In addition to the high-capacity storage facilities based on hydropower technologies, electrochemical solutions are today's candidate for storage for renewable energy sources support. However, sustainability and limited life cycles of batteries are often inhibiting factors. This paper presents a hybrid energy storage system with high life cycle, which is mainly based on compressed air, where the storage and discharge are 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 mean power. A smoothly variable output power is achieved with the help of a supercapacitive auxiliary storage device used as a filter. This 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, this paper presents the characteristics of high-efficiency storage systems where the pneumatic machine is replaced by an oil-hydraulic and pneumatic converter, which is used under isothermal conditions. Practical results are also presented, which are recorded from a low-power pneumatic motor coupled to a small dc generator as well as from the first prototype of the final hydropneumatic system.
