Long‑term hydrogen storage is essential for balancing the variability of renewable electricity and for enabling a low‑carbon energy system. The German project investigates a broad spectrum of storage concepts, from geological formations to chemical carriers, and evaluates their technical performance, economic feasibility, and environmental impact.
Geological storage in salt caverns is the most mature option. Existing natural‑gas caverns can be repurposed to hold up to 1.7 TWh of hydrogen, corresponding to a volume of 550 million m³. The conversion of a representative cavern is estimated to cost between €28 million and €58 million, while the capital expenditure per kilogram of hydrogen ranges from €1.8 to €18, depending on whether only the purge gas is replaced or full site preparation is included. Salt caverns allow rapid injection and withdrawal, but their performance is limited by the need for high pressure and the risk of brine leakage. Porous reservoirs, such as depleted gas or oil fields, offer a more geographically dispersed potential. They can store a similar 1.7 TWh of hydrogen, but the release rate is slow, permitting only one or two charge–discharge cycles per year. Microbial activity can convert hydrogen into methane or hydrogen sulfide, and geochemical reactions with carbonate or sulfate minerals may lead to pore clogging. Capital costs for these sites are comparable to salt caverns, ranging from €1.8 to €18 per kilogram of hydrogen, yet the lack of operational experience creates a significant research gap.
Chemical carriers provide alternative storage routes. Ammonia, produced via the Haber–Bosch process powered by renewable hydrogen, can be stored in large cryogenic tanks of 55 kt per unit, equivalent to about 10 kt of hydrogen or 0.3 TWh. The capital cost is roughly €6.5 per kilogram of hydrogen, and boil‑off is modest at 0.1 % per day. Ammonia’s high volumetric hydrogen density (≈108–123 kg H₂ m⁻³) and low temperature requirement (-33 °C) make it attractive for long‑distance transport, although its combustion releases nitrogen oxides and it is toxic to aquatic life. Methanol offers a different balance: it can be stored at ambient pressure and temperature of -33 °C, has a gravimetric hydrogen density of 17.7 wt %, and can be converted back to hydrogen via catalytic reforming. Methanol’s closed‑loop use of captured CO₂ and its compatibility with existing fuel infrastructure make it a promising candidate, yet the catalytic conversion step remains energy‑intensive.
Other carriers such as liquid organic hydrogen carriers (LOHCs), Fischer‑Tropsch liquids, methane, and dimethyl ether are still in early development stages. Their technical readiness levels range from 4 to 7, and they require further research on catalyst durability, energy efficiency, and integration with renewable electricity.
The project is a collaborative effort between German research institutes, universities, and industry partners, coordinated under a five‑year program (2021–2025) funded by the German Federal Ministry of Education and Research. The consortium’s objectives include the construction of a demonstration facility for porous‑reservoir storage, the integration of electrolyzers with ammonia synthesis units, and the systematic assessment of techno‑economic trade‑offs across all storage modalities. By combining field experiments, laboratory studies, and process modelling, the partnership aims to close the knowledge gaps identified in the technical assessment and to provide a roadmap for scaling up hydrogen storage infrastructure across Germany and the European Union.