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Generating Power Using Geothermal Resources 181

vapor-fluid partitioning) some amount of all species in a solution will exist in the fluid phase. If the concentrations
in the fluid are significant, as for example the Salton Sea brine in Table 5.1, deposition of some solid will inevitably
form on the turbine blades. This is apparent if we consider the solubility of silica (Figure 5.7), which is one of the most
common minerals to be deposited on turbine blades in geothermal power systems. A 5°C drop in temperature results
in a decrease in the solubility concentration by a factor of approximately 0.1–5.0, depending upon the temperature
interval considered. Calcium carbonate is another solute component that commonly impacts turbine efficiency via
the same mechanism. For systems with typical flow rates through the turbines (1.0 kg/s–5.0 kg/s), it becomes only a
matter of time before turbine blade form is compromised sufficiently by deposits of minerals to reduce the efficiency
of turbine performance to unacceptable levels. When that happens, costly turbine overhaul is required. It is for this
reason that it is important to remove a sufficient amount of the dissolved loads from the fluids to allow the longest
possible time between turbine overhauls.
Also affecting efficiency is the presence of gas phases that do not condense. These noncondensable gases (NCG)
diminish the energy that can be transferred to the turbine, which is specifically defined for the thermodynamic
properties of pure steam. The principle NCGs in geothermal steam are CO 2 , H 2 S, and NH 4 . Depending upon the
chemistry of the fluid, removal of NCGs may be accomplished before the steam enters the turbine.
Steam turbines used in geothermal power plants differ from those in conventional power plants powered by fossil
fuels or nuclear power because the steam properties are different. Geothermal systems provide steam at tempera-
tures of 200°C–350°C, in contrast to the >1500°C steam in other power plants. As a result, the turbine design for the
relatively low-pressure geothermal systems is optimized for these less severe conditions.