Energy from subterranean reservoir fluid

1. A method comprising:

monitoring demand on an electrical supply system;

if the monitored demand exceeds a predetermined first value:

enabling fluid to flow substantially under the influence of gravity from a first elevation through a turbine-generator;

in the turbine-generator, converting kinetic energy associated with the flowing fluid into electrical energy;

collecting a portion of the fluid in an area downstream of the turbine generator; and

pumping the collected fluid from the area downstream of the turbine-generator into a subterranean reservoir with an injection pump; and

if the monitored demand drops below a predetermined second value, moving fluid from the subterranean reservoir to the first elevation.

2. The method of claim 1 wherein the injection pump is located between the turbine-generator and the subterranean reservoir, relative to a flow path of the fluid.

3. The method of claim 1 further comprising:

sensing a fluid level within the area downstream of the turbine-generator; and

controlling the injection pump's operation based on the sensed fluid level.

4. The method of claim 1 further comprising:

sensing a pressure associated with the collected fluid in the area downstream of the turbine-generator; and

controlling the injection pump's operation based on the sensed pressure.

5. The method of claim 1 wherein the subterranean reservoir comprises an aquifer.

6. The method of claim 1 wherein the subterranean reservoir comprises a fractured hot dry rock.

7. The method of claim 1 wherein the first elevation is higher than the subterranean reservoir.

8. A system comprising:

a collection of water at a first elevation;

a subterranean reservoir at a second elevation lower than the first elevation;

one or more fluid communication channels to facilitate fluid communication between the collection of water and the subterranean reservoir;

a turbine-generator to create electrical energy from kinetic energy of fluid flowing from the collection of water to the subterranean reservoir inside one of the fluid communication channels;

a fluid collection area downstream of the turbine-generator to at least temporarily collect fluid that has passed through the turbine-generator; and

an injection pump to move accumulated fluid in the collection area to the subterranean reservoir.

9. The system of claim 8 further comprising:

a fluid level sensor to sense a fluid level in the collection area; and

a controller to control the injection pump's operation based on the fluid level sensed by the fluid level sensor.

10. The system of claim 8 further comprising:

a pressure sensor to sense a pressure associated with the fluid collected in the collection area, wherein the sensed pressure provides an indication of fluid level in the collection area; and

a controller to control the injection pump's operation based on the pressure sensed by the pressure sensor.

11. The system of claim 8 wherein the injection pump is located between the turbine-generator and the subterranean reservoir relative to a flow path of the fluid.

12. The system of claim 8 wherein the subterranean reservoir comprises an aquifer.

13. The system of claim 8 wherein the subterranean reservoir comprises a fractured hot dry rock.

14. The system of claim 8 further comprising a pump to move fluid from the subterranean reservoir to the collection of water at the first elevation.

15. A method comprising:

moving fluid from a subterranean reservoir to an elevation above the subterranean reservoir;

using the fluid at the higher elevation for a domestic or industrial application;

after using the fluid for the domestic or industrial application, enabling the fluid to flow from the higher elevation through a turbine-generator substantially under the influence of gravity;

converting kinetic energy of the flowing fluid into electrical energy with the turbine-generator;

collecting fluid that has passed through the turbine-generator at a collection area downstream of the turbine-generator; and

pumping fluid from the collection area into the subterranean reservoir with an injection pump.

16. The method of claim 15 wherein the injection pump is located between the turbine-generator and the subterranean reservoir relative to a flow path of the fluid.

17. The method of claim 15 further comprising:

sensing a fluid level within the collection area; and

controlling the injection pump's operation based on the sensed fluid level.

18. The method of claim 15 further comprising:

sensing a pressure associated with the fluid in the collection area; and

controlling the injection pump's operation based on the sensed pressure.

19. The method of claim 15 wherein the subterranean reservoir comprises an aquifer.

20. The method of claim 15 wherein the subterranean reservoir comprises a fractured hot dry rock.

21. The method of claim 15 wherein the fluid is moved from the subterranean reservoir to the higher elevation through a first fluid communication channel and returned to the subterranean reservoir through a second fluid communication channel.

22. A method comprising:

monitoring demand on an electrical supply system;

if the monitored demand exceeds a predetermined first value:

enabling fluid to flow substantially under the influence of gravity from a first elevation that is higher than a subterranean reservoir through a turbine-generator;

converting kinetic energy associated with the flowing fluid into electrical energy at the turbine-generator;

collecting fluid in a collection area at an elevation lower than the turbine-generator; and

pumping collected fluid from the collection area into the subterranean reservoir; and

if the monitored demand drops below a predetermined second value:

moving fluid from the subterranean reservoir to the first, higher elevation; and

when the fluid is at or near the higher elevation, using the fluid for a domestic or industrial application.