Control of Tethered Airfoils for High–Altitude Wind Energy

246 Pages·2010·9.55 MB·English

POLITECNICO DI TORINO
DOCTORATE SCHOOL
Course in Information and System Engineering – XXI Cycle
A dissertation submitted for the degree of
Doctor of Philosophy
Control of Tethered Airfoils for
High–Altitude Wind Energy Generation
Advanced control methods as key technologies for a breakthrough in
renewable energy generation
LORENZO FAGIANO
Advisors PhDcourseCo–ordinator
ing.MassimoCanale
prof. MarioMilanese prof. PietroLaface
ComplexSystemModelingandControlGroup
Headoftheresearchgroup
prof. MarioMilanese
2009
II
AMaria
III
IV
Note from the author
I would like to point out here that the research activities that I’ve carried out during my
Ph.D. studies have nothing to share with the company named “KiteGen Research s.r.l.”.
The name “KiteGen” has been coined at Politecnico di Torino, well before the founda-
tion of KiteGen Research s.r.l., and it has been the name of the first research project,
funded by Regione Piemonte and coordinated by Politecnico di Torino, aimed to investi-
gatehigh-altitudewindenergyusingpowerkites. ThisisthereasonwhyIreferredtothis
technologyas“KiteGen”inmyPh.D.thesis. KiteGenResearchs.r.l. gavenocontribution
to my research activities and to the related publications. In order to avoid confusion, I’ve
decided to modify my thesis and to refer to the technology with the acronym “HAWE”
(HighAltitudeWindEnergy).
October19th,2010
LorenzoFagiano
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Abstract
This thesis is concerned with the development of an innovative technology of high–
altitude wind energy generation and with the investigation of the related advanced au-
tomaticcontroltechniques. Indeed,theproblemsposedbytheactualenergysituationare
among the most urgent challenges that have to be faced today, on a global scale. One of
thekeypointstoreducetheworlddependanceonfossilfuelsandtheemissionsofgreen-
house gases is the use of a suitable combination of alternative and green energy sources.
Renewable energies like hydropower, biomass, wind, solar and geothermal could meet
the whole global energy needs, with minor environmental impact in terms of pollution
and global warming. However, they are not economically competitive without incen-
tives, mainly due to the high costs of the related technologies, their discontinuous and
nonuniform availability and the low generated power density per unit area. Focusing the
attention on wind energy, recent studies showed that there is enough potential in the to-
tal world wind power to sustain the global needs. Nevertheless, such energy can not be
harvested by the actual technology, based on wind towers, which has nearly reached its
economical and technological limits. The first part of this dissertation is aimed at eval-
uating the potential of an innovative high–altitude wind energy technology to overcome
some of these limitations. In particular, a class of generators denoted as HAWE (High
Altitude Wind Energy) is considered, which exploits the aerodynamical forces generated
by the flight of tethered airfoils to produce electric energy. Numerical simulations, the-
oretical studies, control optimization, prototype experiments and wind data analyses are
employed to show that the HAWE technology, capturing the energy of wind at higher
elevation than the actual wind towers, has the potential of generating renewable energy
available in large quantities almost everywhere, with a cost even lower than that of fossil
energy.
Though the idea of exploiting tethered airfoils to generate energy is not new, it is practi-
cable today thanks to recent advancements in several science and engineering fields like
materials, aerodynamics, mechatronics and control theory. In particular, the latter is of
paramount importance in HAWE technology, since the system to be controlled is non-
linear, open loop unstable, subject to operational constraints and with relatively fast dy-
namics. NonlinearModelPredictiveControltechniquesofferapowerfultooltodealwith
thisproblems,sincetheyallowtostabilizeandcontrolnonlinearsystemswhileexplicitly
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taking into account state and input constraints. However, an efficient implementation is
needed, since the computation of the control input, which requires the real–time solution
ofaconstrainedoptimizationproblem,cannotbeperformedattheemployed“fast”sam-
pling rate. This issue motivates the research efforts devoted in the last decade to devise
more efficient implementations of predictive controllers. Among the possible solutions
proposed in the literature, in this thesis Set Membership theory is employed to derive
off–lineacomputationallyefficientapproximatedcontrollaw,tobeimplementedon–line
insteadofsolvingtheoptimization. Thesecondpartofthisthesisinvestigatesthemethod-
ological aspects of such a control strategy. Theoretical results regarding guaranteed ap-
proximation accuracy, closed loop stability and performance and constraint satisfaction
are obtained. Moreover, optimal and suboptimal approximation techniques are derived,
allowingtoachieveatradeoffbetweencomputationalefficiency,approximationaccuracy
and memory requirements. The effectiveness of the developed techniques is tested, be-
sidestheHAWEapplication,onseveralnumericalandpracticalexamples.
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Acknowledgements
The studies and research activities underlying this dissertation have been funded in part
byMinisterodell’Istruzione,dell’UniversitàedellaRicercaundertheProjects“Advanced
control and identification techniques for innovative applications” and “Control of ad-
vanced systems of transmission, suspension, steering and braking for the management of
thevehicledynamics”andbyRegionePiemonteundertheProjects“Controllodiaquiloni
dipotenzaperlagenerazioneeolicadienergia”and“Powerkitesfornavalpropulsion”.
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Contents
Abstract
VII
Acknowledgements
IX
I High–altitude wind energy generation using controlled airfoils 1
1 Introduction 3
1.1 Globalenergysituation . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1.1 Actualglobalenergysituation . . . . . . . . . . . . . . . . . . . 5
1.1.2 Globalenergyoutlookto2030 . . . . . . . . . . . . . . . . . . . 10
1.2 Windenergytechnology: stateoftheartandinnovativeconcepts . . . . . 16
1.2.1 Actualwindenergytechnology . . . . . . . . . . . . . . . . . . 16
1.2.2 Conceptsofhigh–altitudewindpower . . . . . . . . . . . . . . . 20
1.3 Contributionsofthisdissertation . . . . . . . . . . . . . . . . . . . . . . 21
2 HAWE:High–AltitudeWindEnergygenerationusingtetheredairfoils 25
2.1 Basicconcepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
2.1.1 Theairfoil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
2.1.2 Thecables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.1.3 TheKiteSteeringUnit . . . . . . . . . . . . . . . . . . . . . . . 27
2.2 TheroleofcontrolandoptimizationinHAWE . . . . . . . . . . . . . . 29
2.3 HAWEconfigurationsandoperatingcycles . . . . . . . . . . . . . . . . 29
2.3.1 HE–yoyoconfiguration . . . . . . . . . . . . . . . . . . . . . . . 29
2.3.2 HE–carouselconfiguration . . . . . . . . . . . . . . . . . . . . . 32
2.4 NavalapplicationofHAWE . . . . . . . . . . . . . . . . . . . . . . . . 34
3 ControlofHAWE 37
3.1 HAWEmodels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
3.1.1 Gravityforces . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
3.1.2 Apparentforces . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
X