IN Brief:
- Airengy and Nobian will study a 2.5GWh compressed-air energy storage project in Denmark.
- The proposed plant would use an existing Nobian-operated underground salt cavern.
- Its initial output range is stated as 3MW to 10MW, indicating long-duration operation.
Airengy and Nobian have agreed to assess the feasibility of developing a 2.5GWh compressed-air energy storage installation using an existing Danish salt cavern.
The proposed plant would combine Airengy’s AirBattery technology with a cavern operated by Nobian. Initial plans indicate an electrical output of between 3MW and 10MW alongside the considerably larger energy-storage capacity.
Nobian will lead the cavern-related work, permitting, local stakeholder engagement, and associated infrastructure, while remaining the licence holder and operator of the underground storage volume. Airengy will be responsible for the compression, expansion, electricity-generation, and control systems.
The proposed site is associated with Dansk Salt’s production operations at Mariager. During the initial charging process, compressed air would displace brine from the cavern, and that brine could then be used within the electrified salt-production facility.
Compressed-air energy storage uses electricity to drive compressors, placing air under pressure for later release through an electricity-generation system. Salt caverns can offer large underground volumes, strong containment characteristics, and limited surface land requirements, although suitability depends on geology, cavern geometry, pressure range, and long-term integrity.
The relationship between the proposed 2.5GWh energy capacity and the 3MW to 10MW output points towards a very long-duration operating concept. Final discharge duration will depend on usable pressure, conversion efficiency, thermal management, auxiliary consumption, and the operating limits applied to the cavern.
That profile differs substantially from most lithium-ion projects, which are commonly designed for between one and four hours of full-power discharge. A lower-power, high-energy system could support extended renewable shortages, industrial resilience, scheduled energy delivery, or multi-day balancing rather than rapid high-power cycling alone.
Efficiency will be one of the principal engineering questions. Compression produces heat, while expansion causes cooling, so the way thermal energy is captured, stored, rejected, or reused will influence round-trip performance and equipment selection.
Compressor staging, turbine or expander design, moisture management, pressure losses, and auxiliary loads will also affect the amount of stored electrical energy returned to the grid. A project with very long duration must balance energy capacity against the efficiency and cost of each charging and generating cycle.
Using an existing cavern may reduce some geological-development work compared with creating a new storage volume, but detailed verification remains essential. Cavern condition, salt behaviour, well integrity, pressure cycling, brine handling, subsurface monitoring, and emergency arrangements must all be assessed for the proposed duty.
The electrical connection will determine how the asset can participate in the power system. A 3MW to 10MW rating is modest relative to the stated energy inventory, allowing extended discharge but limiting the plant’s ability to respond to large instantaneous shortfalls.
The feasibility study will therefore need to establish whether that power rating best matches local grid capacity, industrial requirements, market products, or the physical limits of the initial design. Alternative configurations may emerge as the cavern, connection, and commercial model are examined in greater detail.
A related Romanian salt-cavern development is targeting up to 5GWh, indicating broader European interest in geological storage options capable of operating beyond conventional battery durations.
Long-duration storage remains difficult to finance because current markets often reward power and rapid response more clearly than sustained energy delivery. Capacity mechanisms, strategic reserves, balancing products, congestion services, and long-term contracts may all be required to support systems whose greatest value arises during relatively infrequent but extended events.
Compressed-air installations also compete with pumped hydro, flow batteries, thermal storage, hydrogen, and other mechanical systems. Each technology carries different site requirements, efficiency, response characteristics, operating lifetime, environmental considerations, and construction risks.
Salt-cavern CAES is geographically constrained, yet suitable sites can potentially hold energy volumes that would require extensive above-ground electrochemical equipment. Existing industrial caverns may also offer a route to repurpose infrastructure and skills already associated with underground storage.
The Danish proposal remains a feasibility study rather than a committed construction project. Technical performance, cavern suitability, grid connection, planning requirements, capital cost, operating strategy, and commercial revenues must all be established before an investment decision can be reached.
Its combination of underground storage, an electrified industrial facility, and a highly renewable national power system gives the project a practical setting in which to test whether compressed air can provide commercially useful multi-day flexibility.
