Page 593 - Environment: The Science Behind the Stories

pools water temporarily, and the pipe or channel can be run
along the surface or underground. Another method is to flow
river water over a dam small enough not to impede fish pas-
sage, siphoning off water to turn turbines, and then returning
the water to the river. Run-of-river systems are useful in areas
remote from established electrical grids and in regions with-
out the economic resources to build and maintain large dams.
This approach cannot guarantee reliable water flow in all sea-
sons, but it minimizes many impacts of the storage technique.
To better control the timing of flow, pumped-storage
hydropower can be used. In pumped storage, water is pumped
from a lower reservoir to a higher reservoir during times when
demand for power is weak and prices are low. When demand
is strong and prices are high, water is allowed to flow downhill
through a turbine, generating electricity. Although energy must
be input to pump the water, pumped storage can be profitable.
When paired with intermittent sources such as solar and wind
FIGURE 20.23 Switchgrass provides fuel for biopower and is power, it can help balance a region’s power supply by compen-
being studied as a crop to provide cellulosic ethanol. sating for dips in power from these intermittent sources.

bioenergy options may provide promising avenues for sustain-
ably replacing fossil fuels. Hydroelectric power is clean and renewable,
but also has impacts

For producing electricity, hydropower has three clear advan-
Hydroelectric Power tages over fossil fuels. First, it is renewable; as long as precipi-
tation falls from the sky and fills rivers and reservoirs, we can
Next to biomass, we draw more renewable energy from the use water to turn turbines.
motion of water than from any other resource. In hydroelectric Second, hydropower is efficient. It is thought to have an
power, or hydropower, we use the kinetic energy of moving EROI (energy returned on investment) ratio of 100:1 or more,
water to turn turbines and generate electricity. We examined higher than any other modern-day energy source.
hydropower and its environmental impacts in our discussion Third, no carbon compounds are burned in the pro-
of fresh water resources (pp. 421–422). Now we will take a duction of hydropower, so no carbon dioxide or other
closer look at hydropower as an energy source. pollutants are emitted into the atmosphere, and this helps
safeguard air quality, climate, and human health. Fossil
fuels are used in constructing and maintaining dams—
Modern hydropower uses three and recent evidence indicates that large reservoirs release
approaches the greenhouse gas methane as a result of anaerobic decay
in deep water. But overall, hydropower accounts for only a
Most of our hydroelectric power today comes from impound- small fraction of the greenhouse gas emissions typical of
ing water in reservoirs behind concrete dams that block the fossil fuel combustion.
flow of river water and then letting that water pass through the Although it is renewable, efficient, and produces little
dam. Because immense amounts of water are stored behind air pollution, hydropower does exert negative impacts. Dam-
dams, this is called the storage technique. ming rivers (p. 421) destroys habitat for wildlife as riparian
As reservoir water passes through a dam, it turns the areas above dam sites are submerged and those below dam
blades of turbines, which cause a generator to generate elec- sites often are starved of water. Because water discharge is
tricity (FIGURE 20.24). Electricity generated in the powerhouse regulated to optimize electricity generation, the natural flood-
of a dam is transmitted to the electric grid by transmission ing cycles of rivers are disrupted. Suppressing flooding pre-
lines, while the water flows into the riverbed below the dam vents river floodplains from receiving fresh nutrient-laden
and continues downriver. The amount of power generated sediments. Instead, sediments become trapped behind dams,
depends on the distance the water falls and the volume of where they begin filling the reservoir.
water released. By storing water in reservoirs, dam opera- Dams also cause thermal pollution (p. 430), because
tors can ensure a steady and predictable supply of electricity water downstream may become unusually warm if water lev-
at all times, even during periods of naturally low river flow. els are kept unnaturally shallow. Moreover, periodic flushes
An alternative approach is the run-of-river technique, of cold water occur from the release of reservoir water. Such
which generates electricity without greatly disrupting the flow thermal shocks, together with habitat alteration, have dimin-
of river water. Several methods can be used; one is to divert a ished or eliminated native fish populations in many dammed
portion of a river’s flow through a pipe or channel, passing it waterways. In addition, dams generally block the passage of
through a powerhouse and then returning it to the river (FIGURE fish and other aquatic creatures, effectively fragmenting the
592 20.25). This can be done with or without a small reservoir that river and reducing biodiversity in each stretch.

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