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The Practical Limits of Geothermal Resources
The most valuable characteristic of most energy resource is heat, and the
higher the temperature it can produce, the more versatile its application
can be. We use coal instead of wood, for example, because coal has more
heat per pound and is therefore cheaper to transport. Oil has more heat
per pound than coal, and uranium can produce many times more heat than any
fossil fuel. The energy contained in geothermal resources, in contrast,
is at a relatively low heat concentration. This produces some practical
limits to its use, but the resources are attractive nevertheless because
the heat can be obtained more cheaply than from most any alternative.
There is, however, a disadvantage to such low heat concentrations.
Most notably, this disadvantage takes on a spatial connotation; neither
hot water nor steam can be transported long distances economically.
It would not be sensible, for example, to load a tanker with hot water and
then attempt to transport the water across the ocean for use. Even
if we could somehow insulate that water so that it retained its heat energy,
it would have very little value wherever it was offloaded. The practical
limitation of such transportability means that using geothermal resources
is restricted to locations near where they found. In other words, geothermal
energy has to be used where it is initially available, or not at all.
Geothermal energy is a spatially restricted resource.
It is also a depleting resource. To make this clear, let’s refer back to
Figure 3. We see that heat source
overlies an impermeable layer that conducts heat to the replenishing supply
of water. This puts the heated water where we can draw it to the surface
for use. The longer the water remains underground near the heat source,
the hotter it becomes. If we withdraw water too rapidly, however,
it will not have enough time to heat up. Worse, if we withdraw the water
more rapidly than it can be replenished naturally, we will over-tap the
aquifer. Without such water, we would then not have any way of bringing the
heat to the surface.
Such overtapping has occurred. One example comes from The Geysers.
While it appears that the heat source there is sufficient to continue heating
water, the water supply itself is in diminishing supply for conversion to
steam. As a result, several power plants there have been idled. The
remedy has been an on-going model of ingenuity and good sense. Treated
wastewater from nearby communities, including Santa Rosa, is now being pumped
to The Geysers to recharge the aquifer. Not only does evidence strongly
suggest that this is having the desired impact, but it has the additional
benefits of preserving the geothermal investment while disposing of treated
wastewater.
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