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154 Geothermal Energy: Renewable Energy and the Environment

10,000

8000

MW installed capacity 6000

4000

2000

0
1900 1920 1940 1960 1980 2000 2020
Year

FIGUre 9.1 The installed global geothermal generating capacity, from 1916 through 2007. (Data from
Smith, B., Beall, J., and Stark, M., Induced Seismicity in the SE Geysers Field, California. Proceedings World
Geothermal Congress, Kyushu-Tohoku, Japan, 2887–92, 2000; Bertani, R., GeoHeat Center Bulletin, 28,
2007; International Geothermal Association, 2008; Bolton, R. S., Geothermics, 38, 11–29, 2009.)

General FeaTUres oF GeoThermal power GeneraTIon FacIlITIes
Producing power from geothermal energy relies on the ability to convert geothermal heat at depth
to electricity. To accomplish this requires transferring the heat to a surface generating facility that
can efficiently turn thermal energy into electrical power. The equipment necessary to accomplish
this is a piping complex that will bring hot fluids from depth to a turbine facility on the surface
where the thermal energy is converted to kinetic energy in the form of a rotating turbine (see
Sidebar). The kinetic energy of the turbine is then converted to electrical energy using an electrical
generator. The quantity of electrical energy that can be generated from a geothermal fluid is

P = ε gen × ε turb × H , (9.1)
elec
g
where P is the number of watts, which is equivalent to J/s, produced of electrical energy, ε is the
elec
gen
efficiency of the electrical generator, ε turb is the efficiency of the turbine, and H is the rate at which
g
thermal energy is supplied to the turbine. The remainder of this chapter will consider the factors
that determine H . The factors that influence ε and ε turb are the subjects of numerous engineering
gen
g
texts and will not be directly addressed here.
Figure 9.2 is the pressure–enthalpy diagram for water that was previously discussed in Chapter 3.
We will use it as a means to examine the conversion of geothermal heat to electrical power.
The energy available for doing work to rotate the turbine shaft that drives the electrical gen-
erator is the enthalpy of the fluid. Although the pressure and temperature of the fluid in the
geothermal reservoir is the initial energy resource, extraction of fluid from the reservoir, and
transferring it to the turbine results in some energy loss. The primary sources of energy loss are
from conduction of heat from the fluid to the surrounding rock as the fluid flows up the well, fric-
tional losses as the fluid interacts with the well pipe during flow, and the change in gravitational
potential energy. All of these losses are relatively small due to the low thermal conductivity of
the well pipe and casing, the relatively smooth pipe wall and the velocity of the rising fluid (on
the order of meters to tens of meters per second). Collectively, these effects amount to a few