Exergy analysis of isochoric and isobaric adiabatic compressed air energy storage

Adiabatic Compressed Air Energy Storage (ACAES) is an energy storage technology that has the potential to play an important role in the transition to a predominantly renewables-driven net zero energy system. However, the technology has not yet achieved the performance necessary to be widely deployed. One reason is the difficulty in system operation with variable pressure air storage, hence isobaric air storage has been proposed. In this paper, we undertake an exergy analysis of isobaric and isochoric ACAES systems. We are able to track lost work through the system and explore the influences of different design choices. We model two systems in different configurations - one with 3 compression and 3 expansion stages and one with 4 compression and 2 expansion stages - considering both isobaric and isochoric air storage modes for each. Our results illustrate that isobaric systems are likely to have higher round trip efficiency and energy density, at the cost of achieving isobaric storage. The difference between isobaric and isochoric operation is larger for systems with fewer compression stages, while maintaining a higher isentropic turbine efficiency is more important for isobaric systems and systems with fewer expansion stages.