Hydroelectric Power Generation - Renewable Energy: Technology, Economics and Environment - page 371

lower, as one part of the incoming water (i.e. floodways) is diverted, unutilised,
across the dam since a portion of the annual water volume usually has to be re-
leased over the spillway during floods.

Operation behaviour and power curve. The systems performance and thus the
operating performance of a run-of-river power station for the supply of electrical
energy during the course of the year largely depend on the available flow and the
current head. Fig. 8.17 shows the interaction of these components, including the
corresponding turbine flow over the course of one year.
In the example the head rises during the summer months as the tailwater level
drops with a decrease in flow. Because of the conditions at the dam, the headwater
is kept at a constant level. The head correspondingly decreases during the winter
and during spring. The outflow increases and thus the tailwater rises slightly.

Flow Head Turbine flow Power

Fig. 8.17 Operation scheme of a run-of-river power station (see /8-5/)

The turbine flow is linked to the discharge in the river; it therefore decreases
during the summer in line with the lower discharge. As the turbine is only de-
signed for a given maximum flow (so-called design flow), only the design flow
can be utilised, even if the discharge in the river is higher. The additional dis-
charge has to be spilt unutilised. In the example represented in Fig. 8.17, this is
the case in February, March and particularly in April.
The power output of the hydroelectric power station is almost proportional to
the flow through the turbine (Equation (8.5)). Thus in the example given in
Fig. 8.17, it decreases during the summer months, which are characterised by de-
creasing flows. Furthermore, power output is also dependent on the available head
(Equation 8.5)); as the head normally does not change as much as the flow
through the turbine, its influence is generally less significant. The dependency of
the power generation of the hydroelectric power station on the head, however, is
responsible for the slight power output decrease in April - in spite of appropri-
ately high flows.