Environmental Engineering Reference
In-Depth Information
8
Rainfall, like wind, is common but is unevenly distributed across the surface of the earth. Hydro-
electric energy technologies, which in a sense use water for “fuel,” depend on rainfall to create
flowing water that, when falling due to the gravitational pull of the earth, may be used to generate
mechanical power to grind flour, motivate other machinery, or produce electricity. Thus, hydro-
electric energy is produced by the force of falling water (USGS 2011). The movement of water as
it flows downstream creates energy that can be converted into electricity. A hydroelectric power
plant converts this energy into electricity by running water, often held in a reservoir behind a dam,
through a hydraulic turbine connected to a generator. Water exits the turbine and is returned to a
stream or riverbed below the reservoir (USEPA 2011).
The capacity to produce hydroelectric energy is dependent on both the available flow and the
height from which it falls. Building up behind a dam, water accumulates potential energy. The
amount of power that may be extracted from water depends on the volume and the difference
in height between the source and the turbine. This height difference is called head. The amount
of potential energy in water is proportional to the head. Falling water is used to turn a turbine,
as illustrated in
Figure 8.1.
The turbine's rotation spins electromagnets that generate current
in stationary coils of wire. Finally, the current is put through a transformer where voltage is
increased for long-distance transmission over power lines (USGS 2011). Thus, hydropower is
mostly dependent upon precipitation and elevation changes; high precipitation levels and large
elevation changes are necessary to generate significant quantities of electricity. An area such as
the mountainous Pacific Northwest has more head and more productive hydropower plants than
an area such as the Gulf Coast, which might have large amounts of precipitation but is compara-
tively flat (USEPA 2011).
Facilities with a few hundred megawatts to more than 10,000 MWe are generally considered to
be large hydroelectric facilities. Large hydroelectric facilities attempt to maximize the capture
of seasonal runoff and store it until it is needed for irrigation or power generation, so they at-
tempt to impound an entire year's worth of water in large reservoirs. Grand Coulee Dam on the
Columbia River in Washington State is the largest single electric power facility in the United
States (USEIA 2011c), at 550 feet (168 meters) high and 5,223 feet (1,592 meters) long, with
installed generating capacity of over 6,800 MWe and reservoir capacity of over 9.5 million acre-
feet of water (U.S. Bureau of Reclamation 2011a). In addition to generating electric power, Grand
Coulee provides water for approximately 600,000 acres of irrigated farmland in the Columbia
River Basin and is a primary factor in controlling floods on the Columbia River. Hoover Dam,
southeast of Las Vegas, Nevada, is the tallest solid concrete dam in the United States at 726
feet (221.3 meters) high and 1,244 feet (379.2 meters)long, although its installed generating
capacity is a bit less than 2,100 MWe. Hydroelectric plants with greater generating capacity
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