This paper presents a study and experimentation of a hybrid energy storage system prototype, based on Compressed Air, where the charging and discharging is done within maximum efficiency conditions of the volumetric machine. As the 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 on the calculated energetic efficiency performance, as well as measured on a first prototype system.