Hydroelectric Power Generation - Renewable Energy: Technology, Economics and Environment - page 367

Water wheels run with a lower rotational speed of approximately 5 to 8 rota-
tions per minute. Therefore transmissions or belt drives are required to achieve the
higher rotational speed necessary to drive the generator.
Initially, water wheels were mainly made of wood. Soon axles, wheel rims and
spokes made of steel gained acceptance. Up to the present day the blades them-
selves are still made of wood in many cases. They are normally run on bearings
on both sides and should, if there is a risk of heavy icing, be stored in a closed
wheel chamber. Water wheels that run on bearings on one side only, coupled with
a standard planet transmission and a generator unit are a recent development. The
complete wheel-generator-transmission unit can be prefabricated and delivered to
the place of installation. The customer only has to build the water intake, the
flume for the wheel itself and the supporting foundation. The complete unit is put
in and can virtually start operating immediately afterwards.
Outflow and tailrace. The water leaves the turbine into the so-called tailrace. For
reaction turbines, a draft tube is used to make better use of the available head and
reach all the way to the tailwater. The cross-section of the draft tube is increasing
and shaped like a diffuser at the exit to the tailwater. Therefore the speed in the
draft tube is reduced before entering the tailwater; thus part of the kinetic energy
of the out flowing water can be used energetically within the turbines.
Shaft coupling and transmission. The turbine and the generator can be either
directly coupled or indirectly, via a transmission. In large plants, turbine and gen-
erator are installed on the same axis. In this case only a simple coupler is used. In
very small plants, however, transmissions are used in most cases because the in-
crease of the rotational speed through the transmission allows the use of standard,
high-speed generators. Toothed gearing for smaller units and toothed gearing or
belt drives for very small machine units (up to 100 kW) are used in most cases.
Transmission efficiencies, defined as the ratio of the power at the transmission
output shaft and the capacity at the turbine shaft, reach approximately 95 to 98 %.
If the head is low, turbines in small hydroelectric power stations run with rela-
tively low speed at e.g. 75 rpm. For cost reasons, generators are kept relatively
small; they have to run at a much higher speed. Therefore in smaller power sta-
tions a transmission or a belt drive has to be inserted between the turbine and the
generator.
Belt drives have proved to be efficient for up to approximately 50 kW electrical
power and are used for significantly higher power levels nowadays. They are
slightly cheaper than transmissions. But their life spans are generally shorter and
they need more maintenance. As nowadays the belts can be repaired or exchanged
in a very short time by mobile maintenance units, they are often the most cost-
efficient alternative.
Generator. In the generator mechanical energy from the turbine or the transmis-
sion shaft is converted into electrical energy. Synchronous or asynchronous gen-