Novak P. Developments in hydraulic engineering - Vol. 5
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Low-head plants are equipped with axial turbines or, at the present rarely, with high-
speed Francis turbines, thus utilizing heads generally up to around 30 m, less frequently
up to around 50 m. With low-head stations the intake, the machine hall and the
conveyance system in between are usually integrated in a single structure—the
powerhouse.
High-head power stations usually house Francis turbines or Pelton wheels and, owing
to the higher heads, the intake, the conveyance (penstock or shaft) and the powerhouse
are usually separate structures.
3.7 Classification of Plants According to their Role in the Power
Supply System
The operation mode of a hydropower plant depends partly on the available input, i.e. the
natural flow conditions and storage possibilities, and partly on the output demands, i.e.
the consumers’ needs.
(a) Isolated plants, located mostly in remote regions, are not connected to a regional or
national power network; they provide electric energy for a lonely consumer or for a tiny
consumer group. A co-operating plant supplies its power into a regional, national or
multi-national grid and, consequently, its operation mode, beside the physical potential
input, is greatly influenced by both the consumers’ demands and the power available
from all other plants connected to the network (i.e. other hydropower projects, thermal
and nuclear stations). ‘Available’ refers here not only to the capacities but includes also
economical operation.
(b) According to the kind of energy fed into the network the plants can be sorted
roughly into two categories: base-load and peak-load plants. Run-of-river stations
produce base energy and, if pondage is possible, a relatively low quantity of peak
kilowatt-hours too. Storage projects, on the other hand, are mostly peak-load plants,
being suitable to contribute, according to the degree of storage, to daily, weekly and
seasonal peak production of the power system. As the main objective of creating a
reservoir is often, on the contrary, the equalization of the inflows, a consumer needing
uniform base load can also be fully satisfied (e.g. for some kinds of electrochemical,
metallurgical industries). Dumped storage is especially established for peak power
production. Tidal schemes, wave power generators and depression plants can contribute,
without any additional energy conversions, to coverage of base demands only.
Base energy is evidently of lower value than peak energy. Power production, due to
the intricate economic evaluation of the various kinds of producible kilowatt-hours, uses
a cluster of energy types supplying the network.
4 LOW-HEAD RIVER DEVELOPMENTS
Water power development: low-head Hydropower utilization 19
4.1 River Channel Projects
4.1.1 Types of General Layout
The simplest type of low-head river plant consists of two parts: the weir (spillway) and
the powerhouse. Depending on the physical conditions and on the selected type of
structure, the powerhouse may be located within the original channel or in a bay-like
enlargement. The shaping of the bay influences the head losses and the uniformity of
flow distribution to the turbines.
26
The river barrage on a navigable waterway also
incorporates a single or double ship lock, either adjoining the weir or the powerhouse or
situated in a by-pass canal (Fig. 5). According to local needs the project may also
comprise a boat-lock, a fish-pass, a log-chute, a sediment flushing sluice, further intakes
for diverse water requirements and control structures. In some instances, mainly to
simplify construction, the barrage is placed into a cut of the river bend (Fig. 6). To ensure
the safe release of floods, gated weirs are applied almost exclusively on larger
watercourses.
It used to be very common to build an intake structure in front of the powerhouse at
the entrance of the headwater bay, consisting of sill, coarse rack, skimmer wall and
service bridge (Fig. 7). This arrangement has practically been abandoned and is now
adopted only in exceptional cases.
Typical low-head stations fall into four main categories according to the location and
the general layout of the powerhouse:
1. Unit-block and twin arrangement
2. Pier-head power station
3. Submersible power plant
4. Underground location
4.1.2 Unit-Block and Twin Arrangement
The machine sets are installed into an open-air powerhouse block, or divided between
powerhouses adjoining the weir on either side (Fig. 8).
Developments in hydraulic engineering–5 20
FIG. 5. River barrage on navigable
waterway. Upper drawing: ship lock
directly joins the other structures;
Aschach, Danube, Austria, 1964, 2040
m
3
/s, 15 m, 286 MW.
26a,b
Lower
drawing: ship lock in by-pass canal;
Birsfelden, High Rhine, Switzerland,
1954, 1500 m
3
/s, 4·9–9·2 m, 85
MW.
26c
The latter, the so-called twin arrangement, was, in a few cases, chosen when the river at
the power site constituted the frontier between two countries and, although the project
was a joint undertaking, both wanted to possess a separate powerhouse (Fig. 9).
27
Furthermore, it is sometimes useful to increase the flood-discharging capacity of the weir
by locating it into the main current of the river (Fig. 10)
28,28a,28b
Water power development: low-head Hydropower utilization 21
In the conventional solution the block-type power station is equipped with
conventional machine sets, with regular setting, i.e. the roof of the spiral case lies lower
than the lowest headwater level. The conventional aggregate comprises a vertical-shaft
axial (Kaplan or propeller) turbine
FIG. 6. River barrage located in a cut
of the river channel; Tiszalök, Tisza,
Hungary, 1953, 300 m
3
/s, 0–7·5 m, 12
MW.
Developments in hydraulic engineering–5 22
FIG. 7. Power plant with enclosed
headwater bay; Chancy-Pougny,
Rhône, France.
FIG. 8. Unit-block and twin-station
arrangements.
with spiral casing and elbow-type draft tube, coupled directly or by gear-drive to the
generator (Figs 11 and 12).
In order to reduce foundation costs it is, under particular geological conditions,
expedient to aim at a higher setting of the machines and, consequently, a higher location
of the draft tube. If the safety concerning
Water power development: low-head Hydropower utilization 23
FIG. 9. Twin station on a boundary
stretch of a river; Iron Gate,
Yugoslavia/ Rumania, Danube, 8700
m
3
/s, 27·2 m, 2050 MW.
27
FIG. 10. Twin arrangement chosen for
hydraulic reasons; Ybbs-Persenbeug,
Danube, Austria, 1959, 2160 m
3
/s,
10·6 (6–14) m, 200 MW.
28
Developments in hydraulic engineering–5 24
cavitation is not jeopardized (see Section 7), even a so-called siphon setting
29,30
may
prove to be an optimal solution with the spiral case roof elevated above the minimum
headwater level (Figs 13 and 14).
31
A remarkable solution for reducing construction costs has been achieved by the
construction of spillway conduits within the powerhouse proper, thus rendering possible a
reduction in the width of the weir or even its complete omission. The spillway conduits
releasing partly or completely the excess flow Q−Q
p
may serve two more purposes:
firstly, the high-velocity jets, released through the conduits and submerged into the
tailwater, depress the tailwater level at the draft tube exits and consequently increase the
effective turbine head; secondly, in the case of a sudden load rejection, by using
automatic gate control devices synchronized with the turbine governors, the generation of
surges
32,33
(positive in headwater and negative in tailwater) can be avoided or at least
significantly damped. This beneficial effect is of paramount importance on navigable
watercourses. The main types of arrangement are exemplified here by a powerhouse with
elevated
34
(Fig. 15) and deep-located
35
(Fig. 16) spillway conduits. With the diversion-
type of development it is possible for the capacity of the conduits to equal the plant
design flow Q
p
, so that construction of a weir
FIG. 11. Cross section of a typical
low-head powerhouse equipped with
conventional Kaplan machine sets;
Tiszalök plant (see Fig. 6).
Water power development: low-head Hydropower utilization 25
or a spillway at the station is not necessary at all. Outstanding examples for similar
solutions are power plants on the navigable Grand Canal d’Alsace along the upper Rhine.
The head-increasing effect of the overflow jet has been overestimated in the past as it
is restricted to periods when Q>Q
p
and is induced only by that portion of the excess flow
which spills directly in front of the draft tubes (see also Section 4.1.4). The flow through
the adjoining weir, especially in the case of a long dividing wall, has no or only a very
slight bearing on the tailwater level at the draft tube exits.
In the last decades the bulb-type tubular aggregates have become the favoured
machines in block powerhouses for harnessing heads in the lower portion of the low-head
range, i.e. up to about 25 m (Fig. 17).
36
The tubular turbine adapted originally, in its first
version (with a rim-generator), to the submersible powerhouse design has undergone a
multi-phase
Developments in hydraulic engineering–5 26
FIG. 12. Coupling of turbine to
generator.
evolution during the last five decades. Some technological problems have retarded its
wider application so that the more common tubular set at present is the bulb-type turbine.
But a very promising revival of the rim-generator turbines has recently been witnessed,
i.e. its improved version, the so-called STRAFLO turbine operating well for heads of up
Water power development: low-head Hydropower utilization 27
to 15–20 m (Fig. 18).
37
Finally, it has to be mentioned that the S-type tubular turbine
(coupled to an open-air generator) is also favoured in the lower portion of
FIG. 13. Low-head station equipped
with conventional Kaplan units in
elevated (siphon) setting; Vargön,
Göta-Älv, Sweden, 1934, 700 m
3
/s,
3·3–4·3 m, 26 MW. (After a
publication of the KMW turbine
factory, 1946.)
the low-head range and designed, almost without exception,
37a
for low-capacity units
(Fig. 19).
In order to avoid disturbing whirl formation in front of the powerhouse and to obtain a
uniform velocity inflow pattern to the turbines, the proper shaping of the dividing wall
between the powerhouse and the adjoining structure(s)—weir, ship lock, etc.—is of great
importance.
38,38a,38b,53
4.1.3 Pier-Head Layout
The tendency to reduce the enlargement of the river bed required for the powerhouse bay
resulted, about 50 years ago, in a combined design of weir and powerhouse when the
machines were placed in the piers of the weir (Figs 20 and 21), each pier housing one or
Developments in hydraulic engineering–5 28