NOVELTY - The system (1) has a reservoir (200) connected to a storage tank (100), for storing fluid i.e. water (W) and exchanging the stored fluid with the storage tank. A path transfers the stored fluid between the tank and the reservoir. A pump (410) injects the stored fluid into the tank and compresses air (A) in the tank. A generator (420) generates electricity using the injected fluid transferred from the tank to the reservoir. A set of valves (151, 153) controls flows of the stored fluid between the tank and reservoir. A spray device (300) sprays the stored fluid into the tank.    USE - Energy storage system for a power generation system. Uses include but are not limited to a thermoelectric power plant, hydroelectric power plant, solar photovoltaic system, solar thermal generation system, wind power plant, tidal power plant and a wave-power plant.    ADVANTAGE - The system is installed in small scale relative to pumped storage power plants for providing less thermal energy loss and effectively restraining decrease of power generation efficiency due to drop in air temperature, while generating power. The system can effectively smoothen outputs of newly constructed renewable energy generation facilities i.e. wind-power generation facilities and improve living environments of residents nearby. The system provides compression ratio to be uniformly maintained when temperature of the storage tank is maintained as being uniform, so that energy storage capacity can be kept consistent regardless of the season. The system uses a compressor for compressing and recompressing the air by pumping water, so that the system provides improved compression efficiency and generation efficiency relative to compressed air energy storage systems. The system provides expansion of the compressed air that goes through thermodynamic process similar to isothermal expansion process, so that phenomenon of rapidly decreasing power generation efficiency due to expansion of the air and the decrease of the temperature can be effectively prevented. The system provides large amount of water that is stored in the storage tank when variance in temperature is small, thus effectively reducing heat dissipation of a part of stored energy.    DETAILED DESCRIPTION - The reservoir is a lake. The path is a first flow path (150), second flow path (152), third flow path (154), air injection path (510) and an air discharge path (520). An INDEPENDENT CLAIM is also included for a method for storing and recovering energy.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of an energy storage system.    Air (A)    Water (W)    Energy storage system (1)    Storage tank (100)    Flow paths (150, 152, 154)    Valves (151, 153, 155, 157, 511, 522)    Reservoir (200)    Spray device (300)    Pump (410)    Generator (420)    Air injection path (510)    Air discharge path (520)