*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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