Environmental Engineering Reference
In-Depth Information
cover almost 600,000 km
2
, an area nearly twice as large
as Italy, and those used solely or mostly for electricity
generation cover about 175,000 km
2
. This prorates
to about 4 W/m
2
in terms of installed capacity and to
about 1.7 W/m
2
in terms of actual generation. The
power densities of individual projects span 3 OM. There
is a predictable rise of power density with increasing in-
stalled capacity. Projects with 2-99 MW
ei
averaged just
0.4 W/m
2
, those with 500-999 MW
ei
rated 1.35 W/
m
2
, and mean power density for the world's largest
dams (3-18.2 GW
ei
) surpassed 3 W/m
2
(Goodland
1995).
Dams on the lower courses of large rivers impound
huge volumes of water in relatively shallow reservoirs.
The combined area of the world's seven largest reservoirs
is as large as the Netherlands and the top two, Ako-
sombo on the Volta River in Ghana (8730 km
2
) and
Kuybyshev on the Volga River in Russia (6500 km
2
),
approach the size of small countries like Lebanon or Cy-
prus. The power densities of these projects are below 1
W/m
2
(fig. 9.4). In contrast, dams on the middle and
upper courses of rivers have power densities on the order
of 10
1
W/m
2
. Itaip ´ has power density of 9.3 W/m
2
,
Grand Coulee rates nearly 20 W/m
2
, China's Sanxia
(the world's largest at 17.68 GW when finished in
2008) will have about 28 W/m
2
, and some Alpine sta-
tions surpass 100 W/m
2
. The world's record holder will
be the Nepali Arun project, whose 42-ha reservoir and
210 MW installed capacity translates to about 500 W/
m
2
.
All these cited density values are calculated by using in-
stalled capacities; actual generation, determined by water
availability, translates into substantially lower rates. Load
factors of most hydro stations are highly variable, but
the average rates are commonly below 50% (Itaip´ was
9.4 Power densities of the world's largest hydro projects, cal-
culated using total installed capacities. Actual electricity gener-
ation densities depend on variable load factors.
designed for 68%, Sanxia for 53%). Many projects are
built primarily as peaking power facilities. Consequently,
effective power densities of most hydro stations are
merely one-third to one-half of the theoretical rates.
Pumped storage has become an increasingly important
component of hydroelectric generation. It uses night-
time or weekend low-demand power to pump water to
an elevated reservoir to be released during peak-demand
hours through turbines into a lower reservoir. Genera-
tion can start in as little as 10 s.
With heads up to 1400 m (commonly 200-500 m)
and limited load factors, pumped storage generation has
quite high power densities, mostly 1-3 kW/m
2
. These
relatively expensive plants were built first during the
