Task 1 - Reactivity and interactions between H2 and microorganisms

Task 2 & 3 - Numerical simulations and thermodynamic aspects of H2 storage in porous media

Executive Summary

The “Climate change” phenomenon has taken on rather large dimensions culminating in the “Paris agreement” in 2015. One of the main goals of this agreement was to reach net zero emissions by 2050 (Unfccc, 2015). Energy consumption is steadily increasing and consequently the use of carbon (C) based energy sources constitutes a global concern. The Energy Outlook of 2023 highlighted that the four main aims of the global energy’s future are the declining role of hydrocarbons, the rapid expansion in renewable energies, the increase of electrification, and the growing use of low-carbon hydrogen (bp, 2023). The low-carbon hydrogen field is dominated by green hydrogen, produced by electrolysis using renewable power, and blue hydrogen, produced by natural gas or coal in combination with Carbon Capture and Storage (Heinemann et al., 2021). During periods of renewable energy shortage, green hydrogen could be produced from the excess of renewable energy (“Power-to-Gas”), stored, and recovered when electricity generation is needed (“Gas-to-Power”). In this way, the imbalance between supply and demand, that withheld the renewable energy’s dominance over the other energy sources, can be surpassed (Dopffel et al., 2023; Thaysen et al., 2023). Green hydrogen is a very promising fossil fuel substitute, which is expected to embody 60% of low-carbon hydrogen by 2030 (bp, 2023).

Hydrogen has high energy density since its gravimetric energy content is higher (140 MJ/kg) than hydrocarbons, for example natural gas (54 MJ/kg) (Dopffel, Jansen and Gerritse, 2021). However, its low density (0.084 kg/m3 at 20 °C and 0.1 MPa) necessitates the use of larger volumetric capacity reservoirs for its storage in comparison to natural gas when attempting to produce the same amount of energy (Heinemann et al., 2021). The above ground hydrogen storage structures are well developed but their storage capacity and discharge times are limited. The underground storage reservoirs are an economical and feasible storage option for hydrogen (Thaysen et al., 2023). As a liquid, hydrogen has a critical temperature of -239.97 °C and critical pressure of 1.297 MPa, and consequently it is stored in the gaseous phase. At geological storage conditions, hydrogen does not form hydrates but is almost three times more heat conductive than CH4 and CO2. The non-polar nature of hydrogen limits its solubility in water, which increases proportionally to pressure, as well as its density. Generally, there are some characteristics of hydrogen that must be considered for its storage procedure. Firstly, hydrogen has different physical and chemical properties than the gases which are usually stored. It can react with subsurface minerals and fluids, and it might trigger hydrogen consuming microorganisms’ growth. Secondly, after repeated injection-recovery cycles, a change in the stress field in the storage site and a contamination might occur (Heinemann et al., 2021).

Gases can be stored in surface storage facilities, like pipelines or tanks, or subsurface storage sites like depleted reservoirs, caverns, or deep aquifers (Heinemann et al., 2021). The most feasible option for large-scale hydrogen storage is the underground storage in geological formations. These reservoirs are considered unaffected from fire, safe from extreme weather conditions, military actions, or terrorist attacks. They have great storage capacity. Their operational and investment costs are lower than the above ground storage reservoirs and the suitable geological storage sites are numerous (Dopffel, Jansen and Gerritse, 2021). The underground gas storage sites (UGSs) can be categorized regarding either their storage mode or their geological and hydrodynamic features, which are the most common. UGSs according to their features are divided to the porous types and to the non-porous types. The porous type UGSs consist of depleted oil and gas reservoirs, and deep aquifers. Artificial caverns and hard rock caverns belong to the non-porous types UGSs. Other specific and rare solutions are the lined and unlined hard rock storage and abandoned mines (Molíková et al., 2022).