Hydroelectric Energy: Definition, Uses, Advantages & Disadvantages

Chapter 23 / Lesson 12
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  • 0:05 What Is Hydroelectric Energy?
  • 2:16 Water Generates Electricity
  • 4:04 Pros and Cons of…
  • 7:32 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has a Master's degree in Zoology and a Bachelor's in Biology, and has taught college level Physical Science and Biology.

In this video lesson you will understand how water can be used to generate electrical energy. You will also identify some benefits and drawbacks of using this type of renewable resource for energy production.

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!

Dams can have serious impacts on the surrounding environment as well. Think of it this way: There is a busy street that runs through the center of town that seems to move along pretty well. All of a sudden, a large wall gets constructed across the road that reduces it from four lanes to one. Traffic will likely get backed up, and some people will simply not be able to make it to work any longer. And, any homes, plants and animals will be forced to move where the wall was constructed.

A dam has the same effect on a river environment. The water is not allowed to flow downstream as it would without the dam, so it backs up, creating the reservoir behind it. The reservoir can only be made by flooding the land behind the dam, so people, plants and animals are displaced. Since the flow is altered at the dam, the downstream areas get less water than they would normally.

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