Scuola Superiore Sant'Anna Advances in Airborne Wind Energy and Wind Drones

220 Pages·2017·4.04 MB·English

Scuola Superiore Sant’Anna
Tecip Institute, via Moruzzi 1, Pisa, Italy
PhD Thesis in Emerging Digital Technologies
Advances in Airborne Wind
Energy and Wind Drones
Antonello Cherubini
Tutor:
Prof. Marco Fontana
Reviewers:
Prof. Roland Schmehl
Prof. Philip Bechtle
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Preface and
Acknowledgements
WhenIfirststartedmyPhDprogramatScuolaSuperioreSant’Anna
in Pisa I never imagined the vibrant, educational, constructive and
inspiring environment that I found. I still don’t know what makes
thisplacesogoodforresearch,itmightbethebalancebetweensunny
summers and rainy winters, or the left-wing, dirty and warm city, or
the amount of artists and philosophers outside the laboratory, or
simply a strong tradition of science and engineering. After all, this
is where Galileo was from. In Pisa it is easy to meet new people and
share your thoughts with them, I will always remember some of the
bestconversationsofmylifeduringrandomnightswithpeoplewhose
name I can hardly remember. It seems that good brainstorming,
creative thinking, games and art are rooted deep inside this place.
Scuola Sant’Anna is a tiny and beautiful university in Pisa, born
30yearsago, andnowinthetoptenworldrankingoftheyounguni-
versities. Few people outside Italy know it, but whoever has known
Sant’Anna has good words for it. I am glad and proud to have been
part of it and I hope that this thesis and my publications will be a
small piece of the big puzzle of innovation that mankind needs in a
world where resources are always less and demand is always more.
The list of people that I should thank is fairly long and it goes
more or less like this. First, I wish to thank my supervisor, Prof.
Marco Fontana, the kindest of all bosses and a really really bright
mind. Thank you Marco, you are a master to me. Special thanks to
my colleagues Giacomo and Gastone for their deep daily conversa-
tions in our tasty and lovely canteen and their constructive attitude.
Thanks also to: Prof. Roland Schmehl for trusting me, Pietro Fag-
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giani and Eduardo Terzedis for their pasta-time in the Dutch days,
Prof. Philip Bechtle for making me feel an important guest by let-
ting an engineer teach physics in a physics department, Ing. Fabio
Calamita, Nicola Giulietti and Marco Marzot for their patience in
reading some of my works, Ing. Basilio Lenzo for sharing with me
many dance floors and nights, Marcello Corongiu for being an inspi-
ration, Aldo Cattano for asking me to join his team. Many thanks
the numerous players in the Airborne Wind Energy field who gave
me their feedbacks and comments. Thanks to Mr. Riccardo Renna,
former administrator in the Kitegen team, for his support and his
ability to look deep inside people and a final big thank you to my
parents and my girlfriend for never having asked that silly question
‘When are you going to find a real job?’
Hoping that the reader of this thesis will find this work useful, I
wishtoaddthatthefooterofthisthesiscontainsaflipbookanimation
where the reader can already see a dual drone system taking off!
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Abstract
Among novel technologies for producing electricity from renewable
resources, a new class of wind energy converters has been conceived
underthenameofAirborneWindEnergySystems(AWESs)orWind
Drones (WDs). This new generation of systems employs flying teth-
ered wings or aircraft in order to reach winds blowing at atmosphere
layers that are inaccessible by traditional wind turbines.
The economics of AWESs is very promising for mainly two rea-
sons. First, winds high above ground level are steadier and typ-
ically much more powerful, persistent and globally available than
those closer to the ground, and second, the structure of AWESs is
expected to be orders of magnitude lighter than conventional wind
turbines.Theseplantsarethereforeinterestingfortheirpotentialhigh
power density, i.e. ratio between nominal power and weight of re-
quiredconstructions,thatmakesitpossibletoforecastalowLevelized
Cost of Energy (LCOE) for the produced electricity.
Despite this interesting potential, two important issues might be
a major limitation to AWESs development. First, the large require-
ment in terms of airspace and the related safety issues, and second,
the power dissipation through the aerodynamic cable drag. In a
scenario where AWESs beat conventional wind turbines in terms of
LCOE, the large airspace requirement is a logistic constraint that
might slow down the economic development of AWESs, but the rela-
tively small size/weight of AWESs foundations might be a key factor
that enables the development of inexpensive floating offshore plat-
forms thus solving the airspace limitations thanks to practically un-
limited sea area. A significant part of this thesis investigates the
performance of floating offshore AWESs by means of dynamic mod-
els, first with a single Degree of Freedom (D.o.F.) of the floating
platform, then with a more complex multi D.o.F. model.
The second issue, the aerodynamic cable drag, is a physical con-
straint that is already limiting the potential of AWESs. In short,
in order to reach higher altitudes, current AWESs must increase
the cable length, but the power that would be dissipated by sweep-
ing a longer cable through the air exceeds the power that would be
gainedfromstrongerwindsathigheraltitudes. Thispreventscurrent
AWESs from working at very high altitudes where the jet streams
carry up to 15.5 kW/m2 of wind power density.
A possible solution to this second problem is represented by a
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dual Wind Drone architecture in which two aircrafts, with on-board
generators, are connected to the ground with a ‘Y’ shaped tether-
ing; a concept that was first envisioned in 1976. At that time it was
hard to envision a real operation of this system but recent studies
are starting to investigate this concept in more detail. The last two
chapters contain an attempt to estimate the power output of a large
scale dual Wind Drone system and a proposal for a novel take-off
method that might enable a first implementation of the dual Wind
Drone system. It is first introduced a power model that captures
the most significant real-world issues such as the effect of the weight
of airborne components, the limits of structural/electrical elements,
andconsiderstake-offconstraints. Anumericalcasestudyisanalyzed
considering a large scale system in Saudi Arabia. For such a device,
the power curve is computed and, using real wind data, a nominal
power output of approximately 15 MW with 30% capacity factor is
estimated. Finallyanexperimentalcampaignthatwascarriedoutin
TU Delft is described. In that campaign a take-off system for dual
drones inspired to the so called ‘control line flight’ was investigated.
The passive flight stability of a single wind drone in axi-symmetric
configurationisapositiveexperimentalevidencethatencouragesfur-
ther research in dual drone systems.
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Contents
Introduction 13
1 AirborneWindEnergy: stateoftheartandliterature
review 17
1.1 Availability of Airborne Wind Energy . . . . . . . . . 18
1.2 Classifications of Airborne Wind Energy Systems . . . 19
1.3 Ground-Gen Airborne Wind Energy Systems . . . . . 20
1.3.1 Ground-Gen systems architectures and aircraft 23
1.3.2 Fixed-ground-stationsystemsunderdevelopment 26
1.3.3 Moving-ground-stationsystemsunderdevelop-
ment . . . . . . . . . . . . . . . . . . . . . . . . 34
1.4 Fly-Gen Airborne Wind Energy Systems . . . . . . . . 35
1.4.1 Aircraft in Fly-Gen systems . . . . . . . . . . . 35
1.4.2 Fly-Gen systems under development . . . . . . 38
1.5 Crosswind flight: the key to large scale deployment . . 40
1.5.1 Crosswind GG-AWESs . . . . . . . . . . . . . . 42
1.5.2 Crosswind FG-AWESs . . . . . . . . . . . . . . 44
1.6 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 46
1.6.1 Effect of flying mass . . . . . . . . . . . . . . . 46
1.6.2 Rigid vs Soft Wings . . . . . . . . . . . . . . . 46
1.6.3 Take-off and landing challenge . . . . . . . . . 46
1.6.4 Optimal altitude . . . . . . . . . . . . . . . . . 47
1.6.5 Angle of attack control. . . . . . . . . . . . . . 47
1.6.6 Cables . . . . . . . . . . . . . . . . . . . . . . . 48
1.6.7 Business opportunities . . . . . . . . . . . . . . 50
1.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 50
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CONTENTS
2 Simplifiedmodelofoffshoreairbornewindenergycon-
verters 53
2.1 Offshore AWES . . . . . . . . . . . . . . . . . . . . . . 54
2.2 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
2.2.1 Hydrodynamic model . . . . . . . . . . . . . . 57
2.2.2 Aerodynamic model . . . . . . . . . . . . . . . 59
2.2.3 Integrated model . . . . . . . . . . . . . . . . . 61
2.2.4 Control . . . . . . . . . . . . . . . . . . . . . . 62
2.2.5 Combined wind-wave power output . . . . . . . 63
2.3 Case study . . . . . . . . . . . . . . . . . . . . . . . . 65
2.3.1 Geometry . . . . . . . . . . . . . . . . . . . . . 65
2.3.2 Computation of the hydrodynamic coefficients 67
2.3.3 Results . . . . . . . . . . . . . . . . . . . . . . 67
2.3.4 Small aircraft - Small platform . . . . . . . . . 69
2.3.5 Small aircraft - Medium platform . . . . . . . . 69
2.3.6 Small aircraft - Big platform . . . . . . . . . . 69
2.3.7 Big aircraft . . . . . . . . . . . . . . . . . . . . 69
2.3.8 Discussion . . . . . . . . . . . . . . . . . . . . . 70
2.3.9 Transient behaviour . . . . . . . . . . . . . . . 70
2.3.10 Applicability of the results . . . . . . . . . . . 73
2.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 73
3 Dynamic model of floating offshore airborne wind en-
ergy systems 75
3.1 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
3.1.1 Floating platform dynamic model. . . . . . . . 78
3.1.2 Mooring lines model . . . . . . . . . . . . . . . 81
3.1.3 Kite model . . . . . . . . . . . . . . . . . . . . 83
3.2 Case study . . . . . . . . . . . . . . . . . . . . . . . . 86
3.2.1 Simulator . . . . . . . . . . . . . . . . . . . . . 87
3.2.2 Platform and mooring . . . . . . . . . . . . . . 89
3.2.3 Kite and controller . . . . . . . . . . . . . . . . 91
3.2.4 Simulation results . . . . . . . . . . . . . . . . 93
3.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 97
3.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 102
4 Assessment of high altitude dual wind energy drone
generators 103
4.1 Jet stream altitude wind drone system . . . . . . . . . 104
4.2 Model . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
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CONTENTS
4.2.1 Model hypotheses . . . . . . . . . . . . . . . . 108
4.2.2 Geometrical relations . . . . . . . . . . . . . . 109
4.2.3 Distributed drag on fixed cable . . . . . . . . . 109
4.2.4 Crosswind Fly-Gen flight and drone mass . . . 110
4.2.5 Structural equations of dancing cables . . . . . 113
4.2.6 Electrical equations . . . . . . . . . . . . . . . 113
4.2.7 Fixed cable mass and shape . . . . . . . . . . . 114
4.2.8 Power output . . . . . . . . . . . . . . . . . . . 115
4.3 Power curves of a dual wind drone system . . . . . . . 115
4.3.1 Powercoefficientofthehighaltitudewinddrone
system . . . . . . . . . . . . . . . . . . . . . . . 120
4.4 Future works . . . . . . . . . . . . . . . . . . . . . . . 120
4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 125
5 Automatic ‘control-line flight’ for high altitude wind
energy drones 127
5.1 Dynamic model of take off and landing of a single drone129
5.1.1 Position . . . . . . . . . . . . . . . . . . . . . . 130
5.1.2 Attitude . . . . . . . . . . . . . . . . . . . . . . 131
5.1.3 Kinematics . . . . . . . . . . . . . . . . . . . . 131
5.1.4 Force balance . . . . . . . . . . . . . . . . . . . 133
5.1.5 Relative wind velocity and angles of attack . . 133
5.1.6 Aerodynamic coefficients and forces . . . . . . 134
5.1.7 Pitch motion dynamics . . . . . . . . . . . . . 135
5.1.8 Horizontal steady-state flight . . . . . . . . . . 136
5.1.9 Pitch stability. . . . . . . . . . . . . . . . . . . 136
5.1.10 Altitude stability . . . . . . . . . . . . . . . . . 137
5.1.11 Faster than real time integration . . . . . . . . 137
5.2 Test setup . . . . . . . . . . . . . . . . . . . . . . . . . 137
5.3 Automatic flight results . . . . . . . . . . . . . . . . . 139
5.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . 144
A How to use the open source multi d.o.f floating off-
shore AWE simulator 149
A.1 Step by step guide . . . . . . . . . . . . . . . . . . . . 149
A.2 Hydrodynamic preprocessing . . . . . . . . . . . . . . 151
A.3 Input simulation options . . . . . . . . . . . . . . . . . 151
A.4 Input platform and mooring data . . . . . . . . . . . . 153
A.5 Input kite data . . . . . . . . . . . . . . . . . . . . . . 154
A.6 Simulation results . . . . . . . . . . . . . . . . . . . . 155
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CONTENTS
B Cables for Airborne Wind Energy 159
B.1 Cable sag 2D steady state model . . . . . . . . . . . . 159
B.1.1 Differentialsteadystatemodelatconstantten-
sion . . . . . . . . . . . . . . . . . . . . . . . . 160
B.1.2 Without gravity, constant ρ . . . . . . . . . . . 161
B.1.3 Without gravity, variable ρ . . . . . . . . . . . 162
B.1.4 With gravity, constant ρ . . . . . . . . . . . . . 163
B.1.5 Numerical results . . . . . . . . . . . . . . . . . 163
B.2 Partitioned tether fairing . . . . . . . . . . . . . . . . 165
B.2.1 Concept . . . . . . . . . . . . . . . . . . . . . . 165
B.2.2 Experimental procedure . . . . . . . . . . . . . 167
C Fast solver based on non-linear iterative nested loops171
C.1 Pre-processing wind data analysis. . . . . . . . . . . . 171
C.2 Solver processing . . . . . . . . . . . . . . . . . . . . . 172
C.3 Post-processing and hovering constraint . . . . . . . . 173
D Technical drawings of the wind drone experimental
setup 177
Bibliography 217
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