Environmental Engineering Reference
In-Depth Information
Science: Producing Electricity from Flowing
Water—Dams, Tides, and Waves
Water flowing in rivers and streams can be trapped in
reservoirs behind dams and released as needed to
spin turbines and produce electricity.
Solar energy evaporates water and deposits it as water
and snow in other areas through the water cycle. Wa-
ter flowing from higher to lower elevations in rivers
and streams can be controlled by dams and reservoirs
and used to produce electricity (Figure 11-8, p. 243).
This indirect form of renewable solar energy is called
hydropower.
The most popular approach is to build a high dam
across a large river to create a reservoir. Some of the
water stored in the reservoir is allowed to flow
through huge pipes at controlled rates, spinning tur-
bines and producing electricity. Smaller and lower
dams can also be used.
In 2003, hydropower supplied about one-fifth of
the world's electricity, including 99% of that used in
Norway, 75% in New Zealand, 25% in China, and 7%
in the United States (but about 50% on the West
Coast).
Figure 13-36 lists the advantages and disadvan-
tages of using large-scale hydropower plants to pro-
duce electricity.
Because of increasing concern about the harmful
environmental and social consequences of large dams,
the World Bank and other development agencies have
been pressured to stop funding new large-scale hy-
dropower projects. Also, according to a 2000 study by
the World Commission on Dams, hydropower in trop-
ical countries is a major emitter of greenhouse gases.
The reservoirs that power the dams can trap rotting
vegetation, which can emit greenhouse gases such as
carbon dioxide and methane.
T rade-Offs
Large-Scale Hydropower
Advantages
Disadvantages
Moderate to high
net energy
High construction
costs
High efficiency
(80%)
High environmental
impact from flooding
land to form a
reservoir
Large untapped
potential
High CO 2 emissions
from biomass decay
in shallow tropical
reservoirs
Low-cost electricity
Long life span
No CO 2
emissions
during operation
in temperate areas
Floods natural areas
behind dam
Converts land habitat
to lake habitat
May provide flood
control below dam
Danger of collapse
Provides water
for year-round
irrigation of
cropland
Uproots people
Decreases fish
harvest below dam
Reservoir is
useful for fishing
and recreation
Decreases flow of
natural fertilizer (silt)
to land below dam
Figure 13-36 Trade-offs: advantages and disadvantages of
using large dams and reservoirs to produce electricity. Critical
thinking: pick the single advantage and the single disadvan-
tage that you think are the most important.
The greater heating of the earth at the equator than at
the poles and the earth's rotation set up flows of air
called wind. This indirect form of solar energy can be
captured by wind turbines and converted into electric-
ity (Figure 13-37, p. 318).
Since 1990, wind power has been the world's
fastest-growing source of energy, with its use increas-
ing almost sevenfold between 1995 and 2004. Europe
is leading the way into the age of wind energy. About
three-fourths of the world's wind-generated power is
produced in Europe in inland and offshore wind farms
or parks. European companies manufacture 80% of the
wind turbines sold in the global marketplace. Wind
power also is being developed rapidly in India and to
a lesser extent in China.
Much of the world's potential for wind power re-
mains untapped. According to a 2003 Wind Force 12 re-
port, wind parks on only one-tenth of the earth's land
could produce twice the world's projected demand for
electricity by 2020. This estimate does not include the
establishment of wind parks at sea.
x
H OW W OULD Y OU V OTE ? Should we greatly increase our
dependence on renewable large-scale dams for producing
electricity? Cast your vote online at http://biology.brookscole
.com/miller11.
Small-scale hydropower projects eliminate most
of the harmful environmental effects of large-scale
projects. But their electrical output can vary with sea-
sonal changes in stream flow.
We can also produce electricity from water flows
by tapping into the energy from tides and waves. Most
analysts expect these sources to make little contribution
to world electricity production because of high costs
and lack of enough areas with the right conditions.
Science: Producing Electricity from Wind
Since 1995, the use of wind turbines to produce
electricity has increased almost sevenfold.
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