Lesson Video: Wind Power | Nagwa Lesson Video: Wind Power | Nagwa

Lesson Video: Wind Power Physics

In this lesson, we will learn how to describe the advantages and disadvantages of wind turbines, and which locations are suitable for wind farms.


Video Transcript

In this video, we’re talking about wind power. Today, many of the applications of wind power have to do with generating electricity. But that hasn’t always been the case. As early as 1500 years ago, people used power supplied by the wind to do things like grind grain into flour. As technology has developed over time, though, we now think of wind power as a viable option for generating electrical energy. This is done using collections of wind turbines in what are called wind farms. Each turbine consists of a base and then a rotor, typically with three arms, sometimes with two.

Wind farms can be located on shore, on dry ground, as well as offshore. There are advantages and disadvantages to both locations. But before we get into those, let’s consider how an individual turbine works. Say that we have a wind turbine with a three-blade rotor, like the one we’ve sketched here. When the wind is blowing past this turbine, that means that the molecules in the air are in motion. If we think of these molecules as collections of little spheres that have mass, we can see that because these spheres are in motion, they have kinetic energy. So pushed by the wind, these air molecules pick up kinetic energy. And some of them crash into the blades of our rotor.

When they do that, energy is transferred from the wind to the rotor. The wind pushes on the rotor. It’s the same effect as we feel whenever we walk outside on a windy day or when we’re driving in a car and we put our hand out the window. We feel a pressure exerted by the moving air. The same thing happens to this rotor blade. Now, the blades on this rotor are angled so that when they experience a push, they start to turn in a certain direction. As the wind speed picks up, the blades in this turbine spin faster and faster. Now, if we were to look inside our wind turbine, we would see that this rotating blade is connected by a shaft to an electrical generator. And it’s the rotating turbine, which is driven by the wind, which helps to drive electricity generation in the generator. That electrical energy can be transported out of the wind turbine using a wire, which then connects up with other turbines on the farm and delivers this energy to its end users.

Depending on the size of a wind turbine, in particular the size of the blades in its rotor, it can generate more or less electricity. Smaller-sized land-based turbines may generate on the order of 50 kilowatts or 50000 watts of electrical power, while larger turbines, such as the ones installed offshore, can generate in the neighborhood of eight megawatts or eight million watts of electrical power. Along with blade size, wind speed is a big factor in how much electricity a turbine generates. The wind must be moving at a minimum of about seven miles per hour or three metres per second for any electricity to be generated. And for safety reasons, there’s also a maximum wind speed in which turbines are allowed to operate. When wind speeds exceed 55 miles per hour or roughly 24 metres per second, a turbine will automatically shut down, so it’s not damaged by high-speed winds.

As we consider advantages and disadvantages of wind power, this discussion of wind speed brings up one of the disadvantages, unlike other sources of energy, which can operate day or night regardless of weather conditions. Wind turbines obviously need wind. And more than that, the wind has to be travelling between a certain minimum and certain maximum speed. A second disadvantage of wind power is that wind turbines, individually and together, take up lots of space. These turbines can be tens or even hundreds of metres tall. To bring a group of them together in a farm then takes up lots of space both on the ground or on the surface of the water and up into the air. For areas in the world with high-population densities, this can be a problem.

But along with the disadvantages, there are several advantages of wind power. For one thing, once they’re built, wind turbines are cheap to maintain. There aren’t many upgrade or maintenance costs associated with them. Another advantage of wind power is that it produces almost no pollution. Now, we say almost because even though while its operating a wind turbine doesn’t give of any pollution whatsoever, if we consider the entire life cycle of a wind turbine from mining the materials to make it, to creating it, to installing it, to disassembling it once it’s finished use. Early on in that process, during the manufacturing stage, some carbon dioxide is released as part of building a wind turbine. Once it’s operating, though, the turbine produces no carbon dioxide or other greenhouse gases or pollution.

Now that we’ve talked a bit about wind power in general, let’s go back to considering the differences between onshore and offshore wind farms. One of the first differences that may stand out is that offshore wind farms require installing wind turbines in water, sometimes deep water. For this reason, several different support structures have been developed for offshore wind turbines. If the water is shallow enough, the base of the turbine can go right into the sea floor. When the water is deeper on the order of tens of metres, then rigid supports can be connected from the sea floor to the base of the turbine. And then for turbines installed in even deeper water, deeper than about 60 metres, flexible cable stays, which are fixed to the sea floor, can be used to keep the turbine in place and upright.

All the effort required to give offshore wind turbines a good foundation means that onshore wind farms have the advantage of being easier to install. But then offshore wind farms have the advantage that wind speed is typically higher over open ocean or open water than over land. And not only other higher wind speeds over water, but because these turbines don’t use up any land, it can be built bigger with longer blades that catch more of the wind.

As of early 2019, the biggest offshore wind farm in the world consists of 189 turbines located off the west coast of England. This farm generates about one gigawatt or one billion watts of electrical power. When these turbines generate electricity, it’s communicated to the land via undersea cables. And in general, the electricity created by any wind turbine, whether onshore or offshore, has an alternating current. As wind causes turbine blades to rotate in a cycle, that cyclical motion, when communicated to the generator, leads to the generation of AC or alternating current. Let’s get some practice now with these ideas about wind power through an example.

The diagram shows three possible locations where wind turbines could be placed. The average wind speed is 1.2 metres per second in the valley, 5.5 metres per second on the moor, and 7.9 metres per second over the ocean. Which of the three locations is not suitable for wind turbines. Why is this location not suitable for wind turbines?

Okay, so looking at our diagram, we indeed see these three candidate locations for a wind turbine, the valley, the moor, and the sea. And we’re told that, at these three different spots, the wind speed is not the same. In the valley, the wind speed is 1.2 metres per second, while on the moor, it’s 5.5 metres per second. And then over the ocean, it’s fastest of all, 7.9 metres per second. Now, wind speed is a major determining factor for where to put a wind turbine. Up to a certain limit of about 24 metres per second, the faster the wind is blowing, the more power a turbine can generate.

Not only do turbines have a maximum allowable wind speed which is enforced for the safety of the turbine materials, they also require a minimum wind speed in order to generate any electricity at all. That minimum wind speed needed to turn the rotor of the turbine and generate electricity is about three metres per second. But then look at this. The wind speed in the valley is below that speed. This means that if we put our turbine here and the wind blew at this speed, we wouldn’t get any electricity out of it. The wind speed is not reaching the minimum value needed to generate electricity. On the other hand, the wind speeds over the moor and over the sea are high enough to cause our turbine to generate electricity.

So only one of these three locations is not suitable for a wind turbine; it’s the valley. And the reason is that in the valley the wind speed doesn’t reach its minimum required speed. We can write that this way. We can say that wind turbines cannot generate electricity if the wind speed is below approximately three metres per second. This explains which of the three locations where we wouldn’t install a wind turbine and why.

Let’s now look at a second example exercise.

Wind turbines convert the blank energy of the moving air to blank energy.

Okay, in this exercise, we want to fill in the blanks in this sentence. And we can tell that the sentence has to do with converting energy from one form to another. In particular, it’s talking about how wind turbines work and take some type of energy associated with moving air and convert it to another type of energy. If we think about this moving air, we realise that air in motion is air molecules in motion. And since molecules have mass, what we have when the wind blows is we have mass in motion. The type of energy associated with motion is called kinetic energy. Since the moving air is in motion, it has this type of energy. So we’ll write that in in the first blank in our sentence. Now, our sentence reads, wind turbines convert the kinetic energy of the moving air to blank energy.

To see how to fill in this second blank, we can recall the general purpose of wind turbines. These turbines are designed to produce energy that can be fed into the national grid and then used by homes and businesses. The type of energy that goes into this grid is electrical energy. That’s the most useful form of energy that wind turbines can produce. Now that we have both blanks filled in our sentence reads, wind turbines convert the kinetic energy of the moving air to electrical energy. This is how they take a less useful form of energy, the air’s kinetic energy, and convert it to a more useful form, electrical.

Let’s look now at one last example on wind power.

What type of electric current do wind turbines produce?

A great way to start answering this question is to sketch out what a wind turbine looks like as it’s working. Let’s say that this is our wind turbine, and we have wind blowing past it. The pressure of the wind on the turbine blades will cause this whole turbine to start to rotate in a certain direction. As that happens, as the turbine rotates, it’s connected internally to a device called a generator. The generator is what takes the rotational energy of the turning turbine and converts it to electrical energy. And we want to know when that electricity is produced, what kind of current is output?

Going back to the internal workings of our turbine, as the shaft of our turbine rotates, that causes the rotation of a coil of wire inside magnets in the generator. Depending on the angle of this rotating wire more or less electrical current is induced in it. It’s this current, which is the output of the electrical generator. And it goes up and down, depending on the rotation of the turbine. For one position of the turbine and therefore the position of the wire inside the magnets, the current generated is at a maximum value. But then later on, as this wire rotates, the current is at a minimum. But then with rotation continuing, it goes back to a maximum value then back to a minimum then to a max, and so on and so forth.

If we drew a line connecting these points, that line would go up and down like a sine wave. Now, because of the way this generator works, some of this current flows in one direction, and some of it flows in the opposite direction. We could say the current above this horizontal line is positive; it flows one way. And the current below the line is negative; it flows the other direction. And this alternation between positive and negative current happens over and over again. In other words, we have electrical current, which goes from positive to negative and back again in a periodic or regular way. And in fact, that’s the definition of alternating current or AC. Because the electricity from a wind turbine is generated thanks to the rotation of the turbine, that electrical energy is output with an alternating current, a current that reverses direction periodically.

Let’s summarise what we’ve learned in this lesson on wind power. Starting off, we saw that wind power involves converting kinetic energy of moving air into electrical energy. We learn that wind farms harvest wind energy. And these farms can be built onshore or offshore. The advantage of onshore wind farms is that they’re cheaper to build and maintain, while offshore wind farms enjoy higher wind speeds on average. And they also take up no land. An individual wind turbine has a minimum as well as a maximum allowable wind speed. That minimum is about three metres per second or seven miles per hour. And the maximum is about 24 metres per second or 55 miles per hour. And when the turbine is operating within this range, it outputs alternating electrical current.

And lastly, we learned about advantages and disadvantages of wind power in general. Advantages include that wind power generates very little pollution and that wind turbines, once they’re set up, have very low maintenance costs. Disadvantages of wind power are that wind turbines are weather dependent. They require air moving at a certain range of speeds to generate electricity. And also, turbines can take up significant space, especially on land, or space maybe at a premium. This is a summary of wind power.

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