Page 194 - Geothermal Energy Renewable Energy and The Environment

180 Geothermal Energy: Renewable Energy and the Environment

Rotating blades

d

High P and T
steam
Lower P and T
steam

Stationary blades

FIGUre 9s.2 Schematic geometry of the stator–rotator couple along the steam flow path within the turbine.
The geometry of each set of blades is designed for specific temperature and pressure conditions as well as
flow rates, to maximize extraction of energy from the expanding steam. The distance, d, is one of the critical
parameters that influences turbine efficiency.

d* d

FIGUre 9s.3 Deposits (dark, irregular material coating the light gray turbine blades) on the turbine blades
modify the design distance, d. The presence of the deposits add roughness to the blades, which diminishes
the smooth flow of the fluid over the shaped blade and reduces the energy input to the blade. In addition, the
deposits diminish the distance, d, to d*, which reduces the flow volume and hence the total rate of energy
transfer.
wheels than the lower pressure stages, reflecting the decreasing energy density of the fluid at lower pressures—at
lower energy densities (and pressures) a greater blade area is required for efficient energy extraction. Figure 9S.1
shows the configuration for a single stage turbine while Figure 9S.2 shows the schematic relationships and flow path
for a single stator–rotator pair in a turbine.
Modern steam turbine design has benefited from decades of engineering refinement and scientific analysis. The
result is that today’s multistage steam turbines are capable of operating at high efficiencies, commonly in excess of
85% of the theoretical maximum for extracting energy from the available steam resource.
Any modification of the flow path of the expanding steam or the blade geometry, or change in the steam prop-
erties of the incoming steam will diminish the efficiency of the turbine, since its design is optimized for specific
conditions. Of particular concern in geothermal power production is the common occurrence of components other
than H2O in the steam supply. Although achieving the highest steam quality from the geothermal fluid is assiduously
pursued, thermodynamics prevent the formation of pure steam from a solution that contains solutes. Regardless of
how small the partitioning may be for a component between steam and liquid (see Chapter 5 for a discussion of