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
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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
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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
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You just need to go to brilliant.org/EngineeringwithRosie,
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thanks to you for watching. I'll see you in the next video.