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What is Hydroelectric Energy?

Nicholas Amendolare, Elizabeth Friedl
  • Author
    Nicholas Amendolare

    Nicholas Amendolare is a high school and middle school science teacher from Plymouth, Massachusetts. He has a bachelor's degree in environmental science from Worcester Polytechnic Institute and a master's degree in education from Harvard University. He has been a teacher for nine years, has written for TED-Ed, and is the founder of www.MrAscience.com.

  • Instructor
    Elizabeth Friedl

    Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.

Learn the definition of hydroelectric power and see examples. Understand how hydroelectric energy works and its disadvantages and advantages. Updated: 08/27/2021

Hydroelectric Power Definition

The United States Office of Energy Efficiency and Renewable Energy defines hydroelectric energy, also referred to as hydroelectric power, as "a renewable source of energy that generates power by using a dam or diversion structure to alter the natural flow of a river or other body of water." The definition can even be glimpsed within the term itself, which combines the Greek word hydro, which means "water," and the Phoenician word elekron, which means "shining light."

Srisailam Dam Reservoir in India.

Srisailam Dam Reservoir

The history of hydropower goes back thousands of years to the Ancient Greeks, who used water wheels to grind flour, and the Ancient Egyptians, who used water screws for irrigation. But in the modern world, hydroelectric power has been in use since the late 19th century, in facilities like the one pictured above, where it uses the power of moving water to generate electricity.

What Does Hydroelectric Mean to the World?

Hydroelectric power is considered one of the most important energy sources in the world. It is the most common form of renewable energy, producing about 16% of the world's electricity, which is three times as much as wind power and six times as much as solar power. All told, approximately 71% of all of the renewable electricity generated on Earth is from hydropower. In the United States, hydropower is the leading renewable energy source by far, accounting for about 96% of all renewable energy production.

Hydroelectricity has a few major advantages. The first is the high energy density contained by flowing water. The second is that the power source is dispatchable, meaning that power plant operators can regulate the flow of water thereby generating more or less electricity, as needed. This helps hydroelectric power plants to meet the ever-changing demands of electricity customers worldwide. Hydroelectric power plants are particularly common in developed nations such as the United States, China, Brazil, Canada, India, and Russia.

Hydroelectric Energy Examples

Hydroelectric plants come in three main types: impoundment dams, run-of-river facilities, and pump storage. Impoundment dams may be the most iconic. They are massive dams created on major rivers with the goal of collecting a large, deep reservoir of water behind them. These reservoirs serve to store energy, in high volumes and at high pressure, that can then be sent through the dam and used to generate electricity as needed. One famous example is the Hoover Dam, built in the 1930s on the border between Nevada and Arizona.

The Hoover Dam, on the border of Nevada and Arizona.

Hoover Dam

A second common type of hydroelectric plant is called a "run of river" facility. Rather than damming an entire river, these facilities channel a portion of a river into a smaller power plant, leaving the majority of the river's natural flow intact. One example is Chief Joseph Dam in Bridgeport, Washington. Of course, it is important to note that, although these run-of-river facilities are more ecologically friendly, they do not generate nearly as much energy as the impoundment dams discussed above.

Chief Joseph Dam near Bridgeport, Washington.

Chief Joseph Dam

The third and final type of hydroelectric energy is pump storage. These facilities store energy by pumping water from lower-elevation reservoirs to higher-elevation reservoirs. Then, the water can be released downhill and used to generate electricity as needed. They can be combined with sources like solar power to store excess energy during the daytime and then make that energy available when demand peaks in the early evening.

What is the Largest Hydroelectric Energy Plant in the World?

China is now the largest producer of hydroelectric power in the world. It is also home to the world's biggest hydroelectric dam: The Three Gorges Dam Project located in the Xilingxia Gorge, in Hubei province, China.

The Three Gorges Dam, Hubei province, China.

The Three Gorges Dam

Construction began on the Three Gorges Dam in 1994 and it wasn't finished until 2003. The dam is a missive impoundment dam that is almost a mile and a half long. The dam holds back a giant wall of water almost 600 feet high. Beneath the dam, water flows at a rate of 20,000-30,000 cubic feet per second through thirty-two large turbines capable of generating up to 22,500 megawatts, which is something like thirty times the capacity of a typical power plant.

What Is Hydroelectric Energy?

Water is pretty powerful stuff! You know this if you have stood in a flowing stream and felt the water rushing past you. Or, if you've gone whitewater rafting down a raging river. Even mere images of a place like Niagara Falls help you appreciate just how strong moving water is. This is not a new observation, and water has been used to produce different forms of energy for a very long time. When electricity is generated from flowing water, this is called hydroelectric energy, and the prefix 'hydro' refers to water.

Hydroelectric energy is considered a renewable energy resource because it's an energy resource that regenerates in a short time period. You can think of this like the hair on your head. If you get a bad haircut that you don't like, you don't have to worry about it too much because it will grow back in a reasonable amount of time. However, if you lose an arm, you certainly can't regenerate that! Your hair is a renewable resource, whereas your arm is a non-renewable resource.

Sunlight, wind and heat generated from the earth are similar types of renewable energy resources. Just like you can't stop water from flowing, you can't stop the sun from shining or the wind from blowing. The resource 'renews' itself as it is used, and unlike things such as minerals and metals, does not occur on Earth in a limited supply.

Hydroelectric energy is a very important energy source, both nationally and worldwide. About 19% of the world's total electricity production comes from hydroelectric energy and about 7% of the power in the U.S. When we look at hydroelectric energy for the U.S. in terms of renewable energy sources (so comparing this to things like solar power, wind power, geothermal energy and biofuels), it makes up 96% of renewable energy electricity production. That means that almost all of our energy production that comes from renewable energy sources comes from water!

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Water Generates Electricity

So, how exactly is electricity generated from water? This usually occurs through a hydroelectric dam that is built along a flowing waterway. Think of the water at Niagara. The water flowing over the falls is travelling faster than the water approaching the falls. As water travels downhill, it picks up speed and power.

To take advantage of this, dams are constructed along the waterway where there is a large elevation drop. This is exactly why you wouldn't expect to find dams in flat places like Florida, but would expect to find them in the hilly, mountainous regions of the Southwest U.S.

Like going down a slide at the playground, water is pulled downhill by gravity and picks up speed as it goes (you're moving faster at the bottom of the slide than at the top, right?). Inside the dam are turbines that get spun by the moving water - similar to how a pinwheel gets spun by the wind blowing by.

The spinning turbine shafts are connected to a generator, and the spinning of the turbines themselves creates electricity inside the generator. The generator is connected to power lines, which transmit the electricity to homes and buildings just like they do with coal-fired and natural gas power plants.

Just like wind blowing past the pinwheel is not affected by the pinwheel itself, water flowing by the turbines is not affected as it passes through the dam. It flows on downstream as if nothing has happened. You can see why this is such a valuable and widely used energy resource all around the world! However, with every source of energy, there are pros and cons. Let's look at the benefits and drawbacks that come with harnessing the amazing power of moving water.

Pros and Cons of Hydroelectric Energy

As you already know, hydroelectric energy is beneficial because it's a renewable resource. The water is not affected as it flows through the dam and spins the turbines - water will flow whether there's a dam there or not! This also means that there is minimal pollution, unlike the air and water pollution generated from coal and nuclear power plants.

Because water flows along naturally, we also do not have to create any water to run through the dam. Water is provided by nature, and we simply have to sit back and let it do all the work! This also means that maintenance and operational costs are relatively low for the dam itself.

One reason that hydroelectric energy is so popular is its reliability. Wind power is great, but if the wind isn't blowing, you don't generate any electricity. Solar power has a similar issue. If the sun isn't shining, you're not generating any electricity from the sunlight. Dams, however, are specifically designed to hold water behind them. This provides a steady, constant flow of energy from the water that runs through it. So instead of depending on rainfall to deliver water, we know that the water will be there waiting to spin those turbines inside the dam.

While hydroelectric energy can be a very beneficial renewable resource, there are also drawbacks to using water to generate electricity. While the maintenance costs are low after the dam is built, they're very expensive to construct and require large up-front investments.

And you know those reservoirs behind the dam? For the most part, these are a steady, reliable source of water. But they are dependent on water coming from their own sources, such as rain. If it doesn't rain, the reservoir may dry up. It's like having a slow leak in your bathtub. If you sit in there long enough and you don't turn the water back on to refill the tub, eventually you'll be sitting in a dry tub, which doesn't work very well!

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Video Transcript

What Is Hydroelectric Energy?

Water is pretty powerful stuff! You know this if you have stood in a flowing stream and felt the water rushing past you. Or, if you've gone whitewater rafting down a raging river. Even mere images of a place like Niagara Falls help you appreciate just how strong moving water is. This is not a new observation, and water has been used to produce different forms of energy for a very long time. When electricity is generated from flowing water, this is called hydroelectric energy, and the prefix 'hydro' refers to water.

Hydroelectric energy is considered a renewable energy resource because it's an energy resource that regenerates in a short time period. You can think of this like the hair on your head. If you get a bad haircut that you don't like, you don't have to worry about it too much because it will grow back in a reasonable amount of time. However, if you lose an arm, you certainly can't regenerate that! Your hair is a renewable resource, whereas your arm is a non-renewable resource.

Sunlight, wind and heat generated from the earth are similar types of renewable energy resources. Just like you can't stop water from flowing, you can't stop the sun from shining or the wind from blowing. The resource 'renews' itself as it is used, and unlike things such as minerals and metals, does not occur on Earth in a limited supply.

Hydroelectric energy is a very important energy source, both nationally and worldwide. About 19% of the world's total electricity production comes from hydroelectric energy and about 7% of the power in the U.S. When we look at hydroelectric energy for the U.S. in terms of renewable energy sources (so comparing this to things like solar power, wind power, geothermal energy and biofuels), it makes up 96% of renewable energy electricity production. That means that almost all of our energy production that comes from renewable energy sources comes from water!

Water Generates Electricity

So, how exactly is electricity generated from water? This usually occurs through a hydroelectric dam that is built along a flowing waterway. Think of the water at Niagara. The water flowing over the falls is travelling faster than the water approaching the falls. As water travels downhill, it picks up speed and power.

To take advantage of this, dams are constructed along the waterway where there is a large elevation drop. This is exactly why you wouldn't expect to find dams in flat places like Florida, but would expect to find them in the hilly, mountainous regions of the Southwest U.S.

Like going down a slide at the playground, water is pulled downhill by gravity and picks up speed as it goes (you're moving faster at the bottom of the slide than at the top, right?). Inside the dam are turbines that get spun by the moving water - similar to how a pinwheel gets spun by the wind blowing by.

The spinning turbine shafts are connected to a generator, and the spinning of the turbines themselves creates electricity inside the generator. The generator is connected to power lines, which transmit the electricity to homes and buildings just like they do with coal-fired and natural gas power plants.

Just like wind blowing past the pinwheel is not affected by the pinwheel itself, water flowing by the turbines is not affected as it passes through the dam. It flows on downstream as if nothing has happened. You can see why this is such a valuable and widely used energy resource all around the world! However, with every source of energy, there are pros and cons. Let's look at the benefits and drawbacks that come with harnessing the amazing power of moving water.

Pros and Cons of Hydroelectric Energy

As you already know, hydroelectric energy is beneficial because it's a renewable resource. The water is not affected as it flows through the dam and spins the turbines - water will flow whether there's a dam there or not! This also means that there is minimal pollution, unlike the air and water pollution generated from coal and nuclear power plants.

Because water flows along naturally, we also do not have to create any water to run through the dam. Water is provided by nature, and we simply have to sit back and let it do all the work! This also means that maintenance and operational costs are relatively low for the dam itself.

One reason that hydroelectric energy is so popular is its reliability. Wind power is great, but if the wind isn't blowing, you don't generate any electricity. Solar power has a similar issue. If the sun isn't shining, you're not generating any electricity from the sunlight. Dams, however, are specifically designed to hold water behind them. This provides a steady, constant flow of energy from the water that runs through it. So instead of depending on rainfall to deliver water, we know that the water will be there waiting to spin those turbines inside the dam.

While hydroelectric energy can be a very beneficial renewable resource, there are also drawbacks to using water to generate electricity. While the maintenance costs are low after the dam is built, they're very expensive to construct and require large up-front investments.

And you know those reservoirs behind the dam? For the most part, these are a steady, reliable source of water. But they are dependent on water coming from their own sources, such as rain. If it doesn't rain, the reservoir may dry up. It's like having a slow leak in your bathtub. If you sit in there long enough and you don't turn the water back on to refill the tub, eventually you'll be sitting in a dry tub, which doesn't work very well!

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Frequently Asked Questions

What is hydroelectric power and how does it work?

Hydroelectric power is electricity generated via flowing water. Most hydroelectric plants work by channeling flowing water through a large turbine. The spinning turbine is attached to a generator which generates electricity.

What are advantages and disadvantages of hydropower?

The advantages of hydropower include that there is no fuel required, it is renewable and clean, it is dispatchable, and it is reliable. The disadvantages include limited availability, that the world is running out of sites, its high cost, and its ecological costs.

What is an example of hydroelectric power?

Two famous examples of hydroelectric power are the Hoover Dam in the United States and the massive Three Gorges Dam in China.

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