Geothermal power generates electricity by harnessing heat from within the Earth. It is considered a renewable resource, though specific locations can eventually be depleted as the ground cools over many decades. Three main types of geothermal power plants exist - dry steam, flash, and binary cycle plants. The largest geothermal facility in the world is The Geysers in California, while other significant sites include those in Nevada, Indonesia, and Kenya. Geothermal energy also shows promise for desalination and heating applications.

1. GEOTHERMAL POWER
Geothermal power is the use of geothermal heat to generate electricity. It is
often referred to as a form of renewable energy, but because the heat at
any location can eventually be depleted it is by definition not strictly
renewable. Geothermal comes from the Greek words geo, meaning earth,
and therme, meaning heat. Geothermal literally means "earth heat".
Geothermal-generated electricity was first produced at Larderello, Italy, in
1904.
Capacity
By the end of 2005 worldwide use of geothermal energy for electricity had
reached 9.3GWs, with an additional 28 GW used directly for heating. If heat
recovered by ground source heat pumps is included, the non-electric use of
geothermal energy is estimated at more than 100 GWt (gigawatts of thermal
power) and is used commercially in over 70 countries
During 2005 contracts were placed for an additional 0.5 GW of capacity in
the United States, while there were also plants under construction in 11 other
countries.
Resources
Estimates of exploitable worldwide geothermal energy resources vary
considerably. According to a 1999 study, it was thought that this might
amount to between 65 and 138 GW of electrical generation capacity 'using
enhanced technology'.
A 2006 report by MIT that took into account the use of Enhanced
Geothermal Systems (EGS) concluded that it would be affordable to
generate 100 GWe (gigawatts of electricity) or more by 2050, just in the
United States, for a maximum investment of 1 billion US dollars in research and
development over 15 years.
The MIT report calculated the world's total EGS resources to be over 13,000 ZJ,
of which over 200 ZJ would be extractable, with the potential to increase this
to over 2,000 ZJ with technology improvements - sufficient to provide all the
world's energy needs for several millennia.
Although geothermal sites are capable of providing heat for many decades,
eventually specific locations may cool down. It is likely that in these locations,
the system was designed too large for the site, since there is only so much
energy that can be stored and replenished in a given volume of earth. Some
interpret this as meaning a specific geothermal location can undergo
depletion, and question whether geothermal energy is truly renewable.
Although geothermal sites are capable of providing heat for many decades,
eventually they are depleted as the ground cools. The government of
Iceland states It should be stressed that the geothermal resource is not strictly
renewable in the same sense as the hydro resource. It estimates that
Iceland's geothermal energy could provide 1700 MW for over 100 years,
compared to the current production of 140 MW.
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2. Bjarnarflag Geothermal Station in northeast
Iceland
Electrical generation
Three types of power plants are used to generate power from geothermal
energy: dry steam, flash, and binary. Dry steam plants take steam out of
fractures in the ground and use it to directly drive a turbine that spins a
generator. Flash plants take hot water, usually at temperatures over 200°C,
out of the ground, and allows it to boil as it rises to the surface then separates
the steam phase in steam/water separators and then runs the steam through
a turbine. In binary plants, the hot water flows through heat exchangers,
boiling an organic fluid that spins the turbine. The condensed steam and
remaining geothermal fluid from all three types of plants are injected back
into the hot rock to pick up more heat. This is why geothermal energy is
viewed as sustainable. The heat of the earth is so vast that there is no way to
remove more than a small fraction even if most of the world's energy needs
came from geothermal sources.
Distribution
The largest dry steam field in the world is The Geysers, about 90 miles (145 km)
north of San Francisco. The Geysers began in 1960 which has 1360 MW of
installed capacity and produces about 1000 MW net. Calpine Corporation
now owns 19 of the 21 plants in The Geysers and is currently the United States'
largest producer of renewable geothermal energy. The other two plants are
owned jointly by the Northern California Power Agency and Santa Clara
Electric. Since the activities of one geothermal plant affects those nearby,
the consolidation plant ownership at The Geysers has been beneficial
because the plants operate cooperatively instead of in their own short-term
interest. The Geysers is now recharged by injecting treated sewage effluent
from the City of Santa Rosa and the Lake County sewage treatment plant.
This sewage effluent used to be dumped into rivers and streams and is now
piped to the geothermal field where it replenishes the steam produced for
power generation.
Another major geothermal area is located in south central California, on the
southeast side of the Salton Sea, near the cities of Niland and Calipatria,
California. As of 2001, there were 15 geothermal plants producing electricity
in the area. CalEnergy owns about half of them and the rest are owned by
various companies. Combined the plants produce about 570 megawatts.
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3. The Basin and Range geologic province in Nevada, southeastern Oregon,
southwestern Idaho, Arizona and western Utah is now an area of rapid
geothermal development. Several small power plants were built during the
late 1980s during times of high power prices. Rising energy costs have spurred
new development. Plants in Nevada at Steamboat near Reno, Brady/Desert
Peak, Dixie Valley, Soda Lake, Stillwater and Beowawe now produce about
235 MW. New projects are under development across the state.
Geothermal power is very cost-effective in the Rift area of Africa. Kenya's
KenGen has built two plants, Olkaria I (45 MW) and Olkaria II (65 MW), with a
third private plant Olkaria III (48 MW) run by geothermal specialist Ormat.
Plans are to increase production capacity by another 576 MW by 2017,
covering 25% of Kenya's electricity needs, and correspondingly reducing
dependency on imported oil.
Geothermal power is generated in over 20 countries around the world
including Iceland (producing 17% of its electricity from geothermal sources),
the United States, Italy, France, New Zealand, Mexico, Nicaragua, Costa
Rica, Russia, the Philippines (production output of 1931MW (2nd to US, 27% of
electricity), Indonesia, the People's Republic of China and Japan. Canada's
government (which officially notes some 30,000 earth-heat installations for
providing space heating to Canadian residential and commercial buildings)
reports a test geothermal-electrical site in the Meager Mountain–Pebble
Creek area of British Columbia, where a 100 MW facility could be developed.
Desalination
Douglas Firestone began working with evaporation/condensation air loop
desalination about 1998 and proved that geothermal waters could be used
as process water to produce potable water in 2001. In 2003 Professor Ronald
A. Newcomb, now at San Diego State University Center for Advanced Water
Technologies began to work with Firestone to enhance the process of using
geothermal energy for the purpose of desalination. Geothermal Energy is a
primary energy source.
In 2005 some testing was done in the fifth prototype of a device called the
“Delta T” a closed air loop, atmospheric pressure, evaporation condensation
loop geothermally powered desalination device. The device used filtered sea
water from Scripps Institute of Oceanography and reduced the salt
concentration from 35,000 ppm to 51 ppm.
Water injection
In some locations, the natural supply of water producing steam from the hot
underground magma deposits has been exhausted and processed waste
water is injected to replenish the supply. Most geothermal fields have more
fluid recharge than heat, so re-injection can cool the resource, unless it is
carefully managed.
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