Coordinated control of a pre-expander and bypass to reduce throttling losses during compressed air energy storage discharge

To reduce throttling losses at the expander inlet during sliding-pressure discharge in compressed air energy storage (CAES) systems, this study proposes a novel expander configuration that incorporates an upstream pre-expander (T0) and a bypass branch. In the proposed configuration, stored compression heat is used to preheat high-pressure air before pre-expansion in T0, enabling T0 to convert part of the pressure drop that would otherwise be dissipated by throttling into useful work. The bypass branch provides an additional flow-regulation path to maintain feasible main-expander operation as cavern pressure decreases. A large-scale expander unit is adopted as the case study, and a discharge thermodynamic model coupled with operating-parameter optimization and discharge-side exergy evaluation is developed. Two modes are compared: Mode A treats T0 output as additional recovered power, whereas Mode B fixes the total expander output for dispatch-following operation. The results show that the optimized strategy exhibits three phases: throttling matching with the bypass closed, partial-bypass operation with progressive bypass opening, and full-bypass operation with T0 shut down. At the reference power of 700 MW, the proposed configuration increases total delivered energy by 6.48% in Mode A and 5.84% in Mode B, and raises air-side discharge exergy efficiency from 84.87% to 89.46% and 88.90%, respectively. Over the power-demand range of 400–800 MW, the relative energy gain decreases with increasing demand, and the difference between the two modes narrows. These results provide practical guidance for efficient, dispatch-oriented CAES discharge operation.