A Review of Gravity Energy Storage

Abstract

Gravity energy storage, a technology based on gravitational potential energy conversion, offers advantages including long lifespan, environmental friendliness, and low maintenance costs, demonstrating broad application prospects in renewable energy integration and grid peak regulation. This paper reviews the technical principles, characteristics, and application progress of liquid gravity energy storage (LGES), like pumped hydro storage (PHS) and solid gravity energy storage (SGES) systems—tower-based (T-SGES), shaft-type (S-SGES), rail-mounted (R-SGES), and mountain gravity energy storage (M-SGES). PHS, the most mature technology, is widely deployed for large-scale energy storage but faces significant geographical constraints. T-SGES and R-SGES exhibit higher flexibility for diverse terrains, while S-SGES leverage abandoned mines for resource reuse. Despite advantages such as high round-trip efficiency and extended lifecycle, challenges remain in efficiency optimization, high initial investments, and land utilization. Future development of gravity energy storage will require technological innovation, intelligent dispatch systems, and policy support to enhance economic viability and accelerate commercialization.

1. Introduction

With the growing global energy demand and the rapid development of renewable energy, the role of energy storage technology in the energy system is becoming more and more prominent. Wind, solar, and other renewable energy sources are intermittent and fluctuating. Their large-scale application requires efficient and reliable energy storage systems to realize a stable supply of energy. Gravity energy storage, as an energy storage technology based on physical principles, has unique advantages over other energy storage methods, especially in terms of long life, large-scale energy storage, and environmental friendliness. Taking pumped hydro storage (PHS) [1,2] as an example, its operating life can exceed 50 years, far exceeding the 5–15 years of lithium-ion battery energy storage (LIBES) [3,4]. There is no chemical degradation problem and low maintenance cost. Compared to short-term, high-power technologies such as flywheel energy storage (FES) [5,6] or superconducting magnetic energy storage (SMES) [7,8], gravity energy storage capacity can reach GWh levels, making it suitable for grid peaking and long-duration energy storage needs, rather than being limited to second- or minute-scale applications. Economically, they have lower life cycle costs than batteries and hydrogen energy storage (HES) [9], despite a higher initial investment. Materials like water and concrete are cost-stable and readily available. They are not subject to price fluctuations of rare metals. Environmentally, gravity energy storage avoids the mineral mining and waste disposal problems associated with electrochemical energy storage, and pumped storage has a low carbon footprint. Solid gravity energy storage (SGES) [10,11,12] can even utilize waste resources, which is better than compressed air energy storage (CAES) [13,14] that requires fuel assistance. In addition, gravity energy storage technology is highly mature. PHS has a hundred years of application history and strong reliability. The new SGES pilot also shows the potential to break through geographical limitations. Compared to thermal energy storage like HES, which is less efficient, gravity energy storage can reach 70–90% efficiency, with direct and stable output. However, it is less geographically dependent and responsive than Li-ion BES or SMES and still needs to be optimized. The comparison results of several common energy storage methods are shown in

Table 1. Comparison of several common energy storage technologies.

Type	Capacity Level	Energy Density (Wh/kg)	Response Time	Service Life (year)	Efficiency (%)	Levelized Cost (USD/kWh)
LIBES	kW	100–265	ms	10–20	85–95	0.15–0.25
FES	kW	5–30	ms	15–20	85–90	0.25–0.50
SMES	kW	0.5–50	us	20–30	95–98	0.50–2.00
HES	kW/MW	30–50	us	15–25	35–50	0.20–0.50
CAES	MW	30–60	min	20–30	40–70	0.10–0.20
PHS	GW	0.5–1.5	min	40–60	70–85	0.05–0.15
SGES	MW	0.1–0.5	min	30–50	80–90	0.05–0.10

.

Gravity energy storage is a technology that relies on the conversion of gravitational potential energy to realize electric energy storage, and the main forms include PHS and the emerging SGES. Compared with other energy storage methods, gravity energy storage has the following advantages: First, it has a longer lifespan, less wear and tear on the mechanical system, and compared with LIBES, it has a higher cycle life and lower decay rate. Second, the materials of gravity energy storage systems are mostly steel and concrete, which have less environmental impact and are more environmentally friendly than HES. In addition, compared with SMES and FES, gravity energy storage is more economical for large-scale energy storage applications. However, gravity energy storage also has certain disadvantages. First, its energy density is low compared with CAES and LIBES and its energy storage capacity per unit mass is low. Second, it is more restricted by geographical conditions, especially since traditional pumped storage requires suitable terrain. And while solid gravity lift energy storage reduces geographic dependence, it still requires larger infrastructure investments. In addition, gravity energy storage has a slower instantaneous power response, which is not advantageous in scenarios requiring fast dynamic response compared to SMES and FES. Therefore, in future applications, gravity energy storage needs to be combined with intelligent control to optimize scheduling to enhance its competitiveness [15,16].

The fundamental principle of gravity energy storage technology is to achieve the conversion between gravitational potential energy and electrical energy through the lifting and lowering of heavy objects. During the lifting phase, excess electrical energy is converted into gravitational potential energy for storage. When there is a shortage of electricity supply, the stored gravitational potential energy is released and converted back into electrical energy. Gravity energy storage can be categorized into liquid gravity energy storage (LGES) and SGES based on the storage medium. PHS is the most mature form of LGES and has been widely implemented worldwide. However, its expansion is constrained by geographical conditions and environmental impacts. In recent years, emerging gravity energy storage technologies have gained attention. SGES includes tower-based, rail-based, and shaft-based gravity storage systems. These innovative approaches aim to expand the application scenarios of energy storage systems and enhance energy utilization efficiency.

The second part focuses on liquid gravity energy storage. The third part describes solid gravity energy storage. Part 4 presents a comparison between gravity energy storage and the challenges faced. Part 5 summarizes the whole paper. This review summarizes and analyzes the latest research progress in gravity energy storage technology, covering the working principles, technical characteristics, advantages, and challenges of different types of gravity energy storage systems. It also explores their potential applications in future energy systems. By systematically reviewing existing literature, this review aims to provide theoretical support and reference for further research and development of gravity energy storage technology.

2. Liquid Gravity Energy Storage

LGES uses liquid as the storage medium. By moving water between an upper reservoir and a lower reservoir, electrical energy is converted into gravitational potential energy for storage and later released as electrical energy when needed. Its working principle consists of two phases: energy storage and energy release.

During the energy storage phase, excess electricity from the grid—such as surplus wind or solar power during peak generation periods or nighttime low-demand hours—is used to drive water pumps. These pumps transfer water from the lower reservoir to the upper reservoir, converting electrical energy into gravitational potential energy stored in the elevated water mass. The stored energy can be expressed by Equation (1). Where m denotes the mass of the lifted weight, g is the acceleration of gravity, and h is the vertical height at which the weight is lifted. In the energy release phase, water from the upper reservoir flows back down to the lower reservoir. The flowing water drives a turbine, causing it to rotate. The turbine’s mechanical energy is transferred to a generator, which converts it into electrical energy and supplies it to the grid to meet peak electricity demand. Due to factors such as hydraulic head losses, electromagnetic losses in the generator, and turbulence and friction within the pipes, the efficiency of pumped hydro storage typically ranges between 70% and 85%.

$$\mathrm{E}=mgh$$

(1)

LGES is highly dependent on geographical conditions, as its storage capacity is directly determined by the volume of stored water and the elevation difference. However, its simple and reliable operation makes it irreplaceable for large-scale, long-duration energy storage applications. The following sections present specific application cases of liquid gravity energy storage.

2.1. Pumped Hydro Storage

The Fengning PHS power station, located in Hebei Province, China, is currently the largest PHS power station in the world in terms of installed capacity, as shown in Figure 1. The project was constructed in two phases, with a total installed capacity of 3600 MW and a total investment of RMB 18 billion. The station consists of 12 generating units, each with a capacity of 300 MW, making them among the largest single-unit capacities globally. The station features an upper and lower reservoir, with the upper reservoir capable of storing 58 million cubic meters of water and the lower reservoir 61 million cubic meters. The elevation difference between the two reservoirs is 425 m. The water conveyance system consists of high-pressure steel pipelines with a total length of approximately 4.5 km and a diameter of 8.5 m. The power station can reach full-load power generation from standby within just 2 min. It is capable of completing two full charge–discharge cycles per day, with an overall efficiency of 82–85%. Fengning is also the world’s first pumped storage station to surpass 3000 MW in capacity while maintaining an efficiency of over 80%. In the future, the station plans to integrate 500 MW of photovoltaic (PV) and 200 MW of wind power, forming a comprehensive renewable energy storage system that combines solar, wind, and pumped hydro storage.

The Bath County Pumped Storage Station, jointly operated by Dominion Energy and FirstEnergy, is one of the largest energy storage facilities in North America, as shown in Figure 2. Construction of the power station began in 1977, and it was fully commissioned in 1985, with a total investment of $1.6 billion. The station has a total installed capacity of 3003 MW, and at full-load operation for 8 h, it provides an energy storage capacity of 24 GWh. The upper reservoir has a storage capacity of 113 million cubic meters, while the lower reservoir can store 136 million cubic meters. The elevation difference between the two reservoirs is 380 m. The station consists of six generating units, each with a 501 MW capacity. The water conveyance system includes two main tunnels, each with a total length of 1.6 km and a diameter of 8.8 m. The station can ramp up from standby to full-load generation within 5 min and is capable of 1.5 full charge–discharge cycles per day. The overall efficiency of the system is 75–80%. The Bath County Pumped Storage Station plays a crucial role in frequency regulation in the U.S. Pennsylvania-New Jersey-Maryland (PJM) Interconnection, with a response time of 5 min and the ability to smooth ±2% load fluctuations. Due to its long operational history, a variable-speed upgrade is planned for two generating units by 2025, aiming to improve the station’s overall efficiency to 80%. Additionally, the integration of solar, hydro, and pumped storage (PV-hydro hybrid system) is being explored as a future development direction.

The Okutataragi Pumped Storage Power Plant, located in Japan and operated by Kansai Electric Power Company (KEPCO), is one of the country’s most advanced pumped storage facilities with the highest hydraulic head. As illustrated in Figure 3, the plant was commissioned in 1974. The system comprises four units, each rated at 483 MW, utilizing synchronous generators. It achieves full-load power generation from a standstill within 3 min, with a total installed capacity of 1932 MW. The plant provides an energy storage capacity of 14.5 GWh during 7.5 h of full-load operation. The upper reservoir has a storage capacity of 12 million cubic meters, while the lower reservoir holds 14 million cubic meters. The system operates under a hydraulic head of 388 m, supported by a water conveyance system consisting of two vertical shaft penstocks, each 1.2 km in length and 5.2 m in diameter. The plant completes two full charge–discharge cycles daily, achieving a round-trip efficiency of 85%. By integrating variable-speed units with an ultra-high-head design, the Okutataragi plant demonstrates exceptional performance in grid frequency regulation accuracy and renewable energy integration efficiency, offering critical support for power systems with high penetration of renewable energy.

A comparison of the performance of several major pumped storage plants is shown in

Table 2. Comparison of parameters of several major PHS stations.

Parameter	Fengning	Bath County	Okutataragi
Capacity (MW)	3600	3003	1932
Hydraulic head (m)	425	380	388
Efficiency (%)	82–85	75–80	85
Response time (min)	2	5	2.5

. Pumped storage occupies a central position in energy storage systems [17] by virtue of its ultra-large scale, long life, low cost, and mature technology. Its irreplaceability in scenarios such as grid peaking and renewable energy consumption, despite the challenges of geographical constraints [18] and long construction cycles.

2.2. Novel Liquid Gravity Energy Storage

Compared to solid gravity energy storage, pumped hydro energy storage (PHES) suffers from lower energy density. To address this limitation, RheEnergise focuses on developing high-density fluid-based gravity energy storage (HDF-GES) systems. The core innovation lies in replacing water with high-density fluids as the energy storage medium, aiming to enhance energy density and reduce system footprint. RheEnergise employs a proprietary eco-friendly mineral-based fluid with a density of 2.5 ton/m³, approximately 2.5 times that of water, while maintaining viscosity comparable to water. The m in Equation (1) can be expressed as in Equation (2). Where $\rho$ is the density of the lifting weight and V is its volume. This medium theoretically achieves 2.5 times the energy storage density of conventional PHS under ideal conditions. The system utilizes abandoned mines as storage chambers. During periods of excess electricity, motors pump the high-density fluid (HDF) from lower to elevated reservoirs. During discharge, the HDF flows downward through pipelines, driving turbines to generate electricity, and then returns to the lower reservoir, completing a charge–discharge cycle.

$$m=\rho \mathrm{V}$$

(2)

Similarly, Sink Float Solutions (SFS) in The Netherlands employs a dense saline solution circulated between elevated and lower reservoirs for energy storage. SFS aims to integrate storage facilities into urban infrastructure, demonstrating the feasibility of compact, high-density liquid gravity storage. In China, New Energy Let’s Go leverages urban underground utility tunnels to construct distributed liquid gravity storage networks using high-density fluids.

3. Solid Gravity Energy Storage

SGES represents a mechanical energy storage technology operating on principles analogous to LGES. The critical distinction lies in its utilization of solid masses as the energy storage medium rather than fluids. This technology achieves bidirectional energy conversion through controlled vertical displacement of solid weights, transforming electrical energy into gravitational potential energy during charging and reversing the process during discharge. By enabling efficient energy storage and grid-scale peak shaving capabilities, SGES addresses critical challenges in renewable energy integration. These characteristics have positioned SGES as an emerging research focus in sustainable energy storage solutions.

Current SGES implementations can be categorized into four primary configurations: Tower-based SGES (T-SGES) [19], Rail-based SGES (R-SGES) [20], Shaft-based SGES (S-SGES) [21,22], Mountain-based SGES (M-SGES) [23]. While tower and rail configurations have reached prototype or pilot stages, like Energy Vault’s tower systems and ARES’ rail-based demonstrations, shaft-based implementations remain in experimental validation phases. Mountain-based approaches, though theoretically promising for multi-GWh storage potential, require further geotechnical feasibility studies. The diversity of these architectures underscores SGES’ adaptability to varying infrastructure landscapes, from urban environments to post-industrial sites.

3.1. T-SGES

T-SGES is an emerging grid-scale energy storage technology that utilizes the gravitational potential energy of solid gravity to store and release electricity. Due to its advantages of low cost, long life, and environmental friendliness, there are several T-SGES stations around the world used for peak-to-valley regulation, grid frequency regulation, and so on.

The T-SGES system model developed by Swiss energy storage company Energy Vault is illustrated in Figure 4. The system centers on a hexagonal tower integrated with six crane arms. During periods of electricity surplus, excess power drives the system to lift 35-ton concrete blocks and stack them into a 120 m-high tower, converting electrical energy into gravitational potential energy. During electricity shortages, the cranes lower the blocks, with the gravitational force of the weights driving generators to release stored energy as electricity. Leveraging this technology, Energy Vault deployed its first 35 MWh T-SGES system in India in 2019, as shown in Figure 5. The system comprises a massive six-arm crane and multiple 35-ton concrete blocks. It delivers a storage capacity of 35 MWh, a peak power output of 4 MW, and a response time of 2.9 s to initiate power generation. The system achieves a round-trip efficiency of 90% and can discharge continuously for 8–16 h at 4–8 MW, enabling rapid grid demand response.

T-SGES systems offer advantages such as minimal geographical constraints, flexible siting, long-duration continuous power discharge, and fast response speed, making them well-suited to meet grid peak-shaving demands. The storage capacity of these systems can be expanded through the integration of multiple storage towers and modular configurations. However, the key to overcoming development constraints for T-SGES systems lies in mitigating external environmental impacts and ensuring millimeter-level error control.

3.2. S-SGES

S-SGES is an energy storage technology that utilizes underground shafts or abandoned mines for electricity storage and release. Its working principle is similar to that of tower-based gravity energy storage, except that this system employs natural underground shafts or abandoned mines to lift heavy weights. This approach fully leverages existing underground spaces, reduces construction costs, and minimizes surface land occupation. With approximately 1 million abandoned mines globally, S-SGES systems hold significant development potential.

Gravitricity, a UK-based company specializing in gravity energy storage, has proposed an S-SGES system. A schematic diagram of the system is shown in Figure 6. By integrating abandoned mines with energy storage infrastructure, Gravitricity constructed a test site at Leith Port in Edinburgh in 2021. The shaft depth is 15 m, and the total mass of the weight is 50 tons. During the energy storage phase, surplus electricity powers a motor to lift the weight to the top of the shaft, converting electrical energy into gravitational potential energy. During discharge, the weight descends, driving a generator via a gear system to produce electricity. The system has an energy storage capacity of 1 MWh, a peak power of 259 kW, and a round-trip efficiency of 85%. It can undergo tens of thousands of cycles with no significant efficiency degradation and achieves a response time of 0.5 s, meeting grid frequency regulation requirements. Following the successful pilot, Gravitricity initiated a commercial-scale demonstration in 2023 by retrofitting an 800 m-deep coal mine shaft in the Silesia region. The upgraded system is designed for a total capacity of 20 MWh and a peak power of 4 MW, serving local microgrids in mining areas.

The Gravitricity project has validated the feasibility of constructing gravity energy storage systems using abandoned mines. S-SGES offers advantages such as high response speed and long service life, making it an ideal solution for grid frequency regulation and integration with renewable energy storage. However, abandoned mines generally carry risks of collapse, necessitating additional reinforcement. A major technical challenge for S-SGES systems lies in improving the positioning accuracy of motor systems to achieve precise frequency regulation.

3.3. R-SGES

R-SGES is a novel energy storage technology that converts between gravitational potential energy and electrical energy by moving weights along inclined tracks. It achieves energy storage and release through track-based mechanical systems. When grid electricity is surplus, the system activates motors to transport electric trains carrying counterweights upward along the tracks to elevated storage platforms. The energy conversion efficiency during this process depends on motor efficiency and mechanical friction losses, with comprehensive efficiency reaching approximately 80–85%. When grid power input is required, the weights descend along the tracks under gravity, driving generators to produce electricity. The potential energy is converted to electrical energy and fed back into the grid through inverters, achieving a generation efficiency of 85–90%. The system’s overall round-trip efficiency ranges between 70 and 75%. The entire process is chemical-free and environmentally friendly. Suitable for mountainous areas and abandoned mines, several pilot projects of rail-mounted gravity energy storage systems have already been implemented.

The U.S. Advanced Rail Energy Storage (ARES) project, as shown in Figure 7, utilizes sloped terrain in abandoned mines to deploy rail tracks. The tracks feature an inclination of 8–10° and extend approximately 8.85 km. To reduce friction losses from overhead catenaries, the project employs distributed electric locomotives powered via a third rail. A single train consists of two locomotives and seven carriages, with a total weight of 1550 tons. The modular design supports multi-train coordinated scheduling to expand system energy storage capacity. The project’s bidirectional motor system enables rapid switching between charging and discharging modes, facilitating real-time response to grid frequency fluctuations and optimized energy dispatch. The Nevada demonstration project has a total capacity of 12.5 MWh and a peak power output of 50 MW, capable of meeting the hourly electricity demand of 15,000 households. By leveraging pre-existing railway infrastructure, the project reduces capital costs. Following the successful Nevada demonstration, ARES plans to develop a GW-scale energy storage system in California, integrated with photovoltaic plants to enable seamless renewable energy grid integration. The target project has a planned lifespan exceeding 40 years, with an estimated levelized electricity cost below 0.05 USD/kWh.

The ARES project has established itself as a benchmark case for novel gravity energy storage through its efficient rail-based mechanical system. The R-SGES system leverages sloped terrain and pre-existing railway infrastructure for construction, achieving reduced investment costs while maintaining geographical adaptability. As a mechanical system, it offers an extended operational lifespan and lower maintenance requirements. Capable of transitioning from standby to full power output within seconds, it effectively addresses high-frequency frequency regulation demands. However, its overall efficiency and energy density remain relatively low, accompanied by intermittent power output during weight transportation. Consequently, developing lightweight composite materials to minimize energy losses and co-locating with renewable energy plants such as wind and solar farms have emerged as key developmental trends for R-SGES.

3.4. M-SGES

The International Institute for Applied Systems Analysis (IIASA) has proposed M-SGES, a long-term energy storage technology suitable for capacities below 20 MW. This technology utilizes natural elevation differences in mountainous terrain to construct gravity energy storage systems, as illustrated in Figure 8. It achieves mutual conversion between potential energy and electrical energy by transporting bulk materials such as sand and gravel between elevated and lower storage sites. During the energy storage phase, cranes lift sand and gravel via cables from the lower to the elevated storage site, converting redundant electrical energy into increased gravitational potential energy of the weights. In the energy release phase, weights descend along cables from the elevated to the lower storage site, converting gravitational potential energy into electricity through mechanical work, which is then fed into the grid.

Compared to PHS, M-SGES offers higher energy density while fully utilizing steep mountainous terrain. However, the scale of such systems is directly constrained by topography: the greater the height difference between the lower and elevated storage sites and the steeper the mountain slopes, the more pronounced the energy storage performance of the system becomes.

4. Discussion

4.1. Comparison

As discussed in Section 2 and Section 3, PHS, T-SGES, S-SGES, and R-SGES are all long-duration, large-scale energy storage technologies. Their fundamental operational principle involves mutual conversion between gravitational potential energy and electrical energy to achieve energy storage and release. However, notable differences exist among these common gravity energy storage technologies, with comparative results summarized in

Table 3. Comparison of parameters of GES.

Type	PHS	T-SGES	S-SGES	R-SGES
Capacity	GW	MW	MW	MW
Service life (year)	40–60	30–50	30–50	25–40
Efficiency (%)	70–85	80–90	80–85	75–85
Response time (min)	2–5	1–5	1–5	2–10
LCOE (USD/MWh)	60–150	80–130 [24]	70–110 [25]	90–160 [26]
Example	Fengning PHS Station	Energy Vault: Switzerland	Gravitricity: Scotland	ARES: Nevada, US state

.

PHS, the most mature technology, utilizes water level differences between upper and lower reservoirs for energy conversion. It achieves efficiencies of 70–85% and typically has a service life of 50 years. Nevertheless, it faces significant geographical constraints, long construction cycles, and high capital costs. T-SGES offers greater siting flexibility by lifting weights via tower structures for energy storage and release. With efficiencies ranging from 80 to 90% and a lifespan of 30–50 years, it nevertheless encounters challenges in material selection for weights and structural strength. S-SGES leverages natural or abandoned mine shafts for system construction, effectively utilizing existing resources to reduce investment costs. It achieves efficiencies of 80–85% and a lifespan comparable to tower-based systems. However, this technology remains in the demonstration phase, with system scale directly limited by shaft depth and maintenance costs remaining relatively high. R-SGES, currently in the pilot stage, employs railway tracks for infrastructure development, significantly lowering construction costs. With comprehensive efficiencies reaching 75–85%, it represents a relatively high-efficiency storage solution.

While gravity energy storage technologies are finding increasingly broad applications in power grids, significant challenges remain in their development.

4.2. Opportunities and Challenges

4.2.1. Load Stability

In SGES systems, the stability of heavy objects, like concrete or steel blocks, directly affects the operational safety and energy conversion efficiency of the system. During the lifting and lowering process, due to inertia, the weight may oscillate, which affects the operational stability of the rail system. Existing studies have shown that the use of active damping systems or adaptive control strategies can effectively reduce the oscillation. For the T-SGES, strong winds may lead to the deflection of the suspended weights, affecting the alignment accuracy. Solutions include optimizing the aerodynamic design of the weights and adopting an intelligent wind control system [27,28,29]. Uneven load distribution in a multiple-weight co-storage scheme can lead to uneven stress in the system and exacerbate the wear and tear of mechanical components. Optimizing the geometry of the weights and the load management strategy are effective directions for improvement [30,31].

4.2.2. Gravity Energy Storage System Accuracy

The orbital system is the core part of the solid gravity energy storage system, and its precision determines the positioning accuracy of the weight and the overall efficiency of the system. Small deformations or accumulated errors in the orbit may cause the weight to deviate from the ideal path and affect the operational stability. A high-precision track manufacturing process and a regular error calibration mechanism are essential. Rollers or slides in a rail system may suffer friction loss due to long-term use, reducing system efficiency [32]. Studies have shown that the use of low-friction materials, such as polymer composites, and magnetic levitation technology can significantly reduce friction losses [33,34,35]. In addition, in large-scale energy storage applications, multiple heavy objects may run on different tracks, and avoiding track interference and optimizing track switching strategies are key technical challenges. In recent years, machine-learning-based track assignment algorithms have shown greater potential for application in this field.

4.2.3. Control Algorithm Optimization

The optimization of control algorithms is crucial to improving the overall performance of solid gravity energy storage systems, which mainly involves the aspects of weight scheduling, energy management, and system dynamic response. The energy efficiency of the system can be improved by predicting the load demand and rationally arranging the lifting and lowering rhythms of the weights [36,37]. In recent years, intelligent optimization methods based on reinforcement learning have been explored in this field. The use of high-precision sensors, such as laser rangefinders and inertial navigation systems, to monitor the position of heavy loads in real time, and the combination of adaptive control algorithms, such as sliding mode control and fuzzy control, can improve the accuracy of cargo positioning. Solid gravity energy storage systems may have transmission failures or control errors, and intelligent fault diagnosis systems, such as deep learning-based anomaly detection algorithms, can improve the reliability and safety of the system [38,39].

4.2.4. Conclusions

Gravity energy storage, as an emerging large-scale energy storage technology, holds broad development prospects in the global energy transition. Its core advantage lies in utilizing low-cost, recyclable weights such as water and gravel for energy storage, avoiding resource consumption and environmental pollution associated with chemical batteries. In recent years, driven by rapid renewable energy development and global carbon neutrality goals, gravity energy storage has demonstrated significant growth potential.

First, policy support and growing market demand have created development opportunities for gravity energy storage. Governments worldwide are actively promoting renewable energy infrastructure [40,41], while energy storage—critical for grid peak shaving and load balancing—faces increasing demand. Second, technological advancements and cost reductions have enhanced the competitiveness of gravity energy storage. Furthermore, integrating abandoned mines and repurposing aging infrastructure can effectively reduce upfront investments and improve economic viability.

However, the global development of gravity energy storage still faces multiple challenges. First, system efficiency requires optimization, as current round-trip efficiencies remain lower than those of some mature storage technologies. Second, high initial investments persist. The substantial infrastructure costs necessitate economies of scale to reduce unit costs for commercial viability. As a green energy storage solution, gravity energy storage presents coexisting opportunities and challenges. Future success will depend on technological innovation, policy guidance, and optimized business models to drive its large-scale integration into global energy systems.

5. Conclusions

Gravity energy storage achieves energy storage and release through weight lifting and lowering, making it suitable for grid peak regulation and renewable energy integration. Key technologies include PHS, T-SGES, S-SGES, R-SGES, and low-cost long-duration storage, with emerging technologies like M-SGES under exploration. Gravity energy storage offers advantages such as long lifespan, high safety, and environmental friendliness, yet faces challenges including system efficiency optimization, high initial investments, and land use constraints. Future development directions include enhancing technical efficiency, reducing construction costs, and optimizing intelligent control systems. Concurrently, policy support and growing market demand will drive its commercial application, enabling it to play a greater role in the global energy system.