Welcome to this lecture on wind energy technology concepts. I've chosen to give you an overview of the different concepts for extracting wind energy, and not only focusing on the commercial concept. After this lecture you should be able to list the different concepts and describe the mechanisms of their energy extraction. You should also be able to approximate the power output from the different concepts. I have chosen to divide the different concepts for extracting wind energy into four principle categories. Rotating lift-based machines, rotating drag-based machines, flying lift based-machines, and machines that are using flow induced vibration. Let's have a look at the different concepts. The most well known is the horizontal axis wind turbine, where the energy from the wind is extracted from a rotating shaft, that is parallel to the wind. It's rotating due to the lift forces on the air foils of it's blades. Another concept, the blades are rotating in a horizontal plane, and also driven by lift, where the shaft is now vertical. Therefore, it's called vertical axis wind turbine. Another typical vertical axis turbine is the drag base Savonius turbines where the drag on one side is larger than the drag on the other side. And therefore, creates a moment that makes the shaft rotate. The flying lift based machines, one of them is the kite that pulls a cable that then pulls the generator that is mounted on the ground. And the last one, the machines that are using flow induced vibrations is the last category. We'll have a closer look at each of these categories and give some examples in the next couple of slides. First, we will have a look at the efficiencies of the different concepts. The mechanical power is, from any concept, can be calculated from the air density, the swept area from which the energy is extracted off the wind, the wind speed itself cubed and the power coefficient. The well known Betz limit is limiting the power coefficient due to conservation of momentum to about 59%. It's simply due to the fact that the wind turbine extracts the energy from, the momentum from the wind, which slows down the wind speed. And there is a limit on how much you can slow down the wind and still keep an efficient extraction of energy in the swept area. You can of course discuss, if you look at the different concepts, what is this swept area. For example, if you look at a vertical axis turbine you could say that it has two areas of which it extracts. One in the front and one when the blade is passing behind the other blades. Of course, the energy in the latter will not be as much and in the front because it's in the shadow of the first one. So unless there is energy coming from outside into the second plane, it will still have a power coefficient that is limited to the Betz limit. Here are some examples of the horizontal axis wind turbine. This is a very old turbine, one of the old research turbines in Denmark. And they have of course developed far from that time. Another example of a rotating lift based machines, is this vertical axis wind turbine, a Darrieus turbine. Where the energy is now extracted in the bottom of the turbine instead of up at the top of the tower, in the nacelle, from the generator. The swept area are quite easily calculated for these because they are simply given by the rotor diameter or the size of the rotor. The rotating drag-based machine, the most well known is the Savonius. It has the benefit, also being able to extract energy from the bottom, but also that it is a quite safe turbine and can be mounted in urban areas. The swept area is also given by the geometry of the rotor. Here I am showing the power coefficient for the different rotating concepts that have been derived over the years, both experimentally and numerically. These are some of the results that are coming out. On the y-axis, we have the power coefficient, and on the x-axis, we have the tip speed ratio. The tip speed ratio is the ratio between the tangential speed of the rotor tip, due to the rotation divided by the wind speed. You see here that in this case, goes from zero to about seven. We have the modern commercial turbine which is running with a quite high tip speed ratio and it can produce quite high power coefficients between 40% and 50%. We also have the vertical axis turbine, the Darrieus type turbine which also running with quite high tip speed ratio and also produces relatively high power coefficients. In this low range of the tip speed ratio, we find the multi-plated rotors that can produce a high torque but not one very fast. It has a power coefficient up to 30 %, and is therefore also more efficient than the drag base machines, which also have low tip speed ratios. Like these Savonius turbines between 10% and 15% efficiency. The flying lift-based machines are a little bit more difficult to define a power coefficient based on the swept area, because the swept area depends on how you are flying the kite. The principle of operation is that you're flying the kite out, it pulls the cable, and when it's fully extended you pull back the kite and then you fly it out again. When you pull it back you try to minimize the aerodynamics forces and lift on the kite. This is the pumping kite with the cable pull. You can also extract the energy from the kite by mounting a propeller, or you could say the horizontal axis turbines on the kite itself, and then fly it in circles and extract the energy from the generators on the kite. And then you have to transmit the electricity generated through the cable to the ground. For these concepts, the power coefficient or the power produced is based on the area of the kite and the lift coefficient of the kite which is typically in the order of one. And also based on the glide ratio of the kite itself, which is in the range of 5 to 20, depending on the design. A high aspect ratio kite, or plane like this one will have a high glide ratio, while a typical kite that you see on the beaches will have a lower glide ratio. The last one is flow induced vibrations that can generate electricity. For example, by making a beam vibrate and have piezo ceramic material that extract the energy. If we look at the motion itself, of the tip of the airfoil, we see that it is vibrating due to the cross-flow over it. If we look at the area we should use to calculate the power coefficient, we should use the swept area when we look at conservation of momentum. If I take this example from Cornell University and the paper shown here, they have shown that they can generate a couple of milliwatts with a vibration of eight centimeters and a span-wise airfoil that is about 13.6 centimeters. It has the benefit of being able to stop this particular one at very low windspeeds. And if I calculate the power coefficient for this example in this paper, I get a quite low power coefficient. But it has the benefit of being able to start at low windspeeds. It has very few moving parts, and it can therefore be used in, for example, to power remote sensing and other devices. But it's still in milliwatts, we're not talking megawatts. Finally, we come to the summary of this lecture. In this lecture you should have learned that the concept of extracting wind energy can be extracted into four different categories. The rotating lift-based machines, the rotating drag-based machines, the flying lift-based machines, and machines that are extracting the energy due to flow-induced vibrations. The power output from each concept will depend on the area they sweep and their power coefficient, that is in fact how we define the power coefficient. They all subject to conservation of momentum and therefore, they also have each of them different power coefficients
