Environmental Engineering Reference
In-Depth Information
tical area swept by rotating blades 50 m above ground.
Spacing equal to five rotor diameters is enough to avoid
excessive wake interference, but at least twice that dis-
tance is needed for wind energy replenishment in large
installations.
Moreover, no more than 16/27 (59.3%) of the wind's
kinetic energy can be extracted by a rotating horizontal-
axis generator (the Betz limit), and the actual capture
is about 80% of that. For example, in locations with
wind power density averaging 450 W/m 2 (mean value of
power class 4 common in the Dakotas, northern Texas,
Western Oklahoma, and coastal Oregon), machines with
a 50-m high hub would intercept about 7 W/m 2 , but
average 25% conversion efficiency and 25% power loss
caused by wakes and blade soiling would reduce the
actual power output to about 1.3 W/m 2 (Elliott and
Schwartz 1993). Vertical power density of 700 W/m 2 ,
turbine efficiency of 35%, and power loss of 10% would
more than double that rate to about 3.5 W/m 2 . Actual
rates are highly site-specific. Altamont Pass averaged
about 8.4 W/m 2 (D. R. Smith 1987). The most densely
packed wind farms rate up to 15 W/m 2 ; more spread-
out sites 5-7 W/m 2 and as low as 1.5 W/m 2 ; and the
best offshore sites 10-22 W/m 2
and Wright 1999). Geothermal electricity generation be-
gan at Italy's Larderello field in 1902. New Zealand's
Wairakei was added in 1958, California's Geysers in
1960, and Mexico's Cerro Prieto in 1970. Only later did
geothermal generation expand beyond these four pio-
neering high-temperature vapor fields.
By the year 2000 the United States had installed nearly
2.2 GW, the Philippines 1.9 GW, and Italy 785 MW.
The global total was 7.7 GW, and annual generation
reached nearly 52 TWh, implying an average load factor
of 7% (IGA 2005). Power densities of 20-50 W/m 2 are
comparable to those of Alpine hydro stations. Global ca-
pacity of 8 GW is only about 11% of the total that could
be harnessed with existing techniques, and the prospec-
tive potential of 138 GW is less than 5% of the world's
electricity-generating capacity in the year 2000. Geother-
mal energy will remain a globally marginal source of
electricity, but it can be a locally important supplier of in-
dustrial and household heat. U.S. capacity was about 3.7
GW t in 2000, followed by China (2.3 GW t ) and Iceland
(1.5 GW t ). Global capacity was about 15.1 GW t
in the
year 2000 (IGA 2005).
The photovoltaic (PV) effect, whereby electricity gen-
eration of an electrolytic cell made up of two metal elec-
trodes increased when exposed to light, was discovered
by Edmund Becquerel in 1839. In 1873 came the
discovery of the photoconductivity of selenium, which
made it possible for W. G. Adams and R. E. Day to
make the first PV cell just four years later; the conversion
efficiencies of such cells were a mere 1%-2%. The decisive
breakthrough came only in 1954, when a team of Bell
Laboratories researchers produced silicon solar cells that
were 4.5% efficient and raised that performance to 6%
just a few months later. By March 1958, when Vanguard
I became the first PV-powered satellite (0.1 W from
(McGowan and Con-
nors 2000).
Geothermal electricity generation can tap only a tiny
part of the immense flux of the Earth's heat (see section
2.5). Drilling to depths of more than 7 km to reach rock
temperatures in excess of 200 C is now possible, but it
would be economically prohibitive to do that in order to
inject water for steam generation. Consequently, today's
conversion techniques could harness at best about 72
GW of electricity-generating capacity; enhanced recovery
and drilling improvements currently under development
could enlarge this total to about 138 GW (Gawell, Reed,
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