Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation

201 Pages·2012·3.789 MB·English

by  Stefan Emeis

Green Energy and Technology
For furthervolumes:
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Stefan Emeis
Wind Energy Meteorology
Atmospheric Physics for Wind Power
Generation
123
http://avaxhome.ws/blogs/ChrisRedfield
Stefan Emeis
Institutfür MeteorologieundKlimaforschung
Karlsruher InstitutfürTechnologie
Kreuzeckbahnstr. 19
82467Garmisch-Partenkirchen
Germany
ISSN 1865-3529 ISSN 1865-3537 (electronic)
ISBN 978-3-642-30522-1 ISBN 978-3-642-30523-8 (eBook)
DOI 10.1007/978-3-642-30523-8
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Preface
Many books have already been written on converting the kinetic energy of the
wind into mainly electrical energy. This one, written by a meteorologist, will
entirely concentrate on the atmospheric features and phenomena influencing the
generation of electric power from the wind. Such a book is presently—to my
knowledge—unavailable. This book presents part of what is today called ‘energy
meteorology’,apresentlyemergingnewsub-disciplineinthefieldofmeteorology.
IthankSpringerScienceMediafortheinvitationtowritesuchabookwhichis
designed to fit into the series ‘‘Green Energy and Technology’’ which deals with
variousaspectsonrenewableenergies.Thisseriesalreadycomprisesseveraltitles
onwindenergy. Onceagainmostofthese titles are ontechnical aspects butnone
of these concentrates on the meteorological boundary conditions for the conver-
sionofenergyfromthewind.MyspecialthanksgotoClausAscheronofSpringer
who accompanied the preparation of the manuscript and gave invaluable advice.
I am working as a scientist in the discipline of meteorology since the 1980s.
The field of energy meteorology has found my attention for more than 20 years,
althoughtheterm‘energymeteorology’ismuchnewer.Myinterestinthissubject
was initiated during a sabbatical leaveat the Wind Energy Institute ofthe Danish
NationalLaboratory(todaypartoftheDanishTechnicalUniversity,DTU)atRisø
nearRoskilde,Denmark.Here,Imetboundary-layermeteorologyexpertsandsaw
oneofthefirsttestsitesforwindturbines.Essentially,windenergymeteorologyis
a special section of boundary-layer meteorology. I still have very fruitful and
friendly contacts with this renowned Danish research institute. In 1991 in Risø I
alsometthelateStenFrandsenforthefirsttime.Discussionswithhimstartedmy
attentiontothewindparkissue.WhatispresentedhereinChap.6inthisbookisa
much more elaborated version of an idea which was born during that first stay in
Risø. Thus, I dedicate Chap. 6 to him.
Later I worked many years on acoustic profiling of the atmospheric boundary
layerwithSODARdevices.Theseinstrumentsallowforasurface-baseddetection
oftheboundary-layerwindprofilebasedonananalysisoftheDopplershiftofthe
backscattered signal. This is a technique, which captured the interest of the wind
energy community in the 1990 s. In recent years, my experimental activities and
v
vi Preface
expertise have been complemented by the operation of ceilometers, RASS and
wind lidars. In addition, I focussed on the investigation of peculiarities of the
marineboundarylayerfromdatafromtheGermanoffshoremeasurementplatform
FINO1inseveralresearch projects. IamamemberoftheSouthernGermanwind
energy research alliance WindForS.
Themarineboundarylayerprojectshavebeenfundedthroughseveralgrantsby
the German Ministry of the Environment, Nature Protection and Nuclear Safety
(BMU, FKZ 032 99 61, 032 50 50, 032 53 04). These projects within the RAVE
program (Research at Alpha Ventus) were initiated in order to accompany
scientificallytheestablishmentofthefirstGermanoffshorewindparkAlphaVentus,
whichissituatedintheGermanBightroughly45 kmawayfromthenearestcoastin
about30 mdeepwaters.Sixyearsbeforethefirstturbineinstallationbegan,a100 m
meteorologicalmeasurementtower(FINO1)waserectedatthelatersiteofAlpha
Ventusinordertofacilitatethestudiesofthemarineboundarylayer.Muchofthe
informationconcerningthemarineboundarylayerpresentedinthisvolumeisbased
ondataobtainedatthistowerwhichhaseightmeasurementplatformsbetween30
and100 m.TheevaluationofthistowerdatahasmainlybeenperformedbytwoPhD
studentsofmine;MatthiasTürk1andRichardForeman2.Furtherfundingisavailable
throughaprojectleadbySven-ErikGryningfromRisøDTUandwhichispresently
supported by Forsknings—og Innovationsstyrelsen at the Danish Ministeriet for
Videnskab, Teknologi og Udvikling (Sagsnr 2104-08-0025) within the project:
‘‘Large wind turbines—the wind profile up to 400 m’’. The results for urban
boundary layers are partly based on studies funded by the German Ministry of
EducationandResearch(BMBF)intheframeworkoftheAFO2000program.The
data from Graswang in Figs. A.1 and A.2 in Appendix A have been obtained in
theframeworkoftheTERENOprogrammeoftheHelmholtzsocietyfundedbythe
BMBF. The studies on flow over complex terrain have partly been made possible
throughthefinancialsupportbyseveralprivateenterprises.
AdraftversionofthemanuscripthasbeenreadbyBeatrizCañadillas,Richard
Foreman,TomNeumann,andMatthiasTürk.Ithankallofthemfortheirvaluable
suggestions,helpandadvice.Nevertheless,itismetobeblamedforanyerrorsor
inconsistencies.Ihopethat thisbook willhelp tobring the meteorological part in
wind power conversion to a better visibility.We urgently need efficientstrategies
to generate renewable energies for the energy demand of mankind and a better
understanding of the meteorological prerequisites for wind power generation
should be part of this strategy.
Spring 2012 Stefan Emeis
1 Türk, M.: Ermittlung designrelevanter Belastungsparameter für Offshore-Windkraftanlagen.
PhDthesis,UniversityofCologne(2009)(Availablefrom:http://kups.ub.uni-koeln.de/2799/)
2 Foreman,R.: Improved calculation ofoffshore meteorologicalparameters forapplications in
windenergy.PhDthesis,FacultyofMathematics andNaturalSciences, UniversityofCologne
(2012)
Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Scope of the Book. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Overview of Existing Literature . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 History of Wind Energy Generation. . . . . . . . . . . . . . . . . . . . . 3
1.4 Potential of Wind Energy Generation. . . . . . . . . . . . . . . . . . . . 4
1.5 Present Status of Wind Energy Generation. . . . . . . . . . . . . . . . 5
1.6 Structure of This Book. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2 Wind Regimes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.1 Global Circulation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.2 Driving Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.1 Hydrostatic Equation. . . . . . . . . . . . . . . . . . . . . . . . . . 11
2.2.2 Momentum Budget Equations for the Wind. . . . . . . . . . 12
2.3 Geostrophic Winds and Gradient Winds. . . . . . . . . . . . . . . . . . 14
2.4 Thermal Winds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.5 Boundary Layer Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.6 Thunderstorm Gusts and Tornados. . . . . . . . . . . . . . . . . . . . . . 18
2.7 Air Density. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
3 Vertical Profiles Over Flat Terrain. . . . . . . . . . . . . . . . . . . . . . . . 23
3.1 Surface Layer (Prandtl Layer). . . . . . . . . . . . . . . . . . . . . . . . . 25
3.1.1 Logarithmic Wind Profile . . . . . . . . . . . . . . . . . . . . . . 28
3.1.2 Power Law. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
3.1.3 Comparison Between Logarithmic and Power Law. . . . . 33
3.1.4 Vertical Wind Profile with Large Wind Speeds . . . . . . . 40
3.2 Profile Laws Above the Surface Layer. . . . . . . . . . . . . . . . . . . 40
3.2.1 Ekman Layer Equations. . . . . . . . . . . . . . . . . . . . . . . . 41
3.2.2 Inertial Oscillations in the Ekman Layer . . . . . . . . . . . . 42
vii
viii Contents
3.2.3 Vertical Wind Profiles in the Ekman Layer . . . . . . . . . . 42
3.2.4 Unified Description of the Wind Profile
for the Boundary Layer. . . . . . . . . . . . . . . . . . . . . . . . 43
3.3 Spectra. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.4 Diurnal Variation of the Wind Profile . . . . . . . . . . . . . . . . . . . 50
3.4.1 Vertical Profiles of the Weibull Parameters . . . . . . . . . . 51
3.4.2 Low Level Jets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.5 Internal Boundary Layers. . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
3.6 Wind and Turbulence Profiles Over Forests . . . . . . . . . . . . . . . 61
3.7 Winds in Cities. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
3.7.1 Characteristics of Urban Boundary Layers. . . . . . . . . . . 63
3.7.2 Vertical Profiles of Wind and Turbulence . . . . . . . . . . . 65
3.7.3 Special Flow Phenomena in Urban Canopy Layers. . . . . 67
3.8 Summary for Flat Terrain. . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
4 Winds in Complex Terrain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
4.1 Characteristics of Boundary Layers Over Complex Terrain . . . . 76
4.1.1 Mountain and Valley Winds. . . . . . . . . . . . . . . . . . . . . 78
4.1.2 Katabatic Winds. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.2 Wind Profiles Over a Hill. . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.2.1 Potential Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.2.2 Modifications to the Potential Flow: Addition
of an Inner Layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
4.2.3 Modifications to the Potential Flow: Consideration
of Thermal Stability . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.2.4 Weibull Parameters over a Hill. . . . . . . . . . . . . . . . . . . 86
4.3 Wind Profiles Over an Escarpment . . . . . . . . . . . . . . . . . . . . . 87
4.4 Spectra. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.5 Diurnal Variation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4.6 Summary for Complex Terrain . . . . . . . . . . . . . . . . . . . . . . . . 91
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92
5 Offshore Winds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
5.1 Characteristics of Marine Boundary Layers . . . . . . . . . . . . . . . 96
5.1.1 Sea Surface Roughness and Drag Coefficient. . . . . . . . . 96
5.1.2 Fetch and Stability Dependent Wave Formation. . . . . . . 101
5.1.3 Extreme Wave Heights . . . . . . . . . . . . . . . . . . . . . . . . 106
5.1.4 Wave Age . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
5.1.5 Impact of the Vertical Moisture Profile. . . . . . . . . . . . . 109
5.1.6 Annual and Diurnal Variations. . . . . . . . . . . . . . . . . . . 110
5.2 Vertical Profiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
5.3 Extreme Wind Speeds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
Contents ix
5.4 Turbulence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115
5.4.1 Turbulence Intensity . . . . . . . . . . . . . . . . . . . . . . . . . . 116
5.4.2 Wind Speed Variances. . . . . . . . . . . . . . . . . . . . . . . . . 120
5.4.3 Turbulence Length Scales and Inclination Angles. . . . . . 122
5.4.4 Gust Events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
5.5 Weibull Parameter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125
5.6 Coastal Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
5.6.1 Land and Sea Winds. . . . . . . . . . . . . . . . . . . . . . . . . . 128
5.6.2 Low-Level Jets. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
5.7 Summary for Marine Boundary Layers . . . . . . . . . . . . . . . . . . 130
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
6 Physics of Wind Parks. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.1 Turbine Wakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.2 Analytical Model for Mean Wind Speed in Wind Parks. . . . . . . 138
6.3 Analytical Model for Wind Park Wakes. . . . . . . . . . . . . . . . . . 144
6.4 Application of the Analytical Model with
FINO1 Stability Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146
6.5 Risks that a Tornado Hits a Wind Park . . . . . . . . . . . . . . . . . . 147
6.6 Summary for Wind Parks. . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
7 Outlook. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
7.1 Size of Wind Turbines. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
7.2 Size of Offshore Wind Parks . . . . . . . . . . . . . . . . . . . . . . . . . 156
7.3 Other Techniques of Converting Wind Energy . . . . . . . . . . . . . 156
7.4 New Measurement and Modelling Tools to Assess
Wind Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
7.5 Wind Resources and Climate Change . . . . . . . . . . . . . . . . . . . 157
7.6 Repercussions of Large-Scale Wind Power Extraction
on Weather and Climate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
Appendix A: Statistical Tools. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
Appendix B: Remote Sensing of Boundary Layer
Structure and Height. . . . . . . . . . . . . . . . . . . . . . . . . . . 177
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
List of Variables
a reduction factor
constant in stability correction function
power-law exponent
b constant in stability correction function
c wave age
c specific heat
p
c phase speed
ph
c factor in Eq. (3.91)
2
c surface drag coefficient
s
c turbine drag coefficient
t
c effective drag coefficient
teff
d displacement height
f horizontal Coriolis parameter
f* vertical Coriolis parameter
f(x) Weibull function
g gravity
h height of atmospheric layers
k wake decay coefficient
k maximum wave number of spectrum
max
l mixing length
inner layer height (flow over hills)
m mass-specific momentum
n frequency
p air pressure
q specific humidity
r radius of curvature
s distance
t time
u west–east wind component
xi
xii ListofVariables
u 10 m wind speed
10
u west–east component of geostrophic wind speed
g
u hub height wind speed
h
u wind speed from potential flow theory
pot
u gust wind speed
gust
u friction velocity
*
u upstream wind speed (flow over hills)
?
v south–north wind component
v maximum wind speed with 1 year return period
1
v maximum wind speed with 50 year return period
50
v 10 min extreme wind speed with 1 year return period
e1
v 10 min extreme wind speed with 50 year return period
e50
v south–north component of geostrophic wind
g
w vertical wind component
w convective velocity scale
*
x eastward pointing coordinate
abbreviation in stability correction function
y northward pointing coordinate
z vertical coordinate
z roughness length
0
z boundary-layer height
i
z reference height
r
z Ekman layer height
g
z Prandtl layer height
p
z height variable in shape function
m
z height variable in shape function
A
A scale factor in Weibull distribution
A geostrophic scale factor in Weibull distribution
g
A rotor area of a wind turbine
r
B Bowen ratio
B backscatter profile
i
BR buoyancy ratio
C drag coefficient
D
C neutral drag coefficient at 10 m height
DN10
C thrust coefficient
T
D distance
D ageostrophic wind component
u
D ageostrophic wind component
v
D wake width
w
E wind energy
wind
F Weibull distribution function
F components of turbulent friction
x,y,z
G gust factor