1 1 Coal
1 2 TRANSFORMING POST-MINING AREAS FOR GRAVITATIONAL ENERGY STORAGE: POTENTIAL, CHALLENGES AND PROSPECTS *P. Moczko1, L. Antošová10, A. Bajcar2, M. Cała3, T. Dobosz1, M. Galetakis5, Z. Kasztelewicz3, M. Korbut4, M. Kowalczyk1, M. Lazar6, M. Małachowski8, V. Moni10, D. Najgebauer4, A. Ostręga3, S. Papagiannidou9, I. Predoiu7 A. Stopkowicz3, J. Jakóbczyk3 1Department Wroclaw University of Science & Technology, (*Presenting author: przemyslaw.moczko@pwr.edu.pl) 2“Poltegor-Instytut" Instytut Górnictwa Odkrywkowego; 3AGH University of Krakow; 4PGE Górnictwo i Energetyka Konwencjonalna S.A.; 5Technical University of Crete; 6University of Petrosani; 7Oltenia Energy Complex S.A.; 8Four Point Sp. z o.o. TerraEye; 9IGNITORICHIA ACHLADAS S.A.; 10VUHU a.s. ABSTRACT This article explores the potential of post-mining areas as locations for innovative gravity energy storage (GES)systems: RM-GES (Rail-Mounted) and CB-GES (Conveyor-Belt). The assumptions behind these new technologies are also presented, including their scale, technical solutions, and basic technical and operational parameters. The potential locations for such energy storage facilities are presented, along with the criteria used to evaluate them and determine their suitability for the proposed technology. The social context of reusing these areas, resulting from the energy transition, is also discussed. KEYWORDS Energy transition, gravity energy storage, post-mining areas 1. INTRODUCTION 1.1 The reasons and goals of energy transition and the resulting need for energy storage The global energy transition is a fundamental challenge and objective for energy systems, aiming to achieve deep reductions in greenhouse gas emissions while maintaining a secure and reliable energy supply. Its primary driver is mitigating climate change by gradually replacing fossil-fuel-based energy generation with low-carbon alternatives, particularly renewable energy sources [1, 2]. In the European Union, the transformation of the energy system is supported by legally binding climate neutrality targets for 2050 and intermediate targets for renewable energy deployment [3]. During 2022–2024, renewable energy sources accounted for approximately 25 % of electricity generation in the EU, with a binding target to increase renewable energy to 42.5
1 3 % of final energy consumption by 2030 [4]. Wind and solar energy have become the dominant sources of new generation capacity, reflecting rapid technological development and declining costs [5]. However, wind and solar power are inherently variable and only partially predictable, resulting in temporal imbalances between electricity generation and demand. As highlighted by recent system analyses, the demand for energy storage is strongly correlated with the share of renewable energy in the power system, which currently stands at approximately 25 % globally [6]. Increasing the share of renewable energy sources, therefore, requires a substantial expansion of energy storage capacity to maintain system reliability and operational flexibility. According to European studies, installed storage capacity must increase from approximately 89 GW in 2025 to around 200 GW by 2030 and to approximately 600 GW by 2050 to support a highly renewable European electricity system [6, 7]. At the same time, the energy capacity of storage systems is projected to grow significantly, reaching up to 30% of total electricity demand in the EU by 2050 (approximately 2,189 TWh), up from around 11% in 2021 [6]. These numbers underline the critical role of energy storage as an 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
1 4 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 or gravel. In both cases, surplus electricity is used to move mass to a higher elevation, while discharge occurs through regenerative operation of the same electromechanical drive system. Preliminary analyses indicate that such installations can operate at medium scale, with power outputs ranging from several megawatts up to more than 100 MW per site, while requiring surface areas typically below 0.3 km². A key element of the GrEnMine project is the systematic assessment of the European potential for gravity energy storage in post-mining areas. The project aims to identify suitable open-pit mines and associated waste dumps across Europe and to develop a comprehensive database describing their topographical, geological, and infrastructural characteristics. This database will serve as the basis for a multi-criteria evaluation tool to enable the technical and economic assessment of potential GES installations at specific sites. Another major objective of the project is to develop and validate the proposed technology through modelling, laboratory testing, and small-scale demonstration. A pilot gravity energy storage installation will be constructed and tested at the Turów lignite mine, operated by PGE Górnictwo i Energetyka Konwencjonalna S.A. The demonstrator will allow verification of system performance, efficiency, dynamic response, and operational reliability
1 5 under operating conditions, providing essential data for further upscaling and commercial deployment. Beyond its technical scope, GrEnMine explicitly addresses environmental and socioeconomic aspects of the energy transition. By enabling the reuse of post-mining areas, the project supports the concept of transforming post-mining sites into active components of future energy systems. The proposed gravity energy storage technology is characterised by high expected efficiency, fast response times, long service life, and the absence of electrochemical degradation or hazardous materials. As such, it offers a sustainable and socially acceptable pathway for revitalising mining regions while contributing to the flexibility and resilience of renewable-based power systems. 2.2 General data on potential energy storage locations in individual countries Across Europe, the scars of coal and lignite extraction are being reimagined as engines of a renewable future. Vast open‑pit mines, towering overburden dumps, and reclaimed quarries offer a rare combination of large elevation differences, existing grid connections, and industrial access—conditions ideally suited to gravitational energy storage. Rather than filling pits or stabilising spoil heaps at great cost, developers can install modular RM‑GES systems that lift and lower mass to store and release electricity, turning former liabilities into long‑duration assets. Poland’s lignite basins, from Bełchatów’s deep excavations to extensive spoil ridges, exemplify sites where head and proximity to high‑capacity substations make rapid deployment feasible. Greece’s mosaic of active and reclaimed mines and hundreds of aggregate quarries presents a distributed opportunity for regional balancing and seasonal storage. Germany and the Czech Republic add further scale and technical know‑how, creating a network of potential installations that can support grid stability, frequency services, and capacity markets. The appeal of RM‑GES lies in its simplicity and modularity. Systems can be adapted to slope geometry, relocated as mining operations evolve, and scaled to match local demand profiles. Technical challenges—geotechnical stability, groundwater interactions, and permitting—are manageable with targeted surveys, staged pilots, and early stakeholder engagement. Economically, RM‑GES competes where the value of long‑duration storage is recognised and where existing infrastructure reduces connection costs. Repurposing mining landscapes for gravitational storage reframes reclamation as an opportunity. By aligning engineering pragmatism with regional planning and community benefits, former extraction sites can become visible symbols of transition—landmarks where industrial heritage meets renewable resilience. 2.3 Database of potential locations for GES Within the GrEnMine project, a comprehensive geospatial database of selected postmining areas in five European countries has been developed to assess their suitability for Gravitational Energy Storage (GES) technologies. The database serves as a strategic decisionsupport tool that integrates geological, geotechnical, spatial, infrastructure, and socio-economic data within a unified relational-spatial framework.
1 6 The system is based on PostgreSQL 15 and supports spatial objects, spatial indexing, and advanced geoprocessing. The database is designed for interoperability and can be accessed via SQL clients and GIS software (e.g., QGIS), thereby facilitating analytical and modelling workflows. The core data model comprises two main entities: sites and ramps, linked by a one-tomany relationship. The sites table contains general descriptors of mining and post-mining locations, including country, operational status, and selected infrastructure indicators. Each site is georeferenced, enabling spatial filtering and comparative mapping. The ramps table stores detailed geometric and geotechnical parameters required for GES feasibility assessment. These include ramp length, width, elevation difference, inclination, as well as unit weight, cohesion, friction angle, Young’s modulus, Poisson’s ratio, etc. These parameters provide the basis for slope stability analysis and for preliminary estimation of energy storage capacity. At the same time, the database structure allows integration of additional thematic layers and numerical models for advanced assessments. By consolidating spatial and engineering data, the GES database enables technical comparison and ranking of candidate locations. It provides a robust analytical foundation for identifying high-potential post-mining sites suitable for deploying gravitational energy storage to support Europe’s energy transition. 2.4 RM-GES technology concept description Rail-Mounted Gravity Energy Storage (RM-GES) is a mechanical energy storage technology that converts electrical energy into gravitational potential energy by controlling the vertical or inclined movement of solid masses. The RM-GES system consists of heavy storage masses mounted on rail transporters that operate along an inclined track or a network of tracks connecting a lower and an upper station. During charging, surplus electricity from the grid or renewable sources powers an electromechanical system that moves the masses uphill along the rails. Electrical energy is thus converted into gravitational potential energy stored in the elevated masses. During the discharging phase, the same drive system operates in regenerative mode: the controlled descent of the masses drives an electric generator, converting gravitational potential energy back into electricity and feeding it into the power system. A key feature of the RM-GES concept is the use of commercially available electromechanical components, such as motors, generators, power electronics, and railway or conveyor subsystems, adapted for bidirectional operation. This approach enhances system reliability and reduces technological risk. The modular architecture of RM-GES allows flexible scaling of both power and energy capacities by adjusting the number of storage masses, track length, elevation difference, and operating cycles. The visualisation of the RM-GES technology is shown in Figure 1.
1 7 Figure 1 – RM-GES technology concept The technology is characterised by high expected round-trip efficiency (typically 75– 85%), rapid response times of a few seconds, and negligible performance degradation over time, as no electrochemical processes are involved. RM-GES systems can be designed for power ratings ranging from several megawatts to over one hundred megawatts per site, while occupying relatively small surface areas compared to other large-scale storage technologies. By utilising solid materials—potentially including locally available post-mining materials—the RM-GES concept minimises dependence on critical raw materials and avoids environmental risks associated with chemical storage systems 2.4 CB-GES technology concept description The conveyor belt-based gravity energy storage system (CB-GES) stores energy by moving a weight from a lower to an upper elevation. As shown in Figure 2, the system stores energy as gravitational potential by using excess renewable electricity to power a reversible conveyor belt that transports material uphill to an upper stockpile (charging mode). When power is needed, the process reverses: the descending material drives the motor-generator, feeding electricity back into the grid (discharging mode).
1 8 Figure 2 – Principles of operation of a conveyor belt-based energy storage system (CB-GES) CB-GES leverages the existing steep topography and the availability of bulk granular materials (such as sand, gravel, and crushed rock) from former open-pit mines. Its storage capacity is limited only by the size and geometry of the stockpiles and the elevation difference between them. The main parameters that affect the performance of the CB-GES are the height difference between the upper and lower stockpiles, the conveyor belt inclination, the type of conveyed material, the loading factor, the frictional and rolling resistances, and the motor/generator efficiency. 3. EVALUATION AND SELECTION METHODOLOGY OF POTENTIAL LOCATIONS FOR GES 3.1 Factors influencing the applicability of this technology in post-mining areas Geotechnical factors. The identified and selected slopes should be evaluated for stability. Slope stability should be analysed in two dimensions (2D) and/or three dimensions (3D) utilising numerical methods [15]. To perform numerical computations in 2D and 3D, it is necessary to determine the basic geotechnical parameters for each identified geological layer. These parameters should be determined somewhat differently for rock formations and for soil deposits. However, for each layer, it is essential to determine the unit weight, cohesion, angle of internal friction, Young’s modulus, and Poisson’s ratio. If necessary, residual parameters should also be specified. Shear Strength Reductions Technique (SSR) should be applied for that purpose. The stability of selected slopes should be analysed in the “natural” state (without a gravitational energy storage system) and under the additional load from a gravitational energy storage system.
1 9 If the factor of safety (FS) of the slope with the load from the gravitational energy storage system is unsatisfactory (less than 1.5), the possible solutions of subsoil reinforcement should be considered. The final decision on the location of the gravitational energy storage system shall take into account economic and safety factors. As a result, a method for assessing the potential of mass energy storage technology in specific locations within post-mining areas will be developed. The 2D & 3D numerical models will be validated to account for static and dynamic loads from the gravitational energy storage system. Infrastructural factors Gravity energy storage systems based on the cyclic transport of rock masses on slopes can be successfully used in lignite open pits that already have several types of infrastructure and provide relatively large elevation differences. Fundamentally, the efficient operation of these energy storage systems depends on two components: the transport and electrical infrastructures. Their influence is equally evident in the system's storage capacity, installed power, and overall efficiency. The existing transport infrastructure in an open pit is, in most cases, represented by railway lines or conveyor belt systems. The use of these infrastructures reduces investment costs and accelerates the implementation of energy storage systems [16]. The storage capacity of a gravity system is directly proportional to the transported mass and the available elevation difference. Both transport systems encountered in the open pits analysed in the project (conveyor belts and rail transport) are highly energy-efficient, offer automation possibilities, and thus represent solutions for cyclic transport in such applications. The technical condition of the transport infrastructure is a major factor that can limit the application of GES systems, as wear of the rails, rollers, or foundations reduces the maximum allowable load and the operating cycle frequency. In these cases, it is necessary to adapt and reinforce them for operation in an intense cyclic regime, specific to energy storage. The presence of electrical networks near open lignite deposits is an important factor for the applicability of gravitational energy storage systems, since they provide essential infrastructure for energy exchange, system stability, and economic efficiency. The mining areas considered in the project are generally characterised by well-developed electrical infrastructure, a significant advantage for integrating energy storage systems. There is also the possibility of reusing existing electrical equipment, such as drive motors and transformers. Additionally, electric motors can be adapted for reversible operation in motor-generator mode, thereby reducing initial investment [17]. In conclusion, the existing transport infrastructure and the electricity grid significantly affect the applicability of GES systems. Transport infrastructure influences storage capacity and mechanical efficiency, whereas the electricity grid determines installed power and integration into the energy system. The advantages of these infrastructure facilities associated with lignite quarries primarily involve cost reduction and increased sustainability of mining activities. Social factors The assessment of the suitability of post-mining areas for siting gravitational energy storage systems should account for social conditions, as these may influence investment acceptance [18, 19]. Key aspects include the level of public acceptance, the perception of technological risk, and the expected local benefits, such as the development of services and new technical competencies, job creation, the possibility of utilising the existing human capital of
1 10 mining sector employees, and the improvement of the region’s image [20-22]. Post-mining areas, often affected by industrial restructuring and unemployment, may be perceived as opportunities for socio-economic revitalisation [23]. Within the GrEnMine project, public consultations, identification of potential conflicts of interest, assessment of compliance with local development strategies, and educational activities are planned, along with a demonstration of the pilot installation at the PGE Turów Lignite Mine in the second half of 2026. Environmental factors The environmental assessment of locations intended for gravitational energy storage systems should include not only an analysis of impacts on individual environmental components, but also the revitalisation potential of post-mining areas [19, 22]. These areas have already been significantly transformed by mining activities, which, on the one hand, reduce conflicts with sites of high natural value and, on the other hand, create opportunities for their purposeful reuse within the energy transition, particularly for the siting of today’s essential energy infrastructure [24]. In addition, secondary raw materials will be used to construct the demonstrator, in line with circular economy principles [25]. From an environmental impact perspective, it is important that GES technology does not generate pollutant emissions during operation and does not require chemical reagents or batteries, which constitute advantages over electrochemical technologies. Nevertheless, potential impacts during the construction phase — such as earthworks, dust emissions, and transport — must be considered. The actual environmental impact of the GES system designed as part of the GrEnMine project will be assessed during the demonstration. 3.2 Methodology for the evaluation and selection of potential locations in post-mining areas The methodology for evaluating the suitability of a site for RM-GES of CB-GES is based on criteria related to topography, conveyed materials, geotechnical stability, existing infrastructure, and environmental, social, and economic factors. As shown in Figure 3, the methodology begins by defining clear objectives and establishing selection criteria. Relevant data from Digital Elevation Models, geological-geotechnical maps, and environmental, financial, social, and regulatory sources are collected.
1 11 Figure 3 – Flowchart presenting the proposed methodology for site selection regarding the installation of gravity energy storage systems (CB-GES or RM-GES) Subsequently, constraints are applied to eliminate unsuitable areas, such as unstable slopes or flood-prone regions, thereby narrowing the focus to viable sites. Next, the viable options are rigorously evaluated using a weighted approach based on the Analytic Hierarchy Process (AHP), in which expert consultations and pairwise comparisons determine the relative importance of each criterion [26, 27]. Finally, multi-criteria decision analysis (MCDA) is applied to assign normalised scores to potential sites, which are then combined using weighted overlay techniques to assess the suitability of each location for implementation of RM-GES or CB-GES technology. The methodology is then applied to candidate locations, and the suitability values for RM-GES and CB-GES technologies are estimated, culminating in the categorisation of sites as optimal, moderately suitable, or unsuitable. This comprehensive approach ensures a balanced consideration of technical, environmental, and socioeconomic factors in site selection for implementing RM-GES or CBGES. 4. RM-GES TECHNOLOGY DEMONSTRATOR 4.1 The use of evaluation and selection methodology for choosing a demonstrator site The methodology for the selection of suitable sites for GES was applied to four potential sites from European open-pit coal mines: (1) Achlada Lignite Mines, Northern Part – Greece; (2) Rovinari Mining Basin, Tismana Open Pit – Romania; (3) Most Basin, ČSA Mine - Czech Republic; (4) Turów Mine - Poland. Each site was assessed for its suitability for both RM-GES and CB-GES gravity energy
1 12 storage technology. Site No 4 (Turów Mine, Poland) was identified as highly suitable for both RM-GES and CB-GES deployment due to strong geotechnical stability, existing infrastructure, and proximity to the power grid. However, a limited elevation difference slightly constrains energy potential. Site No. 2 (Rovinari Mining Basin, Romania) is well-suited to both RM-GES and CB-GES technologies, owing to stable soil conditions and the potential for integration with existing energy infrastructure. Site No. 1 (Achlada Lignite Mines, Greece) demonstrated high feasibility for CB-GES, with sufficient elevation differences and available space, but it requires geotechnical reinforcement. Finally, site No. 3 (Most Basin, Czech Republic) is considered moderately feasible, given geotechnical and infrastructure constraints, and requires further assessment. According to the above analysis, Turów Mine was selected as a suitable site for implementing RM-GES technology. 4.2 Geotechnical modelling and assessment of the demonstrator site For the 4 locations selected and discussed above, 2D numerical calculations were performed using the shear-strength-reduction method. The calculations were performed using the Finite Element Method or the Finite Difference Method (both methods yield very similar results). Fig. 4 shows the slip-surface locations and safety factors for the four locations. The calculation results clearly indicate that locations 1 (Achlada) and 3 (ČSA) do not meet the required stability indices of FS > 1.5. Locations 2 (Tishmana) and 4 (Turów) meet the stability criteria. It is worth noting that the Turów mine, which was ultimately selected as the location for the RM-GES demonstrator, has the highest stability indices of FS = 3.33. a) Achlada Lignite Miners – Greece, slip surface derived from maximum shear strain at FS =0.88 b) Tishmana open pit – Romania - slip surface derived from maximum shear strain at FS =1.51
1 13 c) ČSA Mine - Czech Republic, profile 7, slip surface derived from maximum shear strain at FS =0.92 d) Turów Open Pit Mine – Poland, slip surface derived from maximum shear strain at FS =3.33 Figure 4 – The results of stability analyses for potential sites (1), (2), (3) and (4) 4.3 Technology demonstrator description The Rail-Mounted Gravity Energy Storage (RM-GES) demonstrator represents a smallscale implementation of a mechanical energy storage technology developed within the GrEnMine project. The demonstrator is designed to validate the technical feasibility, operational safety, and performance characteristics of gravity-based energy storage systems dedicated to post-mining areas. The RM-GES concept is based on the reversible conversion of electrical energy into gravitational potential energy through the controlled movement of a solid storage mass along an inclined rail track. During charging, electrical energy from the grid or renewable sources powers an electromechanical system that transports the mass from the lower station to the upper station. In the discharging mode, the mass descends under gravity, and the same drive system operates in regenerative mode, converting mechanical energy into electrical energy. The demonstrator consists of a rail-mounted transporter chassis, a structural lifting platform carrying the storage masses, a rail track system based on standard railway components, and an electromechanical drive unit equipped with braking and safety systems (Fig. 5). The mechanical structure has been designed using detailed CAD models and verified through finite element analyses, including strength and buckling simulations. These analyses ensure safe operation under static and dynamic loads expected during charging and discharging cycles.
1 14 Figure 5 – RM-GES technology demonstrator concept Particular emphasis has been placed on operational safety, including emergency braking, controlled acceleration and deceleration, and fail-safe modes. The RM-GES demonstrator is located at the Turów open-pit lignite mine, where existing post-mining infrastructure and topography provide favourable conditions for experimental validation. Experimental testing of the demonstrator will focus on efficiency assessment, dynamic response, operational repeatability, and interaction with the supporting ground and infrastructure. The results obtained from the demonstrator will provide essential input for scaling up the RM-GES technology and assessing its applicability as a durable, environmentally friendly energy storage solution for post-mining regions. The demonstrator's general view is shown in Figure 5. 5. CONCLUSIONS AND RECOMMENDATIONS FOR TECHNOLOGY DEVELOPMENT 5.1 The use of evaluation and selection methodology for choosing a demonstrator site Preliminary assessment of the benefits and limitations of the proposed GES technologies in post-mining areas (target scale, comparison of costs, social and environmental impact, etc) is presented below: Target scale of RM-GES The RM-GES (Rail-Mounted Gravity Energy Storage) and CB-GES (Conveyor-Based Gravity Energy Storage) concepts developed within the GrEnMine project are designed primarily for medium-scale deployment in post-mining areas. Target power ratings range from several megawatts up to approximately 100 MW per site, depending on available elevation difference, ramp geometry, and site-specific geotechnical constraints. Energy capacity is modular and can be adjusted by increasing transported mass, track length, or operational cycles, allowing configurations from daily balancing units (tens to hundreds of MWh) to larger multicycle installations. This scalability makes both concepts suitable for grid support, renewable
1 15 integration, and regional flexibility services. The RM-GES and CB-GES concepts represent economically promising, durable storage solutions tailored to the technical and spatial characteristics of post-mining regions. Preliminary comparison of costs with other storage technologies Gravity energy storage (GES) is a relatively new alternative in this field, but cost comparisons can be made with other mature storage technologies. To compare energy storage technologies from this perspective, the levelized cost of storage (LCOS) is used, which accounts for capital costs (CAPEX), operating costs (OPEX), loading costs, end-of-life costs, and total discharged capacity over the technology's lifetime [28]. In principle, the capital costs (CAPEX) of GES can be considered medium to high, while the costs of Li-Ion technology are medium (decreasing), those of PHS technology are very high, and those of Redox Flow batteries are high. Furthermore, the capital costs of GES implemented in post-mining areas are much lower due to existing transport and electrical infrastructure. The operational costs (OPEX) are also lower than those of other storage technologies, given that existing transport systems (rail-mounted or belt-mounted) are mature and have low long-term maintenance costs, and the use of local materials such as sand, gravel, or mine tailings as storage mass further reduces OPEX. Thus, with a lifetime of 30-50 years and a round-trip efficiency of 75-85%, the levelized cost of storage (LCOS) of GES is comparable to that of PHS and much lower than that of the other two technologies considered (Li-Ion and Redox Flow). Social and environmental impact Considering social and environmental factors in the siting of gravitational energy storage systems is crucial to the long-term viability and acceptance of this technology. Postmining areas, owing to their existing transformation and socio-economic conditions, are particularly promising sites for GES development, combining infrastructure and revitalisation functions. Such investments can reduce pressure on greenfield areas, create jobs, and support the energy transition of former mining regions. However, the final assessment of their impacts should be based on the results of the planned technology demonstration at the PGE Turów Mine in the second half of 2026. 6. REFERENCES [1] International Energy Agency (IEA). (2023) Energy Storage Tracking Report. International Energy Agency, Paris. [2] International Energy Agency (IEA). (2021) Net Zero by 2050: A Roadmap for the Global Energy Sector. International Energy Agency, Paris. [3] European Commission. (2021) European Climate Law. Regulation (EU) 2021/1119, Official Journal of the European Union. [4] Eurostat. (2024) Renewable energy statistics. European Commission, Luxembourg. [5] Evans, S., Gabbatiss, J. (2023). Wind and solar were the EU’s top electricity source for the first time ever. Carbon Brief. [6] NWEUROPE. (2021) State of the Art Report on Storage Technologies, Opportunities and Trends. Interreg North-West Europe Programme, Lille.
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1 18 RESEARCH ON RESOURCE RECYCLING OF COAL-MEASURE MINING WASTE AND FORESTRY, AGRICULTURAL, AND LIVESTOCK RESIDUES Liu Jing, Yang Zihai, Wang Kexin, Kong Suli, Li Yifeng (CCTEG Tangshan Research Institute Co., Ltd., Tangshan, Hebei, 063012) (Presenting author: 18903389888@189.cn) ABSTRACT Coal gangue is a solid waste generated during the processes of coal mining and coal preparation. At present, the main treatment method is landfill, which has a significant impact on the environment. In this study, thermophilic aerobic microbial strains are selected and optimized for the rapid and complete fermentation of forestry, agricultural, and livestock residues, such as livestock manure, tree leaves, and crop straw, within 24 hours to produce organic microbial fertilizer; the organic microbial fertilizer is blended with -6 mm coal gangue and tailing coal slime according to a scientific formulation to produce efficient artificial soil. This artificial soil can be used independently as a soilless cultivation substrate. It can also be used to improve low-productivity land such as saline-alkali land, desert land, and loess land. Laboratory tests and small-scale field trials have demonstrated notable effectiveness. Innovative coal preparation technology and equipment are utilized to separate +6 mm coal gangue for underground backfilling, thereby ensuring that gangue is not lifted to the surface. This series of technologies are conducive to the reduction, large-scale reuse, and recycling of coal gangue. This approach holds significant importance for the ecological restoration of mining areas, improvement of low-productivity land, and advancement of agricultural production. KEYWORDS Coal gangue treatment; Organic microbial fertilizer; Resource utilization; Improvement of lowproductivity land The primary chemical components of coal gangue are SiO2 and Al2O3. As a mixture of carbonaceous, argillaceous, and sandy shale, coal gangue contains a small amount of carbon and a certain amount of organic matter and humic acid, along with approximately 30 trace elements, including nitrogen, phosphorus, and potassium. It accounts for 15% to 30% of raw coal production. According to surveys, China's accumulated stockpile of coal gangue exceeds 7 billion tons, with an annual addition of 500 to 700 million tons. Due to constraints in economic conditions, technological capabilities, and external factors, the utilization of coal gangue is primarily limited to applications such as collapse pit backfilling, power generation, and production of building materials. However, the disposal capacity of these applications remains insufficient. Consequently, most coal gangue continues to be stockpiled, leading to a series of issues, including land occupation and environmental pollution. With the increasingly stringent environmental
1 19 protection policies, this situation has severely impacted routine mine production, underscoring an urgent need for large-scale resource utilization. The large-scale disposal and resource utilization of coal gangue has always been a research topic for experts and scholars[1]. The existing technology and utilization approaches have limited processing capacity, making it impossible to carry out large-scale disposal and achieve full resource utilization. Many researchers have conducted extensive and valuable studies and attempts on the soil utilization of coal gangue, striving to convert these materials into artificial soil with soil-like properties suitable for plant growth[3-5] through the continuous decomposition and humification of specific microorganisms[2], thereby transforming waste into a valuable resource. According to surveys, existing technologies for preparing artificial soil from coal gangue mainly include direct inoculation of biological bacteria on coal gangue and direct addition of straw. Due to the very low organic content in coal gangue, limited carbon source conversion, and extremely slow activation and decomposition, it is difficult to form large-scale applications. The 3Rs (Reuse, Reduce, and Recycle) of the circular economy are aimed at achieving harmlessness, reduction, and resource utilization for coal gangue. In this study, based on the physicochemical characteristics of coal gangue, innovative technologies are developed to establish a system that integrates underground reduction and surface quality-graded resource utilization, which can effectively achieve large-scale and harmless disposal of coal gangue. Raw coal is sorted underground with innovative coal preparation technology and equipment. Gangue larger than 6 mm is directly separated and used as backfill in mine roadways, resulting in a reduction at the source. Gangue smaller than 6 mm is lifted to the surface and further divided into low-calorific-value fuel and valuable elements through quality-graded separation, and the remaining tailings are co-processed with efficient organic microbial fertilizer. The residual carbon and beneficial elements in coal gangue, as well as the good water retention, fertilizer retention, and moisture conservation performance of tailing coal slime are fully utilized to produce artificial soil. This artificial soil can be used for direct planting of vegetables and fruits, or as a soilless cultivation substrate. It can also be used for large-scale improvement of lowproductivity land such as saline-alkali land, desert land, and loess land. 1. EFFICIENT AND RAPID FERMENTATION TECHNOLOGY OF FORESTRY AND AGRICULTURAL RESIDUES Forestry and agricultural residues refer to organic matter discarded during the entire production process in agriculture and forestry, including crop straw, branches, livestock manure, and the like. According to surveys, China produces 1 billion tons of crop straw, 3.9 billion tons of livestock manure, and 100 million tons of forestry waste annually. The random discharge of these wastes has further deteriorated the ecological environment in rural areas. In this study, forestry, agricultural, and livestock residues are used as production inputs and carriers for microbial agents due to their abundant resources and high organic matter contents. Through innovative processes, including crushing and fermentation, the rapid and complete fermentation of the raw materials can be achieved. The entire process can effectively shorten
1 20 the fermentation time and improve the fermentation effect, reduce the cost of preparing organic microbial fertilizer, and is essential for accelerating the disposal of mining waste. Microbial decomposition and humification are inherently slow processes. Therefore, continuous screening and optimization of microbial strains are required to ensure their effective integration, survival, and reproduction within the two types of solid waste. The objective is to convert the mineral elements in waste from a solid state into a free state absorbable by plants and enable the effective release of such beneficial trace elements, thereby realizing thorough activation and rapid decomposition of beneficial mineral elements in mining waste, while passivating heavy metals and balancing pH levels. Wang Hanxia[6] found in her research that mixed microbial strains exhibited superior decomposition performance compared to single-strain systems. Liu Ziwei[7] compared the effects of single-, dual-, and triple-microbial inoculants on the improvement of coal gangue-based substrates, and concluded that dual-microbial inoculants yielded better results for mining area soil than a single-microbial inoculant. Traditional aerobic fermentation takes 7-15 days. In the study, phosphate-solubilizing bacteria, thermophilic bacteria, and nitrogen-fixing bacteria suitable for decomposing coal gangue and capable of proliferating vigorously in forestry and agricultural residues were selected to form a composite microbial inoculant, and efficient microbial strains were cultivated and screened through optimal selection and cultivation. Pruned tree branches from the site of CCTEG Tangshan Research Institute, used as experimental materials, were crushed using specialized equipment and fermented efficiently into microbial fertilizer in the fermentation equipment. By measuring changes in nutrient elements and organic matter in the microbial fertilizer, the optimal inoculant amount ratio, optimal fermentation temperature, and optimal fermentation time were determined for complete fermentation of the microbial inoculant. The experimental results indicate that the complete fermentation of forestry, agricultural, and livestock residues is achieved within 24 hours at a composite inoculant amount ratio of 1:10 and a fermentation temperature of 60 °C-80 °C, and all indicators of the produced organic microbial fertilizer meet the national standards for organic fertilizers in China. The product is rich in organic microbial content and exhibits high activity. When subsequently co-processed with coal gangue, it enables the rapid activation and release of nutrient elements from the coal gangue. 2. COAL GANGUE GRADATION AND RAW MATERIAL BLENDING The coal gangue and tailing coal slime used in the experiment were sourced from Tangshan Mine of Kailuan Group. Through testing, the heavy metal content is below the screening limit for soil contamination risk specified by the Soil Environmental Quality – Risk Control Standard for Soil Contamination of Agricultural Land (GB 15618-2018). There are significant differences in the physicochemical properties of coal gangue with different particle sizes. Previous studies[8-9] have shown that an appropriate gradation of coal gangue can improve the substrate's bulk density and porosity, thereby ensuring aeration and promoting crop growth. This study has shown that coal gangue with different particle sizes plays distinct roles in soil utilization. Tailing coal slime (-0.5 mm) has a large specific surface area and
1 21 very good water retention, fertilizer retention, and moisture conservation performance, but it is viscous and has poor permeability. Fine-grained gangue (0.5-6 mm) can increase permeability, but has poor moisture conservation. Large gangue (+6 mm) cannot form a stable soil structure when used directly, and has slow rate of activation and decomposition, which is not conducive to plant growth. Therefore, in the soil utilization of coal gangue, particle sizes below 6 mm are selected for gradation. -6 mm coal gangue is divided into tailing coal slime (-0.5 mm) and fine-grained gangue (0.5-6 mm) according to the particle size. In Table 1, fine-grained gangue, tailing coal slime, soil, plant powder, and microbial fertilizer are mixed in a certain ratio for orthogonal test to form 9 different substrates, designated as R1-R9. Soil collected from the experimental site is used as the control check (CK). Table 1 - Test substrates and compounding ratios Substrate No. Fine-grained gangue Tailing coal slime Soil Plant powder Microbial fertilizer R1 22.3% 44.7% 20% 10% 3% R2 33.5% 33.5% 20% 10% 3% R3 44.7% 22.3% 20% 10% 3% R4 21.7% 43.3% 20% 10% 5% R5 32.5% 32.5% 20% 10% 5% R6 43.3% 21.7% 20% 10% 5% R7 20% 40% 20% 10% 10% R8 30% 30% 20% 10% 10% R9 40% 20% 20% 10% 10% CK 100% The pre-planting soil property indicators of different substrates are presented in Table 2. Table 2 - Pre-planting soil property indicators of different substrates Substrate No. pH Organic matter (g/kg) Rapidly available phosphorus (mg/kg) Available nitrogen (mg/kg) Rapidly available potassium (mg/kg) R1 8.52 59.06 5.57 55 120 R2 8.37 52.05 5.61 58 126 R3 8.41 45.05 5.59 57 131 R4 8.36 57.04 7.17 62 138 R5 8.20 50.49 7.32 65 143 R6 8.27 43.94 7.09 63 140 R7 8.02 52.53 9.41 70 158 R8 7.67 46.93 9.82 76 171 R9 7.73 41.33 9.75 72 166
1 22 CK 6.97 30.00 15.00 65 125 3. EXPERIMENTAL RESULTS AND ANALYSIS 3.1 Influence of Different Substrates on Crop Growth 3.1.1 Influence on emergence rate The pot-based cultivation experiment was initiated in early April 2024 within a greenhouse. The experiment utilized planting boxes measuring 50 cm (L) × 40 cm (W) × 28 cm (H). Pak choi seeds were procured from Tangshan Academy of Agricultural Sciences. During the growth period, the plants were regularly watered and managed under uniform cultivation conditions. Pak choi sprouts 3 to 4 days after sowing, and the emergence rate reaches its highest level in 6 to 7 days. The emergence rates of pak choi in different substrates are shown in Figure 1.Figure 1 indicates that pak choi is able to germinate in any substrate, with an average emergence rate of over 85% and equivalent to that of the CK. The emergence rate of R2, R5, and R8 is higher than that of the CK; the emergence rate of R1, R4, and R7 is slightly lower than that of the CK; the emergence rate of R3, R6, and R9 is close to that of the CK. This indicates that after adding a certain ratio of microbial fertilizer, the ratio of finegrained gangue and tailing coal slime is different, and the overall trend in emergence rate follows R2 > R3 > R1, R5 > R6 > R4, and R8 > R9 > R7. This suggests that the substrates exhibit optimal water retention and aeration properties when the fine-grained gangue is mixed with tailing coal slime in a 1:1 ratio. A high ratio of fine-grained gangue results in a weak water retention capacity, adversely affecting seed root development. A high ratio of tailing coal slime reduces substrate aeration, consequently decreasing the emergence rate. Figure 1 - Emergence rate of pak choi in different substrates It should be noted that in previous experiments, when the ratio of microbial fertilizer exceeded 15-20%, seedlings wilt and the emergence rate dropped significantly. Taking into account the price factor of microbial fertilizer, we have ultimately determined its maximum ratio as 10%. 3.1.2 Influence on growth performance of pak choi
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