A conceptual thermo-hydraulic framework and techno-economic optimization of a hybrid compressed air–pumped hydro–gravity energy storage system for utility-scale solar integration

This study develops an integrated thermo-hydraulic framework for the design, performance assessment, and optimization of a hybrid Compressed Air–Pumped Hydro–Gravity Energy Storage (CAPHGES) system. The proposed framework simultaneously models gas compression and expansion processes, hydraulic interactions within the water column, and the motion of the gravity piston, enabling a comprehensive evaluation of system performance from thermodynamic, hydraulic, operational, and economic perspectives. A utility-scale 2.5 MW solar photovoltaic power plant in Iraq is considered as a representative case study to demonstrate the applicability of the proposed methodology. The effects of six key design variables, including well diameter, well height, piston height, initial storage pressure, booster pressure ratio, and water-to-air volume ratio, are systematically investigated. Comprehensive single-objective and multi-objective optimizations are then performed to minimize the levelized cost of storage (LCOS), maximize volumetric energy density (VED), and improve round-trip efficiency (RTE). The selected multi-objective Pareto design, consisting of a 2.00 m well diameter, 300.0 m well height, 100.0 m piston height, 14.93 MPa initial preset pressure, 1.83 water-to-air volume ratio, and 1.50 booster pressure ratio, achieves an RTE of 70.32%, a VED of 1.458 kWh/m 3 , and an LCOS of 0.658 CNY/kWh. Compared with the baseline design, the optimized system reduces LCOS by 17.3% while increasing VED by 228.5%. Parametric analysis reveals that the booster pressure ratio is the most influential design variable, significantly improving RTE, VED, and LCOS, while the initial preset pressure primarily affects system compactness at the expense of efficiency and cost. Sensitivity analysis further reveals that the electricity tariff is the most influential economic parameter, followed by the discount rate and O&M ratio, while construction costs and system lifetime have relatively minor impacts. Uncertainty analysis using Monte Carlo simulation (1000 runs) confirms that LCOS remains within a competitive range (0.573–0.747 CNY/kWh) under typical uncertainties, with a mean value of 0.660 CNY/kWh. The results provide practical design guidelines for developing compact and cost-effective long-duration hybrid energy storage systems for large-scale renewable energy integration.