Dead-volume management in compressed-gas energy storage and recovery systems

1. A method of increasing efficiency of an energy-recovery process performed in a first cylinder assembly and a second cylinder assembly in which gas is collectively expanded from an initial pressure to a final pressure, the method comprising:

pre-compressing gas in the first cylinder assembly to approximately the initial pressure;

following the pre-compression, admitting compressed gas at the initial pressure into the first cylinder assembly, the pre-compression reducing coupling loss during the admission of compressed gas, wherein the admission of compressed gas comprises (i) withdrawing compressed gas from a storage reservoir, (ii) withdrawing heat-transfer fluid from the storage reservoir, (iii) mixing the compressed gas and the heat-transfer fluid within a mixing chamber, separate from the storage reservoir, to form a foam at the initial pressure, and (iv) admitting the foam into the first cylinder assembly;

substantially isothermally expanding the gas in the first cylinder assembly to a mid-pressure between the initial pressure and the final pressure by mingling, within the foam, substantially all of the gas with heat-transfer fluid in the first cylinder assembly, thereby maintaining substantially all of the gas at substantially the same temperature during expansion;

completing a partial expansion cycle by exhausting (i) only a portion of the expanded gas and (ii) at least a portion of the heat-transfer fluid out of the first cylinder assembly into a mid-pressure vessel at the mid-pressure;

mingling compressed gas and heat-transfer fluid within the mid-pressure vessel to form a mid-pressure foam at the mid-pressure;

admitting mid-pressure foam into a second cylinder assembly;

expanding the compressed gas of the mid-pressure foam within the second cylinder assembly to the final pressure;

exhausting gas at the final pressure from the second cylinder assembly; and

repeating the foregoing steps at least once, thereby performing at least one additional expansion cycle.

2. The method of claim 1 , wherein the heat-transfer fluid is sprayed into the gas within the mixing chamber, the mixing chamber comprising a screen therewithin.

3. The method of claim 1 , wherein the heat-transfer fluid forms a foam with the gas within the mixing chamber after the compressed gas and the heat-transfer fluid are each withdrawn from the storage reservoir from a different outlet and transferred to the mixing chamber.

4. The method of claim 1 , wherein, after expansion of the gas within the second cylinder assembly, at least a portion of the heat-transfer fluid is exhausted out of the second cylinder assembly.

5. The method of claim 1 , wherein expanded gas is exhausted to the ambient atmosphere from the second cylinder assembly.

6. The method of claim 1 , wherein admitting compressed gas into the first cylinder assembly comprises actuating a valve to establish a connection between the first cylinder assembly and the mixing chamber, the pre-compression reducing an actuation energy required to actuate the valve.

7. The method of claim 1 , wherein at least a portion of the gas that is pre-compressed is within dead volume of the first cylinder assembly.

8. The method of claim 7 , wherein (i) the first cylinder assembly comprises an interior volume bounded by two opposing ends, (ii) substantially an entire length of the first cylinder assembly between the two opposing ends is swept by a solid piston during expansion therein, and (iii) the dead volume is disposed within a volume between the piston and an end of the first cylinder assembly when the piston is disposed at a limit of travel at the end of the first cylinder assembly.

9. The method of claim 1 , wherein exhausting only a portion of the expanded gas out of the first cylinder assembly comprises exhausting substantially all of the expanded gas in the first cylinder assembly that is not within dead volume thereof.

10. The method of claim 9 , wherein (i) the first cylinder assembly comprises an interior volume bounded by two opposing ends, (ii) substantially an entire length of the first cylinder assembly between the two opposing ends is swept by a solid piston during expansion therein, and (iii) the dead volume is disposed within a volume between the piston and an end of the first cylinder assembly when the piston is disposed at a limit of travel at the end of the first cylinder assembly.

11. The method of claim 1 , wherein an amount of the gas that is pre-compressed is substantially less than an amount of the gas expanded in the first cylinder assembly.

12. The method of claim 1 , further comprising monitoring at least one of a temperature, a pressure, or a position of a boundary mechanism within the first cylinder assembly during at least one of gas expansion or gas exhaustion, thereby generating control information.

13. The method of claim 12 , further comprising utilizing the control information in a subsequent expansion cycle to control at least one of the pre-compression, expansion, or exhaustion steps.

14. The method of claim 1 , wherein the gas expansion drives a load connected to at least one of the first cylinder assembly or the second cylinder assembly.

15. The method of claim 14 , wherein the load is a mechanical crankshaft or a hydraulic pump/motor.

16. The method of claim 1 , wherein the gas expansion generates electricity.

17. The method of claim 1 , wherein exhausting only a portion of the expanded gas out of the first cylinder assembly comprises (i) monitoring at least one of a temperature, a pressure, or a position of a boundary mechanism within the first cylinder assembly, thereby generating control information, and (ii) based at least in part on the control information, discontinuing the gas exhaustion, thereby trapping a remnant portion of the expanded gas within the first cylinder assembly.

18. The method of claim 17 , wherein the remnant portion of the expanded gas is determined such that a pre-compression step of a subsequent expansion cycle compresses the remnant portion to approximately the initial pressure.