Deep Underground Science and Engineering (DUSE) is pleased to present this special issue highlighting recent advancements in underground large-scale energy storage technologies. This issue comprises 19 articles: six from our special issue “Underground large-scale energy storage technologies in the context of carbon neutrality”, 11 from regular submissions on related topics, and two from early regular submissions. These contributions include five review articles, one perspective article, and 13 research articles. The increased volume of this issue and later issues reflects DUSE's commitment to addressing the rapid growth in submissions and the current backlog of high-quality papers.

Global energy demand has experienced steady growth in recent decades. While renewable energy capacity has expanded exponentially, fossil fuels remain the dominant energy source, currently accounting for approximately 80% of global primary energy consumption. In major economies, renewables now contribute over 20% of the energy mix. The dual challenges of meeting rising energy demand and reducing greenhouse gas emissions are driving the global transition toward renewable sources. However, the inherent intermittency and instability of renewables—such as solar, wind, marine, geothermal, and biomass energy—create a dynamic imbalance between energy supply and demand. To address this, technologies like power-to-gas, power-to-liquids, and solar-to-fuel have been developed. Among these, energy storage serves as a critical buffer. Energy storage technologies facilitate the spatiotemporal redistribution of energy, maintaining a dynamic balance between generation and demand. This enables higher penetration of renewable sources, improves power system stability, enhances overall efficiency, and reduces the environmental impact of energy generation.

Large-scale energy storage is essential for a better integration of renewable sources, balancing supply and demand, enhancing energy security, improving grid management, and advancing the transition to a low-carbon economy. Underground reservoirs offer significant potential for storing large volumes of fluids with minimal environmental and societal impact, thereby making large-scale energy storage feasible. Viable underground energy storage technologies include compressed air energy storage (CAES), underground pumped hydro storage (UPHS), underground thermal energy storage (UTES), underground gas storage (UGS), and underground hydrogen storage (UHS). These technologies require suitable geological formations, such depleted hydrocarbon reservoirs, porous aquifers, salt formations, engineered rock caverns, and abandoned mines, each forming distinct production chains.

A perspective article entitled “A novel technological conception of integrated large-scale CO2 storage, water recovery, geothermal extraction, hydrogen production, and energy storage” (DOI: 10.1002/dug2.70055) discusses the integration of underground energy storages with carbon capture, utilization, and storage (CCUS) production chains. The article proposes an integration system through three interconnected modules: (1) upstream CO2-enhanced water recovery (CO2-EWR), (2) midstream green hydrogen synthesis, and (3) downstream energy utilization. This represents a novel technological concept for underground energy storage via CO2 reutilization.

The selection of an appropriate underground energy storage technology depends on the site-specific screening. Technical criteria must be established to evaluate the feasibility, safety, and economic viability for each reservoir type and technology. A review article entitled “Critical technologies in the construction of underground artificial chamber for compressed air energy storage systems” (DOI: 10.1002/dug2.70064) provides examples of such assessments. Different energy storage facilities face distinct challenges. For instance, the review “Gels for CO2 geo-storage and conformance control: A systemic review of behavior and performance” (DOI: 10.1002/dug2.70027) addresses sealing issues in CO2 geo-storage. Another review examines challenges, models, and algorithms for flow and transport simulations in deep reservoirs (DOI: 10.1002/dug2.70006). Further reviews discuss the application of artificial intelligence in geothermal energy extraction from hot dry rock (DOI: 10.1002/dug2.70018) and outline geothermal resource exploration and development in North Africa (DOI: 10.1002/dug2.70042). Collectively, these reviews synthesize key knowledge points in underground energy storage.

Seepage and diffusion: The article “Study on the diffusion and migration law of CO2 sequestrated in abandoned coal mine goaf” (DOI: 10.1002/dug2.70002) investigated CO2 adsorption characteristics in residual coal via isothermal adsorption tests, enabling accurate calculation of adsorbed CO2 volumes. The research also examines CO2 diffusion behavior in the coal matrix and observes preferential flow, providing an experimental and theoretical foundation for assessing CO2 storage capacity in coal goafs.

Cavern leakage safety: Leakage is a critical safety concern for energy storage caverns. The article “Leakage risk assessment system for salt cavern hydrogen storage” (DOI: 10.1002/dug2.70011) comprehensively evaluates leakage risk, considering the unique structural aspects of salt caverns and the inherent challenges of hydrogen storage, and proposes corresponding risk control and preventive measures.

Bentonite performance: In the context of deep geological disposal of high-level radioactive wastes, bentonite serves as a key backfilling buffer material. The study “Elevated temperature effects on swelling pressure of compacted Bentonite” (DOI: 10.1002/dug2.12145) experimentally investigates the performance of compacted bentonite under elevated temperature. Results indicate that increasing temperature reduces swelling pressure, accompanied by pore water exchange within the clay's microstructure. Water transfer from micropores to macropores is identified as a key controlling process in high-density compacted bentonite, influencing both swelling pressure and hydraulic conductivity.

CO2 sequestration and reutilization: This topic is represented by four articles, including the aforementioned study on CO2 diffusion in coal goafs. Other contributions include a simulation of CO2-water two-phase fluid displacement using the phase field method (DOI: 10.1002/dug2.70019), an evaluation of CO2 storage potential in CO2-enhanced oil recovery in Subei Basin, China (DOI: 10.1002/dug2.12150), and a thermal-hydraulic-mechanical coupling analysis of potential well damage induced by CO2 injection (DOI: 10.1002/dug2.70014). These studies address flow mechanisms, induced damage, and instability across various production chains in porous media.

CAES in LRC caverns: This section includes three research articles. One investigates the stability of a CAES cavern converted from a horseshoe-shaped roadway in an abandoned coal mine, exploring initial damage effects and shape transformation (DOI: 10.1002/dug2.70041). Another presents an elastoplastic analysis of surrounding rock from excavation to CAES operations (DOI: 10.1002/dug2.70062), proposing a novel approach for lining material selection and structural design. A third study examines the impact of damage on the stability and airtightness of lined rock caverns for CAES (DOI: 10.1002/dug2.70066), exploring the consistency among CAES structure, rock mass and operation parameters.

Salt cavern storage: Salt caverns, valued for their large volume and natural sealing capability, are suitable for both CAES and hydrogen storage. Three articles address associated challenges. One studies H2S generation due to thermochemical sulfate reduction during hydrogen storage in depleted gas reservoirs and explores mitigation measures (DOI: 10.1002/dug2.70000). Another investigates the feasibility of using insoluble sediment as a thermal storage medium in a CAES-thermal storage coupled system within salt caverns (DOI: 10.1002/dug2.70056).

The remaining two research articles, “Ground vibration isolation using mass scatters” (DOI: 10.1002/dug2.12130) and “Study on damage and failure characteristics of loaded gas-bearing rock-coal-rock combination structures” (DOI: 10.1002/dug2.12129), report on technical developments in ground vibration isolation and explore the failure mechanisms of composite rock structures.

DUSE highly appreciated the efforts of the Guest Editors for the success of this Special Issue. The Guest Editors are Dr. Jifang Wan (Deep Underground Technology Center, China Energy Digital Technology Group Co., Ltd., China), Associate Professor Tao Meng (Taiyuan University of Science and Technology, China), Professor Wei Liu (Chongqing University, China), Professor Xilin Shi (Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, China), Professor Maria Jose Jurado (Geosciences Barcelona CSIC, Spanish National Research Council, Spain), and Dr. Reza Taherdangkoo (Institute of Geotechnics, Technische Universität Bergakademie Freiberg, Germany).

The advancements presented in this issue offer valuable insights into cutting-edge research directions. It is acknowledged that several important technologies—such as compressed air energy storage in aquifers, hydrogen storage in LRC cavern, and hybrid systems combining CAES with hydro-pumped storage—are not covered in this collection. The Editors aim to broaden the journal's coverage of deep underground energy storage, encompassing conceptual technology roadmaps, site screening, fundamental scientific research, new material development, engineering implementation, and operation management, thereby supporting the development of integrated production chains for each technology. The publications in this issue provide a foundation for further innovation. The Editors cordially invite researchers and industry professionals to advance this critical field through continuous high-quality contributions, ultimately promoting safer and more sustainable deep underground energy storage practices worldwide.