1. Geothermal Energy for Electric Power Generation
Samuel A. Gray
Introduction: Geothermal Energy use for Electrical Power Generation
- The amount and temperature steam needed for power generation
- History of Geothermal Energy application for Electric Power Generation
1.3 Total U.S. Installed capacity of Electric Power Production from Geothermal Energy.

2. Source of Geothermal Energy
Geothermal Energy is sustainable and renewable energy
For commercial use, only useful reservoirs are those that produce hydrothermal
Steam is created beneath Earth surface when water comes in contact with hot bodies such as magma chambers, and hot rocks
Geothermal energy comes from the heat deep down inside Earth
A magma chamber is a large underground liquid pool of liquid rock found beneath Earth’s surface trapped in porous rocks. The molten rock in those chambers is under great pressure
If the rocks around the magma fracture, volcanic eruption results on the surface of Earth.
Most of the known magma chambers are close to the surface but, they are hard to detect.
Yellowstone magma chamber: Wikimedia [5]

3. Geological Regions with Rich Hydrothermal Resources
Estimated Subterranean temperature at depths of 6 km.
U.S. Geothermal map [8].
Geothermal resources are classified as low, intermediate, and high enthalpy resources [Source: Renewable and Sustainable Energy Reviews. Arif Hepabali]
* Geological regions where magma chambers are close enough to the surface are most useful for geothermal power plants
Global Energy Institute [10]. Hottest geological regions of the world.

4. Geothermal Energy for Electric Power Generation
Larderello, Italy, in 1904 as an experiment.. Became commercial in 1913 with a capacity of 250 KW. Currently, it has 487 MW capacity
In the U.S. the first geothermal power plant was built in Geysers in 1960.
First plant to use geothermal energy to produce electric power was installed in Larderello, Italy, in 1904 as an experiment. It lit a bulb
Prince Piero Ginori Conti proved the viability of geothermal power plant technology at the dry steam facility in Larderello, Italy.
It became commercial in 1913 with a capacity of 250 KW.
In the U.S., first geothermal power plant was built in Geysers, Northern California in The geothermal plants in Geysers field have been operating for more than 50 years.
Photograph courtesy of ENEL SpA Posted 30 Sep 2001

5. Types of Geothermal Power Plant Designs
Dry Steam Power Plant
Geothermal resources are classified as low, intermediate, and high enthalpy resources [Source: Renewable and Sustainable Energy Reviews. Arif Hepabali]
Temperature is used as a classification parameter because it is the earliest to measure and understand.
In geothermal flash power plant, high-pressure geothermal water separates into steam and water as it rises through the pipe and pressure drops. The steam and water are separated in a surface tank, called steam separator. The steam is delivered to the turbine and the turbine powers a generator.
The steam is delivered to the turbine, and the turbine powers the generator. The condensed water is injected back into the reservoir.
As of 2012, 900 MW of installed 3187 MW geothermal capacity in the U.S. is comprised of Steam-flash power plants.
In Dry Steam power plant, steam alone is produced directly from geothermal reservoir and is used to drive the turbines that power the generator.
As of 2012, Dry-Steam power plants account for 1585 MW (almost 50%) of geothermal installed capacity in the U.S.
Flash Steam Power Plant
Source: Energy Almanac: [7]

6. Types of Geothermal Power Plant Designs
Geothermal resources are classified as low, intermediate, and high enthalpy resources [Source: Renewable and Sustainable Energy Reviews. Arif Hepabali]
Temperature is used as a classification parameter because it is the earliest to measure and understand.
Binary-Cycle power plants have made it possible to produce electricity from geothermal reservoirs with temperature lower than 150oC (302oF).
Binary-Cycle plants use an Organic Rankine-Cycle system. Geothermal water is used to heat another liquid (working fluid) with lower boiling point than water. Liquids such as isobutane are used for this purpose.
A heat exchanger transfers heat from water to the working fluid. When the working fluid vaporizes, the work output from the expansion of the vapor drives the turbine-generator set. The geothermal water is then injected back into the ground.
As of 2012, Binary-Cycle power plants make up about 702 MW of the installed geothermal capacity.
Binary Cycle Power Plant.
Source: Energy Almanac: [7]

7. Geothermal Power Plant Capacity by Technology
Total U.S. Geothermal Installed Capacity by Technology (MW)
Geothermal Energy Association, [1]

8. California Geothermal Power Plant
It is the largest complex of geothermal power in the world
Geysers geothermal power plants use Dry Steam technique.
Today, total of 14 power plants, draw steam from 350 wells, and produce 725 MW enough to power homes
Geysers today has a capacity of 725 MW from 14 plants
The plants meet nearly 60% of the average power demand for California’s North Cost Region
The plants use evaporative cooling method. The water resource is replenished by directing treated sewage water to the Geysers region and injecting it into the ground.
Geysers condensers of geothermal power plants. [4]

9. Global Installed Geothermal Capacity
Globally, 12.8 GW online at geothermal power plants in 24 countries as of February 2015.
Established Geothermal Power Installed Capacity (MW) [February 2015].
February 2015 Annual U.S & Global Geothermal Installed capacity [7]

10. Geothermal Power Plants Advantages
Geothermal power plants are reliable and predictable. [1].
They are considered baseload plants because they are available 24/7 regardless of environmental conditions
They are dispatchable. Capacity is available when you need it
High Capacity Factor
Baseload (also base load, or baseload demand) is the minimum amount of power that a utility or distribution company must make available to its customers, or the amount of power required to meet minimum demands based on reasonable expectations of customer requirements.Nov 15, 2011
Baseload plants are the production facilities used to meet some or all of a given region's continuous energy demand, and produce energy at a constant rate, usually at a low cost relative to other production facilities available to the system. (From Wikipedia)
The net capacity factor of a power plant is the ratio of its actual output over a period of time, to its potential output if it were possible for it to operate at full nameplate capacity continuously over the same period of time.
Geothermal, nuclear, and coal-plants are baseload plants
Geothermal is capable of producing constant power 365 days/year, 24 hours/day
In geothermal development, 1MW is equivalent to electric power used by 1000 homes

11. Comparison of Energy Sources
Capacity Factor of a power plant = (Actual Output) /((Period of Time) * (Potential continuous output )
Example: For wind farm with installed capacity of 60 MW, if it generated 60,000 MW-h in 2010, then the capacity factor can be calculated as
Capacity factor = (60,000 MW-h) / ((366 days) x (24 h/day) x (60 MW)) = = 11.38%
Renewable Energy World [1]

12. Benefits and Main Issues
- No burning fossil fuel
- High Capacity Factor
- Very low operating expense
Main issues
- Areas with idle geology are very rare; most places require deep holes be drilled
- Very high initial investment. Many places, it is not economically viable
- Steam from geothermal reservoirs carry some dangerous gases like H2S
- Some geothermal wells suddenly stop producing steam or hot water.
- Overtime, the heat of geothermal well runs out.

13. References
[1] Annual U.S. Geothermal Power Production and Development Report. April
[2] Union of Concerned Scientists. How Geothermal Energy Works [3] Renewable Energy World.
[4] Calpine Corporation:
[5] Wikimedia: [6] Geothermal Energy Association: [7] Energy Almanac:
[8] U.S. Geothermal map.
[9] Renewable and Sustainable Energy Reviews, Arif Hepbasli [10] Global Energy Institute: