Rosemary Barnes: Welcome to engineering with
Rosie. This is the second part of my vertical axis wind turbine series. And in this video, I'm going
to show you how to analyze the aerodynamics of a vertical axis wind turbine. And talk about some of
the design options to get around the challenges faced by the vertical axis wind turbines that I
discussed in the previous video on this topic. So if you didn't look at that one yet, then you can
check it out here. The basic idea of a vertical axis turbine is the same as a horizontal axis one
the wind exerts a force on the blades, which turns a shaft connected to a generator, which generates
electricity. So I have talked before on this channel a lot about horizontal axis turbines and
how that design evolution has kind of narrowed into a pretty standard design. But that's not the
case for vertical axis wind turbines, there is still a huge range of variety, but they can be
split into two broad categories, the drag type and the lift type, also known as Savonius, and
Darrieus type. So if you saw the video that I made on the design evolution of horizontal axis wind
turbines, you might remember that the old fashioned windmills use drag to push the sails
around. And modern wind turbines use aerodynamic blades that create a lift force from the wind,
which is a much more efficient way to capture energy from the wind. There are similar
aerodynamics at play in vertical axis turbines. So for a given size of turbine, you should get a lot
more power out of a lift type turbine than a drag type one. For vertical axis turbines where
efficiency matters, Like if you're trying to generate as much electricity as possible, you
would choose a lift type turbine. To analyze these we use the same aerodynamic theory as airplanes or
horizontal axis turbines, it's just the orientation is different. So like I said before, there are really a lot of
different configurations of vertical axis turbines. Sometimes the lift ones that lift type
machines are all lumped together and called Darrieus turbines. But the machine that Darrieus
designed and patented in 1926 was of the egg beater type with curved blades. You can also have
straight blades arranged so they look like the letter H. These are known as h-type turbines. Or
you can take the H shape and kind of twist the blades into a helical shape. So it looks a bit
like a piece of DNA. And this will help reduce some of the problems with this type of turbine
that I'll talk about later. And people have made vertical axes turbines in pretty much every kind
of shape you can think about: V configuration, delta configuration, Diamond configuration,
giromill configuration, you get the picture. Alright, so there are heaps of different kinds of
vertical axis turbines. But I'm going to choose the H type arrangement to talk about the
aerodynamics and operating principle, just because I think that's the simplest to analyze. And it's
also pretty common amongst the recent current commercial turbines. But you can apply the same method to the other
types as well, it's just a bit more involved, because you can't just make a single calculation
for the whole blade length, you would need to split the blade into sections and analyze each one
separately, and then sum them up pretty much like we did in the BEM method for horizontal axis wind
turbine analysis. So I'm just going to do a really simplified
analysis here, I just want to show you the operating principles in enough detail to get the
feeling for how it works and what some of the problems are, I will recommend a few books and
papers that you can read to get a rigorous and correct analysis. And if you really want to see a
video, including all of the aerodynamic theory and the equations, then let me know in the comments,
and I'll try to make that happen, but wanting they'll be a lot of maths in that video. So we'll start by looking at a single blade. The
basic aerodynamic principle, it's the same for an aeroplane or a horizontal axis turbine, the blade
is an aerofoil, and when air flows over it, it creates a lift force perpendicular to the wind
direction, a component of this force is in the right direction to create a torque that will
rotate the shaft. So far, so good. And we have a simpler situation than in a horizontal axis
turbine because the entire length of the blade sees the same local airflow speed and the same
angle of attack. So this means a blade can be straight more like an aeroplane wing, and it
doesn't need to be twisted and tapered like a horizontal axis wind turbine blade does. But it is a bit more complex than that because the
blade does rotate around at shaft axis. So the local airflow is, is affected by the rotational
speed as well as the wind and the wind speed. And there is also some inflow to be taken into
account, but I'm just ignoring that for today's really simplified analysis. So at different points in its path, the blade is
oriented differently relative to the wind, you can add the wind speed and the rotational velocity
vectors to find the local airflow speed and angle of attack and then use trigonometry to resolve the
lift and drag forces into into normal and tangential components. So let's look at what happens as the rotor turns.
We'll start with a blade on the downwind side of the rotor so it's perpendicular to the wind
direction - it's crossing the wind. We add the wind and blade velocity vectors and use
trigonometry to find the resultant wind speed and angle of attack. And then you can resolve that to
find the force acting in the tangential direction, which turns the rotor. As the blade turns further
around, the angle of attack gets smaller, meaning the lift force and torque gets smaller too. At 90
degrees, when the blade is parallel to the wind, there's no angle of attack. And assuming we have a
symmetrical airfoil, then there's no lift only a small drag force which acts opposite to the
direction of the rotation, so we have a small negative torque here. Then as we rotate further,
the angle of attack continues to decrease. So it's negative, making our lift force in the opposite
direction relative to the blade. But since the blade is facing the other way, now we still have a
component of that force creating a positive torque that will turn the rotor. Depending on the tip speed ratio, which is the
ratio of the blade speed to the wind speed, you may or may not see the angle of attack go beyond
the stall angle. And if that does happen, if the angle of attack exceeds the stall angle, then the
flow separates, creating a violently swelling flow. In that case, the lift force suddenly
decreases and the drag force suddenly increases and this swirling turbulent flow causes a lot of
vibrations in the blade. These constantly changing forces are the most
challenging part of vertical axis turbine blade design, as they can cause several problems. The first is that it's very hard on the blade
structure and the other turbine components to survive these loads. When loads change frequently,
it's called fatigue loading. And just like the name suggests, but fatigue loading can wear
components out quickly. Even if the forces themselves are small. It's the same effect as when
you bend a paperclip repeatedly back and forth. Even small bends will eventually break the
paperclip if you repeat them enough. A second consequence of the different forces as a
blade moves around its path is that the torque that the generator sees there is constantly This
is called torque ripple and it causes problems for a lot of turbine components. It causes vibrations
and noise and shortens the lifetime of the machine. And as well as causing problems for the
generator, the fact that you have the most efficient angle of attack in just one location per
rotation means that the rest of the time the blade is generating less torque than the optimal so you
have compromised efficiency. So that's what happens if you have the most simple
design of a straight blade that's fixed at a certain angle, you have a very simple design, but
two major problems dynamic stall and severe fatigue loading that goes with that and you also
have reduced efficiency and torque ripple because the blade is only oriented optimally for a small
portion of the rotation. But you can change the design slightly to reduce
these problems. The dynamic stall behavior of vertical axis turbines has been the cause of all
lot of failures for vertical axis turbines over the years. Early vertical axis turbine blades are
mostly made of aluminium and their fatigue lives were perhaps not very well understood at the time,
which led to a lot of early failures on vertical axis blades, which kind of caused a widespread
assumption that this was just an inherent problem with vertical axis wind turbines. But modern
composite materials like fiberglass and carbon fiber and analysis tools like CFD and FEA,
together with the experience of early failures have helped to reduce these issues a lot. So another design option to mitigate issues with
fatigue loading is to make the blades in the egg beta shape of the Darrieus turbine. This
arrangement means the blades are loaded nearly exclusively in tension which suits composite
materials very well, because they perform much better in tension than compression. So a composite
structure loaded only in tension can be really light. So far, we only looked at a single but adding more
blades will reduce the variability of the output as not all of the blades will stall at the same
time. But you'd need a pretty large number of blades to totally eliminate the top variation and
then that would get expensive. You can achieve a similar effect by making the
blades helical. So there is a section of the blade at every location along the circumference. So the
torque that that kind of a turbine will produce should be a lot more constant than one with
straight blades. But of course, the manufacturing complexity for these like double helix shaped wind
turbines is a lot higher than for a simpler straight bladed design. Another way to reduce top variation is to control
the blade pitch. So instead of rigidly fixing the blades in place, you would allow them to pitch to
keep the local angle of attack close to the most efficient one. This approach means that the torque
is a lot more constant and it can also increase the efficiency a lot compared to a fixed blade.
And there's no need for the blades to ever stall so you should decrease the fatigue loading the
blades are subjected to, potentially making the structural design simpler. But of course, to get
this extra efficiency and simpler structural design, we need to add complexity to achieve it in
the form of fast responding, active control for example. In my opinion, this active blade pitch control is
the most promising way to improve vertical axis wind turbine design. It's probably too complex to
be cost effective for small, small turbines. But for large ones, I think that if any vertical axis
turbine is going to become widespread, it's going to be one with this kind of technology. My take on
vertical axis turbines is that they can be simpler, but in their simpler configuration, they
are less efficient. And so usually designers end up trying to increase efficiency. And in doing
that they increase complexity. So we end up with designs that are not necessarily any less complex
than horizontal axis machines. So I don't think that they're going to replace
utility scale horizontal axis turbines, but they could fill niches that horizontal axis turbines
don't suit. So this kind of niche might be distributed generation, places where noise is an
issue and maybe off grid installations in places that have turbulent wind conditions. But what do you think? Do you like vertical axis
turbines by any specific locations? Do you think that there are advantages or disadvantages that I
didn't mention here? And don't forget to tell me if there's a specific type of vertical axis
turbine that you'd like me to look into in a future video. You can find lots of other wind
turbine design videos on my channel, plus a lot of other clean energy technologies. So don't forget
to subscribe and I'll see you in the next video.
Love it. Thanks for sharing :)