This paper presents the study and experimentation of a hybrid energy storage system based on Compressed Air, where the storage and discharge are done within maximum efficiency conditions for the volumetric machine. As these maximum efficiency conditions impose the level of the converted power, an intermittent time-modulated operation mode is applied to the thermodynamic converter to obtain a variable converted power. A smoothly variable output power is achieved with the help of a supercapacitive auxiliary storage device used as a filter. The paper describes the concept of the system and reports the theoretical evaluation of its energetic efficiency, as well as the measured performances form a first experimental prototype.