy revolution. A so-called "Lighthouse" within this initiative is the "Wind-Hydrogen Coupling" project, whose major goals are: (a) increase the efficiency of conversion of the excess wind energy to Hydrogen via water electrolysis and (b) the development of new concepts for industrial scale Hydrogen storage and their scientific validation.

The following processes and systems are tested and/or further developed in order to ensure the storage of Hydrogen produced by conversion from excess wind energy:

• storage in existing distribution and transport gas network (in the medium term about 10 % vol.)
• storage as fuel in a large number of small distributed tanks - storage in large-volume underground caverns (i.e. salt caverns)
• storage in underground porous media (i.e. depleted oil & gas reservoirs and suitable aquifers)

In the framework of the Lighthouse "Wind-Hydrogen-Coupling" project, the mission of the joint project H2STORE ("Hydrogen to store") is to research the possible impact of Hydrogen on the mineralogical, geochemical and petrophysical properties of siliciclastic reservoirs and caprocks of largely depleted gas reservoirs. This joint project is composed of 6 Subprojects (SP), each with different scientific emphasis each; the German Research Centre for Geosciences (GFZ) hosts the following Subprojects:

SP 4 (Lead: H. Würdemann): Characterization of microbial processes in reservoir rocks - relevance for geological Hydrogen storage

SP 5 (Lead: A. Liebscher): Physico-chemical interactions between Hydrogen, formation fluids and reservoir rock and their impact for organic and anorganic dissolution and precipitation processes

SP 6 (Lead: M. Kühn): Numerical simulation of geochemical reactions between Hydrogen, formation fluids and rock-forming minerals

SP 5 is conducted at the Centre for Geological storage (CGS). Project runtime is 3 years (1.8.2012 - 31.7.2015)

In the framework of SP 5, laboratory experiments under site-specific teperature and pressure conditions with potential reservoir rocks and Hydrogen-saturated formation fluids or Hydrogen atmosphere are undertaken in order to investigate the impact of Hydrogen on the rock characteristics. Focus is set on possible reactions leading to dissolution or precipitation of mineral phases in the different investigated rocks. Such processes are qualitatively and whereas possible also quantitatively described, and constitute the basis for numerical simulations of complex reservoir-specific processes. The simulations are conducted within SP 6.

The laboratory experiments are achieved using an autoclave specially tailored and certified for Hydrogenating (Büchi "MidiClave" type 3) with pressures until 300 bar and temperatures until 250 °C. Core samples from different potential reservoir rocks (Ketzin, Wilsum, Altmark, Bad Langensalza, Fahner Höhe, Rockensußra, Ratzel) will be treated.