Hydroelectric Power Generation - Renewable Energy: Technology, Economics and Environment - page 356

quired in very narrow streams for the dam to use the entire width of the river
as a floodway.
• Twin block power station. Here, the powerhouses are situated on both sides of
the river (i.e. detached design; Fig. 8.6). Such plants are normally built on
border rivers, as it enables both countries to generate energy separately.
• Power station in pier. In a pier-type power station the mechanical installations
and thus the powerhouse are identical with the piers that support the gates of
the barrage (i.e. detached design; Fig. 8.6). This type of power station is char-
acterised by favourable flow conveyance characteristics and a space-saving
design.
• Submersible power station. Power station and dam are built in one block
(Fig. 8.6). Thus the space required for machine groups and the required dam
are reduced to a minimum. Therefore the plant fits in well with the surround-
ing landscape. There are hardly any plant parts visible above the headwater
level.
If several run-of-river power stations are set up in a river directly one after the
other, they form a power station chain. In extreme cases the backwater curve of
the impoundment of a power station reaches upstream into the tailwater area of
the next upstream power station; there are hardly any free flowing reaches. In
some cases such power station chains are equipped with a large storage reservoir
at the beginning or compensation reservoirs in between; they have quite large vol-
umes with the capacity to store water for a certain amount of time to optimise per-
formance.
Medium-head plants. The medium-head plants exclusively built as barrages
mainly consist of a dam and a powerhouse at their base. Thus these plants use the
head created by the dam, which can be between 20 and approximately 100 m
high. The average discharges used by the turbines are partly obtained by appropri-
ate reservoir management. If medium-head plants are built as diversion-type
power plants, they sometimes use water from one or several streams, which is led
through channels, free-flow or low-pressure tunnels to a compensation and stor-
age reservoir and from there through pressure tunnels or shafts to the powerhouse.
High-head plants. High-head plants have a head of between 100 and 2,000 m
(max.). They can be found in low and high mountain ranges and are normally
equipped with a reservoir to store the inflowing water. The flow rates are rela-
tively low. In contrast to the low-pressure plants, where the available power is a
result of large flows, power is a result of high heads in this case. As the available
water often comes from very small catchment areas, the effort it takes to capture
water in the reservoirs is sometimes quite significant. Often, small streams are di-
verted from parallel valleys into the valley with the reservoir.
High-pressure plants can be planned as diversion-type or dam power stations.
Diversion-type high-head power stations divert the water from the reservoir
through tunnels or low pressure pipes via a so-called surge tank (reduction of wa-