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The Greening of IT
256 How Companies Can Make a Difference for the Environment

Hydropower produces no waste and does not produce carbon dioxide
(CO2), a greenhouse gas. Much of the following information comes from
the Web site http://en.wikipedia.org/wiki/Hydroelectricity.
Although large hydroelectric installations generate most of the
world’s hydroelectricity, small hydro schemes are particularly popular in
China, which has more than 50 percent of world small hydro capacity.
Some jurisdictions do not consider large hydro projects to be a sustain-
able energy source due to human and environmental impacts, though
this judgment depends on the definition of sustainability used.
Most hydroelectric power comes from the potential energy of dammed
water driving a water turbine and generator. In this case, the energy
extracted from the water depends on the volume and on the difference in
height between the source and the water’s outflow. This height differ-
ence is called the head. The amount of potential energy in water is pro-
portional to the head. To obtain very high head, water for a hydraulic
turbine can be run through a large pipe called a penstock.
Pumped storage hydroelectricity produces electricity to supply high-
peak demands by moving water between reservoirs at different eleva-
ptg
tions. At times of low electrical demand, excess generation capacity is
used to pump water into the higher reservoir. When there is higher
demand, water is released back into the lower reservoir through a tur-
bine. Pumped storage schemes currently provide the only commercially
important means of large-scale grid energy storage and improve the
daily load factor of the generation system. Hydroelectric plants with no
reservoir capacity are called run-of-the-river plants, because it is not
then possible to store water. A tidal power plant makes use of the daily
rise and fall of water due to tides; such sources are highly predictable,
and if conditions permit, construction of reservoirs can also be dispatch-
able to generate power during high-demand periods.
Annual electric energy production depends on the available water
supply. In some installations, the water flow rate can vary by a factor of
10:1 over the course of a year.

Small-Scale Hydroelectric Plants
Small hydro plants are those producing up to 10 megawatts, although
projects up to 30 megawatts in North America are considered small
hydro and have the same regulations. A small hydro plant might be con-
nected to a distribution grid or might provide power only to an isolated
community or a single home. Small hydro projects generally do not