Page 276 - Geothermal Energy Renewable Energy and The Environment

The Geothermal Energy Future: Possibilities and Issues 267

location on the planet could, in principle, construct geothermal power production facilities since
temperatures in excess of 130°C can be found everywhere at depth. The magnitude of this resource
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can be understood by considering the amount of energy available in 1 km of rock that is at 250°C.
Temperatures of this magnitude can be found at depths between 5 and 10 km under half the area of
the United States. The amount of energy that can be extracted, Q is a function of the heat capacity,
ex,
3
C of the rock (J/m -K), the number of degrees by which the temperature is decreased in the power
p,
production cycle, ∆T, the density of the rock, ρ, and the rock volume, V,
Q = V × ρ × C × ∆T. (14.1)
p
ex
3
For 1 km of rock with a density of 2550 kg/m and a heat capacity of 1000 J/kg-K, the extracted
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energy as a function of the ∆T is shown in Figure 14.4. Also shown is the amount of thermal
energy that could be extracted if only 1% of the rock volume were available for energy production.
Temperatures in excess of 150°C occur within 10 km of the surface over more than 90% of the area
of the United States and the amount of energy available from that resource is shown in Figure 14.4
by the bracketed portion of each curve.
Integrating all of the available energy as a function of depth is difficult because of the irregular
heat distribution and the absence of subsurface data in many parts of the United States. Figure 14.5
shows the distribution of temperatures in the United States at a depth of 6 km, estimated on the basis
of measured geothermal gradients, surface heat flow, thermal conductivities, and the thickness of
sedimentary rocks overlying crystalline basement. The temperature distribution is irregular, reflect-
ing the effects of geological processes that have shaped the continent. Particularly significant are
the effects of rifting in the western United States. The high temperatures in the Basin and Range
Province typify the effects of this process. Taking into account the obvious variability, it has been
estimated (Tester et al. 2006) that down to a depth of 10 km in the subsurface of the United States
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there is a thermal energy resource base in excess of 13 million exajoules (1 exajoule = 10 J). It has

10 18
150°C
Total available energy
10 17

1% extraction
Joules 10 16
150°C

10 15

10 14
0 50 100 150 200 250
Temperature difference (°C)
FIGUre 14.4 The amount of thermal energy that could be extracted from 1 km of rock, as a function of
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the temperature difference between initial and final states. The top curve defines the total thermal energy that
would be extracted from the entire rock volume and the lower curve represents the extracted energy from 1%
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of the rock volume. These calculations assume the same C p at 150 C and 250 C, and that the minimum final
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temperature is 15 C.
o