Technical feasibility analysis of using axial compressor as turbine for pumped thermal energy storage systems with air and sCO2 working fluids

Pumped thermal energy storage (PTES) can improve renewable-energy integration, but its capital cost is strongly influenced by the utilization of key rotating components. This study examines a high-commonality baseline in which a mature axial-flow compressor is repurposed as a reversible turbomachine by operating it in forward rotation as a compressor and in reverse rotation as a turbine, without changing the main geometry. The National Aeronautics and Space Administration (NASA) Stage 37 axial compressor is selected as a reference configuration. Three-dimensional CFD simulations are performed in ANSYS CFX for both operating modes using air and supercritical CO2 (sCO2) as working fluids under boundary conditions for PTES charging and discharging. The numerical approach is validated against published experimental data for the air-compressor case. Under a self-consistent, feasibility-oriented set of charging and discharging boundary conditions, the coupled operating constraints of a closed-loop cycle—mass-flow continuity, power balance, and inter-component thermodynamic matching—are assessed for combined compressor-turbine operation. The baseline geometry shows severe mismatch: in the charging cycle, the reverse-rotating turbine is choked over a wide back pressure range and its flow capacity exceeds the compressor's stable operating range; in the discharging cycle, the compressor's minimum stable flow rate is higher than the turbine's maximum flow rate. A coordinated control strategy combining adjustable guide vanes (IGV in compressor mode/OGV in turbine mode) and rotational-speed control is therefore evaluated. Adjustable guide vanes can effectively regulate inlet pre-swirl to improve aerodynamic matching and broaden the compressor's stable operating range, reducing residual swirl at the turbine outlet and improving performance. Speed control significantly changes flow capacity and is essential to establish an operable overlap region. A relatively wide feasible operating region can be obtained for the charging cycle, whereas the discharging cycle remains more restrictive. Overall, the present results should be interpreted as a baseline operability assessment of a high-commonality reversible-turbomachinery concept rather than an optimized PTES design recommendation. Further improvement in discharging-mode performance and overall cycle efficiency will require independent hot/cold-side scaling and integrated thermo-aerodynamic optimization.