1298 enabling technology for deep decarbonisation. At present, large-scale electricity storage is dominated by pumped-hydroelectric storage (PHS), which accounts for approximately 96–98% of global installed storage capacity, corresponding to roughly 200 GW worldwide [8, 9]. All other commercial and emerging storage technologies together account for only 2–4% of installed capacity [10]. Battery energy storage systems, particularly lithium-ion technology, are the fastest-growing segment, with installed battery capacity in the EU reaching approximately 36 GWh and round-trip efficiencies of 85–95 % [11, 12]. Nevertheless, challenges related to material availability, lifecycle sustainability, and long-term storage duration motivate the development of other energy storage solutions. In this context, gravity-based energy storage systems are receiving increasing attention as a mechanical storage option capable of medium-scale, long-duration operation. These systems store energy as gravitational potential energy by lifting solid masses with electrical power and recover it during controlled descent [13]. A potentially long service life characterises gravity-based storage concepts, negligible performance degradation, and the use of low-cost, widely available materials [13, 14]. 1.2 Possible use of post-mining areas for energy storage Post-mining areas constitute a significant and largely underutilised resource for deploying energy storage technologies. Across the European Union, more than 3,200 open-pit mines with surface areas exceeding 0.5 km² have been identified, offering substantial topographical and geotechnical potential for energy storage applications. These areas provide opportunities for technical reuse while supporting regional redevelopment. Gravity energy storage (GES) systems are particularly well-suited to post-mining sites. The GES installations are expected to operate at approximately 0.5-100 MW, with response times of a few seconds and round-trip efficiencies of 70%-85 %. Unlike electrochemical batteries, gravity storage systems do not suffer capacity degradation and can enable long-term energy storage without significant energy loss [13]. In addition to technical advantages, reusing post-mining areas for energy storage aligns with European policy objectives for a just transition. Research and innovation programmes, such as the EU Research Fund for Coal and Steel, explicitly promote the transformation of former mining sites into hubs for energy services, including energy storage, to support economic diversification and preserve industrial expertise in affected regions. 2. GRENMINE PROJECT CONCEPT AND OBJECTIVES 2.1 General concept and objectives of the GrEnMine project GrEnMine (Gravitational Energy storage in the post-Mine areas) is a research and innovation project funded by the European Union under the Research Fund for Coal and Steel (RFCS). The project is implemented by an international scientific–industrial consortium led by Wrocław University of Science and Technology, comprising universities (AGH University of Krakow, Technical University of Crete, University of Petrosani), research institutes (POLTEGOR Institute, VUHU), mining companies (Lignitorichia Achladas S.A., Oltenia Energy Complex S.A., PGE GiEK), and innovative companies (Four Point). Its core objective is to develop, assess, and demonstrate gravity energy storage (GES) technology specifically for post-mining environments. The general concept of the GrEnMine project is to convert electrical energy into gravitational potential energy by lifting large masses, then recover the energy during controlled descent. Unlike conventional pumped-hydro storage, the proposed solution does not rely on large water reservoirs and can therefore be deployed in locations with unfavourable hydrological conditions or limited water availability. The concept leverages the characteristics of post-mining areas, including significant elevation differences, extensive surface areas, and existing geotechnical and energy infrastructure. Within the GrEnMine framework, several technological configurations of gravity energy storage are investigated. These include rail-based systems (RM-GES), in which solid masses are transported along inclined rail tracks, and conveyor-based systems (CB-GES), which use on-site bulk materials such as sand
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