Optimized combined cycle compressed air system for large-scale energy storage

Energy storage is a critical enabler for achieving a 100% renewable energy system, yet existing large-scale technologies suffer from limitations in efficiency, cost-effectiveness, scalability, and environmental impact. This paper proposes an optimized isobaric adiabatic Combined Cycle Compressed Air Energy Storage concept that enhances system profitability and reliability. The proposed system, which stores air in a hard rock cavern — affordable due to operation at constant pressure — utilizes large-scale membranes filled with volatile CO 2 as cushion fluid for compressed air, achieving the corresponding phase change through an innovative heat pump cycle, ensuring stable cavern pressure during air injection and extraction. Additionally, an advanced compressor without intercooling is used, which allows for higher power if thermal stresses can be managed, and hard-rock thermal energy storage is integrated to maximize efficiency. The article presents the conceptual design, a thermodynamic analytical model, and a parametric study, evaluating the system’s scalability and efficiency. A comparative techno-economic analysis is also conducted against existing energy storage technologies. The results demonstrate that the proposed approach offers a viable pathway to overcoming current CAES limitations while achieving higher round-trip efficiency and cost-effectiveness. • CAES system fully optimized for hard-rock caverns, including thermal and fluid layout. • Heat pump cycle enables efficient CO 2 phase-change and stable cavern pressure. • Auxiliary loads are minimized via optimized thermal integration and mass ratios. • Constant-pressure operation achieves > 80% round-trip efficiency. • Techno-economic study confirms scalability and financial viability of the concept.