Environmental Engineering Reference
In-Depth Information
Generator
Steam
Turbine
Steam
Separator
Cooling
tower
Condenser
Cold water
Water
Pump
Production
well
Injection
well
Geothermal reservoir
Figure 7.4
A diagram of the flow of fluids in a geothermal electric power plant.
of nearly 6000 MW, the United States accounts for 44%, most of which is located in northern
California. In 1977, the average capacity factor for U.S. geothermally generated electricity
was 15.6%.
Geothermal heat is recovered in many locations where the reservoir temperature is insufficient
for economical power generation but where the low-temperature heat may be used for space heating
or industrial processing. The heat recovery capacity of this type of geothermal energy supply is listed
in the last column of Table 7.4. Assuming an average thermal efficiency of 20% for geothermal
electric power generation, the figures in Table 7.4 suggest that nearly three-quarters of the world's
geothermal heat recovery is used to generate electricity.
A typical geothermal electric power plant is sketched in Figure 7.4. A production well supplies
pressurized warm water, a water and steam mixture, or only steam, depending upon the temperature
and pressure conditions in the underground deposit. This fluid is fed to a separator, where steam
is separated from liquid. The liquid water is pumped back into the ground via an injection well,
while the steam is fed to a low-pressure turbine that generates electrical power. The turbine exhaust
steam is condensed in a condenser, where cool water from a cooling tower removes heat from the
exhaust stream. The condensed steam is reinjected into the ground. These systems typically supply
steam at 160-180
◦
C and a pressure of 0.6-0.9 MPa and have thermal efficiencies of 20-25%.
More elaborate systems than that shown in Figure 7.4, which improve the thermal efficiency, have
been constructed, albeit at higher capital cost.
10
A geothermal plant transfers heat from a hot underground reservoir to the earth's surface,
where it can be profitably utilized to produce electrical power or for other heating purposes. If it
is to be a source of renewable energy, this heat must not be removed at a faster rate than it can be
replenished by conduction from deeper in the earth. In the regions where geothermal resources have
10
See Dickson, Mary H., and Mario Fanelli, Eds., 1995.
Geothermal Energy.
Chichester: John Wiley & Sons.
