System and method for energy storage and retrieval

1. A system for storing and releasing energy, comprising:

a generator/motor subsystem;

a hydraulic pump for pumping hydraulic fluid between first and second pump ports in response to force applied by the generator/motor subsystem and also capable of imparting force to the generator/motor subsystem in response to hydraulic fluid being caused to flow between the first and second pump ports;

a first compression/expansion vessel for exchanging hydraulic fluid with the hydraulic pump via the first pump port;

a second compression/expansion vessel for exchanging hydraulic fluid with the hydraulic pump via the second pump port;

a gas storage subsystem for selectively storing gas from, or releasing gas to, the first and second gas compression/expansion vessels;

an internal heat exchanger within each of the first and second compression/expansion vessels for exchanging heat between the hydraulic fluid and the gas therein, each internal heat exchanger having a very large thermal mass relative to the mass of gas within the first and second compression/expansion vessels;

a control subsystem for controlling at least a plurality of gas valves associated with the gas storage subsystem to switch between the storing and releasing,

wherein, during storing, responsive to hydraulic fluid being pumped by the hydraulic pump between the first and second pump ports, gas is caused to be compressed in the gas storage subsystem thereby to store energy, and

wherein, during releasing, responsive to pressure from release of compressed gas from the gas storage subsystem, hydraulic fluid is forced to flow between the first and second pump ports thereby to release energy.

2. The energy storage/retrieval system of claim 1 , wherein the gas storage subsystem comprises:

a high pressure gas chamber for selectively exchanging gas with the first or second compression/expansion vessel responsive to open/closed states of respective gas valves; and

a low pressure gas chamber for selectively exchanging gas with the first or second compression/expansion vessel responsive to open/closed states of respective gas valves.

3. The energy storage/retrieval system of claim 2 , wherein each internal heat exchanger comprises a mesh or metal foam and the density of the mesh or metal foam is substantially increased in the top portion of the respective vessel where the heat of compression or expansion is greatest.

4. The energy storage/retrieval system of claim 3 , wherein the mesh or metal foam is substantially denser than the gas under compression.

5. The energy storage/retrieval system of claim 4 , wherein each internal heat exchanger fills less than about 25 percent of the interior volume of the respective vessel.

6. The energy storage/retrieval system of claim 5 , wherein each internal heat exchanger fills about 3 percent of the interior volume of the respective vessel.

7. The energy storage/retrieval system of claim 4 , wherein the aggregate density of the mesh or metal foam is 1 gm/cc and the fill factor is less than 25% of the available volume in the expansion/compression chamber.

8. The energy storage/retrieval system of claim 3 , wherein each internal heat exchanger is a metal foam.

9. The energy storage/retrieval system of claim 8 , wherein substantially all pores in the metal foam are less than about 5 cubic millimeters in size.

10. The energy storage/retrieval system of claim 3 , wherein each internal heat exchanger is a highly porous sintered plug of metal.

11. The energy storage/retrieval system of claim 3 , wherein each internal heat exchanger is a coil.

12. The energy storage/retrieval system of claim 11 wherein each internal heat exchanger comprises at least one of copper, aluminum.

13. The energy storage/retrieval system of claim 3 , further comprising an external heat exchanger in contact with the hydraulic fluid for exchanging heat between the hydraulic fluid and the ambient.

14. The energy storage/retrieval system of claim 1 , further comprising a flywheel associated with the generator/motor subsystem.

15. The energy storage/retrieval system of claim 1 , further comprising a power conditioning module associated with the generator/motor subsystem for conditioning electrical power from the generator/motor subsystem during the releasing.

16. The energy storage/retrieval system of claim 1 , wherein the hydraulic pump is configured to rotate in the same direction whether hydraulic fluid is flowing from the first pump port to the second pump port or from the second pump port to the first pump port.

17. The energy storage/retrieval system of claim 1 , further comprising pressure transducers for communicating data to the control system regarding gas pressure within each of the first and second compression/expansion vessels, and within the gas storage subsystem.

18. The energy storage/retrieval system of claim 1 , wherein at least one of the first and second compression/expansion vessels houses a respective liquid level transducer for communicating data to the control system regarding hydraulic liquid level within the compression/expansion vessel.

19. The energy storage/retrieval system of claim 1 , wherein the hydraulic fluid is a non-flammable, non-reactive fluid.

20. The energy storage/retrieval system of claim 19 , further comprising, between the gas and the hydraulic fluid within each of the first and second compression/expansion vessels, a second fluid that is non-flammable and non-reactive, and is lower in density and has higher impermeability than the hydraulic fluid.

21. The energy storage/retrieval system of claim 20 , wherein the second fluid is an ethylene glycol/water/inhibitor based coolant.

22. The energy storage/retrieval system of claim 20 , wherein the hydraulic fluid and the second fluid are separated by an isolator piston.

23. The energy storage/retrieval system of claim 20 , wherein the hydraulic fluid and the second fluid are directly adjacent to each other.

24. The energy storage/retrieval system of claim 22 or claim 23 , wherein the first and second compression/expansion vessels are configured such that the heavier of the hydraulic fluid and the second fluid is below the lighter of the hydraulic fluid and the second fluid.

25. The energy storage/retrieval system of claim 23 , wherein the second fluid is an ionic fluid with a density of about 1.5 gm/cc.

26. The energy storage/retrieval system of claim 25 , wherein the ionic fluid is selected from the group consisting of: EMIM BTA, BMIM BTA, and BMMIM BTA.

27. The energy storage/retrieval system of claim 1 , wherein the generator/motor subsystem comprises a unitary generator/motor.

28. The energy storage/retrieval system of claim 1 , wherein the generator/motor subsystem comprises both a generator and a motor, each of which are mechanically connected to the hydraulic pump.