Development of mined, lined rock cavern for gaseous hydrogen storage

Clean hydrogen is recognised as an energy carrier that will play a major role in the decarbonisation of European energy systems, as it can substitute fossil fuels in hard-to-abate sectors. Several governments and institutions have announced ambitious plans for developing a hydrogen economy. The European Union has notably set a 2030 target of 40 GW of electrolysers producing 10 million tonnes of renewable hydrogen to be added to 10 million tonnes of imported clean hydrogen.

These substantial quantities of hydrogen will require aboveground and underground storage capacities. Notably, underground hydrogen storage will provide a means for fulfilling these large-scale storage needs as it presents advantages in terms of environmental protection, energy security, safety, and economically, in terms of CAPEX (for high storage capacity) and OPEX. Underground storage CAPEX is highly dependent on targeted capacities, operating envelopes (namely required flowrates), available geology, needs for purification, and on storage technologies. However, an estimation of the orders of magnitude for costs is as follows:

• According to the Clean Hydrogen Partnership project HYSTORIES^[1] (2022), storage solutions based on porous reservoirs have an estimated cost of about 20€/kg (+/- 50%) and are only valid for very large quantities, whilst SRIA KPIs (2022) present a target value of 5€/kg in 2030 for porous reservoirs (storage capacity not provided; 120 bar compression);
• Salt caverns technology costs are estimated at approximately 35€/kg (+/- 50%) and are applicable for moderate to large quantities, whilst SRIA KPIs (2022) present a target value of 30€/kg in 2030 for salt caverns (storage capacity > 3000 tons);
• Storing hydrogen in mined, lined rock caverns is more difficult to assess as the methodology is not fully understood yet. Initial assessments estimate costs between 250€/kg (large quantities, in very good rock conditions) and 500€/kg (large quantities, in good rock conditions). However, costs could be both higher or lower, depending on conditions. Nonetheless, these costs remain attractive when compared to costs for surface storage techniques while also addressing concerns that are present for such techniques (e.g. safety, security, etc.).

Whether these storage capacities will be scattered or centralised remains an open question, but many analysts consider that a variety of storage unit sizes will be required including large and centralised storage.

Salt caverns or porous geological traps offer possibilities for massive hydrogen storage needs as a more cost-effective large-scale hydrogen storage solution. However, applications are limited to locations with suitable geology. In the EU, the number of such locations is limited. Thus, for regions without suitable geology, mined, lined rock caverns may be considered as a suitable technological solution for gas and liquid storage.

The design and safe operation of European hydrogen storage in mined, lined rock caverns requires the development of shared, dedicated standards and guidelines. Amongst the challenges are the choice of a hydrogen-compatible liner material (e.g. steel), the behavior of this material in cycle fatigue^[2] situations, the selection of optimised concrete or other materials to cushion the liner against the rock mass and protect it from the effects of the environmental degradation (e.g. corrosion), and other potential impacts, and an understanding of how varying geological lithologies will interact with the cyclical pressure differences. Steel is likely to be chosen for the liner based on lessons learned from manufacturing, installation, and operation processes. However, other materials may also be explored and compared to steel.

Understanding the impact of constructing new caverns as opposed to utilising previously constructed caverns on environment, safety, energy security, and economics is also a topic of interest.

Project results are expected to contribute to all the following expected outcomes:

• Generate knowledge on the mechanical behaviour of a complex liner (concrete, steel, etc.) in combination with the geomechanical behaviour of the surrounding rock for a mined, lined rock cavern subject to cycling conditions and natural hazards (e.g., earthquakes);
• Provide design principles and operation envelopes to be used by decision makers when assessing CAPEX and OPEX of mined, lined rock caverns in various conditions (rock mass quality, commercial needs, accessibility, security considerations, etc.);
• Make hydrogen storage systems that are fit for purpose and that can reduce the cost and improve the efficiency of hydrogen supply across Europe available to industry;
• Facilitate international collaborations to generate and apply knowledge that can improve underground hydrogen storage operations that contribute to hydrogen sustainability and reduce associated costs;
• Contribute to maintaining European leadership for large-scale hydrogen storage solutions, with particular focus on assessing the opportunities to understand what makes a previously built cavern best suited for purpose, as well as to understand the dynamics of building mined, lined rock caverns in a diverse set of potential geological lithologies (e.g. gneiss, granite, carbonates, sandstones, basalts). Furthermore, identify and define which geological, geotechnical, and hydrological parameters are best suited for large-scale underground hydrogen storage;
• Provide 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;
• Motivate technical and economic revitalisation of areas with abandoned and/or underutilised cavern infrastructure (e.g. tunnels, natural gas caverns, mines, etc.) in Europe.

Project results are expected to contribute to the following objectives (KPIs of the Clean Hydrogen JU SRIA are not applicable as such):

• Undertake research activities on underground storage to validate the performance in different geologies, to identify better and more cost-effective materials and to encourage improved designs;
• Support the development of Regulations Codes and Standards (RCS) for hydrogen technologies and applications, focusing on standards for assessing the life span of a mined, lined rock cavern for hydrogen storage;
• Organise safety, Pre-Normative Research (PNR) and RCS workshops.

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.

This topic focuses exclusively on gaseous hydrogen – liquid hydrogen is not considered because of its extremely low temperature requirements.

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).

Building on the results of previous activities, proposals should, as relevant, provide recommendations and dissemination for updated and/or developing new standards at EU and international levels. Projects are encouraged to involve the relevant standardization bodies, for example through liaison organisations^[3]. In addition, the outcomes of, but not only, project MefHySto^[4], supported by the under the EURAMET research programme, maybe of relevance.

For additional elements applicable to all topics please refer to section 2.2.3.2

Activities are expected to start at TRL 3 and achieve TRL 5 by the end of the project - see General Annex B.

The JU estimates that an EU contribution of maximum EUR 5.00 million would allow these outcomes to be addressed appropriately.

The conditions related to this topic are provided in the chapter 2.2.3.2 of the Clean Hydrogen JU 2025 Annual Work Plan and in the General Annexes to the Horizon Europe Work Programme 2023–2025 which apply mutatis mutandis.

[1] https://cordis.europa.eu/project/id/101007176

[2] Understood as material fatigue under a range of operational demands

[3] https://www.cencenelec.eu/media/Guides/CEN-CLC/cenclcguide25.pdf

[4] Metrology for Advanced Hydrogen Storage Solutions, https://www.euramet.org/european-metrology-networks/energy-gases/activities-impact/projects/project-details/project/metrology-for-advanced-hydrogen-storage-solutions This project has developed standards-based solutions to support the development of advanced hydrogen storage technologies.