How do wind turbines work with such thin blades?

James - Wind turbines and old fashioned windmills are both designed to capture wind energy, but advancements in technology have led to more efficient designs over time. Here to help me answer your question is professor of engineering at the University of Durham, Simon Hogg. Hello Simon.

Simon - Hello. There are two things that really determine the amount of power that a wind turbine is able to extract from the wind. The size of the turbine, so the swept area of the blade, the circle within which the rotor rotates. The bigger that is, the more air the turbine interacts with, the more power you get. And then the other thing is the amount that the air is deflected. I don't recommend you do this, but if you're driving a car down a road and if you put your hand out the window of the car and then if you tilt your hand such that you deflect the air flow downwards, what you feel is a force on your hand pushing it upwards. And that's in effect all a sail or an airfoil does. It's responsible for deflecting the airflow to create lift. And the extent to which it's able to do that depends upon the careful design of the airfoil. So on modern wind turbines, there's an awful lot of engineering design and innovation that goes into the shapes of the airfoils on the blades that are used in order to maximise the amount that they're able to deflect the air. Whereas on older designs of windmills, they are by no means as efficient. If you look at the blades, they're not aerofoils. They can be sails or even wooden structures, much more basic designs and therefore much less efficient.

James - Interesting. And while I've got you, another thing that will always strike you if you look at a wind turbine is the consistent number of blades. It always seems to be three, whereas in a traditional windmill you may see more. Why is that?

Simon - So imagine that we didn't have to worry about unbalanced effects and imagine that you've had a wind turbine with just one blade on it. Quite a large portion of the air will just pass straight past the turbine through the area where the blade is going to rotate at some point in the future, but not deflected at all. So that's not going to be a good aerodynamic design. But the number of blades you need depends upon how fast the blades are going because clearly the faster the blade is sweeping round, the greater proportion of the air that's flowing through the turbine is going to be affected by that blade. And you may not realise it when you look from the ground, but typically the big wind turbines that you see now when you drive around, the blade tips are going round about 200 miles an hour. So they're actually going very fast. And that's why you only see three blades. Look at aircraft propellers there. You often see two blades, one, sometimes three, sometimes four. It's the same reason.