The deep underground salt cavities have the advantage of allowing storage of hydrogen at higher pressures than surface installations. Worldwide, in the 2020s there were 4 functional sites of salt caverns used as hydrogen deposits, three in the United States and one in the United Kingdom, at an average of 600,000 m3 capacity each at pressures varies between 50 ÷ 150 bar in the United States and 3 x 70,000 m3 at a pressure of 45 bar in the United Kingdom [1, 2]. From 2020 until now, there is an increase in the salt deposit development for hydrogen storage, expecting to account for about 18% of global energy demand by 2050 [3]. Near Vadu locality, Constanta County, Romania, there are sald deposits located between 705 m and 905 m, respectively 1542 m and 1778 m, offshore in the Black Sea continental shelf. The study consists in determining the storage capacity, at the two drilling depths, for reliable projects of hydrogen storage, for Romania's Energy Strategy, leading to a reduction in the carbon footprint at 70% in 2050 compared to the levels recorded in 2008 of CO2 emissions per transport unit for all the civilian or military transport ships [4, 5].

This work presents an integrated energy production and storage system based on Compressed Air Energy Storage (CAES), implemented within a surface lignite quarry. The proposed approach enables the reintegration of closed, degraded, or closed mining areas into the economic circuit while supporting environmental restoration, including the conservation of local flora and fauna. The concept capitalizes unused quarry surfaces and tailings dumps by installing photovoltaic panels on these deposits. In addition, artificial underground caverns are created at the quarry face using ecological concrete and segmented linings made of prefabricated reinforced concrete rings, designed for compressed air storage. In Romania, existing CAES initiatives are limited, mainly consisting of small-scale projects of up to 1 MWh, alongside a larger proposed project of approximately 300 MWh using external vessels, often associated with long investment payback periods. This study proposes a scalable alternative by integrating renewable energy generation with large-scale storage directly within lignite quarry perimeters. The system aims to achieve an estimated annual energy production of approximately 100 GWh per cavern, offering both economic and environmental benefits.