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.
