must be installed to meet the peak demand while, on average, a consider-
able part of this machinery is standing idle.
The choice of type of power system depends on the kind of fuel
available, its costs, availability of suitable sites, etc. While the fuel costs of
a hydroelectric plant are virtually nil, the construction costs of civil engin-
eering works are usually much greater than those of a thermal plant owing
to the additional costs of the impounding structures, penstocks, long trans-
mission lines, etc.
Thermal plants are more efficient to run at full load and hence are
suitable for generating continuously near capacity to carry the base load.
A hydroelectric plant can be put into operation in a time of a few seconds
to 4–5 min, whereas at least 30 min are required to start up and load a
thermal system. Hydroelectric plants are therefore very suitable for
meeting variations in load with little waste of power. In an ideal intercon-
nected system the thermal stations would be used to generate up to the
maximum base load, with the hydrostation providing peak power.
Nuclear power plants are equally unsuitable for variable load operation,
as the reactors cannot be easily controlled to respond quickly to load changes,
and hence are used as baseload plants at a load factor of at least 80%.
Because of lack of bulk, nuclear fuel transport costs are negligible and hence
nuclear power plants are advantageous in locations where conventional fossil
fuels and hydropower are unavailable. Although nuclear power plants, unlike
fossil-fuel thermal plants, do not need expensive air-pollution control systems,
safety and waste disposal remain a serious consideration.
12.4 Some fundamental definitions
The gross head, H 0 , at a hydroelectric plant is the difference in water level
between the reservoir behind the dam and the water level in the tail race.
498 HYDROELECTRIC POWER DEVELOPMENT
Fig. 12.1 A typical demand curve
