Energy storage technologies are playing an essential role in recent decades due to the increased penetration of renewable power. Compressed air energy storage (CAES) is among the most promising energy storage technologies. However, conventional CAES system has two main drawbacks, i.e. the heat loss during air compressing and variable working condition during expansion caused by air pressure drop. In this paper, a combined air and hydro energy storage (CAHES) system is proposed and simulated. The system realizes isothermal compression during charging through enhanced heat transfer in an external heat exchanger, and reduces air pressure drop during expansion through heat compensation. Simulation results show that the exergy efficiency of a typical CAHES system reaches 65.3 %, which is significantly higher than a conventional CAES system. Two major influencing factors on system efficiency and energy density are compression volume ratio and allowed pressure drop during expansion.
