Performance Test of a 9 MW-Class Three-Stage Axial CO2 Compressor

Abstract Performance testing of a three-stage 9 MW axial CO2 compressor has been successfully completed at the closed-loop CO2 compressor test facility at the University of Notre Dame. The design speed, inlet pressure, inlet temperature, and mass flowrate of the compressor were 19,800 rpm, 2.77 MPa, 98 °C, and 125.9 kg/s, respectively. The compressor performance was evaluated from 60% to 100% of the design speed. Each speedline covered operating points from choke to near stall except for at 60% speed, where the compressor was throttled into rotating stall. Blade tip clearance and blade vibrations were measured during the compressor operation to ensure the safety of the compressor. Calibrated capacitance probes were used to establish average running tip clearances of 0.38 mm (1.2% of span), 0.5 mm (1.9% of span), and 0.5 mm (2.4% of span) for stage 1, stage 2, and stage 3, respectively. Blade vibrations were measured using tip timing probes. A six engine-order synchronous blade vibration of the first bending mode was observed at the first stage rotor around 73–82% speed. Stall testing at 60% speed revealed a single stall cell, which extends from the first stage rotor to the third stage rotor, and is rotating at 60.4% of the shaft speed. The compressor stall at 60% caused significant shaft vibration. Hence, the stall line at 70% and higher speeds was extrapolated from stall testing at 60% and lower speed to avoid potential damage of the test rig from stall or surge. The measured peak isentropic efficiency of the compressor was 91.5%, 91.1%, 90.6%, 90.0%, 85.3% at 60%, 70%, 83%, 90%, and 100% speeds, respectively. Considering increases in efficiency with decreases in mass flowrate and that the performance test above 70% was stopped with some safety distance away from the stall point to avoid damage, the peak isentropic efficiency is expected to be above or close to 90% at 100% speed as well. The measured performance successfully demonstrated the performance benefit and usefulness of the axial compressor for grid-scale energy systems such as pumped thermal energy storage (PTES).