uphill all night long, when there is excess power in the electrical grid
produced by fossil fuel– and nuclear-powered generating facilities. The
water is then allowed to flow back down through a penstock to spin a
turbine generator. The reservoir may only be large enough to provide
generating capacity for several hours per day. The plant may be brought
on-line quickly, however, when power is needed to meet peak demands.
Even though this type of plant operates at a net energy loss, it is relatively
inexpensive to construct, compared to a base-load plant that produces
power all the time. The fact that this type of plant allows for supplemental
power when the utility needs it most makes it economical.
Hydroelectricity and the environment
Hydroelectricity produces no fossil fuel pollutants, and the facilities
are fairly quiet and perhaps less obtrusive than other generating plants.
Hydroelectric power plants, however, are polluting. They just produce a
different form of pollution by having the potential to alter the ecology
of rivers.
If a river is dammed, migratory fish may not be able to return
upstream to their spawning grounds. To counteract this problem, a series
of fish lifts were installed in the Susquehanna River in Pennsylvania in
the 1990s to help American shad return upstream. The fish lifts are in
operation for only a two- to three-week period out of the year. They are
like giant elevators that lift the fish and water up onto the top of the dam,
where they dump into a sluiceway. The fish can then swim again until
they reach the next dam, several miles upstream, where another fish lift
has been installed. They instinctively swim toward a downstream current,
which is artificially created during the times the fish lift is in operation.
The fish lift is an example of the extent to which utilities may be required
to accommodate nature in exchange for damming the river. The utilities
may also be responsible for maintaining the river level at a safe depth for
recreational boating and water sports. Draining the river too quickly could
also create fish kills, as fish and other aquatic life become trapped
in areas that could stagnate and evaporate if not replenished with
flowing water.
Tidal Energy
Tidal energy is another form of hydroelectric energy that offers
tremendous generating potential if it can be harnessed. The gravitational
pull of the moon and the rotation of the earth cause the changing tides.
Significant tide changes occur about twice daily. There are only a few
places in the United States where the tide change is so significant that it
can be used to generate power.
How a tidal power plant works
The concept of a tidal power plant is simple. Make use of the flowing
tide to spin a generator in one direction during the rush of high tide, and
then spin the generator in the opposite direction with the exiting water
during low tide. Of course, a natural bay that could be sealed by a perma-
nent or floating dam known as a barrage is most ideal for using tidal
power. See Figure 5-13. The extreme capital cost of tidal energy and the
lack of ideal locations are the primary limitations to more extensive use of
Barrage: A perma-
nent or floating
dam that seals a
natural bay in
order to use tidal
power.
Renewable and Inexhaustible Energy Sources 109
Fish lift: A giant
elevator-like device
installed on
dammed rivers to
help fish return
upstream during
spawning season.
G R E E N T E C H
In addition to the
damage to the rivers’
ecological systems,
the construction of
dams also impacts
surrounding land.
Lower lands are used
in the construction of
dams, so any forests
and life within those
forests are affected.