Performance assessment of Adiabatic Compressed Air Energy Storage (ACAES) power plants integrated with packed-bed thermocline storage systems

Among energy storage technologies, compressed air energy storage (CAES) systems have undergone a real development since the 70s, although only two large-size commercial plants are operating worldwide. CAES systems allow very large energy storage to be performed, accumulating compressed air to be used for electrical energy generation. In recent years, A-CAES (Adiabatic Compressed Air Energy Storage) plants have had an important role. This technology allows the storage of the thermal energy released during air compression to be used for heating the compressed air during electricity generation, avoiding the consumption of fossil fuels. The main objective of this paper is to propose an innovative system solution for large-size A-CAES plants. The proposed configuration is characterized by: (i) a compression train based on two axial compressors constantly operating at design conditions and a centrifugal compressor fully devoted to managing the pressure variation, (ii) a thermocline thermal energy storage (TES) system based on a packed bed of solid material located between the low-pressure and high-pressure compressors, (iii) an expansion train based on a high-pressure radial turbine and a low-pressure axial turbine. TES performance was evaluated with integration with the A-CAES plant through a dedicated numerical simulation model. Operating modes for managing the high-pressure and low-pressure turbines through air throttling and high-pressure turbine bypassing were also studied. Finally, an in-depth analysis of the off-design behaviour of the different A-CAES components was carried out. Globally the A-CAES round trip efficiency exceeds 0.7-0.75.