Power Plant: March 2010

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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.

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.

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.

Here are the basic components of a conventional hydropower plant:

Hydropower plant parts

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.

Fig 1. Dam

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).

Power Developed by a Turbine

Pt = 9.81 x Q x H x ? (KW)
where:
Q = Discharge, m3/sec
H = Net Head, m
h = Efficiency of Turbine

Types of turbine:

Kaplan

Fig 2 Kaplan Turbine

* Power: 100 Kw to 7 Mw
* Head: from 1.80m to 25m
* Runner blades : 4 /5 /6
* Diameter: 700 to 4000mm
* Simple or double regulation
* Arrangement:
* Vertical
* S type
* Horizontal ( Pit)
* Inclined simple regulated
* Syphon intake

Francis

Fig 3. Francis Turbine

* Power: 100 Kw to 15 Mw
* Head: from 15m to 200m
* Diameter: 250 to 3500mm
* Arrangement:
* Vertical shaft
* Horizontal shaft
* Semi spiral casing or full spiral casing
* Double francis (2 runners )

Pelton

Fig 4. Pelton Turbine

* Power: 100 Kw to 10 Mw
* Head: from 100m to 1000m
* Diameter : up to 1800mm
* Arrangement:
* Vertical 3 jets /4 jets
* Horizontal 1 jet /2 jets
* Double (horizontal 4 jets)

Generators

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

Fig 5. Generator

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.

Inside a hydropower plant generator

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:

* Shaft
* Excitor
* Rotor
* 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.

Transformer

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

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.)

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.

The Largest Hydroelectric Power Plant:

The largest hydroelectric power plant in the world is the Itaipu power plant, jointly owned by Brazil and Paraguay. Itaipu can produce 12,600 megawatts.
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.