Geothermal Electric Energy

Geothermal ("earth heat") energy has tremendous potential for producing electricity. About 8,000 megawatts (MW) of geothermal electricity are currently produced around the world, including about 2,800 MW of capacity in the United States. Today's technology produces electricity from hydrothermal (hot water/steam) resources. In the future, we may be able to use the heat of the deep, hot, dry rock formations of Earth's crust, and possibly the even deeper, almost unlimited energy in Earth's magma.

How it works

Two basic types of geothermal power plants are used today: steam and binary.

Steam Electric Plants

Steam plants use very hot (more than 300° F) steam and hot water resources (as found at The Geysers plants in northern California—the largest geothermal electricity producer in the world).  The steam either comes directly from the resource, or the very hot, high-pressure water is depressurized ("flashed") to produce steam.  The steam then turns turbines, which drive generators that generate electricity.  The only significant emission from these plants is steam (water vapor).  Minute amounts of carbon dioxide, nitric oxide, and sulfur are emitted, but almost 50 times less than at traditional, fossil-fuel power plants.

Binary Electric Plants

Binary plants use lower-temperature, but much more common, hot water resources (100° F – 300° F).  The hot water is passed through a heat exchanger in conjunction with a secondary (hence, "binary plant") fluid with a lower boiling point (usually a hydrocarbon such as isobutane or isopentane).  The secondary fluid vaporizes, which turns the turbines, which drive the generators.  The remaining secondary fluid is simply recycled through the heat exchanger.  The geothermal fluid is condensed and returned to the reservoir.  Because binary plants use a self-contained cycle, nothing is emitted.  Because these lower-temperature reservoirs are far more common, binary plants are the more prevalent.

Challenges

Although geothermal power plants have many features in common with more traditional power plants, they also pose special challenges: non-condensable gases and minerals in the geothermal fluid, need for a greater amount of heat rejection, use of hydrocarbon fluids, and lack of cool water to cause condensation.

Researchers are working on new technologies that will improve heat-exchange efficiency, lower the equipment-damaging effects of the sometimes corrosive geothermal fluid, and improve the plant's condensing capability.  This research ismaking geothermal plants more efficient, thereby bringing down the cost of geothermal electricity.

Sources:  Department of Energy Department National Renewable Energy Laboratory and Energy
Efficiency and Renewable Energy