The Role of Underground Salt Caverns in Renewable Energy Peaking: A Review

Abstract

To address the inherent intermittency and instability of renewable energy, the construction of large-scale energy storage facilities is imperative. Salt caverns are internationally recognized as excellent sites for large-scale energy storage. They have been widely used to store substances such as natural gas, oil, air, and hydrogen. With the global transition in energy structures and the increasing demand for renewable energy load balancing, there is broad market potential for the development of salt cavern energy storage technologies. There are three types of energy storage in salt caverns that can be coupled with renewable energy sources, namely, salt cavern compressed air energy storage (SCCAES), salt cavern hydrogen storage (SCHS), and salt cavern flow battery (SCFB). The innovation of this paper is to comprehensively review the current status and future development trends of these three energy storage methods. Firstly, the development status of these three energy storage methods, both domestically and internationally, is reviewed. Secondly, according to the characteristics of these three types of energy storage methods, some key technical challenges are proposed to be focused on. The key technical challenge for SCCAES is the need to further reduce the cost of the ground equipment; the key technical challenge for SCHS is to prevent the risk of hydrogen leakage; and the key technical challenge for SCFB is the need to further increase the concentration of the active substance in the huge salt cavern. Finally, some potential solutions are proposed based on these key technical challenges. This work is of great significance in accelerating the development of salt cavern energy storage technologies in coupled renewable energy.

1. Introduction

In the context of sustained global economic growth and expanding population, global energy demand is showing a persistent upward trend [1,2]. Simultaneously, environmental pollution is becoming increasingly serious [3,4], and the use of traditional fossil fuels has exacerbated the phenomenon of global warming [5,6], triggering the international community’s deep concern for environmental protection and sustainable development [7,8]. In recent years, renewable energy, as a form of clean and renewable energy, has gradually become the focus of global attention [9,10]. At the 21st United Nations Climate Change Conference, 178 countries from around the world gathered to sign the historic Paris Agreement [11,12]. The signing of this agreement marks a broad consensus in the international community to address climate change issues. The agreement sets an ambitious and long-term goal of guiding countries to take effective measures to significantly reduce greenhouse gas emissions, aiming to limit the increase in global average temperature to less than 2 degrees Celsius [13] in this century, and to further limit it to 1.5 degrees Celsius [14], so as to slow down the trend of global warming and protect the Earth’s ecological environment. In order to realize this ambitious goal, collaboration on a global scale has become particularly important. Countries need to work together to advance the process of carbon neutrality [15,16], achieve net-zero emissions, and reach a climate-neutral state. In this process, the concepts of peak carbon and carbon neutrality have gradually gained wide acceptance and recognition globally [17,18]. As the largest developing country and one of the leading carbon dioxide emitters, China is well aware of its responsibility to address climate change. Therefore, China has actively committed to reaching peak carbon dioxide emissions by 2030 [19] and, using this as a benchmark, is striving to move toward a carbon-neutral vision by 2060 [20]. To achieve this goal, China is actively promoting a cleaner and low-carbon transformation of its energy structure by vigorously developing renewable energy and reducing the use of traditional fossil fuels such as coal and oil [21,22]. Although coal and oil still account for a high proportion of total energy consumption, it is worth noting that the share of clean energy consumption is steadily increasing year by year. This trend is attributed to the Chinese government’s strong focus on clean energy development [23] and the formulation of a series of incentive policies [24]. In the future, as new energy technologies continue to advance and costs continue to fall, the proportion of clean energy in the energy consumption structure is expected to climb further, providing strong support for the realization of China’s vision of carbon neutrality [25].

Promoting the development of renewable energy sources, such as wind and solar energy, is a key initiative in China’s implementation of its energy-saving and emission-reduction strategy and the promotion of a low-carbon transformation of its energy structure [26,27]. Among the many renewable energy options, solar energy [28], wind energy [29], pumped storage [30], and geothermal energy [31] are regarded as high-quality alternatives to traditional energy sources due to their huge potential, showing a broad development prospect. In recent years, China’s installed capacity of wind power and PV has shown significant growth (Figure 1). Specifically, China’s installed wind power capacity has increased significantly from 145 GW in 2015 to 441 GW in 2023, with a compound annual growth rate of up to 15% during the period. This remarkable achievement has enabled China to maintain its position as the global leader in total installed wind power capacity for 23 consecutive years. Meanwhile, China’s PV installed capacity has also experienced rapid growth, expanding rapidly from 43 GW in 2015 to 610 GW in 2023, with a compound annual growth rate of 39%, topping the world’s cumulative installed PV capacity for 7 consecutive years. The rapid development of these renewable energy sources is mainly due to the fact that they are environmentally friendly, with zero carbon emissions in the power generation process [32,33]. However, the inherent intermittency and instability characteristics of renewable energy present significant challenges to the stability and reliability of the power system when integrated in large volumes [34,35]. To ensure the stability and reliability of renewable energy power supply, the power system must have a high degree of flexibility and a fast response mechanism to effectively deal with the impact of large-scale renewable energy access. The key to solving this problem fundamentally lies in realizing large-scale and reliable energy storage [36,37]. To match the reliability of traditional hydrocarbons and coal-based energy sources, it has become imperative to establish an effective energy storage system to realize the release of energy on demand [38,39].

Large-scale underground energy storage is intended to be clearly differentiated from smaller energy storage facilities, such as various battery systems and small above-ground storage containers [41,42]. Large-scale underground energy storage offers unique advantages over these smaller storage facilities; it can store large amounts of energy carriers [43], has a lifetime of several decades [44], and is economical [45] and feasible [46]. The emergence of this technology presents new development opportunities in the field of energy storage. Currently, large-scale underground energy storage facilities encompass a wide range of underground formations or reservoirs, each of which has its own unique characteristics and offers a variety of options for energy storage. Among these, salt caverns [47], aquifers [48], depleted oil and gas reservoirs [49], and hard mines [50] are the more common underground energy storage spaces. Depleted oil and gas reservoirs, as a special category of underground energy storage resources, refer to those oil and gas reservoirs that have been exploited to the depletion stage or exploited to a certain extent and decommissioned. These reservoirs are often utilized to construct underground natural gas reserves to store large quantities of natural gas resources after appropriate modifications [51]. This type of utilization not only improves the efficiency of resource utilization but also provides a new pathway for energy storage.

Aquifers, another important underground energy storage space, generally consist of permeable water-bearing sands and impermeable overburden [52]. This special geological structure makes aquifers capable of storing large amounts of water resources and also offers the possibility of gas storage [53]. Through rational extraction and utilization, aquifers can be an important option for underground energy storage. Mines, as the residual space after the development of mineral resources, also have huge potential for energy storage [54]. These mines have large resources and are widely distributed, providing abundant site resources for underground energy storage [55,56]. Through appropriate modification and utilization, mine caverns can become ideal sites for storing various energy sources [57]. Among all the underground energy storage options, salt caverns have attracted much attention due to their unique energy storage properties [58]. Salt caverns, as underground spaces formed in salt rock rich in NaCl by aqueous mining methods, have many advantages that make them favorable for energy storage [59,60]. The construction process of a salt cavern is illustrated in Figure 2. Salt rock is a widely distributed sedimentary rock, with burial depths ranging from a few hundred meters to several thousand meters. The extremely low porosity [61] and permeability [62] of salt caverns make them tight; at the same time, their self-healing ability and excellent plasticity [63] allow them to recover quickly when subjected to external forces, thus ensuring their mechanical stability. These characteristics make salt caverns one of the ideal choices for underground energy storage [64,65]. Salt cavern energy storage technology makes full use of the existing underground salt layer resources, which not only reduces the cost investment but also reduces land use and damage to the natural landscape. This technology is in line with the core concept of sustainable development and is therefore widely recognized internationally as one of the most suitable options for energy storage [40]. Currently, salt caverns have been widely used in a variety of fields, such as natural gas [66], oil [67], compressed air energy storage [68], hydrogen [69], flow battery [70], solid waste treatment [71], and carbon dioxide geological storage [72]. According to statistics, there are approximately 100 constructed salt cavern gas storage facilities worldwide, with a working gas capacity exceeding 33 billion cubic meters [73]. At the same time, salt cavern oil reserves, salt cavern compressed air energy storage (SCCAES) power generation stations, salt cavern hydrogen storage (SCHS) reservoirs, and other successful cases are also emerging. Globally, three large SCCAES plants have been successfully put into operation: the Huntorf CAES plant in Germany, the McIntosh CAES plant in the USA, and the Jintan CAES plant in China. With regard to the construction of SCHS reservoirs, the UK, the US, and Germany have all built them one after another. Of these, the US has built the most, with three SCHS reservoirs. In China, significant progress has also been made in large-scale underground energy storage technology. Several salt cavern storage reservoirs have been built in Jintan in Jiangsu Province and Jianghan in Hubei Province, with a cumulative total of more than 40 chambers in operation and a working gas capacity of more than 1 billion cubic meters [74]. These underground energy storage facilities not only enhance the safety and reliability of energy storage but also provide strong support for local economic development. It is worth mentioning that large-scale underground energy storage technology provides a high degree of safety and security because it avoids the problem of possible damage to energy storage facilities on the surface [75]. This feature gives underground energy storage technology a unique advantage in dealing with natural disasters, wars, and other emergencies. With the continuous progress of technology and deepening of application, large-scale underground energy storage technology is expected to play a more important role in the future.

Currently, salt caverns have been widely and deeply applied in the field of large-scale underground energy storage. However, their role as an energy storage method for peak shaving and valley filling for coupling renewable energy has received limited research attention. The key challenges in this field have yet to be deeply explored and summarized. Specifically, the energy storage application of salt caverns in renewable energy peak shaving and valley filling primarily includes three forms: SCCAES, SCHS, and salt cavern flow battery (SCFB). These three types of energy storage methods exhibit varying degrees of differences and uniqueness in terms of their mechanisms, their current development status, and the main challenges they face. To promote the further development of these three types of salt cavern energy storage technologies, this study aims to comprehensively review the current status of salt cavern applications in compressed air energy storage, hydrogen storage, and flow batteries. It explores the key challenges faced by each energy storage method and proposes potentially effective solutions to these challenges. The core objective of this work is to accelerate the practical application of salt cavern energy storage technology in large-scale energy peak shaving and valley filling and to contribute to the promotion of scientific and technological innovation and sustainable development in the field of energy storage.

2. Pathways for Renewable Energy Storage Utilizing Salt Caverns

2.1. Salt Cavern Compressed Air Energy Storage

SCCAES technology, which is a highly efficient way of storing and converting energy [76], works on the principle that when the power grid is in the low ebb, the technology utilizes the abundant power to drive electric motors, which, in turn, drive multi-stage compressors (Figure 3). These compressors draw in ambient air and compress it to high pressures in a step-by-step process. The high-pressure air is then injected into a sealed underground salt cavern for storage. This process effectively utilizes the electrical energy available during low-demand periods, facilitating efficient energy storage [77]. When electricity demand peaks, the high-pressure air stored in the salt caverns is released. This high-pressure air is first depressurized through a throttle valve and adjusted to the required pressure level at the inlet of the expander [78]. At the same time, the heat storage medium from the hot tanks is fed into the pre-stage heat exchangers of the various expansion stages to heat the high-pressure air at the inlet of the expander [79]. The heated high-pressure air enters the multi-stage turbine expander to perform work, driving the turbine to rotate and the generator to generate electricity [80]. This process not only realizes the release of energy but also provides valuable power resources for the power grid, effectively relieving the pressure of power consumption during peak hours [81].

Three large-scale SCCAES plants have been successfully put into operation globally (Figure 4). Among these, the Huntorf CAES plant in Germany, as the world’s first commercially operated SCCAES plant, has been in good operating condition since it was put into operation in 1978. The plant employs two salt caverns as energy storage containers, buried at a depth of about 600 m, with a total volume of 3 × 10⁵ cubic meters [82]. The operational pressure range is between 4.6 and 7.2 MPa, with an energy storage duration-to-discharge duration ratio of approximately 4:1. The compressor power of the unit is 60 MW, while the output power of the release unit is up to 321 MW after upgrading, demonstrating the strong capability of energy storage and release. Following this, the McIntosh CAES plant in Alabama, USA, is the world’s second SCCAES plant in commercial operation [83]. The plant has been upgraded from the Huntorf energy storage plant in Germany with the addition of an expander exhaust waste heat reuse system. By installing a heat exchanger in the expander exhaust flue, the heat carried by the expander exhaust is transferred to the compressed air stream released from the storage cavern, thus saving natural gas consumption. The McIntosh CAES plant was put into commercial operation in 1991, with a compressor unit of 50 MW and an expansion generator unit with an output of 110 MW. The salt cavern is buried at a depth of 300 m, has a total volume of 5.6 × 10⁵ cubic meters, and is capable of storing energy continuously for 41 h and outputting electricity to the outside world continuously for 26 h, which has demonstrated excellent performance. In China, significant advancements in SCCAES technology have also been achieved. On 26 May 2022, China’s first non-complementary combustion CAES power plant was put into operation. The first phase of this plant features an installed power generation capacity of 60 MW, an energy storage capacity of 300 MWh, and a long-term construction scale of up to 1000 MW. The power plant has an overall energy conversion efficiency of more than 60% due to its ability to collect and utilize thermal energy during operation, demonstrating a high degree of energy efficiency. The total volume of the salt cavern is about 2.2 × 10⁵ cubic meters, and the depth of the cavern is about 1000 m.

The successful operation of these power stations not only validates the feasibility and economy of SCCAES technology but also provides a new solution for global energy storage and grid peaking. The emergence of this technology has enabled the grid to be more flexible in dealing with peaks and troughs of electricity consumption, effectively improving the stability and reliability of the grid [84]. In addition, according to the latest industry news, new SCCAES power plant projects have been planned in locations such as Feicheng in Shandong Province, Changzhou in Jiangsu Province, and Huaian in Jiangsu Province [41]. The advancement of these projects signals that SCCAES technology will be more widely applied and developed in the future. With the continuous progress of technology and further cost reduction, it is believed that this technology will inject new vitality into the field of global energy storage and grid peaking.

2.2. Salt Cavern Hydrogen Storage

Hydrogen energy, as a highly promising new form of energy, is attracting global attention due to its many significant advantages [85]. These advantages include, but are not limited to, its clean and environmentally friendly nature, renewability, high energy density, and wide range of sources [86,87]. In the context of the global energy transition, hydrogen energy is regarded as a bright pearl in the future energy system and is expected to lead a new revolution in the energy field [88]. Hydrogen energy can be prepared in a variety of ways, among which the use of electricity generated from renewable energy sources, such as wind and solar energy, to produce hydrogen through the electrolysis reaction of water (Figure 5) is a highly promising method [89,90]. In this process, electrical energy from renewable energy sources is efficiently converted into chemical energy and stored in hydrogen, thereby enabling the efficient conversion and storage of energy. The overall efficiency of water electrolysis for hydrogen production can reach 60–75%, which is a figure that fully demonstrates the technology’s sophistication and practicality [91]. Even more strikingly, hydrogen energy can not only serve as a transitional energy source and play a bridging role in the existing energy system [92] but can also be an ideal energy storage method to support the development of renewable energy in the long term. This characteristic makes hydrogen energy irreplaceable in the future energy landscape [93].

Compared with other ways of storing hydrogen, SCHS has multiple advantages, such as a large energy storage capacity [94], low storage cost [95], and high safety and sustainability [96] (Figure 5). These advantages make SCHS one of the most important directions for hydrogen storage in the future. However, it is worth noting that the development of SCHS is still in its infancy worldwide. Although three countries have successfully established salt cavern hydrogen storage facilities and accumulated substantial operational experience, there is currently a lack of such infrastructure domestically. For instance, the UK constructed a SCHS facility in Teesside as early as 1972, utilizing three salt caverns with a total storage capacity of approximately 1 million cubic meters of pure hydrogen [97]. This facility has operated safely for over 50 years. The stored hydrogen is primarily used for the production of ammonia and methanol. The United States has also built three salt cavern hydrogen storage reservoirs in Texas at depths between 800 and 1500 m that are mainly used in the petrochemical industry. One notable project, the Spindletop hydrogen storage project in the United States, was put into operation in 2014 at a depth of approximately 1340 m [41]. In addition, the SCHS project in Cologne, Germany, stores hydrogen in the form of hybrid hydrogen, and although the purity of hydrogen is not high, this innovative attempt provides a new idea for SCHS.

Table 1. Information about SCHS projects worldwide [41,69].

Project Information	Geological Structure	Purity/%	Operating Condition	Depth/m	Volume/m3	Current Condition
Teesside/Britain	Salt layer	95	45 MPa	365	210,000	Operating
Clemens/U.S.	Salt dome	95	7–13.7 MPa	1000	580,000	Operating
Moss Bluff/U.S.	Salt dome	95	5.5–15.2 MPa	1200	566,000	Operating
Spindletop/U.S.	Salt dome	95	6.8–20.2 MPa	1340	906,000	Operating
Kiel/Germany	Salt cavern	60	8–10 MPa	/	32,000	Closing

lists the relevant parameters of foreign SCHS reservoirs in detail, offering valuable references for further development. As indicated in the table, the safe operation of these SCHS reservoirs fully proves that storing hydrogen in salt caverns is a safe and feasible large-scale storage solution.

Despite the limited number of SCHS reservoirs currently in operation globally, the technology’s enormous potential for growth cannot be ignored. With the continuous advancement of hydrogen energy technologies and the increasing global demand for clean energy, the number of SCHS facilities is expected to expand in the future. Countries around the world are also actively exploring innovative applications of SCHS technology to promote the sustainable development of the hydrogen energy industry. In summary, hydrogen energy, as a new form of energy, is leading the global energy transition with its unique advantages and significant development potential. SCHS, as one of the key directions in hydrogen energy storage, is poised to play an increasingly important role in the future energy landscape. We have reason to believe that in the near future, SCHS technology will contribute significantly to achieving global goals for clean, low-carbon, and sustainable energy development.

2.3. Salt Cavern Flow Battery

As a cutting-edge energy storage technology, the salt cavern flow battery (SCFB) cleverly combines the electrochemical energy storage mechanism of the flow battery with the natural advantages of the geological structure of salt caverns [98]. The core working principle of this technology lies in the use of huge salt caverns as storage containers, in which the positive or negative active materials carry out reversible redox reactions on the electrode surfaces, enabling efficient conversion between chemical energy and electrical energy (Figure 6b). The overall efficiency of salt cavern flow batteries can reach more than 80%. Specifically, chemical energy is stored in the electrolyte solution in the form of active material ions, and the use of the saline solution as the electrolyte not only improves the safety of the system but also significantly enhances its environmental friendliness [99,100]. Salt caverns, with their large liquid storage capacity, provide unique conditions for SCFB technology to meet the needs of large-scale installed energy storage capacity, enabling it to show great potential for application in meeting the challenges of large-scale energy storage and renewable energy peaking [101].

In the global field of SCFB research, Germany undoubtedly holds a leading position. In 2017, German energy giant Ewe Gasspeicher GmbH, in collaboration with the Friedrich Schiller University of Jena, announced a landmark plan to build an SCFB demonstration project [98] called “brine4power” (Figure 6) in two salt caverns with a volume of nearly 100,000 cubic meters in the Jemgum region. Once completed, the project is expected to have a storage capacity of up to 700 MWh and an output of up to 120 megawatts MW, which would be enough to meet the city of Berlin’s electricity needs for a full hour, making it the largest battery energy storage system in the world. Compared to other energy storage methods, such as converting electricity into compressed air or hydrogen storage, SCFB directly stores and utilizes electricity through “brine4power”, which significantly improves the energy conversion efficiency of the overall system.

In contrast, research on SCFB in China is still in its infancy, with no public reports of actual projects yet, and the amount of relevant academic literature is extremely limited, with only single-digit research results available. The research direction in China mainly focuses on two main segments; the first is exploring the geological characteristics of salt caverns. For example, Ding et al. [70] designed a dual-chamber SCFB system based on two salt mines in the Jintan area, combining renewable energy and all-vanadium flow battery technology. After thorough analysis and evaluation, the system shows good performance in terms of roof stability, waist deformation control, and volume loss rate, which verifies its safe and reliable energy storage potential. Second, research is conducted on the design and optimization of electrolytes. Wang et al. [102] successfully synthesized an unsymmetrical two-electron viologens with high-solubility 3-(1′-(2-hydroxyethyl)-[4,4′-bipyridin]-1,1′-diium-1-yl) propane-1-sulfonate bromide ((SO3)V(OH)Br). When paired with a derivative of (2,2,6,6-tetramethylpiperidin-1-yl) oxyl (TEMPO), experimental results showed that this electrolyte formulation exhibited excellent overall battery performance, achieving an energy efficiency of up to 78%. In addition, Huang et al. [98] systematically summarized the eight key issues facing the construction of SCFB considering the characteristics of salt cavern formations and electrolyte properties. These issues include the tightness, conductivity, ions, and temperature of the salt caverns and the selection, permeability, corrosion, and concentration of the electrolytes; in addition, the study proposed several potential solutions to these issues. Meanwhile, it also identifies research directions for the future development of SCFB technology.

Despite the absence of successfully operational commercial projects for SCFB globally, this technology is widely regarded as a promising energy storage solution due to its unique advantages and significant application value. With the rapid development of renewable energy and the increasing demand for energy storage technologies, SCFB is gaining considerable attention in the industry as a novel energy storage approach that combines long-duration storage capabilities, high safety, and cost-effectiveness. It is expected to achieve further breakthroughs in both technological innovation and practical applications. The advancement of this technology will undoubtedly provide strong technical support and assurance for the transformation of the global energy structure and the promotion of sustainable development strategies.

3. Key Technical Challenges of Different Salt Cavern Energy Storage Methods

3.1. Key Challenges of SCCAES

SCCAES technology offers unique advantages, including high energy density, long storage duration, and significant environmental friendliness. These features align closely with China’s current national strategy promoting energy conservation and emission reduction, indicating a broad development space and application potential within the country. However, compared to other energy storage technologies, such as flow batteries and pumped hydro storage, SCCAES exhibits a relatively low overall efficiency of approximately 60% [103]. This efficiency is contingent upon the effective collection and utilization of thermal energy during system operation, presenting a potential bottleneck that could hinder the further promotion and development of SCCAES technology. Moreover, SCCAES projects require substantial initial capital investment, encompassing various aspects, such as the exploration and extraction of salt caverns, the purchase and installation of high-performance compressors, and the construction of complex energy storage systems. These high costs undoubtedly increase the economic pressure on projects and pose challenges to their commercialization. More importantly, the SCCAES technology has more stringent requirements on the geological conditions of salt mines, and not all regions have suitable salt resources, which, to a certain extent, restricts the geographical scope of application of the technology.

In response to the urgent need to improve the efficiency of SCCAES, strengthening technological innovation and investment in research and development is particularly important. By optimizing the technical details of the energy storage and release processes, enhancing the overall efficiency and operational stability of the system becomes key to overcoming the current efficiency bottleneck. At the level of surface equipment, the development of more advanced and reliable compressors, energy storage systems, and control systems is crucial for improving the overall performance of SCCAES. These technological innovations can not only enhance energy conversion efficiency but also increase the system’s reliability and durability, thereby further reducing long-term operational costs. To alleviate the financial pressure of SCCAES projects, multiple approaches can be taken. On the one hand, by optimizing the mining and utilization strategy of salt caverns, the cost of mining salt caverns can be reduced, and the efficiency of resource utilization can be improved. On the other hand, exploring more cost-effective energy storage system construction solutions can reduce unnecessary resource waste and lower construction costs. At the same time, the government can attract more social capital to invest in the SCCAES projects by providing incentives such as subsidies and tax incentives to promote the effective allocation and use of funds. Given the geographical distribution limitations of salt resources, the expansion of SCCAES technology applications is also worth considering. In addition to traditional power peak shaving, the potential applications of SCCAES in industrial energy storage and distributed energy systems should be actively explored. Expanding the application areas will not only improve the economy and market competitiveness of SCCAES technology but also provide a broader space for its development. In the actual deployment of SCCAES power stations, the burial depth of salt caverns is a key factor that needs to be carefully considered. The three existing SCCAES plants have burial depths of 600 m, 300 m, and 1000 m, and these differences have significant impacts on the power output and overall efficiency of the plants. The output power of SCCAES plants is mainly affected by the pressure difference between the inside and outside of the salt cavern and the gas recovery efficiency. On the one hand, improving the gas recovery efficiency means that more compressed gas can be effectively utilized in the power generation process; on the other hand, maintaining an appropriate pressure difference is also essential to guarantee the overall efficiency of the SCCAES system. However, if the burial depth of the salt cavern is too shallow, it may lead to inadequate tightness of the caprock, whereas an excessively deep burial could increase the risks borne by the well casing. Therefore, considering China’s unique geological characteristics, including thin salt layers and numerous interlayers, it is recommended that the optimal burial depth for SCCAES power plants be controlled within the range of 500 to 1000 m in order to balance efficiency, safety, and economic viability.

In summary, although SCCAES technology faces multiple challenges, such as efficiency, cost, and geographical constraints, its future development prospects are still worth looking forward to through technological innovation, cost optimization, application field expansion, and reasonable deployment strategy adjustment. With ongoing technological advancements and continuous policy support, SCCAES is expected to play an increasingly important role in the energy transition and sustainable development, both in China and globally.

3.2. Key Challenges of SCHS

Hydrogen storage in salt caverns, as an advanced hydrogen energy storage technology, faces a series of complex and severe challenges in its practical application and dissemination, despite its significant advantages. These challenges are mainly due to the unique physical and chemical properties of hydrogen and the dynamic changes in the geological environment of salt caverns [104]. Specifically, hydrogen has a very high compression and diffusion coefficient compared to natural gas and air, as well as a low viscosity, which makes it more susceptible to leakage during storage [105]. To complicate matters, salt caverns, as a storage medium, may be subjected to multiple factors, such as geological stress and temperature changes, under long-term geological action, which may lead to a gradual weakening of their structural stability. This phenomenon is particularly noticeable in critical areas such as wellbores, casing, and cement rings, increasing the risk of hydrogen leakage (Figure 7). The overall efficiency of hydrogen stored in salt caverns is not high, in the order of 40–60%. This is due to the fact that hydrogen energy is subject to significant energy losses in the process of production, storage, transmission, and use. Therefore, compared with natural gas and air storage, hydrogen storage imposes more stringent requirements on the comprehensive performance of salt caverns, and it is urgent to construct a feasibility evaluation system for the site selection of hydrogen storage in salt caverns. Moreover, during the complex processes of hydrogen storage and extraction, hydrogen may undergo a series of chemical reactions with other residual chemicals in the salt cavern. These reactions may not only generate impurities, thereby reducing the purity of hydrogen and affecting storage efficiency, but they may also pose a potential threat to the cavern’s original structure. For example, under high-temperature conditions, metal oxides in the salt cavern may undergo reduction reactions with hydrogen, resulting in the precipitation of metals and water. This process not only consumes hydrogen but may also deteriorate the internal environment of the cavern [106]. Additionally, the underground piping must withstand dynamic temperature and pressure fluctuations while also facing the coupled effects of multi-medium corrosion, significantly increasing the risk of hydrogen embrittlement and corrosion, which further threatens the safety of hydrogen storage in salt caverns.

In view of the characteristics of small hydrogen molecules and extremely strong permeability and diffusion, it is suggested that a multi-scale research method be adopted to deeply investigate the adsorption, diffusion, and permeation mechanisms of hydrogen in hydrogen-adjacent geological bodies. Based on these research results, the development and application of new piping materials can be promoted to optimize the tightness structure of salt cavern geological bodies, thus effectively reducing the risk of hydrogen leakage [107]. This research path not only provides key evaluation indexes for the suitability of salt caverns for hydrogen storage but also lays a solid experimental and theoretical foundation for the construction of a feasibility evaluation system for the siting of SCHS reservoirs. Meanwhile, considering the chemical reactions of hydrogen with rock layers and microorganisms in salt caverns, the far-reaching impacts of these reactions on the rock pore structure, composition, permeability, and pore water composition should be revealed in depth, as they are the key factors determining the overall tightness performance of salt caverns. In addition, the source and activity of microorganisms and their colony growth evolution laws under different hydrogen storage conditions need to be systematically investigated to provide a scientific basis for inhibiting the unfavorable reactions between hydrogen and microorganisms. In view of the fact that SCHS reservoirs are located deep underground, typically between hundreds of meters and 2000 m in a complex environment, they are subject to not only the surrounding pressure and temperature but also the chemical effects of multi-phase media erosion. Therefore, an in-depth study of the damage and corrosion mechanisms of underground pipeline materials under multi-condition coupling is crucial, followed by targeted material improvements to enhance the sealing performance of SCHS systems. It is worth noting that hydrogen has a relatively low volumetric energy density, roughly one-third that of natural gas, which directly results in higher storage costs for gaseous hydrogen compared to natural gas. To improve the economy and efficiency of storage, it is recommended to appropriately widen the operating pressure range of SCHS, which is an adjustment that helps to reduce the cost of storage per unit volume in salt caverns compared to conventional salt cavern gas storage. Despite the challenges of hydrogen storage in salt caverns, the salt rock surrounding salt caverns is still considered ideal for hydrogen storage due to its chemical inertness, which does not react with hydrogen, as well as the tightness of the salt caverns themselves.

In conclusion, the feasibility and safety of SCHS depend on a thorough understanding of the physicochemical properties of hydrogen, accurate evaluation of the salt cavern geological environment, and the development and application of novel pipeline materials. Through interdisciplinary integrated research and technological innovation, it is anticipated that existing challenges can be overcome, thereby promoting the rapid development and widespread application of SCHS technology.

3.3. Key Challenges of SCFB

Compared with SCCAES and SCHS, the most significant difference in SCFB is that it has not yet been successfully applied in demonstration projects, which undoubtedly reveals that SCFB has encountered many engineering challenges in the process of promotion. Although the construction of SCFB inevitably faces a number of problems caused by the complex interaction between the geological characteristics of salt caverns and the physicochemical properties of the electrolyte, one of the most prominent problems that need to be solved is the low concentration and uneven distribution of active materials in the huge salt caverns [98]. This problem stems from the huge volume of salt caverns, which usually range from tens to hundreds of thousands of cubic meters. The large storage capacity makes it difficult for active materials to effectively accumulate during the charging process, leading to a natural decline in concentration and highly uneven distribution, which severely restricts the efficiency and performance of the discharge process. In addition, the flow rate of the electrolytes discharged from the wellhead of the salt cavern is relatively low. Referring to the traditional gas injection and brine discharge process, the flow rate of brine discharge is generally maintained in the range of 200–300 cubic meters per hour, which is obviously insufficient for the SCFB system, and it directly restricts the enhancement of its power output, making it difficult to satisfy the expectations of the demand side of the electricity. What is more complicated is that during the operation of SCFB, the dynamic pressure difference between the internal pressure of the salt cavern and the ground stress changes, prompting the electrolytes to extensively diffuse and penetrate the rock layers around the salt cavern, and because of the design of the safety distance of the mine pillar, the electrolytes in the adjacent salt cavern cannot be mixed with each other, which undoubtedly increases the complexity of the management of the system.

The problem of low concentration and uneven distribution of active materials in the salt caverns is essentially due to the fact that the energy of the SCFB is stored in the ions of the active materials. These ions undergo reversible redox reactions during the discharge process, thus converting chemical energy into electrical energy. The ions will have a concentration value in the solution. In the huge storage space of salt caverns, if electrolytes of different valence states of the same electrode are stored in the same salt cavern, then this will result in a low concentration and uneven distribution of active materials. To solve this problem, Shi et al. [98] proposed a novel SCFB system (Figure 8), which cleverly deploys two salt caverns at the positive or negative electrode for storing the electrolytes before and after the redox reaction to ensure that the electrolytes of the same valence state are stored in the same salt cavern, thus effectively avoiding the problem of uneven concentration. At the same time, although the huge space of the salt caverns provides the SCFB system with the potential of a large capacity, insufficient electrolyte outflow has become a key factor that restricts its power output. For this reason, it is recommended to increase the number of boreholes or expand the diameter of the wellbore in order to increase the unit flow rate of electrolytes and ensure that sufficient active materials are involved in the redox reaction so as to meet the demand for power output. In the face of the rapid development of emerging technologies, we should adopt an open attitude and have the courage to innovate the existing system rather than letting the new technology accommodate the old technological framework or system, which is the key to promoting technological progress. Regarding the diffusion and permeation of the electrolytes in the surrounding rock layers of the salt cavern, detailed laboratory experiments should first be conducted to investigate the diffusion and permeation mechanisms of the electrolytes in the salt rock and its interlayers. Subsequently, a large-scale model should be constructed using numerical simulation technology to simulate the actual diffusion and permeation range of the electrolytes, which will provide a theoretical basis for the safe design of the mine column in the SCFB system. In addition, the impact of the geological conditions of the salt cavern on electrolyte properties cannot be ignored. Through comprehensive geological exploration and engineering techniques, the conditions of the salt cavern can be optimized, and the adverse effects minimized. In particular, for unavoidable impurity ions in the salt cavern, such as K⁺, Ca²⁺, Mg²⁺, CO₃²⁻, SO₄²⁻, Fe²⁺, and so on, experiments should be conducted to study the impact of each ion on the electrochemical performance of the electrolytes. It should be clarified which ions are favorable, which are harmful, and which have a neutral effect on the system; at the same time, the safety thresholds of the harmful ions should be determined to ensure that they do not adversely affect the stable operation of the SCFB system. This series of research results will provide strong technical support and theoretical guarantee for the healthy development of SCFB technology and promote a more important role in the future energy storage field.

In summary, the main challenges faced by the SCFB belong to the cross-cutting category of multiple disciplines, and it is necessary for scholars in the field of electrochemistry research and salt cavern energy storage research to work together to promote the landing of this technology and its industrial application, so as to make new contributions to the national dual-carbon strategy and the use of new energy sources.

4. Comparison of Underground Large-Scale Energy Storage Methods

4.1. Environmental and Safety Risk Analysis of Salt Cavern Energy Storage

Underground salt cavern energy storage is a technology that utilizes underground salt rock reservoirs to store energy and is particularly suitable for storing energy sources, such as natural gas, oil, hydrogen, and flow batteries. The technology offers high energy density and flexibility and has significant potential for renewable energy regulation and load balancing. However, underground salt cavern energy storage comes with certain environmental risks and safety issues. The following are some of the major environmental risks, safety hazards, and prevention strategies.

There are three main environmental risks. The first is groundwater contamination. Underground salt cavern energy storage usually utilizes the voids in salt rock formations to store gas or other energy sources. The tightness of the salt rock layer is critical to the effectiveness of the energy storage system. However, the presence of cracks or other imperfections in salt rock formations can lead to leakage of the storage medium and consequent contamination of groundwater resources [108]. In particular, if hydrogen is used as a storage medium, this may pose a risk of chemical reaction or leakage. In order to avoid groundwater contamination, the selection and development of salt caverns must be subject to rigorous geological investigations to ensure that salt rock formations are sufficiently confined and stable. At the same time, regulations require the regular monitoring of groundwater quality, as well as the complete confinement and safe treatment of salt caverns. In addition, salt cavern energy storage facilities are often equipped with testing systems to monitor groundwater changes in real time and to repair any anomalies that are detected. The second is the impact of seismic activity on the stability of salt caverns. The mining and injection process of underground salt cavern energy storage may have a certain impact on the surrounding geological environment, especially in seismically active areas. Although the salt rock itself has good plasticity and pressure resistance, excessive pressure changes or man-made operations may lead to the rupture of the rock formation, which, in turn, may trigger seismic activities or induce microseismicity. To reduce seismic risk, salt cavern energy storage projects need to avoid development in seismically active areas or take additional measures for seismic monitoring and data analysis. A detailed seismic risk assessment needs to be carried out prior to construction to ensure that all operations are carried out within the set safe pressure range to avoid unnecessary load accumulation. Third is the need to take up a large amount of underground space. Underground salt cavern energy storage projects need to take up a large amount of underground space and generally have limited options for being located in surface areas. Although this technology does not directly occupy a large amount of space on the surface, it has a large impact on the indirect use of land, especially in densely developed areas. The construction of energy storage facilities, pipeline laying, etc., may have some impact on the local ecological environment and land use. Reasonable site selection can minimize the occupation of surrounding land resources and ensure that the energy storage facility will not disrupt the ecological balance through rational land planning and environmental impact assessment. In addition, environmental protection measures (e.g., green cover, greening, etc.) are taken to restore the environment around the facilities to ensure that the land is used in a sustainable manner.

There are three main risks regarding the safety of salt cavern energy storage. The first is the risk of hydrogen leakage. Unlike natural gas and air, hydrogen has an extremely high compression coefficient and diffusion coefficient, as well as a low viscosity, and these characteristics make it more prone to leakage during storage. Liu et al. et al. studied hydrogen leakage in layered salt rocks [109]. The results of the study showed that hydrogen leakage is more extensive in interlayers with higher permeability, and the leakage extent is directly proportional to the permeability of the interlayer. Furthermore, in order to increase the tightness of the salt cavern hydrogen storage reservoir, it is recommended that the permeability of the interlayer should be less than 1 × 10⁻¹⁷ m² for single cavern hydrogen storage and less than 1 × 10⁻¹⁸ m² for multi-cavern hydrogen storage. In addition, hydrogen itself is highly flammable, which may lead to fire, explosion, and other major safety incidents in the event of a leak. In addition, hydrogen leaks may pollute the surrounding environment and pose a threat to the safety of facility personnel. To minimize the risk of hydrogen leakage, hydrogen storage systems should be designed to ensure tightness and use high-standard sealing materials and techniques to avoid hydrogen leakage. Equipment and pipework should be regularly inspected and maintained. In addition, hydrogen storage depots should be equipped with real-time hydrogen concentration monitoring systems to detect any abnormal leaks. Once a leak occurs, emergency response measures can be triggered in a timely manner. The second is the design of pressure control and safety valves for the storage reservoir system. Pressure variations in underground salt cavern energy storage are inevitable, especially when injecting or releasing hydrogen, and improper operation may result in too high or too low pressure within the salt cavern, which, in turn, may lead to problems such as system malfunction, leakage, or collapse. For safe and smooth operation, the energy storage system should be equipped with strict pressure monitoring and regulation equipment to ensure that it operates within the set safety range. In particular, drastic pressure fluctuations should be avoided during gas injection and release. Further, the gas storage system shall be equipped with an automatic safety valve, so that when the pressure exceeds the safety range, the valve can automatically close to prevent accidents. The third is long-term stability monitoring. Over time, the salt caverns may undergo changes, such as the dissolution, compression, or structural damage of the salt rock, which may lead to leakage of the storage medium or other safety hazards. In order to make the energy storage system more reliable, geological monitoring equipment (e.g., seismic wave detection, pressure sensors, etc.) can be used to regularly check the changes in the salt rock layer to ensure that it remains stable. If potential risks are detected, immediate remedial measures are taken, or the salt caverns are repaired and closed.

Furthermore, to ensure the safe operation of salt cavern energy storage facilities, a series of regulatory frameworks and safety standards have been developed by governments and relevant agencies. For example, in the United States, salt cavern energy storage facilities are subject to regulations and standards developed by the Federal Energy Regulatory Commission (FERC); in Germany, salt cavern energy storage facilities are subject to regulations and standards developed by the German Energy Regulatory Agency. These regulatory frameworks and safety standards cover all aspects of the siting, construction, operation, and maintenance of salt cavern energy storage facilities, ensuring their safety and reliability. As a viable energy storage technology, underground salt cavern energy storage has great potential for its application in renewable energy systems, but it comes with some environmental and safety risks. Reasonable sitting, strict safety design, continuous monitoring and maintenance, and emergency response plans are the keys to ensuring the safe operation of underground salt cavern energy storage facilities. With technological advances and improved management, underground salt cavern energy storage is expected to become an important part of the future energy system, supporting the broader green energy transition.

4.2. Comparative Analysis with Alternative Energy Storage Technologies

Underground large-scale energy storage technologies, especially in the context of energy transition and large-scale power storage, are becoming an important area of research. Four common underground large-scale energy storage technologies are salt caverns, depleted oil and gas reservoirs, aquifers, and lined chambers. Each technology has its own specific characteristics, advantages and disadvantages, and applicable scenarios. The following is a comparative analysis of these four energy storage technologies, focusing on aspects such as efficiency, cost, and scalability, as shown in

Table 2. A comprehensive comparison of four underground large-scale energy storage methods.

Energy Storage Method	Overall Efficiency	Cost	Construction Cost	Operating Cost	Maintenance Cost	Scalability	Advantage	Disadvantage
Salt cavern	60–80%	USD 50–USD 100/MWh	Moderate, RMB 300–800/m3	Low to moderate	Low	Excellent	High efficiency, proven technology, large-scale energy storage	Higher construction costs and geographically limited resources
Depleted oil and gas reservoir	50–75%	USD 60–USD 120/MWh	Moderate, RMB 200–600/m3	Moderate	Moderate	Moderate	Use of existing facilities, low initial costs	Less efficient, limited resources, uncertain long-term stability
Aquifer	50–70%	USD 70–USD 150/MWh	Low to moderate, RMB 200–600/m3	Low to moderate	Moderate	Moderate	Wide distribution of resources, low initial costs	Inefficiency, limited by aquifer conditions, risk of contamination
Lining chamber	55–75%	USD 100–USD 200/MWh	High, RMB 300–800/m3	High	High	Moderate	Large space, high safety	High construction costs, limited by geological conditions

.

Salt cavern energy storage is a technology that uses salt caverns formed by natural salt layers (usually rock salt layers) underground to store energy. Salt caverns formed underground by salt formations are ideally suited for storing gases or liquids and are well-sealed and stable. The overall energy efficiency ranges from 60 to 80%. Particularly when applying liquid flow batteries for energy storage, efficiencies of around 80 percent can be achieved. Initial construction costs are high and consist mainly of the excavation of the salt caverns and the construction costs of the facilities. Depending on the location and quality of the salt layer, the cost ranges from USD 300 to USD 800 per cubic meter. Construction costs can be significantly reduced if already mined salt mines can be retrofitted. Operation and maintenance costs are relatively low, but regular inspections and maintenance are required. Scalability is good. Salt caverns usually have a large energy storage capacity and can be flexibly expanded. They are particularly suitable for storing large-scale energy for long-term large-scale use and have important applications, especially in the fluctuating storage of wind and solar energy.

Energy storage technologies for depleted reservoirs utilize the subsurface space of oil and gas fields that have been developed and cannot be further exploited for energy storage. Overall energy efficiency ranges from 50 to 75%. Initial construction costs are moderate, as many depleted reservoirs already have infrastructure in place (e.g., wellheads and pipelines), which saves a portion of the initial investment. Operation and maintenance costs are moderate, mainly due to the high energy consumption for compressing gas and extracting energy and integrity checks of the wellhead unit. Scalability is limited, as depleted reservoirs are geographically and quantitatively limited, and the energy storage capacity and geological structure of each reservoir varies, making them less scalable than salt cavern storage.

Aquifer energy storage technology uses underground aquifers to store energy. Initial construction costs are relatively low because aquifer resources are widely distributed, but the permeability and stability of the aquifer need to be rigorously assessed. Overall energy efficiency ranges from 50 to 70%. Operating costs are lower than salt cavern storage but face larger maintenance costs, especially if the aquifer is contaminated or leaked. Scalability is moderate and can be expanded over a large area, especially in water-rich regions. However, it is limited by the geological conditions of the aquifer and environmental factors.

Energy storage in lined chambers is a method of utilizing these underground spaces for energy storage by manually excavating chambers in underground rock formations and lining and reinforcing them. Overall energy efficiency ranges from 55% to 75%. Initial construction costs are high, requiring cavern excavation and lining treatments, and are usually suitable for areas with good geological conditions. Costs for operation and maintenance are high, requiring high pressure and temperature conditions to be maintained and high energy losses. Although lined chambers can provide a large energy storage space, their scalability is somewhat limited due to the cost of excavation and the geological conditions required, resulting in moderate scalability.

In summary, the construction costs of these four underground large-scale energy storage methods vary depending on the specific requirements of the project, the technical options, the geological conditions, and the economic level of the region in which they are located. Therefore, the specific construction costs need to be confirmed by a detailed project feasibility study. The construction cost of salt cavern energy storage does not show any advantage among these four types of energy storage, instead, the upfront investment requirement of aquifers is lower. This is because aquifers are relatively rich in resources and can be constructed through relatively simple underground works. It is suitable for most areas, and the cost of resource extraction is low. When the construction of these four types of energy storage facilities is completed, salt cavern storage shows an advantage in terms of operation and maintenance, which should require the least amount of money. The reason for this is that salt caverns are well sealed, and it is very difficult for the stored energy to leak, so the compression and release process is more efficient, and there is less energy loss. This also makes it less maintenance-intensive, requiring only regular inspections and ensuring that there are no major geological changes or structural problems. In terms of long-term economic viability, salt cavern energy storage and energy storage in depleted oil and gas reservoirs are highly economically viable because of their low construction and operating costs and large storage capacity. Aquifer storage, although economical, is limited by groundwater resources and aquifer conditions and may have an impact on the groundwater environment. Energy storage in lined chambers may have higher construction and operating costs but may still be economically viable in areas with favorable geological conditions and high market demand. In addition, salt cavern energy storage has shown advantages in terms of efficiency and scalability, making it one of the most promising underground energy storage technologies for large-scale, long-term energy storage systems.

5. Conclusions

To accelerate the development of salt cavern energy storage technologies for coupling renewable energy sources for peak shaving and valley filling, this study provides a comprehensive overview of the current state of development and major technical challenges of SCCAES, SCHS, and SCFB. In response to these technical challenges, this study proposes some potential solutions. The conclusions are as follows:

(1)

The key challenge of SCCAES is that its overall energy efficiency is only about 60%. It is recommended to develop more advanced core equipment, such as compressors, energy storage systems, and control systems, which play a critical role in improving energy efficiency. Reducing the cost of ground equipment should be the focus of future research. Based on the considerations of tightness and output power for SCCAES, it is recommended that the burial depth of the salt cavern should range from 500 to 1000 m.

(2)

Unlike natural gas and air, hydrogen has extremely high compression and diffusion coefficients. This makes the biggest challenge for SCHS the risk of hydrogen leakage. Conducting in-depth research on the diffusion and permeation mechanisms of hydrogen in the surrounding rock of salt caverns, which could also serve as the theoretical foundation for the development of SCHS pipeline materials, is recommended. The development of new materials should be directed toward superior tightness and corrosion resistance. Due to the relatively low volumetric energy density of hydrogen, it is recommended that the operating pressure of SCHS should be appropriately widened, which can help reduce the storage cost per unit volume of salt caverns.

(3)

The key challenge of SCFB is the low and uneven distribution of active materials within the huge salt caverns. This challenge can be addressed by deploying two salt caverns at each of the positive or negative electrodes of the SCFB to store the electrolyte before and after the redox reaction. It is essential to thoroughly investigate the impact of impurity ions (such as K⁺, Ca²⁺, Mg²⁺, CO₃²⁻, SO₄²⁻, and Fe²⁺) in the salt cavern on the electrochemical performance of the SCFB. Considering the temperature conditions of the electrolyte working environment, it is recommended that the burial depth of the salt cavern range from 500 to 1500 m. Developing electrolytes with better electrochemical properties is key to facilitating SCFB development.