welcome to engineering with rosie
vertical access wind turbines i have had
quite a few requests for a video on this
topic people wanting to know about their
aerodynamics so they can design
one and people wanting to know whether
they are more efficient than horizontal
axis turbines
so here is that video i'm going to talk
about how they differ to horizontal axis
wind turbines and
advantages and disadvantages i'm going
to be following up with a video on
vertical axis wind turbine aerodynamic
analysis and design
and i will put a link here when i
release that
so i want to start by saying that
vertical axis wind turbines are pretty
much a totally separate field from
normal horizontal axis turbines i
don't actually know anyone who works
with both horizontal and vertical axis
turbines in fact
i know probably hundreds of engineers
who work on horizontal axis turbines
and only one who works on vertical axis
and i fall into the first category
professionally i've only ever worked on
horizontal axis turbines
and vertical axis has only been a
personal interest for me
so that's the reason why it's taken me
over six months from the first time
someone suggested this video until i
actually
was able to make it i had to learn about
it myself
so vertical access turbines are
appealing for really different
applications than horizontal ones
if you want to put a wind turbine on
your roof or in your yard for example
a vertical axis turbine is likely to be
a better choice than a horizontal axis
turbine
so for this reason a lot of diy and
off-grid enthusiasts are interested in
vertical axis turbines
and that's actually where i started
looking into it myself i wanted to make
a backyard turbine just for the
engineering challenge
now as i've gotten to know more about
vertical axis turbines i've learned
that they can be very simple or they can
be efficient
but they can't really be both and by the
end of this video
um you'll understand why i think that
and you can tell me if you agree or not
okay so let's define what is a vertical
axis wind turbine
pretty much all of the large turbines
you see around these days are horizontal
axis
that means that the center of the rotor
and the shaft it rotates is horizontal
parallel with the direction of the wind
so vertical axis wind turbines flip that
with the rotating shaft
oriented vertically perpendicular to the
direction of the wind
and actually it's this last point that
defines vawts that that axis of rotation
is perpendicular to the wind
so a design that looks like a hamster
wheel where the shaft is horizontal but
perpendicular to the wind would
normally actually be grouped with a
vertical axis turbines
so there are a lot of different types of
vertical axis wind turbines but they all
fit into two broad categories either
drag type or
lift type also known as cevonias and
darius type to give a five second
explanation of the difference between
the two
dragon types of onions are very simple
and very inefficient
and lift tight vertical axis turbines
are more complex and more efficient
so what about in comparison to
horizontal axis turbines
there are a few access of vertical axis
turbines that can be simpler than
horizontal axis designs
a lot of the complicated components that
are located like hundreds of meters up
in the air for a horizontal axis turbine
can be located at the ground level for a
vertical axis one
so the generator and the gearbox for
example they have to be located at the
end of the shaft and for
a horizontal axis wind turbine that's at
the top of the tower
but for vertical axis wind turbines um
the end of the shaft is on the ground so
you can put these complex components
there
which makes maintenance much easier and
cheaper
so next the aerodynamics of a vertical
axis turbine can result in a simpler
blade geometry
i've talked a lot on this channel about
horizontal axis wind turbine design and
especially
about the aerodynamic design of their
blades
it's complicated because the blades are
rotating around the horizontal axis
so the local air flow speed is different
along the length of the blade
and that's why they need to be tapered
and twisted to account for this
so that's perhaps the most obvious
reason to think of a vertical axis
instead
the blade aerodynamic design can be much
simpler
another benefit of the vertical access
is that the blade tip speeds
are usually lower than on horizontal
axis when turbine blades where the local
airflow over the blade is faster at the
blade
tip than the root due to the rotational
speed so for horizontal axis turbines if
you keep the rotational speed the same
but increase the blade length
then the tip speed increases and this is
a challenge for wind turbine designers
because the turbine noise is strongly
related to the blade tip speed
so vertical axis wind turbines can be a
lot quieter than horizontal axis
turbines
another design aspect of bawts that can
be simpler than hawts
is that it doesn't necessarily matter
what direction the wind comes from
because the blades are rotating around a
vertical axis whether the wind comes
from the front or the back or the side
it doesn't matter and because of this
you don't need to yaw them to face the
wind like you do with a horizontal axis
turbine
so you can get rid of that whole your
system and this
omnidirectional nature also means that
they tend to perform better in turbulent
or gusty winds than horizontal axis
turbines do
so these are very common wind conditions
in urban environments or
low to the ground so you may have
noticed that horizontal axis
turbines are getting taller and offshore
turbines are becoming more common
both of these trends are a way to reach
higher quality wind resources
vertical excess turbines aren't as badly
affected by turbulence or gusts
and together with the fact that they're
often less noisy than similar horizontal
axis turbines
it can make them a good alternative for
distributed generation devices in
in urban environments but
you need to bear in mind that even if
you can improve the efficiency of
turbines in these kinds of wind
resources
you're only capturing a greater share of
much
smaller potential their maximum energy
capture from
wind depends on the wind speed cubed so
that means if you double the wind speed
there's potentially eight times
more energy available so it's much
easier to get more energy by moving to a
higher quality wind site than it is by
trying to make a more efficient turbine
in a low quality site
so you would need to have reasons other
than efficiency to want to put wind
turbines in
urban locations where the winds where
the wind resource isn't as good
and so what about efficiency are
vertical axis wind turbines
as efficient as horizontal axis turbines
or maybe even
more efficient because they can capture
energy on the upward side of the rotor
and again on the downwind side so for
cervonius type machines the answer is
simple
because they are drag based um and they
have this
issue of the returning blades drag
fighting the advancing blades
they're less efficient they're only able
to reach about half the bets limit which
is the maximum theoretical power that a
horizontal axis turbine can extract
i have a whole video about the bets
limit and how much energy is in the wind
so you can check that out here
if you want to know more about that
theory but
darius type turbines are lift-based and
their orientation means that in addition
to capturing some energy from the upwind
blades you
do also capture some from the downwind
blade it's like a kind of double dipping
of course the upwind blades have already
extracted some of the energy from the
wind so by the time it reaches the
downwind side it's slower with less
energy
um in a typical actually existing
vertical axis wind turbine and by
actually existing i mean
not a sketch on the back of an envelope
or a computer simulation that someone
made a youtube video about
in these real vertical axis turbines
about 80 of the turbines power is
generated by the upwind blades and 20
by the downwind blades and so far
vertical axis wind turbines are
all a lot less efficient than a standard
horizontal axis one
i haven't seen any commercial vertical
axis wind turbines promising
over 41 efficiency and actually most of
them
are in the 30s you compare that to most
modern
horizontal axis utility scale turbines
and they're in the high 40s
but are they necessarily less efficient
or have we just not seen a great
bawt design yet is the maximum
efficiency of a vertical axis wind
turbine
limited by the bets limit the same as a
horizontal axis one
the answer that is no the theory doesn't
apply and there isn't an equivalent
theory for vertical axis turbines at
least as far as i could find
okay so maybe they can be more efficient
but how might we actually get some of
this increased efficiency
you would need to improve on today's
designs which are all less efficient
than
horizontal axis wind turbines so to do
this you'd probably want to increase the
rotor diameter so you could get as much
separation between
the upwind and the downwind blades and
hopefully have better wind quality on
the downwind side
but making the diameter larger will
cause lots of practical problems which
you'll end up adding complexity to solve
there's several other ways that you can
increase the efficiency of vertical axis
wind turbines and i will be going
through those
in the in the next video but in all
cases to get the extra efficiency you
need to add complexity
so i would want to see a functioning and
reliable prototype of a new vertical
axis design
that claims to beat horizontal axis
efficiencies
before i was willing to believe that it
was going to shake up the industry too
much
so if you want more information on wind
turbine aerodynamics then have a look at
the playlist i made on that topic
and um i'll link to that in the
description and if you're interested in
other engineering issues related to wind
energy and other green energy topics
then please subscribe and check out the
other videos on my channel and i'll see
you in the next one