Exploring compressed air energy storage in abandoned flooded coal mine: Thermodynamic analysis and applicability study

Global renewable energy generation has been steadily increasing, necessitating commensurate expansion in energy storage infrastructure. Utilizing abandoned coal mines fo compressed air energy storage (CAES) presents a promising solution. Considering the widespread occurrence of high water levels in southern China's coal mines, a novel flooded coal mine roadway compressed air energy storage (FM-CAES) system is proposed. This system leverages water pressure to maintain constant air pressure, thereby enhancing efficiency and minimizing leakage. The influence of key input parameters on the FM-CAES system efficiency is comprehensively analyzed, with particular focus on average air leakage rate (AALR) and operational performance under varying roadway depths and permeability conditions. The results indicate that the thermal efficiency of the FM-CAES system reaches 71.53%, with an energy storage density (ESD) of 29.72 MJ/m 3 . Increased water pressure within the roadways improves efficiency and extends charge/discharge durations, while airflow rate has minimal impact. Improved compressor isentropic efficiency further boosts the system performance. Deeper roadways exhibit lower AALR and reduced efficiency during initial storage phases, though efficiencies converge across depths in later stages. The ESD increases by approximately 4.3-6.7 MJ/m 3 for every 100-m increase in depth. Critical permeability thresholds are identified at 1×10 -17 m 2 for surrounding rock and 1×10 -19 m 2 for limited-thickness linings, below which the AALR stabilizes. Compared to conventional constant-volume CAES systems in coal mines, the FM-CAES system, which utilizes the constant pressure of floodwater, exhibits an approximate 9% improvement in efficiency, along with significantly enhanced ESD and discharge depth. • A novel FM-CAES system using flooded coal mines is thermodynamically assessed. • Air leakage and efficiency are analyzed under varying rock and lining permeabilities. • FM-CAES offers higher energy density and air confinement than constant-volume CAES. • FM-CAES shows stable efficiency with low air leakage after long-term storage. • Abandoned mines facilitate green energy storage, aiding renewable grid integration.