Geothermal Energy Power Generation

1. Geothermal EnergyPower Generation San Jose State University FX Rongère March 2009

2. Geothermal Power Generation in the World • Located along the main plate boundaries

3. Hydrothermal Sites Source: Boyle, Renewable Energy, 2nd edition, 2004

4. The Geysers (CA) Power generation: 1,400 MWe

5. Caenozoic: Sedimentary non marine Sedimentary marine Volcanic Mezozoic Paleozoic Intrusive Igneous Rocks Granite Ultramafic The Geysers Geology The Geysers

6. Geologic Time C B A

7. The Geysers Geology • Typical Porosities of ground material

8. Geysers Geothermal resource A steam reservoir is located about 2,000 meters under the surface

9. Geothermal resource at the Geysers The Geysers

10. Dry Steam Cycle • At Geysers, the steam is dry (250oC – 30 bars). It is directly used to drive steam turbines

11. Steam Cycle • It is an open Rankine Cycle (the ground is the Steam Generator)   

12. Power by the turbine  

13. h-s Diagram • Enthalpy variation may directly be read on the diagram   

14. Steam turbines cannot resist to more than 12% of water drops in low pressure steam Multi-stage steam turbine are used Generated Power • Computations of the power by the turbine • Isentropic (Perfect) Turbine • Actual Turbine Intersection of vertical from and P=0.056 bars

15. Heat rejected at the Condenser • Energy balance on the condenser  

16. Rejected Heat • Computation of the heat rejected at the condenser

17. Water Re-injection • In 1997 and 2003 a re-injection system was built to offset the depletion of the Geysers steam reservoir. It provides 19 M Gallons per day (=832 kg.s-1). This re-injection should covered a power generation of 830 MW

18. Casa Diablo • Located by the Mammoth Mountain on the East side of the Sierra Nevada About 40MW by three power plants

19. Long Valley Caldera

20. Bishop Tuff Porosity: 48% to 65% Source: J. Roberge Permeability study of pumice samples from the Bishop Tuff, Long Valley Caldera, CA American Geophysical Union, Spring Meeting 2004, abstract #V21A-07

21. Caldera Formation • The caldera was formed 760,000 years ago by the explosion of a volcano

22. Alimentation of the Hydrothermal Reservoir

23. Hydrology • Water flow starts in the west by the Mammoth Mountain and continues to the southeast toward Crowley Lake. • Reservoir temperatures decline from 230°C near the Inyo Craters to 50°C near Crowley Lake

24. Geothermal resource at Casa Diablo Casa Diablo The Geysers

25. Water Extraction • Because of the low temperature and no impermeable cap, there is no steam and the system is not pressurized • Hot water is pumped from the reservoir to run the Power Plant

26. Binary Cycle Power Plant • At Casa Diablo, the thermal water temperature is only 170oC. The Power Plant uses a Binary Cycle with Isobutane as the working fluid

27.     Why the iso-butane? • Evaporation temperature is lower than for steam • Higher pressure at the turbine -> Cheaper turbine

28. Enthalpy-Entropy Diagram    

29. Summary • Fluid state at each connection point:

30.   Power delivered by the turbine

31. Heat transferred at the Condenser • Energy balance on the condenser  

32. Water Branch of the Condenser • We can assess the flow of water required to cool the condenser Tw-inis imposed by the weather conditions, sizing for the hottest day: 30oC (dry Aero-Condenser) Tw-outis limited by the temperature in the condenser

33. Graph Enthalpy-Temperature • The two lines cannot intersect because heat goes naturally from the hotter to the colder fluid (Second law of Thermodynamics)   n-butane out water in

34. Effect of dry Aero-Condensor • Air dew point defines the evaporation • Wet-bulb temperature defines the lowest temperature for water evaporating in an air stream Wet-bulb Temperature Dew Point

35. Effect of dry Aero-Condensor • With evaporative cooling towers the temperature of water entering the condenser would have been 15oC • This would allow a condenser temperature of 20oC rather than 35oC leading to a gain of 2 points in conversion rate

36. Pump • The pump brings the n-butane back to high pressure • Its power is much lower than the Turbine power because the fluid is liquid

37. Evaporator • Energy balance on the evaporator

38. Water Branch of the Evaporator • We can assess the flow of water required to cool the evaporator Ta-inis imposed by the water temperature of the Geothermal resource Ta-outis limited by the temperature in the evaporator

39. Graph Enthalpy-Temperature • The two lines cannot intersect because heat goes naturally from the hotter to the colder fluid (Second law of Thermodynamics) in water out  n-butane 

40. Summary • Balance of Energy and Entropy on each component • Conversion rate:

41. Imperial Valley • Located between Salten Sea and the Gulf of California Salten Sea units 3 and 4 570 MW by 15 power plants

42. Imperial Valley Geology About 300oC at -1,000 m

43. Cenozoic: Sedimentary non marine Sedimentary marine Volcanic Mezozoic Paleozoic Intrusive Igneous Rocks Granite Ultramafic Geology

44. Geothermal resource at Casa Diablo Casa Diablo Salten Sea The Geysers

45. Steam Flash Cycle • Simple flash cycle: 30bars – 230oC

46. Simple Steam Flash Vap 230oC 50 bars, 300oC Liq

47. Steam Flash Cycle • Double flash cycle

48. Double Steam Flash Vap 10 bar Liq

49. Geothermal Power in California In Operation: 2,400 MWe Total Potential: 4,900 MWe Source: California Geothermal Energy Collaborative/GeothermEx, 2006

50. Hot Dry Rock (HDR) • Injection of water in a dry hot rock • A Hot Rock Source: Granite that is generating abnormally high internal heat from the natural radioactive decay of minerals • An insulating blanket of sediments, that effectively entraps the heat generated from the buried granite. • Adequate fracturing of the hot dry rock source that allows circulation of a horizontal fluid flow regime Source: http://www.geothermal-resources.com.au/exploration.html