Opportunities and challenges of large-scale salt cavern hydrogen storage in China coupled with renewable energy sources

Abstract

To expedite China’s pursuit of the “dual-carbon” goal, a gradual transition from traditional fossil energy to renewable energy sources is imperative for the nation’s energy mix. Hydrogen energy, poised to become a pivotal component of the future energy industry, offers myriad advantages, including diverse sources, high efficiency, cleanliness, and high energy density. Salt caverns present distinct benefits for underground storage. This research synthesizes the current development trajectories of renewable energy and hydrogen energy in China, summarizing the advantages, disadvantages, and economic comparisons of various underground hydrogen storage methods, with a particular emphasis on the merits of salt cavern hydrogen storage. Furthermore, it reviews the current state and opportunities for salt cavern hydrogen storage both domestically and internationally, underscoring its substantial potential within China. Ultimately, the research identifies three major scientific and technological challenges associated with hydrogen storage in salt caverns in China and envisions future directions for this technology. The findings of this research are anticipated to contribute to the development of a hydrogen storage strategy in salt caverns that aligns with China’s national conditions.

Graphical Abstract

Open in new tabDownload slide

1 Introduction

In recent years, environmental problems, such as the greenhouse effect, smog, and haze caused by emissions from fossil fuel wastes, have become increasingly serious, and fossil energy sources, being nonrenewable, will eventually be exhausted [1].To transition the energy structure from traditional nonrenewable fuels to renewable and clean energy, countries worldwide have initiated plans to develop the hydrogen energy industry. Hydrogen is recognized as a high energy density and versatile energy source, serving as a crucial medium for promoting the clean, low-carbon, and efficient utilization of energy, as well as for achieving the optimal allocation of heterogeneous energy sources across regions and seasons. The government emphasized that the hydrogen industry is a critical development direction for future industries and strategic emerging sectors. Accelerating the development of the hydrogen energy industry is a strategic choice to address global climate change and achieve the goals of “carbon peak and carbon neutrality.” It is also a significant initiative to implement the spirit of the 20th CPC National Congress, expedite the green transformation of the development model, and accelerate the planning and construction of a new energy system. It is projected that by 2060, China’s annual hydrogen demand will increase from 33.42 million tons to 130 million tons, accounting for 29% of total energy consumption [2, 3]. As the crucial link between hydrogen production and utilization, hydrogen storage is a key technology to advance the hydrogen energy industry [4]. However, it also represents a bottleneck that restricts its development. As is shown in Fig. 1, there are various states of hydrogen storage at the current stage, including high-pressure gaseous hydrogen storage, low-temperature liquid hydrogen storage, solid hydrogen storage, and organic liquid hydrogen storage [6]. Due to limited ground space and high costs, the scale of ground hydrogen storage is generally small. From a long-term developmental perspective, effective measures must be taken to enable large-scale storage for hydrogen energy to fully realize its role in achieving carbon peaking and carbon neutrality.

Underground hydrogen storage (UHS) offers significant advantages, including large-scale capacity, long cycle times, and the ability to store energy across seasons, making it a crucial development direction for large-scale hydrogen storage technology [7]. Among various types of UHS reservoirs, salt cavern hydrogen storage (SCHS) reservoirs are considered one of the most promising due to their excellent sealing performance and high hydrogen storage pressure [8]. These reservoirs can hold nearly 100 million m³ of hydrogen and maintain a purity of over 95%. China possesses abundant salt rock resources. The country has already constructed five salt cavern natural gas storage reservoirs and two salt cavern compressed air energy storage power stations [9, 10], gaining substantial experience in building salt cavern gas storage facilities. Currently, domestic SCHS is still in its initial stage, with a weak and unsystematic research foundation, and no demonstrative engineering projects for SCHS have been undertaken. This paper addresses the critical need for large-scale hydrogen storage, summarizes the structural changes in hydrogen energy in China, integrates the development history of hydrogen storage both domestically and internationally, and analyzes the advantages of underground SCHS. It also explores the technical challenges of SCHS in China, examines the overall feasibility based on the characteristics of China’s layered salt rocks and the experience of constructing salt cavern natural gas storage, and provides relevant recommendations for the development of SCHS in China. The roadmap is shown in Fig. 2.

2 Analysis of renewable energy development and hydrogen energy structure in China

2.1 The development of renewable energy in China

China’s renewable energy power generation is developing robustly. According to data published on the State Council’s website, China’s renewable energy installed capacity exceeded 1.3 billion kW in 2023, accounting for 53.9% of the country’s total installed power generation capacity. This marks a doubling of the installed capacity scale since the end of 2017, with an addition of 134 million kW of new installed capacity, representing 76.1% of the country’s new power generation capacity. China has been the world’s largest investor in renewable energy for 8 consecutive years, and its installed capacities for wind power and photovoltaic energy are the highest globally. According to the sustainable energy development strategy implemented by China, Huang et al. [11] indicated that by around 2030, renewable energy will occupy a significant position in China’s overall energy system as one of the mainstream energy sources. By around 2050, renewable energy is expected to become one of the dominant sources of energy, thereby fundamentally transforming the energy structure.

Taking wind energy as an example, China’s installed wind power capacity has developed rapidly and has become the world’s largest wind power market. As shown in Fig. 3, by 2022, China’s total installed wind power capacity reached 365 GW. However, China’s wind curtailment phenomenon has also been increasing, reaching its peak in 2016. According to data released by the National Energy Administration (NEA), in 2016, China’s abandoned wind power amounted to 49.7 billion kilowatt-hour (kWh), equivalent to 53% of the Three Gorges Dam’s full annual power generation (93.5 billion kWh) that year(shown in Fig. 4). In Gansu, where wind curtailment is particularly severe, the wind curtailment rate exceeded 43%, with 10.4 billion kWh of wind power being abandoned, triggering widespread concern in the industry and society at large.

The problem of wind curtailment has caused a significant waste of clean energy and has gradually become a major factor restricting the development of wind power. Therefore, establishing a SCHS system to store wind energy and provide electricity during peak periods has emerged as an effective way to reduce the waste of clean energy such as wind power. This approach promises substantial economic and environmental benefits for society.

2.2 Status of China’s hydrogen industry

The main methods of hydrogen production include hydrogen from coal, natural gas, industrial byproducts, and the electrolysis of water. The cost of hydrogen production from coal is about RMB 10 yuan/kg (calculated at a coal price of RMB 450 yuan/t), making it the lowest-cost method among current hydrogen production techniques. As shown in Fig. 5a, coal-based hydrogen production accounts for 62% of the total hydrogen production in China, making it the most significant method at this stage. The cost of hydrogen production from natural gas is about RMB 15 yuan/kg (calculated at a natural gas price of RMB 2.5 yuan/Nm³), and the average cost of hydrogen production from industrial byproducts ranges from RMB 9 to 22 yuan/kg. These two methods account for 19% and 18% of China’s hydrogen production, respectively. In contrast, the current cost of hydrogen production from the electrolysis of water is more than RMB 30/kg (calculated at an electricity price of RMB 0.4 yuan/kWh) [13]. Due to its high cost, this method accounts for only 1% of China’s hydrogen production capacity.

Depending on the source of production and carbon emissions, hydrogen can be divided into three types: gray, blue, and green hydrogen (shown in Fig. 5a). Gray hydrogen is produced directly from fossil energy sources and industrial byproducts, resulting in large amounts of carbon dioxide emissions. It is currently the most dominant method of hydrogen production. Blue hydrogen is produced from gray hydrogen in combination with carbon dioxide capture, utilization, and storage (CCUS) technology, serving as an important stage in the transition from gray to green hydrogen. Although blue hydrogen can reduce carbon emissions by up to 90%, the stringent conditions required for carbon dioxide sequestration greatly limit its development. Green hydrogen is produced using renewable energy sources (such as solar or wind power) to generate electricity, which is then used in the electrolysis of the water process. This method can achieve net-zero carbon emissions [14], but it is currently costly. Presently, the primary source of hydrogen in China is gray hydrogen. Based on the development of the demand-side industry and the improvement of the industrial chain, a gradual transition from gray hydrogen to green hydrogen is considered a better approach. This involves prioritizing the use of byproduct hydrogen and achieving comprehensive resource utilization.

2.3 China’s hydrogen energy status

China has a significant consumption of hydrogen energy, accounting for 53.7% of global hydrogen energy consumption in 2022. Hydrogen energy is primarily used in the industrial sector, with a share of 66%. The largest application is as an industrial feedstock for the production of ammonia, at 37%, followed by hydrogen for methanol and oil refining, at 19% and 10%, respectively. It is expected that by 2060, hydrogen for industrial use will continue to dominate the national hydrogen energy application field, with about 77.53 million tons, accounting for 59.5% of the total hydrogen demand. However, the application field will gradually shift from traditional uses like ammonia production, synthetic methane, and oil refining to steelmaking, transport, and electric power. Hydrogen use in the transport sector is projected to rise to about 40.65 million tons, accounting for 31.2% of the total demand, while usage in the construction and electric power sectors will rise to 4.5% and 4.6%, respectively.

• (i) In the industrial sector, hydrogen will be used both as a feedstock and as an energy source. As a raw material, hydrogen energy will be widely used in the steel, chemical, petrochemical, and other industries, replacing coal, oil, and other fossil fuels. As an energy source, hydrogen energy will be utilized in fuel cell technology for cogeneration to meet the demand for distributed industrial power and heat. It is expected that the demand for hydrogen energy in the industrial sector will exceed 35 million tons by 2050 [15].

• (ii) In the field of transportation, hydrogen fuel cell vehicles and lithium battery vehicles will “play their respective roles and do their best” to jointly promote the role of new energy vehicles as substitutes for traditional fuel vehicles, initiating a wave of new energy changes in the transport sector. Due to the advantages of hydrogen fuel cell vehicles, such as long driving range, short refueling time, high energy density, and low-temperature resistance, they have more potential in heavy freight transport, long-distance transport, public transport, and aerospace in cold regions. It is expected that the demand for hydrogen in the transport sector will be close to 40 million tons by 2050 [16].

• (3) In construction and other fields, household hydrogen fuel cells, fuel cell emergency power supplies, and other technical equipment are also expected to achieve large-scale application, with the demand for hydrogen projected to be close to 20 million tons by 2050 [17].

To sum up, by 2050, the hydrogen demand of the whole society may be close to 100 million tons (equivalent to about 380 million tons of standard coal). The Energy Research Institute of the National Development and Reform Commission (NDRC) estimates that if the “2°C” carbon reduction scenario is achieved, hydrogen demand could further increase to 150 million tons or even higher, an increase of more than 50%.

3 Development of underground hydrogen storage in salt caverns

3.1 Geologic storage options

3.1.1 Comparison of geological features

In geological structures, the carriers of UHS can be caverns or caves, which can store gas due to the impermeability of the surrounding rock, or porous media formations, such as depleted gas reservoirs and aquifers, which have high porosity and good permeability, making them suitable for the rapid injection and extraction of gas [18, 19]. Generally, UHS reservoirs are classified into four types: depleted oil and gas reservoirs, aquifers, abandoned mines, and salt caverns [20, 21].

Depleted oil and gas reservoirs:

These reservoirs use gas or oil reservoirs after depletion for gas storage. Due to their low cost and reliable operation, depleted reservoirs are the most commonly used method for natural gas storage, with about 500 such reservoirs, accounting for 72% of the total number of gas storage reservoirs [22]. Geologically, this type of reservoir has proven to be capable of storing natural gas. The main difference with hydrogen storage lies in the physical and chemical properties of the storage medium. UHS in depleted reservoirs, therefore, has a broader development prospect [23].

Aquifers:

The usual reservoir of an aquifer is a porous and permeable sandstone or carbonate rock, generally containing salt or fresh water in the pores, with gas enriched at the top and sealed by a cap. There are about 77 aquifer gas reservoirs worldwide, accounting for 11% of the total number of underground gas reservoirs [22]. Although there are no examples of pure hydrogen storage in underground aquifers, numerical modeling studies and analysis of influencing factors have demonstrated the feasibility of storing hydrogen in aquifers [24].

Abandoned mines:

These are noncompliant mines that have been closed due to depletion of mineral resources, mine gas outcrops (a phenomenon where crushed coal and gas are ejected in large quantities into the extraction space), failure to meet safe mining requirements, and other regulations. Using this method for energy storage does not require the excavation of new underground space but effectively utilizes the existing underground mining space as a gas storage reservoir. Due to the complexity of geological conditions and extraction requirements, abandoned coal mine hydrogen storage technology is still in its primary stage, with both basic theories and key technologies needing further research [25].

Salt caverns:

Compared with sandstone, conglomerate, and mudstone, salt rock has insufficient density, poor compressive strength, and a modulus of elasticity, making its mechanical properties particularly unique. Salt rock is incompressible, has good creep characteristics [26], self-repairs damage, and has stable mechanical properties [27], allowing it to adapt to changes in storage pressure in gas storage reservoirs. Additionally, salt rock’s solubility in water makes the construction of salt rock cavities more economical. Therefore, salt rock is internationally recognized as an ideal medium for energy storage. As shown in Fig. 6, salt cavern gas storage has been widely used to store a variety of energy substances.

The advantages and disadvantages of different types of geological reservoirs are shown in Table 1. Depleted reservoirs and aquifers offer substantial capacity for hydrogen storage; however, they present limitations such as hydrogen leakage through rock pores and fault channels, along with potential chemical contamination. Hard rock chambers exhibit high adaptability, but the surrounding rock is prone to tensile damage under high-frequency loading and unloading operations. At shallow depths, localized tensile plastic zones, tensile cracks, and damage from rock fatigue can significantly impact both the stability and airtightness of the chamber. Repurposing existing roadways and chambers in abandoned mines for energy storage conserves land resources and reduces preliminary construction costs, yet safety issues must be carefully addressed to prevent leakage and collapse.

Compared to other geological storage reservoirs, salt caverns possess low permeability, favorable creep properties [29], effective damage recovery, and ease of excavation. Additionally, salt rock is inert to hydrogen. These characteristics enable SCHS reservoirs to perform multiple injection and extraction cycles annually, thereby playing a crucial role in peak shifting and short-term energy storage needs, making them the optimal choice for large-scale UHS [30, 31].

3.1.2 Economic comparison

This section compares three hydrogen storage methods—rock caverns, aquifers, and salt caverns—in terms of hydrogen storage costs. Table 2 labels the total capital cost model for the three geological types, which allows for a preliminary economic comparison of these hydrogen storage methods, highlighting the advantages of SCHS.

After calculating the cost of hydrogen storage, the cost of electricity generation from SCHS and repowering is analyzed. Hydrogen has a high energy density, up to about 142 MJ/kg, theoretically, a unit mass of hydrogen can generate electricity of about 40 kWh/kg. Currently, fuel cells are the most efficient way of utilizing hydrogen energy, and theoretically, they can be operated at 100% thermal efficiency, and the chemical energy efficiency of the current fuel cells is generally in the range of 40%–60%. Calculated at 50% efficiency, the power generation capacity per unit mass of hydrogen through fuel cell stacks can be up to 20 kWh/kg (about 1.8 kWh/Nm³). Thus, the cost of electricity generation from renewable energy coupled with SCHS and repowering can be accounted for. The cost of kWh is shown in Table 3 in the case of annual hydrogen production of 2 million Nm³ in the electrolyzer.

It can be seen that the current technology route of hydrogen production from electrolytic water combined with UHS and repowering has a high power generation cost of about 1.88 yuan/kWh. Analysis of the cost components reveals that the cost of electricity and the cost of equipment account for the main part of the cost, accounting for 61.1% and 25.6% of the total cost, respectively.

The cost of kWh is analyzed under the following cost change scenarios: Case 1: the current cost of hydrogen repowering from electrolytic water; Case 2: the cost of equipment is reduced to half of the current cost; Case 3: the price of electricity is reduced to 0.2 yuan/kWh; Case 4: hydrogen storage from excess electricity from renewable energy generation; Case 5: Case 2 + Case 3; Case 6: Case 2 + Case 4; Case 7: Case 6 + fuel cell efficiency increase to 80%. The results are shown in Fig. 7.

It can be found that the cost of regeneration can be reduced to 0.64 yuan/kWh by using the excess electricity that cannot be fed into the grid during the peak season of renewable energy to produce hydrogen from electrolytic water for energy storage, and the cost of kWh of this technology can be further reduced to 0.49 yuan/kWh, which is close to the current price of electricity and has a practical application prospect. If the efficiency of the fuel cell is further improved to 60%, the cost of electricity will be further reduced to 0.43 yuan/kWh, which is basically the same as the current price of electricity, and the green and clean characteristics of green hydrogen power generation will make this technology route competitive compared with traditional fossil energy power generation.

3.2 State of development of underground hydrogen storage in other countries

Countries in Europe and the USA began exploring the engineering of SCHS as early as the 1970s and 1980s, leading to the construction of five SCHS reservoirs. As shown in Table 4, the UK was the first country to use salt caverns to store high-purity hydrogen, with the Teesside hydrogen storage reservoir storing about 760 tons. In the USA, three SCHS reservoirs have been built at Clemens Dome, Moss Bluff, and Spindletop, with storage capacities of 6210, 6020, and 13 100 tons, respectively [35]. These four salt cavern reservoirs store high-purity hydrogen with a concentration of more than 95%. Additionally, the Kiel reservoir in Germany was used to store town gas with a hydrogen content of 62%. These reservoirs were primarily used for hydrogen in chemical plants and were built on a smaller scale. In recent years, with the launch of national hydrogen energy strategies in various countries, European nations have increasingly focused on the study of UHS. For example, the large-scale underground hydrogen energy storage project “HyUnder” was initiated by 12 units from seven European countries, including Germany, France, and the UK. This project conducted a study on the use of underground salt caverns for hydrogen storage in a European context. It assessed the potential for long-term storage of renewable electricity using hydrogen storage in underground salt caverns on a European scale. The project established a methodology for assessing the economics of hydrogen storage for automotive hydrogen, natural gas refueling, industrial plants, and power generation. It also analyzed the hydrogen storage market for the period 2025–2050. Furthermore, the new pilot project “H₂ Research Caverns,” recently initiated by the HYPOS Institute in Germany, is studying the potential for long-term storage of renewable electricity through UHS in central Germany. This project aims to build a research platform for hydrogen storage in salt caverns. Site selection research and test demonstrations of underground SCHS reservoirs are also accelerating, with more than 20 projects currently in progress or planned for construction. Canada conducted a preliminary assessment of four potential SCHS sites in Ontario, recommending the conversion of existing natural gas storage reservoirs or old salt caverns to hydrogen storage [36].In Poland, a preliminary assessment was conducted to evaluate the hydrogen storage potential in 27 salt mounds and the stratified salt rock formations of the Zatoka Gdanska area, screening potential sites for hydrogen storage [37–39]. Based on the HyUnder project, four potential SCHS sites were identified in Romania [40], while six sites were screened in France, with commercial operation models developed for these storage facilities [41]. In the Netherlands, a SCHS facility is planned for construction in the Zuidwending region, with an expected operational date of 2028 [42].

3.3 Conditions and development status of hydrogen storage in domestic salt caverns

China is rich in salt mine resources, with nearly 105 proven mineral deposits primarily distributed in Qinghai, Jiangxi, Jiangsu, Shandong, Yunnan, Henan, Hunan, and other regions [43]. Salt mine resources follow the basic law of sea salt in the east, lake salt in the west, and well mine salt in the center. In 2014, the national output of salt mine reached 4.8 × 10⁷ tons, and based on this data, it is estimated that about 2.0 × 10⁷ m³ of space can be generated from the mining area annually. Further estimations suggest that salt mining has created more than 3.0 × 10⁸ m³ of empty space over the past 10 years, presenting a significant potential for the reconstruction of SCHS.

The development of salt cavern gas storage in China started relatively late, with preliminary demonstrations beginning in 1999. The construction of related projects commenced in 2000 with PetroChina’s Jiangsu Jintan salt cavern gas storage reservoir and now has over 20 years of development. Currently, five salt cavern natural gas storage reservoirs are in operation, with more than 10 projects under construction or proposed. The use of salt caverns to store hydrogen and natural gas shares many similarities. The West-to-East Gas Transmission Jintan Salt Cavern Reservoir Project has been operational since 2007, maintaining safe and stable operations. Additionally, the Sichuan Gas Transmission to the East Jintan Salt Cavern Storage Reservoir and the Jianghan Salt Cavern Storage Reservoir Project have also begun operations. As shown in Fig. 8, more than 40 salt cavern gas storage reservoirs with a total volume of nearly 1.5×10⁷ m³ are currently operational. Moreover, salt cavern gas storage projects in Jiangsu Huai’an Salt Mine, Hubei Yunying Salt Mine, Henan Pingdingshan Salt Mine, Shandong Tai’an Salt Mine, and Hebei Ningjin Salt Mine have entered the feasibility study and pilot test stages. This marks a significant phase in the construction of salt cavern gas storage reservoirs in China. In the field of engineering where salt cavern gas storage has been successfully practiced, many scholars have made outstanding progress in the areas of fine detection of layered salt rocks, mechanical behavior of layered salt rocks, surface subsidence, and interlocking damages of layered salt rock outgrowth clusters, software for making cavities in layered salt rocks, and the theory of risk and protection of layered salt cavern storage, Shi et al. [45] carried out tests on the tensile strength of muddy interlayers under different concentrations of brine immersion conditions. The results show that the lower concentration of cavity brine environment is conducive to the weakening of the tensile strength of the interlayer, which in turn promotes the collapse of the interlayer. Subsequently, a mechanical model for the collapse of the interlayer in the water-soluble cavity-making process was established, and the calculation method of the ultimate span of the interlayer was proposed. Wei et al. [46, 47] have studied the creep and contraction law of the Jianghan ultra-deep salt cavern and evaluated the sealing of the Jianghan ultra-deep salt cavern based on the on-site monitoring data such as the closed well holding pressure of the Jianghan salt cavern. Ban et al. [48] have optimized the cavity-building process, including the circulation mode, displacement, and pipe-column combination of multi-interlayered salt cavern reservoir construction. A roofing process for multi-interlayered salt cavern gas storage construction is also proposed. Yang et al. [49] conducted comprehensive research on the construction of underground gas reservoirs in layered salt rock under challenging circumstances, resulting in the development of a range of theories and technologies, including deformed salt cavern repair, giant-thickness interlayer control, and micropermeable interlayer sealing. These researches in interlayered salt rock reservoirs provide valuable experience for the engineering practice of hydrogen storage in salt caverns in China.

Domestic research on hydrogen storage in underground salt caverns is still in its infancy, but many scholars have already conducted relevant studies. For instance, Liu et al. [50] studied the layered salt rock in Jintan, Jiangsu Province, China. They combined the large-scale storage of surplus electricity from renewable energy generation, represented by wind energy, and analyzed and evaluated the feasibility of the salt rock in Jintan salt cavern as a potential site for UHS from aspects such as geological storage, stability, and cavern densities. The salt layer of the Jintan salt cavern has good sealing properties, meeting the requirements for constructing SCHS. Fang et al. [51] analyzed the benefits of the integrated hydrogen production-storage-use scheme under the multifunctional SCHS for the Anning salt mine in Yunnan Province and the power structure of Yunnan Province. Zhu et al. [44] used the entropy weighting method to calculate the weights of evaluation indexes and evaluated the suitability of hydrogen storage in salt caverns based on Topsis. Bai et al. [12] employed the coupling of hierarchical analysis and the entropy weighting method to carry out energy storage planning for 20 salt mines in China, selecting three locations as potential sites for hydrogen storage in salt caverns. Li et al. [52] used numerical simulations to evaluate the stability of hydrogen storage in salt caverns with different burial depths and optimized the key operating parameters. Additionally, Song et al. [53] investigated the physical properties and flow characteristics of multicomponent gases in UHS systems and evaluated the hydrogen tightness of UHS by simulating different storage strategies and gas components. These studies help assess the feasibility and site selection for hydrogen storage in salt caverns in China and lay the foundation for stable hydrogen storage in salt caverns.

4 Prospects for hydrogen storage in salt caverns in China

4.1 Coupling renewable energy with salt cavern hydrogen storage for power generation systems

The coupling of renewable energy and underground hydrogen energy storage technology is a powerful support for accelerating the realization of China’s “dual-carbon target” and further expanding the scale of China’s renewable energy power generation [33]. SCHS technology is an ideal large-scale energy storage solution for the future, as it has high energy storage density, is green, nonpolluting, and can theoretically realize the whole process of self-cycling. The basic technology roadmap is shown in Fig. 9.

The hydrogen production unit consumes excess power generated due to the seasonal and fluctuating characteristics of renewable energy. During the peak season of renewable energy, electrolysis of water produces hydrogen, creating green hydrogen to manage renewable energy storage peaks. This hydrogen is then pressurized by a hydrogen compressor and stored in a salt cavern. The hydrogen storage unit consists of an underground salt cavern and a hydrogen compressor. The salt cavern is ideal for storing hydrogen, which is a highly permeable gas, due to the dense and low permeability of the salt cavern’s salt layer. Hydrogen stored in the salt caverns can be used in a variety of applications. It can serve as a secondary energy source, supplied to users through fuel cell repowering; as a raw material for chemical production or hydrogen refueling stations for use in hydrogen fuel cell vehicles; or injected into natural gas pipelines to increase the calorific value of the gas.

As there is a high overlap between hydrogen and natural gas in the application field, while salt cavern storage is very mature in the natural gas field, the construction demand of SCHS can be predicted with reference to the natural gas underground reserve. Fig. 10 summarizes the natural gas underground reserve rate (underground storage capacity/annual consumption) in major countries around the world. Countries in Europe and the USA have overall high natural gas underground reserve ratios, all exceeding 15%, with most around 20%–30%. The underground reserve rate in Austria and Hungary is more than 100%. China’s underground storage construction development is relatively late, as of 2021 China’s natural gas reserve rate is only 4%. The 2018 NDRC and the NEA issued the “Opinions on accelerating the construction of gas storage facilities and improving the Market mechanism for gas storage and peaking auxiliary services,” which requires that the gas supply enterprises have a gas storage capacity of no less than 10% of their annual contracted sales volume. Based on this and with reference to the fact that China’s built salt cavern natural gas storage capacity accounts for 20% of the total storage capacity under the ground, China’s SCHS capacity demand will reach 260.6 × 10⁴ tons in 2060 [55].

4.2 Hydrogen storage opportunities in Chinese salt caverns

In recent years, a series of national, provincial, and local policies have been issued to support the development of the hydrogen energy industry and promote the realization of the “dual-carbon” goal [56, 57]. The white paper “China’s Energy Development in the New Era 2020,” issued by the State Council, emphasizes the importance of accelerating the development of the green hydrogen energy industry. To achieve this goal, it is necessary to accelerate the entire process of the green hydrogen industry chain, including green hydrogen production, storage, transport, and utilization. Additionally, it is essential to strengthen the research, development, and introduction of related technologies and equipment. This indicates that the development of China’s hydrogen energy industry is set to speed up, and the development prospects of hydrogen storage are immense [58].

According to the “White Paper on China’s Hydrogen Energy and Fuel Cell Industry 2019,” it is expected that by 2050, the share of hydrogen energy in all energy sources in China’s energy market will reach 10%. This implies that China will need close to 60 million tons of hydrogen by then and will generate more than 10 trillion yuan of annual economic output for the industry. Currently, China has become the world’s largest hydrogen producer, demonstrating strong hydrogen production capacity with an annual production of about 33 million tons of hydrogen. In March 2022, the Development and Reform Commission and the NEA released the “Medium- and Long-Term Plan for the Development of Hydrogen Energy Industry (2021–2035),” which formally defines the role and status of hydrogen energy in China’s energy structure. For the first time, it also presents a master plan for the large-scale application of hydrogen storage, guiding the development of China’s hydrogen energy industry in a clear direction.

Furthermore, countries such as the USA, the UK, and Germany have earlier research and more practical experience in the construction, operation, and comprehensive use of SCHS reservoirs, promoting high-quality development in this field. On 23 August 2021, SINOPEC established an innovative hydrogen refueling station in Chongqing, utilizing hydrogen storage well technology to store hydrogen approximately 150 m below ground level. Capable of supplying 100 kg of hydrogen per day, this major breakthrough has created a new wave in the application of UHS technology in China [59]. In the same year, a compressed air energy storage project was successfully implemented in the Jintan salt cavern of China Salt. According to the national long-term planning and deployment, China will form five major gas storage groups in the Northeast, North China, the middle and lower reaches of the Yangtze River, the Pearl River Delta, and Southwest China in the future, forming an energy reserve network that “protects the key points and radiates to the whole country,” which will strongly guarantee the energy security of the country and contribute to the stable supply of energy. The involvement of these energy companies continues to drive the development of the energy storage industry in China, providing new momentum and opportunities for the development of hydrogen storage in underground salt caverns in China.

5 Technical challenges of hydrogen storage in salt caverns

Compared with natural gas, helium, and compressed air, hydrogen has a high diffusion capacity and a wider explosion limit, and it is also chemically active and can react biochemically with minerals and microorganisms in the salt cavern, which brings many new challenges to the construction of SCHS. This section summarizes three technical challenges around the main concerns and risks of SCHS, which are described below:

5.1 Evaluation of cavern leakage in salt caverns

The sealing of the cavern must be closely monitored during the construction of the salt cavern to ensure the normal use and operation of the reservoir, so the sealing test is very important, and it is the key to assess the sealing of the cavern [60]. Wellbore tubing, cementing quality, salt cavern pressure, and sealing of the cap and interlayer are the main factors affecting the sealing of the gas reservoir. After research, it is found that there are three locations where leakage occurs in the salt cavern as follows: (i) leakage of wellbore tubing column and fittings; (ii) leakage of production casing shoe; and (iii) leakage of the cavern. There are many causative factors that cause leakage in the above three locations, such as geological factors, engineering factors, process factors, management factors, etc. The evaluation of factors affecting the sealing of salt cavern caverns is shown in Table 5.

Hydrogen has a small molecular weight, low viscosity, and high diffusion capacity, and can pass through rubber and latex tubing at high temperature and through metals such as palladium, nickel, and steel at high temperatures. Compared to natural gas and high-pressure air, which are usually stored in salt cavern reservoirs, hydrogen is more prone to leakage and requires a higher degree of salt cavern containment [61].

5.2 Wellbore integrity testing and evaluation

As shown in Fig. 11, wellbore integrity testing is used to establish a comprehensive and accurate understanding of the wellbore’s integrity by identifying the specific locations and types of tool failures, such as those affecting casing, injection tubing, and packers [62, 63]. The types of integrity failures are particularly diverse, with common issues including corrosion of the tubing column, perforation, cracking, debonding, wireline seal failures, and packer seating failures, as depicted in Fig. 11. Therefore, wellbore integrity testing must be capable of locating and identifying leaks and must remain unaffected by gases and corrosive fluids.

Hydrogen is colorless and odorless, and unlike methane, its flame is difficult to observe with the naked eye during the daytime after combustion, necessitating the use of detection tools. Currently, mainstream detection tools are primarily used in laboratories, making them costly and not widely adopted in industry. A summary and evaluation of deep gas well production tubing integrity testing technology reveal that common wellbore integrity testing methods include the pressure balance method, electromagnetic corrosion detection, downhole acoustic wave and temperature logging, distributed fiber optics, mechanical seat sealing test pressure, and micro-temperature difference detection (shown in Table 6). Each method has its advantages. Electromagnetic corrosion detection is effective only for large aperture leakage points, while mechanical sealing test pressure necessitates section-by-section testing, which is unsuitable for deep wells. Pre-installed distributed optical fibers can provide real-time monitoring of gas well dynamics. Acoustic detection is sensitive to noise interference near the well bottom and surface, with its efficacy determined by the sensitivity of the micro-temperature differential logging sensor and the degree of leakage. The effectiveness of pressure balance inversion algorithms and isotope tracing is primarily influenced by the applied mathematical methods. Currently, isotope tracing, cross-section flow detection, and spiral logging have limited applications [64]. Relying on a single method cannot clearly determine the main cause of wellbore tubular column failure. However, integrating downhole acoustic wave and temperature logging technology with distributed fiber-optic detection technology can effectively detect wellbore integrity [65].

In addition, one of the most significant challenges currently facing wellbores is hydrogen corrosion. This phenomenon has the potential to significantly impair the durability of wellbore materials, leading to a range of detrimental effects, including hydrogen blistering, hydrogen embrittlement, and hydrogen cracking of steel. Molecules such as stored hydrogen (H₂) or generated hydrogen sulfide (H₂S) decompose and react on the surface of the material, forming hydrogen atoms which then chemisorb onto the metal surface. In this atomic form, hydrogen can accumulate at the location of defects beneath the metal surface. This accumulation results in the generation of significant internal pressure in the vicinity of the defects, leading to plastic deformation [66]. Cementing plays an essential role in maintaining the integrity of the well. Reitenbach et al. [58] conducted a comprehensive investigation into the durability, corrosion, and environmental risks associated with microbial metabolism of the materials utilized for well completion. Furthermore, hydrogen penetration through the cement ring represents a significant potential risk. The mechanical strength of cement is reduced when it is exposed to extreme loading conditions due to corrosion caused by pressure, thermal expansion, and volume changes. Kutchko et al. [67] examined the pattern of change in cement under the influence of acid gases (H₂S–CO₂) and pure CO₂ under simulated reservoir conditions. The findings revealed that temperature and pH had a significant impact on the porosity and permeability of the cement. The chemical degradation of cement by CO₂ is referred to as carbonation. In the context of UHS, the carbonation process is contingent upon the quantity of CO₂ present in the rock minerals and formation fluids. If carbonation persists, calcium carbonate undergoes conversion to calcium bicarbonate [Ca(HCO₃)₂] [68], as demonstrated by Teodoriu et al. [69], which is a water-soluble product that results in a reduction in cement strength.

5.3 Biochemical reactions in salt caverns

SCHS should take microbial growth in the subsurface into stability evaluation. UHS may promote hydrogen-consuming bacterial and archaeal growth [70], thereby reducing the gas storage capacity of salt rocks [71]. The three main types of microbial reactions that have a large impact on the hydrogen storage process are the following: sulfate reduction reactions that produce H₂S after consuming hydrogen [72], the conversion of CO₂ to CH₄ by oxidizing hydrogen [62], and the reaction of hydrogen consumption by iron-reducing bacteria [73]. The main side effects brought by underground microbial reactions [22] are (i) the metabolism of microorganisms leads to a decrease in hydrogen content; (ii) active microorganisms reduce sulfate to generate H₂S gas, causing steel corrosion; (iii) microorganisms cause mineral precipitation, which reduces the pore space of the underground reservoir, leading to a decrease in permeability and a decrease in the injection capacity; (iv) the microbial reaction products may diffuse along with the geological cracks or the cap layer and thus cause leakage.

This phenomenon has been studied by many scholars: Gregory et al. [74] have shown that hydrogen produced by both biotic and abiotic interactions can be consumed by microorganisms. In addition to indigenous subsurface communities, exotic microorganisms can be introduced from surface gases or drilling fluids during storage. Microorganisms have been implicated in the hydrogen cycle of consumption, production, and corrosion. There are a number of recurring microorganisms that are considered to be major hydrogen consumers, such as methanogenic bacteria, sulfate-reducing bacteria, and acetic acid bacteria. The loss of hydrogen is primarily attributable to microbiological reactions that facilitate the conversion of H₂ into gases such as CH₄ or H₂S. In the Underground SunStorage project in Austria, the microbial activity resulted in the loss of 3% of the hydrogen. In the Beynes project in France, the reduction of hydrogen (H₂) was observed to be 17% over a period of 7 months. However, the concomitant increase in methane (CH₄) indicates that H₂ was converted to CH₄. While the rise in methane is advantageous in terms of calorific value, it is not aligned with the green and low-carbon concept over the long term and introduces some uncontrollable variables. Conversely, H₂S produced by microorganisms also presents a risk for hydrogen storage. As the density of microorganisms increases, the formation of biofilms or mineral precipitates may result in the obstruction of pores, thereby reducing the capacity for hydrogen injection. Berta et al. [75] found that during the process of sulfate reduction and acetate generation, hydrogen was consumed faster but no methane was produced, and the rate of reaction was not related to the partial pressure of hydrogen, and the consumption of hydrogen only stopped when the concentration of brine was increased to more than 35 g/L, indicating that certain microorganisms would be inhibited from growing in a high salt environment. High pressure also had an effect on microbial metabolism. These studies suggest that parameters such as temperature, brine concentration, and pressure can be used to control microbial growth. Although it has been shown that some methanogenic bacteria are extremely salt tolerant and can survive in brines [76]. However, the lack of CO₂, a reactant in the salt caverns, also limits the activity of methanogenic bacteria when storing high-purity hydrogen.

6 Conclusions and outlook

Large-scale hydrogen storage can help mitigate major issues with renewable energy generation, such as intermittency, seasonality, and geographical constraints. It meets the demand for hydrogen and promotes the hydrogen economy to achieve the green goal of dual carbon emission reduction. This work presents relevant hydrogen storage technologies and the current development of renewable energy in China. Through various analyses and comparisons, salt caverns have great prospects in the direction of large-scale UHS. More and more countries are investing heavily globally, while China is still in the beginning stage of large-scale SCHS projects. Considering the current status of hydrogen energy industry and policy in China and the demand for SCHS, the scientific and technological challenges of SCHS in China are reviewed and the following conclusions are obtained:

• i. The rapid development of renewable energy in China provides excellent potential for hydrogen production. The hydrogen energy industry is also growing rapidly. The amount of hydrogen production in 2060 will grow from 3781 × 10⁴ tons in 2022 to 13 030 × 10⁴ tons. At present, hydrogen production in China is mainly based on hydrogen production from coal, and its main use is as a chemical raw material for ammonia synthesis, methanol production, and oil refining. In the future, the application scenarios of hydrogen energy will gradually shift to various fields such as metallurgy, transport, construction, and electric power. Large-scale hydrogen production, storage, use, and power generation through renewable energy sources can greatly promote China’s green energy development and energy structure adjustment, and improve China’s energy security. It can also lead to the rapid development and technological progress of advanced manufacturing industries. Underground hydrogen energy storage can support and match China’s future demand for increased renewable energy capacity.

• ii. Multidimensional studies show that salt caverns are extremely suitable for hydrogen storage. Foreign salt cavern construction started earlier and has built a small number of small-scale SCHS reservoirs. In contrast, China’s salt cavern storage construction began later. However, after 20 years of development, China has put into production five salt cavern natural gas storage facilities and two salt cavern pressure gas storage power stations, laying a solid foundation for the construction of SCHS projects.

• iii. Hydrogen has strong permeability and high activity, posing significant challenges for the construction of SCHS. Currently, there are three primary challenges in this field: hydrogen leakage in salt caverns, control of wellbore integrity of SCHS reservoirs, and biochemical reaction in salt caverns. Addressing these key scientific issues will provide the theoretical foundation and technological support necessary for the development of large-scale SCHS in China.

Currently, SCHS research is still in its initial stages. China should leverage the momentum of global energy development and changes to advance the theoretical and experimental research on SCHS as soon as possible. By capitalizing on the rapid development trend of the hydrogen energy industry, China can accelerate the demonstrative construction of SCHS facilities, fill technical gaps both domestically and internationally, occupy the forefront of science and technology, and lead the development direction of large-scale geological hydrogen storage research.

Author contributions

Weizheng Bai (Methodology [Supporting], Writing—original draft [Lead]), Xilin Shi (Funding acquisition [Lead], Writing—review & editing [Lead]), Shijie Zhu (Conceptualization [Equal], Methodology [Equal]), Xinxing Wei (Methodology [Equal], Supervision [Equal]), and Yashuai Huang (Methodology [Equal], Supervision [Equal])

Conflict of interest statement

None declared.

Funding

This work was supported by the Excellent Young Scientists Fund Program of the National Natural Science Foundation of China (No. 52122403); Natural Science Foundation of Wuhan (No. 2024040701010062); National Natural Science Foundation of China (No. 52374069); and Youth Innovation Promotion Association CAS (No. Y2023089).

Data Availability

No new data were generated or analyzed in support of this research.

References