The primary challenge to the integrity of a mined, lined rock cavern used for hydrogen storage is the cyclical fatigue, within which hydrogen embrittlement can play a role.

Cyclic strains are induced by the loading/unloading of gas in combination with the confining pressure exerted by the surrounding geological and hydrological environment. These strains can be significant enough to cause plastic deformation of the liner. Additionally, the operational cycling conditions leads to liner (e.g. steel, concrete, etc.) fatigue in addition to having an impact on the surrounding rock mass itself. This fatigue is known as “low-cycle fatigue” (large strain, limited number of cycles).

Proposals should address the technical challenges stemming from combining large strains, fatigue conditions, and hydrogen service on the liner, the surrounding concrete, and the encompassing rock masses. Therefore, industrial development of this concept for hydrogen storage requires studies, tests and a combination of laboratory and field demonstrations.

To overcome the gaps mentioned above, proposals should address the following:

• Generate knowledge of steel behaviour when subject to cycling conditions in hydrogen environment under a range of operational demands. This may include simulations based on rupture mechanics, fracture propagation, plasticity theory, etc. This should also include validation by testing;
• Generate knowledge on the corrosion of steel over time including the potential for crevice corrosion and pitting that could result in failure. Damage resulting from H2 embrittlement, or impurities within the H2 of the steel liner may also be considered. This includes knowledge generation on hydrogen quality after storage and withdrawal from the mined, lined rock cavern. This may include hydrogen analysis under simulated cavern conditions in the laboratory using material from the lined rock cavern in the test reactor or by testing gas samples from a field demonstration;
• Generate knowledge on appropriate concrete compositions for cycle fatigue under a range of operational demands, as well as to best protect the integrity of both the steel liner and the surrounding rock mass. Alternative binders to Ordinary Portland Cement should be considered, to improve the environmental footprint while creating a concrete with higher durability. This may include simulations on fracture propagation, porosity/permeability analyses, as well as laboratory and/or field testing;
• Design the concrete buffer slurry ensuring that it is designed to be space filling in such a way that it does not introduce stress/strain concentrations. It will likely require high pumpability, alongside good self-compacting properties with high gravitational stability. The use of expanding agents in the concrete mix may be considered through testing, to improve space filling properties and potentially pre-stress the steel liner;
• Generate knowledge on how variations in geological conditions (e.g. lithology, depth, stress, temperature, etc.) impact both the short- and long-term performance of the storage site. This may include complex numerical simulations of the full storage system, taking into account fracture generation and propagation, fatigue, etc., as well as analogue modeling in the laboratory and/or field testing in a variety of representative geological conditions;
• Provide guidelines for the selection of steel grades (including welds) for hydrogen services in mined, lined rock caverns. This may include simulations and testing. Challenges associated with welds including potential damage due to the presence of residual stresses and heterogenous microstructures may be considered;
• Develop recommendations for a standardised design for new mined, lined rock caverns, and best practices for converting existing caverns for hydrogen storage. This design should include underground and aboveground installations dedicated to the storage activity (hydrogen treatment, compression, piping, metering). Connecting lines between the cavern and the aboveground installations should also be covered. Additionally, it is important to consider the impact of natural hazards (e.g. earthquakes) on the entire system (e.g. steel liner, concrete, rock mass, etc.);
• Understanding potential monitoring methods, including the storage site and surrounding rock mass, should be considered. Ideally, any field testing carried out would include various potential monitoring methods to understand advantages and disadvantages of each approach. Monitoring methods should be able to able to indicate potential failure, as well as other changes within the mined, lined rock cavern storage system (i.e. steel liner, concrete, rock mass, etc.);
• Ascertain the design through a comprehensive set of simulations. A physical proof of concept (POC) should also be proposed. The parameters for the POC should be ascertained through a combination of numerical modelling, and laboratory testing. The proposal for a POC may be either or a combination of 1) an above ground test that could be utilised to explore the impact of cycling hydrogen within a storage container on the various non-subsurface components (e.g., steel, concrete) and/or 2) a series of tests designed to understand the impact of different geological conditions. Other POC approaches can be proposed provided they significantly improve the level of confidence in the concept;
• Define construction methods for a mined, lined rock cavern;
• Define cavern acceptance test procedure of the mined, lined rock cavern with a focus on how geological uncertainty may impact this;
• Provide a comprehensive risk analysis covering construction, operation, and geomechanical risks taking into account an understanding of the economic, environmental, energy security, and safety considerations;
• Define guidelines/protocols to support Storage System Operators (SSOs) in the identification and management of risk associated to the storage of hydrogen in mined, lined rock caverns. The guidelines should also propose a fast-track procedure which will allow the SSOs to have a preliminary qualitative assessment of the hydrogen storage feasibility, considering the main relevant factors, as well as assist SSOs in the identification of the optimum storage sites including preferential geological/hydrological conditions; These guidelines should be seen as replication tools of the methodologies developed and demonstrated in the project in sites in other European regions with different subsurface (and operational) characteristics, ensuring an exhaustive coverage of the different European sites’ specifics;
• Develop techno-economic analyses considering the application of this large-scale solution in a number of different use-case studies including dynamic simulations. Possibilities include, but are not limited to: 1) on-grid applications where mined, lined rock caverns support the EU hydrogen grids in transporting and managing the daily intermittent (e.g., solar, wind) hydrogen production, 2) off-grid applications, where the storage solution is directly connected to an end-user (e.g., industrial use cases, maritime transportation, etc.) and its hydrogen demand, 3) hybrid solutions wherein temporary hydrogen storage may be beneficial, but that use by the grid may also be beneficial (e.g., integrated renewable energy systems).