An energy storage system (1), which comprises a weight (2) hanging from a cable (3, 10) connected to a first rotation shaft (5) mechanically coupled to a second rotation shaft (6) of a motor-generator assembly (7) connected to the power grid. When there is a surplus of electrical energy in the grid, the motor-generator (7) consumes energy, operating as a motor to rotate the first shaft (5) in a winding direction of the cable (3, 10), whereas, when there is a shortage of electrical energy in the grid, the motor-generator (7) supplies energy, operating as a generator utilising the rotation of the first shaft (5) in an unwinding direction of the cable (3, 10) when the free fall of the weight (2) is allowed. When the weight (3) is displaced immersed in water, in addition a compressed-air system can be utilised to modify the buoyancy of the weight.

System for generating electricity from a fluid, the device comprising a wing profile structure comprising a first side configured to generate a low pressure and a second side configured to generate a high pressure from the fluid passing along the wing profile; wherein the first side comprises at least one first aperture; wherein the second side comprises at least one second aperture; wherein between the at least one first aperture and the at least one second aperture, a fluid connection is provided through the wing profile structure; wherein between the at least one first aperture and the at least one second aperture an electricity generating device is positioned such that fluid flowing from the second high pressure side to the first low pressure side through the fluid connection passes the electricity generating device for generating electricity from the passing fluid.

An energy storage system (1), which comprises a weight (2) hanging from a cable (3, 10) connected to a first rotation shaft (5) mechanically coupled to a second rotation shaft (6) of a motor-generator assembly (7) connected to the power grid. When there is a surplus of electrical energy in the grid, the motor-generator (7) consumes energy, operating as a motor to rotate the first shaft (5) in a winding direction of the cable (3, 10), whereas, when there is a shortage of electrical energy in the grid, the motor-generator (7) supplies energy, operating as a generator utilising the rotation of the first shaft (5) in an unwinding direction of the cable (3, 10) when the free fall of the weight (2) is allowed. When the weight (3) is displaced immersed in water, in addition a compressed-air system can be utilised to modify the buoyancy of the weight.