Lesson Video: Hydroelectric Power Physics

In this lesson, we will learn how to describe the advantages and disadvantages of generating electricity from hydroelectric dams.


Video Transcript

In this video, our topic is hydroelectric power. And right away, we can see from the name “hydroelectric” how this energy production method works. It involves taking the energy stored in water and converting it into electrical energy. There are lots of ways of doing this on a smaller scale. We see an example of that in our picture. But our focus is going to be on the larger scale, on very large hydroelectric facilities.

As we get started, here’s how we can define hydroelectricity. It’s an energy production method that involves converting the energy stored in water to electrical energy. And when we talk about generating electrical energy, we know that this particular method is one of very many options. Other methods include burning coal, burning gas, using sun energy, using wind energy, and nuclear energy. And the list goes on well beyond this.

In any given energy production method, we’re interested to know what are the advantages and disadvantages of that particular approach. To see what these are in the case of hydroelectricity, let’s consider a standard facility that generates energy this way. These facilities are located where water is already in motion, streams or typically larger rivers. So let’s say this is our river and we’re looking at it from a side-on view. The first step in creating hydroelectricity is to block the river by building a wall. This wall is called a dam. And when we build one, the water starts to pile up behind it.

Basically, as the river continues to flow, a big pool of water called a reservoir is built up behind the dam. As the reservoir fills up, the water towards the top is elevated higher and higher. This water is gaining gravitational potential energy. And our facility is going to use this energy, the energy stored in the water, to create electrical energy. The way we do it is by opening up a pathway for the water to fall through. And then in that pathway, we put what’s called a turbine, a device that spins thanks to the force of the water on it. This rotating turbine is connected to an electrical generator. And since the turbine is rotating, this provides the energy to power the generator and create electricity. It’s this electrical energy which is sent out using power lines to be used by homes and businesses.

So we see how this energy production process works. The gravitational potential energy that’s stored in the water elevated in the reservoir is converted to kinetic energy, energy of motion, as the water falls through the dam. And that movement turns the turbine, which then powers the generator to generate electricity. In terms of how much electricity a plant like this can generate, it depends on two factors. First, it depends on how far the water falls before it interacts with the turbine. So that means the taller our dam wall is, the more the water will fall and the more energy we can get out of it. And then, the second factor is how much water passes through the dam per unit time. The more water we have moving through the dam falling at given distance, the more energy is produced.

This first factor of how far the water falls has to do with how tall the dam is designed to be. While the second factor is more so determined by the river itself: a big river with lots of water flowing through it will naturally have a higher flow rate than a smaller one. This means that some of the most productive hydroelectric plants in the world are built on some of the largest rivers. When we compare this energy production method to other approaches, a few advantages stand out.

The first advantage is that once a hydroelectric plant is created, it’s fairly cheap to maintain. The dam along with the turbine and the generator within it don’t need much in the way of maintenance. So long as water continues to flow into the reservoir as expected, a facility like this won’t need much more investment to continue delivering electrical energy. To see another advantage of hydroelectric power, let’s consider this opening here, where the water in the reservoir falls through the dam. It’s entirely possible to control the size of this opening to make this as big in this case or as small in this case as we want. And that opening size is something that we can change on command. We’re able then to let through more or less water. And we know that the effect of that will be generating more or less electricity.

So let’s say we’re running our plant like this with a gate up in this position, so only a little bit of water falls through. But maybe for the moment, that’s all right that that amount of water falling through the dam generates all the electricity that’s needed. But then, what if the demand increases? We know this happens, for example, at certain times of day. When that happens, we can adjust to the increased demand by opening our gate a bit farther. That way, more water falls through our dam and more electricity is generated. This change in our production capacity happens very quickly. It happens as quickly as we’re able to move this gate up and down. So that’s another advantage of hydroelectric plants that they’re able to respond quickly to changes in electricity demand.

Along with the advantages of this method though, there are also disadvantages. We said that hydroelectric plants are inexpensive to maintain. And that’s true. But it turns out that they’re very expensive to build. Along with this, the creation of a hydroelectric plant creates a reservoir of water what’s like a man-made lake where there wasn’t one before. This can have significant ecological effects on the area around the dam. That includes flooding an area which up until the dam was built was above water. Plants, which are used to living above water, are then suddenly below water, and they begin to rot. Rotting plant matter releases greenhouse gases into the atmosphere. Methane and carbon dioxide. And compounding this effect to produce all of the concrete needed to build the dam, processes are required that themselves release significant amounts of CO2. So building and installing a hydroelectric facility involves the production of greenhouse gases.

Like all methods of energy production, hydroelectric power has some advantages and some disadvantages. We can learn even more about these facilities by thinking about the power that they produce. In our side-on sketch of a hydroelectric facility, right now, we see one turbine and one generator. But if we look at such a facility from another perspective from the front, then we can see that if the river is wide enough, we can have more than one turbine and more than one generator. With multiple turbines and generators going, the total output for this facility is the sum of the output from each turbine and generator pair.

Say, for example, that each of these generators was able to produce 50 million watts, 50 megawatts of electrical power. So then, this first generator produces 50 million watts. And so does the second one and the third and the fourth and the fifth and the sixth so that the facility overall outputs 300 million watts, 300 megawatts of power. And, by the way, some of the most powerful turbines in the world are able to generate not 50 million watts but 700 million watts of power. We’ve seen that turbines do this by rotating about an axis through their center. This causes the connection shaft to rotate, which connects the turbine and the generator. This constant rotation means that the electricity generated by the generators is itself constantly changing in phase. That means that it moves in regular cycles. In other words, the current is alternating current.

From an efficiency perspective, these turbines and generators can be very good. Modern hydroelectric facilities are able to convert up to 90 percent of the gravitational potential energy stored in the water in the reservoir to electrical energy. So that’s a bit about the power produced and the current generated by hydroelectric facilities.

Let’s now take a look at an example exercise to get some practice with these ideas.

What type of electric current do the turbines in a hydroelectric dam produce?

Okay, to answer this question, we will want to know a bit about how the turbines in a hydroelectric dam work. The basic idea with a hydroelectric dam is if we start with a flow of water, say, a river flowing along, then we want to block that flow, using a barrier that’s called a dam. When we do this, water starts to pile up behind the barrier. It forms a pool that’s called a reservoir. We can design the dam so that the only way for water to go from the reservoir to the riverbed is to pass through the dam in a particular channel. And then right in the middle of that channel, we put what’s called a turbine.

A turbine is a device with angled fins so that when water falls through it, the turbine starts to rotate. As the turbine rotates, so does the shaft that connects the turbine to what’s called a generator. This is where the electrical energy in this hydroelectric dam is actually produced.

The amount of electricity produced by the generator depends on the particular angle of the rotating shaft. And we know that angle is constantly changing, since the shaft is constantly turning. So sometimes, the electricity produced is at a high point and then a little while later at a low point. But then, as the shaft continues to rotate, the production goes back to a high point, and so on and so forth. The electrical current that the generator produces goes up and down in a regular pattern. And not only that, but half of the time, the current is moving in one direction. And the other half of the time, it’s moving in the reverse direction.

The current generated above this dotted line we could consider positive and the current below it, we could consider negative, which just means it’s moving the opposite way. All this shows us that the particular type of electrical current generated by a turbine is an alternating current. This is electrical current that regularly — periodically — changes direction. So that’s the answer to our question.

Now, let’s take a look at a second example exercise.

Which of the following correctly describe the advantages of building a hydroelectric dam? a) Building a hydroelectric dam releases no carbon dioxide into the atmosphere. b) Building a hydroelectric dam releases no methane into the atmosphere. c) Hydroelectric dams can respond quickly to changes in energy demand. d) Hydroelectric dams are very cheap to build. e) The running costs of a hydroelectric dam are very low.

Okay, we’re being asked to describe the advantages of building a hydroelectric dam. Now, the starting point for most hydroelectric dams is a river. And let’s say this is our river right here. When we create a dam, we put a big concrete wall in the way of the river. The effect of this is that the river water starts to pile up behind this dam. And when that happens, it’s not uncommon for flooding to occur. After all, before the dam was in place, the water in the river just occupied this narrow riverbed. But now, it’s all collected up in a gigantic pool, called a reservoir.

This means that land that used not to be underwater now is. And this includes the plants that grow on that land. When these plants are underwater for long enough, they start to rot. And in the process, they release greenhouse gases, both methane and carbon dioxide. As we look at our first two answer options, we see that these options claim that building a hydroelectric dam releases no carbon dioxide and no methane. But the decaying plants released both of these gases. And along with that, the production of all the concrete needed to build the dam releases CO2 as well.

So building a hydroelectric dam does release carbon dioxide, which means we won’t choose option a as an advantage. And it also tends to release methane. So we won’t choose option b. Option c says that hydroelectric dams can respond quickly to changes in energy demand. And if we think about it, this dam can be designed so it releases different amounts of water from the reservoir. And it’s the amount of water running through the dam that affects how much electrical energy the facility produces. So if there’s a change in energy demand, either an increase or a decrease, the dam can be changed so that the water flow through it responds to that. And this can happen very quickly, as quickly as moving a mechanized gate. So option c does correctly describe an advantage of building a hydroelectric dam.

Let’s move on to options d and e. Option d says, “Hydroelectric dams are very cheap to build.” Well, when we consider the manufacturing costs of building a gigantic wall of concrete and then installing the turbines and the generators that help produce electricity, on the front end, when they’re being built, hydroelectric dams are actually very expensive. They’re very expensive to set up, to build. But they’re inexpensive to maintain once they’re built. We won’t choose option d as an answer choice.

The last option says that the running costs of a hydroelectric dam are very low. Once the dam is built, the turbines and generators are installed, and the reservoir is filled, not much needs to happen from then on for the hydroelectric facility to generate electricity. The costs at the start are very large. But once electricity is being produced, it can be produced fairly inexpensively from then on. The cost of maintaining a hydroelectric dam is low. So in describing the advantages of building a hydroelectric dam, one advantage is these dams can respond quickly to changes in energy demand. And another is that their operating costs are very low.

Let’s summarize what we’ve learned about hydroelectric power. Starting off, we saw that hydroelectric power is electrical power that’s produced from energy stored in water. We saw further that hydroelectric power is generated when falling water causes turbines to spin, which powers electric generators. Due to this rotation of the turbines, the current generated by hydroelectric dams is alternating current. And lastly, we saw that there are advantages and disadvantages to generating energy this way. The advantages are that hydroelectric dams are quick to respond to changes in energy needs. Along with this, they’re also inexpensive to maintain once they’re built. And disadvantages are that it is expensive to build a hydroelectric dam. And as well, the construction process typically involves the release of carbon dioxide, a greenhouse gas, into the atmosphere.

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