[MUSIC] So now we are going to explore wind energy, how we're going to extract the energy of motion, of wind. You remember that the sun when it shines on Earth and warms the surface of the water or the surface of land, that water and land acquire all that energy and start warming up, start satisfying its heat capacity. All matter has a capacity to store heat. The temperatures start rising and start transferring the heat, remember conduction, now, when it's in contact with the other, to the wind to the air, to the gases. The gases expand and move up by differences in density and then compounded by the motion of the Earth around its axis, there are preferred directions of motion. And that actually what it does is that it produce wind currents, and the wind currents have mass, have velocity. So if they have mass and velocity, they have kinetic energy, and we would like to see if we can get the kinetic energy out of the wind. So what we are going to use are fundamentally a technology called wind turbines. So turbines actually implies that there is a rotation axis, and there are some blades that can interact with the wind. How that happens, well, that's what we would like to explore with you today. So fundamentally, it is a device that can take the kinetic energy of the wind and transform it into electrical energy. That transformation make this sort of device a transducer, it sort of transduce one form of energy to another, in this case, kinetic energy of the motion of wind into electrical energy that we can store in the our batteries. So this device have, of course, some blades that are the one that are going to interact with the wind, and the blades made it to rotate, so it needs some sort of an electric motor. Actually the electric motor of choice, it's usually something called a permanent magnet DC motor, although sometimes the people use AC Motors. And what happened is that all these blades, the blades that are connected to the rotation axis in that electrical motor, the cross section of the blades looks like the wing of an airplane, that it is thick at the front and then tail down to a thinner and thinner cross section until it ends up in the back. Imagine that this is the cross section of a wing, the wind comes and phases the wing section. It divides in two flows one flow on top and another flow at the bottom. The distance that it has to travel on the top, it's longer than the distance it has to travel at the bottom. So it is sort of a longish on the top of the wing section than in the bottom. So what happen is that the molecules of air on the top are separated by a larger distance as they try to speed up to meet the ones that are going at the bottom and join at the end of the wing section. So since the molecules are farther apart on the top than at the bottom, they hit the surface of the wing section less. So the number of molecules per unit area, per unit time hitting the top is less than the one hitting the bottom, but actually number of molecules per unit time hitting an area, it's called pressure. So actually the pressure in the bottom, it's higher than the pressure in the top, and the motion, it's in the direction of the force, not the acceleration, it's in the direction of the force. So it tends to move the blade in this direction. Of course, it's tied to a rotation axis, so it cannot just go straight like this. It has to go at the same distance all the time describing a circle, and that it's what produce that circular motion that it's coupled to the DC electric motor, and it will produce electric power. We need, besides these blades by the way, the blades are usually three, and the whole idea is that if there are two blades and the windmill, the turbine, it's located on the top of a tower or a structure. One of the blades are always at some point in the rotation, it's going to be covering the blade, the structure that it's holding the turbine in place. So only one is actually producing power. So in order to avoid that, they will try to increase the number of times that it goes around. So it add another blade, and there are turbines that have seven blades, five blades, maybe there's some that has many more blades. And the three blades will make the center of the electric motor rotate. You remember the electric motor has this axis that is connected to the three blades, and this axis, it's called the rotor for obvious reasons. Now it's the one that rotates. The part of the motor that is attached to the tower, it's called the stator because it stays in there. It doesn't really move. In the stator, they have the three permanent magnets, and they have three. Why they have three? Has nothing to do with the three blades of the turbine. It has to do that the angular displacement around the cylindrical case of the DC motor. It's a circle, and the circle, you know that the angular displacement, it's 2 Pi radians, 360 degrees. So if it's 3 pi radian, I mean 2 Pi radians, we divide 2 Pi radians by 3, and each two-thirds of Pi, we will placed a magnet, a permanent magnet. And then that's in the stator. Now in the rotor, we will put three also every two-third Pi, we are going to put a coil, an electrical coil. So what happen is than when an electrical coil moves in the presence of a magnetic field of a permanent magnet, it induces a current in the conductor. The conductors are going to be copper wires, and it's going to produce a current. So here is the stator, here's the permanent magnet, as the coil moves, it start intercepting more and more magnetic field lines. It start producing more and more current, it goes through a maximum when it is in front of the permanent magnet and then start going down. So it is a sinusoidal node, it's like the waves in the ocean. There is a maximum. There's a minimum, there's a maximum, there's a minimum. But there's three of them, so we say, and they are not producing power exactly the same time. They produce power one slightly before or after the other, so we say that the electric motor produced a three-phase AC power. Of course, most of the time you want to charge a battery. So if you're producing AC power in your electric motor connected to your windmill, you have to change it from AC to DC. That phenomena is called rectification, and you can buy rectifiers. They are not particularly expensive. They are very reliable. All you have to be careful is that the amount of power that you're going to generate in your windmill, it's match to the amount of power that your rectifier can handle. Once you do that, you can use it very reliably, and there's no problem. There's a problem, talking about problems, there is a problem with windmills. And it is that usually small windmills are not particularly efficient, or they are not particularly important in the production of power. And so a scale, it's really important. So the power that we can extract from the motion of a mass of gases, the wind, it's fundamentally proportional to the cross-sectional area of the blades that are interacting with that air current. So the larger the turbine blades and the larger the circle that the tip of the turbine blades are described, the more power you can get out of the wind. That's why you see that most of the commercially available wind turbines are really large, are many decades of meters in diameter, and you see them going very, very, very slow. Of course, because you are moving a very large mass, so you need a very large force. It is really important to remember that if you're going to go wind, you would like to buy the largest possible wind turbine you can afford because the amount of power is scaled directly with that cross-sectional area.
