Geo, meaning earth, and thermal, meaning heat, is a naturally occurring energy in the form of heat under the surface of the earth. This energy source can be only a few feet below the surface, in water that comes to the surface of the ground, in hot rocks miles below the surface, or even further down in molten rock called magma. This energy originates from radioactive decay deep within the earth’s crust.
“The Geysers”, near San Francisco, having a generating capacity of
1360MWe, is the largest geothermal electric plant in the U.S.
It is one of only two locations in
the world where a high-temperature,
dry steam is found that can be directly
used to turn turbines and generate electricity
(the other being Larderello, Italy). The Geysers is comparable to the
hydroelectric Hoover dam project, which has a generating capacity of
1,345MWe. Nuclear and coal – fired
power plants may have generating
capacities on the order of 1000MWe
California's geothermal power plants produce about 40 percent of the world's geothermally generated electricity. U.S. geothermal power plants have a total generating capacity of 2,700 megawatts and produce electricity at 5˘ to 7.5˘ per kilowatt-hour. Iceland gets about one-third of its total energy from geothermal resources (2; 3).
Geothermal power plants have sulfur-emissions rates that average only a few percent of those from fossil-fuel alternatives. The newest generation of geothermal power plants emits only 0.3 lb of carbon (as CO2) per MW-hr of electricity generated. This is 1000 times lower than that for a plant using natural gas (methane) and and even more for a coal- fired plant. Nitrogen oxide emissions are much lower in geothermal power plants than in fossil power plants. Nitrogen-oxides combine with hydrocarbon vapors in the atmosphere to produce ground-level ozone, a gas that causes adverse health effects and crop losses as well as smog (4).
Only hot water and natural steam reservoirs are being used today to create large amounts of electricity. Many of the hot water reservoirs, particularly those of higher temperature and salinity, pose the potential for contamination of the soil by salination if the extracted water is not reinjected into the ground. There is also the risk of aquifer disruption when large amounts of water are extracted from the ground. Gaseous air pollutants such as hydrogen sulfide can be liberated into the atmosphere by some hot water reservoirs and by natural steam. But again, this is often less than that emitted by other energy sources. Other possible environmental effects include induced seismic activity if water is injected into dry rock formations or if explosive fracturing techniques are used in normally impermeable rock formations (3).
There are many geothermal sources in Alaska but only a few are available for producing good quality steam for direct use with a turbine, for producing electricity, and those that are available generally are not always in the place they are most needed. For use of steam directly from the source it is preferable the steam be well over 200°C, those at 200°C or lower will generally require the use of a binary cycle. A binary cycle plant is similar in construction to a direct-use-plant but the main difference is in the medium that goes through the turbine. Because steam that is lower than 200°C should not be sent through the turbine, a substance that vaporizes at much lower temperatures should be used as the medium through the turbine. The low
temperature steam is sent through a heat exchanger in which the other substance, usually iso-butane, is on the other side of the heat exchanger and then this substance is sent through the turbine. The provided map shows some geothermal sources, in the form of surfacing water, in Alaska. The map reveals that the most productive sources for good quality steam at higher temperatures are on the Alaska Peninsula in the close proximity of volcanic activity. The closest source, over 200°C, to Alaska’s largest city is across Cook Inlet from Anchorage and is well over 100 miles away. This is an obvious inconvenience since electric lines would need to be routed such a long distance and around such an obstacle as Cook Inlet. Most of the geothermal springs in Alaska are under 200°C and currently are being used to heat homes locally, for recreation at resorts, or are not being tapped. Low ground temperatures near the surface and permafrost limits the use of heat pumps. Because geothermal energy is associated with low emittance of SO2, CO2 and other pollutants and because there are many geothermal sources in the state, there is potential.