Geothermal Energy - Environmental Impacts of Renewable Energy - page 258

To transmission
lines

AC

Secondary
coil

Primary
coil

Step-up transformer

Electric
generator

Turbine

Condenser

Ground-
water
pump

Flash
tank

Injection
well

Production well

Earth

FIGURE 6.4

Simplified functional diagram of a flash steam geothermal power plant.

F flash s team P ower P lants
Figure 6.4 illustrates a flash steam geothermal power plant. Fluid is sprayed into a
tank held at a much lower pressure than the fluid, causing some of the fluid to rapidly
vaporize, or “flash.” The vapor then drives a turbine, which drives a generator. If any
liquid remains in the tank, it is returned to the groundwater pump to be forced down
into the Earth again so it can be flashed again to extract more energy.

b inary c ycle P ower P lants
In a binary cycle geothermal power plant (see Figure 6.5 ), water is pumped into the
Earth and comes back up hot, just as it does in the flash steam system. Instead of
going into a flash tank, however, the hot water enters a heat exchanger (see Figure
6.5 ) , where most of its energy is transferred to another fluid, the binary liquid. This
fluid can be water, but more often it is a volatile liquid resembling refrigerant that
boils easily into vapor at a lower temperature than the water. The liquid-to-vapor