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Geothermal
Geothermal Technology
Most currently operating geothermal electricity plants are either "steam" or "binary cycle." If the geothermal temperatures are sufficiently hot, dry steam from the earth can be used directly to drive a steam turbine – such as is done at The Geysers geothermal complex in Northern California.
More typically, hot water is "flashed" into steam by reducing the ambient pressure, and the resulting steam is used to drive a turbine. This is a relatively simple process (at least relative to a binary cycle process), but requires high-temperature water (above 150 degrees C). Solids and other contaminants in the steam can also cause environmental problems.
Lower-temperature resources (100-150 C) can use a binary cycle system, in which hot brine (a mix of water and various minerals) is pumped from geothermal wells. The heat of the brine is extracted with a heat exchanger, and the cooled brine is then reinjected into the earth. While this is a more expensive and complicated process it has the advantage of useful production at lower temperature sites.
The DOE has a detailed description of the technologies and cost breakdowns.
| Type of technology |
U.S. installed capacity in MW
|
| Dry steam |
1140 |
| Flash |
740 |
| Binary |
220 |
Source: Kutscher 2000
Another approach to geothermal, "hot dry rock," is intriguing but not yet commercially viable. The idea is to tap the vast amount of heat contained in deep rock formations that do not contain water or steam, and introduce surface waters as the medium for heat transfer.
The technical challenges are many, including drilling down to these hot dry rocks, injecting and then recovering some sort of heat transfer fluid (probably water), and fracturing the rocks to allow the fluid to penetrate. Although the resource potential is large, the high costs and technical challenges will keep this technology uneconomical for some time.
Virtually all existing United States geothermal power plants are more than 5 MW. Active research into "small" (usually defined as less than 5 MW) geothermal power plants, for both on-grid and off-grid applications is in progress. Such plants could be located at sites that could also make use of direct heat, increasing the overall cost-effectiveness.
For example, a federally funded research project is examining the feasibility of installing a 1.2 MW binary plant in Nevada that will also provide process heat for a vegetable dehydration plant. In general, however, electricity costs for small geothermal plants will be quite high, and therefore such plants will make economic sense only in applications where the direct heat is of great value.
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