Geothermal Power

 

Geothermal Links:
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Geothermal Resources Council
National Renewable Energy Laboratory
California Energy Commission Geothermal Program
Geothermal Energy Program
 
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           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 (1).

           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.

           Information provided by the International Geothermal Association, last updated in 1999, indicates that there was a  plan to build a 15MWe plant at Unalaska, Alaska.  With a population of over 4,300 and containing one of the United States' most productive fishing ports, Unalaska would be a great place to test geothermal electric generation in Alaska.  Electricity currently costs  $0.24/kWh  in Unalaska.  State subsidized power cost equalization benefits provided for many remote villages in Alaska, including Unalaska, help to reduce electrical costs there because the cost of producing energy is so high.  The current mode of electric generation at Unalaska is via diesel generators.  One of the main obstacles to implementing a geothermal plant there is the difficult terrain that separates the source and the city.  This would increase capital costs.   

 

References:

1)       California Energy Commission
2)     National Renewable Energy Laboratory: Geothermal Energy
3)      Ristinen, R. and Kraushaar, J., 1999, Energy and the Environment, John Wiley and Sons, NYC  
4)     Center for Renewable Energy and Sustainable Technology