Rooftop solar is one of the big
success stories of the energy transition. My home country
Australia is leading the way here, one in four houses now has
solar panels on its roof. When you add up all the rooftop solar
in Australia, it is a larger contributor to the grid than any
single generator or power station. This has happened
partly because of early government support and partly
because of successful efforts to reduce permitting time and
installation costs. And of course, it helps that Australia
is simply a very sunny place. Look at the last year of
electricity grid across the eastern electricity grid in
Australia. There's plenty of solar power in every single
week, even in the middle of winter. So we have a kind of
easy in Australia, but what about in Northern Europe, like
Germany or Denmark where I used to live, having lived there for
five years, I can tell you that there was a lot more opportunity
for windsurfing than sunbathing there. It's generally a dark and
windy place compared to Australia. But if you travel
through northern Germany and into Denmark, although the
highways are lined with utility scale wind farms, if you look at
the houses, they have solar panels, not wind turbines on
them. So why has an urban wind taken off like urban solar has?
In this video, we're going to talk about some of the reasons
why rooftop wind turbines or urban wind in general is a way
harder challenge than rooftop solar, even in windy, dark
places like Denmark. And yes, I will also talk about
circumstances where small scale wind does make sense. And
include a few urban wind technologies that you can use if
you decide that small scale wind is for you. And I know from the
comments that that is a lot of you. So first up, I want to start
with the quality of renewable energy resources that's
available on roofs. The quality of a solar resource, or how much
sun there is, is measured in solar irradiation, which on this
map is the annual average in kilowatt hours per square meter.
To get a good resource, you just need to find a sunny place and
then tilt your solar panel so it faces the sun directly. If
you're looking to put a solar panel on your roof, you'll need
to avoid shading from nearby trees, buildings or hills and
choose a face of your roof that's close to the optimum tilt
angle. And if you can achieve that, you'll end up with a solar
resource that is pretty close to what a nearby solar farm would
achieve. And if you're in a location with a bad solar
resource, say Kolding, Denmark where I used to live, that has
about 930 kilowatt hours per square meter, that is a little
less than one half of the power available in a good location,
say Canberra where I live now, which has 2100 kilowatt hours
per square meter. And so you'll get a little less than half of
the power from a solar panel in Kolding than you would from the
same solar panel in Canberra. Now let's look at wind. The
quality of a wind resource is usually assessed by looking at
average annual wind speed. Close to Kolding, Denmark, the mean
annual wind speed is about nine and a half meters per second at
150 meters above the ground, and 4.3 meters per second at 10
meters above the ground. So in this last sentence, there were
two clues as to why we don't see much urban wind. The first clue
was where I said the wind speed "close to Kolding" and not the
wind speed "in Kolding." And that's because winds in urban
areas are slower than in surrounding areas. Tall
buildings block the wind and slow it and make it more
turbulent. So wind speeds in urban areas are commonly like
about 30 to 40% lower than those in the nearby countryside. The
second clue was the fact that the wind speed was significantly
higher at 150 meters above the ground compared to 10 meters
above the ground. And this is because of wind shear. As I
mentioned before wind close to the ground gets slowed by
buildings and also by trees and hills and stuff. As you move up
and away from these obstacles. There's nothing to slow the wind
down so it's faster. That is one of the main reasons why turbines
and wind farms are getting taller and taller towers. A wind
farm wind turbine can have a tower height of 150 meters and
reach those 9.5 meters per second winds whereas an urban
wind turbine say on top of a roof or in a backyard would be
closer to the 10 meter height, which says 5.2 meters per second
in open terrain, and probably something like 4.3 meters per
second in a built up area. So the short urban wind turbine
will get wind speeds that are only about 46% of the tall wind
farm wind turbine even when they're really close by each
other. And it gets worse because the energy in 4.3 meters per
second wind is not even 46% of the energy in 9.5 meters per
second wind. In fact it's less than 10% as much. That's because
the power available in wind is related to the cube of the wind
speed. There's 525 watts in a one meter swept area with 9.5
meters per second wind compared to 50 watts in a one meters per
squared area with 4.3 meters per second wind. So even with a wind
turbine of the exact same size, you'll get 10 times less power
from the urban wind turbine compared to the utility scale
one. Let's complete our comparison by heading back to
Canberra where the wind speed data gives us 5.6 meters per
second at 150 meters and 3.5 meters per second at 10 meters.
If we put a wind turbine that sweeps a one square meter area
on top of a Canberra roof, it would say 3.5 meters per second
average wind speed and generate 25.6 watts of power at that wind
speed, the Kolding site has an average wind speed of 4.3 meters
per second, and the same turbine on a Kolding roof will generate
50 watts of power at that wind speed. And if you could put
those towers slightly out of town on 150 meters towers, a one
square meter turbine would see 5.6 meters per second in
Canberra, and 9.5 meters per second in Kolding and generate
110 watts in Canberra, and 525 watts in Kolding. So those two
factors, the reduction in wind speed close to the ground and in
urban areas, those two factors combined with the fact that wind
power varies with a cube of wind speed make up most of the reason
why urban winds does not make as much sense as urban solar. But
there are a few other reasons that I'll go into quickly. Now,
let's stay on the wind and solar resource maps and take a closer
look at the Canberra region, the solar irradiation varies from
about 1800 kilowatt hours per square meter to 2100 kilowatt
hours per square meter. It's basically the same whether I'm
on my roof or in the biggest solar farm near the city, the
Royalla solar farm, whereas the average wind speed varies from
about 2 meters per second to 10 meters per second. That's about
two meters per second in the calmest area and around 3.5
meters per second in the parts where most Canberrans live, to
over nine meters per second at 150 meters in the highest wind
speed areas, which is basically right where I am now. Surprise,
surprise, that's where they installed a wind farm. And I
will say that when I left town, I would have said it was
basically a windless day, but out here it's pretty windy.
Because the wind speed varies a lot and the power depends on the
cube of the wind speed, there is a lot more to be gained by
picking really good spots to put your wind turbines compared to
solar panels which can go basically anywhere that you've
got space for them as long as they're not shaded. The next
reason I want to look at is the nature of a wind turbine versus
a solar panel. Wind turbines rotate, which means moving parts
a certain amount of noise and vibrations and the wind forces
that turn the blades around necessarily cause a large force
that the base of the tower has to support. Industrial wind
turbines have service technicians doing maintenance
and repairs on at least an annual basis. And they have
large concrete foundations to resist the thrust loads that are
trying to push the turbine over. If you want to put a turbine on
top of your roof, then you'll need to make sure that the roof
structure can withstand it and all the noise and vibrations
will travel into the supporting structure, which sometimes can
be a big nuisance. Solar panels, on the other hand are pretty
light, they're silent, their vibration-less and they're close
to maintenance-free over their lifetime apart from possibly
needing to clean dust or snow off on occasion. Okay, so that's
a bunch of reasons why urban wind doesn't make sense. But I
did promise at the start that I would include examples of where
it does make sense. So the first and most obviously beneficial
situation where you're definitely going to want to
install small wind turbine is off-grid homes or micro-grids
that need to generate all their power on their own. This adds
value or diversity of resources to help spread out the
generation between day and night and summer and winter, you can
get by with a lot less energy storage or backup diesel
generation if you include wind energy, even in a location whose
wind resource is not that great. The next place you'd likely want
to use smaller urban wind is if you're located in a site that
actually does have high wind speeds. So rural properties,
like on the coast of Jutland are one example of this. There are
small wind turbines everywhere over there on small farms and
rural properties. And I spoke to a couple of farmers who have
them and they were thrilled with the performance and money it
saved them compared to buying on the grid. Another place where
urban wind might make sense is architectural buildings with
wind turbines whose purpose is to inspire or look cool or any
other reason that isn't to actually generate meaningful
amounts of reasonably priced electricity. I mean, these
buildings are just cool. And I have no doubt that they could
have gotten the same amount of renewable energy much cheaper by
building a normal large wind turbine somewhere else. But then
they could also house the same number of hotel rooms or offices
more cheaply just by building a lot of concrete rectangles.
There is a place in the world for things that are just cool
and for things that inspire people. Finally, if you enjoy
DIY or backyard engineering as a hobby and want to learn about
wind energy, then an urban wind turbine is a great project for
you. I have personally learned so much from small wind energy
projects that definitely didn't make any economic sense for the
amount of energy generated. And I highly recommend these
projects to anyone with an interest in wind energy or
engineering in general. So if you fall into one of these
categories, then I would like to refer you to this book by Hugh
Piggott. And if you're buying a small wind turbine, I would
recommend getting it from the list of certified wind turbines
whose performance has been validated. A big thank you to
the engineering with Rosie Patreon team who supported this
video like all others. If you would like to
join us, then there is a link in the description. You can join us
on Patreon and on the discord server to chat about all things
renewable energy, we would love to welcome you. I work a lot
with wind energy and less with solar power. So for this video,
I turned to Brilliant, who are the sponsor of this video. And I
did their solar course to brush up on the relevant topics,
Brilliant's a website and app with over 60 interactive courses
in math, science and related topics like engineering. I like
Brilliant because it lets you learn interactively. It's not
about memorizing, it's about doing. And that's what real
world STEM and especially engineering is all about. Some
of the topics and the solar course are things that I know
well from my job as a clean energy expert. Some of it was a
reminder of stuff I studied at school, and some of it was brand
new. Brilliant has something for all knowledge levels. If you've
never studied STEM, or if it's been a really long time, then
you can go back to basics. And even if you're an expert in
closely related fields, there's still something to learn. You
can get started for free and for Engineering with Rosie viewers,
Brilliant's offering 20% off an annual subscription for the
first 200 viewers to sign up. You just need to go to
brilliant.org/EngineeringwithRosie, I'll put a link in the
description. Thanks Brilliant for coming on board as a new
sponsor of the channel and thanks to you for watching. I'll
see you in the next video.
