Welcome to Engineering with Rosie.
Today's topic is part of the wind energy series.
And we're going to find out about the two
different ways that you can regulate a
wind turbine's power output: stall regulation or
pitch regulation.
So first let's start by explaining what that
means:
to regulate a wind turbine's power output.
A wind turbine generates electricity from
the wind.
Wind moving over the wind turbine blade
generates a lift force at a distance from the hub.
This force times distance creates torque
which is what turns the blades.
The wind turbine blades are connected to a shaft,
which is connected to a generator.
The generator takes that mechanical energy
From the rotating shaft and converts it to electricity.
But the amount of power the generator can
handle is limited and when wind speeds are
very high there is more power available than
the generator can handle.
And so to avoid overloading it, there needs
to be a way to reduce the amount of power
coming from the blades to the generator.
And there are two main ways of doing this:
stall regulation or pitch regulation.
The first way, most common on early turbines
up to about the 1990s is stall regulation.
So, what is stall?
The aerodynamic effects that make a wind turbine
blade rotate are the same as the effects that
make an aeroplane wing fly.
When an aerofoil is inclined relative to airflow
it creates a lift force that acts perpendicular
to the airflow and also a drag force that
acts parallel, opposite to the direction of
the wind.
In aerodynamics, the amount of incline relative
to the wind is called the angle of attack.
An increase in this angle increases the amount
of lift.
But only up to a point.
And that point is called the stall angle.
For an aeroplane to climb, the lift force
needs to increase.
So the pilot will pitch the front of the aeroplane
up.
But she needs to be careful not to climb too
steeply.
Because if she increases the angle of attack
beyond the stall angle the lift force will
suddenly decrease and the aeroplane will start
to drop.
What's happening here is at moderate angles
of attack the air flows smoothly along the
aerofoil.
And the inclination causes some rotational
component to the flow, which causes low pressure
on the top of the profile relative to the
bottom.
The profile gets pushed up by the high pressure
on the bottom and pulled up by the lower pressure
on the top.
But at a certain point the airflow won't stay
attached to the profile, but will instead
separate.
And when it does that then there is suddenly
less circulation and so less lift.
At the same time, the separated flow is very
turbulent which means that it is violently
swirling.
This suddenly increases the drag force at
the same time as the lift force decreases.
And this effect is called stall.
On a stall regulated wind turbine blade this
sudden decrease in lift force means that there
is less torque turning the blades.
Ok so now we know what stall is and we know
why it decreases the power produced by a turbine.
But how do they make sure that the blade stalls
when wind speeds get too high?
Well actually you don't need to do anything.
If the blade is fixed in place, then as the
wind gets stronger, the angle of attack will
automatically increase towards stall.
I'll explain that a bit more because maybe
it's not that obvious why that happens.
I mentioned that a wind turbine blade flies
just like an aeroplane wing does.
But the environment that it's operating in
is a bit different because the turbine blade
is rotating.
In addition to the airflow in the wind direction,
the blade also sees airflow in the direction
of rotation.
To find the local angle of attack you need
to add these two wind speed vectors to get
the resultant wind speed and direction.
As the wind speed increases it changes the
angle of attack closer to stall.
If the blades are fixed and the rotational
speed stays the same then when the wind speeds
gets high enough the blade will start to stall
which reduces the power.
This is a very simple way to regulate power.
The blades can be rigidly attached as they
don't need to twist to change the angle since
that happens automatically as the wind speed
increases.
But there are also some big disadvantages.
When the blade stalls, it causes turbulent
swirling airflow which is quite unpredictable
and causes a lot of vibrations and extreme
forces that can shorten the lifetime of the
blades and other turbine components.
So let's move on now to the second way that
you can regulate power on a wind turbine,
that's called pitch regulation.
Pitch regulated blades deal with the problem
differently.
As the wind speed increases to the point where
power needs to be regulated, instead of allowing
the angle of attack to increase to stall,
a pitch regulated blade will actually twist
the blade to decrease the angle of attack.
This also reduces the lift force and so it
limits the power.
But because it avoids stall the airflow stays
attached to the blade so there are less vibrations,
and the extreme forces are lower for a pitch
regulated blade compared to a stall regulated
one.
But to be able to use pitch regulation you
need to be able to twist the blades relative
to the airflow.
This means you can't rigidly attach the blades
at the hub.
They need to be mounted on bearings to be
able to turn.
And that is the main reason why it took engineers
a while to move to pitch regulation as the
norm.
The pitch bearings operate in a really challenging
environment compared to most applications
for bearings.
They need to be able to transfer really big
forces from extreme winds and they need to
have a long design lifetime.
It's very difficult to replace a pitch bearing,
so they should last the entire life of the
wind turbine which is usually 20 to 30 years.
There are other advantages though, to being
able to twist the blades, aside from pitch
regulation.
You can also twist the blades to start up
in low wind speed.
You can twist the blades out of the wind in
very high wind speeds.
And you can twist it to use it as a brake.
In turbines that had fixed blades, they had
to add tip brakes.
And that meant adding hydraulics through the
length of the blade which was not necessarily
reliable or easy to maintain.
So for large modern turbines the benefits
of being able to twist the blades mean it's
worth the added complexity so these days the
only stall regulated turbines you see are
either really old or sometimes in small turbines
for home or farm usage where simplicity is
really important.
So, there you have it.
Those are the two ways that you can regulate
wind turbine power.
Let me know in the comments if there was a
concept that you found difficult to understand
or maybe there's another concept that you'd
like me to cover and I can make a new video
on that.
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your friends.
I'll see you next time.
