Numerical Modelling of Wave Energy Converters. State-of-the-Art Techniques for Single Devices and Arrays

287 Pages·2016·21.02 MB·English

by  Matt Folley

NUMERICAL MODELLING OF
WAVE ENERGY CONVERTERS
NUMERICAL
MODELLING OF
WAVE ENERGY
CONVERTERS
State-of-the-Art Techniques for
Single Devices and Arrays
Editedby
M F
ATT OLLEY
SchoolofPlanning,ArchitectureandCivilEngineering,
Queen’sUniversityBelfast,Belfast,NorthernIreland
AMSTERDAM (cid:129) BOSTON (cid:129) HEIDELBERG (cid:129) LONDON
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Contributors
M. Alves WavEC - Offshore Renewables, Lisbon, L. Kregting School of Planning, Architecture and
Portugal Civil Engineering, Queen’s University Belfast,
D.Causon SchoolofComputing,Mathematicsand Belfast,NorthernIreland
DigitalTechnology,ManchesterMetropolitanUni- C.Mingham SchoolofComputing,Mathematicsand
versity,Manchester,England Digital Technology, Manchester Metropolitan Uni-
B. Child Wave and Tidal Energy Advisory, DNV versity,Manchester,England
GL,Bristol,UnitedKingdom L. Qian School of Computing, Mathematics and
J. Davidson Centre for Ocean Energy Research, DigitalTechnology,ManchesterMetropolitanUni-
versity,Manchester,England
MaynoothUniversity,Maynooth,Ireland
B. Elsa¨ßer School of Planning, Architecture and P.Ricci OrwellOffshore,London,UnitedKingdom
Civil Engineering, Queen’s University Belfast, (Formerly at Global Maritime Consultancy Ltd.,
Tecnalia Research and Innovation and Instituto
Belfast,NorthernIreland
SuperiorTe´cnico,Lisbon)
C.B.Ferreira DNVGL:Energy,RenewablesCertifi-
J.V. Ringwood Centrefor OceanEnergy Research,
cation(WaveandTidal),London,UnitedKingdom
MaynoothUniversity,Maynooth,Ireland
C.J. Fitzgerald University of Oxford, Oxfordshire,
UnitedKingdom V. Stratigaki Department of Civil Engineering,
GhentUniversity,Ghent,Belgium
M. Folley School of Planning, Architecture and
P. Troch Department of Civil Engineering, Ghent
Civil Engineering, Queen’s University Belfast,
University,Ghent,Belgium
Belfast,NorthernIreland
D. Forehand School of Engineering, University of S. Vaughan School of Engineering, The University
Edinburgh,Edinburgh,UnitedKingdom of Edinburgh, Edinburgh, Scotland; DNV GL:
Energy, Renewables Certification (Wave and
S. Giorgi Centre for Ocean Energy Research,
Tidal),London,UnitedKingdom
MaynoothUniversity,Maynooth,Ireland
J.P. Kofoed Department of Civil Engineering,
AalborgUniversity,Aalborg,Denmark
ix
C H A P T E R
1
Introduction
M. Folley
School of Planning, Architectureand Civil Engineering, Queen’sUniversity Belfast,Belfast, Northern
Ireland
1.1 THE CHALLENGE OF WAVE with acknowledgement of the collaborative
ENERGY effort that hasmadethis bookpossible.
The potential for extracting and using the
energy in ocean waves has been recognised for 1.2 A SHORT HISTORY OF THE
atleast200years,withthefirstpatentforawave NUMERICAL MODELLING OF WECs
energyconverter(WEC)beingsubmittedbyMon-
sieur Girard and his son in 1799. In this patent, It was not until after the first oil crisis in
energywasextractedfromthewavesbyresisting 1974 that serious scientific attempts were made
the heaving motion of a ship using a lever tonumericallymodeltheresponseofWECsand
mountedonthedockside.Undoubtedly,thisidea estimatetheirpotentialpowercapture.Although
would have worked if it had ever thefirstarticleonthepotentialforwaveenergyis
been constructed, but Monsieur Girard and his generally attributed to Salter (1974), the funda-
sonwouldhavelackedthenumericaltoolstoesti- mentaltheoryforWECswasfirstproducedinde-
matethepowergenerationwithanyaccuracy.Up pendentlybyEvans(1976),Mei(1976)andBudal
to the 1970s, designs for WECs continued to be (1977).Thistheorywastheneffectivelyusedover
proposed, and some prototypes were even con- thenextfiveyearstodevelopnumericalmodelsof
structed at the beginning of the 20th century. WECsandWECarraysinthefrequencydomain
However,ingeneralthesedesignscouldbecon- (Chapters2and8)andtimedomain(Chapters3
sidered as fruits of intuition and empirical and 8), as well as semianalytical methods for
research,unsupportedbyanynumericalanalysis. modellingarraysofWECs(Chapter9).
This chapter first provides a short history Duringthenext15–20years,upto1997,numer-
of the numerical modelling of WECs before icalmodelsofWECsandWECarrayscontinued
looking at the current challenges and future to be developed, but without any significant
developmentsinthefield.Thechapterthendis- developmentinthetypesofmodellingtechniques
cusseswhythebookhasbeenwrittenandhowit used. Towards the end of this period sufficient
should be used. Finally, the chapter finishes computing power became available that the
NumericalModellingofWaveEnergyConverters 1 Copyright#2016ElsevierInc.Allrightsreserved.
http://dx.doi.org/10.1016/B978-0-12-803210-7.00001-3
2
1. INTRODUCTION
hydrodynamic coefficients for arbitrary shapes Most recently two additional modelling tech-
couldbedeveloped,thefirstexampleofthisbeing niquesforWECshavebeendeveloped.Thefirst
themodelofanoscillatingwatercolumn(OWC) of these techniques, spectral-domain modelling
byLeeetal.(1996).However,thisonlyincreased (Chapter 4), was first implemented by Folley
thescopeandaccuracyofthepossiblenumerical andWhittaker(2010),whilstthesecondofthese
models rather than representing a fundamental techniques, model identification (Chapter 7),
development. Up to this time all of the models wasfirstimplementedbyDavidson(2013).Both
ofWECsandWECarrayshadbeenbasedonlin- of these modelling techniques are focused on
earpotentialflowtheory;thenin1997amodelofa achievingacomputationallymoreefficientWEC
WECbasedonfullynonlinearpotentialflowthe- model,ratherthanincreasingthemodelfidelity.
(cid:1)
ory(Chapter5)waspublishedbyClement(1997),
whichcoincidentallywasalsoamodelofanOWC.
The next significant advancement in the 1.3 CURRENT CHALLENGES AND
modelling of WECs came in 2004 with the use FUTURE DEVELOPMENTS
of a computational fluid model (CFD) of a
WEC(Chapter6)thatsolvedtheincompressible The current key challenge in the numerical
Euler equations for the flow around an OWC modelling of WECs (one that it is hoped this
(Minghametal.,2004).However,althoughother bookwillgosomewayinmeeting)istoidentify
CFD models were also developed around that which, from the wide range of modelling tech-
time (Alves and Sarmento, 2005), it was not niques available, is most appropriate for a par-
reallyuntil2016,withtheincreaseincomputing ticular WEC concept and modelling objective.
power, that the production of CFD models of One reason for this wide range of potential
WECs became more common-place. It would modelling techniques is that there is a wide
seem that this has been enabled significantly rangeofWECconceptswithverydifferentsizes
by the availability of the open source software and operating principles, with each concept
OpenFOAM (www.openfoam.com), which making potentially different demands on the
allows developers to share their code and modelling technique. A second reason is that
advancements,anadvantagethatwasnotprevi- themodellingofWECsisarelativelynewfield,
ously available, as each developer worked on compared to many other fields such as naval
their ownparticularsoftware tool. architecture,andsothereisnocanonofmodel-
2007saw,totheauthor’sknowledge,thefirst ling techniques that have been publically
implementations of WECs in wave propagation acknowledgedasacceptablebythewaveenergy
models that would allow the far-field effect of community.Consequently,anyonenewtowave
WECs and WEC arrays to be determined. energy is likely to find it difficult to determine
Millar et al. (2007) produced the first example how best to numerically model their
of a phase-averaging wave propagation model particular WEC.
to include WECs (Chapter 11), whilst Intimately linked tothechallengeofidentify-
Venugopal and Smith (2007) produced the first inganappropriatemodellingtechniqueismodel
example of a phase-resolving model to include validation.Fundamentally,withoutvalidationof
WECs(Chapter10).TherepresentationofWECs a modelitisdifficultto fully assess its accuracy
in a phase-resolving model was subsequently and true suitability. Unfortunately, at this point
improved by Beels and Troch (2009) to enable onlyafewWECshavebeendeployedatfullscale
themodellingofarrayinteractionsin2009,whilst and no, or very limited, data is publically avail-
Silverthorne and Folley (2011) did the same for ablefromthesedeploymentsthatcouldbeused
phase-averagingmodelsin2011. to provide validation of a numerical model. An
3
1.4 WHYTHISBOOK
alternative potential source of validation data is aretobeexpected.Thekeychallengewiththese
wave-tank testing, withthe clear understanding hybridmodelsistoconfiguretheinputsandout-
that scaling issues mean that some differences puts from each of the models so that there is a
may exist between this data and what would seamless transfer of information in both direc-
be expected at full scale. However, even wave- tions. For example, bidirectional coupling could
tankdatasuitableforthevalidationofnumerical beusedtomodelaWECarray,withaCFDmodel
modelsisrelativelyrare,withonlyasmallnum- that defines the local flow around each WEC,
ber of cases being published. Moreover, the whilsta(non)linearpotentialflowmodelisused
author is unaware of any comparative analysis topropagatethewavesbetweentheWECs.This
of the validation of numerical models to deter- hybrid model may be expected to accurately
mine their relative fidelity for a particular WEC model any flow separation around each WEC,
andwavecondition.Itisclearthatalackofrig- whilstminimisingtheissueswithnumericaldif-
orous and critical validation of the numerical fusion that can occur with the propagation of
models limits our current ability to fully assess waves in a CFD model. Moreover, it would be
the potential and relative merits of the different expectedthatthemodelwouldbecomputation-
numerical modelling techniques available. This ally less demanding. Of course, the concept of
makes it a clear and present challenge for the hybridmodelsisnotnovel;however,theauthor
numericalmodellingofWECs. isunawareoftheiruseinthemodellingofWECs
Considering future developments in the (althoughbythetimeyouarereadingthisbook,
numericalmodellingofWECs,itisobviousthat hopefullythiswillnolongerbethecase).
the seemingly unrelenting increase in comput-
ing power will have an impact. The particular
areas where this is likely to be significant is in
thedevelopmentofCFDmodelsandtheassess- 1.4 WHY THIS BOOK
ment of the mean annual energy production
(MAEP), both of which are generally limited to Currently, other than attending a specialist
some extent by the computational resources course on the modelling of WECs, the only
available to the modeller. However, whilst optionavailabletosomeonenewtowaveenergy
greater computing power may be expected to is to work through the current literature in the
provide an incremental advancement in the field. However, this wave energy literature is
numerical modelling of WECs, it is not consid- extremelydispersed,beingcontainedinthepro-
ered likely toresult in anykind of step change. ceedings of specialist conferences such as the
Itwouldseemthatwhenevercomputingpower highlyrecommendedEuropeanWaveandTidal
increases, the expectations of the numerical Energy Conference (EWTEC) series and in aca-
modelsalsoincrease,whilstmanyoftheunder- demic journals such as Applied Ocean Research
lying issues with the individual numerical and Ocean Engineering. This complicates the
modelling techniques will remainthe same. work of studying the numerical modelling of
Anexcitingfuturedevelopmentinthenumer- WECsasthesepaperscanbedifficultandcostly
icalmodellingofWECsislikelytobetheproduc- to obtain.Asecond issue with thecurrentliter-
tionofhybridmodelsthatusethebestelements ature is that it can be opaque and difficult to
fromseparatemodelsandcombinethemtopro- interpret.Thisisbecauseinmanycasestheliter-
duceahigherfidelityormoreefficaciousmodel. ature describes ongoing research that may be
Thistrendhasalreadystartedwiththeinclusion incompleteornotfullyvalidated.Inthecontext
of WECs in wave propagation models (Babarit of the conference in which the work was pre-
andFolley,2013);however,morehybridmodels sented this may be acceptable; however, it can
4
1. INTRODUCTION
significantly complicate interpretation for a the modelling of WECs, which allows the text
reader who did not attend the conference, or to be more succinct and hopefully more
may be reading the paper a number of years readable.
after its publication. In extremis, the current It may reasonably be questioned why WECs
wave energy literature can appear contradic- need to be treated differently to other marine
tory. Whether the literature is actually contra- structures, for which a number of books on the
dictory, or simply appears to be due to subject already exist. From a fundamental per-
differing contexts, is to some extent irrelevant spectiveitistruethataWECcanbeconsidered
becauseforsomeonenewtothefielditisimpos- as simply another marine structure; however,
sible to tell thedifference. the modelling objective for WECs is generally
This book has been written and compiled in very different. Specifically, it is typically the
response to all of the points discussed here. power extracted from the waves that is impor-
Thus, it is intended that this book will provide tant in modelling a WEC, whilst for a general
a single compendium of the techniques marine structure the structural responses or
currently used for modelling WECs. Necessar- forces are typically required. Moreover, it is
ily, if only to limit its size, it does not go into common in the modelling of marine structures
the intimate details of each method, but con- to either assume that they are large with small
fines itself to a clear exposition of the different motions so that a potential flow model can be
modellingtechniquesavailable.Thus,thefocus used, or small with larger relative motions so
is on the fundamental characteristics of each that the Morisonequation can beused. Neither
technique, together with its inherent limita- of these assumptions is typically reasonable
tions. From this it is anticipated that the reader forWECsthatarenormallyrelativelylargewith
will be able to assess whether the technique is relatively large motions. Notwithstanding this
suitable for the particular WEC and modelling difference, all of the modelling techniques may
objective they are interested in. However, each alsobesuitableformodellingmarinestructures.
chapter contains extensive references that However,theimportantdistinctioninthetextis
can beused by thereaderfor further investiga- thatinthisbookthefocusisontheir suitability
tion once the modelling technique of interest for modelling WECs, rather than their general
has been identified. In addition, the book does application.
not go into detail in the modelling of compo-
nents of WECs that are adequately covered in
other texts, even though they may have a large
1.5 HOW TO USE THIS BOOK
influence on performance, but limits itself to
howmodelsofthesecomponentsmaybeincor-
This book isseparated into fourparts.
poratedinaWECmodel.Thus,thehighlycom-
plex field of modelling moorings is not (cid:129) Part Adeals with the modelling of WECs
included in this book, nor does it include the wherethehydrodynamicforcesarebasedon
modelling of pneumatic, hydraulic or electrical linear potential flow, whichis themost
machines that may be used to extract power commonmethod used for modelling the
fromaWEC;thereaderisdirectedtothemany WEC hydrodynamics. Specifically,this part
booksthatalreadyexistonthesesubjectswhere containschapterson thefrequency-domain
more in-depth information can be found. Con- modelling ofWECs,the time-domain
sequently, the book can focus specifically on modelling ofWECsand the spectral-domain
5
1.5 HOWTOUSETHISBOOK
modellingofWECs.Currently,WECmodels As much as is reasonably possible, all the
whose hydrodynamics are based onlinear modelling technique chapters in the book have
potentialflowprobablyrepresentover90%of the same layout. Thus, they typically start with
models produced. However,their ubiquity anintroductiontotheparticularmodellingtech-
should notdisguisethe fact that they have a nique that contains details of its fundamental
number oflimitations that need to be principles followed by the application of the
recognised. technique to modelling WECs. At the end of
(cid:129) Part B deals with the modelling ofWECs each chapter there is typically a section on the
using techniques other than linear potential limitations of the modelling technique and a
flow.Thus, this part containschapterson bullet-point summary of thechapter.
nonlinear potential flowmodels, Thisbookhasbeenwrittensothateachchap-
computational fluid dynamics modelsand ter can be read largely independently of all the
model identification.Naturally, there is no other chapters. However, there are clearly a
strong link betweenthese chaptersexcept number of links between the different chapters
that they describemodels that are notbased andtheselinksarereferencedinthetextwhere
onlinear potential flow. appropriate. Although the chaptersin the book
(cid:129) Part C deals with the modellingof arraysof can be read in any order it is anticipated that
WECs. This part contains chapterson the the order in which the chapters are read will
semi-analyticalmodellingofWECarrays,the depend on the perspective of the reader. Three
modellingof arraysasan extensionof distinct perspectives have been identified—the
techniquesusedforsingleWECs,WECarray model specifier, the model developer and the
modellingbasedin phase-resolving wave model interpreter—although it is likely that
propagation modelsandWECarray any reader will actually have a combination of
modellingbasedon phase-averaging wave one or more of these perspectives.
propagation models.The modelling of WEC A model specifier will typically know their
arraysisparticularlyimportantforlargescale modelling objectives and so is most likely to
developmentofwaveenergyasitisrequired be a developer that is looking to identify the
notonly for the prediction ofa wave farm’s modelling technique that is most appropriate
energy yield, but also for assessing what the forthecurrentchallengesintheircurrentdevice
environmental impact may beof large-scale or project development. It is anticipated that
extractionof wave energy. this type of reader will start in Part D of the
(cid:129) Part D deals with the use of the WEC book, where the applications of the modelling
modelling techniques described in Parts A techniques are detailed, and then refer to the
to C to achieve particular modelling appropriatemodelsinPartsAtoCasrequired.
objectives. This part contains chapters on Conversely, it is expected that a model devel-
the control of WECs, the calculation of the oper,mostlikelytobeanengineerorcomputer
mean annual energy production (MAEP), programmer, will start with the chapter on the
the estimation of the structural loading and particularmodellingtechniquethattheyintend
the modelling of the environmental impact to use in Parts A to C of the book and then
of WECs. A particular focus of the chapters subsequently look at Part D when considering
in this part is the identification of the howthismodelmaybeappliedtoachieveapar-
appropriate WEC modelling technique for ticular modelling objective. Finally, a model
the particular modelling objective. interpreter, who is likely to be an assessor or
6
1. INTRODUCTION
potentialinvestor,islikelytoreadboththechap- Structural Funds Programme 2007–13) under
teronthemodellingtechniqueused,inPartsA Grant EI/CF/2011/1320, and Science Founda-
toC,andalsotherelevantchapter inPart Don tionIrelandunderGrantNo.13/IA/1886.
the application to the particular modelling The work presented in Chapter 10 (Phase-
objective. resolving wave propagation array models) has
been supported by the PhD funding grant of
Dr. Vasiliki Stratigaki by the Research Founda-
1.6 ACKNOWLEDGEMENTS tion Flanders (FWO), Belgium. Furthermore,
part of the work presented in this chapter has
Theproductionofthisbookhasbeenacollab- been supported by the FWO research project
oration between many of the currently leading 3G029114. The experimentaldata used for vali-
experts in wave energy. The knowledge that dating numerical methods presented in this
these experts have developed in this field chapter have been obtained during the ‘WEC-
over the yearsisa result of significant and sus- wakes’ project, supported by the European
tained support from a range of sources that Community’s Seventh Framework Programme
deserveacknowledgementfortheircontribution through thegrant tothebudgetofthe Integrat-
to wave energy in general and this book in ingActivityHYDRALABIVwithintheTransna-
particular. tional Access Activities, Contract no. 261520,
Specifically,Chapter3hasbeenlargelybased and by the Research Foundation Flanders
on research material previously developed by (FWO)-Contract Number FWO-KAN-15
the author when working at Instituto Superior 23 712N.
(cid:1)
Tecnico supported by the EC WAVETRAIN ThewritingforChapter12(Controloptimisa-
Research Training Network Towards Competi- tionandparametricdesign)wassupportedbya
tive Ocean Energy, contract No. MRTN-CT- projectfundedbyEnterpriseIreland(IrishGov-
2004-50166 and subsequently at Tecnalia ernment and the European Union under Ire-
Research and Innovation partially funded land’s EU Structural Funds Programme 2007–
by the Department of Industry, Innovation, 13) under Grant EI/TD/2009/0331, the Irish
CommerceandTourismoftheBasqueGovern- Research Council, and Science Foundation Ire-
ment (ETORTEK Program). Although not land under Grant No. 12/RC/2302 for the
directly involved in the writing of the Chapter MarineRenewable Ireland (MaREI)centre.
3,theauthorrecognizesthecontributionarising Finally, Chapter 15 (Environmental impact
fromfruitfulcollaborationwithAnto´nioFalca˜o, assessment) was written by authors supported
Jean-Baptiste Saulnier, Joseba Lopez Mendia by the UK Engineering and Physical Research
and Imanol Touzon among others. Then Chap- CouncilgrantEP/J010065/1,togetherwithwel-
ters4(SpectralDomainModels)and8(Conven- come contributions from Graham Savidge, for-
tional Multiple Degree-of-Freedom Array merly ofQueen’sUniversity Belfast.
Models) have been produced by authors who
were supported by the UK Engineering and
Physical Science Research Council under the References
SuperGen Centre for Marine Energy Research
project (grantEP/I027912/1). Alves, M., Sarmento, A., 2005. Non-linear and viscous
diffraction response of OWC wave power plants.
ThewritingofChapter7 (IdentifyingModels
In: 6th European Wave and Tidal Energy Conference,
UsingRecordedData)wassupportedbyaproject
Glasgow.
fundedbyEnterpriseIreland(IrishGovernment Babarit,A.,Folley,M.,2013.OnthemodellingofWECsin
and the European Union under Ireland’s EU wave models using far field coefficients. In: 10th