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Introduction
Geothermal energy, literally the heat of the earth, is everywhere.
Its widespread availability has made this resource popular for centuries,
and today we know we can use it for many purposes, including making electricity.
In the United States, the most attractive sites for electricity are in
the west, but in one form or the other, geothermal energy can be developed
just about any where.
Regardless of where you are, there is geothermal energy beneath your
feet; just drill down far enough and it will eventually get hotter. The
deeper you go, generally the hotter it gets. The average, or “normal” gradient
is an increase of about 1-2 degree F for every 100 feet you drill down.
This means that if you drill down 1,000 feet, the temperature will rise
by 10-20 degrees. This is, of course, only a roughly accurate statement.
In practice, you will find temperatures both higher and lower than the normal
gradient. Geothermal prospectors, like any other prospectors, try to find
places where the resource they seek is available for the lowest cost and
with the least amount of trouble. This means they gravitate to where
it is hotter faster.
And there are plenty of places to look. The first places that attract
attention are those locations where geothermal energy is obvious, such
as where there are hot springs or geysers, perhaps volcanoes. After
these sites are exhausted, exploration gets a bit more complicated, but
it is still relatively easy. Geologists concentrate on the young and active
places in the world; that is, where there are earthquakes, mountains, and
faults, anywhere where the conditions are right for the earth’s heat to
get closer to the surface. After all, drilling is expensive, so where
the heat is close to the surface, we do not have to drill so far down. And
this means, costs will be less.
As anyone knows who has descended into a deep mine, temperatures increase
as one travels toward the center of the earth. If we could get all
the way there, the temperature would reach to more than 7600 F. While some
of this heat is left over from the fiery origins of the earth itself, most
of this heat originates with radioactive decay deep below the surface.
Where we can bring some of this heat to the surface, and control it, we
can substitute it for the heat we presently get from coal, oil, natural
gas, and uranium.
The total geothermal resource is temptingly large. It has been estimated
that 42 million thermal megawatts (42 x 1012 watts) are radiated into space
continuously. While this is an enormous amount of heat, it is radiated
over the entire earth surface. This means that the temperature is
usually too diluted to be useful for any practical application.
Figure 1 - Cutaway of the Earth, showing its four principal levels. Source:
Geothermal Information Agency
Figure 2 - Temperatures Increase with Depth. Source: Geothermal
Education Association
However, in some places, the heat is concentrated and much closer to the
surface, and that means that we have a better chance to drill wells deep
enough to bring it up for our use. The trick is to find these places.
In some sites this heat finds its way to the surface where we can see it,
such as a volcano or a geyser. One such place in the United States
is in Yellowstone National Park, where perhaps half the world’s geysers are
located—such as Old Faithful. Another place where geothermal energy is on
vivid display is at Volcano National Park on the Big Island of Hawaii, where
the Kilauea volcano has been erupting continuously for over a decade.
If we look at the whole planet, the most conspicuous location for geothermal
energy is the “Ring of Fire” that largely encircles the Pacific Ocean.
Here are located the majority of volcanoes and major earthquake faults.
Here also we find concentrated the hottest temperatures, often hot enough
to generate electricity.
There are, however, many other places—such as Bath, England, and Spa,
Luxembourg—where geothermal energy is used for recreational and domestic
purposes. These are known as “direct uses”, and the example of this use is
the capital of Iceland, where virtually all heating needs are provided by
geothermal energy.
The principal ingredients in a geothermal system are fairly simple. There
has to be a heat source and some way to get the heat toward the surface.
This usually means there has to be some cracks and faults in the rock,
as well as water to carry the heat upward. If the heat reaches all
the way to the surface, there might be a fumarole, a hot spring, or a geysers.
Without faults and cracks, there would be no way for the water to rise.
The upper impermeable layer traps the heat and prevents it from dissipating
to the atmosphere. Beneath the trapped water, there is usually an impermeable
layer separating the aquifer from the heat source itself. This configuration
is depicted in the accompanying drawing.
Figure 3 - A Generalized Schematic Drawing of the
Principal Ingredients of a Geothermal System.
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