Hydroelectric Power Generation - Renewable Energy: Technology, Economics and Environment - page 358

in Fig. 8.4) fish ladder cannot be used in this turbine. But the outflow from the
turbine ought to be combined in a useful way with the attracting flow for the fish
ladder. The advantage of constant or seasonally varying in-stream flows is that
comparatively simply built - and therefore relatively cheap - turbines can be used
because of the constant discharge.
8.2.3 System components
The turbine, the generator, and the transmission that might be required in some
cases (Fig. 8.2), are all located in the powerhouse. Furthermore, structural and
mechanical components and other system elements are required. In the following
they are going to be described and discussed according to current state-of-the-art
of technology. The technical information generally applies to medium and larger-
size power stations, but it also applies to small hydroelectric power stations in
most cases as well. The latter, however, might have a series of special compo-
nents. Those will be mentioned separately - if relevant.
Dam, weir or barrage. The dam has to bank up the water in order to enable a
controlled supply of water from the impounded water body to the power station.
Thus the natural head of a stream is concentrated in one place. Upstream of the
barrage or the dam is the reservoir with a certain storage volume. The dam and
spillway need to be able to convey floods and maintain the water level at the re-
quired elevation in times of low water.
Dams can be built in many ways; they can take the form of fixed weirs, bar-
rages, earth or rockfill embankments, masonry or concrete dams. Weirs can be
fixed or movable, whereas barrages always have movable gates.
For smaller hydroelectric power stations the selection of retaining works de-
pends on whether the headwater needs to be kept at a constant level. For very low
heads and for plants in run-of-river design, the former is usually the case. Bar-
rages or weirs with moveable gates have to be used in that case. These maintain a
constant water level upstream of the dam and the turbine is regulated such that
this water level is maintained. If the inflow to the impoundments exceeds the de-
sign discharge of the turbines, the gates have to be opened partly to release the
excess inflow directly into the riverbed without further increasing the headwater
level.
Mainly fixed weirs, fitted with flap gates and inflatable rubber dams are used in
this case (Fig. 8.7). Flaps, often called fish-belly flap gates because of their pro-
file, are driven hydraulically or by racks or chains; during a flood they are low-
ered on to the dam. Inflatable dams consist of a very firm, multi-layered rubber
membrane and are filled with either air or water. A connected pump regulates the
level of the dam crest using the internal pressure.
In the recent past inflatable dams have been increasingly accepted and - as
long as there was no risk of vandalism - also proved to be reliable; however, the