Video: Tidal Power

In this video, we will learn how to describe the advantages and disadvantages of generating electricity from tidal barrages.

13:50

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.

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