Small-scale adiabatic compressed air energy storage: Control strategy analysis via dynamic modelling

The increasing capacity of variable renewable energy sources fosters the importance of electric energy storage. This paper is focused on exploring Compressed Air Energy Storage (CAES) as a suitable solution for smallmedium size stationary applications. A small-scale Adiabatic Compressed Air Energy Storage system with an artificial air vessel has been analysed and different control strategies have been simulated and compared through a dynamic model in Simcenter AMESim (R), by identifying the most appropriate ones to improve the performance in off-design conditions. The built dynamic model allows simulating the real fluids behaviour, heat exchange, and thermofluid-mechanical inertia of its components. The proposed system is equipped with a three-stage volumetric compressor and a four-stage radial turbine. The use of variable speed compressor and turbine for both charging and discharging processes and the introduction of throttling to improve the compressor working point have been compared. Regarding the variable speed, the advantages resulted negligible for the expansion phase (+0.1% on the generated energy), while they were remarkable in the compression one. An enhancement of about 1% on the roundtrip efficiency was achieved with the variable-speed compressor and this solution demonstrated to better follow a power profile. The throttling valve was introduced to enable the reciprocating compressor to work close to nominal conditions when the pressure of the air vessel is lower than the nominal one. It allowed to strongly reduce the compression energy consumption, reaching an enhancement of about 2.3% on the round-trip efficiency. The combined use of the variable speed and throttling valve in the charging process has proven to be the best solution to reduce the compression energy consumption (-3.3%). The roundtrip efficiency enhancement of this solution (+1.8%) is lower than that obtained with the exclusive use of the throttling valve (+2.3%) because of the presence of the inverter efficiency but, simultaneously, it enables the compression group more flexible and to follow a given power profile.