48
2. Formulating
the Problem
Fig. 2.3 Relation between storage volume of a reservoir and head.
pected that in mixed hydro-thermal systems the hydro plants normally
would be assigned to base-load duty. However, in practice hydro plants
are more likely to be used to meet peaking requirements rather than for the
base load because (1) limitations on the available storage capacity of the
reservoir and large mandatory releases and other requirements are such
that the hydroelectric plant is not capable of providing the base load, and
(2) the time required to start or shut down the generation of electricity
in a hydroelectric plant is relatively short compared to that for a thermal
plant. It is more economical to start and shut down a hydroelectric plant
than a thermal plant.
The net revenue for a period from electric energy might be expressed as
follows (in words) :
net revenue — (dollars/MW-hr)(energy output) — generating costs
and the energy output might be
energy output = 8.50 y( Σ
A jm Q imt )Zijt
m
where Ζ φ is the head in reservoir j in month i of year t and 8.50 is a constant appropriate for the reservoir. If some of the water released by-passes
the turbines, then the fraction passing through the turbines y is less than 1.