Hydroelectric Power Generation - Renewable Energy: Technology, Economics and Environment - page 349

some cases a headrace, plus the powerhouse and tailrace (Fig. 8.1; see also Chap-
ter 8.2 and Fig. 8.2). The flow is led to the turbine via the intake structure, the
headrace and penstock. Afterwards it streams through the draft tube into the tail-
race.

Energy line

Energy loss
Velocity head

Headwater

1

Pressure
head

2

Screen

Pipe
hanger Stoplogs

Generator

Velocity head

Quick-action

stop valve

Energy line

Tailwater

3

5

4

Geodetic elevation
of the stream line

Draft tube

Reference level

Fig. 8.1 Physical correlations in a hydroelectric power station

In Fig. 8.1 the lines enable the graphic representation of the Bernoulli equation.
The dotted line represents the geodetic level of the water flowing through the hy-
droelectric power station. The so-called energy line is at the top left corner of the
diagram. It shows the locations and respective energy losses. The distance to the
broken line below the energy line corresponds with the kinetic energy of the wa-
ter. This becomes apparent at the intake structure, where the water flow increases
due to the narrowing of the cross-section and the kinetic energy therefore in-
creases at the same time. The difference between the geodetic level and the bro-
ken line is the pressure energy level.

Intake. The intake structure is the connection between headwater and penstock or
turbine. In the example (Fig. 8.1) there is a screen at the entrance of the intake
structure keeping floating debris out of the plant. Furthermore the intake structure
has stoplogs and a quick-action stop valve. The stoplogs enable the hydroelectric
power station to be drained during maintenance work. The quick-action stop valve
stops the flow into the hydroelectric power station in the case of an accident.
Within the intake structure, a partial conversion of potential energy into kinetic
energy takes place (reference point 1 to reference point 2; see Fig. 8.1). Because
of local energy losses in the intake and the flow resistance at the screen, part of
the energy is lost before it can be utilised in the turbine. These losses are com-
bined in Equation (8.3) as the loss correction value, ξ IS , for the intake structure. As
the headwater flow velocity usually can be neglected, the respective term on the
left side of the Equation (8.3) is dropped. The water density ρ Wa can be regarded