Excess energy from non-programmable renewable sources can be stored by using hydrogen produced by electrolysis as an energy carrier, to be fed into existing natural gas infrastructure. This strategy attracts great interest at a national and international level, in the prospect of a progressive replacement of fossil natural gas with so-called renewable gases. However, the storage of large quantities of hydrogen still poses some challenges that have to be carefully assessed with a view to strongly develop the technologies for the production from non-programmable renewable sources. Hydrogen storage in underground tanks is a desirable option in the medium-long term, which will require identifying and selecting the most suitable sites from a technical, safety and product stability point of view. The analysis of the microbiological processes that can have an impact on the underground storage natural gas and hydrogen mixtures suggests investigating the presence of microorganisms within rocks of different types and origins, in order to assess the risks of contamination and degradation of the stored gas. This document describes the criticality due to the presence of microorganisms in underground gas storage tanks and reports the results of the investigation on the presence of microbial pools in different types of national soil which could be of interest as potential sites to host hydrogen storage tanks. It also describes the procedure developed for sampling microorganisms within superficial and deep porous rocks. The results confirm the widespread and significant presence of different types of sulphate-reducing, hydrogenotrophic bacteria and methanogenic archaea in all the samples analyzed. The metagenomic analysis of the results shows that the genera of the microorganisms found differ depending on the chemical-physical characteristics of the rocks. Each of the aforementioned types of microorganisms is responsible for a different transformation of hydrogen into another molecule (methane, acid, etc.) with resulting critical issues in terms of loss of energy and corrosion problems.
The activity conducted therefore suggests the need for a more in-depth and systematic screening of the geochemical and microbiological characteristics of potential hydrogen storage sites on the Italian territory, in order to assess the potential risk of loss and transformation of the stored gas; it is also advisable to model the kinetics of the possible bio-chemical reactions in these environments according to the various groups of microorganisms identified.