hydropower plants produce about 24 percent of the world's
electricity and supply more than 1 billion people with power.
The world's hydropower plants output a combined total of
675,000 megawatts, the energy equivalent of 3.6 billion
barrels of oil,
according to the National
Renewable Energy Laboratory. There are more than 2,000
hydropower plants operating in the United States, making
hydropower the country's largest renewable energy source.
Photo courtesy U.S. Bureau of
Reclamation The outflow from
the hydropower plant at the Hoover Dam on the
In this edition of HowStuffWorks,
we'll take a look at how falling water creates energy and
learn about the hydrologic cycle that creates the water flow
essential for hydropower. You will also get a glimpse at one
unique application of hydropower that may affect your daily
The Power of Water
While the use of
hydropower peaked in the mid-20th century, the idea of
using water for power generation dates back thousands of
years. A hydropower plant is essentially an oversized
water wheel, which was first used by the Greeks.
More than 2,000 years ago, the Greeks are said to
have used a water wheel for grinding wheat into flour.
These ancient water wheels are like the turbines of
today, spinning around as a stream of water hits the
blades. The gears
of the wheel ground the wheat into
When watching a
river roll by, it's hard to imagine the force it's carrying.
If you have ever been white-water rafting, then you've felt a
small part of the river's power. White-water rapids are
created as a river, carrying a large amount of water downhill,
bottlenecks through a narrow passageway. As the river is
forced through this opening, its flow quickens. Floods are
another example of how much force a tremendous volume of water
Hydropower plants harness water's energy and use simple
mechanics to convert that energy into electricity.
Hydropower plants are actually based on a rather simple
concept -- water flowing through a dam turns a turbine, which
turns a generator.
Here are the basic components of a conventional hydropower
Dam - Most hydropower plants rely on a dam that
holds back water, creating a large reservoir. Often,
this reservoir is used as a recreational lake, such as Lake
Roosevelt at the Grand
Coulee Dam in Washington State.
Intake - Gates on the dam open and gravity
pulls the water through the penstock, a pipeline that
leads to the turbine. Water builds up pressure as it flows
through this pipe.
Turbine - The water strikes and turns the large
blades of a turbine, which is attached to a generator above
it by way of a shaft. The most common type of turbine for
hydropower plants is the Francis Turbine, which looks like a
big disc with curved blades. A turbine can weigh as much as
172 tons and turn at a rate of 90 revolutions per minute
(rpm), according to the Foundation
for Water & Energy Education (FWEE).
Generators - As the turbine blades turn, so do a
series of magnets inside the generator. Giant magnets rotate
past copper coils, producing alternating current (AC)
by moving electrons. (You'll learn more about how the
generator works later.)
Transformer - The transformer inside the
powerhouse takes the AC and converts it to
Power lines - Out of every power plant come four
wires: the three phases of power being produced
simultaneously plus a neutral or ground common to all three.
Power Distribution Grids Work to learn more about power
Outflow - Used water is carried through
pipelines, called tailraces, and re-enters the river
Photo courtesy U.S. Bureau of
Reclamation The shaft that
connects the turbine and
The water in the reservoir is considered stored
energy. When the gates open, the water flowing through the
penstock becomes kinetic energy because it's in motion.
The amount of electricity that is generated is determined by
several factors. Two of those factors are the volume of
water flow and the amount of hydraulic head. The
head refers to the distance between the water surface and the
turbines. As the head and flow increase, so does the
electricity generated. The head is usually dependent upon the
amount of water in the reservoir.
Pumped Storage The
majority of hydropower plants work in the manner described
above. However, there's another type of hydropower plant,
called pumped-storage plants. In a conventional
hydropower plant, the water from the reservoir flows through
the plant, exits and is carried down stream. A pumped-storage
plant has two reservoirs:
Upper reservoir - Like a conventional hydropower
plant, a dam creates a reservoir. The water in this
reservoir flows through the hydropower plant to create
Lower reservoir - Water exiting the hydropower
plant flows into a lower reservoir rather than re-entering
the river and flowing downstream.
Using a reversible turbine, the plant can pump water
back to the upper reservoir. This is done in off-peak hours.
Essentially, the second reservoir refills the upper reservoir.
By pumping water back to the upper reservoir, the plant has
more water to generate electricity during periods of peak
Inside the Generator The heart of the
hydroelectric power plant is the generator. Most hydropower
plants have several of these generators.
Photo courtesy U.S. Bureau of
Reclamation The giant
generators at Hoover Dam produce more than 2,000
The generator, as you might have guessed, generates the
electricity. The basic process of generating electricity in
this manner is to rotate a series of magnets inside coils of
wire. This process moves electrons, which produces electrical
Inside a hydropower plant
The Hoover Dam has a total of 17 generators, each of which
can generate up to 133 megawatts. The total capacity of the
Hoover Dam hydropower plant is 2,074 megawatts. Each generator
is made of certain basic parts:
As the turbine turns, the excitor sends an
electrical current to the rotor. The rotor is a series
of large electromagnets
that spins inside a tightly-wound coil of copper wire, called
the stator. The magnetic field between the coil and the
magnets creates an electric current.
In the Hoover Dam, a current of 16,500 volts moves from the
generator to the transformer, where the current ramps up to
230,000 volts before being transmitted.
The largest hydroelectric power plant in the world
is the Itaipu power plant, jointly owned by
Brazil and Paraguay. Itaipu can produce 12,600
The second largest hydroelectric power plant is the
Guri power plant, located on Caroni River in
Venezuela. It can produce 10,300 megawatts.
The largest U.S. hydroelectric power plant is the
Grand Coulee power station on the Columbia River
in Washington State. It can produce 7,600 megawatts and
is currently being upgraded to produce 10,080 megawatts.
Sources: U.S. Bureau of Reclamation
and the National Renewable Energy
plants take advantage of a naturally occurring, continuous
process -- the process that causes rain to fall and rivers to
rise. Every day, our planet loses a small amount of water
through the atmosphere as ultraviolet rays break water
molecules apart. But at the same time, new water is emitted
from the inner part of the Earth through volcanic
activity. The amount of water created and the amount of
water lost is about the same.
At any one time, the world's total volume of water is in
many different forms. It can be liquid, as in oceans, rivers
and rain; solid, as in glaciers; or gaseous, as in the
invisible water vapor in the air. Water changes states as it
is moved around the planet by wind currents. Wind currents are
generated by the heating activity of the sun.
Air-current cycles are created by the sun shining more on the
equator than on other areas of the planet.
Air-current cycles drive the Earth's water supply through a
cycle of its own, called the hydrologic cycle. As the
sun heats liquid water, the water evaporates into vapor
in the air. The sun heats the air, causing the air to rise in
the atmosphere. The air is colder higher up, so as the water
vapor rises, it cools, condensing into droplets. When
enough droplets accumulate in one area, the droplets may
become heavy enough to fall back to Earth as
The hydrologic cycle is important to hydropower plants
because they depend on water flow. If there is a lack of rain
near the plant, water won't collect upstream. With no water
collecting up stream, less water flows through the hydropower
plant and less electricity is generated.
Californians, who rely on power from the Pacific Northwest,
have realized what can happen when there is a lack of
precipitation. During the winter of 2000-2001, there was a
less-than-average amount of snowfall. A lack of snow and rain
in that region caused a decrease in power production by
hydropower plants, so there was not as much electricity to
sell to California. Read How
California's Power Crisis Works for more information.
Hydroelectric Footwear The basic idea of
hydropower is to use the power of a moving liquid to turn a
turbine blade. Typically, a large dam has to be built in the
middle of a river to perform this function. A new invention is
capitalizing on the idea of hydropower on a much smaller scale
to provide electricity for portable electronic devices.
Inventor Robert Komarechka of Ontario, Canada, has
come up with the idea of placing small hydropower generators
into the soles of shoes. He believes these micro-turbines will
generate enough electricity to power almost any gadget. In May
2001, Komarechka received a patent for his unique foot-powered
Photo courtesy U.S. Patent and Trademark
Office Image from patent No.
6,239,501: Footwear with hydroelectric generator
There's a very basic principle to how we walk: The foot
falls heel-to-toe during each step. As your foot lands on the
ground, force is brought down through your heel. When you
prepare for your next step, you roll your foot forward, so the
force is transferred to the ball of your foot. Komarechka
apparently noticed this basic principle of walking and has
developed an idea to harness the power of this everyday
There are five parts to Komarechka's "footwear with
hydroelectric generator assembly," as it's described in its
Fluid - The system will use an electrically
Sacs to hold the fluid - One sac is placed in the
heel and another in the toe section of the shoe.
Conduits - Conduits connect each sac to a
Turbine - As water moves back and forth in the
sole, it moves the blades of a tiny turbine.
Microgenerator - The generator is located between
the two fluid-filled sacs, and includes a vane rotor,
which drives a shaft and turns the generator.
person walks, the compression of the fluid in the sac located
in the shoe's heel will force fluid through the conduit and
into the hydroelectric generator module. As the user continues
to walk, the heel will be lifted and downward pressure will be
exerted on the sac under the ball of the person's foot. The
movement of the fluid will rotate the rotor and shaft to
An exterior socket will be provided to connect wires to a
portable device. A power-control output unit may also be
provided to be worn on the user's belt. Electronic devices can
then be attached to this power-control output unit, which will
provide an steady supply of electricity.
"With the increase in the number of battery-powered,
portable devices," the patent reads,"there is an increasing
need to provide a long-lasting, adaptable, efficient
electrical source." Komarechka expects that his device will be
used for powering portable
players, GPS receivers
and two-way radios.
For more information on hydropower plants and related
topics, check out the links on the next page.