How does it work?
Geothermal energy in powerplants is used to generate
electricity (not direct heating of space). Geothermal power plants use
hydrothermal resources, steem and water, which is required to be at
extremely high temperatures to be used efficiently. T
hese resources are
extracted by drilling wells into the earth about one to two miles deep,
and piping the steem or hot water to the surface.
Hydrothermal resources come from either dry steem
well, or hot a water well. A dry steem or hot rock powerplant first
wells and pumps the steem from the resevoir. The vapor is not
contaminated and is between (1050-1220° F). The steem passes
through a turbine and expands which rotates the blades and generates
electricity. Waist heat is generated in the cooling tower, but the
generation is highly
efficient. The most common version of this power plant is the Godfrey
Boyle. This process needs water to enject into the rock, which takes
more time and energy to transport.
In a flash steem plant hot water and steem from
Gysers (about 300° F) is converted into steem to generate
the
turbines. This process is common in California because it has many
gysers to use. Steam either comes directly from the source, or the very
hot and high-pressure water is depressurized "flashed" to produce
steam. The majority of geothermal powerplants are flash steem. Only
significant emission from these plants is steam (water vapor). Small
amounts of carbon 
dioxide,
nitric oxide, and sulfur are emitted from the gysers and below earth,
but almost 50 times less
thanat traditional, fossil-fuel power
plants.Energy produced 
this way currently costs about 4-6 cents per
kWh.
Binary power plants use the lowes temperature
(100-300° F). heat is transfered from geothermal hot water
to
another liquid by a heat exchanger. This second or "binary" liquid if
often not water, but a liquid with a lower point of vaporization,
usually a hydrocarbon. The teperatures dont have to be quite as high
because the liquid has a lower boiling point. Both the secondary fluid
and the geothermal fliud is recycled. Water is condenced and returned
to the resevoir, and the binary fluid is recycled through the heat
exchanger. Lower temperature resevoir are more common, making bunary
plants more prevelant. Energy produced by binary plants currently cost
5 to cents per kWh.