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.
