Environmental Engineering Reference
In-Depth Information
operational costs are low, there are no sulfur or nitrogen dioxide emis-
sions or particulate air pollution, and thermal pollution is limited com-
pared to nuclear plants. Hydroelectric plants also tend to have longer
economic lives than fuel-fi red generation; some plants still in service were
built 50 to 100 years ago.
Because fairly large rivers are best suited for the generation of hydro-
power, geography is a major control on the location of large dams.
With the exception of Arizona and the Colorado River, all of the states
with the highest hydropower generation are in the rainy and mountain-
ous northwestern states or are tapping the abundant moisture in the
mountainous Appalachian region.
Water supply may not be the same year-around, so the electric
energy produced can vary seasonally. In some installations the water
fl ow rate can vary by a factor of 10:1 over the course of a year. This
variation can be overcome with pumped storage. Low water fl ow and
times of peak demand can be accommodated by moving water between
reservoirs at different elevations. At times of low electrical demand,
excess generation capacity is used to pump water into a higher res-
ervoir. When demand is higher, water is released back into the lower
reservoir through a turbine. Pumped storage schemes currently provide
the only commercially important means of large-scale energy storage
in hydropower facilities. Hydroelectric plants with no reservoir capac-
ity are called run-of-the-river plants, since it is not possible to store
water.
Hydroelectric plants are not without environmental drawbacks.
Despite the fact that they do not burn fossil fuels, they produce global-
warming carbon dioxide and methane gas, and in some cases this can
have a climatic effect worse than that of plants running on fossil fuel.
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Methane is twenty times more effective at trapping heat than carbon
dioxide. Emissions vary from dam to dam, and scientists do not agree
on the quantitative importance of the methane release. The gases are
released from the trees and other plants that rot after they are carried
into the reservoir behind the dam. After the fi rst pulse of decay when
the organic debris releases carbon dioxide, the plant matter settles to the
fl oor of the reservoir and is buried, where it decomposes in the absence
of oxygen and generates methane gas.
There are seasonal changes in water depth in most reservoirs, and in
the dry season when the water level in the reservoir drops, plants colo-
nize the area around the reservoir. Subsequently the plants are drowned
when the water level rises in the rainy season. Low areas around a
