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Electricity generation

High temperature geothermal resources are particularly valuable for the electricity they can produce. Over 8,000 MW of installed geothermal electricity generation capacity worldwide, in about 25 countries. Even with today’s technology, an estimated 72,000 MW that can be developed. Enhanced technology could provide nearly 140,000 MW of electrical generating capacity, according to the Geothermal Energy Association.

Several power plant designs are available. These include dry steam, flash steam and binary cycle systems.  The simplest type of geothermal power plant uses dry steam. It is the suitable design where the geothermal steam is not mixed with water.  Once production wells are drilled to the aquifer, the superheated steam (180-350 C) is directed against turbine blades, which in turn turns the generator, making electricity. Spent water is directed back underground via reinjection wells, a step that helps avoid surface contamination while recharging the aquifer.
         
 
Figure 5 - World geothermal potential. Source: Geothermal Energy Association. http://www.geo-energy.org

Figure 6 - The approximately 25 countries that have active geothermal development for the generation of electricity. There are at least 25 additional countries with such potential. Source: Geothermal Education Office. http://geothermal.marin.org
Because steam temperatures are low when compared to fossil- and nuclear-fueled power plants, energy conversion efficiency for geothermal power plant tends to be l ower, around 30%.  An efficient coal-burning power plant—by contrast—operates at efficiencies approaching 40%, while a gas-fired combined-cycle plant has efficiencies above 50%.  On a more positive note, geothermal power plants tend to have high “capacity factors”; that is, they operate most of the time, even more than coal.

Flash-steam power plants are more complicated, more expensive and somewhat less efficient than are steam-dominated facilities, owing to the additional step. The biggest difference between this design and the dry version is that the heat is contained in liquid water, not steam, and it is usually at a lower temperature.


In a flash-steam power plant the hot water is brought to the surface under pressure to maintain its liquid state. The liquid is then sprayed into a flash tank that is held at a much lower pressure.  At that point, the liquid vaporizes—or flashes—to steam and is directed to the turbine.  A variant of this process uses the remaining fluid in the tank that itself can be flashed to steam.  This steam then drives a second, lower-pressure turbine.  Efficiencies in such plants are typically 20-25%.

The third major type of geothermal generating system is called a binary cycle.  It is generally more expensive than the other two systems, but it is often necessary where geothermal fluids are particularly caustic, such as in the Imperial Valley of California, or where the resource is not hot enough to produce steam efficiently. The greatest difference in the binary plant is that it employs a heat exchanger. First, the geothermal fluid is passed through the heat exchanger. Then, the secondary fluid—such as isobutene--is vaporized and expanded through a turbine to generate electricity. The working fluid is then condensed and recycled for another cycle. All geothermal fluid is then reinjected, maintaining everything in a closed-cycle system.

Figure 7 – Dry-steam power plant. This system is used at geothermal 
resource sites where dry steam is available. Steam is vented out of the 
turbine to the atmosphere. As with all the designs, cooling water is 
evaporated to increase power plant efficiency.  The Largest such 
developments are at The Geysers, 90 miles north of
San Francisco, California;  and Larderello, 50 miles west of Siena, Italy.
Larderello was the first  geothermal power plant in the world.





Figure 8 - A single-flash geothermal power plant. Flash-steam power plants are much more common than dry-steam facilities.

Figure 9 - A binary cycle power plant can use caustic and lower-temperature water than any
other type of geothermal power plant.
Binary cycle power plants, despite their additional step, have several advantages. First, they allow the utilization of lower temperature resources. Second, corrosion problems are avoided.  However, the large pumps required consume a significant percentage of the power output of the plants. The unit sizes are typically much larger than the 50-150 MW range of the other plants.