12.2 Worldwide hydroelectric power development in perspectiveThe total installed hydropower capacity worldwide is about 740 GW and
the energy produced annually is about 2770 TWh i.e. approximately 19%
of the world’s electricity supply (Aqua-Media International, 2004).
The gross theoretical annual potential worldwide for primary
hydropower (i.e. excluding pumped storage schemes) is over 40 000 TWh of
which about 14 000 TWh is considered to be technically possible and about
8000 TWh economically feasible. (Bartle and Hallowes, 2005). The above
figures hide, of course, the large variability of the potential and installed
hydropower across continents and countries due to morphology, climate
and economic development. Thus, e.g. in Norway over 99% of the total
power supply is from hydropower whereas in the UK it is only about 3%.
Some very large hydropower schemes are under construction or have
been recently completed. The Three Gorges project when completed (in
2009) will have 26 turbine generator units of 700 MW each, totalling
18 200 MW installed capacity producing 84.7 TWh annually; there is also
provision on site for a future expansion by 6 further units. The Ghazi
Barotha project in Pakistan, completed in 2004 (which features a 52 km
long lined channel), has a generating capacity 1450 MW with an average
output of 6600 GWh. However, in many countries the major emphasis in
new hydropower development is on pumped storage schemes (see Section
12.5) and particularly on small hydro plants (see Section 12.11).
12.3 Power supply and demand
Demand for electrical power varies from hour to hour during the day,
from day to day, and from year to year; the power demand is defined as
the total load which consumers choose, at any instant, to connect to the
supplying power system. The system should have enough capacity to meet
the expected demand, in addition to unexpected breakdowns and mainte-
nance shutdowns. A daily load (demand) curve for a typical domestic area
is shown in Fig. 12.1.
Base load is the load continuously exceeded, whereas the average load
is the area under the curve divided by the time. The load factor over
a certain period is the ratio of the average load to the peak load and
is expressed as a daily, weekly, monthly or yearly value. A single station
connected to an industrial plant may have a load factor of, for example 80%.
In a country where the supply is distributed through a national grid system
for a diversity of uses, the annual load factor may be of the order of 40%.
It must be appreciated that low load factors represent a degree of
inefficiency, as sufficient capacity in the form of generating machinery