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Use of Geothermal Resources: Environmental Considerations 241

N
0

–60
–20 –80 0
–40
–40

–20

0 –40 –20
0
–60
–20
2 km
Taupo

0
Lake taupo

FIGUre 12.8 Contours of the subsidence rates (in mm/yr) in the vicinity of Taupo, New Zealand. The loca-
tions of wells for geothermal power generation are shown by the partially filled circles. At each of these sites
there usually are multiple wells. (From Allis, R., Bromley, C., and Currie, S., Geothermics, 38:169–80, 2009.)

such instances, if permanent radar scatterers can be identified that are independent of the vegetative
effects. This method is described by Eneva et al. (2009) and they employ it to map the subsidence
rate in a portion of the Salton Sea area.
Ground subsidence generally has little impact in geothermal regions, particularly if reinjection
of the produced mass is employed. However, remote sensing techniques such as satellite interferom-
etry may provide the means to rapidly assess if subsidence problems are developing and how best
to mitigate them.

waTer Use

As noted in Chapter 9, a significant quantity of water is required to achieve the energy production
rates necessary to efficiently extract energy from a geothermal fluid. The flow path followed by a
geothermal fluid at a generating site generally involves the following sequence of stages:

• High pressure mixed steam and liquid flow from the wellhead to the separator.
• High pressure steam is separated from condensed liquid and piped to the turbine.
• Steam expands into the turbine, dropping in pressure and temperature with some
condensation.
• Condensate is separated from steam as steam flows through multiple turbine stages.
• Steam exits the turbine through the turbine exhaust and is condensed in the condenser.
• Condensed water from the condenser is cooled in the cooling tower.

Cooling towers spray water into flowing air to result in evaporative cooling. Throughout this
process about 60–80% of the original steam that enters the turbine ends up evaporating to the atmo-
sphere. The remaining fluid is collected and reinjected.