Page 277 - Handbook of Energy Engineering Calculations

percent, while the absolute head drops 53.8 percent (from 3.9 to 1.8 m). This
is why the tail-water level is so important in small hydro installations.
Figure 3c shows how station costs can rise as head decreases. These costs
were estimated by the department of energy (DOE) for a number of small
hydro-power installations. Figure 3d shows that station cost is more sensitive

to head than to power capacity, according to DOE estimates. And the
prohibitive costs for developing a completely new small hydro site mean that
nearly all work will be at existing dams. Hence, any water exploitation for

power must not encroach seriously on present customs, rights, and usages of
the water. This holds for both upstream and downstream conditions.

3. Outline machinery choice considerations
Small-turbine manufacturers, heeding the new needs, are producing a good
range of semistandard designs that will match any site needs in regard to

head, capacity, and excavation restrictions.
The Francis turbine, Fig. 2, is a good example of such designs. A
horizontal-shaft Francis turbine may be a better choice for some small
projects because of lower civil-engineering costs and compatibility with

standard generators.
Efficiency of small turbines is a big factor in station design. The problem
of full-load versus part-load efficiency, Fig. 4, must be considered. If several
turbines can fit the site needs, then good part-load efficiency is possible by

load sharing.

FIGURE 4 Steep Francis-turbine efficiency fall-off frequently
makes multiple units advisable.