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The Forms of Geothermal Energy
Geothermal energy has a simply yet unique characteristic. It is all heat.
While the energy within uranium, coal, oil, and natural gas must first be
converted to heat, geothermal resources arrive as heat directly from the
earth, ready to use. Other than any chemicals that might be extracted
from hydrothermal systems, the value of geothermal energy is its heat; it
cannot be reformulated into plastics, fertilizers, or anything else. If the
temperatures are high enough, it can be used to generate electricity. At
lower temperatures, it may be used in some “direct” way, such as for heating.
The vapor and hot water forms of geothermal energy are together known as
hydrothermal systems, and they are the only forms that are commercially available.
However, two other forms of geothermal energy are being eyed as resources
of the future. They include hot-dry rock and geopressured systems. HDR resources
occur when heat is available in the rock but where no water is available
in the rock to bring that heat to the surface. Geopressured systems are comprised
of heavily compressed sediments that contain heat, methane, and pressurized
hot water.
Figure 4 - A schematic drawing of a hot-dry rock system. Even if
the rock beneath the surface contains a great deal of heat, the only way
to get the heat to the surface is to pump water down, let it pass through
artificial or natural fractures in the rock and on to a withdrawal well and
then bring the heated water to the surface. Los Alamos National Laboratory
experimented with this approach for over two decades at their Fenton Hill
site on the southern flank of the Valles Caldera in New Mexico. Simple
in concept but difficult to accomplish commercially, the potential of this
form of geothermal energy is many times that of the hydrothermal resources.
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