Romania is becoming one of SEE’s most interesting storage markets because its renewable expansion is colliding with a lack of long-duration flexibility. Lithium-ion batteries are entering the system, but Romania is also testing deeper storage solutions. The agreement between Hagag Europe and Airengy to develop compressed-air electricity storage in Romanian salt caverns points to a different type of flexibility asset: one designed to store energy for days rather than hours.
The planned project uses Romania’s natural salt caverns to store compressed air. Electricity is used to inject air into underground caverns, which provide large storage volumes, natural airtightness and relatively low operating costs. When the system requires power, the air is released and used to drive electricity-generating turbines through a hydraulic system. The technology sits within the category of Long Duration Energy Storage, or LDES, which becomes more valuable as renewable penetration increases.
The financial envelope is significant but not unmanageable by infrastructure standards. The total estimated investment is €55mn. The first phase would create about 200 MWh of storage capacity at an estimated cost of €4.5mn, with operations expected within 12–18 months from the start of construction. The second phase targets approximately 25 MWof discharge capacity and around 5 GWh, or 5,000 MWh, of total storage, with an additional investment of about €50mn.
The project structure is also notable. Hagag Europe will provide rights to use the salt caverns, while Airengy will design, construct and operate the storage technology. This separates resource ownership from technology delivery and could become a replicable model in countries with suitable underground formations.
Romania’s need is straightforward. The country is experiencing a wave of photovoltaic investment and high power prices, but it lacks enough storage to manage volatility. Short-duration batteries can help with daily balancing, but they cannot fully solve multi-day renewable swings. Salt-cavern compressed-air storage offers a different proposition: lower reliance on critical minerals, long asset life and the ability to store electricity over longer periods.
For investors, this is still a technology and execution risk. The economics depend on spreads, capacity payments, balancing-market design and financing terms. But Romania is right to test the model. A power system built around solar, wind, gas and interconnection will need several storage layers. Batteries provide speed. Pumped hydro provides scale. Salt-cavern storage may provide duration.
