Environmental Engineering Reference
In-Depth Information
The subsurface heat is generated by the decay of radioactive elements
in the crust itself and at greater depths from molten rock within the
earth's mantle and core. It is transported upward from these areas by
convection and conduction. Sometimes molten rock is very near the
surface, and we see evidence of its heat as volcanic eruptions or, less
dangerous, as geysers. The fl uid shooting into the air at a geyser is liquid
water and water vapor emanating from molten rock at shallow depths,
combined with normal groundwater that has been heated by the hot
rock. The water in geysers is dangerously hot at the surface and tends
to be at least 300ºF to 400ºF at depths less than 300 feet.
Geothermal capacity in the United States in 2009 was 3,100 mega-
watts.
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In 2009, a report by the Geothermal Energy Association identi-
fi ed 144 projects underway in fourteen states, from Alaska to Florida
and from Hawaii to Mississippi. When developed, these projects could
provide nearly 7,000 megawatts of new electric power—enough electric-
ity to meet the needs of roughly 7 million homes. In large areas of the
western United States, temperatures are hot at depths of just a few
hundred feet, and this heat has been tapped into for many decades. In
Oregon, Utah, and Idaho, geothermal heat is used for power generation
and space heating, providing about 600 megawatts of heat, roughly
enough energy to heat and cool more than 400,000 homes. Nevada has
fi fteen geothermal power plants that generate enough electricity for
73,000 homes, and two dozen new plants are under development. One
company in 2008 in Nevada announced plans to deliver 1,000 mega-
watts of geothermal power to Los Angeles and Las Vegas from massive
natural steam zones in northwestern Nevada. In California, hot waters
from 350 wells at the Geysers, 72 miles north of San Francisco, supply
5 percent of the state's power needs. (The billows of steam at the locality
prompted the fi rst European visitor in 1847 to believe he had discovered
the gates of hell.) On the volcanic island of Hawaii, 25 percent of the
electrical supply comes from geothermal energy.
The Bureau of Land Management says that a dozen western states
could generate 5,500 megawatts of geothermal energy from 110 plants
by 2015, and that number could rise by another 6,000 megawatts by
2025. Geothermal power plants such as the Geysers have system avail-
abilities of at least 95 percent, much higher than plants fueled by coal,
oil, natural gas, or uranium. And unlike wind and solar systems, a
geothermal plant works night and day. However, their effi ciencies are
only 8 to 15 percent, less than half that of coal plants. High construc-
tion expenses and the low effi ciencies make the cost of hydrothermal
