Video Transcript
In this video, we’re talking about
tidal power. The idea behind tidal power is to
use the energy associated with the raising and lowering water levels of tides and to
convert that energy into a useful form, electrical energy that can be sent out
through the national grid. To get an idea for how all this
works, we’re going to focus on a particular tidal power station.
On the coast of France, in the
province of Brittany, there’s a town called Saint-Malo, which helps to connect the
open sea with a river known as the Rance River. At this location, when the tide is
rising, the water level increases in this direction, from the sea towards the
river. And then, about six hours later as
the tide goes out, water level decreases in that direction. This pattern is repeated for every
high and low tide, water level rising and then receding at this narrow point. With so much energy going into
raising and lowering the level of the water in this narrow band, the idea was
developed to convert the energy stored in the water to electrical energy. This is done by means of what is
called a tidal barrage.
A tidal barrage is like a dam that
goes across the mouth of a river or a bay. It uses the fact that water level
is constantly increasing and decreasing as tides come in and go out to generate
electricity. At this particular location, in
1966, the Rance Tidal Power Station was completed. At the time and for several
decades, it was the largest tidal power station in the world and generated about 240
million watts of power. To get a better sense for how a
tidal power station produces electricity, let’s take a side-on view of this
barrage.
Looking at it this way, we see that
the barrage is like a dam. It separates water on the sea side
from water on the river side. This barrier is made mostly of
concrete, but we can see that the wall isn’t continuous. On either side, there’re gaps in
the wall. Then right now, those gaps are
blocked by gates, called sluice gates. These gates have the ability to be
raised to let water through or lowered to block water from passing through the
barrage. In between the sluice gates, we see
a turbine connected to a power generator. When the blades of the turbine
rotate, that helps to drive the generator and produce electricity.
For the sake of clarity, we’ve only
drawn one generator in our sketch, but the Rance Power Station actually has 24 of
these. For a tidal barrage like this one,
the whole power generation scheme relies on an imbalance in water levels on either
side of the barrage. When the water levels are the same,
like they are now, even if we were to raise up our sluice gates, no water would flow
through the barrage. That’s because there’s no pressure
imbalance pushing the water from one side to the other. The water pressure on the sea side
of the barrage is just as great and in the opposite direction as the water pressure
on the river side. Even if the gates were up, there’s
no net or overall pressure on the water to move through the barrage, so it
wouldn’t.
But all that changes when we start
to account for the raising and lowering of water levels due to tides. Let’s say that there’s a high tide
coming in from the sea. That means water will be moving in
this direction. As water flows that way, it begins
to pile up against the tidal barrage, and the water level rises. When that starts to happen, we see
a growing height difference between the water level on one side of the barrage and
on the other. And when we reach high tide, that
height difference is as large as it will ever get.
Let’s think once more about the
water pressure on either side of the barrage. On the left-hand side, on the sea
side, the height of our water column — we could call it — is this tall. All that water pushing down on
itself creates a significant pressure pushing the water from the left to the
right. In comparison, the height of water
on the river side is less. And that means the pressure of the
water on the river side is less as well. If the overall water pressure
pushing from the sea side to the river side looks like this, we could say the
overall water pressure pointing the other way looks like that.
In other words, there’s now a net
water pressure that tends to push the water through the barrage. And if we raise up the sluice
gates, that’s exactly what happens. The higher-pressure water on the
sea side starts to flow through to the river side. When that happens, when water is
flowing past the blades of the turbine, the turbine starts to rotate. And this generates electricity in
the generator. If we think about this process from
an energy perspective, what we’re doing is we’re taking the gravitational potential
energy stored up in the water high up on the sea side. And then, as the water flows
through the barrage, that potential energy becomes kinetic energy. It’s this moving water we noted
that turns the turbine and generates electrical energy.
So then, we have a two-step
process. Gravitational potential energy
becomes kinetic energy. And that is converted into
electrical energy thanks to the generator. By looking at our sketch though, we
can tell that this process won’t continue for long. That’s because as water flows
through the barrage, the water level on the sea side starts to decrease. The closer the water on either side
of the barrage is, the less energy it has to move past the turbine until eventually
the levels are equal again. And water stops flowing through the
barrage. And this means that the turbine
stops spinning, so electricity stops being generated by this power station.
At this point, the sluice gates go
back down. And now, water can’t flow from one
side to the other, even if it wanted to. Thanks to the cyclical nature of
tides though, it won’t be long before the water level on either side of our barrage
is uneven once more. This is because after the tide came
in, the next step is for the tide to go out for there to be a low tide. This causes the water height on the
sea side of the barrage to decrease. And as that continues on, we once
more reach a point where the water pressure on either side of the sluice gates is
uneven. This time, the pressure from the
river side is greater than the pressure from the sea side.
Now, when we raise up the sluice
gates, water tends to flow that way, river to sea. And once again, this causes the
turbine blades to start to rotate. And any time that turbine is
rotating, it means electricity is being generated. Regarding this electricity, it’s
created by a generator inside this housing, which is connected to the turbine by a
rotating shaft. As the turbine turns, the shaft
rotates with it. And it’s the cyclical motion of
that shaft which allows for electricity production. And just like that shaft, the
current produced by the generator moves in cycles as well. At one moment, the electrical
current produced will be at a maximum value. And then, it will drop down to a
minimum value and then come back up to a maximum, and so on.
If we were to plot this current on
a graph, we could see that half of it moves in one direction through its
circuit. We could call that the positive
direction. And the other half moves the
opposite way. We could call that the negative
direction. Since the current regularly
switches direction, we can see that the current made by these generators is
alternating current. And all that comes back to the way
that this electricity is generated through the rotation of a turbine.
So then, getting back to our tidal
barrage, we’ve got water emptying out of the river side, causing that water level to
decrease until, like before, the water levels on either side of the barrage even
out. So water stops flowing through,
which means the turbine stops turning and electricity is no longer generated. At this point, the sluice gates
drop back down and the cycle starts again.
Now, we know that tidal barrages
are one of many different options for generating electrical energy. And every single one of those
options has both advantages and disadvantages. When we think about the advantages
of a tidal barrage, one is that there’re no fuel costs associated with running a
tidal barrage. We’ve seen that all the power
needed to turn the turbine and generate electricity is provided by the moving
water. No external fuel supply, which
could be costly, is needed.
Another advantage is that tidal
barrages produce energy very reliably. Every time a tide comes in or goes
out, which is about once every six hours, we can count on electrical energy from a
barrage. A final advantage, and it’s a
significant one, is that once a tidal barrage is built, it releases no
pollution. It doesn’t give off any carbon
dioxide or methane or any other greenhouse gases. And in fact, it doesn’t release any
unwanted by-products from its normal operation.
Now, let’s think about how these
advantages are counterbalanced by some disadvantages. It turns out that as water passes
back and forth through a tidal barrage, the sediments and nutrients in the water can
be affected. A barrage may cause erosion to
happen or its opposite, the buildup of sediment. This filtering process can affect
local plant life. And then, when plants are affected,
so is the wildlife around them. Another disadvantage of tidal
barrages is that there are few places in the world to build them. They require special circumstances
where a large body of water, like a sea or an ocean, narrows down quite quickly in a
bay or a river. These are the sorts of places where
tidal water heights vary the most, but there just aren’t that many of them on
Earth.
And then, one last disadvantage of
tidal barrages is that they only produce electricity periodically. Yes, that production is
reliable. But even when a barrage is working
properly, there’re still many hours in the day when it’s not producing any
energy. This means that if we had an energy
need at all hours of the day and night, a barrage couldn’t completely meet that
need. It only supplies electricity some
of the time when its turbines are rotating. Now that we’ve seen how a tidal
barrage works and thought through some of the advantages and disadvantages to them,
let’s get a bit of practice with these ideas through an example.
The four diagrams in the image show
a cross section of a tidal barrage. When water from one side of the
barrage passes through the turbines to the other side, the turbines generate
electricity. In which two of the diagrams shown
will water flow from one side of the barrage to the other?
Okay, we see these four diagrams
labelled (a), (b), (c), and (d). Each diagram shows a cross section
of a tidal barrage. On one side, on the left in each
case, we have water from the sea. And then, on the right side, we
have water from the river estuary behind the barrage. An estuary, by the way, is a
transitional zone where water goes from being in a very large body of water, such as
a sea or an ocean, into a smaller narrow one, such as a river or a bay. But anyway, based on what we see in
these four diagrams, we want to identify which two of them will have water flowing
from one side of the barrage to the other. In other words, in two of these
four diagrams, water will not tend to flow. And in two of them, it will.
When we think about what makes
water flow through a barrage in the first place, we can recall that it has to do
with an imbalance of water pressure on either side of the barrage. For example, let’s take a closer
look at diagram (b). In this diagram, we see that the
height of the water on the sea side is less than the height of the water on the
river estuary side. This has an impact on the pressure
on water on either side of this tidal barrage. If we think about a bit of water
right here on the sea side that has a certain height of water column above it
pushing down on it, that height of water directly translates into some amount of
water pressure at that point. And the taller the stack of water
on top of that point, the greater the pressure will be.
If we consider a point at the same
elevation on the river estuary side, we can see that this point has much more water
stacked on top of it, so to speak, whereas on the sea side, we have this much water
pushing down on that spot. On the river estuary side, we have
this much. This means that the water pressure
on the river estuary side will be greater than that on the sea side. And if there’s an imbalance of
pressure, that means water will tend to move. Since the water pressure on the
river estuary side is greater, water will tend to flow from right to left through
the turbine like this.
We see then that, at least in the
case of diagram (b), water will tend to flow from one side of the barrage to the
other. This was because of the imbalance
in water pressure on either side of the barrage, which had to do with the uneven
water heights on either side. And that’s the key. Uneven water heights on either side
of a tidal barrage create uneven pressures. And when the pressure on either
side is not the same, water will tend to move through the barrage.
So then, looking at the other
diagrams, we want to find a situation where water level on either side of the
barrage is again uneven, like it is in diagram (b). In diagram (a), we see these water
levels are the same, so the water pressure will be the same and no water will
flow. And it’s the same thing in diagram
(c), even though, compared to (a), the water level is higher in this case. The important point is that it’s
the same on either side of the barrage, so water won’t tend to flow through it. But when we come to diagram (d), we
see that, here, the water level on either side is different. That difference, like we’ve seen,
will create a pressure imbalance. In this case, the greater pressure
will be on the sea side. So the water will tend to flow
through the turbine in this direction.
Knowing this, we now have the
answer to our question. In diagrams (b) and (d), water will
flow from one side of the barrage to the other. And we saw that this is due to a
pressure imbalance which is created by uneven water heights on either side of the
barrage.
Let’s summarize now what we’ve
learned about tidal power. Starting off, we saw that the
energy of tidal water can be converted to electrical energy using what’s called a
tidal barrage. The way a tidal barrage works is it
uses differences in water level on either side of the barrage to create flow through
that powers a generator. And like any energy-generation
method, tidal barrages have advantages as well as disadvantages.
In the advantages column, tidal
barrages have no fuel costs. They produce electricity reliably
with the tides. And once they’re built, they don’t
release any pollution. And on the disadvantages side,
tidal barrages tend to impact their local environment. There aren’t many places in the
world to build them. And even though they do produce
electricity reliably, they don’t produce it constantly. There are large chunks of the day
and night when a tidal barrage is producing no electricity. This is a summary of tidal power
obtained through tidal barrages.
