Energy storage system
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.
1. 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.
2. The energy storage system according to claim 1 , wherein the ballast is located in a lower portion of the weight, the water reservoir is located in an intermediate portion of the weight and the compressed-air reservoir is located in an upper position of the weight.
3. The energy storage system according to claim 1 , wherein the weight further comprises:
a first compressed-air filling opening arranged in the compressed-air reservoir in communication with the outside;
a second exhaust opening arranged in an upper portion of the water reservoir in communication with the outside;
a third water inlet/outlet opening arranged in a lower portion of the water reservoir in communication with the outside; and
a fourth air inlet opening arranged between the compressed-air reservoir and the water reservoir.
4. The energy storage system according to claim 3 , wherein the third water inlet/outlet opening comprises a tipping valve located on an internal side of the water reservoir and equipped with a float so that, when the water reservoir is filled with water, the float keeps the tipping valve open, whereas, when the water reservoir is filled with air, the weight thereof causes the valve to close provided that pressure outside the water reservoir is lower than the pressure inside the water reservoir.
5. The energy storage system according to claim 3 , wherein the fourth air inlet opening comprises a pressure valve configured to open when the pressure in the water reservoir reaches a predetermined value.
6. The energy storage system according to claim 1 , wherein the ballast is a sand-filled reservoir.
7. The energy storage system according to claim 1 , wherein the vessel comprises a hull comprising at least one vertical opening a top end of which is located above a waterline of the vessel, the cable which the weight hangs from passing through the vertical opening.
8. The energy storage system according to claim 7 , wherein a cross section of the vertical opening is larger than a cross section of the weight in such a manner that, in the upper position, an upper portion of the weight is located above the waterline.
9. The energy storage system according to claim 7 , wherein the weight is cylindrical in shape with rounded ends.
10. The energy storage system according to claim 1 , wherein the weight is at least one U-shaped weight, a width of the U-shaped weight being larger than a beam of the vessel, the weight hanging from additional cables connected to ends of arms of the U-shaped weight in such a manner that, in the upper position of the weight, ends of the arms of the U-shaped weight are located above the waterline.
11. The energy storage system according to claim 10 , which further comprises at least one cable passing through a vertical opening in the hull of the vessel and connected to the U-shaped weight.
12. The energy storage system according to claim 10 , wherein the weight comprises two compressed-air reservoirs located at ends of the arms of the U-shaped weight.
13. The energy storage system according to claim 1 , wherein there are a plurality of weights cables of which are connected to each other by means of rigid connections to prevent the cables from entangling with one another, wherein the rigid connections are able to slide along the cables.
14. The energy storage system according to claim 1 , which further comprises an automatic device for cutting the cable in case of emergency.
15. The energy storage system according to claim 14 , which further comprises a tracer configured to locate the weight in the event that the cable is cut.
16. A procedure for operating an energy storage system, which comprises a vessel in which a first rotation shaft is installed mechanically coupled to a second rotation shaft of a motor-generator assembly connected to a power grid in such a manner that a weight can alternate between an upper position and a lower position, wherein a path of the weight between the upper position and the lower position is controlled by a cable when it is wound or unwound and the weight is immersed in water, which comprises the following steps:
when there is a surplus of electrical energy in the power grid, operating the motor-generator assembly as a motor to actuate the first rotation 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, allowing free fall of the weight in the direction towards the lower position taking advantage the rotation of the first rotation shaft in an unwinding direction of the cable to operate as a generator of the motor-generator assembly,
wherein the weight comprises a ballast, a water reservoir and a compressed-air reservoir so that, when the motor-generator assembly operates as a motor causing the weight to rise in a direction towards the upper position, water is drained from the water reservoir by feeding compressed air coming from the compressed-air reservoir; and
when the motor-generator assembly operates as a generator to generate power by allowing the weight to free fall in the direction towards the lower position, the water reservoir is filled with water.
17. The procedure for operating an energy storage system according to claim 16 , when the weight has a first compressed-air filling opening arranged in the compressed-air reservoir in communication with the outside; a second exhaust opening arranged in an upper portion of the water reservoir in communication with the outside; a third water inlet/outlet opening arranged in a lower portion of the water reservoir in communication with the outside; and a fourth air inlet opening arranged between the compressed-air reservoir and the water reservoir, comprises the following steps:
when the weight is located in the upper position, filling the compressed-air reservoir with compressed air through the first opening and filling the water reservoir with water through the third opening while keeping the second opening open;
closing the second opening and allowing the free fall of the weight towards the lower position to generate electrical energy by means of the motor-generator assembly operating as a generator;
when the weight is located in the lower position, filling the water reservoir with compressed air through the fourth opening draining the water through the third opening; and
hoisting the weight towards the upper position consuming electrical energy from of the motor-generator assembly operating as a motor.