This paper presents a hybrid energy storage system based on Compressed Air Energy Storage (CAES), where the charging and discharging is done within maximum efficiency conditions. As the maximum efficiency conditions impose the level of 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, the power-electronic interface circuits and especially the Maximum Efficiency Point Tracking (MEPT) algorithm and the strategy used to vary the output power. In addition, the paper will present the characteristics of a high efficiency storage device where the pure pneumatic machine is replaced by an oil-hydraulics and pneumatics converter, used under isothermal conditions. Practical results are also presented, recorded from a low-power pneumatic motor coupled to a small DC generator, as well as from a first prototype of the final hydraulic/pneumatic system.