Environmental Engineering Reference
In-Depth Information
50
45
40
Pump Storage
(pumping)
Pump Storage
(generating)
Gas Turbines
35
Least efficient steam turbines
30
25
20
Steam Turbines
15
10
Most efficient steam turbines
Nuclear
5
0
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-5
Figure 7.3
Typical generator scheduling to meet demand
Figure 7.3 shows that hourly demand throughout the day can be met by a variety of gen-
erating plant in a typical power system. Nuclear generation, being relatively infl exible, is
allocated fi rst. Pumped storage reservoirs are fi lled using low cost electricity during the night-
time period. More effi cient gas and steam thermal generators are next in merit, and fi nally
fast response gas turbines and hydro pump storage are used to meet demand during peak
periods. In the next section, how the so-called 'optimum economic dispatch' is arrived at is
discussed.
7.3.2 Optimum Economic Dispatch
The aim is to load generating sets in response to demand changes so as to minimize the cost
of supply. This is known as
optimum economic dispatch
(OED) and over the years sophisti-
cated OED methods have been developed to identify the minimum cost solution and at the
same time satisfy numerous engineering and operational requirements and constraints. Here
the basic ideas that provide guidance in the task of adjusting the load on individual conven-
tional or RE generators will be investigated in order to achieve a minimum production cost
on a short term basis.
A crude way of applying economic dispatch is to assume that the cost of operation of a
plant is a linear function of the plant loading level, i.e.
=+′
where
C
A
is the cost in £/h for the
A
th plant,
K
A
and
CKKP
A
A
A
A
K
A
are constants related to the
capital and fuel plant cost components respectively and
P
A
in MW is the power deliv-
ered by the plant. To determine the optimum load sharing between two plants A and
B when the total demand on the system is
P
d
the linear cost curve for unit A is plotted
normally as in Figure 7.4, while the axis of the unit B curve is rotated by 180 °. The
origin
O
A
of unit A is placed at a distance from
O
B
equal to the total load
P
d
=
P
A
+
P
B
.
′
