Welcome to engineering with Rosie. Today I want to talk
about how wind turbine designers take care of a really important
issue. How do they make sure the blades don't hit the tower. I'm
going to talk about how to choose whether to put the blades
upwind or downwind of the tower. And then some other things that
you can do to keep the blades well out of the way of the
tower, including rotor tilt and overhang, cone angle and then
something that I think is particularly clever design
solution, pre-bending the blades away from the tower.
Okay, so I think it is pretty obvious why it's important to
make sure that wind turbine blades don't hit the tower. But
maybe it's not so obvious why that is even something that you
need to worry about. Wind turbine blades are usually
pretty flexible, they can easily bend like 10, or even 20% of
their length when they're under very strong winds. Of course,
you can just make them stiffer. But to do that, you would need
to add a lot of material which cost money and makes the blades
heavy. It means that every other part of the turbine needs to be
strengthened to carry the heavier blades. There's also
some advantages with flexible blades that the the extreme wind
loads aren't as high on a flexible blade as a really stiff
one. So designers want to make the blade strong enough to
withstand the strong wind loads, but still a bit flexible.
So the next most obvious question is, why don't we just
put the blades on the downwind side of the tower? Then when the
wind blows on the blades, it will bend them away from the
tower. So in that case, it wouldn't matter how stiff they
are. And actually there are turbines like this, such as this
little two bladed turbine that I visited recently.
So behind me, we have a small wind turbine, which is sold for
farm or home usage, this one is connected to a household of five
people. And it creates more than enough electricity for that
house and the excess is sold to the grid. So there are a few
really interesting design features about this turbine. And
obviously, you can see it only has two blades, whereas most
large modern turbines have three. But the one I want to
talk about today is the fact that the blades are located
downwind, you can see from the direction my hair is blowing,
that the wind is coming from behind me and the blades on this
turbine. They're actually located on the downwind side of
the tower. And the advantage of putting them on the downwind
side is that if there's a strong gust of wind then the direction
that the blades are going to bend is away from the tower, so
you know it's never going to hit the tower.
So that solution works really well for that small turbine that
we just saw but modern utility scale turbines pretty much all
have their blades on the upwind side of the tower. Why? Why are
wind turbine blade structural designers making their job
harder than it needs to be? So to answer that question, let's
take a trip to a wind turbine test site to have a look at some
modern turbines and find out why designers want to put the blades
upwind and then what steps they take to make sure that even
though the blades are upwind, they never bend and hit the
tower.
Remember, when we saw the small two bladed turbine, we talked
about how its blades were on the downwind side of the tower so
that they wouldn't bend and hit the tower and strong wind? Well,
there's a lot of other reasons why it's better to - or more
efficient to - have the blades on the upwind side of the tower
and nearly all modern industrial turbines are made in this way.
So what are the reasons that make it better or more efficient
to have the blades on the upwind side of the tower instead of the
downwind? The first issue is that if the blades are downwind
of the tower, then the tower is blocking wind from hitting the
blade when it's directly behind it. Now, that might not sound
like a very big problem, I mean the tower is pretty skinny, it
only covers a pretty small portion of the area that the
blades sweep. Can that make such a difference? The problem isn't
so much that there's a loss of power production from that
blocked wind, it's more that when the blade is behind the
tower in its shadow, so to speak, then the blade it won't
bend as much as when it's out of the shadow in the wind. So every
time the blade passes the tower, it goes from bending away to the
tower to suddenly bending towards it when it goes past and
then back away from it again when it's out of the shadow. Now
this is bad for the blade structure because all of the
times that the blade bends over its lifetime they add up and
they could eventually break the blade. This is called fatigue
loading. It's the same effect as when you bend a paperclip back
and forth. Each individual bend may not be enough to damage it,
but after enough bends the paperclip or the blade will
break. So wind turbine blade designers are very concerned to
reduce the fatigue loading on a blade. Locating the blades
downwind of the tower, it also causes an audible thump every
time the blade passes the tower which is no doubt really
annoying for anybody who has to listen to it.
Okay, so those are pretty compelling reasons to locate the
blades upwind of the tower, you wouldn't want to have the
blade's lifetime reduced due to fatigue loading from the
constant bending in the tower shadow. But you also wouldn't
want it to break because it's smashed into the tower. So if we
can't just make the blades stiff enough that they don't bend at
all, then how do wind turbine engineers make sure the blades
won't hit the tower?
There are a few things that they do. First, the hub that the
blades are attached to is sticking out away from the
tower, and the end of this big box, which is called the
nacelle. Obviously, the further the overhang, the more the
blades can bend before they hit the tower. But a really big
overhang increases bending moments on the shaft and then
nacelle bedplate, which complicates the design of these
components. So you don't want to make the overhand too big.
Second, you can tilt the rotor disc away from the tower to
increase clearance that way. Of course, the more you tilt, the
less area of wind your blades are sweeping, which reduces the
amount of power available to the turbine. So you need to keep the
tilt angle small, but angles of around five or six degrees, give
extra blade tower clearance with only a very small reduction in
power.
Third, you can change the cone angle. The blades on a wind
turbine, they aren't mounted strictly in plane to sweep a
flat disc, they're angled slightly away to make like very
shallow cone shape. So the more that you angle them out,
obviously the more clearance that you get between the blade
and the tower. But once again, the bigger the angle, the
smaller the swept area becomes. So cone angles are usually kept
small, maybe around two or three degrees.
And the final method that wind turbine designers use to make
sure that the blades don't hit the tower is my personal
favorite. I think it's a really clever design solution. So I
want to show you, I found some good examples of this back at
the test center. So let's go back there now to see.
Now I'm next to some blades, which are sitting on the ground.
And you can see one really interesting feature about them.
The tips, they're bent away from the ground. So to avoid the
problem of blades hitting the tower and strong winds, they
pre-bend them so that the tips start out bent away from the
tower. And then when the wind blows, they have some extra
distance to go before they would hit the tower.
So that's the options for how wind turbine designers can keep
the blades from hitting the ower. I like the pre-bend o
tion the best because it d esn't cause much penalty in ae
odynamic efficiency or st uctural design challenges as th
other options do. Though it is more challenging to m
nufacture and transport the b ades like this. So are these t
e only option that turbine d signers have to stop blades h
tting towers? Actually, in s me of the more modern t
rbines, they can monitor the p sition of the blades in real
ime. And if the blade is going o get too close to the tower,
hen they can change the blade ngle so it moves away as it
asses the tower and then back o its normal position once it's
assed. This means that blades an be more flexible and
otentially lighter, which helps ith other aspects of the
urbine design. But it makes a ontrol system more complicated.
nd of course it needs to be eally reliable because the
onsequences of getting it rong could be very serious.
If you were designing a wind turbine, how would you make sure
the blades don't hit the tower? Using one of the methods I've
talked about here or have you got a totally different idea?
Please tell me in the comments. And also feel free to ask for
more explanation on anything I've talked about in this video
or to suggest another topic.
Remember that if you want to see more videos like this, then you
should like and subscribe and share with your friends. I'll
see you next time.