Coordinated operational optimization of a variable-speed pre-expander and bypass for compressed air energy storage

The discharge process of compressed air energy storage (CAES) is challenged by inlet throttling loss and off-design mismatch caused by continuously declining cavern pressure. To address this issue, a coordinated operational optimization integrating a variable-speed pre-expander with a bypass is proposed. CFD-derived performance maps for the pre-expander and four main-expander sections are coupled with an air-side thermodynamic model and assessed under three aerodynamic-power-constrained discharge modes: fixed main-expander power (Mode A), fixed total unit power (Mode B), and sliding-pressure operation with a minimum power constraint (Mode C). Relative to matched main-expander-only baselines, the constant-speed pre-expander–bypass configuration increases cumulative aerodynamic work by 6.54%, 6.05%, and 3.10% in Modes A–C, respectively, while variable-speed optimization raises these gains to 6.69%, 6.27%, and 3.13%. In Modes A and B, the improvement results from additional heating and a 65.65%–79.95% reduction in throttling exergy destruction. In Mode C, it mainly reflects expansion-ratio redistribution and lower flow-dependent pressure losses, yielding an air-side discharge exergy efficiency of about 90.6%. Compared with constant-speed operation, variable-speed operation further increases the ME-referenced cumulative-work gains by 0.15, 0.22, and 0.03 percentage points in Modes A, B, and C, respectively. When the corresponding whole-train work increments are normalized by the cumulative pre-expander outputs, they represent 2.27%, 3.09%, and 0.50%, respectively. A practical two-stage rule—3150 rpm while an upstream throttling margin is available and 3300 rpm after this margin is exhausted—reproduces the stepwise-optimized Mode A and B gains within 0.03 percentage points.