Hydroelectric Power Generation - Renewable Energy: Technology, Economics and Environment - page 365

into the buckets (Fig. 8.12). During this process the entire pressure energy of the
water is converted into kinetic energy when leaving the nozzle. This energy is
converted into mechanical energy by the Pelton wheel; the water then drops more
or less without energy into the reservoir underneath the runner.
Fig. 8.12 Power station with a Pelton turbine (see /8-11/)
The Pelton turbine is characterised by a relatively flat efficiency curve
(Fig. 8.9). It is therefore suitable for highly variable flows between approximately
10 and 100 % of the design discharge.
Pelton turbines can also be used in small hydroelectric power stations - but
usually for lower heads than in large hydroelectric power stations (Fig. 8.8).
Heads from 30 m can be enough to drive small Pelton turbines. For larger dis-
charges, models with several nozzles (up to 6) are also used.
Cross-flow turbines. In cross-flow turbines the water reaches the runner after a
simple guide vane and flows through the runner from the outside to the inside and
then to the outside again after having passed through the interior of the wheel
(Fig. 8.13). The runner is built like a roll and separated into two chambers in the
direction of the axis at a ratio of two to one, thus allowing for better efficiencies at
partial flows.
In a way similar to the Pelton turbine, the cross-flow turbine is particularly
suitable for highly varying flows. The roll-shaped runner can be driven - depend-
ing on the available discharge - by opening only the small, the large, or both
chambers under partial load. Thus the turbine efficiency curve is very flat (see
Fig. 8.9). However, the absolute efficiency is not as high as for other turbine de-
signs. Additionally, very low heads cannot be used entirely. Because of its good
adjustment possibilities to varying flows and the simple, robust - and therefore
inexpensive - design, the cross-flow turbines are often built into small hydroelec-
tric power stations. For medium and low heads a draft tube is necessary to im-
prove head utilisation.