Energy storage system

An energy storage system, which comprises a vessel in which a first rotation shaft is installed, to which a cable is connected from which a weight hangs, mechanically coupled to a second rotation shaft of a motor-generator assembly connected to a power grid in such a manner that, by winding and unwinding the cable around the first shaft, the weight can alternate between an upper position and a lower position, a path of the weight between the upper position and the lower position being immersed in water, wherein the energy storage system is configured that:

• when there is a surplus of electrical energy in the power grid, the motor-generator consumes energy, operating as a motor to rotate the first shaft in a winding direction of the cable to hoist the weight in the direction towards the upper position and,
• when there is a shortage of electrical energy in the power grid, the motor-generator supplies energy, operating as a generator utilising the rotation of the first shaft in an unwinding direction of the cable that occurs when a free fall of the weight is allowed in the direction towards the lower position,
• wherein
• the weight comprises a ballast, a water reservoir, and a compressed-air reservoir, the energy storage system being configured such that:
• when the motor-generator operates as a motor to consume power causing the weight to rise in the direction towards the upper position, water is drained from the water reservoir by feeding compressed air coming from the compressed-air reservoir to increase the buoyancy of the weight;
• and
• when the motor-generator operates as a generator to generate power allowing the weight to fall in the direction towards the lower position, the water reservoir (22) is filled with water to reduce the buoyancy of the weight.