Environmental Engineering Reference
In-Depth Information
Small scale hydro schemes show little variability in output from minute to minute, but can
change substantially over hourly or daily cycles due to sudden rainfall. As already mentioned,
the output is also highly sensitive to the time of year.
If a large number of small scale hydro schemes are connected to an integrated electricity
network their infl uence on the minute to minute operation of the power system would be
negligible due to the statistical smoothing effect due to the aggregation of uncorrelated short
term variations.
Turbine Designs
All hydo systems exploit the potential energy of the water. Engineers use the term head,
H
,
for the height through which the water is allowed to fall; often in practice this becomes the
net or effective head, refl ecting frictional losses. Potential energy is simply
mgH
where
g
is
the acceleration due to gravity; for a volume
V
of water density
gH
. The power
in kilowatts associated with a fl ow rate
Q
m
3
/s falling through an effective head of
H
metres
is thus given by:
ρ
this is
V
ρ
= ρ
Two different approaches to turbine design exist: impulse turbines like the Pelton wheel
extract kinetic energy from the fl ow through impact on cups mounted on the turbine wheel,
while reaction turbines like Francis and Kaplan designs, which in contrast to impulse turbines,
operate submerged in water. Impulse turbines are suited to high pressure/head and can have
effi ciencies around 90%, whereas reaction turbines run faster and are suited to lower heads
and have higher effi ciencies. Turbine effi ciency is conventionally plotted, as in Figure 2.2,
as a function of the specifi c speed
N
S
=
n
(
P
)
H
5/4
, where
P
is the output power in kW,
H
is
the effective head in metres and
n
is the rotational speed in revolutions per minute (rpm).
The formula for the specifi c speed can be understood by considering a turbine scaled down
to produce 1 kW of power at a head of 1 m for then its runner will rotate at the specifi c speed
equal to its speed in rpm, i.e.
N
S
=
n
.
Figure 2.2 shows example effi ciency curves for different turbine types. The speed of the
runner blade relative to the water striking it is critical for the effi ciency of the turbine. Propel-
ler type turbines run best when their blade tips move faster than the water. In the case of the
PQgH
96
94
92
90
88
86
84
82
80
Francis turbines
Kaplan turbines
1 - jet Pelton turbines
0
76
152
228
304
380
456
532
608
684
Figure 2.2
E f fi ciency curves for different turbine types
