34C3 preroll music
Christoph Sieg: The idea is now to go from
space back to earth and try to use drones
– so autonomous flying vehicles – for
power generation. So this is the second
part. So the outline here is …
Applause
Christoph: Thank you very much.
Applause
Christoph: So the outline is that first I
will introduce the source here and
motivate why it is a good idea to harvest
high altitude winds and produce energy
from them. The technological part will
come in the second part here. This is
about the technology which is called
airborne wind energy. And in a third part
I want to show how you can build a wind
drone for low cost for yourself and
experiment with this kind of technology.
So let's start with the first part. And
here as a reminder is the conventional
energy supplier wish list, so probably
what your global players in conventional
energy would think about it or tell you:
They would say that is a surely clean-
enough resource and, meaning on timescales
here, it is exploitable of the order of
one human life expectancy, it's
controllable especially economically and
politically, it is depreciable
economically and it leads so to a very
high profit for some players.
Unfortunately there's also the
technological part and here sometimes it's
driven by hope, saying it will be OK. But,
as we know, it might be mostly harmless.
So as we see here for instance there are
catastrophes like Chernobyl. This is after
the catastrophe where you have the
memorial for the people who died. Then you
have scenarios during the catastrophe
here. This is Deepwater Horizon being like
desperately tried to extinguish the fire
by the US Coast Guard and Fire Brigades.
And of course – what I don't have to
mention here, but in times of fake news
it's important to mention – we are before
the catastrophe. So this is here a plot of
the carbon dioxide concentration in the
atmosphere taking from ice. And as you see
here the ice ages give this variations
over 500,000 years, and now we are at this
spot here that points up. And if you
resolve this into the time scale, extent
this time scale from the last thousand to
2,000 years here – so we are here at this
spot at 2000, year 2000 – then you see
that this rise has started at the
industrialization. So it's a clear sign
that we have to do something. And we have
to do it quickly.
Applause
Christoph: So now let's try to propose
something which can be part of the
solution, namely sustainable energies. And
here's a wish list of what probably you
would think it should be: It should be
sustainable, ubiquitous, continous,
accessible and profitable at the very end.
So does such a source exist? And first I
should define what it means. So
sustainable means it should serve present
needs without compromising the future –
and this is clearly not what we are doing
now – so it should be available on
timescales which are like the lifetime of
our central star if possible. It should be
ubiquitous, meaning that it should be
present almost on any location on earth so
that we can without a very complicated
long-range infrastructure have access to
the energy. It should be continuous,
meaning it should be present at almost any
day time and seasons, so that we can plan
of what we produce. And of course it
should be accessible, meaning it can be
tapped by the technology and lead to a
significant contribution to our energy
mix. And profitable should of course also
be. So does it exist? And the answer is
yes and I want to show that this airborne
wind energy can be a big part of it. So
here I have a table of some sustainable
energy sources and the wishlist items are
written here and I put some of the
sustainable sources. So there is fusion,
there is solar energy – terrestrial and
also the spacial energy which was
presented by Anja and by Stefan before –
hydro energy, geothermal energy and
conventional wind energy; where by
conventional wind energy I mean wind
energy up to approximately 100 meter which
is the hub height of wind turbines,
approximately. And as you can see some of
these items here are not fulfilled by all
these different approaches. So for example
the spacial energy is clearly not
ubiquitous, because you have this beam as
we heard which is just like basically
hitting a certain spot on the earth and
there are transferred into energy, so you
have to distribute this energy. Also it is
not yet accessible. On the other hand wind
energy – here conventional wind energy –
is not ubiquitous, because you can only
select certain spots. And it is not
continuous, because you cannot really plan
when the wind is blowing and when it's not
blowing. So let's add to this list what is
called high altitude wind. And high
altitude wind is clearly sustainable,
because it's also wind energy – so it's
like driven as all the other wind energy
as well. And high altitude here means to
go to heights which are above 200 meters
and try to drain energy from these winds.
So let me argue why it is a ubiquitous
source. And for this Philip who is also
here and part of the team – I'm very happy
he has made this very nice plot here which
shows the western part of Europe and it
shows the ratio of wind power which you
can extract at an optimal height which
should be below 1,000 meter – so this is
just an arbitrary at the moment limit – to
say that we can have a system which can
basically get up to thousand meter height
and compare it to the wind energy which is
basically available at hundred meter. And
in this plot you can see at the coastline
there is a line here and this line is the
line where in the interior you have
already a doubling of the wind power. So
meaning at the coast line itself if you go
to higher altitude you have the double
wind power available then at hundred
meter. Even better, directly at the coast
line there is another line which is a
factor of four better. So as soon as you
put your wind turbines on land side, you
will be a factor of, you have access to a
factor of four higher wind power. And
here, in the region slightly south of
Leipzig, there's another line, this is a
factor of eight where you become better in
wind power, in high altitudes wind power.
So, seeing that the coastal regions have
already a factor of four in this ratio
better and the inland between four and
eight. Oh, the wrong sign. Sorry, they
should be reversed of course. So, saying
that here the site of conventional wind-
energy harvesting, which are now very
limited, and where you put for instance
all the wind turbines in the north, they
become much more accessible if you go to
higher heights. Because there you can
basically use all the land sites. So this
is where you have more sites available
when you harvest at optimal height. And
here, as an example about why it is a
continuous source, you see a time
distribution of the wind velocity in
January 2016 in Leipzig. The wind velocity
is here increasing from yellow to red, and
the altitude is displayed here, and this
is the time scale of the month. And what
you can see is, at hundred meter height
you have almost like only in the lower
parts you have winds, whereas, if you go
to higher heights you have the reddish
parts where you have high wind velocities.
So this shows that continuity is already
improved if you go to higher altitude,
especially for land sites. And this is
almost impossible for conventional wind
turbines. You would have to build a mast
higher and much, much bigger structures.
And also, what is displayed here is the
optimal harvesting height. So this is the
height, again below thousand meter, where
it would be optimal to harvest wind at a
certain time, displayed over the whole
month. And if one goes from this plot to
the histograms, so to the time
distribution of the different wind
velocities, you get this picture here. So
this are the spots the histograms of 100,
170, 500, 1000 meter, and of the optimal
height, so if you adjust your height. And
one of the things that you can see is that
the mean is clearly shifted to higher wind
velocities if you increase the height. And
also, if you harvest at optimal altitude
you shift the whole probability
distribution to the right. So and what
increases there is that the fraction of
time below five meter per second, which is
like the the time where the cut in wind
speed for a wind turbine, so you would
like starting produce energy, the
probability to have such winds is
increased from 76% to 87%, which is quite
a lot of increase. So adjusting to varying
optimal harvesting height is not only
almost, but is really impossible for
conventional wind turbines. So one has to
find another technology, which is better
and can give you access to this higher
altitude winds. So this is the plot again
from before. So I have now a little bit
motivated why the source is ubiquitous and
continuous. Now the question is, is it
accessible, and how it is accessible. And
this is the technological part which is
called airborne wind energy. So how do we
access these high altitude winds. So on
for these, let's come back to the design
challenges, which would be necessary to go
to higher height. So high altitude means,
that you just cannot just increase your
tower, and have more torque on your
foundation, and just scale up the system.
So you should avoid proliferation of mass
and proliferation of the tower and
foundation. And also, varying altitude
means you shouldn't have passive,
stabilizing, static structures, but find
something which can vary. So just as an
example here this is the sky walk in the
Grand Canyon, and this is already a quite
scary lever arm which you have. And if, in
comparison, you take your modern wind
turbine, you rotate it by 90 degrees, and
compared it in size to this, you can see
what kind of torque will be, like will act
on the foundation. So this is already a
very big piece of technology you have
here. So we have to do better, and this is
the second part, namely airborne wind
energy, so the technology itself. So the
first slide is probably the most important
of this part because it explains the
idea behind this technology. So you take
autonomous drones, which are the most
flexible connected to the ground via
tethers, and extract wind energy via these
drones. So how does it work? So look at
this conventional wind turbine here. You
have most of the energy is produced by the
outer part of the wings. They are rotating
with the highest velocity, and at the same
time you have the highest the largest
lever arm. So you produce most of the
energy in the outer part. The inner part
is more or less passive, stabilizing
structure. So you remove that structure
and replace it by something which is
flexible, and the first which comes to
mind probably is a tether with which you
attach it to the ground. And then you have
just the active part here, which is now an
aircraft, moving in this circle, which
before was circulated by the wing tips, to
extract your energy. This is the
principle. So how do we bring down the
power when circulating this aircraft? So
we have to, in some way, transform it to
electric power. So there are, which are
not shown in the picture before, lighter
than air systems. So you just basically
take a balloon, you put your wind turbine
at high altitude, and extract the power.
And here the tether can clearly serve as
the power line. But what we can also do is
crosswind flight, which was shown in the
picture before. So here you have a moving
aircraft, which can move in something
which is called the drag mode, meaning
that you have onboard generators on the
aircraft. So essentially it's a propeller
aircraft, but the propellers are reversed
in repeller mode, so that the repellers
produce energy for you. And then the
tether serves as power line. So this
principle is shown here. So here you can
see the generators and then the power is
brought down by the tether. In the second
part, second strategy, is using the so
called lift mode So here you have ground
based generators and the tether itself
transmits the power, there are no power
lines in the tether. So here you use that
the power is given by the pulling force
times the reel out velocity of the tether.
So you circulate in some patterns with
your aircraft and you use the lift force
acting on the aircraft to unreal this
tether from a drum, and at the drum, on
the base station, there's a generator
attached which helps you to get the
energy, to transform the energy into
electrical energy. And of course, at some
point the tether is maximally reeled out
and then you have to have to go to a reel
in phase, where with minimal energy you
reel in the tether again, and start
periodically this phase again. So these
are the concepts, and there's a whole zoo
of airborne wind energy devices and
proposals, which show that this technology
is still in a very early stage of being
developed. So you have people here flying
figure-of-eight patterns with the
aircraft. So some things are lighter than
air turbines, which look very exotic like
this one, probably this one you have seen
in media already. Proposals like this
here. There are quad copters, which
produce the energy by rotating of their,
of the propellers here. And all kind of
exotic lever arm and aircrafts which you
can use. So let's bring a little bit of
more order into the technology, into the
proposals. And one of the things I want to
discuss, which is very promising, is what
is called crosswind flight. So here as a
example is a comparison of a conventional,
lighter-than-air system with the big wheel
in London. So this is one of the biggest
wind turbines. And the harvesting area is,
so the effective area of such a wind
turbine is the swept area of your
propellers, essentially. So now let's look
what happens if you move an aircraft
instead through the wind. Then the picture
of before is like of that size. And if you
take an aircraft, which has the same wing
area as the wing areas of the propeller
here, you're harvesting area is of that
size. It's much bigger. And the reason for
this is, that the effective area is now
given by the wing area times a
coefficient, which is the square fraction
of the lift to drag coefficient of the
aircraft times the lift coefficient
itself. And this factors of the order of
200. So it increases the efficiency of
your of your wings dramatically. This was
already found by Loyd in 1980. And you can
now ask "Why does it take 30 years from
this idea to first systems?". And the
answers is, in this community for is
probably a very interesting is "Why are
these prototypes are appearing only 30
years later?". It's because sufficient
computer power. So for the control
algorithms, which allow you to control
such flight modes, was not available. So,
as an example, here's an illustration of
one of the current leaders in the field
called AMPYX POWER, showing a crosswind
airborne wind energy system versus a
conventional system. So here's the
conventional wind turbine for two
megawatts. And the conventional this is a
conventional system. And the airborne wind
system is, this is the ground station, and
this is the aircraft. So one of the things
which are, I mean, visible in this picture
is that it has much less like even sight
impact in the environment. So having
something like this is much less
disturbing from the even from the
aesthetic point of view, than this huge
wind turbine. So now the next step would
be to look closer to the technology and
see what are the AWE system components
that you need, that you need to build such
a device. So first of all, there is the
drone or the fixed-wing aircraft. We have
seen that it's very good to have large
lift and small drag coefficients, so you
need something which is like a rigid
glider, more or less. On board you need
sensors, like accelerometer, gyroscope,
GPS, receiver, barometer, and a pitot tube
to measure the air the air speed. And this
is to determine the system state, that
then is like reacted on by the control
surfaces, in the case of an aircraft by
ailerons flaps and the rudder. Moreover,
you need of course a microcontroller and
algorithms which do the state estimation.
So from the sensor data they compute the
state of the system, meaning it's
position, altitude, velocity. And you have
to navigate. So and of course you might
need something like a propeller for
takeoff, landing, and energy generation in
case of drag mode. The second thing is of
course the ground station. So here you
need the drum for tether wind-up. You need
a motor which eventually has to be
transformed into generator mode if you
have the lift mode. You need power
converters, also microcontrollers and
algorithms which synchronize your ground-
station operation with the drone; and you
need a runway, catapult or something alike
for takeoff and landing. So far it looks
quite simple, but the devil is in the
detail. And here I found a nice quote a
colleague of mine – (uninteligble name) –
has done in one of his talks,
and I liked it very much because it
displays very well what challenges have to
be still overcome. So it starts with
"Theory is when nothing works but everyone
knows why." and to demonstrate this let's
have a look at this video here which is
one of the flight attempts of one of the
companies: So the aircraft lifts off,
there's no sound … yet. Now there is
sound.
Background music of shown video
Speaker in shown video:
Abort! Abort! Abort!
Soft laughter
Christoph: Yeah. And the desperation of
the founder was clearly hearable at the
end. And you could see that the tether
ruptured. And then there was no way to
recover that most of the aircraft was
lost. Second: "Sometimes practice is when
everything works but no one knows why." So
there are also positive surprises. And
here is a launch, a catapult launch, for
an aircraft which now uses weight.
Background noise of shown video
Laughter
Christoph: So a positive surprise for a
test. And finally, sometimes if you
combine theory and practice then "nothing
works but no one knows why". This is where
the complication really is: The devil is
in the detail. And here you can see a
video from a flight which is crosswind
flight: Everything seems normal …
Dramatic background music of shown video
Christoph: … and then the prototype is
again lost. So this is complicated. So but
there is a lot of progress and so I want
to come closely, very quickly introduce
the current industrial status. So I focus
on three companies which work on that: So
one of them is Enerkite in Berlin, and
they have now a system which is basically
stationed on such a truck and this is a
crosswind system of a passive wing. So it
steered via three tethers and it produces
up to 30 kilowatts of energy. Then you
have Ampyx Power. They have here the
launching site in the Netherlands and they
are currently producing this aircraft
here. This type which is a crosswind
system in lift mode. And at the end will
produce up to 250 kilowatts of power. This
is under construction. And, finally, there
is Google X Makani in California. And they
have built a drag-mode aircraft – here –
which is flying. And I can show you a
video that they have on their home page –
very nicely. Where they show a flight so
that you can see that the 600 kilowatt
system is working. Here you see the
onboard propellers. You can see the
tether. Down here this is from the tether
attachment point. So the things are
working. There are prototypes. But one of
the things which are important is: one has
to "test, test, test" and get experience
from tests. So "experience is what you get
when you were expecting something else".
You really. So what does it mean? So we
have to test, analyze, adapt the systems.
So because many – as you could see from
this design variations in the zoo which
I've shown – many of the concepts are
still open. So for example the design of
the airframe. If you use a biplane, a
flying wing or anything alike – or
something totally different – is still
open. The tether construction – what kind
of materials to use – is still open. The
materials in itself is still open for the
aircraft etc. etc.. The mode of operation
– that means takeoff, landing and direct
versus lift mode – is still an open
question. What is the best thing to
realize for industrial products? And then
control hardware and software algorithms
have to be tested thoroughly. Of course
it'd have to be certified by the aerospace
agencies, of course. You want to have a
failsafe. So what you have to do is you
want to even, I mean, have total losses in
experiment. You want to do the experiments
wich would lead to a total loss of your
system. So here comes the idea that
instead you should build a cheap and
disposable test platform instead of a
largely scaled-up system first, before you
build the expensive prototype and do tests
on them. And this brought us to the idea
to provide a low-cost open-source test
platform where everybody at home can build
his own wind drone. And this is the third
part of the talk. So the do-it-yourself
wind drone. What are the ingredients here?
So first you need a drone, so here I want
to show the airframe and reinforcement
hack which is necessary to prepare your
airframe for the additional forces by
adding the tether. Then there's a ground
station and here I want to motivate why
the drone is essentially behaving like a
fish – in this case a barracuda. The next
thing is navigation on curved manifold is
very important because you have like a
constraint coming from the tether. And
finally you need something for control
which is the autopilot. So in this case
it's the ardupilot open-source project
which we adapted. So let's come to the
airframe-reinforcement-hack. So what you
use: Take your favourite polystyrene
airframe – so in this case it's an Easy
Star II – and glue the wings together.
This is the lower side of the wings. You
put in there a carbon rod – here in this
part – and you stabilize it with racks
which you glue into the slits we can see
here. And then you wrap carbon in the
forward part of it where the most of the
aerodynamic force is attached. Then you
have the carbon ???? ???? wind around your
tether. And you install additional tubes
for fixing the wings on the fuselage. So
the fuselage is here. We cut off the
engine blocks, included additional carbon
rods. So you can put these carbon rods on
these carbon rods here, and fix everything
with screws. So to show you how that looks
like and what are the size of this model
is: So here is the original-size aircraft
with carbon. And you can later – if you
want – pass by the assembly area and look
at it and have a look at it and touch it.
So this is how it looks decomposed into
different components: So again wings and
so on and so on, the servos for the
control surfaces. The central unit here is
the Pixhawk autopilot. So there's a
microcontroller which contains some of the
sensors: You have a GPS sensor, in
addition you have a telemetry antenna for
data – for data transfer to the ground
station. And you have RC control for
manual control when you switch out of auto
mode to have manual control in emergency
situations – or if you want to make other
kind of flight tests. So now this is the
drone itself. So now is the question what
to do with the ground station. And here
let's look why the drone behaves as a
fish: Because what it does is, like in
fishing, you would need a free-moving
tether; it has to be fast and fail-safe
reeled in and reeled out; and it should
remain twist free so that it doesn't give
any knots if it is not under tension. And
the thing which we came up with wich best
serves for our needs at the moment is an
off-shore fishing reel.
Applause
Christoph: So and you need offshore here
because the drum has to be perpendicular
to the rod: This guarantees you like
reload phases twist free on the
tether. Other fishing rods have the drum
aligned with the rod, and then you
accumulate twist on the tether which can
lead to knots, lead to knots and then …
it's not a good idea. It will destroy your
tether. So and this is the first flight
test. So we were very enthusiastic and
started the first flight test. And here it
is.
Indistinct voice in shown video
Voice in shown video: OK. … Hinterher?
Laughter
Visv: Versuch mal rauszugehen. Manual?
Achtung, Achtung! Versuch ihn zu fangen.
Na gut.
Beeping in shown video
Christoph: OK.
Laughter
Christoph: So unfortunately it did not
work.
Applause
Christoph: So what happens? This was the
result: The tail was broken. And because
the tether apparently wrapped around the
back of the aircraft and then it became
uncontrollable. So we came up with what do
we do: If you don't know any further, any
better, use carbon! So we put some carbon
on the lower part of the of the fuselage
to reinforce it. And then of course you
have to think about writing your
navigation code, to navigate if you are
under tethered flight. So here is the
receipe for how to do it: So first you
take one git clone of ardupilot – this
autumn open-source software. You
take one curved 2-dimensional manifold –
it's essentially giving us a hypersurface
embedded in 3-dimensional Euclidean space.
In case of constant tether length this is
just a semi-hemisphere as to which is
centered around your ground station. Then
you take a planar curve which you want to
fly along – or curved segments – and a
pinch of Differential Geometry to wrap it
on the sphere, to make this curve
appearing on the sphere. You take a little
bit of Classical Mechanics for the flight
control to transfer the curve
accelerations into actually control-
surface motions. And then you need, of
course, 12 dozen coffee for doing so. You
put everything together into – of course
not the coffee – into the computer algebra
system and stir well, and let the CPU bake
it at 100 degrees, and then you come up
with a smooth – at least C¹ – curve.
Applause
Christoph: So the curve is shown here. So
it's … this is one part of a figure-8
pattern. So the other part would be behind
here. It's composed of two geodesic
segments and one turning segment and they
are C¹ glued together here. And these are
the equations: So you can find in the
paper – I don't want to go into detail. So
now you have to modify the source code of
this ardupilot project. So here there are
highlighted the patterns which you
basically have to … where you have to do
modifications: You have to implement new
flight modes and change some of the
control algorithms. And then you come up
with the next flight test. And here is the
next attempt.
Music and propellor sounds
Voice in shown video: Beim Auswerten
müssen wir sehen, ob wir dann verschiedene
wählen.
Music ends
Applause
Christoph: The whistling sound you have
heard at the end is the tether being
dragged through the air. So there was
really tension on the tether. And you can
also see this if you do a data analysis on
the flight data later. So yes for example
multiple possibilities. You have a lot of
data which is possible to analyze. So the
autopilot this was very very it's very
very nicely done in this open-source
project: So they have a data file with all
primary and secondary data you can use for
your analysis. So for instance this is the
flight curve of different flight modes
which we used. You have the altitude of
the aircraft, you can look to deviations
in radial and transverse directions. You
can look to tether tension – or like a
measure for tether tension – by looking to
the length variation of the tether. And
you can of course do time series analysis
of how your figure-8 pattern has flown
along. And that is what you can do with
this very very nice autopilot open-source
software which is available when … written
by many many people on the internet. So
the question which remains is: After all
of this is, will it be a fail-safe to
100%? And the answer is nope, it will not!
It will … there will be of course
accidents happen. But the thing is:
Nothing is failsafe. And so here's a
standard wind turbine and look for
yourself.
Laughter
Christoph: You see there is no 100%
guarantee, but we have to try very hard to
get it as failsave as possible. So yeah
this is essentially it. That was the talk.
So what I want to say is that the current
status of airborne wind energy can be seen
here by a nice book on the Springer page
which you can download here. And we are
very very happy to have any kind of
critical remarks, input to help in
developing the system further. So please
if you want, look to this web page,
there's a lot of information including a
paper and we will be very happy for any
kind of help. And finally I would again
stress that we could rely on this
tremendous work of the open-source
community working on this autopilot
project that has helped us to realize this
project in very short time; so very happy
about this. And I want to thank of course
Phillip Bechtle, who is here, and Thomas
Gehrmann and Maximillian Schulz-Herberg,
the students, and Udo Zillmann, who can
not be here, for working on this project
and putting so much work also into it.
Thank you very much for your attention!
Applause
H: We can have two more on the microphones
here and here – one and five – so two
questions. The first one, please!
Question: So you talk, so you talked a lot
about powered – and not powered –, but
controlled flight. How does it compare –
energy wise – to uncontrolled flight?
Basically putting a propellor on a kite?
Answer: So the thing is the propellor on
the kite … with kite you mean, I guess,
non-rigid structures. So meaning that the
first question is how do you want to put a
propeller on a kite if it's non rigid. So
that is a question which goes back to you.
So because that is something is not clear
to me. But in any case rigid air
frame is harder to control than a
kite. So there are people who work with a
kite. And by kite surfing or if you do
like steer normal kites from the ground.
You know it's like moving not that fast in
the wind field, so it's easier to control.
This is a big benefit of kites. And also
the weight is a big benefit. But the power
output – because of the bad or worse lift-
to-drag coefficient – is unfortunately not
that efficient as a rigid aircraft. So you
want to go to the rigid air craft.
H: If you leave the room now, please be
quiet because we have questions and
answers here! Number three please, and
that is the last question I'm afraid. But
you can ask questions after the talk.
Q: I want to go back to the space part. I
was wondering … there are some ideas about
bootstrapping like a solar station on the
moon and then like shipping, I don't know,
hydrogen or like pre-charged lithium
batteries back to earth and back and
forth. Is it like realistic or not really?
A by Anja Kohfeldt (previous talk): I
think also this approach would be quite
expensive. And you have to install this
infrastructure on the moon first, and you
have to establish the flight base back and
forward. Realistic is a thing, you know.
At the end that's a question of money and
investment. And I'm not sure whether this
would pay out, but we haven't analyzed
this kind of approaches, yet.
H: Thank you! So thank you very very much
Stefan, Anja and Christoph! Give them a
warm applause again please!
Applause
Stefan: Thank you!
Outro music
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