Gravity Energy Storage and Its Feasibility in the Context of Sustainable Energy Management with an Example of the Possibilities of Mine Shafts in Poland

Abstract

This paper discusses the viability and efficiency of gravity energy storage (GES) systems utilizing abandoned coal mine shafts in Poland as a new frontier of energy management within the broader framework of sustainable energy transition. After a thorough analysis of shaft infrastructure, economic factors, and regulatory environment, the research demonstrates how GES is in line with circular economy and sustainability principles yet there are certain technical and financial limitations—smaller lifting capacities and expensive adjustments, for instance—that are currently stalling its large-scale adoption. The results highlight the importance of harmonizing such repurposing efforts with the available renewable energy infrastructure and call for aggressive policy, technological, and funding efforts to sustain the conceptual promise with actual fulfilment.

1. Introduction

The concept of storing electrical energy has long been understood, initially applied in systems aimed at stabilizing electrical performance, such as those in spark-ignition engine vehicles—later adapted to hybrid drive technologies. Building on this foundation, the notion of storing already produced energy in dedicated energy storage systems began to take shape. For years, pumped-storage hydroelectric power plants have been implemented in large-scale energy systems [1], serving the role of stabilizing the grid by capturing surplus energy during periods of low demand and releasing it when the system experiences an energy deficit [2].

Since 2015, the idea of utilizing gravity for electrical energy storage has been increasingly discussed in the literature [3,4]. Gravity-based energy storage systems function by capturing the potential energy that arises when an object is elevated, with gravity acting as the driving force behind this energy accumulation. These systems use surplus electricity from the grid to raise a heavy mass, thereby storing energy in the form of gravitational potential, which can later be released and transformed back into electrical energy through a generator [5]. This system can be used in mines when their basic function is stopped, including closed coal mines [6]. The technology of gravity-based energy storage in mining environments is still in the development phase, although initial implementations have already begun to emerge [7]. One such solution is the gravity energy storage system (GESS), which operates on a similar principle to that of pumped hydro storage. However, rather than relying on water reservoirs, the GESS utilizes columns of loose bulk materials—such as sand, gravel, or other granular substances—positioned at varying heights to store and release energy [8].

The main goal of this article is to investigate the feasibility and functionality of energy storage in mine shafts using gravity energy storage technology in the context of sustainable energy management and the ongoing energy transition.

In this paper, the following research questions (RQs) were formulated:

• RQ1: What are the technical and economic limitations and the potential opportunities for implementing gravity energy storage in mine shafts in Poland?
• RQ2: To what extent can gravity energy storage in mine shafts represent a competitive solution compared to other energy storage methods, such as pumped-storage power plants or battery systems?
• RQ3: Is the concept of using mine shafts as gravity energy storage facilities consistent with the assumptions of sustainable energy transition and the plans for restructuring the mining industry in Poland?

Subsequent parts of this paper present theory background, contextual applicability, the research method, and findings from the analysis of gravity energy storage (GES) in Polish mine shafts. In the Background section, how energy storage is conducted in the context of overall energy transition is explained wherein EU policy action and technology innovation are the context where GES is an applied solution. The Study Method section distinguishes between theory and practical application according to rudimentary physics formulae and environmental limitations—legal, economic, and infrastructural—to analyse mine shaft conversion practicability. In the Results of Analysis section, the authors show quantitative results for which they prove that although theoretically energy can be conserved by utilizing gravity in mine shafts, their efficacy is rather low due to lifting capacity, shaft geometry, and maintenance. The Discussion section places these findings within the context of policy alignment, circular economy potential, and relative performance versus competing technologies and concludes that GES is a slightly better technology at scale. The Conclusion then combines these findings to ensure that although GES aligns with sustainability goals and policy instruments, its application in Poland requires focused, hybrid solutions based on focused technological and financial support.

2. Background

Energy storage systems have been known and used for several centuries. In recent years, the growing momentum of global climate policies combined with the expansion of renewable energy sources (RES) has significantly accelerated interest in energy storage technologies. These storage systems are essential to the ongoing energy transition, as they facilitate the effective integration of RES into power systems while contributing to grid stability and reliability [9,10,11,12].

The importance of ensuring access to energy is underscored by the United Nations 2030 Agenda for Sustainable Development, which identifies it as the seventh of the 17 Sustainable Development Goals [13]. Energy storage is seen as the future of zero-emission energy systems and forms the foundation for implementing the green transition in economies and industries. The effective regulation of energy storage in the European Union requires balancing technological innovations with appropriate legal frameworks [14]. International cooperation and strong partnerships are essential for developing sustainable technologies [15]. Many regions of the world, particularly the EU, are striving to create an internal electricity market based on non-fossil fuel energy sources. Directive (EU) 2019/944 [16] sets out rules for the generation, transmission, distribution, supply, and storage of electricity, as well as consumer protection measures, with the aim of creating integrated, competitive, consumer-centred, flexible, fair, and transparent electricity markets within the European Union. It was amended in 2024 by Directive (EU) 2024/1711.

It is estimated that over 75% of greenhouse gas emissions in the EU currently come from energy production and use across various sectors of the economy, which underscores the conviction that achieving the ambitious climate targets for 2030 and 2050 is a critical EU strategy [17]. Achieving net zero emissions by 2050 is seen as a key objective for limiting global warming and meeting the goals of the Paris Agreement [18]. The new energy policy is only possible thanks to technological breakthroughs, including new storage possibilities such as pumped-storage hydropower, battery and hydrogen storage, and, notably, gravity storage in mine shafts (following the decommissioning of coal mines) [3,19,20,21,22,23].

Gravity storage systems are part of the broader category of mechanical energy storage systems. According to the principles of their operation, storage systems can be classified as chemical, thermal, or mechanical. Among mechanical storage systems, those that utilize potential energy, kinetic energy, or compression energy are in use (see Figure 1). Within the category of potential energy storage systems are the gravity-based storage systems in mine shafts. The storage capacity of these systems is determined by two parameters: the mass of the load (or loads) and the available height difference between the resting levels of the load [3,24,25,26,27,28].

Energy storage in former mining sites—particularly through the innovative approach known as underground gravity energy storage (UGES)—offers a promising solution for long-term energy retention. This technique repurposes decommissioned mines into storage facilities by using gravitational potential energy, thereby contributing to the advancement of a more sustainable and resilient energy system [UGES]. It is considered in the restructuring scenarios for the mining industry alongside the management of methane [29,30,31]. In EU countries, including Poland, the decommissioning of hard coal mines has been an ongoing process for years, aimed at gradually closing the mines in line with energy transition objectives and market changes. In Poland, there is a plan to close 12 mines by the end of 2049. On 28 May 2021, in Katowice, the government, trade unions, mining municipalities, and mining companies signed a social agreement [32]. According to the Energy Policy of Poland (PEP2040), the coal phase-out rate from 2030 to 2040 is projected to be 65% (2025), 56% (2030), 38% (2035), and 28% (2040) [33]. According to INSTRAT data from December 2024, there were 19 hard coal mines and four lignite mines operating in Poland [34]. The transition of mining in Poland is the main climate policy in this country [35]. Of the innovative energy storage solutions in the European (EU) new energy policy, pumped-storage hydropower plants could be used with closed mines. In Poland, existing facilities of this type currently offer an energy storage capacity of around 8 GWh, with projections indicating that this capacity could potentially be expanded, although only by a few times at most. Following the planned cessation of lignite extraction at the Turów open-pit mine, there has been a proposal to construct a pumped-storage plant with an installed capacity of 2300 MW and a total storage potential of up to 160 GWh over the next three decades—an amount still considered modest in scale. The above limitations mean that other methods and facilities for storing the electricity are being sought, and mainly gravitational, pressure, and electrochemical ones are considered [36].

Energy storage facilities could present an opportunity for decommissioned mines, fulfilling the primary function of energy storage while also offering additional, secondary functions, such as the utilization of existing electrical infrastructure within the closed mine, the reuse of shaft and hoisting equipment, the retention of some mining-specialized jobs at the site of the closed mine, and the deferral of shaft decommissioning costs until after the storage facility has completed its operation. Of course, establishing such storage facilities requires significant direct investment costs, including purchasing and installing technology, draining water from decommissioned mine shafts, maintaining shaft ventilation for monitoring and operational needs, and covering the expenses of maintenance and repair work within the shaft. In the case of storage systems with multiple (divided) loads, it is also necessary to build loading and unloading systems for the weights and masses both at the surface and underground [25,37,38,39,40,41,42].

The gravity-based solution (GES) is comparable to a pumped-storage power plant, except that water is replaced with composite blocks or “mobile masses” made from inexpensive, locally sourced materials (including waste materials such as discarded wind turbine blades). The composite blocks and the lifting mechanism for storing and releasing energy are central to gravity-based storage systems. The operational process involves transferring energy from renewable sources to motors that raise the blocks (the Gravitricity technology). When electricity demand arises, the elevated blocks are lowered, and the kinetic energy generated during their descent is converted into electrical power. Gravity-based energy systems are particularly suitable for locations where traditional water-pumping methods are impractical. They are also applicable in post-mining areas that offer appropriate vertical elevation and sufficient quantities of granular bulk materials [43]. One such example is the Gravitricity technology, which operates by raising and lowering heavy weights—up to a combined mass of 12,000 tons—within deep mine shafts to store and release energy. Recent research by Imperial College London has shown that the electricity discharged by a commercially available 10 MW lithium-ion battery system would cost USD 367 (283 GBP)/MWh over its lifetime, compared to USD 171/MWh for the Gravitricity system. According to Recharge, the gravity-based technology is currently being piloted in the United Kingdom, Finland, Poland, the Czech Republic, and South Africa, where mine shafts can reach depths exceeding 2000 m [43,44,45].

Among the concepts for building gravity energy storage systems in the workings of inactive mines are the following solutions: underground pumped-storage hydropower plants using mine workings as, at minimum, the lower water reservoir; gravity storage systems with a single fixed weight; gravity storage systems with multiple fixed weights; gravity storage systems with containers filled with a working liquid as the load; gravity storage systems with containers for loose ballast material; gravity storage technologies utilizing the gravity of water flow; gravity storage technologies based on pressure gravity; and others that are currently in the technological development phase [3,43].

Here is an overview of international examples and projects related to various types of gravity energy storage, particularly those making use of the infrastructure of inactive mines:

• (i) Underground pumped-storage power plants in mine workings: In Finland, in the inactive Pyhäsalmi mine, the deepest mine shaft in Europe (with a depth of almost 1.5 km), a project is underway to build an underground pumped-storage power plant. The planned installation will have a capacity of 75 MW and an energy storage capacity of 530 MWh, utilizing the existing shafts and workings as the lower water reservoir. The project has received financial support from the European Union amounting to EUR 26.3 million [44].
• (ii) Gravity energy storage systems with a single or multiple weights: Energy storage technology based on lifting and lowering heavy weights in mine shafts (research on adapting this technology is currently underway in inactive mines such as Velenje in Slovenia, Pyhäsalmi in Finland, Darkov in the Czech Republic, and Grube Teutschenthal in Germany) [45].
• (iii) The world’s first gravity energy storage systems utilizing gravity blocks—constructed from waste materials such as construction debris, coal ash, or fiberglass—and designed for an operational lifespan of up to 50 years, have been established in China. A notable example is the China Tianying Rudong Gravity Energy Storage Project, located in Yangkou town, Rudong county, Nantong. This pioneering facility, with an initial capacity of 26 MW, is expected to reach a total energy storage capability of 100 MWh once it becomes fully operational by the end of 2025 [46].
• (iv) In Australia, the difference in height between two water reservoirs is used to store and generate electricity. (Snowy 2.0 project: The Snowy 2.0 project involves tunnelling through 27 km (17 miles) of rock to create a huge pipeline connecting two bodies of water. The elevation difference is 700 m (2296 feet). The energy gravity system would provide about 10% of Australia’s electricity production at full output [47].)
• (v) A recent study by the International Institute for Applied Systems Analysis (IIASA) introduced a novel concept for energy storage, involving the transportation of sand into decommissioned underground mines. This approach, known as underground gravity energy storage (UGES), generates electricity by lowering sand into the mine and harnessing its gravitational potential through regenerative braking systems [26]. According to the findings, a single mine could offer an energy storage capacity ranging from 10 to 100 GWh, depending on the use of 5 to 50 million tonnes of sand. The deeper and wider the shaft, the greater the potential for power generation, while the overall size of the mine determines the maximum energy that can be stored [26,48].

The growth in the use of renewable energy sources has demonstrated the need to build and develop efficient and large-scale energy storage solutions. The extensive infrastructure of coal mines can provide the basis for repurposing mine facilities as energy storage. Several equivalent solutions are under development.

2.1. Examples of Energy Storage in Coal Mines

2.1.1. Pumped Hydro Energy Storage (PHES)

One of the most established methods of large-scale energy storage is pumped storage (PHES). In PHES systems, energy storage is achieved by pumping water from the lower reservoir to the upper reservoir during off-peak hours and releasing it during peak demand hours. A PHES power plant has been proposed at the Bełchatów opencast lignite mine in Poland. Pits serving as the lower reservoir are used and the upper reservoir is to be on heaps. Key parameters affecting the efficiency of this investment include reservoir volume, useful head, and energy losses. Evaporation and groundwater recharge rates also affect the efficiency of this system. GIS tools [49,50] and the Colebrook–White [51] formula were used to measure and determine the key parameters. The estimated energy storage potential is between 16.4 and 36.2 GWh per cycle, with an efficiency of around 75%. This could generate between 9.7 and 13.7 TWh of electricity per year, equivalent to 45% of the output of a nearby power station [52].

Underground pumped hydro storage power plants (UPHS) can be used in underground coal mines. These use the voids in closed coal mines as lower reservoirs. This method is particularly effective when combined with renewable energy sources such as wind or solar power. A case study in Asturias, Spain, demonstrated the feasibility of integrating UPHS with a 60 MW wind farm. The system, which had an investment cost of EUR 193 M, could generate EUR 54 M in benefits over 40 years, while reducing CO₂ emissions by 29,000 tonnes per year [53]. In China, the suitability of a combination of wind farms and coal mines in pumped hydro storage (PHS) was analysed. Storage capacities and efficiencies were analysed. The results presented indicate that such systems are technically feasible in regions rich in wind and solar energy [54].

Underground gravity energy storage (UGES) is an innovative method using sand as a storage medium. During off-peak hours, sand is lifted and stored above ground, and during peak demand hours it is lowered into underground tanks to generate electricity. A major advantage of UGES is the relatively long energy storage time. At the same time, the use of sand eliminates the risk of water pollution and provides a cost-effective solution with relatively low investment costs. The global potential of UGES is estimated at 7 to 70 TWh [26].

2.1.2. Compressed Air Energy Storage (CAES)

Compressed air energy storage (CAES) involves compressing air during off-peak hours and storing it in underground caverns. During peak demand, the compressed air is expanded to generate electricity. Coal mines, with their existing underground voids, are ideal locations for CAES systems. The Adiabatic CAES (A-CAES) method stores both the compressed air and the heat generated during compression, enabling more efficient energy recovery. A study in a Spanish coal mine showed that A-CAES systems can work. However, there may be minor geo-mechanical problems. These systems are estimated to have a fairly high capacity of more than 10,000 cycles [55,56]. Combining CAES with thermal energy storage (TES) systems can increase efficiency. The hybrid system in the post-mining shaft achieved an energy output of 140 MWh at 5 MPa, with a round-trip efficiency of 70.44% [57].

2.1.3. Battery Storage Systems

Lithium–iron–phosphate batteries are increasingly used in coal mines for energy storage [58]. These batteries offer a high performance and long life, making them suitable for both energy storage and generation. Research has focused on optimising battery configurations, charging mechanisms, and battery management systems. Experimental data have shown that these systems can effectively improve charging performance and extend battery life [59].

2.1.4. Hybrid Systems and Integration

Integrating multiple energy storage technologies can increase the overall efficiency and reliability of coal mine-based systems. For example, combining CAES with TES or UPHS with wind power can create hybrid systems that optimise energy storage and generation. A study on industrial energy management systems highlighted the benefits of integrating large-scale energy storage with smart grid concepts [60,61]. This approach improves power supply reliability, operational efficiency, and environmental friendliness [62]. In summary, each of the methods described offers unique advantages, and their integration with renewable energy sources and smart grid systems can further enhance their efficiency and environmental benefits. As the energy transition [63] continues, repurposing coal mines for energy storage is a sustainable and economically viable solution.

The use of underground energy storage is also associated with a number of environmental risks. In the event of leaks of the chemicals used, groundwater can be contaminated [64]. Greenhouse gas emissions are no less of a threat. The use of CAES systems using compressed air heating promotes CO₂ emissions [65]. This places an additional burden on the efficiency of these installations. Using the underground spaces of mines for energy storage can result in damage to geological structures, micro-impacts, and the consequent disturbance of the land surface. These effects are often irreversible and involve not only environmental but also financial and social costs [37]. This can lead to damage to surface infrastructure. When adapting existing mine workings for energy storage, the environmental effects associated with the business must also be taken into account, such as noise or the disturbance of existing ecosystems. Therefore, when developing technologies using the existing underground infrastructure of mines, these constraints must be kept in mind.

As important as the environment is, the safety of these installations must also be considered. Underground storage facilities are less vulnerable to fire, terrorist attack, or warfare [66].

3. Study Method

Feasibility was analysed in two areas:

• Theoretical feasibility refers to the compliance of a proposed project with fundamental natural laws—such as those of physics, mechanics, or thermodynamics—indicating that the concept is scientifically sound and does not violate established principles [67].
• Practical or situational feasibility, on the other hand, indicates that the project or process can be realistically implemented within a given context, taking into account specific local conditions such as time, location, and available resources.

A practically feasible project must also be theoretically feasible (Figure 2).

The practical feasibility of theoretically feasible projects is influenced by restrictions occurring at the time and place of the intended implementation of this project in the form of the following:

• Financial or, more broadly, economic constraints;
• Restrictions resulting from applicable standards and legal regulations at the time and place of project implementation;
• Limitations resulting from the availability and skills of human resources along with the existing patterns of social culture in the place and time of the project;
• Local constraints such as terrain, climate, geological structure or locally available infrastructure.

A practically feasible solution must take into account and not contradict the existing constraints. This means that the range of acceptable (practically feasible) solutions must not conflict with any of the groups of restrictions (Figure 3).

Calculations were based on basic formulas for calculating potential energy and kinetic energy as well as the dependence of the path on time and the speed of movement, assuming that the mass of the weight, the permissible speed of the weight, and the depth of the shaft are known, i.e., using the following [68]:

$$E=m\times g\times h$$

(1)

$$E={\displaystyle \frac{m\times v^2}{2}}$$

(2)

$$h=v\times t\hspace{1em}\hspace{1em}\mathrm{w}\mathrm{h}\mathrm{e}\mathrm{r}\mathrm{e}:(t={\displaystyle \frac{h}{v}})$$

(3)

where m—mass, v—speed, h—shaft working height, g—gravity, and E—Energy.

Due to the lack of extensive experience with the use of gravitational energy storage, especially in mine shafts, it is important to determine the expected technical parameters of the project in order to assess the purposefulness of such a project and its functionality. The technical parameters have a direct bearing on the industrial application of the technology and its cost-effectiveness. Compared to the locally obtainable parameters in the project, it is possible to obtain information about one of the important areas of functionality and the purposefulness of undertaking the project.

Local constraints may make it impossible to achieve an economically effective scale for the project.

It should be emphasized that the lack of practical experience with gravitational energy storage makes it difficult to use common tools in performing functionality studies.

4. Results of Analysis

The vast majority of ideas assume the use of the shaft as a hollow tube and the existing shaft hoist drive as part of the energy storage installation. Mine shafts used to transport people, raw material, and other materials with a high efficiency and speed of movement require the forced movement of conveyances (cages or skips), which results in equipping them with steel structures or ropes to guide these conveyances (Figure 4).

The shaft reinforcement limits the dimensions of the weight, which translates into the capacity of energy storing. Moreover, shaft hoists used in the Polish mines use Koeppe pulleys, the design of which are optimized for pulling large loads upwards or balanced loads in both directions. An additional complication in such a system is the use of stabilizing ropes, which makes it impossible to lower heavy loads to the shaft bottom [69].

The main argument for building gravity energy storage facilities in shafts of liquidated mines is the low investment costs for setting up such a storage facility because the shaft exists and there is an electric reversible drive that can be used. The low expected costs of using such an installation are notable in light of the long expected life cycle—about 50 years (Figure 5).

Shaft hoists have structural limitations on the transported weight (in the conditions of Polish mines these are a maximum of 80 tons, and most often only 30–35 t) and, in the case of simple shaft adaptation shaft energy storage, this is a critical value.

This significantly limits the amount of energy stored in such conditions, the increase of which will require significant investment costs. Moreover, the existing shafts require constant service and maintenance, and in the long expected period of their further use, i.e., 50 years, they will also require additional costs related to their natural wear and tear. For the operation of the gravity energy storage system in the mine shaft, it will be necessary to secure the possibility of inspection and repair, and thus ventilation of all necessary workings. Water flows into each mine, even after its closure, until the water table equalizes the natural water level in the ground. This means that it will also be necessary to drain all workings and installations necessary for the operation of the gravity storage facility.

The calculations showed that the power obtained for a 70 Mg load when lowering at a speed of 5.0 m/s and with 100% efficiency would be approximately 2750 MW for approximately 100 s, and the amount of theoretically produced energy would be less than 80 kWh (0.08 MWh). To obtain storage capacity as stated in the description of the surface energy storage facility, it would be necessary to lift and lower thousands of tons of masses in a single shaft—this requires completely different technical solutions. Simply, mine shaft adaption as a gravity energy storage system is feasible but ineffective in capacity and energy volume (Figure 6).

Taking into account the use of hard coal mine shafts in the Upper Silesian Coal Basin as gravitational energy storage, their basic utility parameter in the form of useful depth was analysed. It should be noted that this depth of lowering the weight and its mass determine the amount of energy stored in the shaft. In Figure 7, the quantitative division of shafts in active hard coal mines according to the active depth in the ranges of 120 m between the shallowest and the deepest shaft is shown.

It should be noted that the depths of mine shafts in Polish mines are insufficient for long-term operation with energy return. As an example, it can be pointed out that the deepest shaft at the weight-lowering speed v = 1 m/s will generate power for only 1290 s (21.5 min) It should be emphasized that less than 10% of these shafts are not equipped with hoisting devices and therefore do not have any steel structures in their cross-section. This is beneficial for building a gravity storage facility from scratch. It also does not have permanent hoisting devices that could be adapted to the energy storage system. In shafts that are equipped with hoisting devices (sometimes several in one shaft), most shaft hoists are driven by a drive wheel (Koeppe wheel). This makes it difficult to adapt the hoisting machine to work as a gravitational energy storage system due to the risk of rope slippage on the drive wheel. In the drive wheel hoisting machines (Koeppe), working weight in Polish coal mines does not exceed 50 tons (Figure 8).

With the high technical complexity of the gravitational energy storage in the mine shaft, not only with the use of existing shaft hoists, the amount of energy that can be stored is very small. Therefore, it should be pointed out that this idea, although logically correct, is not scalable.

5. Discussion

The findings of this study provide an overall picture of the viability of gravity energy storage (GES) in mine shafts, showing both the potential and the constraints of this new energy storage technology under Poland’s energy transformation context. The results of this analysis support those of Aneke and Wang [1] and Hossain et al. [9], who refer to the fact that energy storage systems play a significant role in stabilizing the power grid, particularly with the expanding uses of variable renewable energy sources. Nevertheless, aside from overall compatibility with the objectives of sustainable development and the theoretical possibility of Polish energy storage, the technical and cost feasibility of practical application within existing mine shafts is low.

Technically, studies identify substantial constraints in terms of the ability of existing shaft infrastructure to provide the massive loads necessary for optimal gravity storage. In particular, the existing shaft hoisting systems, as configured for use in mining operations, are not capable of lifting and lowering the enormous composite blocks or “mobile masses” contemplated for gravity storage use [3,24]. This agrees with the work of Siostrzonek [24] and Kula et al. [25], where they show that shaft equipment limitations—e.g., restricted lifting capacity and structure design—overwhelmingly limit the volume of energy which can be stored by methods based on gravity. Moreover, the analysis conducted in this research supports Hunt et al.’s [26] discovery, that considering the low theoretical cost savings from the reuse of mine infrastructure and the extended operating life, practical figures are negligible compared to other electricity storage technologies.

Policy and sustainability-wise, the plan to reuse abandoned mine shafts as a facility for gravity energy storage is extremely aligned with the general goals of the EU climate and energy policies, as outlined in the European Commission’s Clean Planet for All strategy [17] and the directives for the internal electricity market [16]. In addition, it aligns with the ideals of the circular economy since it seeks to reuse existing industrial structures to facilitate the uptake of renewable energy systems and the shift towards a low-carbon economy [3,28,29,30,31]. This strategic alignment is most apposite to Poland, where the phased closure of the mines, as in Poland’s Energy Policy 2040 (PEP2040) [33], provides room for a reengineering of utilizing current mining infrastructure.

Its broader significance is that although gravity energy storage in mine shafts is theoretically possible as a novel, site-specific store, it would not be competitive with tried-and-tested systems like PHES or contemporary BESSs (battery energy storage systems). PHES is the yardstick against which mechanical energy storage is measured [1,3,26] with its established capability for large-scale delivery and established dependability. In like manner, lithium-ion and other batteries, more expensive in the initial instance, are more flexible and capable of accommodating variable renewable sources of power [59]. Therefore, gravity storage in mine shafts is better described as a complementary technology that might find application in special cases where attributes of the mine shaft and local grid load make it economically feasible.

Gajdzik et al. [70,71] offer context to this evaluation that is useful, as they mention Poland’s comparatively high electricity and heat demand in its steel sector, a pre-eminent industrial energy consumer. Their research illustrates that gravity storage, in theory, would be capable of functioning as a balancing agent for such energy-intensive processes, especially within a strategy of energy management. The current energy sources that can be realized in existing mine shafts would fall short of the requirements of large industrial consumers, hence casting aspersions on the efficacy of GES as a standalone source to feed heavy industry. Stecuła et al. [72] highlight the disruptive potential of digital management and urban AI in the implementation of city energy solutions. This is consistent with our findings that, although GES in mine shafts has modest technical potential, its application would heavily be facilitated by smart digital control systems to maximize energy flows, regulate loads, and incorporate storage activities into overall grid requirements. These technological solutions would assist in alleviating, in part, the operation inefficiencies indicated in our study, most importantly in relation to the finite time and low capacity of energy yield feasible in the mine shaft-based GES systems considered.

The low scalability and low yield nature of GES results in our study are also reflected in the treatment of Gajdzik et al. [73], who studied the CO₂ emission intensity of the steel process. Their research indicates that the decarbonization of emissions-intensive sectors like cement will demand large-scale, solid energy solutions—a challenge which today’s mine shaft-type GES designs cannot easily meet. Hybrid solutions like those of Qin et al. [74] and Tardy et al. [75] in this case become increasingly pertinent. Qin et al. [74] consider innovative integrated energy management technologies that incorporate storage with peak load management in excavators at mines, and Tardy et al. [75] emphasize the worth of hybridizing wind and pumped hydro storage to achieve enhanced energy resilience for isolated mining operations. Our findings indicate that similar hybrid or multi-technology approaches may be needed to achieve GES in mine shafts as a worthy contributor to the transformation of Poland.

Jiang et al. [76] and Lyu et al. [77] offer additional evidence for the innovative reuse of mining facilities and resources for thermal and mechanical energy storage. Their results validate the conceptual attractiveness of GES as a mine life extension measure—a theme most central to our endeavour. Nonetheless, as our findings show, the particular running and maintenance issues of sizing shaft hoist arrangements to fit gravity-based energy storage, as well as power output finiteness, are overarching concerns. These results are in agreement with those of Yixi et al. [78], who highlight the necessity for optimization and system responsiveness in matching industrial loads with variable renewable energy sources—a goal which the gravity-based storage systems we examined meet in only partial measure.

Our research also corresponds to the issues of Zhao et al. [79] and Liu et al. [80], who refer to the time-dependent degradation and life-cycle cost effect of energy storage in the mining environment. For Polish shaft mines, that means ongoing investment in shaft maintenance, water drainage, and ventilation—expenses which would have the effect of largely stripping gravity-based storage systems of their already slim economic competitiveness. These life-cycle issues are again emphasized further by Li et al. [81] and Zhang et al. [82], whose energy storage optimization for flexible industrial and mining load work substantiates the limitation in using gravity-based storage as a stand-alone-technology solution.

Tang et al. [83] and Nuñez et al. [84] further substantiate this point with better control structures and phase-change material-based storage in the mining context, respectively. Even if these studies discuss other storage media and conditions of operation, they confirm our overall conclusion: that gravity energy storage in mine shafts, as a standalone solution, does not have the operational variability and storage capacity to become a leading balancing mechanism in the energy mix of Poland in the future. In place of that, as Ochmann et al. [85] and Yang et al. [86] observe in their study of post-mining energy systems, a more holistic approach involving various types of storage and numerical optimization techniques is also expected to be more effective.

Schmidt et al. [87] and Liao et al. [88] also corroborate the necessity of a strong geomechanical and technical analysis during retrofitting mining structures to energy storage, supplementing our own evidence towards intensive reinforcement and monitoring in Polish instances. Finally, Sheng et al. [89] present the promise of big data-based management in optimizing energy storage performance, providing a sequence of future studies that would assist in improving the operational performance of gravity-based energy storage systems in spite of the technical and economic constraints of their implementation.

Technological path dependency theory, in which the inertia caused by previous design choices and operational models is emphasized, is a robust interpretative tool to explain these findings [90,91,92,93]. As the paper shows, shafts in mines were intended to transport material and people rather than lift and lower huge composite weights in repeated cycles. This historical context has given rise to hoisting technology focusing on speed, security, and dependability for personnel transport but being too weak and energy-inefficient to be appropriate for extensive-scale energy storage purposes [94,95]. Thus, however theoretically desirable the gravity energy potential of deep mining shafts is, underlying technical and design limitations create tall hurdles to utilizing them as feasible repositories.

The calculations and mechanical analysis in this study highlight how this historical path dependency actually carries over into current technical issues. The limitations of existing shaft hoists on carrying load and the structural modifications needed in order to provide for energy storage operations are some points that reflect an evident clash between initial design purpose and current repurposing goals. This is the key argument from technological path dependency theory: that the risks and costs of retro-fitting existing systems generally outweigh the benefits of reuse if new functionality significantly differs from existing functionality [96,97,98]. In gravity energy storage, the need for precision-controlled raising and lowering of heavy composite blocks through a restricted shaft space is a challenge not readily overcome without widespread and expensive technological upgrading—upgrades, perhaps, that would make the concept economically noncompetitive [99,100,101,102,103,104,105].

The general implications of the work extend beyond the particular framework of Polish mine shafts. The research proposes that the process of integrating previous industrial infrastructure into new energy grids in the future should be considering the limitation by technological path dependency right from the planning stage onward. As much as recycling old assets is a central tenet of circular economy principles and consonant with higher goals of sustainability, the pragmatic realization of such projects must also reconcile these visions against deeply ingrained boundaries of incumbent technical regimes. Policymakers and engineers can thus craft more realistic and affordable approaches that respect the original design constraints and functional purpose of existing infrastructure while advancing the transition toward cleaner and more sustainable energy systems. Important conclusions that follow from the technical calculations in Section 4 are the gap between the theoretical cycle-by-cycle energy output of a gravitational fall and the system energy needs of a national grid power supply network. An estimate by the research suggests that a 70 Mg load dropped at 5 m/s under ideal circumstances discharges less than 80 kWh per cycle—i.e., no more than approximately 0.08 MWh of power (Figure 6). Even considering round-the-clock operation, the total energy input of such a system would still be minute compared to that of existing technology like pumped-storage hydropower, which with much better scalability and efficiency can provide hundreds of MWh per cycle [26,52]. For instance, the 2300 MW Turów pumped-hydro scheme, assumed to supply up to 160 GWh, shows the statistical disparity between shaft-based gravity systems and more large-scale, traditional solutions [36]. The numbers emphasize an important consideration: despite how theoretically consistent mine-based GES systems are with low-carbon policy aims, even at the barest minimum levels they are not capable of achieving such that they would be likely viable as primary or secondary grid-scale storage means.

This conclusion is further confirmed by the statistical depth distribution of Polish mine shafts (Figure 7). As ARP SA data illustrate, the majority of the shafts are less than 600 m deep, and less than 10% are both deep enough and not burdened by the hoisting structure, i.e., Koeppe wheels or guiding frames [25,69]. Since stored potential energy is proportional to mass and depth (Formula (1)), these depth limitations alone restrict the practicality of delivering significant energy capacity. This technical limitation is then further exacerbated by the low permissible mass loadings of Polish shafts, normally ranging from 30 to 80 tons, which significantly lower the energy output per cycle. Conversely, innovations such as Gravitricity, which is tested in deeper mines of over 1000 m, are far more promising in their application [43,44] but do not have widespread applicability to the Polish situation. Upgrading the infrastructure for energy storage under existing technological conditions thus does not statistically justify either peak-shaving on a daily basis or long-term grid stabilization, and thus investment claims are questionable.

Lifecycle considerations raise further questions of long-term economic viability for gravity storage in mine shafts. Polish mine shaft hoists were not designed for the cyclical dynamic and stresses of energy storage operations [25,69], implying a greater mechanical wear profile for round-the-clock load-lifting regimes. Extrapolated to a notional 50-year operating lifetime—a common energy transition planning assumption [26]—these systems would likely have gigantic maintenance and retrofitting expenses. This result is consistent with findings from studies by Liu et al. [80] and Sheng et al. [89], which highlight lifecycle deterioration and the economic penalty of time-dependent material degradation in energy storage facilities. Analogously, the requirements of constant dewatering and the ventilation of closed shafts have fixed costs of operation that also offset the anticipated advantages of the reuse of infrastructure [27,37]. As such, despite optimistic assumptions on circular economy advantages, technical and economic factors presented here reduce the statistical accuracy of the long-term cost–benefit analysis commonly applied to justify the installation of GES within heritage mining infrastructure.

The feasibility lessons from this research chart the fine line between the promise and challenge of GES systems in the context of Poland’s ongoing energy transition. Technically, the research indicates that although GES presents a novel solution for using discarded coal mine shafts, the significant adaptation of existing shaft hoisting gear—i.e., reinforcing load-carrying capacity and compliance with safety specifications—is a colossal operation and engineering task. Such limitations imply the necessity of thorough technical examination and appraisal studies prior to undertaking any large-scale projects involving mine-based GES systems.

At the policy level, the work contributes to the strategic value of integrating circular economy principles—like recycling old mining infrastructure—into Poland’s strategy of decarbonization. Yet it is also a warning that technological path dependencies offered in the present design and operation of old industrial plants can potentially limit the real application of these aims significantly, without major policy and financial incentives. Therefore, the conclusions of this study are of particular interest to energy policymakers, regulators, and business executives, and present an earthed view on the importance of balancing environmental aspirations with technical feasibility and economic imperatives in the quest for climate neutrality and energy security.

6. Conclusions

The principal quantitative findings of the study confirm that the potential energy return from the gravity energy storage (GES) devices on installed Polish mine shafts is tightly constrained by structural and operational problems. In particular, for one 70-ton load descending at 5 m/s, the maximum theoretical power return is around 2.75 MW for about 100 s, delivering less than 0.08 MWh of stored energy per cycle. Since the majority of shafts in Polish hard coal mines are less than 600 m, and acceptable payloads range from 30 to 80 tons, the calculated storage capacity is on the margin. The study also indicates that less than 10% of available shafts do not have hoisting equipment that would render retrofitting, and thus scalability limiting, unfeasible. These values, compared to national grid or industrial energy requirements, are indicative of the technical feasibility of GES only to a very limited extent and under very exceptional circumstances.

This article proves that, theoretically, gravity energy storage in Polish mine shafts is possible within reach of circular economy and sustainability goals, but in practice, it is significantly hindered, directly answering all three research questions posed (RQ1–RQ3). In order to respond to RQ1, the paper presents an exhaustive characterization of economic and technical limitations—i.e., the low rates of hoisting, limited depths, and prohibitively costly retrofitting of the installed infrastructure—combined that constrain energy supply to levels far below desired levels to enable useful grid support. Such remarks suggest justification by quantitative modelling and operating estimates to demonstrate that even on the optimistic assumption, single-cycle energy production is marginal (<0.1 MWh), so mass application is not possible without costly modifications [25,26,69]. For RQ2’s scenario, the comparison explicitly situates GES as far less competitive than potential alternatives in the guise of pumped-storage hydropower (PHES) or battery energy storage systems (BESSs), both of which possess higher scalability, flexibility, and energy density [36,59]. That shaft-based GES is not even a competitive player on the terms of these technologies even when under the hypothesis of infrastructure reuse underscores its niche status within Poland’s energy storage portfolio. Lastly, in RQ3, while the concept of recycling mine shafts is consilient with national and EU-level policy—i.e., the European Green Deal, Circular Economy Action Plan, and Poland’s Energy Policy to 2040 (PEP2040)—pragmatic reality depends on an immense amount of technological and financial intervention. Therefore, as much as GES in mine shafts is appealing to the green transformation and just transition principle, the study concludes that for it to be applied, it has to be extremely discriminatory and based on hybrid solutions derived from the incident and public policy tools to reconcile sustainability aspirations with economic and operational requirements.

The novelty of this paper lies primarily in its focus on the Polish context, which has thus far remained underexplored in the international literature on gravity energy storage (GES). While an increasing number of studies point to the limited scalability of GES in existing mine shafts, this article makes a unique contribution by offering a detailed technical and economic analysis tailored to the geological, infrastructural, and industrial conditions specific to Poland. Its most significant added value is the in-depth quantification of operational parameters such as shaft load capacity, working depth, operation time, and the unit amount of energy that can be stored. This enables a realistic assessment of the deployment potential of GES in a country where mining sector restructuring and the challenge of repurposing post-mining infrastructure represent urgent issues within the broader framework of a just energy transition.

To best address the given weaknesses of GES in Polish mine shafts, subsequent studies will have to design specific hybrid system configurations and simulation regimes aimed at the physical and operational limitations of the facilities. A viable option is the integration of gravity energy storage with high-efficiency battery systems (BESSs), where GES serves for rapid-response, short-duration oscillations and batteries for longer-duration capacity levelling—thus optimizing response time and the depth of discharge. Another potential model is integration with compressed air energy storage (CAES), utilizing the mine shaft for both mass conveyance and as a pressure vessel, leveraging existing voids for dual utilization. Artificial intelligence algorithms and digital twin technology could be employed to dynamically manage the flow of energy, forecast peak demand, and optimize the load cycles of the hoisting machinery in real time. These proposals, based on techno-economic modelling, would give practical blueprints for making GES from a conceptually viable but mediocre solution into a real-world contributor to Poland’s energy transition strategy.

The results of this research contrast sharply between the policy ambitions of sustainable energy transitions—specifically, those preferring circular economy approaches and infrastructure repurposing—and the economic and technical realities of gravity energy storage (GES) in Polish mine shafts. Although the strategic fit to EU policy objectives, like the European Green Deal and Clean Planet for All strategy, and national policy objectives like Poland’s Energy Policy 2040 (PEP2040) is indubitable, the deployment of GES in this format is faced with stringent feasibility limitations.

The full reuse of retired mining infrastructure for energy storage from a policy perspective sits very well within the principles of resource efficiency and just transition. It also addresses the socio-economic imperative of stretching the life cycle of industrial infrastructure and maintaining jobs in ex-mining areas. Our analysis illustrates, however, that alignment of such is, at a minimum, symbolic. Existing shaft geometry, low lifting capacities, and structural limitations are all such that—despite firm political will—effective storage capacity per installation is at best marginal, and unlikely to fulfil national or regional flexibility ambitions.

Policy rhetoric consequently needs to shift from one of blanket optimism to one of a more cautious and evidence-informed stance. In particular, mine shaft reuse needs to be repositioned not as a scalable energy technology but as a niche or complementary technology. Potential applications for selective use could be remote applications, microgrids, or industrial parks where special geology and infrastructural circumstances render GES feasible on a limited scale. In such applications, shaft-based GES can help with system resiliency and peak-shaving, particularly if combined with other technologies like compressed air energy storage (CAES) or battery energy storage systems (BESSs).

The limitations of the study are related to the availability of empirical evidence on the gravity energy storage systems applied in Polish mine shafts and their impact on the generalizability of the results. The further analysis is mainly based on theoretical calculations and case study comparisons without any real-world demonstration projects confirming the technical and economic viability of such systems.

Future studies must focus on field tests and pilot projects to verify the theoretical models of gravity energy storage in retired mine shafts and to determine the technical parameters for actual application. Research is also warranted to assess the prospects of integrating gravity-based storage with other forms of renewable energy and hybrid storage systems, i.e., coupling gravitational storage with battery or compressed air options. In addition, the study of sophisticated digital monitoring and control technologies can be applied to maximize the operating performance and long-term viability of gravity energy storage systems in mine infrastructure.