Compressed-air energy-storage system

1. A method for producing energy from a compressed-air energy-storage system, the method comprising:

delivering compressed air from a cavern to a high-pressure inlet line including a three-way valve,

directing, by the three-way valve, a first portion of the compressed air to a variable-nozzle expander;

producing mechanical power by partially expanding the first portion of the compressed air from a first pressure to a second pressure in the variable-nozzle expander, the first pressure being higher than the second pressure;

maintaining the second pressure of the partially expanded first portion of the compressed air within a range of pre-set pressure values by configuring the variable-nozzle expander to control the second pressure;

delivering the partially expanded first portion of the compressed air at the second pressure to a combustion chamber;

directing, by the three-way valve, a second portion of the compressed air directly to the combustion chamber via an auxiliary line;

mixing fuel with the partially expanded first portion of the compressed air from the variable-nozzle expander and the second portion of the compressed air in the combustion chamber;

combusting the air and fuel mixture to produce a high temperature, pressurized combustion gas;

producing mechanical power by expanding the high temperature, pressurized combustion gas in a turbine;

using the variable-nozzle expander and the turbine to drive a first electric machine and a second electric machines, respectively, to produce electric power;

providing a first air compressor configured to be driven by the first electric machine, wherein the first air compressor is connected to the cavern by a first compressed-air line path; and

providing a second air compressor configured to be driven by the second electric machine, wherein the second air compressor is connected to the cavern by a second compressed-air line path, different from the first compressed-air line path.

2. The method according to claim 1 , further comprising maintaining the second pressure of the partially expanded first portion of the compressed air at a substantially constant value using the variable-nozzle expander.

3. The method according to claim 1 , further comprising:

providing compressed air to the cavity using at least one of the first and second air compressors during an air-compressing phase; phase before and/or after delivering the compressed air from the cavern to the high-pressure inlet line.

4. The method according to claim 1 , wherein the first and second electric machines are reversible electric machines.

5. The method according to claim 4 , comprising:

selectively using the first and second reversible electric machines to:

convert the mechanical power into electric power, or

convert electric power from an electric distribution grid into mechanical power for driving the first and second air compressors.

6. A compressed-air energy-storage system comprising:

a cavern;

a high-pressure inlet line;

at least one variable-nozzle expander configured to:

partially expand an airflow from a first pressure to a second pressure lower than the first pressure, and

maintain the second pressure within a pressure range around a pre-set constant pressure value, wherein the high-pressure inlet line connects the cavern to the at least one variable-nozzle expander;

the compressed-air energy storage system further comprising:

at least one combustion chamber configured to receive a fuel and the partially expanded airflow from the at least one variable-nozzle expander and combust the partially expanded airflow and fuel to produce a combustion gas;

at least one turbine configured to receive the combustion gas from the at least one combustion chamber and expand the combustion gas producing mechanical power;

a first electric machine configured and arranged to transform mechanical power produced by the at least one variable-nozzle expander into electric power; and

a second electric machine configured and arranged to transform mechanical power produced by the at least one turbine into electric power;

a first air compressor configured to be driven by the first electric machine, wherein the first air compressor is connected to the cavern by a first compressed-air path; and

a second air compressor configured to be driven by the second electric machine, wherein the second air compressor is connected to the cavern by a second compressed-air line path, different from the first compressed-air line path;

a three-way valve interposed between the cavern and the at least one variable-nozzle expander, wherein a portion of the high-pressure inlet line extends from the three-way valve to the at least one variable-nozzle expander; and

an auxiliary line extending directly from the three-way valve to the combustion chamber.

7. The compressed-air energy-storage system according to claim 6 , wherein the at least one variable-nozzle expander is configured to maintain the second pressure at a substantially constant value.

8. The compressed-air energy-storage system according to claim 6 ,

wherein the first electric machine and the second electric machine are configured and arranged to transform electric power from an electric distribution grid into mechanical power.

9. The compressed-air energy-storage system according to claim 8 , wherein the first electric machine is configured and arranged to drive the first air compressor when transforming electric power from the electric distribution grid into mechanical power and the second electric machine is configured and arranged to drive the second air compressor when transforming electric power from the electric distribution grid into mechanical power.

10. The compressed-air energy-storage system according to claim 6 , wherein the at least one variable-nozzle expander and the at least one turbine have two independent shafts.

11. The compressed-air energy-storage system according to claim 6 , wherein the first electric machine and the second electric machine are reversible electric machines and are further arranged and configured to selectively transform mechanical power from the at least one variable-nozzle expander and the at least one turbine, respectively, into electric power, or produce mechanical power when powered by an electrical distribution grid.

12. The compressed-air energy-storage system according to claim 6 , wherein the first air compressor is selectively connectable to and disconnectable from the first electric machine and the second air compressor is selectively connectable to and disconnectable from the second electric machine.

13. The compressed-air energy-storage system according to claim 6 , wherein the second pressure fluctuates less than 20% about the pre-set constant pressure value.