Environmental Engineering Reference
In-Depth Information
Figure 7.1
Geothermal Power Plant
Geothermal Power Plant
Generator
Turbine
3.
2
4
Steam
Cooling
tower
Injection
well
Hot water
1
5
Source: USEPA 2011.
s&LASHSTEAMPLANTSPULLDEEPHIGHPRESSUREHOTWATERINTOLOWERPRESSURETANKSANDUSETHE
resulting flashed steam to drive turbines. Steam is separated from a hot water resource and
used for power when the temperature of the resource is above 180°C. This allows extrac-
tion of some high-pressure steam through “flashing” in steam separators to run the turbine
generator. The steam portion is used in turbines and remaining hot water is treated as waste
or reinjected into the reservoir (IFC 2007, 13).
s"INARYCYCLEPLANTSPASSMODERATELYHOTGEOTHERMALWATERNEARASECONDARYmUIDWITHA
much lower boiling point than water in a heat exchanger. This causes the secondary fluid to
flash to vapor, which then drives a turbine (USDOE 2011). When the resource temperature
is below 180°C, a secondary cycle using a low-boiling-point fluid such as isobutene, isopen-
tane, pentafluoropropane, or an ammonia-water mixture is used to exchange heat between
the hot geothermal fluids and a turbine (IFC 2007, 13). The two liquids are kept completely
separate through use of a heat exchanger, which transfers the heat energy from the geothermal
water to the working fluid. The secondary fluid expands into gaseous vapor. The force of the
expanding vapor, like steam, turns turbines that power generators (Geothermal Energy As-
sociation 2011). Plants that use a closed-loop binary-cycle release no fluids or heat-trapping
emissions other than water vapor, which may be used for cooling. With closed-loop systems
such as the binary-cycle plant, there are no emissions; everything brought to the surface is
returned underground (UCS 2009).
s)NSOMEFACILITIESBOTHmASHINGANDBINARYPROCESSESAREUSEDTOINCREASEOVERALLEFlCIENCY
of electricity production (IFC 2007, 13). In this type of plant, the portion of the geothermal
water that “flashes” to steam under reduced pressure is first converted to electricity with a
backpressure steam turbine and the low-pressure steam exiting the backpressure turbine is
condensed in a binary system (Geothermal Energy Association 2011).
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