Multi-Objective Optimization of Expansion Trains in CAES: Incorporating Organic Rankine Cycles for Improved Efficiency

Abstract This paper focuses on the expansion train designed for the compressed air energy storage (CAES) concept under development in the EU-funded ASTERIx-CAESar project. The system integrates concentrated solar thermal energy from a high-temperature (800 °C) volumetric central-receiver into a hybrid storage configuration, combining low-temperature thermal energy storage (LT-TES) with compressed air storage (CAS) and high-temperature thermal energy storage (HT-TES). Electricity from the grid powers compressors during low-price periods, storing compressed air and recovering compression heat in LT-TES. Solar heat is stored in HT-TES. During discharge, preheaters and reheaters supply stored energy to the expansion train. Residual energy in the exhaust of the low-pressure turbine reduces round-trip efficiency; therefore, a bottoming waste heat recovery unit based on organic Rankine cycle (ORC) technology is assessed. Multiple air-cooled configurations are modeled for expander exit temperatures (EET) of 300–600 °C, using organic fluids and steam in subcritical, transcritical, and supercritical layouts. Scale effects on expander type (screw or axial) and isentropic efficiency are considered for capacities from 1 to 100 MWe. A multi-objective optimization of the bottoming cycle considers technical and economic aspects to maximize efficiency and heat recovery by adjusting vapor generator pressure/temperature. A global optimization of the expansion train identifies the optimal cycle configuration for each EET and scale, integrating the waste heat recovery system with a two-stage turbine. Recommendations to improve CAES system efficiency are provided.