Load
Turbine
Generator
Heat exchanger
with working fluid
Rock layers
Production
Injection
well
well
(c)
FiGUre 3.3 (See color insert.) (Continued) Three methods of recovering geothermal energy:
(c) binary cycle. (Adapted from Union of Concerned Scientists, How Geothermal Energy Works,
Union of Concerned Scientists, Cambridge, MA, 2012; National Renewable Energy Laboratory.
Planta Solar 20. http://www.nrel.gov/csp/solarpaces/project_detail.cfm/projectID=39.)
40
Water for Energy and Fuel Production
In general, the choice of the approach depends on the nature of the geothermal
resource. If water comes out as steam, the dry steam approach is used. If water
comes out as hot water at high enough temperature, the flash steam approach is used.
Since most resources produce hot waters, an exchanger design (third approach) is
more prevalent in common practice. The largest geothermal system now in operation is a steam-driven plant in an area called Geysers, north of San Francisco,
California [13,14].
Geothermal energy (steam) can sometimes be accompanied by impurities that
are harmful to the environment. The open systems such as geysers can emit some
air pollutants such as hydrogen sulfide and trace amounts of arsenic and minerals
along with steam. For the power plant run by hot water system such as Salton Sea
reservoir in Southern California, a significant amount of salt can be built up in the
pipes, which must be removed. This salt can be either put into landfill or reinjected
back into the ground. The closed loop binary cycle system has no emission problem
because everything is returned back to the ground.
Besides electricity, the geothermal heat can also be used to heat and cool homes,
heat greenhouses, dry out fish and de-ice roads, improve oil recovery, and heat fish
farms, spas, and local resorts among other applications. In Iceland, almost every