Hydroelectric Power Generation - Renewable Energy: Technology, Economics and Environment - page 363

ised in combination; thus it can be ensured that the individual machines are
mainly used at design discharge.
Bulb turbines are Kaplan turbines with an almost horizontal axis. Their axial
flow does not require any change of direction of the flow. This principle reduces
hydraulic losses. Here, the generator is located directly in front of the turbine
within a steel bulb. The generator can be accessed via a shaft. Their design is
similar to the Straflo turbine described in Fig. 8.10. However, the bulb is bigger,
as it accommodates the bearings of the turbine and the generator.
Bevel-gear turbines are built in a similar way. The submerged housing only
contains a bevel-gear pair that converts the rotation of the turbine shaft to the gen-
erator axis that is vertical to the turbine axis; the generator is attached to the tur-
bine housing and freely accessible.
The draft tube of S-turbines is bent down significantly immediately behind the
almost horizontal located turbine. The turbine shaft is led outside to the generator;
thus this is freely accessible, i.e. for maintenance works.
Straflo-turbines - Straflo is the abbreviation for straight flow - have a similar
system design as Kaplan turbines and operate with external rim generators; i.e. the
rotor sits on a ring attached to the runner blades of the turbine. This design is also
shown in Fig. 8.10. In the steel bulb behind the runner is only the bearing of the
turbine. The disadvantage of this design is the costly sealing between the runner
and the generator.
Because of their design a flat efficiency curve at an overall high level can be
realised for Straflo-turbines (Fig. 8.9). Due to their double regulation, all Kaplan
turbines and derived designs can be operated over a broad partial-load range (30
to 100 % of the design power) with comparably high efficiency levels.

Mounting cran

Generator
stator

Trash rack

Guide vanes

Runner

Draft tube

Generator
rotor

Fig. 8.10 Run-of-river power station with Straflo turbine (see /8-9/)

Francis turbines. In Francis turbines - they are typical reaction turbines - the wa-
ter flows radially from the guide vanes into the scroll case, over the runner blades,