Welcome to Engineering with Rosie.
Today's topic is part of the wind energy series.
We're going to discuss why do wind turbines have 3 blades.
This is definitely the most common question that I get asked about wind energy.
And I've actually found that there are a tonne of different explanations out there
as to why this is.
If you ask 5 engineers in the wind industry you'll probably get 5 different answers.
And the reason for that is that there is no single reason why wind turbines have 3 blades.
It's a tradeoff between a whole bunch of different factors.
So let's look into some of those factors and find the answer
why do wind turbines have 3 blades.
The very short answer is that
you want the smallest number of blades that is both balanced and stable.
More than 3 blades means that materials, manufacturing, transport and installation costs
are higher than they need to be.
And in the other direction, a single blade is unbalanced
and 2 blades is not stable when it is turning to face a changing wind direction.
Now if you're interested in the longer answer, keep watching.
And we're going to find out about the way that
structural design, materials, manufacturing, transport, installation,
balance, stability and control considerations
are all tradedoff
which nearly always results in engineers choosing a 3-bladed design.
Ok, I want to start by talking about aerodynamic efficiency, because sometimes people ask:
"what is the most efficient number of blades for a turbine to have?"
And the answer to that question is that there is no answer.
You can have an aerodynamically optimised turbine with any number of blades.
You can design a turbine with 1 blade, 3 blades, 6 blades, even 100 blades.
And they can all have the same efficiency if you change the width of the blade
or the speed that they rotate.
Now normally rotational speed needs to be kept below a certain limit,
because the faster blades move through the air, the more noise they make.
This needs to be limited,
so designers can't just make the blades turn as fast as they want.
Once you know the rotational speed,
you can change the width of the blade to make the
aerodynamic design optimal
for whatever number of blades you want.
So if it isn't anything to do with aerodynamic efficiency,
then how do engineers choose the number of blades?
I mentioned that it is the result of a number of design tradeoffs.
And one of the most important of these is structural considerations.
We've seen that having more blades means that you need to make them skinnier,
so you might think that they would use less materials.
But actually, really skinny blades are hard to make stiff enough.
To get a long skinny blade to be as stiff as a fatter blade of the same length,
you either need to add more material, or change the material.
So you could swap fibreglass for carbon fibre, which is stiffer but more expensive.
Another reason that designers prefer fewer blades is that manufacturing, transport and
installation of a lot of blades costs more than it does for a small number of blades.
So for these reasons, it's cheaper and easier to have a small number of blades.
But that still doesn't answer why we normally see 3 blades.
If we want the smallest number possible, then why not just 1 blade?
That is the smallest number after all.
Now the reason why we don't often see 1 blade or 2 blades is to do with balance and stability.
A single blade is not balanced.
In low wind speeds, a single blade would just sort of hang there in the 6 o'clock position
and it's really hard to get it started.
So in the past when they had single bladed designs,
they had to add a counterweight to balance out the blade,
so you end up adding weight and complexity for no gain in efficiency,
and what's the point in that?
Now, 2 blades is balanced.
So you don't have that problem of needing to add a counterweight.
But the challenge with 2 blades is that it's difficult to turn the turbine
to face a changing wind direction.
You can think of the turbine with the blades at 3 and 9 o'clock
like a figure skater with her arms straight out,
and the turbine with its blades in 6 and 12 o'clock is like a skater with her arms straight
up.
If she moves from horizontal to vertical arms her speed will go from slow to fast, right?
It's the same with a turbine.
As the blades rotate from one position to the other, if you keep the turning force constant,
then the speed changes as the blades move from vertical to horizontal.
And this creates some challenges for the system that turns the turbine
and also for tower structural design,
as that jerky turning action can excite the natural frequency of the tower
and start it wobbling.
So there you have it,
that's why wind turbines usually have 3 blades.
But not always.
None of the problems that I mentioned are unsolvable.
And in some situations, it makes more sense to have 2 blades than 3.
One common example is small turbines that value simplicity.
This turbine has 2 blades that are fixed in the middle.
They can be assembled in one piece, which makes installation easy.
Since the turbine is very small,
it can use a system of springs to allow the blades to tilt
as a way to smooth out the changing forces when the turbine rotates to face a changing
wind direction.
And at the other end of the scale,
really large turbines for offshore wind have very high costs for transport and installation
so some turbine manufacturers believe that 2 blades is the better design for them
because the savings from transport and installation outweigh the extra cost needed
to deal with those stability issues during turning.
And in the future, as blades continue to get longer,
perhaps we'll see a desire to move to slimmer blades
to avoid them getting too wide.
Already, the biggest blades are way over 5 metres at their widest point.
Which means you need a special factory with high roofs
so that you're going to be able to turn them over.
So perhaps designers will decide to move
to a larger number of blades
to get the width down?
I don't know if that will ever happen, but good engineering means constantly reassessing
these tradeoffs
as the design requirements change.
Thanks for watching today and I hope you found this explanation helpful.
If there were any concepts that didn't make sense to you
or another topic that you would like to cover
then please let me know in the comments, and maybe I can make a video on that.
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See you next time.