Multi-Objective Optimization of Power Electronics and Generators of Airborne Wind Turbines

303 Pages·2017·14.55 MB·English

Diss. ETH No. 24559
Multi-Objective Optimization of
Power Electronics and Generators
of Airborne Wind Turbines
A thesis submitted to attain the degree of
DOCTOR OF SCIENCES of ETH ZURICH
(Dr. sc. ETH Zurich)
presented by
CHRISTOPH GAMMETER
MSc ETH
born on 30.01.1985
citizen of Zu¨rich, Switzerland
accepted on the recommendation of
Prof. Dr. Johann W. Kolar, examiner
Prof. Dr. Hans-Peter Nee, co-examiner
2017
ETH Zurich
Power Electronic Systems Laboratory
Physikstrasse 3 | ETL I16
8092 Zurich | Switzerland
http://www.pes.ee.ethz.ch
© 2017 by Christoph Gammeter
Fu¨r meine Frau

Acknowledgments
First and foremost, I would like to thank Prof. Johann W. Kolar
for giving me the opportunity to be part of the excellent work-
ing environment he has set up at the Power Electronic Systems (PES)
Laboratory of ETH Zurich. I thank Prof. Dr. Hans-Peter Nee for his
interestinmythesis. Isincerelyappreciatethathefoundtimetotravel
to Zurich to be the co-examiner at my PhD defense.
A great thanks goes to Gabriel Ortiz, who took me under his wing
andtaughtmeagreatdealbefore,during,andaftermymasterthesisat
PES. I want to thank Florian Krismer for supervising me for the first
part of my PhD. I’m grateful for the wealth of knowledge he always
found time to share. I also thank Arda Tu¨ysu¨z for teaching me an
abundance of things about electrical machines, his support with the
FEM simulations and for always keeping up the good mood.
A great thanks also goes to Jonas Huber, who accompanied me on
my journey at PES from the very beginning until the very end and
always kept my spirit up. I’ve had the pleasure to share an office with
Michael Leibl, Lukas Schrittwieser, David Boillat, and Jon Azurza An-
derson. Thank you Michael for all you’ve taught me about inductor,
transformer,andfilterdesign(andallthechocolate). ThankyouLukas
for sharing all your expertise in micro-controller, FPGA and SoC pro-
gramming, and your insight on industrial relevance. Thank you all for
thefruitfuldiscussionsandthegoodtimesbesideswork. ThankyouRo-
manBosshardfororganizing/helpingmeorganizethePESski-weekend
yearafteryearandensuringthatthegoodcustomsandsocialactivities
were being kept alive at the institute.
Each interaction with every single current and past PES member
hascontributedtomakingmystayatPESunforgettable. Theexcellent
spirit of work at PES is a result of the great collaboration and mutual
support among its team members: Daniel Rothmund, Dominik Bortis,
Dominik Neumayr, Mattia Guacci, Mario Mauerer, Matthias Kasper,
Maurus Kaufmann, Michael Flankl, Pedro Bezerra, Hirofumi Uemura,
Ralph Burkart, Thomas Guillod, Toke Andersen, Patricio Cortes, Ju-
lianB¨ohler,JannikSch¨afer,MichalisAntivachis,PapamanolisPantelei-
mon, Spasoje Miric, Tobias Wellerdieck, Marcel Schuck, Patricio Per-
alta, Thomas Holenstein, Pascal Pu¨ntener, Oliver Knecht, and Yanick
Lobsiger. I would like to thank all of them for all the good times and
interesting technical discussions we had. I enjoyed the time we spent
together at PES or at the various events that we organized outside of
work. I also want to thank Ivan Subotic for his help during the last
part of my PhD.
Ihadthechancetobethethesissupervisor/co-supervisorforplenty
of students. I would like to thank all of them for the good work they
did. Among them, I would like to acknowledge the support of Yan-
nick Drapela, Clemens Stadlinger, Raphael Bernhard and Franziska
Bosshard, whose works contributed directly to this thesis. And a spe-
cial thanks goes to Yannick for sharing the knowledge he gained from
his time at the AMZ and consequently teaching me about the design
an construction of electric machines.
I would like to thank Peter Albrecht, Peter Seitz, Monica Kohn-
Mu¨ller, Prisca Maurantonio, Roswitha Coccia, Damaris Egger, Yvonne
Schnyder-Liebherr and Beat Seiler for maintaining the clockwork orga-
nization of PES. A big thank you goes to Claudia Stucki and Markus
Berger for providing us with high quality IT services.
Thanks to Martin Vogt, Daniel Wegmann and Stefan Brassel from
the D-ITET Workshop for their excellent high precision work and al-
ways lending a helping hand.
I huge thanks to my brother Stephan for all his support, and espe-
cially for taking up the hobby of CNC-machining. Without the access
to his Tormach PCNC the timely construction of the electric machines
would not have been possible.
Finally, I would like to thank my family for everything that I am
and everything that I have. And a special thanks to my wife for all
her love and support, even while I’m writing this PhD thesis on our
honeymoon.
Croatia, July 2017
Christoph Gammeter
Abstract
Increasingconsumptionofelectricenergy,environmentalissues,and
limited availability of fossil fuels have led to a multitude of devel-
opments related to the generation of electricity from renewable energy
sources. One innovative system in this context is the Airborne Wind
Turbine (AWT), which generates electricity from high altitude winds.
Highaltitudewindsareknowntobemorestableandfasterthanwinds
closetoground-leveland,thus,enableamorereliableandeffectivegen-
erationofelectricenergy. AWTsrepresentaradicallynewandfascinat-
ing concept for future harnessing of wind power. This concept consists
of realizing only the blades of a conventional wind turbine (CWT) in
theformofapowerkiteflyingathighspeedperpendiculartothewind.
Furthermore, the AWT is essentially a flying wing with a significantly
lowerconstructioneffortofthepowergenerationsystemmakingiteco-
nomically favorable to CWTs. The AWT considered in this thesis is a
flying wing with air powered on-board power generators and is tied to
thegroundwithanapproximately1kmlongtether. Becauseofthehigh
flightspeedofthepowerkite,severaltimestheactualwindspeed,only
a very small swept area of the on-board turbines is required according
to Betz’s Law, which yields turbines of low weight for the generation
of a given electric power. The tether, a fiber and cable combination,
provides both the required mechanical strength and the electrical link
to the ground station, which is connected to the medium voltage (MV)
grid. For takeoff and landing of the power kite, the turbines act as
propellers and the generators as motors, i.e. electric power is supplied
so that the system can be maneuvered like a helicopter. The great-
est challenge with respect to the realization of the electrical system of
the AWT is to achieve light-weight generators, power converters, and
a light-weight cable. In the present work the configuration of power
electronics converters for the implementation of a 100kW AWT is con-
sidered. Themajoraspecthereisthetrade-offbetweenpower-to-weight
ratio (kW/kg) and efficiency.
The multi-objective optimization of the electrical machine required
for the AWT system is detailed first. Presented investigations include
the analysis of the power-to-weight ratio versus efficiency limits, γ-η
ParetoFronts,ofradialandaxialfluxmachinetopologiesemployingan-
alytical models. Analytical models, describing the electromagnetic and
thermalbehavioraresummarized. Asimpleoptimizationisperformed,
since the use of analytical models, instead of finite element methods,
vii
Abstract
allowstheevaluationofthewholedesignspaceinacomputationallyef-
ficientmanner. Theresultsrevealthatthebestperformanceinthiscase
is achieved with a RFM Halbach inrunner reaching a power-to-weight
ratio of, γ ≈ 6.3kW/kg, at an efficiency of η ≈ 95%. Experimental
results validate the proposed design procedure.
Modelingofpowerelectronicsformulti-objectiveoptimizationisde-
tailedwithafocusonanalyticalsemiconductorlosscalculationsandthe
optimizationofforcedconvectioncoolingsystems,composedoffanand
parallel plate fin heat sinks. The presented investigations detail the
optimization of the heat sink’s fins with respect to minimum weight
and the selection of a suitable fan for minimum overall system weight.
A new analytical cooling system model is introduced. The calculated
results are compared to the results determined with a pre-existing an-
alytical model and Finite Element Method (FEM) simulations. The
comparison to experimental results demonstrate the accuracy improve-
ments achieved with the proposed methods. It is shown that compared
to commercially available products a weight reduction of ≈ 50% is
achieved.
Avoltagesourceinverter(VSI)isdesignedemployingthepreviously
detailed models. A VSI prototype is built, which achieves a power-to-
weight ratio of 18.9kW/kg and shows that the weight contribution of
the VSI to the total weight of the AWT is small.
The fundamentals of the tether design are explained to outline the
importance of a MV power transmission between the airborne wing of
theAWTandthegroundstationinterfacetothemains. Thismotivates
the design, the implementation, and the experimental verification of a
minimum weight input series output parallel (ISOP) structured Dual
Active Bridge (DAB) converter for the AWT system. The main power
componentsoftheDABconverter,inparticularthebridgecircuits,the
actively cooled high frequency (HF) transformer and inductor, and the
cooling system, which largely contribute to the total system weight,
are designed and realized based on multi-objective considerations, i.e.
with respect to weight and efficiency. Furthermore, the design includes
all necessary considerations to realize a fully functional prototype, i.e.
it also considers the auxiliary supply, the control for stable operation
of the system, which also comprises an input filter, over the specified
operating range, and the startup and shutdown procedure. These con-
siderations show the complex interactions of the various system parts
andrevealthatacomprehensiveconceptualizationisnecessarytoarrive
viii
Abstract
atareliableminimumweightdesign. Experimentalresultsvalidatethe
proposed design procedure. The prototype features a power-to-weight
ratio of 4.28kW/kg (1.94kW/lb) and achieves a maximum full-load ef-
ficiency of 97.5%.
In conclusion, the essential results of the work are summarized. To
enablethereadertomakesimplifiedcalculationsandacomparisonofa
CWTwithanAWT,theaerodynamicfundamentalsofbothsystemsare
summarized in highly simplified form in an Appendix, and numerical
values are given for the 100kW system discussed in this work.
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