Unit 11, Lesson 11.3, Exercise B ≤2.21
Part 3 OK. So let’s look at some of the most important practical points about wind turbine design. First of all, how do we make wind turbines start? When there is no wind, the blades are stationary, of course. In order to ensure that the turbine will start, when the wind starts to blow, there has to be some torque applied to the blades. To achieve this, we have to twist the blades near to the centre (or ‘root’) of the blade. By twisting the blades, a small amount of torque will be applied when the wind starts to blow, and hopefully this is enough to start the rotor spinning!
A second point we have to think about is how to ensure that the rotor points into the wind. If the rotor doesn’t point into the wind, we will not extract the maximum energy from the wind and we might have a problem getting started, so we need a mechanism to make sure this happens. Most large turbines use a small motor to turn the whole machine into the wind. This is called ‘yaw control’ (that’s Y-A-W), because yaw is side-to-side movement.
A third issue is ‘pitch control’. Pitch is, of course,
up and down movement. Pitch control varies the angle of the blades relative to the plane of rotation. This works in a similar way to the ailerons, or small wing tabs, on an aircraft. By adjusting the pitch, we change the aerodynamics of the blade and this means we can extract optimum power at different wind speeds. Manwell, McCowan and Rogers explain in a lot more detail how pitch control works, and many other aspects of wind turbines.
Fourthly, we have the problem of too much wind. Wind turbines are, obviously, sited in windy locations, but inevitably such locations sometimes experience extremely high winds. Excessive wind can damage a turbine by making the blades rotate too quickly. Modern turbines have wind sensors, including fibre-optic sensors to monitor tiny changes in the shape of the blades when the wind is gusting. They also have a form of radar which can predict the arrival of a gust. When excessive wind is detected or predicted, the blades are turned slightly so they catch less wind and are therefore protected.
One last thing. In order to turn the turbine power
into electrical power, we need an electrical generator. The problem is, we can extract more power if we allow the turbine to turn at different speeds (depending on the wind speed), but many
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generators which are directly connected to the grid need to run at a fixed speed, because the electrical grid is at a fixed frequency. Fortunately, there are some ways to get around this. One approach is to convert the generator output into DC, and then recreate the fixed frequency mains AC using power electronics. This involves extra cost and some power lost in the conversion, but this has to be traded against the extra wind energy that can be extracted by variable speed operation. Another approach is to use a cleverly designed electrical machine called a doubly-fed induction generator (DFIG). But we’ll have to talk about that next time!
So, today we have seen some of the technical aspects of designing wind turbines. As I said at the beginning, there are other aspects, including environmental concerns. I suppose for people outside the mechanical engineering field, the question is: Do the benefits of wind turbines outweigh any problems they may cause? Some people claim that wind turbines are ugly, kill birds and hardly generate any power. But that just isn’t true. It’s quite clear that the benefits far outweigh the perceived problems. Most of the research has concluded that, if turbines are sited correctly – out of sight, in areas with sufficient average wind, and away from major bird migration routes – they do not create extra problems. The evidence lies in the increasing use of wind power for electricity generation, from around 28 quadrillion BTUs (British Thermal Units) in the year 2000 to more than 40 quadrillion in 2009.
Now, I’m going to set you a task to do some further research on wind turbines. Perhaps you think that there is only one kind of modern wind turbine, but in fact there are two, although one is much more popular than the other. The well- known type is the horizontal-axis wind turbine or HAWT. The axis is horizontal, which means that the turbine blades look like the hands on a clock. The other type is the vertical-axis wind turbine or VAWT. I’d like you to do some research and find out the advantages and disadvantages of each kind, the HAWT and the VAWT.
Unit 11, Lesson 11.3, Exercise E ≤2.22
So, today we have seen some of the technical aspects of designing wind turbines. As I said at the beginning, there are other aspects, including environmental concerns. I suppose for people outside the mechanical engineering field, the question is: Do the benefits of wind turbines outweigh any problems they may cause? Some
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