Environmental Engineering Reference
In-Depth Information
Outlet. Reaction turbines (e.g. Kaplan turbines, Francis turbines) enable a better
utilisation of the head using a draft tube.
How this system operates can be demonstrated by following the stream line of
current from the tailwater to the turbine outlet. The tailwater energy line is deter-
mined by its geodetic level and the ambient pressure (Fig. 8.1). When entering the
tailwater, the water looses its remaining kinetic energy through turbulence. This is
shown in Fig. 8.1 by the drop of the energy line at reference point 5. If - as shown
in Fig. 8.1 - the turbine outflow and the draft tube outlet (reference points 4 and
5) are at the same geodetic level, the Bernoulli equation between these two points
can be drawn up in a simplified way according to Equation (8.6).
2
2
p
v
p
v
4
+
Wa ,
4
=
5
+
Wa ,
5
(8.6)
ρ
g
2g
ρ
g
2g
Wa
Wa
Since the cross-section of the flow at the end of the draft tube is bigger than
that directly behind the turbine, v Wa, 5 has to be less than v Wa, 4 . The draft tube
causes a reduction of the flow velocity before entering the tailwater. Thus the
pressure p 4 at the turbine outlet has to be lower than the pressure determined at the
tailwater, at the end of the draft tube p 5 . Ultimately, this leads to a decrease in
losses due to turbulences and thus a better head utilisation.
Overall system. In a hydroelectric power station hydraulic losses mainly occur in
the intake structure, the penstock and possibly in the outflow (balance point 1 to
balance point 5; Fig. 8.1). The actual usable water power P Wa,act is calculated by
deducting the various loss terms from the theoretical water power (i.e. losses in
the intake structure, the penstock, the outflow); this can be described with Equa-
tion (8.7).
v
2
v
2
v
2
(
)
Wa ,
2
Wa ,
3
Wa ,
5
P
=
ρ
g q
h
h
ξ
ξ
(8.7)
Wa ,act
Wa
Wa
HW
TW
IS
PS
2g
2g
2g
The losses are thus dependent on the flow velocity and can consequently be
minimised with an optimised plant design and layout. The power that can finally
be obtained at the turbine shaft is a result of the actual available water power and
the turbine efficiency.
8.2 Technical description
Based on the discussed physical correlations for the use of hydroelectric power,
the technical requirements of hydroelectric power generation are described below.
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