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People have been harnessing the power of wind for thousands of years, the earliest recorded
evidence of this can be seen over 1000 years ago in Persia. But these machine have evolved
from simple devices used to crush grain and pump water to towering monsters generating enough electricity to power entire towns.
Traditional wind turbines come in many shapes
and designs, but they have all given way to a fairly consistent 3 blade design. Today we
are going to answer the question. Why do wind turbines have 3 blades?
We can imagine each blade as a wrench tightening a nut. If we increase the length of the wrench
we can generate more torque. That’s the force that causes rotation. Likewise if we
add a second wrench we can apply even more force. The same principle applies to wind
turbines. So naturally you may think, why not add as many blades and make them as long
as possible. The biggest wind turbine has a diameter of
164 metres. Each blade weighs 33 tonnes and each cost hundreds of thousands of dollars
to manufacture. So more blades equals more weight and more
cost to manufacture. So maybe less blades is better?
Let’s compare our standard 3 blade design to its immediate competitors a 2 blade turbine
and a 4 blade turbine. Let’s assume they have the same blade design for now.
We can easily eliminate the 4 blade design with a quick cost analysis. Each of these
blades costs a lot of money. Adding a fourth blade provides such a marginal increase in
performance that it does not justify the additional cost,
So its down to 2 and 3 blades.
A 2 bladed design can match the performance of a 3 bladed design by increasing
the chord of the blade by 50%, which eliminates the cost advantage so that’s pointless.
Or we can increase its rotational speed by 22.5%. Turbines with 2 blades will spin faster
in the same wind, due to the reduced drag they experience, but spinning faster
is a negative. Let’s explore why. A faster spinning blade will generate more
noise. This is what a wind turbine sounds like up-close. Yeah people don’t like living
next to these things, so we need to minimise the noise they make, by minimising their speed.
Next we need to worry about centrifugal forces. As the blades spin faster, their apparent
weight increases. Thus the central hub and the blades need to be stronger to resist the
additional stress, again this adds cost. This is what can happen when a turbines breaks
fail in strong winds. So a 3 blade design can generate more power
at slower rotational speeds than a two blade design, while being more cost efficient than
a four bladed option. So the 3 bladed design is our goldilocks choice. Not too much, not
too little.