Geothermal - Energy Policy Analysis: A Conceptual Framework - page 135

are methane, carbon dioxide, and hydrogen sulfide. Carbon dioxide and hydrogen sulfide
are denser than air and can collect in pits, depressions, or confined spaces. These gases are
a poisonous hazard for people working at geothermal power stations or bore fields. Methane
and carbon dioxide are also greenhouse gases, contributing to climate change. Consequently,
wastewater effluents and gases are usually reinjected into the reservoir or its periphery to
minimize air emissions, potential for subsidence, and surface and groundwater contamination.
Construction of cooling and settling ponds with lagoon covers to capture and scrub gases is
sometimes necessary for circumstances in which the reinjection of wastewater fluids and gases
is not possible (IFC 2007, 12).
Depending on characteristics of the produced water and design of the facility, cooling towers
may use geothermal fluids or borrow from surface water sources for circulation. Hazardous solid
waste may be generated from sulfur precipitates in the condensate which must be removed and
properly stored on site before disposal (IFC 2007, 12). Sometimes the resulting elemental sulfur
can then be used as a nonhazardous soil amendment and fertilizer feedstock (DiPippo 2008, 152;
Kagel, Bates, and Gawell 2007).
Scrubbers reduce air pollution emissions but produce a watery sludge high in sulfur and va-
nadium, a heavy metal that can be toxic in high concentrations. Additional sludge is generated
when hot water steam is condensed, causing dissolved solids to precipitate out. This sludge is
generally high in silica compounds, chlorides, arsenic, mercury, nickel, and other toxic heavy
metals (Brower 1992, 151).
Usually the best disposal method is to inject liquid wastes back into a porous level of a geo-
thermal well. Although this technology is more expensive than conventional open-loop systems, in
some cases it may reduce scrubber and solid waste disposal costs enough to provide a significant
economic advantage. This technique is especially important at geopressured power plants because
of the sheer volume of wastes they produce each day. Wastes must be injected well below fresh
water aquifers to make certain there is no transport between usable water and waste-water strata.
Leaks in well casings at shallow depths must also be prevented (Reed and Renner 1994, 19-23;
Brower 1992, 152).
Depletion of Geothermal Resources
The largest geothermal electric power generating system in operation in the United States is a
steam-driven plant north of San Francisco in an area called the Geysers. The first well for power
production was drilled in 1924, but significant development did not occur until the 1970s and
1980s. By 1990 twenty-six power plants had been built, with a combined capacity of more than
2,000 MWe (UCS 2009). The process of extracting geothermal fluids (which include gases, steam,
and water) for power generation typically removes heat from natural reservoirs at over ten times
their rate of replenishment. This imbalance may be partially improved by injecting waste fluids
back into the geothermal reservoir. It is particularly difficult at dry steam reservoirs, where there
is little to reinject after the steam has been utilized (DiPippo 2008, 291).
Because of rapid development in the 1980s, and the technology used, the steam resource was
significantly depleted after 1988. Today, it has a net operating capacity of 725 MWe. Plants at the
Geysers use an evaporative water-cooling process to create a vacuum that pulls steam through
a turbine, producing power more efficiently than some other geothermal plants. However, this
process loses 60 to 80 percent of the steam to the air, without reinjecting it underground. To
remedy the situation, in 2003 various stakeholders partnered to create the Santa Rosa Geysers
Recharge Project, which involves transporting 11 million gallons per day of treated wastewater