Proposals should address the following:

• Demonstrate how a smart large-scale underground hydrogen storage, with a potential storage capacity of at least 1,000 tonnes H2 (for salt cavern: working mass of hydrogen stored, pure hydrogen considered/for depleted gas field or aquifers: mass of hydrogen recovered from the storage) integrated with renewable hydrogen source can contribute to higher integration of renewable electricity for hydrogen production (directly connected to power generation from renewable energy sources (RES);
• Demonstrate the transformation/conversion process of already existing underground storage from natural gas to hydrogen storage or the use of other geological structures for hydrogen storage;
• Define and analyse the interfaces of large-scale hydrogen underground storage with other elements like hydrogen producers, hydrogen consumers and/or a hydrogen dedicated distribution & transmission network;
• Analyse the sector coupling plus interaction of hydrogen underground storage in the future H2 network and overall future energy system network (including future energy scenarios) in terms of efficiency and possible technical operation modes of the hydrogen storage;
• Make recommendations about the technical and economic reproducibility of the process to other sites in EU;
• Improve overall energy and economic efficiency of the integrated system.

The proposed cyclic test program should include:

• End-to-end testing to qualify the performance, the integrity, the environmental impact and the safety of the underground storage, and the associated aboveground infrastructure;
• End-to-end testing to qualify the purity of the hydrogen recovered from the underground storage and the efficiency of the purification facilities (fluids & treatment process) required to deliver the hydrogen to the downstream parts of the value chain.
• For caverns:
  • At least a total number of 100 injection & withdrawal cycles of different pressure & volume variation. This cycling program should be representative of future operating conditions of an underground hydrogen storage in an assumed highly flexible hydrogen market. To that end, the cycling program should demonstrate the ability of underground storage to meet hourly temporal correlation;
  • Short and long-duration storage cycles (including superimposed cycles) with different net ramping rates, amplitudes, periodicities, and stand-by periods. (It is not expected to fill the cavern to 100% then empty it entirely for all 100 cycles).
• For depleted gas fields:
  • At least a complete storage cycle (injection phase followed by the withdrawal phase) that are representatives of the operating conditions of a future underground hydrogen storage in a depleted gas field in compliance with future EU regulation on hydrogen market.

Proposals should also:

• Address health, safety and environmental considerations, and proof compliance to international standards (e.g. quality);
• Include plans for transport & distribution of hydrogen from / to storage site;
• Address sustainability and circularity aspects.