Hydropower Plants

Worldwide, 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.

We will learn 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 life.

The Power of Water
Simple Beginnings
Use of hydropower peaked in the mid-20th century, but the idea of using water for power generation goes back thousands of years. A hydropower plant is basically an oversized water wheel.
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 as a stream of water hits the blades. The gears of the wheel ground the wheat into flour.

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 can have.
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 plant:

1. 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.

2. 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.

3. 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).

4. 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.)

5. Transformer - The transformer inside the powerhouse takes the AC and converts it to higher-voltage current.

6. 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. (Read How Power Distribution Grids Work to learn more about power line transmission.)

7. Outflow - Used water is carried through pipelines, called tailraces, and re-enters the river downstream.

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 the pumped-storage plant. 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:

1. Upper reservoir - Like a conventional hydropower plant, a dam creates a reservoir. The water in this reservoir flows through the hydropower plant to create electricity.

2. 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 consumption.

Inside the Generator

The heart of the hydroelectric power plant is the generator. Most hydropower plants have several of these generators.

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 current.

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:

1. Shaft
2. Excitor
3. Rotor
4. Stator

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.

Hydropower 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 precipitation.

Hydropower Basics

Flowing water creates energy that can be captured and turned into electricity. This is called hydropower. Hydropower is currently the largest source of renewable power, generating nearly 10% of the electricity used in the United States.
The most common type of hydropower plant uses a dam on a river to store water in a reservoir. Water released from the reservoir flows through a turbine, spinning it, which, in turn, activates a generator to produce electricity. But hydropower doesn't necessarily require a large dam. Some hydropower plants just use a small canal to channel the river water through a turbine.
Another type of hydropower plant—called a pumped storage plant—can even store power. The power is sent from a power grid into the electric generators. The generators then spin the turbines backward, which causes the turbines to pump water from a river or lower reservoir to an upper reservoir, where the power is stored. To use the power, the water is released from the upper reservoir back down into the river or lower reservoir. This spins the turbines forward, activating the generators to produce electricity.

Types of Hydropower

Impoundment

An impoundment facility, typically a large hydropower system, uses a dam to store river water in a reservoir. The water may be released either to meet changing electricity needs or to maintain a constant reservoir level.

Diversion

A diversion, sometimes called run-of-river, facility channels a portion of a river through a canal or penstock. It may not require the use of a dam.

Pumped Storage

When the demand for electricity is low, a pumped storage facility stores energy by pumping water from a lower reservoir to an upper reservoir. During periods of high electrical demand, the water is released back to the lower reservoir to generate electricity.

Sizes of Hydropower Plants

Facilities range in size from large power plants that supply many consumers with electricity to small and micro plants that individuals operate for their own energy needs or to sell power to utilities.

Large Hydropower

Although definitions vary, DOE defines large hydropower as facilities that have a capacity of more than 30 megawatts.

Small Hydropower

Although definitions vary, DOE defines small hydropower as facilities that have a capacity of 0.1 to 30 megawatts.

Micro Hydropower

A micro hydropower plant has a capacity of up to 100 kilowatts (0.1 megawatts).

Turbine Technologies

There are many types of turbines used for hydropower, and they are chosen based on their particular application and the height of standing water—referred to as "head"—available to drive them. The turning part of the turbine is called the runner. The most common turbines are as follows:

Pelton Turbine

A Pelton turbine has one or more jets of water impinging on the buckets of a runner that looks like a water wheel. The Pelton turbines are used for high-head sites (50 feet to 6,000 feet) and can be as large as 200 megawatts.

Francis Turbine

A Francis turbine has a runner with fixed vanes, usually nine or more. The water enters the turbine in a radial direction with respect to the shaft, and is discharged in an axial direction. Francis turbines will operate from 10 feet to 2,000 feet of head and can be as large as 800 megawatts.

Propeller Turbine

A propeller has a runner with three to six fixed blades, like a boat propeller. The water passes through the runner and drives the blades. Propeller turbines can operate from 10 feet to 300 feet of head and can be as large as 100 megawatts. A Kaplan turbine is a type of propeller turbine in which the pitch of the blades can be changed to improve performance. Kaplan turbines can be as large as 400 megawatts.

How Hydropower Works

Most hydropower projects use a dam and a reservoir to retain water from a river. When the stored water is released, it passes through and rotates turbines, which spin generators to produce electricity. Water stored in a reservoir can be accessed quickly for use during times when the demand for electricity is high.

Dammed hydropower projects can also be built as power storage facilities. During periods of peak electricity demand, these facilities operate much like a traditional hydropower plant — water released from the upper reservoir passes through turbines, which spins generators to produce electricity. However, during periods of low electricity use, electricity from the grid is used to spin the turbines backward, which causes the turbines to pump water from a river or lower reservoir to an upper reservoir, where the water can be stored until the demand for electricity is high again.

A third type of hydropower project, called "run of the river", does not require large impoundment dams (although it may require a small, less obtrusive dam). Instead, a portion of a river's water is diverted into a canal or pipe to spin turbines.

How Hydropower is Used

Hydropower is currently the largest and least expensive source of renewable electricity produced in the United States.

Large and small-scale hydropower projects are most commonly used by clean power generators to produce electricity. Our Buying Clean Electricity section provides information on buying electricity generated from hydro and other renewable resources in your state.

Many large-scale dam projects have been criticized for altering wildlife habitats, impeding fish migration, and affecting water quality and flow patterns. As a result of increased environmental regulation, the National Hydropower Association forecasts a decline in large-scale hydropower use through 2020. Research and development efforts have succeeded in reducing many of these environmental impacts through the use of fish ladders (to aid fish migration), fish screens, new turbine designs, and reservoir aeration. Although funding has been limited, current research focuses on the development of a "next generation turbine, which is expected to further increase fish survival rates and improve environmental conditions.

A very small hydropower (called microhydro) project can also be installed to meet the electricity needs of a single home or small business, and is especially useful for those in remote areas. Our Making Your Own Clean Electricity section provides more information on issues involved with producing your own electricity.

Reference
Web site :
http://people.howstuffworks.com/
http://www.eere.energy.gov/RE/hydropower.html

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