T.C. Çukurova University Faculty of Engineering and Architecture Department of Electrical and Electronics Engineering Graduation Thesis A Survey on Renewable Energy for Electric Generation By Kasım Zor 2004514018 Advisor Prof. Dr. Mehmet Tümay May-2010 ADANA
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A Survey on Renewable Energy for Electric Generation
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T.C.
Çukurova University
Faculty of Engineering and Architecture
Department of Electrical and Electronics Engineering
Graduation Thesis
A Survey on Renewable Energy for Electric Generation
By
Kasım Zor
2004514018
Advisor
Prof. Dr. Mehmet Tümay
May-2010
ADANA
I
CONTENTS I
LIST OF FIGURES XII
LIST OF TABLES XX
ABSTRACT XXI
ACKNOWLEDGEMENTS XXII
1 INTRODUCTION 1
1.1 Renewable Energy at a Glance 1
2 ELECTRIC GENERATION FROM RENEWABLE ENERGY SOURCES 3
2.1 Solar Energy and Electric Generation 3
2.1.1 Introduction 3
2.1.2 Solar Thermal Power System 3
2.1.2.1 Energy Collection 5
1) Parabolic Trough 5
2) Central Receiver 7
3) Parabolic Dish 9
2.1.2.2 Solar Chimney Power Plant 10
2.1.2.3 Commercial Power Plants 12
2.1.2.4 Potential Technology Developments and Recent Trends 13
2.1.2.5 Future Expectations 14
1. Short Term: Present to 2020 14
2. Medium Term: 2020 to 2035 15
3. Long Term: After 2035 16
II
2.1.3 Phovoltaics 16
2.1.3.1 Introduction 16
2.1.3.2 PV Cell 17
1) PV Cell Technologies 19
a. Crystalline Silicon Solar Cells 19
b. Thin Film Solar Cells 20
c. Concentrator Cell 21
2.1.3.3 Module and Array 23
2.1.3.4 Array Design 26
1) Sun Intensity 26
2) Sun Angle 27
3) Shadow Effect 28
4) Temperature Effect 30
5) Effect of Climate 32
6) Electrical Load Matching 32
7) Sun Tracking 34
8) Peak-Power Operation 37
9) System Components 38
2.1.3.5 Power Electronics for Photovoltaic Power Systems 40
1) Stand-alone PV Systems 41
1.1) Battery Charging 42
1.1.1) Batteries for PV Systems 42
1.1.2) PV Charge Controllers 43
1.1.2.1) A Series Charge Regulators 44
1.1.2.2) Shunt Charge Regulators 45
1.1.2.3) DC-DC Converter Type Charge
Regulators 45
1.1.3) Maximum Power Point Tracking 47
1.1.4) Analog Control 48
1.1.5) Digital Control 49
1.2) Inverters for Stand-alone PV Systems 49
2) Hybrid Systems 54
III
2.1) Series Configuration 54
2.2) Switched Configuration 56
2.3) Parallel Configuration 58
2.4) Control of Hybrid Energy Systems 60
3) Grid-connected PV Systems 62
3.1) Inverters for Grid-connected Applications 64
3.2) Inverter Classifications 64
3.3) Inverter Types 66
3.3.1) Line-commutated Inverter 66
3.3.2) Self-commutated Inverter 67
3.3.3) Inverter with High Frequency
Transformer 68
3.3.4) Other PV Inverter Topologies 69
3.3.4.1) Multilevel Converters 69
3.3.4.2) Non-insulated
Voltage Source 70
3.3.4.3) Non-insulated
Current Source 70
3.3.4.4) Buck Converter with Half-bridge
Transformer Link 71
3.3.4.5) Flyback Converter 71
3.3.4.6) Interface Using Paralleled
PV Panels 72
3.4) System Configurations
3.4.1) Central Plant Inverter 73
3.4.2) Multiple String DC/DC Converter 73
3.4.3) Multiple String Inverters 74
3.4.4) Module Integrated Inverter 75
3.5) Grid-compatible Inverters Characteristics 75
3.5.1) Protection Requirements 77
2.1.3.6 Potential Technology Developments and Recent Trends 79
1) Dye-sensitized Solar Cells 80
IV
2) Organic and Nanotechnology Solar Cells 80
2.1.3.7 Future Expectations 81
1) Short Term: Present to 2020 81
2) Medium Term: 2020 to 2035 82
3) Long Term: After 2035 82
2.2 Wind Energy and Electric Generation 84
2.2.1 Introduction 84
2.2.2 Wind Speed and Energy 85
2.2.2.1 Power Extracted from the Wind 87
2.2.2.2 Effect of Hub Height 90
2.2.3 Wind Power Systems 92
2.2.3.1 System Components 92
1) Towers 94
2) Turbine 99
3) Blades 100
2.2.3.2 Turbine Rating 101
2.2.3.3 Power vs Speed and TSR 102
2.2.3.4 Maximum Power Operation 105
1) Constant-TSR Scheme 105
2) Peak-Power-Tracking Scheme 106
2.2.3.5 System Design Trade-Offs 107
1) Turbine Towers and Spacing 107
2) Number of Blades 109
3) Rotor Upwind or Downwind 110
4) Horizontal vs Vertical Axis 111
2.2.4 Power Electronics for Modern Wind Turbines 111
1) Wind Energy Conversion 111
2) Modern Power Electronics and Converter Systems 115
2.1) Power Electronic Devices 115
2.2) Power Electronics Converters 116
V
3) Generator Systems for Modern Wind Turbines 118
3.1) Fixed-Speed Wind Turbines 118
3.2) Variable-Speed Wind Turbines 122
3.2.1) Variable-Speed Wind Turbines with Partially Rated
Power Converters 123
3.2.1.1) Dynamic Slip-Controlled Wounded Rotor
Induction Generator 124
3.2.1.2) Doubly Fed Induction Generator 124
3.2.2) Full Scale Power Electronic Converter
Integrated Systems 125
4) Control of Wind Turbines 127
4.1) Active Stall Wind Turbine with Cage Rotor Induction
Generators 127
4.2) Variable Pitch Angle Control with Doubly Fed
Generators 128
4.3) Full Rated Power Electronic Interface
Wind Turbine Systems 130
5) Electrical Topologies of Wind Farms Based on Different Wind
Turbines 131
6) Integration of Wind Turbines into Power Systems 134
6.1) Requirements of Wind Turbine Grid Integration 135
6.1.1) Frequency and Active Power Control 135
6.1.2) Short Circuit Power Level and Voltage Variations 135
6.1.3) Reactive Power Control 137
6.1.4) Flicker 138
6.1.5) Harmonics 138
6.1.6) Stability 139
6.2)Voltage Quality Assessment 140
6.2.1) Steady-State Voltage 140
6.2.2) Voltage Fluctations 141
6.2.2.1) Continuous Operation 141
6.2.2.2) Switching Operations 142
VI
6.2.3) Harmonics 142
2.2.5 Environmental Aspects 143
2.2.5.1 Audible Noise 143
2.2.5.2 Electromagnetic Interference 144
2.2.5.3 Effect on Birds 145
2.2.5.4 Other Impacts 145
2.2.6 Potential Catastrophes 146
2.2.6.1 Fire 146
1) Lightning Strike 146
2) Internal Fault 146
2.2.6.2 Earthquake 147
2.2.7 Potential Technology Development and Recend Trends 148
2.2.7.1 Potential Technology Development 148
2.2.7.2 Recent Trends 152
1) Small Wind Systems 153
2) System-Design Trends 153
2.2.8 Future Expectations 154
2.2.8.1 Short Term: Present to 2020 154
2.2.8.2 Medium Term: 2020 to 2035 155
2.2.8.3 Long Term: After 2035 156
2.3 Geothermal Energy and Electric Generation 158
2.3.1 Introduction 158
2.3.2 Geothermal Resources 159
2.3.2.1 Model of a Hydrothermal Geothermal Resource 159
2.3.2.2 Other Types of Geothermal Resources 161
1) Hot Dry Rock 162
2) Geopressure 163
3) Magma Energy 165
2.3.3 Geothermal Power Plants 167
2.3.3.1 Direct Steam Power Plants 169
VII
2.3.3.2 Flash Steam Power Plants 172
1) Single Flash Power Plants 172
2) Double Flash Power Plants 174
2.3.3.3 Binary Cycle Power Plants 175
2.3.3.4 Hybrid Power Plants 178
1) Hybrid Single-Flash and Double-Flash Systems 178
a) Integration of These Systems 178
b) Combined System 179
2) Hybrid Flash-Binary Systems 180
a) Combined Plants 181
b) Integrated Flash-Binary Plants 181
2.3.4 Benefits of Geothermal Energy 183
2.3.5 Potential Technology Development and Recent Trends 183
2.3.5.1 Potential Technology Development 183
2.3.5.2 Recent Trends 184
1) Enhanced Geothermal Systems 184
2.3.6 Future Expectations 186
2.3.6.1 Short Term: Present to 2020 186
2.3.6.2 Medium Term: 2020 to 2035 186
2.3.6.3 Long Term: After 2035 187
2.4 Ocean Energy and Electric Generation 188
2.4.1 Introduction 188
2.4.2 Tidal Energy 188
2.4.2.1 Introduction 188
2.4.2.2 Basic Physics 189
2.4.2.3 Tidal Energy Status 190
1) Tidal Barrages 191
1.1) Principles of Operation 191
1.2) Single-Basin Tidal Barrages 191
a) Ebb Generation 191
VIII
b) Flood Generation 191
c) Two-Way Generation 192
1.3) Double-Basin Tidal Barrages 192
1.4) La Rance, France 193
2) Tidal Current Turbines 193
2.1) Principle of Operation 193
a) Kinetic Energy Extraction 193
2.2) Turbine Technologies and Concepts 195
a) DeltaStream Turbine 196
b) Evopod Tidal Turbine 196
c) Free Flow Turbines 197
d) Gorlov Helical Turbine 198
e) Lunar Energy Tidal Turbine 198
f) Neptune Tidal Stream Device 199
g) Nereus and Solon Tidal Turbines 200
h) Open Centre Turbine 201
i) Pulse Tidal Hydrofoil 201
j) SeaGen 202
k) Stingray Tidal Energy Converter 203
l) Tidal Fence Davis Hydro Turbine 204
m) TidEl Stream Generator 204
n) Tidal Stream Turbine 205
2.4.2.4 Current Issues on Tidal Energy 206
1) Tidal Barrage Systems 206
2) Tidal Current Turbines 206
2.4.2.5 Future Developments 208
1) Tidal Barrage Systems 208
2) Tidal Cyrrent Turbines 208
2.4.3 Wave Energy 209
2.4.3.1 Introduction 209
2.4.3.2 Wave Resources 210
2.4.3.3 The Various Technologies 210
IX
1) The Oscillating Water Column 211
a. Fixed-structure OSW 211
b. Floating-structure OSW 214
2) Oscillating Body Systems 215
a. Single-body Heaving Buoys 216
b. Two-body Heaving Systems 219
c. Fully Submerged Heaving Systems 222
d. Pitching Devices 222
e. Bottom-hinged Systems 227
f. Many-body Systems 228
3) Overtopping Converters 229
4) Electrical Equipment 231
2.4.3.4 Wave Energy Transmission Concepts for Linear Generator
Arrays 232
1) System Description 232
a. Base Unit 232
b. System Options 233
c. Connection Schemes 236
2.4.3.5 Conclusion 238
2.4.4 Ocean Thermal Energy Conversion 239
2.4.4.1 Introduction 239
2.4.4.2 Design Requirements for OTEC Systems 241
2.4.4.3 OTEC Power Systems 242
2.4.4.4 Applications for OTEC 245
2.4.4.5 Advantages and Disadvantages of OTEC System 246
1) Advantages 246
2) Disadvantages 246
2.4.4.6 Perspectives 246
2.5 Biomass Energy and Electric Generation 248
2.5.1 Introduction 248
X
2.5.2 Biomass Technologies 248
2.5.2.1 Gasification-based Biomass 248
2.5.2.2 Direct-fired Biomass 252
2.5.2.3 Biomass Co-firing 254
2.5.3 Status of Technology 256
2.5.4 Biomass Future 258
2.6 Hydropower Energy and Electric Generation 263
2.6.1 Introduction 263
2.6.2 Hydropower Plant Models and Control 263
2.6.3 Hydroelectricity 265
2.6.4 Micro Hydropower Station 268
2.6.4.1 A Hydropower Station Under Study 268
2.6.5 Hydropower’s Future in a Fluid Energy World 269
2.7 Hydrogen Energy and Electric Generation 273
2.7.1 Introduction 273
2.7.2 Electrical Energy Storage 273
2.7.3 Electrolyser 275
2.7.4 Hybrid Systems 276
2.7.4.1 Solar-hydrogen Energy Systems 276
2.7.4.2 Wind to Hydrogen System 278
3 FINANCIAL AND ECONOMIC VIEW OF RENEWABLE ENERGY 280
3.1 Cost of Renewable Energy Systems 280
3.2 Wind Power Cost 287
3.3 Solar Power Cost 289
3.3.1 Solar Photovoltaic Cost 289
3.3.2 Concentrating Solar Power 292
XI
3.4 Geothermal Power 295
3.5 Biopower Cost 296
3.6 Costs in 2020 297
REFERENCES 302
XII
LIST OF FIGURES
Figure Name of the Figure Page
1.1 Solar Thermal Power Plant Schematic for Generating Electricity 4
1.2 Parabolic Trough 5
1.3 Principle of a Parabolic Trough Solar Power Plant 7
1.5 Central Receiver 7
1.6 Operational Schematic of Planta Solar 10 8
1.7 PS10 Solar Thermal Power Plant, Sevilla, Spain 8
1.8 Principle of a Dish-Stirling System 9
1.9 California Edison 25kW Dish-Stirling System 10
1.10 Principle of the Solar Chimney Power Plant 10
1.11 Solar Heat Induced Wind Chimney Power Plant 11
1.12 PV Effect Converts the Photon Energy into Voltage across the P-N Junction 17
1.13 Basic Construction of PV Cell with Performance Enhancing Features 18
1.14 Crystalline Silicon Wafers 19
1.15 Amorphous Silicon Thin Film 21
1.16 Lens Concentrating the Sunlight on a Small Area Reduces the Need for 22
Active Cell Material
1.17 Several PV Cells Constitute a Module and Several Modules Constitute an Array 23
1.18 Construction of PV Cell 24
1.19 PV Module Mounting Methods 25
1.20 I-V Characteristic of PV Module Shifts down at Lower Sun Intensity, 26
with Small Reduction in Voltage
1.21 Photoconversion Efficiency vs Solar Radiation 27
1.22 Kelly Cosine Curve for PV Cell at Sun Angles form 0° to 90° 28
1.23 Shadow Effect on One Long Series String of an Array 29
1.24 Bypass Diode in PV String Minimizes Power Loss under Heavy Shadow 30
1.25 Effect on Temperature on I-V Characteristic 30
1.26 Effect of Temperature on P-V Characteristic 32
XIII
Figure Name of the Figure Page
1.27 Operating Stability and Electrical Load Matching with Constant-resistive Load 33
and Constant-power Load
1.28 Dual-Axis Sun Tracker Follows the Sun throughout the Year 34
1.29 Actual Motor of the Sun Tracker 35
1.30 Sun-Tracking Actuator Principle 36
1.31 Peak-Power-Tracking PV Power System Showing Major Components 39
1.32 Stand-alone PV System 40
1.33 PV-diesel Hybrid System 41
1.34 Grid-connected PV System 41
1.35 Nominal Number of Battery Cycles vs DOD 43
1.36 Series Charge Regulator 44
1.37 Shunt Charge Regulator 45
1.38 Buck Converter 46
1.39 Boost Converter 46
1.40 Buck-Boost Converter 46
1.41 Typical Power/Voltage Characteristic for Increased Insolation 47
1.42 PV Array and Load Characteristic 48
1.43 Single-Phase Inverter 51
1.44 A Stand-alone Three-phase Four-wire Inverter 51
1.45 Typical Inverter Efficiency Curve 52
1.46 Bi-directional Inverter System 53
1.47 Series Hybrid Energy System 55
1.48 Switched PV-diesel Hybrid Energy System 57
1.49 Parallel PV-diesel Hybrid Energy System 58
1.50 Operating Modes for a PV Single-Diesel Hybrid Energy System 60
1.51 Voltage Source Inverter 65
1.52 Line-commutated Single-phase Inverter 66
1.53 Self-commutated Inverter with PWM Switching 67
1.54 PV Inverter with High Frequency Transformer 68
1.55 Half-bridge Diode-clamped Three Level Inverter 69
XIV
Figure Name of the Figure Page
1.56 Non-insulated Voltage Source 70
1.57 Non-insulated Current Source 70
1.58 Buck-converter with Half-bridge Transformer Link 71
1.59 Flyback Converter 72
1.61 Converter Using Parallel PV Units 72
1.62 Central Plant Inverter 73
1.63 Multiple String DC/DC Converter 74
1.64 Multiple String Inverters 74
1.65 Module Integrated Inverter 75
2.1 Horizontal-axis Wind Turbine Showing Major Components 86
2.2 Vertical-axis 33 m Diameter Wind Turbine Built and Tested by DOE/Sandia 87
National Laboratory during 1994 in Bushland
2.3 Rotor Efficiency vs V0/V ratio has a single maximum 89
2.4 Rotor Efficiency vs V0/V ratio for rotors with different numbers of blades 90
2.5 Wind Speed Variations with Height over Different Terrain 91
2.6 Baix Ebre Wind Farm and Control Center, Catalonia, Spain 92
2.7 Nacelle Details of a 3.6MW /104m Diameter Wind Turbine 94
2.8 A Large Nacelle under Installation 95
2.9 A 600kW Wind Turbine and Tower Dimensions with Specifications 96
2.10 A 600kW Wind Turbine and Tower Dimensions with Specifications 96
2.11 Tower Heights of Various Capacity Wind Turbines 97
2.12 WEG MS-2 Wind Turbine Installations at Myers Hill 98
2.13 Wind Turbine Torque vs Rotor Speed Characteristic 102
at Two Wind Speeds V1 and V2
2.14 Wind Turbine Power vs Rotor Speed Characteristic 103
at Two Wind Speeds V1 and V2
2.15 Rotor Efficiency and Annual Energy Production vs Rotor TSR 104
2.16 Maximum Power Operation Using Rotor Tip Speed Control Scheme 105
2.17 Maximum Power Operation Using Power Control Scheme 106
2.18 Optimum Tower Spacing in Wind Farms in Flat Terrain 107
XV
Figure Name of the Figure Page
2.19 Original Land Use Continues in a Wind Farm in Germany 108
2.20 Main Components of a Wind Turbine System 112
2.21 Power Characteristics of a Fixed Speed Wind Turbines 114
2.22 Roadmap for Wind Energy Conversion 114
2.23 Development of Power Semiconductor Devices in the Past and in the Future 115
2.24 Circuit Diagram of a Voltage Source Converter with IGBTs 116
2.25 Waveforms of a Bi-directional Active and Reactive Power of a VSC 117
2.26 Wind Turbine Systems without Power Converter, 119
but with Aerodynamic Power Control
2.27 The Startup of a Fixed-speed Wind Turbine 121
2.28 Wind Turbine Topologies with Partially Rated Power Electronics 123
and Limited Speed Range
2.29 Torque and Speed Characteristics of Rotor Resistance Controlled 124
Wound Rotor Induction Generator
2.30 Wind Turbine Systems with Full-scale Power Converters 126
2.31 Control of Wind Turbine with DFIG System 129
2.32 Basic Control of Active and Reactive Power in a Wind Turbine 131
with a Multipole Synchronous Generator System
2.33 Wind Farm Solutions 132
2.34 A Simple System with an Equivalent Wind Power Generator 136
Connected to a Network
2.35 World Total Installed Capacity in 2001-2010 148
2.36 Top 10 Countries by Growth Rate in 2008 and 2009 149
2.37 Top 10 Countries by Total Capacities in 2008 and 2009 150
2.38 Top 5 Countries in Offshore Wind 152
2.39 Continental Distribution 2007-2009 152
2.40 Areas of Potential Wind Power Technology Improvements 155
3.1 A Geothermal Reservoir 159
3.2 Schematic Model of a Hydrothermal Geothermal System 160
3.3 Ideal Hot Dry Rock Production Scheme 162
XVI
Figure Name of the Figure Page
3.4 Cross-section Schematic of a Geopressured Reservoir 164
3.5 Conceptual Design of Long Valley Magma Energy Exploratory Well 167
3.6 A Geothermal System 168
3.7 Turbine Generator 168
3.8 Lardarello, Tuscany, Northern Italy 169
3.9 Principle of Dry Steam Power Plant 170
3.10 The Geysers Power Plant, California 171
3.11 Simplified Flow Diagram for a Dry Steam Power Plant 172
3.12 Principle of Flash Steam Power Plant 173
3.13 East Mesa, California 173
3.14 Simplified Flow Diagram of a Single Flash Geothermal Power Plant 174
3.15 Double-Flash Power Plant Diagram 175
3.16 Principle of Binary Cycle Power Plant 175
3.17 Binary Power Plant Heat Exchanger 176
3.18 Schematic Diagram of Binary Power Plant 177
3.19 Integrated Single and Double Flash Facility 179
3.20 Combined Single and Double Flash Plants 180
3.21 Combined-Flash Binary System 181
3.22 Integrated Flash-Binary Plants 182
3.23 Simplified Representation of an EGS system where water is circulated 185
through hot dry rock and heat is mined in a closed loop
4.1 The Effect of the Moon on Tidal Range 190
4.2 Tidal Turbine against an Offshore Wind Turbine 194
4.3 Tidal Turbine Fundamental Types 195
4.4 Delta Stream Turbine 196
4.5 Evopod Tidal Turbine 197
4.6 Free Flow Turbine 197
4.7 Gorlov Helical Turbine 198
4.8 Lunar Energy Tidal Turbine 199
4.9 Neptune Tidal Stream Device 199
XVII
Figure Name of the Figure Page
4.10 Nereus Tidal Turbine 200
4.11 Solon Tidal Turbine 200
4.12 Open Centre Turbine 201
4.13 Pulse Tidal Hydrofoil 202
4.14 SeaGen 203
4.15 Stingray Tidal Energy Converter 203
4.16 Tidal Fence Davis Hydro Turbine 204
4.17 TidEl Stream Generator 205
4.18 Tidal Stream Turbine 205
4.19 The Various Wave Energy Technologies 211
4.20 Cross-sectional View of a Bottom-standing OWC 212
4.21 Schematic Representation of the Backward Bent Duct Buoy 214
4.22 Norwegian Heaving Buoy in Trondheim Fjord, 1983 216
4.23 Swedish Heaving Buoy with Linear Electrical Generator 217
4.24 L-10 Wave Energy Converter with Linear Electrical Generator 218
4.25 Schematic Representation of the IPS Buoy 220
4.26 Wavebob 221
4.27 The PowerBuoy Prototype Deployed off Santana, Spain, in 2008 221
4.28 Schematic Representation of the Archimedes Wave Swing 222
4.29 The Duck Version of 1979 Equipped with Gyroscopes 223
4.30 The Three-unit 3x750kW Pelamis Wave Farm 224
in Calm Sea off Northern Portugal, in 2008
4.31 Side and Plan Views of the McCabe Wave Pump 225
4.32 Front and Side Views of the PS Frog Mk 5 225
4.33 Schematic Representation of the Saraev 226
4.34 The Swinging Mace in Three Angular Positions 227
4.35 Oyster Protoype 228
4.36 Schematic Plan View of the Tapered Channel Wave Power Device 230
4.37 Plan View of Wave Dragon 231
4.38 Linear Generator with Point Absorber 232
XVIII
4.39 Single Line Diagram for Base Unit 233
4.40 System Option 1 233
4.41 System option 2 234
4.42 System Option 3 234
4.43 System Option 4 235
4.43 System Option Cable Cross vs Complexity 235
4.44 One Cable from Base Unit to Shore 236
4.45 One Cable from Farm to Shore 236
4.46 One Cable from Cluster to Shore 237
4.47 Subclusters and Clusters with Cable to Shore 237
4.48 Ocean Temperature Resource for OTEC 240
4.49 Diagram of Closed-cycle OTEC Plantship 241
4.50 Diagram of Open-cycle OTEC Power System 242
4.51 Layout Diagram of OTEC-1 Subsystems (Castellano,1981) 243
4.52 Hyberid-cycle OTEC Power System 244
4.53 Block Diagram of All Applications from OTEC Technology 245
4.54 Futurist Project Based on OTEC Technology 246
5.1 Biomass Gasification Combined Cycle System Schematic 249
5.2 Low-pressure Direct Gasifier 250
5.3 Indirect Gasifier 251
5.4 Direct-fired Biomass Electricity Generating System Schematic 253
5.5 Biomass Co-firing Retrofit Schematic for a Pulverized Coal Boiler System 254
5.6 Integrated Gasifier Combined Cycle 260
6.1 General Layout Form of a Hydropower Plant 264
6.2 Block Diagram Form of a Hydropower Plant 264
6.3 Huge Turbine Engines inside the Hoover Dam in Black Canyon, Nevada 265
6.4 Aeiral View of Hoover Dam, Nevada, Creating the Reservoir Lake Mead 266
6.5 Autonomous Variable Speed Micro Hydropower Station 269
7.1 Energy Supply Structure 274
7.2 Paths for Hydrogen 274
7.3 Hydrogen Filling Station Network with Electrolyser as Controllable Load 276
XIX
7.4 Solar-hydrogen Energy System 277
7.5 System without Hydrogen 277
7.6 System with Hydrogen 278
7.7 Construction of the 1.65MW Wind Turbine at the Morris Research Center 278
8.1 Projected 2010 Costs of Wind with Production Tax Credit 288
8.2 Wind Capacity Factor in 2006 by Region and Vintage of Wind Facility 288
8.3 PV Power Costs as Function of Module Efficiency and Cost 289
8.4 Fractional Energy PV Rooftop supply curves 290
for the Three U.S. Interconnections
8.5 Price, customer cost after subsidy, and number of PV installations per year in 291
California under California Energy Commission incentive programs
8.6 Supply curves describe the potential capacity and current busbar costs in terms 293
of nominal levelized cost of energy (LCOE) of concentrating solar power
8.7 Concentrating solar power supply curve based on 20 percent availability of city 294
peak demand and 20 percent availability of transmission capacity
8.8 Geothermal Supply Curve 295
8.9 Levelized Cost Estimate for Biomass and Solar PV Systems in 2010 and 2020 298
8.10 Levelized Cost Estimate for Wind and Solar Thermal Systems in 2010 and 2020 299
8.11 Learning Curve for PV Production 300
XX
LIST OF TABLES
Table Name of the Table Page
1.1 Comparison of Alternative Solar Thermal Power Technologies 10
1.2 Comparison of 10MWe Solar-II and 100MWe Prototype Design 12
1.4 Kelly Cosine Values of the Photocurrent in Silicon Cells 28
2.1 Friction Coefficient α of Various Terrains 91
2.2 World’s Major Wind Turbine Suppliers in 2004 100
2.3 Wind Turbine Topologies Market in 2001 127
2.4 Comparison of Four Wind Farm Topologies 134
2.5 Noise Levels of some Commonly Known Sources Compared with Wind Turbine 143
2.6 Offshore Overall Capacity in 2009 151
3.1 HDR Projects Worldwide 163
3.2 Estimates of U.S. Geothermal Resource Base to 10km depth by category 186
8.1 Current Cost Assumptions for Renewable Technologies (2007) 280
8.2 2020 Cost Projections and Comparisons 282
8.3 Levelized Cost of Energy for New Plants Coming Online in 2012 285
XXI
ABSTRACT
Energy production is a field that routes countries strategically and politically. Due to
the forthcoming global warming danger, and the extinction of fossil fuels, renewable energy
will extend the life length of next generations of humanity.
By means of energy, electricity generation technologies with respect to the
developments in science and engineering improve their efficiency day by day. Because of the
vitality of energy in global meaning, the feature of being inexhaustible of renewable energy
makes it popular and leads the new investments.
Meanwhile, utilization of renewable energy resources attracts and triggers the usage of
technologies which are environmentally friendly. Despite the installation and operation costs,
governments support investors and enterprisers by directing them to renewables, hence CO2
emissions is being reduced and in the near future, our dreams of having a green earth may be
realized optimistically.
Consequently, in this thesis, a general survey which focuses on the generation of
electricity from renewable sources such as solar, wind, geothermal, ocean, biomass,
hydropower and hydrogen resources is presented.
XXII
ACKNOWLEDGEMENTS
I would like to express my gratitude to the following people who have helped me
throughout this thesis duration, and my past up to now.
Prof. Dr. Mehmet Tümay and Assist. Prof. Dr. K. Çağatay Bayındır for giving me the
opportunity to work on this thesis. I would also like to thank Assist. Prof. Dr. K. Çağatay
Bayındır for his guidance, patience and support during thesis.
I would like to thank my teachers in primary, secondary and high school level who
taught me being true hearted, loyal and honest.
Special thanks to my best friends, Caner Yıldırım and Emrah Mehmet Güllüoğlu,
because of their endless support anytime I needed in the past and I never give up this
brotherhood which will exist in the future.
Finally, I would like to appreciate and present my everlasting thanks to my family for
their support financially and mentally, especially my mother, Ayşe Zor, because of her
unconditional love ,which can not be repaid, makes me proud everytime.
1
1 INTRODUCTION
1.1 Renewable Energy at a Glance
Energy is a vital element in human life. A secure, sufficient and accessible supply of
energy is very crucial for the sustainability of modern socities. The demand for the provision
of energy is increasing rapidly worldwide and the trend is likely to continue in the future.
Electricity producing systems presently in use across the world can be classified into
three main categories: fossil fuels, nuclear power and renewables.
Fossil fuels in their crude form, i.e. wood, coal and oil have traditionally been an
extensive used energy source. The present energy supply mainly depends on fossil energy
resources. The priority is to produce and transport fossil fuels in the most economical fashion
and to convert them cheaply into other types of energy in central power stations. The main
advantage of fossil fuel-based energies is their ready availability. Fossil energy can be used
wherever there is a consumer demand.
Nuclear power due to a number of reasons is not accesible to the vast majority of the
world and has found its application only within developed countries.
Renewable energy sources are energy resources that are inexhaustible within the time
horizon of humanity. Renewable energy resources are easily accessible to mankind around the
world. Renewable energy is not only available in wide range, but are also abundant in nature.
Renewables contribute less to air pollution, reduce the human health damages and can balance
the use of fossil fuels in order to save these for other applications and future use.
In 2005, the worldwide electricity generation was 17450 TWh out of which 40%
originated from coal, 20% from gas, 16% from nuclear, 16% from hydro, 7% from oil and
only 2% from renewable sources i.e. small hydro, wind, geothermal, etc. Renewable energy
sector is meeting at present 13.5% of the global energy demand and it is now growing faster
than the growth in overall energy market.
Some long-term scenarios postulate a rapidly increasing share of renewable
technologies (made up of solar, wind, geothermal, modern biomass, as well as the more
traditional source i.e. hydro). Under these scenarios renewables could meet up to 50% of the
2
total energy demand by mid-21st century with appropriate policies and new technology
developments.[1]
3
2 ELECTRIC GENERATION FROM RENEWABLE ENERGY SOURCES
2.1 Solar Energy and Electric Generation
2.1.1 Introduction
Among renewable sources, solar energy comes at the top of the list due to its
abundance, and more evenly distribution in nature than any other renewable energy types
such as wind, geothermal, hydro, wave and tidal energies.
The sun is the central point of our solar system; it has probably been in existence for 5
billion years and is expected to survive for a further 5 billion years. Nuclear fusion processes
create the radiant power of the sun. Various influences of the atmosphere reduce the
irradience, thus values measured on the surface of Earth are usually lower than the solar
constant.
In direct or indirect fashion, the sun is responsible for nearly all the energy sources to
be found on Earth. All the coal, oil and natural gas were produced by decaying plants millions
of years ago. In other words, the primary fossil fuels used today are really stored solar energy.
The heat from the sun also drives the wind, which is another renewable source of
energy. Wind arises because Earth‘s atmosphere is heated unevenly by the sun. The only
power sources that do not come from the sun‘s heat are the heat produced by radioactive
decay at Earth‘s core; ocean tides which are influenced by the moon‘s gravitational force; and
nuclear fusion and fission.
This inexhaustible source of solar energy can be utilized directly by solar thermal or
photovoltaic systems.[2]
2.1.2 Solar Thermal Power System
The solar thermal power system collects the thermal energy in solar radiation and uses
it at high or low temperatures. Low temperature applications include water and room heating
for commercial and residential buildings. High temperature applications concentrate the sun‘s
heat energy to produce steam for driving electrical generators. Concentrating solar power
4
(CSP) technology has the ability to store thermal energy from sunlight and deliver electric
power during dark or peak-demand periods.
Figure 1.1 Solar Thermal Power Plant Schematic for Generating Electricity(Ref:3)
Figure 1.1 is a schematic of a large scale solar thermal power station. In such a plant,
solar energy is collected by thousands of sun-tracking mirrors, called heliostats, which reflect
the sun‘s energy to a single receiver atop a centrally located tower. This enormous amount of
energy that is focused on the receiver tower is used to melt a salt at high temperature. The hot
molten salt is stored in a storage tank and used when needed to generate steam and drive a
turbine generator. After generating steam, the used molten salt, now at low temperature, is
returned to the cold salt storage tank. From here the salt is pumped to the receiver tower to be
heated again for the next thermal cycle. The usable energy extracted during such a thermal
cycle depends on the working temperatures. The maximum thermodynamic conversion
efficiency that can be theoretically achieved with the hot side temperature Thot and the cold
side temperature Tcold is given by the Carnot efficiency, which is as follows:
5
(1.1)
where the temperatures are in degrees Kelvin. A higher hot side working temperature and a
lower cold side exhaust temperature give higher plant efficiency to convert the captured solar
energy to electricity. The hot side temperature, however, is limited by the properties of the
working medium. The cold side temperature is largely determined by the cooling method and
the environment available to dissipate the exhaust heat.
A major benefit of this scheme is that it incorporates thermal energy storage for
several hours with no degradation in performance or for longer with some degradation. This
feature makes this technology capable of producing high value electricity in order to meet
peak demands. Moreover, compared with the solar photovoltaic system, the solar thermal
system is economical and more efficient because it eliminates use of costly PV cells and
alternating current (AC) inverters. It is, however, limited to large scale applications.[3]
2.1.2.1 Energy Collection
CSP research and development focuses on three types of concentrators, which use
different kinds of concentrating mirrors to convert the sun‘s energy into high temperature heat
energy:
1) Parabolic Trough
Figure 1.2 Parabolic Trough(Ref:3)
6
Parabolic trough power plants the first type of solar thermal power plant technologies
operating commercially. Nine large power plants called SEGS I to IX (Solar Electric
Generation System) were commissioned in California between 1984 and 1991. These power
plants have a nominal capacity of between 13.8 and 80 MW each, producing 354 MW in total.
The parabolic trough collector consists of large curved mirrors, which concentrate the
sunlight by a factor of 80 or more to a focal line. A series of parallel collectors are lined up in
rows 300–600 metres long. Multiple parallel rows form the solar collector field. The
collectors moved on one axis in order to follow the movement of the sun; this is called
tracking. A collector field can also be formed by long rows of parallel Fresnel collectors. In
the focal line of the collectors is a metal absorber tube, which usually is embedded into an
evacuated glass tube to reduce heat losses. A special selective coating that withstands high
temperatures reduces radiation heat losses.
In the Californian systems, thermo oil flows through the absorber tubes. These tubes
heat the oil to 400°C. A heat exchanger transfers the thermal energy from the oil to a water–
steam cycle (also called the Rankine cycle). A pump pressurizes the water and an economizer,
vaporizer and a superheater jointly produce superheated steam. This steam expands in a two-
stage turbine; between the high- and low-pressure parts of this turbine is a reheater. The
turbine itself drives an electrical generator that converts the mechanical energy into electrical
energy; the condenser after the turbine condenses the steam back to water, which allows the
closing of the cycle by feeding this water back into the initial pump.
Solar collectors can also produce superheated steam directly. This makes the thermo
oil superfluous and reduces costs due to savings associated with not using the expensive
thermo oil. Furthermore, heat exchangers are no longer needed. However, direct solar steam
generation is still at the prototype stage.
One important advantage of solar thermal power plants is that they can operate with
other means of water heating and thus a hybrid system can ensure security of supply. During
periods of insufficient irradiance, a parallel burner can be used to produce steam. Climate-
compatible fuels such as biomass or hydrogen produced by renewable energy can also fire
this parallel burner.
Figure 1.3 shows the principle of a parabolic trough solar power plant.[4]
7
Figure 1.3 Principle of a Parabolic Trough Solar Power Plant(Ref:4)
2) Central Receiver (Power Tower)
Figure 1.5 Central Receiver(Ref:3)
In the central receiver system, an array of field mirrors focus sunlight on a central
receiver mounted on a tower. To focus sunlight on the central receiver at all times, each
heliostat is mounted on a dual-axis sun tracker to seek a position in the sky that is midway
between the receiver and the sun. Compared with the parabolic trough, this technology
produces a much higher concentration and hence a higher temperature of the working
medium, usually a salt. Consequently, it yields higher Carnot efficiency and is well suited for
utility-scale power plants of tens of hundreds of megawatt capacity.
The first commercial plant is an 11 MW steam receiver plant developed by Abengoa
and inaugurated in March 2007 near Sevilla, Spain. Known as PS10, the plant has a 114-
8
meter tower and 624 heliostats, each 120 square meters. The plant uses a saturated steam
receiver and includes a 20 MWp water storage component. The developer reports a solarto-
electric conversion efficiency of 17 percent. Spain‘s electric feed-in law, set at 18 euro ¢/kWh
at all times, and EU and government subsidies for the plant totaling 6.2 million euros were the
main drivers for the plant. A 20 MW power tower plant is under construction adjacent to
PS10 at the Solúcar Solar Park. The solar field will consist of 1,255 heliostats, each 120
square meters, and a 160 meter high tower. Like PS10, the PS20 receiver will use steam
technology.[3]
Figure 1.6 Operational Schematic of Planta Solar 10 (PS10)(Ref:39)
Figure 1.7 PS10 Solar Thermal Tower Power Plant, Sevilla, Spain(Ref:39)
9
3) Parabolic Dish (Dish-Stirling Technology)
Figure 1.8 Principle of a Dish-Stirling System(Ref:38)
So-called Dish–Stirling systems can be used to generate electricity in the kilowatt
range. A parabolic concave mirror (the dish) concentrates sunlight. A two-axis tracked mirror
tracks the sun with the required high degree of accuracy. This is necessary in order to achieve
high efficiencies. The receiver at the focus is heated to 650°C. The heat absorbed drives a
Stirling motor, which converts the thermal energy into mechanical energy that is used to drive
a generator producing electricity. If sufficient sunlight is not available, combustion heat from
either fossil fuels or bio-fuels can also drive the Stirling engine and generate electricity. The
system efficiency of Dish–Stirling systems can reach 20 per cent or more. Some Dish–Stirling
system prototypes have been tested successfully in a number of countries; however, the cost
of electricity generation using these systems is much higher than that of trough or tower
power plants. Large-scale production might achieve significant cost reductions for Dish–
Stirling systems. Figure 1.8 shows the principle of a Dish–Stirling system.[4]
10
Figure 1.9 California Edison 25kW Dish-Stirling System(Ref:3)
The three alternative solar thermal technologies are compared in Table 1.1.[3]
Technology Solar Concentration Operating
Temperature
Thermodynamic
Cycle Efficiency
Parabolic Trough 100 300-500oC Low
Central Receiver 1000 500-1000 oC Moderate
Dish-Stirling 3000 800-1200 oC High
Table 1.1 Comparison of Alternative Solar Thermal Power Technologies(Ref:3)
2.1.2.2 Solar Chimney Power Plant
Figure 1.10 Principle of the Solar Chimney Power Plant(Ref:4)
11
A solar chimney power plant has a high chimney (tower), with a height of up to 1000
metres. This is surrounded by a large collector roof, up to 5000 metres in diameter, that
consists of glass or clear plastic supported on a framework. Towards its centre, the roof curves
upwards to join the chimney, creating a funnel. The sun heats up the ground and the air under
the collector roof, and the hot air follows the upward slope of the roof until it reaches the
chimney. There, it flows at high speed through the chimney and drives wind generators at the
bottom. The ground under the collector roof acts as thermal storage and can even heat up the
air for a significant time after sunset. The best efficiency of solar chimney power plants is
currently below 2 per cent. It depends mainly on the height of the tower. Due to the large area
required, these power plants can only be constructed on cheap or free land. Suitable areas
could be situated in desert regions. However, the whole power plant has additional benefits, as
the outer area under the collector roof can also be utilized as a greenhouse for agricultural
purposes. As with trough and tower plants, the minimum economic size of a solar chimney
power plant is in the multi-megawatt range. Figure 1.10 illustrates the principle of the solar
chimney power plant.[4]
In the arid flat land of southeastern Australia, EnviroMission of Melbourne has plans
for a solar wind tower (Figure 1.11). The sun-capture area is a 11-km2 glassroof enclosure.
The concrete chimney is 140 m in diameter and 1000 m tall. At the top, 32 wind turbines add
to a total capacity of 200 MW of electric power.[3]
Figure 1.11 Solar Heat Induced Wind Chimney Power Plant(Ref:37)
12
2.1.2.3 Commercial Power Plants
Commercial power plants using the solar thermal system are being explored in
capacities of a few hundred MWe. Based on the experience of operating Solar-II, the design
studies made by the National Renewable Energy Laboratory (NREL) have estimated the
performance parameters that are achievable for a 100-Mwe commercial plant. Table 1.2
summarizes these estimates and compares them with those achieved in an experimental 10-
MWe Solar-II power plant. The 100-Mwe prototype design studied showed that an overall
(solar radiation to AC electricity) conversion efficiency of 23% could be achieved in a
commercial plant using existing technology. For comparison, conventional coal thermal
plants typically operate at 40% overall efficiency, and the PV power systems have an overall
efficiency of 6 to 8% with amorphous silicon, 12 to 15% with crystalline silicon, and 20 to
25% with new thin-film multijunction PV cell technologies.
Performance Paramater Solar-II Plant 10 MWe (in
%)
Commercial Plant 100
MWe (in %)
Mirror reflectivity 90 94
Field efficiency 73 73
Mirror cleanliness 95 95
Receiver efficiency 87 87
Storage efficiency 99 99
Electromechanical
conversion efficiency of
generator
34 43
Auxiliary components
efficiency
90 93
Overall solar-to-electric
conversion efficiency
16 23
Table 1.2 Comparison of 10-MWe solar-II and 100-MWe prototype design (Ref:3)
13
The major conclusions of the studies to date are the following:
1. Designing and building plants with capacities as large as 200 MWe is possible,
based on the demonstrated technology to date. Future plants could be larger. A 200-MWe
plant would require about 3 mi2 of land.
2. The plant capacity factors up to 65% are possible.
3. About 20% of the conversion efficiency of solar radiation to AC electricity is
achievable annually
4. The thermal energy storage feature of the technology can meet peak demand on
utility lines.
5. Leveled energy cost is estimated to be 7 to 9 cents/kWh.
6. The capital cost of $2000/kWe for the first few commercial plants, and less for
future plants, is estimated. The fuel (solar heat) is free.
7. A comparable combined-cycle gas turbine plant would initially cost $1000/kWe,
and then the fuel cost would be added every year.
Compared with PV and wind power, solar thermal power technology is less modular.
Its economical size is estimated to be in the range of 100 to 300 MWe. The cost studies at
NREL have shown that a commercially designed utility-scale power plant using central
receiver power tower technology can produce electricity at a cost of 6 to 10 cents/kWh,
depending on the size. [3]
2.1.2.4 Potential Technology Developments and Recent Trends
A number of new CSP plants are under development or planned. In Spain, Abengoa is
constructing a 20 MW power tower plant next to the PS10 plant. Recent developments
include the AndaSol trough project, which is the first large-scale trough plant in Europe and
the first anywhere with molten salt storage. The salt is a mixture of 60 percent sodium nitrate
and 40 percent potassium nitrate. The Spanish government plans to have 10 GW of CSP
within the next 5 to 7 years. There are a number of upcoming projects for CSP in the United
States, particularly in California, which has an aggressive renewable portfolio standard (20
percent of investor-owned-utilities‘ loads to be served by renewables in 2010, with the same
target intended for public utilities). A number of utilities in the Southwest have formed a
consortium to pursue 250 MW of new CSP plants. The CSP industry estimates 13.4 GW
14
could be deployed for service by 2015. Purchase agreements for CSP of about 4 GW in the
United States had been signed as of February 2009, but there is probably twice that capacity
in planned projects. An evolving technology that relies on solar concentration is high-
temperature chemical processing. The concentrating component of these systems is identical
to that of concentrated solar thermal processes for power generation, but the receiver placed at
the focus of the concentrating reactor is designed to include a chemical reactor. These systems
can provide long-term storage of intermittent solar energy, such as storage in the form of fuel
or a commodity chemical. The global research community is pursuing a number of multiple-
step cycles, including production of hydrogen using water as the feedstock; decarbonization
of fossil fuels; gasification of biomass; production of metals including aluminum; and
processing and detoxification of waste. These systems are most likely to become cost-
competitive when a cost is associated directly with reduction in carbon emissions.[5]
2.1.2.5 Future Expectations
1) Short Term: Present to 2020
CSP technologies are commercially available, and in the past few years new plants
have been deployed in the United States and abroad, with trough systems dominating the U.S.
CSP market. With nearly 4 GW of signed purchase agreements and additional planned
projects, along with favorable financial policies, it is reasonable to expect significant growth
by 2020. Most of the new plants are solar-only plants and do not include fossil fuel backup
on-site. During this timeframe, with the anticipated growth rate, CSP plants will continue to
provide peaking power. With even more expanded growth, CSP technologies will probably be
hybridized with fossil fuel-fired components to share the generation portion of a fossil fuel
facility, as well as continue to serve as peaking plants.
In the short term, incremental design improvements will drive down costs and reduce
uncertainty in performance predictions. With more systems installed, there will be increased
economies of scale, both for plant sites and for manufacturing. Increasing the reflector size
and working with low-cost structures, better optics, and high-accuracy tracking may reduce
the cost of the heliostat or dish concentrators. There may also be design improvements in
receiver technology. Until 2020, long term thermal storage, extending over days rather than
15
hours, will not be a major roadblock. However, new storage technologies will be needed in
the longer term to make solar dispatchable. Storage technologies, such as concrete, graphite,
phase-change materials, molten salt, and thermocline storage, show promise. The number of
molten salt tanks providing thermal storage on the order of hours will likely increase, as
ancillary equipment such as pumps and valves are improved for greater reliability. Molten salt
receivers, which provide storage at about 550°C to power a turbine, can extend storage up to
12 hours, but there are no molten salt receiver plants at this time.
Availability of water may not be a major deterrent, as water withdrawals are not large
with CSP. However, CSP consumes at least as much water as some conventional generation
technologies. The primary water uses at a Rankine steam solar power plant are for condensate
makeup, cooling for the condenser, and washing of mirrors. Historically, parabolic trough
plants have used wet-cooling towers for cooling. With wet-cooling, the cooling tower makeup
represents approximately 90 percent of the raw water consumption. Steam cycle makeup
represents approximately 8 percent of raw water consumption, and mirror washing represents
the remaining 2 percent. Dust-resistant glass is being explored as a possible means to reduce
the mirror washing requirement.[5]
2) Medium Term: 2020 to 2035
New demands on existing transmission systems may require new or upgraded lines.
Longer-term storage on the order of days will be needed if CSP is to be a major source of
electricity. Research and development will continue to accelerate design improvement and
drive down manufacturing costs. Development of less expensive yet durable optical materials
will help control cost and water use, including selective surfaces for receivers in towers and
dishes, transparent polymeric materials that are cheaper than glass, and reflective surfaces that
prevent dust deposition.[5]
16
3) Long Term: After 2035
In the longer term, the use of concentrated solar energy to produce fuels and thus
provide storage via a number of reversible chemical reactions is promising. Fuels produced
from concentrated solar energy may provide a means of generating electricity during periods
of low insolation or at night. Much of the scientific work to date has focused on the
production of hydrogen and synthesis gas through various processes, including direct
thermolysis of water and a number of metal oxide reduction/oxidation cycles. Direct water
splitting is not feasible, because the required temperatures exceed the capability and material
limits of modern concentrating systems, and separation of the products at such temperatures is
impractical. Multiple-step metal oxide reactions are more promising. A two-step process
involves endothermic dissociation of a metal oxide (MxOy) to the metal (M) and oxygen in a
solar reactor, followed by hydrolysis of the metal to produce hydrogen and the corresponding
metal oxide. Carbothermal reduction in a solar reactor reduces the required operating
temperature and yields syngas. The process is technically feasible, but has not been
demonstrated at production scale. Gasification of cellulosic biomass is another promising
route to produce synthesis gas.[5]
2.1.3 Photovoltaics
2.1.3.1 Introduction
Photovoltaic means the direct conversion of sunlight to electricity. The common
abbreviation for photovoltaic is PV.
The history of photovoltaics goes back to the year 1839, when Becquerel discovered
the photo effect, but in that century the technology was not available to exploit this discovery.
The semiconductor age began about 100 years later. After Shockley had developed a model
for the p–n junction, Bell Laboratories produced the first solar cell in 1954. The efficiency of
this cell was about 5 per cent. Initially, cost was not a major issue, because the first cells were
designed for space applications in order to convert sunlight to electricity for earth-orbiting
satellites.
17
In the following years, solar cell efficiency increased continuously; laboratory silicon
solar cells have reached efficiencies of around 25 per cent today. The main material used in
the construction of solar cells is still silicon, but other materials have been developed, either
for their potential for cost reduction or their potential for high efficiency. Costs have
decreased significantly in recent decades; nevertheless, photovoltaic electricity generating
costs are still higher than the costs of conventional power plants. Due to high growth rates in
the photovoltaic sector, cost reduction will continue.
Photovoltaics offer the highest versatility among renewable energy technologies. One
advantage is the modularity. All desired generator sizes can be realized, from the milliwatt
range for the supply of wristwatches or pocket calculators to the megawatt range for the
public electricity supply.
Many photovoltaic applications are built into consumer appliances or relate to leisure
activities or off-grid site supply, for example, telecommunications or solar home systems. In
several countries, particularly in Japan and Germany, large governmental programmes were
initiated, advancing grid-connected installations. Tens of thousands of grid-connected systems
that have been installed since the early 1990s have proven the suitability of the technology.
The potential for photovoltaic installations is enormous. Theoretically, PV systems could
cover the whole electricity demand of most countries in the world.[4]
2.1.3.2 PV Cell
The physics of the PV cell is very similar to that of the classical diode with a pn
junction.
Figure 1.12 PV Effect Converts the Photon Energy into Voltage across the P-N Junction(Ref:3)
18
When the junction absorbs light, the energy of absorbed photons is transferred to the
electron–proton system of the material, creating charge carriers that are separated at the
junction. The charge carriers may be electron–ion pairs in a liquid electrolyte or electron–hole
pairs in a solid semiconducting material. The charge carriers in the junction region create a
potential gradient, get accelerated under the electric field, and circulate as current through an
external circuit. The square of the current multiplied by the resistance of the circuit is the
power converted into electricity. The remaining power of the photon elevates the temperature
of the cell and dissipates into the surroundings.
Figure 1.13 Basic Construction of PV Cell with Performance Enhancing Features(Ref:3)
Figure 1.13 shows the basic cell construction. Metallic contacts are provided on both
sides of the junction to collect electrical current induced by the impinging photons. A thin
conducting mesh of silver fibers on the top (illuminated) surface collects the current and lets
the light through. The spacing of the conducting fibers in the mesh is a matter of compromise
between maximizing the electrical conductance and minimizing the blockage of the light.
Conducting foil (solder) contact is provided over the bottom (dark) surface and on one edge of
the top surface. In addition to the basic elements, several enhancement features are also
included in the construction. For example, the front face of the cell has an antireflective
coating to absorb as much light as possible by minimizing the reflection. The mechanical
protection is provided by a cover glass applied with a transparent adhesive.[3]
19
1) PV Cell Technologies
In comparing alternative power generation technologies, the most important measure
is the energy cost per kilowatthour delivered. In PV power, this cost primarily depends on two
parameters: the PV energy conversion efficiency, and the capital cost per watt capacity.
Together, these two parameters indicate the economic competitiveness of the PV electricity.
The conversion efficiency of the PV cell is defined as follows:
(1.2)
The primary goals of PV cell research and development are to improve the conversion
efficiency and other performance parameters to reduce the cost of commercial solar cells and
modules. The secondary goal is to significantly improve manufacturing yields while reducing the
energy consumption and manufacturing costs, and reducing the impurities and defects.[3]
a) Crystalline Silicon Solar Cells
Historically, crystalline silicon (c-Si) has been used as the light-absorbing
semiconductor in most solar cells, even though it is a relatively poor absorber of light and
requires a considerable thickness (several hundred microns) of material. Nevertheless, it has
proved convenient because it yields stable solar cells with good efficiencies (11-16%, half to
two-thirds of the theoretical maximum) and uses process technology developed from the huge
knowledge base of the microelectronics industry.
Figure 1.14 Crystalline silicon wafers(Ref:6)
20
Two types of crystalline silicon are used in the industry. The first is monocrystalline,
produced by slicing wafers (up to 150mm diameter and 350 microns thick) from a high-purity
single crystal boule. The second is multicrystalline silicon, made by sawing a cast block of
silicon first into bars and then wafers. The main trend in crystalline silicon cell manufacture is
toward multicrystalline technology. For both mono- and multicrystalline Si, a semiconductor
homojunction is formed by diffusing phosphorus (an n-type dopant) into the top surface of the
boron doped (p-type) Si wafer. Screen-printed contacts are applied to the front and rear of the
cell, with the front contact pattern specially designed to allow maximum light exposure of the
Si material with minimum electrical (resistive) losses in the cell.
The most efficient production cells use monocrystalline c-Si with laser grooved,
buried grid contacts for maximum light absorption and current collection. Each c-Si cell
generates about 0.5V, so 36 cells are usually soldered together in series to produce a module
with an output to charge a 12V battery.[6]
b) Thin Film Solar Cells
The high cost of crystalline silicon wafers (they make up 40-50% of the cost of a
finished module) has led the industry to look at cheaper materials to make solar cells.
The selected materials are all strong light absorbers and only need to be about 1micron
thick, so material‘s costs are significantly reduced. The most common materials are
amorphous silicon (Figure 1.15) (a-Si, still silicon, but in a different form), or the
polycrystalline materials: cadmium telluride (CdTe) and copper indium (gallium) diselenide
(CISorCIGS).
Each of these three is agreeable to large area deposition (on to substrates of about 1
meter dimensions) and hence high volume manufacturing. The thin film semiconductor layers
are deposited on to either coated glass or stainless steel sheet.
Thin film technologies are all complex. They have taken at least twenty years,
supported in some cases by major corporations, to get from the stage of promising research
(about 8% efficiency at 1 cm2 scale) to the first manufacturing plants producing early product.
21
Figure 1.15 Amorphous silicon thin film(Ref:6)
Amorphous silicon is the most well-developed thin film technology to-date and has an
interesting avenue of further development through the use of "microcrystalline" silicon which
seeks to combine the stable high efficiencies of crystalline Si technology with the simpler and
cheaper large area deposition technology of amorphous silicon.
However, conventional c-Si (crystalline silicon) manufacturing technology has
continued its steady improvement year by year and its production costs are still falling too.
The emerging thin film technologies are starting to make significant in-roads in to grid
connect markets, particularly in Germany, but crystalline technologies still dominate the
market. Thin films have long held in low power (<50W) and consumer electronics
applications, and may offer particular design options for building integrated applications.[6]
c) Concentrator Cell
In an attempt to improve conversion efficiency, sunlight is concentrated tens or
hundreds of times the normal intensity by focusing on a small area using low-cost lenses
(Figure 1.16).
22
Figure 1.16 Lens Concentrating the Sunlight on a Small Area Reduces the Need for Active Cell
Material (Ref:3)
A primary advantage of this is that such a cell requires a small fraction of area
compared to the standard cells, thus significantly reducing the PV material requirement.
However, the total sunlight collection area remains approximately the same for a given power
output. Besides increasing the power and reducing the size or number of cells, the
concentrator cell has the additional advantage that the cell efficiency increases under
concentrated light up to a point. Another advantage is its small active cell area. It is easier to
produce a high-efficiency cell of small area than to produce large-area cells with comparable
efficiency. An efficiency of 37% has been achieved in a cell designed for terrestrial
applications, which is a modified version of the triple-junction cell that Spectrolab developed
for space applications. On the other hand, the major disadvantage of the concentrator cell is
that it requires focusing optics, which adds to the cost. Concentrator PV cells have seen a
recent resurgence of interest in Australia and Spain.[3]
23
2.1.3.3 Module and Array
The solar cell described previously is the basic building block of the PV power
system. Typically, it is a few square inches in size and produces about 1 W of power. To
obtain high power, numerous such cells are connected in series and parallel circuits on a panel
(module) area of several square feet (Figure 1.17).
Figure 1.17 Several PV Cells Constitute a Module and Several Modules Constitute an Array(Ref:3)
The solar array or panel is defined as a group of several modules electrically
connected in a series–parallel combination to generate the required current and voltage.
Figure 1.18 shows the actual construction of a module in a frame that can be mounted on a
structure.
24
Figure 1.18 Construction of PV Cell: (1) Frame, (2) Weatherproof Junction Box, (3) Rating Plate, (4) Weather Protection for 30 year life, (5)
thin polycrystalline silicon films, and organic inks.
Concentrator systems use only direct, rather than diffuse or global, solar radiation;
therefore, their areas of best application (e.g., in the southwestern United States) are more
limited than those for flat plates. There is also ongoing research to improve the long-term
reliability of concentrator systems and to develop standard tests for concentrator cells and
systems. Thus, most of today‘s remote and distributed markets for PV systems are not suitable
for concentrator systems.
By far the fastest-growing segment of the PV industry is that based on casting large,
multicrystalline ingots in some crucible that is usually consumed in the process.
Manufacturers routinely fabricate large multicrystalline silicon solar cells with efficiencies in
the 13 to 15 percent range; small-area research cells are 20 percent efficient. Silicon ribbon or
sheet technologies avoid the costs and material losses associated with slicing ingots. The
present commercial approaches in the field are the edge-defined, film-fed growth of silicon
ribbons and the string ribbon process. Full-scale production of silicon modules based on
micron-sized silicon spheres was recently announced. In this process, submillimeter-size
silicon spheres are bonded between two thin aluminum sheets, processed into solar cells, and
packaged into flexible, lightweight modules. Another approach uses a micromachining
technique to form deep narrow grooves perpendicular to the surface of a 1- to 2-mm thick
single-crystal silicon wafer. This technique results in large numbers of thin (50 μm), long
(100 mm), and narrow (nearly the original wafer thickness) silicon strips that are processed
into solar cells just prior to separation from the wafer. In another technique, a carbon foil is
pulled through a silicon melt, resulting in the growth of two thin silicon layers on either side
of the foil. After the edges are scribed and the sheet is cut into wafers, the carbon foil is
burned off, resulting in two silicon wafers (150 μm thick) for processing into solar cells.
Thin-film technologies have the potential for substantial cost advantages over wafer-
based crystalline silicon, because of factors such as lesser material use due to direct band
gaps, fewer processing steps, and simpler manufacturing technology for large-area modules.
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Thin-film technologies commonly require less or no high-cost crystalline Si. Many of the
processes are high throughput and continuous (e.g., roll-to-roll); they usually do not involve
high temperatures and, in some cases, do not require high-vacuum deposition equipment.
Module fabrication, involving the interconnection of individual solar cells, is usually carried
out as part of the film-deposition processes. The major systems are amorphous silicon,
cadmium telluride, and copper indium diselenide (CIS) and related alloys. Future directions
include multijunction thin films aimed at significantly higher conversion efficiencies, better
transparent conducting oxide electrodes, and thin polycrystalline silicon films. [5]
1) Dye-sensitized Solar Cells
The dye-sensitized solar cell has its foundation in photochemistry rather than in solid-
state physics. In this device, also called the ―Grätzel cell‖ after its Swiss inventor, organic dye
molecules are adsorbed on a nanocrystalline titanium dioxide (TiO2) film, and the nanopores
of the film are filled with a redox electrolyte. The dyes absorb solar photons to create an
excited molecular state that can inject electrons into the TiO2. The electrons percolate through
the nanoporous TiO2 film and are collected at a transparent electrode. The oxidized dye is
reduced back to its initial state by accepting electrons from the redox relay via ionic transport
from a metal counter-electrode; this completes the circuit and electrical power is delivered in
the external circuit. Dye-sensitized solar cells are very attractive, because of the very low cost
of the constituent materials (TiO2 is a common material used in paints and toothpaste) and the
potential simplicity of their manufacturing process. Additionally, sensitized solar cells are
tolerant to impurities, which allow ease in scaling up the production. Laboratory-scale devices
of 11 percent efficiency have been demonstrated, but larger modules are typically less than
half that efficient. Stability of the devices (e.g., dye materials and electrolyte) while
maintaining high efficiency is an ongoing research issue. [5]
2) Organic and Nanotechnology Solar Cells
Organic semiconductors hold promise as building blocks for organic electronics,
displays, and very low-cost solar cells. In an organic solar cell, light creates a bound electron-
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hole pair, called an exciton, which separates into an electron on one side and a hole on the
other side of a material interface within the device. Polymers, dendrimers, small molecules
and dyes, and inorganic nanostructures are materials that can be used. Organic solar cells can
be about 10 times thinner than thin film solar cells. Consequently, organic solar cells could
lower costs in four ways: low-cost constituent elements (e.g., carbon, hydrogen oxygen, and
nitrogen sulfur); reduced material use; high conversion efficiency; and high-volume
production techniques (e.g., high-rate deposition on roll-to-roll plastic substrates). Organic
solar cells are the focus of DOE‘s research goals for 2020. Research examples in organic solar
cells include quantum dots embedded in an organic polymer, liquid-crystal (smallmolecule)
cells, and small-molecule chromophore cells. Solar cell efficiencies to date are modest (less
than 3 to 5 percent). Unresolved problems associated with this technology include large
optical bandgap, unoptimized band offset, and fast degradation rate due to photoxidation,
interfacial instability delamination, interdiffusion, and morphological changes.
The use of nanotechnology for PV is especially promising, because the optical and
electronic properties of the materials could be tuned by controlling particle size and shape.
They may be easy to manufacture when the nanoparticles are produced by means of chemical
solution. Some of these concepts are already being pursued commercially. Long-term stability
of these devices is another major issue to resolve, along with increasing the efficiency.[5]
2.1.3.7 Future Expectations
1) Short Term: Present to 2020
Currently, polycrystalline silicon PV technologies are well developed and
commercially available. Today, the PV industry is capacity-limited. Given its higher cost than
fossil-based electricity now and for the foreseeable future, deployment of the existing PV
technology will only be constrained by the extent of financial incentives and the absence of
policies that encourage use of solar electricity technology in the nation‘s electricity mix.
Improvement in thin-film efficiencies, which are lower-cost but lower efficiency compared
with Si-base cells, is important for the development of this technology.
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Balance-of-systems costs must be brought down significantly to reduce the whole cost
of a solar electricity system. For example, in California at present, approximately 50 percent
or more of the total installed cost of a rooftop PV system is not in the module cost, but in the
costs of installation, and of the inverter, cables, support structures, grid hook-ups, and other
components. These costs must come down through innovative system-integration approaches,
or this aspect of a PV system will set a floor on the price of a fully installed PV system, either
free-standing or in a rooftop installation. In addition, PV interface devices must improve,
including integrated PV inverters; disconnect, metering, and communications interfaces;
direct PV-DC devices such as Dcdriven end-use devices; and master controllers for use in
buildings with PV, storage, and end users. [5]
2) Medium Term: 2020 to 2035
Cost reductions are needed through new technology development and in the
manufacturing that will accompany the scale-up of existing PV technologies. For example,
new technologies are being developed to make conventional solar cells by using
nanocrystalline inks of precursor as well as semiconducting materials. New cell structures are
being investigated to produce higher efficiency at lower cost.
Thin film technologies have the potential for substantial cost reduction over current wafer-
based crystalline silicon methods, because of factors such as lower material use (due to direct
band gaps), fewer processing steps, and simpler manufacturing technology for large-area
modules. Thin film technologies have many advantages, such as high throughput and
continuous production rate, lower-temperature and non-vacuum processes, and ease of film
deposition. Even lower costs are possible with plastic organic solar cells, dye-sensitized solar
cells, nanotechnology-based solar cells, and other new photovoltaic technologies. [5]
3) Long Term: After 2035
Widespread deployment of PV technology will depend on the ability to reach scale in
manufacturing capacity and achieve cost reductions using technologies for ultralow-cost
module production at acceptable efficiency. Reaching ultralow costs will probably require
learning-curve-based cost reduction, along with development of future generations of PV
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materials and systems to increase efficiency. Next-generation PV cells will most likely have
structures that will make optimal use of the total solar spectrum to maximize light-to-
electricity conversion efficiency.[5]
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2.2 Wind Energy and Electric Generation
2.2.1 Introduction
Wind energy is an indirect form of solar energy in contrast to the direct solar energy.
Solar irradiation causes temperature differences on Earth and these are the origin of winds.
The wind itself can be used by technical systems. Wind can reach much higher power
densities than solar irradiance: 10 kW/m2 during a violent storm and over 25 kW/m
2 during a
hurricane, compared with the maximum terrestrial solar irradiance of about 1 kW/m2.
However, a gentle breeze of 5 m/s (18 km/h, 11.2 mph) has a power density of only 0.075
kW/m2.
The history of wind power goes back many centuries. Wind power was used for
irrigation systems 3000 years ago. Historical sources give evidence for the use of wind power
for grain milling in Afghanistan in the 7th century. These windmills were very simple systems
with poor efficiencies compared to today‘s systems. In Europe, wind power became important
from the 12th century onwards. Windmills were improved over the following centuries. Tens
of thousands of windmills were used for land drainage in The Netherlands in the 17th and
18th centuries; these mills were sophisticated and could track the wind autonomously. In the
19th century numerous western windmills were used in North America for water pumping
systems. Steam powered machines and internal combustion engines competed with wind
power systems from the beginning of the 20th century. Finally, electrification made wind
power totally redundant. The revival of wind power began with the oil crises of the 1970s. In
contrast to the mechanical wind power systems of past centuries, modern wind converters
almost exclusively generate electricity. Germany became the most advanced country for wind
technology development in the 1990s. State of the art wind generators have reached a high
technical standard and now have powers exceeding 4 MW. The German wind power industry
alone has created more than 45,000 new jobs and has reached an annual turnover of more than
€3500 million.
The high growth rate of the wind power industry indicates that wind power will reach
a significant share of the electricity supply within the next two decades, and not only in
Germany and Denmark (the other significant centre of development). Therefore, the main
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deciding factors are the legislative conditions. For instance, the Renewable Energy Sources
Acts in Germany and Spain were the basic conditions for the wind power boom in these two
countries. In most countries the potential for wind power utilization is enormous. Germany
could provide one-third of its electricity demand and the UK could theoretically cover even
more than its whole electricity demand with wind power.
Germany can be taken as example of the rapid development of wind power and its
integration into the electricity supply structures. Most of the established utilities fear the
competition and complain about the problems with line regulation that result from
fluctuations in wind power; however, some utilities have demonstrated that improved wind
speed forecasts can solve these problems. Even some environmental organizations protest
against new wind installations. Their reasons are conservation, nature or noise protection;
indeed, some of their arguments are justifiable. On the other hand, wind power is one of the
most important technologies for stopping global warming. No doubt, wind generators change
the landscape, but if we do not get global warming under control, coastal areas that would be
protected by the reduction in global warming resulting from wind generator installation will
most likely not exist far into the future.[4]
2.2.2 Wind Speed and Energy
The sun heats up air masses in the atmosphere. The spherical shape of the Earth, the
Earth‘s rotation and seasonal and regional fluctuations of the solar irradiance cause spatial air
pressure differentials. These are the source of air movements that create winds.
Technically, the wind turbine captures the wind‘s kinetic energy in a rotor consisting
of two or more blades mechanically coupled to an electrical generator. The turbine is mounted
on a tall tower to enhance the energy capture. Numerous wind turbines are installed at one site
to build a wind farm of the desired power generation capacity. Obviously, sites with steady
high wind produce more energy over the year. [4]
Two distinctly different configurations are available for turbine design, the horizontal-
axis configuration (Figure 2.1) and the vertical-axis configuration (Figure 2.2).
The horizontal-axis machine has been the standard in Denmark from the beginning of
the wind power industry. Therefore, it is often called the Danish wind turbine.
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Figure 2.1 Horizontal-axis Wind Turbine Showing Major Components(Ref:3)
The vertical-axis machine has the shape of an egg beater and is often called the
Darrieus rotor after its inventor. It has been used in the past because of its specific structural
advantage.
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Figure 2.2 Vertical-axis 33 m Diameter Wind Turbine Built and Tested by DOE/Sandia National
Laboratory during 1994 in Bushland, TX. (Ref:3)
However, most modern wind turbines use a horizontal axis design. Except for the
rotor, most other components are the same in both designs, with some differences in their
placements.[3]
2.2.2.1 Power Extracted from the Wind
The actual power extracted by the rotor blades is the difference between the upstream
and downstream wind powers. Using Equation 2.1, this is given by the following equation in
units of watts:
(2.1)
where
Po = mechanical power extracted by the rotor, i.e., the turbine output power,
V = upstream wind velocity at the entrance of the rotor blades, and
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Vo = downstream wind velocity at the exit of the rotor blades.
Let us leave the aerodynamics of the blades to the many excellent books available on
the subject, and take a macroscopic view of the airflow around the blades. Macroscopically,
the air velocity is discontinuous from V to Vo at the ―plane‖ of the rotor blades, with an
―average‖ of ½(V + Vo). Multiplying the air density by the average velocity, therefore, gives
the mass flow rate of air through the rotating blades, which is as follows:
(2.2)
The mechanical power extracted by the rotor, which drives the electrical generator, is
therefore:
(2.3)
The preceding expression is algebraically rearranged in the following form:
(2.4)
The power extracted by the blades is customarily expressed as a fraction of the upstream wind
power in watts as follows:
(2.5)
where
(2.6)
Due to Equation 2.5, we can say that Cp is the fraction of the upstream wind power that is
extracted by the rotor blades and fed to the electrical generator. The remaining power is
dissipated in the downstream wind. The factor Cp is called the power coefficient of the rotor
or the rotor efficiency.
For a given upstream wind speed, Equation 2.6 clearly shows that the value of Cp depends on
the ratio of the downstream to the upstream wind speeds (Vo/V). A plot of power vs. (Vo/V)
shows that Cp is a single-maximum-value function (Figure 2.3).
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Figure 2.3 Rotor Efficiency vs V0/V ratio has a single maximum. Rotor efficiency is the fraction of
available wind power extracted by the rotor and fed to the electrical generator. (Ref:3)
It has the maximum value of 0.59 when the Vo/V ratio is one third. The maximum power is
extracted from the wind at that speed ratio, i.e., when the downstream wind speed equals one
third of the upstream speed. Under this condition (in watts):
(2.8)
The theoretical maximum value of Cp is 0.59. Cp is often expressed as a function of the rotor
tip-speed ratio (TSR) as shown in Figure 2.4. [3]
TSR is defined as the linear speed of the rotor‘s outermost tip to the upstream wind speed.
The aerodynamic analysis of the wind flow around the moving blade with a given pitch angle
establishes the relation between the rotor tip speed and the wind speed. In practical designs,
the maximum achievable Cp ranges between 0.4 and 0.5 for modern high speed two-blade
turbines, and between 0.2 and 0.4 for slow-speed turbines with more blades. If we take 0.5 as
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the practical maximum rotor efficiency, the maximum power output of the wind turbine
becomes a simple expression (in watts per square meter of swept area):
(2.9)
Figure 2.4 Rotor Efficiency vs V0/V Ratio for Rotors with Different Numbers of Blades. Two blade
rotors have the highest efficiency. (Ref:3)
2.2.1.2 Effect of Hub Height
The wind shear at a ground-level surface causes the wind speed to increase with height
in accordance with the following expression:
(2.8)
where
V1 = wind speed measured at the reference height h1,
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V2 = wind speed estimated at height h2, and
α = ground surface friction coefficient.
The friction coefficient α is low for smooth terrain and high for rough ones. The values of α
for typical terrain classes are given in Table 2.1, and their effects on the wind speed at various
heights are plotted in Figure 2.5.
Terrain Type Friction Coefficient α
Lake, ocean, and smooth, hard ground 0.10
Foot-high grass on level ground 0.15
Tall crops, hedges and shrubs 0.20
Wooded country with many trees 0.25
Small town with some trees and shrubs 0.30
City area with tall buildings 0.40
Table 2.1 Friction Coefficient α of Various Terrains(Ref:3)
Figure 2.5 Wind Speed Variations with Height over Different Terrain. Smooth, Low-friction Terrain with
Low α Develops a Thinner Layer of Slow Wind near the Surface and High Wind at Heights(Ref:3)
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It is noteworthy that the offshore wind tower, being in low-α terrain, always sees a
higher wind speed at a given height and is less sensitive to tower height.[3]
2.2.3 Wind Power Systems
2.2.3.1 System Components
The wind power system comprises one or more wind turbine units operating
electrically in parallel. Each turbine is made of the following basic components:
• Tower structure
• Rotor with two or three blades attached to the hub
• Shaft with mechanical gear
• Electrical generator
• Yaw mechanism, such as the tail vane
• Sensors and control
Because of the large moment of inertia of the rotor, design challenges include starting, speed
control during the power-producing operation, and stopping the turbine when required. The
eddy current or another type of brake is used to halt the turbine when needed for emergency
or for routine maintenance.
In a modern wind farm, each turbine must have its own control system to provide
operational and safety functions from a remote location (Figure 2.6).
Figure 2.6 Baix Ebre Wind Farm and Control Center, Catalonia, Spain. (Ref:3)
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It also must have one or more of the following additional components:
• Anemometers, which measure the wind speed and transmit the data to the controller.
• Numerous sensors to monitor and regulate various mechanical and electrical parameters. A
1-MW turbine may have several hundred sensors.
• Stall controller, which starts the machine at set wind speeds of 8 to 15 mph and shuts off at
50 to 70 mph to protect the blades from overstressing and the generator from overheating.
• Power electronics to convert and condition power to the required standards.
• Control electronics, usually incorporating a computer.
• Battery for improving load availability in a stand-alone plant.
• Transmission link for connecting the plant to the area grid.
The following are commonly used terms and terminology in the wind power industry:
Low-speed shaft: The rotor turns the low-speed shaft at 30 to 60 rotations per minute (rpm).
High-speed shaft: It drives the generator via a speed step-up gear.
Brake: A disc brake, which stops the rotor in emergencies. It can be applied mechanically,
electrically, or hydraulically.
Gearbox: Gears connect the low-speed shaft to the high-speed shaft and increase the turbine
speed from 30 to 60 rpm to the 1200 to 1800 rpm required by most generators to produce
electricity in an efficient manner. Because the gearbox is a costly and heavy part, design
engineers are exploring slow-speed, direct-drive generators that need no gearbox.
Generator: It is usually an off-the-shelf induction generator that produces 50- or 60-Hz AC
power.
Nacelle: The rotor attaches to the nacelle, which sits atop the tower and includes a gearbox,
low- and high-speed shafts, generator, controller, and a brake. A cover protects the
components inside the nacelle. Some nacelles are large enough for technicians to stand inside
while working.
Pitch: Blades are turned, or pitched, out of the wind to keep the rotor from turning in winds
that have speeds too high or too low to produce electricity.
Upwind and downwind: The upwind turbine operates facing into the wind in front of the
tower, whereas the downwind runs facing away from the wind after the tower.
Vane: It measures the wind direction and communicates with the yaw drive to orient the
turbine properly with respect to the wind.
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Yaw drive: It keeps the upwind turbine facing into the wind as the wind direction changes. A
yaw motor powers the yaw drive. Downwind turbines do not require a yaw drive, as the wind
blows the rotor downwind.[3]
1) Towers
The wind tower supports the rotor and the nacelle containing the mechanical gear, the
electrical generator, the yaw mechanism, and the stall control.
Figure 2.7 Nacelle Details of a 3.6MW / 104 m Diameter Wind Turbine(Ref:3)
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Figure 2.7 depicts the component details and layout in a large nacelle, and Figure 2.8 shows
the installation on the tower.
Figure 2.8 A Large Nacelle under Installation. (Ref:3)
The height of the tower in the past has been in the 20 to 50 m range. For medium- and large-
sized turbines, the tower height is approximately equal to the rotor diameter, as seen in the
dimension drawing of a 600-kW wind turbine (Figure 2.9).
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Figure 2.10 A 600 kW Wind Turbine and Tower Dimensions with Specifications(Ref:3)
Small turbines are generally mounted on the tower a few rotor diameters high.
Otherwise, they would suffer fatigue due to the poor wind speed found near the ground
surface. Figure 2.11 shows tower heights of various-sized wind turbines relative to some
known structures.
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Figure 2.11 Tower Heights of Various Capacity Wind Turbines(Ref:3)
Both steel and concrete towers are available and are being used. The construction can
be tubular or lattice. Towers must be at least 25 to 30 m high to avoid turbulence caused by
trees and buildings. Utility-scale towers are typically twice as high to take advantage of the
swifter winds at those heights.
The main issue in the tower design is the structural dynamics. The tower vibration and
the resulting fatigue cycles under wind speed fluctuation are avoided by the design. This
requires careful avoidance of all resonance frequencies of the tower, the rotor, and the nacelle
from the wind fluctuation frequencies. Sufficient margin must be maintained between the two
sets of frequencies in all vibrating modes.
The resonance frequencies of the structure are determined by complete modal
analyses, leading to the eigenvectors and eigenvalues of complex matrix equations
representing the motion of the structural elements. The wind fluctuation frequencies are found
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from the measurements at the site under consideration. Experience on a similar nearby site
can bridge the gap in the required information.
Big cranes are generally required to install wind towers. Gradually increasing tower
height, however, is bringing a new dimension in the installation (Figure 2.12).
Figure 2.12 WEG MS-2 Wind Turbine Installation at Myers Hill(Ref:3)
Large rotors add to the transportation problem as well. Tillable towers to nacelle and
rotors moving upwards along with the tower are among some of the newer developments in
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wind tower installation. The offshore installation comes with its own challenge that must be
met.
The top head mass (THM) of the nacelle and rotor combined has a significant bearing
on the dynamics of the entire tower and the foundation. Low THM is generally a measure of
design competency, as it results in reduced manufacturing and installation costs. The THMs
of Vestas‘ 3-MW/90-m turbine is 103 t, NEG Micon‘s new 4.2-MW/110-m machine is 214 t,
and Germany‘s REpower‘s 5- MW/125-m machine is about 350 t, which includes extra 15 to
20% design margins. [3]
2) Turbine
Wind turbines are manufactured in sizes ranging from a few kW for stand-alone
remote applications to a few MW each for utility-scale power generation. The turbine size has
been steadily increasing. The average size of the turbine installed worldwide in 2002 was over
1 MW. By the end of 2003, about 1200 1.5-MW turbines made by GE Wind Energy alone
were installed and in operation. Today, even larger machines are being routinely installed on a
large commercial scale, such as GE‘s new 3.6-MW turbines for offshore wind farms both in
Europe and in the U.S. It offers lighter variable-speed, pitch-controlled blades on a softer
support structure, resulting in a cost-effective foundation. Its rated wind speed is 14 m/sec
with cutin speed at 3.5 m/sec and the cutout at 25 m/sec. The blade diameter is 104 m with
hub height 100 m on land and 75 m offshore. In August 2002, Enercon‘s 4.5-MW wind
turbine prototype was installed near Magdeburgh in eastern Germany. It has a 113-m rotor
diameter, 124-m hub height, and an egg-shaped nacelle. Its reinforced concrete tower
diameter is 12 m at the base, tapering to 4 m at the top. Today, even 5-MW machines are
being installed in large offshore wind farms. The mass of a 5- MW turbine can vary from 150
to 300 t in nacelle and 70 to 100 t in the rotor blades, depending on the manufacturing
technologies adopted at the time of design. The most modern designs would naturally be on
the lighter side of the range.
Turbine procurement requires detailed specifications, which are often tailored from the
manufacturers‘ specifications. The leading manufacturers of wind turbines in the world are
listed in Table 2.2, with Denmark‘s Vestas leading with 22% of the world‘s market share. The
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major suppliers in the U.S. are GE Wind (52%), Vestas (21%), Mitsubishi (12%), NEG
Micon (10%), and Gamesha (3%). [3]
Supplier % Share of the Market
Vestas, Denmark 22
GE Wind, U.S. 18
Enercon, Germany 15
Gamesha, Spain 12
NEG Micon, Denmark (Being acquired by
Vestas, still separate trade names)
10
Table 2.2 World’s Major Wind Turbine Suppliers in 2004(Ref:3)
3) Blades
Modern wind turbines have two or three blades, which are carefully constructed
airfoils that utilize aerodynamic principles to capture as much power as possible. The airfoil
design uses a longer upper-side surface whereas the bottom surface remains somewhat
uniform. By the Bernoulli principle, a ―lift‖ is created on the airfoil by the pressure difference
in the wind flowing over the top and bottom surfaces of the foil. This aerodynamic lift force
flies the plane high, but rotates the wind turbine blades about the hub. In addition to the lift
force on the blades, a drag force is created, which acts perpendicular to the blades, impeding
the lift effect and slowing the rotor down. The design objective is to get the highest lift-to-
drag ratio that can be varied along the length of the blade to optimize the turbine‘s power
output at various speeds. The rotor blades are the foremost visible part of the wind turbine,
and represent the forefront of aerodynamic engineering. The steady mechanical stress due to
centrifugal forces and fatigue under continuous vibrations make the blade design the weakest
mechanical link in the system. Extensive design effort is needed to avoid premature fatigue
failure of the blades. A swift increase in turbine size has been recently made possible by the
rapid progress in rotor blade technology, including emergence of the carbon- and glass-fiber-
based epoxy composites. The turbine blades are made of high-density wood or glass fiber and
epoxy composites.
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The high pitch angle used for stall control also produces a high force. The resulting
load on the blade can cause a high level of vibration and fatigue, possibly leading to a
mechanical failure. Regardless of the fixed- or variable-speed design, the engineer must deal
with the stall forces. Researchers are moving from the 2-D to 3-D stress analyses to better
understand and design for such forces. As a result, the blade design is continually changing,
particularly at the blade root where the loading is maximum due to the cantilever effect.
The aerodynamic design of the blade is important, as it determines the energycapture
potential. The large and small machine blades have significantly different design
philosophies. The small machine sitting on a tower relatively taller than the blade diameter,
and generally unattended, requires a low-maintenance design. On the other hand, a large
machine tends to optimize aerodynamic performance for the maximum possible energy
capture. In either case, the blade cost is generally kept below 10% of the total installed
cost.[3]
2.2.3.2 Turbine Rating
The method of assessing the nominal rating of a wind turbine has no globally accepted
standard. Many manufacturers have, adopted the combined rating designations x/y, the
generator‘s peak electrical capacity followed by the wind turbine diameter. For example, a
300/30-kW/m wind system means a 300-kW electrical generator and a 30-m diameter turbine.
The specific rated capacity (SRC) is often used as a comparative index of the wind turbine
designs. It measures the power generation capacity per square meter of the blade-swept area,
and is defined as follows in units of kW/m2:
(2.10)
The SRC for a 300/30 wind turbine is 300/ Π x 152 = 0.42 kW/m2. It increases with
diameter, giving favorable economies of scale for large machines, and ranges from
approximately 0.2 kW/m2 for a 10-m diameter rotor to 0.5 kW/m
2 for a 40-m diameter rotor.
Some aggressively rated turbines have an SRC of 0.7 kW/m2, and some reach as high as 1
kW/m2. The higher- SRC rotor blades have higher operating stresses, which result in a shorter
fatigue life. All stress concentration regions are carefully identified and eliminated in high-
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SRC designs. Modern design tools, such as the finite element stress analysis and the modal
vibration analysis, can be of great value in rotor design.
Turbine rating is important as it indicates to the system designer how to size the
electrical generator, the plant transformer, and the connecting cables to the substation and the
transmission link interfacing the grid. The power system must be sized on the peak capacity
of the generator. Because turbine power depends on the cube of the wind speed, the system-
design engineer matches the turbine and the generator performance characteristics. This
means selecting the rated speed of the turbine to match with the generator. As the gearbox and
generator are manufactured only in discrete sizes, selecting the turbine‘s rated speed can be
complex. The selection process goes through several iterations, trading the cost with benefit
of the available speeds. Selecting a low rated speed would result in wasting much energy at
high winds. On the other hand, if the rated speed is high, the rotor efficiency will suffer most
of the time.[3]
2.2.3.3 Power vs Speed and TSR
The typical turbine torque vs. rotor speed is plotted in Figure 2.13.
Figure 2.13 Wind Turbine Torque vs Rotor Speed Characteristic at Two Wind Speeds, V1 and
V2(Ref:3)
It shows a small torque at zero speed, rising to a maximum value before falling to nearly zero
when the rotor just floats with the wind. Two such curves are plotted for different wind
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speeds V1 and V2, with V2 being higher than V1. The corresponding power vs. Rotor speed at
the two wind speeds are plotted in Figure 2.14.
Figure 2.14 Wind Turbine Power vs Rotor Speed Characteristic at Two Wind Speeds, V1 and
V2(Ref:3)
As the mechanical power converted into the electric power is given by the product of the
torque T and the angular speed, the power is zero at zero speed and again at high speed with
zero torque. The maximum power is generated at a rotor speed somewhere in between, as
marked by P1max and P2max for speeds V1 and V2, respectively. The speed at the maximum
power is not the same speed at which the torque is maximum. The operating strategy of a
well-designed wind power system is to match the rotor speed to generate power continuously
close to the Pmax points. Because the Pmax point changes with the wind speed, the rotor speed
must, therefore, be adjusted in accordance with the wind speed to force the rotor to work
continuously at Pmax. This can be done with a variable-speed system design and operation.
At a given site, the wind speed varies over a wide range from zero to high gust. We define tip
speed ratio (TSR) as follows:
(2.11)
where R and ω are the rotor radius and the angular speed, respectively.
For a given wind speed, the rotor efficiency Cp varies with TSR as shown in Figure 2.15.
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Figure 2.15 Rotor Efficiency and Annual Energy Production vs Rotor TSR(Ref:3)
The maximum value of Cp occurs approximately at the same wind speed that gives peak
power in the power distribution curve of Figure 2.14. To capture high power at high wind, the
rotor must also turn at high speed, keeping TSR constant at the optimum level. However, the
following three system performance attributes are related to TSR:
1. The maximum rotor efficiency Cp is achieved at a particular TSR, which is specific to
the aerodynamic design of a given turbine. As was seen in Figure 2.4, the TSR needed
for maximum power extraction ranges from nearly one for multiple-blade, slow-speed
machines to nearly six for modern high-speed, two-blade machines.
2. The centrifugal mechanical stress in the blade material is proportional to the TSR. The
machine working at a higher TSR is necessarily stressed more. Therefore, if designed
for the same power in the same wind speed, the machine operating at a higher TSR
would have slimmer rotor blades.
3. The ability of a wind turbine to start under load is inversely proportional to the design
TSR. As this ratio increases, the starting torque produced by the blade decreases.
A variable-speed control is needed to maintain a constant TSR to keep the rotor efficiency at
its maximum. At the optimum TSR, the blades are oriented to maximize the lift and minimize
the drag on the rotor. The turbine selected for a constant TSR operation allows the rotational
speed of both the rotor and generator to vary up to 60% by varying the pitch of the blades.[3]
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2.2.3.4 Maximum Power Operation
In general, operating the wind turbine at a constant TSR corresponding to the
maximum power point at all times can generate 20 to 30% more electricity per year.
However, this requires a control scheme to operate with a variable speed to continuously
generate the maximum power. Two possible schemes for such an operation are as follows:
1) Constant-TSR Scheme
In this scheme the machine is continuously operated at its optimum TSR, which is a
characteristic of the given wind turbine. This optimum value is stored as the reference TSR in
the control computer. The wind speed is continuously measured and compared with the blade
tip speed. The error signal is then fed to the control system, which changes the turbine speed
to minimize the error (Figure 2.16).
Figure 2.16 Maximum Power Operation Using Rotor Tip Speed Control Scheme(Ref:3)
At this time the rotor must be operating at the reference TSR, generating the maximum power.
This scheme has the disadvantage of requiring the local wind speed measurements, which
could have a significant error, particularly in a large wind farm with shadow effects. Being
sensitive to the changes in the blade surface, the optimum TSR gradually changes with age
and environment. The computer reference TSR must be changed accordingly many times,
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which is expensive. Besides, it is difficult to determine the new optimum TSR with changes
that are not fully understood or easily measured.[3]
2) Peak-Power-Tracking Scheme
The power vs. speed curve has a single well-defined peak. If we operate at the peak
point, a small increase or decrease in the turbine speed would result in no change in the power
output, as the peak point locally lies in a flat neighborhood. In other words, a necessary
condition for the speed to be at the maximum power point is as follows:
(2.12)
This principle is used in the control scheme (Figure 2.17).
Figure 2.17 Maximum Power Operation Using Power Control Scheme(Ref:3)
The speed is increased or decreased in small increments, the power is continuously measured,
and ΔP/Δω is continuously evaluated. If this ratio is positive that means we get more power
by increasing the speed, the speed is further increased. On the other hand, if the ratio is
negative, the power generation will reduce if we change the speed any further. The speed is
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maintained at the level where ΔP/Δω is close to zero. This method is insensitive to errors in
local wind speed measurement, and also to wind turbine design. It is, therefore, the preferred
method. In a multiple-machine wind farm, each turbine must be controlled by its own control
loop with operational and safety functions incorporated.[3]
2.2.3.5 System-Design Trade-Offs
When the land area is limited or is at a premium price, one optimization study that
must be conducted in an early stage of the wind farm design is to determine the number of
turbines, their size, and the spacing for extracting the maximum energy from the farm
annually.
1) Turbine Towers and Spacing
Figure 2.18 Optimum Tower Spacing in Wind Farms in Flat Terrain(Ref:3)
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Large turbines cost less per megawatt of capacity and occupy less land area. On the
other hand, fewer large machines can reduce the megawatthour energy crop per year, as
downtime of one machine would have larger impact on the energy output. A certain turbine
size may stand out to be the optimum for a given wind farm from the investment and energy
production cost points of view.
Tall towers are beneficial, but the height must be optimized with the local regulations
and constrains of the terrain and neighborhood. Nacelle weight and structural dynamics are
also important considerations.
When installing a cluster of machines in a wind farm, certain spacing between the
wind towers must be maintained to optimize the energy crop over the year. The spacing
depends on the terrain, wind direction, wind speed, and turbine size. The optimum spacing is
found in rows 8 to 12 rotor diameters apart in the wind direction, and 2 to 4 rotor diameters
apart in the crosswind direction (Figure 2.18).
Figure 2.19 Original Land Use Continues in a Wind Farm in Germany(Ref:3)
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The average number of machines in wind farms varies greatly, ranging from several to
hundreds depending on the required power capacity of the farm. The preceding spacing rules
would ensure that the turbines do not shield those further downwind. Some wind farms have
used narrow spacing of five to six rotor diameters in the wind direction. One such farm in
Mackinaw City, MI, has reported the rotors in downwind direction running slower due to the
wake effect of the upwind rotors.
The wind power fluctuations and electrical transients on fewer large machines would
cost more in the filtering of power and voltage fluctuations, or would degrade the quality of
power, inviting penalty from the grid.
Additionally, it includes the effect of tower height that goes with the turbine diameter,
available standard ratings, cost at the time of procurement, and wind speed. The wake
interaction and tower shadow are ignored for simplicity. Such optimization leads to a site-
specific number and size of the wind turbines that will minimize the energy cost.[3]
2) Number of Blades
One can extract the power available in the wind with a small number of blades rotating
quickly, or a large number of blades rotating slowly. More blades do not give more power,
but they give more torque and require heavier construction. A few fast-spinning blades result
in an economical system. Wind machines have been built with the number of blades ranging
from 1 to 40 or more. A one-blade machine, although technically feasible, gives a supersonic
tip speed and a highly pulsating torque, causing excessive vibrations. It is, therefore, hardly
used in large systems.
A very high number of blades were used in old low-TSR rotors for water pumping and
grain milling, the applications requiring high starting torque. Modern high-TSR rotors for
generating electric power have two or three blades, many of them with just two, although the
Danish standard is three blades.
The major factors involved in deciding the number of blades are as follows:
• The effect on power coefficient
• The design TSR
• The means of limiting yaw rate to reduce the gyroscopic fatigue
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Compared to the two-blade design, the three-blade machine has smoother power
output and a balanced gyroscopic force. There is no need to teeter the rotor, allowing the use
of a simple rigid hub. Three blades are more common in Europe, where large machines up to
a few MW are being built using the three-blade configuration. The practice in the U.S,
however, has been to use the two-blade design. Adding the third blade increases the power
coefficient only by about 5%, thus giving a diminished rate of return for the 50% more blade
weight and cost. The two-blade rotor is also simpler to erect, because it can be assembled on
the ground and lifted to the shaft without complicated maneuvers during the lift. The number
of blades is often viewed as the blade solidity. Higher solidity ratio gives higher starting
torque and leads to low-speed operation. For electric power generation, the turbine must run
at high speeds as the electrical generator weighs less and operates more efficiently at high
speeds. That is why all large-scale wind turbines have low solidity ratio, with just two or three
blades.[3]
3) Rotor Upwind or Downwind
Operating the rotor upwind of the tower produces higher power as it eliminates the
tower shadow on the blades. This results in lower noise, lower blade fatigue, and smoother
power output. A drawback is that the rotor must constantly be turned intothe wind via the yaw
mechanism. The heavier yaw mechanism of an upwind turbine requires a heavy-duty and
stiffer rotor compared to a downwind rotor.
The downwind rotor has the wake (wind shade) of the tower in the front and loses
some power from the slight wind drop. On the other hand, it allows the use of a free yaw
system. It also allows the blades to deflect away from the tower when loaded. Its drawback is
that the machine may yaw in the same direction for a long period of time, which can twist the
cables that carry current from the turbines.
Both types have been used in the past with no clear trend. However, the upwind rotor
configuration has recently become more common.[3]
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4) Horizontal vs Vertical Axis
In the horizontal-axis Danish machine, considered to be classical, the axis of blade
rotation is horizontal with respect to the ground and parallel to the wind stream. Most wind
turbines are built today with the horizontal-axis design, which offers a cost-effective turbine
construction, installation, and control by varying the blade pitch.
The vertical-axis Darrieus machine has different advantages. First of all, it is
omnidirectional and requires no yaw mechanism to continuously orient itself toward the wind
direction. Secondly, its vertical drive shaft simplifies the installation of the gearbox and the
electrical generator on the ground, making the structure much simpler. On the negative side, it
normally requires guy wires attached to the top for support. This could limit its applications,
particularly at offshore sites. Overall, the vertical-axis machine has not been widely used,
primarily because its output power cannot be easily controlled in high winds simply by
changing the blade pitch. With modern low-cost variable-speed power electronics emerging in
the wind power industry, the Darrieus configuration may revive, particularly for large-
capacity applications.
The Darrieus has structural advantages compared to a horizontal-axis turbine because
it is balanced. The blades only ―see‖ the maximum lift torque twice per revolution. Seeing
maximum torque on one blade once per revolution excites many natural frequencies, causing
excessive vibrations. Also a vertical-axis wind turbine configuration is set on the ground.
Therefore, it is unable to effectively use higher wind speeds using a higher tower, as there is
no tower here.[3]
2.2.4 Power Electronics for Modern Wind Turbines
1) Wind Energy Conversion
The development in wind turbine systems has been steady for the last 25 years and
four to five generations of wind turbines exist. The main components of a wind turbine
system, including the turbine rotor, gearbox, generator, transformer, and possible power
electronics, are illustrated in Fig. 2.20.
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Figure 2.20 Main Components of a Wind Turbine System(Ref:8)
The turbine rotor converts the fluctuating wind energy into mechanical energy, which
is converted into electrical power through the generator, and then transferred into the grid
through a transformer and transmission lines.
Wind turbines capture the power from the wind by means of aerodynamically
designed blades and convert it to rotating mechanical power. The number of blades is
normally three and the rotational speed decreases as the radius of the blade increases.
For megawatt range wind turbines the rotational speed will be 10–15 rpm. The
weightefficient way to convert the low-speed, high-torque power to electrical power is to use
a gearbox and a generator with standard speed. The gearbox adapts the low speed of the
turbine rotor to the high speed of the generator. The gearbox may be not necessary for
multipole generator systems.
The generator converts the mechanical power into electrical energy, which is fed into a
grid through possibly a power electronic converter, and a transformer with circuit breakers
and electricity meters. The connection of wind turbines to the grid is possible at low voltage,
medium voltage, high voltage, and even at the extra high voltage system since the
transmittable power of an electricity system usually increases with increasing the voltage
level. While most of the turbines are nowadays connected to the medium voltage system,
large offshore wind farms are connected to the high and extra high voltage level.
The electrical losses include the losses due to the generation of power, and the losses
occur independently of the power production of wind turbines and also the energy used for
lights and heating. The losses due to the power generation of the wind turbines are mainly
losses in the cables and the transformer. The low-voltage cable should be short so as to avoid
high losses. For modern wind turbine system, each turbine has its own transformer to raise
voltage from the voltage level of the wind turbines (400 or 690 V) to the medium voltage. The
transformer is normally located close to the wind turbines to avoid long low-voltage cables.
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Only small wind turbines are connected directly to the low-voltage line without a transformer
or some of small wind turbines are connected to one transformer in a wind farm with small
wind turbines. Because of the high losses in low-voltage lines, large wind farms may have a
separate substation to increase the voltage from a medium voltage system to a high voltage
system. The medium voltage system could be connected as a radial feeder or as a ring feeder.
At the point of common coupling (PCC) between the single wind turbines or the wind
farm and the grid, there is a circuit breaker for the disconnection of the whole wind farm or of
the wind turbines. Also the electricity meters are installed usually with their own voltage and
current transformers.
The electrical protective system of a wind turbine system needs to protect the wind
turbine and as well as secure the safe operation of the network under all circumstances. For
the wind turbine protection, the short circuits, overvoltage, and overproduction will be limited
to avoid the possibly dangerous damage to the wind turbine system. Also the system should
follow the grid requirements to decide whether the wind turbine should be kept in connection
or disconnected from the system. Depending on the wind turbine operation requirement, a
special relaymay be needed to detect if the wind turbine operates in a grid connection mode or
as an autonomous unit in an isolated part of the network due to the operation of protection
devices.
The conversion of wind power to mechanical power is done aerodynamically as
aforementioned. It is important to control and limit the converted mechanical power at higher
wind speed, as the power in the wind is a cube of the wind speed. The power limitation may
be done by stall control (the blade position is fixed but stall of the wind appears along the
blade at higher wind speed), active stall control (the blade angle is adjusted in order to create
stall along the blades), or pitch control (the blades are turned out of the wind at higher wind
speed).
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Figure 2.21 Power Characteristics of a Fixed Speed Wind Turbines: (a)Stall Control, (b)Active Stall
Control, and (c)Pitch Control(Ref:8)
Fig. 2.21 shows the power curves of different types of turbine rotor power limitation methods.
It can be seen that the power may be smoothly limited by rotating the blades either by pitch or
by active stall control while the power limited by the stall control shows a small overshoot,
and this overshoot depends on the aerodynamic design. The possible technical solutions of the
electrical system are many and Fig. 2.22 shows a technological roadmap starting with wind
energy/power and converting the mechanical power into electrical power. It involves
solutions with and without gearbox as well as solutions with or without power electronic
conversion.[8]
Figure 2.22 Roadmap for Wind Energy Conversion (PE=Power Electronics, DF=Doubly Fed) (Ref:8)
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2) Modern Power Electronics and Converter Systems
Many types of wind turbines, such as variable speed wind turbine systems, use power
electronic systems as interfaces. Since the wind turbine operates at variable rotational speed,
the electric frequency of the generator varies andmust therefore be decoupled from the
frequency of the grid. This can be achieved by using a power electronic converter system.
Even in a fixed speed system where the wind turbines may be directly connected to the grid,
thyristors are used as soft-starters.
2.1) Power Electronics Devices
Power electronics has changed rapidly during the last 30 years and the number of
applications has been increasing, mainly due to the developments of semiconductor devices
and microprocessor technology. For both cases higher performance is steadily given for the
same area of silicon, and at the same time the price of the devices is continuously falling.
Three important issues are of concern in using a power electronic system. These are
reliability, efficiency, and cost. At the moment the cost of power semiconductor devices is
decreasing 2–5% every year for the same output performance. Fig. 2.23 shows some key self-
commutated devices and the area where the development is still on going.
Figure 2.23 Development of Power Semiconductor Devices in the Past and in the Future(Ref:8)
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The only power device that is no longer under development (see Fig. 2.23) is the
silicon-based power bipolar transistor because MOS-gated devices are preferable in the sense
of easy control. The breakdown voltage and/or current carrying capability of the components
are also continuously increasing. Also, important research is going on to change the material
from silicon to silicon carbide. This may dramatically increase the power density of power
converters, but silicon carbide based transistors on a commercial basis, with a competitive
price, will still take some years to appear on the market.[8]
2.2) Power Electronic Converters
Power electronic converters are constructed by power electronic devices, driving,
protection and control circuits. A converter, depending on the topology and application, may
allow both directions of power flow and can interface between the load/generator and the grid.
There are two different types of converter systems: grid commutated and self commutated
converter systems. The grid commutated converters are mainly thyristor converters, 6 or 12 or
even more pulse. This type of converter produces integer harmonics which in general requires
harmonic filters. Also thyristor converters are not able to control the reactive power and
consume inductive reactive power.
Figure 2.24 Circuit Diagram of a Voltage Source Converter (VSC) with IGBTs(Ref:8)
Self commutated converter systems are mainly pulse width modulated (PWM)
converters, where IGBTs (Insulated Gate Bipolar Transistor) are mainly used. This type of
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converter can control both active power and reactive power. That means the reactive power
demand can be delivered by a PWM-converter. The high frequency switching of a PWM-
converter may produce harmonics and interharmonics. In general these harmonics are in the
range of some kHz.Due to the high frequencies, the harmonics are relatively easier to be
removed by small size filters. Fig. 2.24 shows a typical power electronic converter consisting
of self commutated semiconductors such as IGBTs and Fig. 2.25 shows the waveforms of
different operation modes.[8]
Figure 2.25 Waveforms of Bidirectional Active and Reactive Power of a VSC: (a)Active Power Flow from the AC System to the Converter DC Side, (b)Active Power Flow from the Converter DC Side to
the AC System, (c)The Converter Generating Reactive Power, (d)The Converter Consuming Reactive Power(Ref:8)
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3) Generator Systems for Wind Turbines
Both induction and synchronous generators can be used for wind turbine systems.
Induction generators can be used in a fixed-speed system or a variable-speed system, while
synchronous generators are normally used in power electronic interfaced variable-speed
systems. Mainly, three types of induction generators are used in wind power conversion
systems: cage rotor, wound rotor with slip control by changing rotor resistance, and doubly
fed induction generators. The cage rotor induction machine can be directly connected into an
ac system and operates at a fixed speed or uses a full-rated power electronic system to operate
at variable speed. The wound rotor generator with rotor-resistance-slip control is normally
directly connected to an ac system, but the slip control provides the ability of changing the
operation speed in a certain range. The doubly fed induction generators provide a wide range
of speed variation depending on the size of power electronic converter systems.
3.1) Fixed-Speed Wind Turbines
In fixed-speed wind turbines, the generator is directly connected to the mains supply
grid. The frequency of the grid determines the rotational speed of the generator and thus of the
rotor. The generator speed depends on the number of pole pairs and the frequency of the grid.
The ―Danish Concept,‖ of directly connecting a wind turbine to the grid, is widely used for
power ratings up to 2.3 MW. The scheme consists of a squirrel-cage induction generator
(SCIG), connected via a transformer to the grid. The wind turbine systems using cage rotor
induction generators almost operate at a fixed speed (variation of 1–2%). The power can be
limited aerodynamically by stall control, active stall control, or by pitch control. The basic
configurations of three different fixed speed concepts are shown in Fig. 2.26. The advantage
of wind turbines with induction generators is the simple and cheap construction. In addition,
no synchronization device is required. These systems are attractive due to cost and reliability,
but they are not fast enough (within a few ms) to control the active power. There are some
other drawbacks also: the wind turbine has to operate at constant speed, it requires a stiff
power grid to enable stable operation, and it may require a more expensive mechanical
construction in order to absorb high mechanical stress since wind gusts may cause torque
pulsations in the drive train and the gearbox. Other disadvantages with the induction
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generators are high starting currents and their demand for reactive power. They need a
reactive power compensator to reduce (almost eliminate) the reactive power demand from the
turbine generators to the grid. It is usually done by continuously switching capacitor banks
following the production variation (5–25 steps).
Figure 2.26 Wind Turbine Systems without Power Converter, but with Aerodynamic Power Control: (a)Pitch Controlled[System 1], (b)Stall Controlled[System II], and (c)Active Stall Controlled [System III]
(Ref:8)
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Connecting the induction generators to power system produces transients that are short
duration, very high inrush currents causing both disturbances to the grid and high torque
spikes in the drive train of wind turbines with a directly connected induction generator.
Unless special precautions are taken, the inrush currents can be up to 5–7 times the
rated current of the generator; however, after a very short period (less than 100 ms), the
current peak may be considerably higher, up to 18 times the normal rated current. A transient
like this disturbs the grid and limits the acceptable number of value of all wind turbines. All
three systems shown in Fig. 2.26 use a thyristor controller, the soft starter (not shown in Fig.
2.26), in order to reduce the inrush current. The current limiter, or soft starter, based on
thyristor technology, typically limits the highest rms value of the inrush current to a level that
is two times below that of the generator rated current. The soft starter has a limited thermal
capacity and so it is short circuited by a contactor, which carries the full load current when the
connection to the grid has been completed. In addition to reducing the impact on the grid, the
soft starter also effectively dampens the torque peaks associated with the peak currents and
hence reduces the loads on the gearbox.
An example is shown here to illustrate the startup of a soft-starter-fed induction
generator. The induction machine has 2MW rated power, 690 V/1700A rated phase voltage
and rated line current, respectively (delta connection). The induction machine is connected via
a soft starter to the supply voltage below synchronous speed (1450 rpm). The starting firing
angle for the soft starter is 120◦. The equivalent diagram of this system is shown in Fig.
2.27(a). The electromagnetic torque and the rotational speed of the high-speed shaft during
the startup are presented in two cases: direct startup and using a soft starter. Fig. 2.27(b)
shows the simulation results for the direct startup, while Fig. 2.27(c) shows the results when
the machine is connected to the grid via a soft starter. When the induction machine is
connected directly to the grid, high starting torque is observed. Large oscillations in the shaft
speed can be seen in Fig. 2.27(b). By using a soft starter, the inrush currents and therefore the
high starting torque are limited and the shaft speed is smoothed as shown in Fig. 2.27(c).[8]
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122
Figure 2.27 The Startup of a Fixed-speed Wind Turbine: (a)Equivalent Diagram of a Fixed-speed Wind Turbine to Show the Startup, (b)Electromagnetic Torque and Shaft Speed during the Direct
Startup of a 2MW Induction Machine (c)Electromagnetic Torque and Shaft Speed during the Startup of a 2MW Soft-starter-fed Induction Machine(Ref:8)
3.2) Variable-Speed Wind Turbines
In variable-speed systems the generator is normally connected to the grid by a power
electronic system. For synchronous generators and for induction generators without rotor
windings, a full-rated power electronic system is connected between the stator of the
generator and the grid, where the total power production must be fed through the power
electronic system. For induction generators with rotor windings, the stator of the generator is
connected to the grid directly. Only the rotor of the generator is connected through a power
electronic system. This gives the advantage that only a part of the power production is fed
through the power electronic converter. This means the nominal power of the converter
system can be less than the nominal power of the wind turbine. In general the nominal power
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of the converter may be 30% of the power rating of the wind turbine, enabling a rotor speed
variation in the range of 30% of the nominal speed. By controlling the active power of the
converter, it is possible to vary the rotational speed of the generator and thus of the rotor of
the wind turbines.
3.2.1) Variable-speed Wind Turbines with Partially rated Power Converters
By using wind turbines with partially rated power converters the improved control
performance can be obtained. Fig. 2.28 shows two such systems. The generator for wind
turbine systems shown in Fig. 2.28 is an induction generator with a wounded rotor.
Figure 2.28 Wind Turbine Topologies with Partially Rated Power Electronics and Limited Speed Range. Rotor-resistance Converter [System IV] and Doubly-fed Induction Generator [System V]
In Fig. 2.28(a) an extra resistance is added in the rotor, which can be controlled by
power electronics. The variation of rotor resistance produces a group of torque-speed
characteristics as shown in Fig. 2.29.
Figure 2.29 Torque and Speed Characteristics of Rotor Resitance Controlled Wound Rotor Induction
Generator(Ref:8)
This is known as the dynamic slip control and gives typically a speed range of 2–5%. The
power converter for the rotor resistance control is for low voltage but high currents. At the
same time an extra control freedom is obtained at higher wind speeds in order to keep the
output power fixed. This system still needs a soft starter and reactive power compensation.[8]
3.2.1.2) Doubly Fed Induction Generator
A doubly fed induction generator (DIFG) using a medium scale power converter is
shown in Fig. 2.28(b). Slip rings are making the electrical connection to the rotor. If the
generator is running super-synchronously, electrical power is delivered to the grid through
both the rotor and the stator. If the generator is running sub-synchronously, electrical power is
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delivered into the rotor from the grid. A speed variation of ±}30% around synchronous speed
can be obtained by the use of a power converter of 30% of nominal power. Furthermore, it is
possible to control both active (Pref) and reactive power (Qref), which gives a better grid
performance, and the power electronics enables the wind turbine to act as a more dynamic
power source to the grid.
The DFIG system does not need either a soft starter or a reactive power compensator.
The system is naturally a little bit more expensive compared to the classical systems shown
before in Figs. 2.26 and 2.28(a). However, it is possible to save money on the safety margin
of gear and reactive power compensation units, and it is also possible to capture more energy
from the wind.[8]
3.2.2) Full Scale Power Electronic Converter Integrated Systems
The wind turbines with a full-scale power converter between the generator and the
grid give the added technical performance. Fig. 3.5 shows four possible systems with full-
scale power converters.
The systems shown in Figs. 2.30(a) and 2.30(b) are characterized by having a gearbox. The
wind turbine system with a cage rotor induction generator and full-rated power electronic
converters is shown in Fig. 2.30(a). Usually, a back-to-back voltage source converter is used
in order to achieve full control of the active and reactive power.
The synchronous generator shown in Fig. 2.30(b) needs a small power converter for
field excitation. Multipole systems with the synchronous generator without a gear are shown
in Figs. 2.30(c) and 2.30(d). The last system is using permanent magnets, which are becoming
cheaper and thereby attractive. All four systems have almost the same controllable
characteristics since the generator is decoupled from the grid by a dc link. The power
converter to the grid enables the system to control active and reactive power very fast.
However, the disadvantage is a more complex system with more sensitive electronic parts.[8]
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Figure 2.30 Wind turbine systems with full-scale power converters. (a) Induction generator with gear [System VI], (b) Synchronous generator with gear [System VII], (c) Multipole
Table 2.3 Wind Turbine Topologies Market in 2001(Ref:8)
4) Control of Wind Turbines
Overall, the power can be controlled by means of the aerodynamic system and has to
follow a set point given by a dispatch center or locally, with the goal to maximize the
production based on the available wind power. The power control system should also be able
to limit the power. Controlling a wind turbine involves both fast and slow control.
4.1) Active Stall Wind Turbine with Cage Rotor Induction Generators
In principle, an active stall wind turbine is a stall turbine with a variable pitch angle.
The main difference between a stall turbine and an active stall turbine is a pitch system for
variable pitch angles, which allows the stall effect to be controlled. An active stall wind
turbine has to pitch in a negative direction to limit the power when the electrical power of the
wind turbine exceeds nominal power. The active stall system basically maintains all the
characteristics of a stall-regulated system. Large wind farms such as Nysted (170MW
installed capacity) have been built with active stall wind turbines.
The generator of an active stall turbine can be a simple squirrel cage induction
generator directly connected to the grid. In order to compensate for the output power factor, a
capacitor bank is used. A soft starter is used only during the startup sequence of the generator
in order to limit the inrush currents and hence reduce the high starting torque.
The maximum power output of the active stall turbines can be maintained at a constant
value. In addition, the aerodynamic efficiency Cp can be optimized to a certain extent. The
active stall control can improve the efficiency of the overall system. The flexible coupling of
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the blades to the hub also facilitates emergency stopping and start up. One drawback of the
active stall controlled wind turbine compared to the passive stall one is the higher price,
which is due to the pitching mechanism and its controller.
The implemented active stall wind turbine controller achieves good power yield with a
minimum of pitch actions. Once the overall mean wind speed is at a constant level, pitch
angle adjustments are rarely necessary, allowing the controller to optimize the pitch angle as
often as possible.
Depending on the pitch system, the lost power (due to slow control) may be justified
by reduced stress and wear in the pitch system and reduced fatigue loads on the wind turbine.
This applies both to power optimization, where the controller strives for maximum power
yield by using the moving average of the wind speed signal to find the appropriate pitch angle
in a lookup table, and to power limitation where the power output is controlled in a closed
control loop. With a slow control system, substantial over-power in the power limitation
mode may cause a problem. This may be avoided by an over-power protection feature.[8]
4.2) Variable Pitch Angle Control with Doubly Fed Generators
The variable speed DFIG wind turbine is a widely used concept today. The control
system of a variable speed wind turbine with DFIG mainly functions to
• control the power drawn from the wind turbine in order to track the wind turbine optimum
operation point,
• limit the power in the case of high wind speeds, and
• control the reactive power exchanged between the wind turbine generator and the grid.
Two hierarchical control levels are related to each other with different bandwidths,
namely, DFIG control level and wind turbine control level. An example of an overall control
scheme of a wind turbine with a doubly fed generator system is shown in Fig. 2.31.
The DFIG control, with a fast dynamic response, contains the electrical control of the
power converters and of the DFIG. The wind turbine control, with slow dynamic response,
supervises both the pitch system of the wind turbine as well as the active power set point of
the DFIG control level.
12
9
Fig
ure
2.3
1 C
ontro
l of W
ind T
urb
ine w
ith D
FIG
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A vector control approach is adopted for the DFIG control, while two crosscoupled
controllers are used to control the wind turbine. These controllers are speed and power
limitation controllers. Their goals are to track the wind turbine optimum operation point, to
limit the power in the case of high wind speeds, and to control the reactive power exchanged
between the wind turbine generator and the grid.
Below maximum power production, the wind turbine will typically vary the speed
proportionally with the wind speed and keep the pitch angle θ fixed. At very low wind, the
speed of the turbine will be fixed at the maximum allowable slip in order not to have
overvoltage. A pitch angle controller will limit the power when the turbine reaches the
nominal power. The generated electrical power is controlled by the doubly fed generator
through the rotor-side converter. The control of the grid-side converter simply keeps the dc-
link voltage fixed. Internal current loops in both converters are used with typical PI-
controllers. The power converters to the grid-side and the rotor-side are voltage source
inverters.
The significant feature of the control method is that it allows the turbine to operate
with optimum power efficiency over a wide range of wind speeds. Moreover, because of the
design of this control method, small changes in generator speed do not lead to large power
fluctuations and unnecessary transitions between power optimization and power limitation
modes. A gain scheduling control of the pitch angle is also implemented in order to
compensate for the nonlinear aerodynamic characteristics.[8]
4.3) Full Rated Power Electronic Interface Wind Turbine Systems
Cage induction generators and synchronous generators can be integrated into the
system by full rated power electronic converters. As shown in Fig. 2.32, a passive rectifier
and a boost converter are used in order to boost the voltage at low speed. It is possible to
control the active power from the generator. A grid inverter interfaces the dc-link to the grid.
Here it is also possible to control the reactive power to the grid. The system is able to control
reactive and active power quickly and then the turbine may take part in the power system
control.[8]
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Figure 2.32 Basic control of active and reactive power in a wind turbine with a multipole
synchronous generator system(Ref:8)
5) Electrical Topologies of Wind Farms Based on Different Wind Turbines
In many countries, energy planning with a high penetration of wind energy is going
on, which includes large wind farms. These wind farms may present a significant power
contribution to the national grid, and therefore, play an important role in power quality and
the control of power systems.
Consequently, high technical demands are expected to be met by these generation
units in order to perform frequency and voltage control, the regulation of active and reactive
power, and quick responses under power system transient and dynamic situations. For
example, it may be required to reduce the power from the nominal power to 20% power
within 2 s. The power electronic technology is again an important part in both the system
configurations and the control of the wind farms in order to fulfill these demands. Also, the
overall performance of a wind farmwill largely depend on the types of the wind turbines
installed as well as the topology of the electrical system. Some possible electrical
configurations of wind farms are shown in Fig. 2.33.
A wind farm equipped with power electronic converters, as shown in Fig. 2.33(a), can
perform both real and reactive power control and also operate the wind turbines in variable
speed to maximize the energy captured as well as reduce the mechanical stress and noise.
Such a system is in operation in Denmark as a 160MWoff-shore wind power station.
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133
Figure 2.33 Wind farm solutions. (a) DFIG system with ac-grid [System A], (b) Induction generator with ac-grid [System B], (c) Speed controlled induction generator with common dc-bus
and control of active and reactive power [System C], (d) Speed controlled induction generator with common ac-grid and dc transmission [System D] (Ref:8)
Fig. 2.33(b) shows a wind farm with induction generators. A STATCOM can be used
to provide the reactive power control to meet the system reactive power control requirements,
and it can help to control the voltage, as well as, provide the reactive power demand of the
induction generators in the wind farm.
For long distance transmission of power from off-shore wind farms, HVDC may be an
interesting option. In a HVDC transmission, the low or medium ac voltage at the wind farm is
converted into a high dc voltage on the transmission side and the dc power is transferred to
the onshore system where the dc voltage is converted back into ac voltage as shown in Fig.
2.33(d). For certain power level, a HVDC transmission system, based on voltage source
converter technology, may be used in such a system instead of the conventional thyristor-
based HVDC technology. The topology may even be able to vary the speed on the wind
turbines in the complete wind farm. Another possible dc transmission system configuration is
shown in Fig. 2.33(c), where each wind turbine has its own power electronic converter and so
it is possible to operate each wind turbine at an individual optimal speed. A comparison of the
topologies of these four wind farms is given in Table 2.4. As it can be seen, the wind farms
have interesting features so as to act as a power source to the grid. Some have better abilities
than others. The overall considerations will include production, investment, maintenance, and
reliability.
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There are other possibilities, such as field excited synchronous machines or permanent
magnet synchronous generators, that may be used in the systems shown in Fig. 2.33(c) or
2.33(d). In the case of a multipole generator, the gearbox may be removed.[8]
FARM
CONFIGURATIONS
PARK A PARK B PARK C PARK D
Individual Speed Control Yes No Yes No
Control Active Power
Electronicaly
Yes No Yes Yes
Control Reactive Power Yes Centralized Yes Yes
Short Circuit (Active) Partly Partly Yes Yes
Short Circuit Power Contribute Contribute No No
Control Bandwidth 10-100ms 200ms to 2s 10-100ms 10ms to
10s
Standby-function Yes No Yes Yes
Softstarter Needed No Yes No No
Rolling Capacity on Grid Yes Partly Yes Yes
Redundancy Yes Yes No No
Investment + ++ + +
Maintenance + ++ + +
++ Cheaper, +More Expensive
Table 2.4 Comparison of Four Wind Farm Topologies(Ref:8)
6) Integration of Wind Turbines into the Power Systems
Large-scale integration of wind turbines may have significant impacts on power
system operation. Traditionally, wind turbines are not required to participate in frequency and
voltage control. However, in recent years, attention has been increased on wind farm
performance in power systems. Consequently, some grid codes have been defined to specify
the requirements that wind turbines must meet in order to be connected to the grid. Examples
of such requirements include the capability of contributing to frequency and voltage control
by continuously adjusting active power and reactive power supplied to the transmission
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system, and the power regulation rate that a wind farm must provide. Some of the
requirements can be dealt with by implementing control schemes in certain types of wind
turbines, such as reactive power control with wind turbines having power electronic
converters. Many research activities have been conducted in this area.
6.1) Requirements of Wind Turbine Grid Integration
6.1.1) Frequency and Active Power Control
The electrical supply and distribution systems used worldwide today are based on ac
systems (50 or 60 Hz). The frequency of a power system is proportional to the rotating speed
of the synchronous generators operating in the system. The generators in the same ac system
are synchronized, running at the same speed. Increasing the electrical load in the system tends
to slow down the generators and reduce the frequency. The task of frequency control of the
system is to increase or reduce the generated power so as to keep the generators operating in
the specified frequency range. However, renewable resources can only produce when the
source is available. For wind power, this is when and where the wind blows. This
characteristic is important when the amount of wind power covers a large fraction of the total
demand for electricity energy in the system. In order to be able to increase the power output
for frequency control, a wind turbine may have to operate at a lower power level than the
available power, which means low utilization of the wind energy resources. One way to
improve the situation may be the use of ―energy storage‖ technologies, such as batteries,
pump storage, and fuel cells, though the speed of response will vary depending on the energy
storage technology. So far large-scale, cost-effective energy storage technologies are yet to be
developed.[8]
6.1.2) Short Circuit Power Level and Voltage Variations
The short circuit power level at a given point in an electrical network is a measure of
its strength, and although it is not a direct parameter of voltage quality, it has a significant
influence. The ability of the grid to absorb disturbances is directly related to the short circuit
power level.
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Considering a point in the network, the voltage far away from the point may not be
influenced by the conditions at this point. Zk is the equivalent impedance between the
concerned point and the remote location. Uk, is a nominal voltage of the point, and the short
circuit power level Sk in MVA can be found as U2
k /Zk. Strong and/or weak grids are terms
often used in connection with wind power installations. If the impedance Zk is small, the
voltage variations at PCC will be small (the grid is strong), but if Zk is large, the voltage
variations will be large (the grid is weak).
Figure 2.34 A simple system with an equivalent wind power generator connected to a network.
(a) System circuit and (b) phasor diagram(Ref:8)
Fig 2.34 illustrates an equivalent wind power generation unit, connected to a network
with short circuit impedance Zk. The network voltage at the assumed remote busbar and the
voltage at the point of common coupling (PCC) are Us and Ug, respectively. The output power
and reactive power of the generation unit are Pg and Qg, which correspond to a current Ig:
(2.13)
The voltage difference ΔU between the system and the connection point is given by
(2.14)
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ΔU is related to the short circuit impedance, the real and reactive power output of the wind
power generation unit. It is clear that the variations of the generated power will result in
variations of the voltage at PCC.
Equation (2.14) indicates the relationship between the voltage and the power
transferred into the system. ΔU can be calculated with load flow methods as well as with
other simulation techniques. The voltage at PCC should be maintained within utility
regulatory limits. Operation of wind turbines may affect the voltage in the connected network.
If necessary, appropriate precautions should be taken to ensure that the wind turbine
installation does not bring the magnitude of the voltage outside the required limits.[8]
6.1.3) Reactive Power Control
Conventional reactive power concept is associated with the oscillation of energy
stored in capacitive and inductive components in a power system. Reactive power is produced
in capacitive components and consumed in inductive components.Asynchronous generator
can either produce or consume reactive power by controlling the magnetizing level of the
generator, i.e. a high magnetizing level results in high voltage and production of reactive
power.
The current associated with the reactive power flow causes system voltage drop as
aforementioned and also power losses. Furthermore, large reactive currents flowing in a
power system may cause voltage instability in the network due to the associated voltage drops
in the transmission lines. Therefore, reactive power control is important. The induction
generator based wind turbines are the consumer of reactive power. To minimize the power
losses and to increase voltage stability, these wind turbines are compensated to a level
depending on the requirements of the local utility or distribution company. For wind turbines
with PWM converter systems, the reactive power can be controlled by the converter. For
example, these wind turbines can have a power factor of 1.00 and also have the possibility to
control voltage by controlling the reactive power (generation or consumption of reactive
power).[8]
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6.1.4) Flicker
Voltage variations caused by fluctuating wind power generationmay cause voltage
quality problems. Fluctuations in the system voltage (in terms of rms value) may cause
perceptible light flicker depending on the magnitude and frequency of the fluctuation. This
type of disturbance is called voltage flicker, commonly known as flicker.
The allowable flicker limits are generally established by individual utilities. Rapid
variations in the power output from a wind turbine, such as generator switching and capacitor
switching, can also result in variations in the rms value of the voltage. At certain rate and
magnitude, the variations cause flickering of the electric light. In order to prevent flicker
emission from impairing the voltage quality, the operation of the generation units should not
cause excessive voltage flicker.
Flicker evaluation based on IEC 1000-3-7 gives guidelines for emission limits of
fluctuating loads in medium voltage and high voltage networks. The basis for the evaluation
is a measured curve giving the threshold of visibility for rectangular voltage changes applied
to an incandescent lamp. The level of flicker is quantified by the shortterm flicker severity Pst,
which is normally measured over a 10-min period. Disturbances just visible are said to have a
flicker severity factor of Pst = 1. Furthermore, a long-term flicker severity factor Plt is defined
where Plt is measured over 2-h periods.
Determination of flicker emission can be done on the basis of measurement. IEC
61000-4-15 specifies a flickermeter, which can be used to measure flicker directly. The flicker
emissions may be estimated with the coefficient and factors, cf (Ψk, va) and kf(Ψk) obtained
from the measurements, which are usually provided by wind turbine manufacturers.[8]
6.1.5) Harmonics
Harmonics are a phenomenon associated with the distortion of the voltage and current
waveforms. Any periodical function may be expressed as a sum of sinusoidal waveforms with
different frequencies including the fundamental frequency and a series of integer multiples of
the fundamental component. Depending on the harmonic order these may cause damage of
various kinds to different type of electrical equipment. All harmonics cause increased currents
and possible destructive overheating in capacitors as the impedance of a capacitor goes down
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in proportion to the increase in frequency. The higher harmonics may further give rise to
increased noise in analogue telephone circuits. The harmonic distortion is expressed as total
harmonic distortion (THD). THD and individual harmonics should meet the system
requirements.
The pulse width modulation (PWM) switching converters are used in most variable
speed wind turbine technologies today. The switching frequency is typically around a few
kilohertzs. The high-frequency harmonics are small in magnitude and are easier to be
removed by filters.[8]
6.1.6) Stability
The problem of network stability is often associated with different types of faults in
the network, such as tripping of transmission lines (e.g. overload), loss of production capacity,
and short circuits. Tripping of transmissions lines due to overload or component failure
disrupts the balance of power (active and reactive) flow. Although the capacity of the
operating generators may be adequate, large voltage drops may occur suddenly. The reactive
power flowing through new paths in a highly loaded transmission grid may force the voltage
of the network in the area down beyond the border of stability. Often a period of low voltage
is followed by complete loss of power. Loss of production capacity obviously results in a
large,momentary, power imbalance. Unless the remaining operating power plants have
enough ―spinning reserve,‖ that is, generators are not loaded to their maximum capacity, to
replace the loss within very short time, a large frequency and voltage drop will occur,
followed by complete loss of power. One way of dealing with this situation is to disconnect
the supply to some areas or some large consumers, so as to restore the power balance and to
limit the number of consumers affected by the fault.
Short circuits have a variety of forms, from the one-phase earth fault caused by trees,
to the three-phase short circuit with low impedance in the short circuit path. Many of these
faults are cleared by the relay protection of the transmission system, either by disconnection
and fast reclosure, or by disconnection of the equipment in question after a few hundred
milliseconds. In all the situations, the result is a short period with low or no voltage followed
by a period when the voltage restores. A large wind farm in the vicinity will see this event and
may be disconnect from the grid if no appropriate control has been implemented. This leads to
140
the situation ―loss of production capacity.‖ The disconnection of the wind farm will further
aggravate the situation and therefore, in some grid codes, wind turbines and wind farms are
required to have the ability of ride through. Studies show that different wind turbines may
have different control methods during the transients.[8]
6.2) Voltage Quality Assessment
The assessment of the impacts from integrating wind turbines may be performed
according to the methods given in the IEC 61400-21 /2/ to determine the acceptability of such
integration. Methods include:
• steady-state voltage
• flicker
• harmonics
6.2.1) Steady-State Voltage
The grid and wind turbine voltage should be maintained within the utility limits.
Operation of a wind turbine may affect the steady-state voltage in the network. It is
recommended that load-flow analyses be conducted to assess this effect to ensure that the
wind turbine installation does not bring the magnitude of the voltage beyond the required
limits of the network. In general, some extreme case of the loads and the wind turbine
production may be checked for compatibility, such as
• low loads and low wind power,
• low loads and high wind power,
• high loads and low wind power, and
• high loads and high wind power.
Depending on the scope of the load-flow analysis, a wind turbine installation may be
assumed as a PQ node, which may use 10-min average data (Pmc and Qmc ) or 60-s average
data (P60 and Q60) or 0.2-s average data (P0.2 and Q0.2).
A wind farm with multiple wind turbines may be represented with its output power at
the PCC. Ten-minute average data (Pmc and Qmc) and 60-s average data (P60 and Q60) can be
141
calculated by simple summation of the output from each wind turbine, whereas 0.2-s average
data (P0.2 and Q0.2) may be calculated according to (2.15) and (2.16):
(2.15)
(2.16)
where Pn,i and Qn,i are the rated real and reactive power of the individual wind turbine and Nwt
is the number of wind turbines in the group.[8]
6.2.2) Voltage Fluctuations
There are two types of flicker emissions: the flicker emission during continuous
operation and the flicker emission due to generator and capacitor switchings. Often, one or the
other will be predominant.
The flicker emissions from a wind turbine installation should be limited to comply
with the flicker emission limits. However, different utilities may have different flicker
emission limits. The assessments of the flicker emissions are described below.
6.2.2.1) Continuous Operation
The flicker emission from a single wind turbine during continuous operation may be
estimated by
(2.17)
where cf (Ψk, va) is the flicker coefficient of the wind turbine for the given network impedance
phase angle Ψk at the PCC and for the given annual average wind speed va at hub-height of
the wind turbine.
A table of data produced from the measurements at a number of specified impedance
angles and wind speeds can be provided by wind turbine manufacturers. From the table, the
flicker coefficient of the wind turbine for the actual Ψk and va at the site may be found by
applying linear interpolation. The flicker emission from a group of wind turbines connected to
the PCC is estimated by (2.18):
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(2.18)
where cf,i (Ψk, va) is the flicker coefficient of the individual wind turbine, Sn,i is the rated
apparent power of the individual wind turbine, and Nwt is the number of wind turbines
connected to the PCC.
If the limits of the flicker emission are known, the maximum allowable number of
wind turbines for connection can be determined.[8]
6.2.2.2) Switching Operations
The flicker emission due to switching operations of a single wind turbine can be
calculated as
(2.19)
where kf(Ψk) is the flicker step factor of the wind turbine for the given Ψk at the PCC. The
flicker step factor of the wind turbine for the actual Ψk at the site may be found by applying
linear interpolation to the table of data produced from the measurements by wind turbine
manufacturers.
The flicker emission from a group of wind turbines connected to the PCC can be
estimated from
(2.20)
where N10,i and N120,i are the number of switching operations of the individual wind turbine
within 10-min and 2-h periods, respectively, kf,i (Ψk) is the flicker step factor of the individual
wind turbine, and Sn,i is the rated apparent power of the individual wind turbine. Again, if the
limits of the flicker emission are given, the maximum allowable number of switching
operations in a specified period, or the maximum permissible flicker emission factor, or the
required short circuit capacity at the PCC may be determined.[8]
6.2.3) Harmonics
A wind turbine with an induction generator directly connected to the grid is not
expected to cause any significant harmonic distortions during normal operation. Only wind
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turbines with power electronics need to be checked concerning harmonics. The harmonic
current emission of such wind turbine system is normally given in the power quality data
sheet, while the limits for harmonics are often specified for harmonic voltages. Thus harmonic
voltages should be calculated from the harmonic currents of the wind turbine, which requires
the information of the grid impedances at different frequencies.[8]
2.2.5 Environmental Aspects
2.2.5.1 Audible Noise
A wind turbine with an induction generator directly connected to the grid is not
expected to cause any significant harmonic distortions during normal operation. Only wind
turbines with power electronics need to be checked concerning harmonics. The harmonic
current emission of such wind turbine system is normally given in the power quality data
sheet, while the limits for harmonics are often specified for harmonic voltages. Thus harmonic
voltages should be calculated from the harmonic currents of the wind turbine, which requires
the information of the grid impedances at different frequencies.
Source Noise Level (dB)
Elevated Train 100
Noisy Factory 90
Average Street 70
Average Factory 60
Average Office 50
Quiet Conversation 30
Table 2.5 Noise Levels of Some Commonly Known Sources Compared with Wind Turbine(Ref:3)
The table indicates that the turbine at a 50-m distance produces no noise higher than
the average factory. This noise, however, is a steady noise. Additionally, the turbine makes a
louder noise while yawing under the changing wind direction. In either case, the local noise
ordinance must be complied with. In some instances, there have been cases of noise
complaints reported by the nearby communities. Although noise pollution is not a major
144
problem with offshore wind farms, it depends on the size whether or not one can hear the
turbines while operating. It has also been suggested that the noise from the turbines travels
underwater and disturbs sea life as well.
In general, there are two main sources of noise emitted from the wind turbine. One is
mechanical, which is inherent in the gearing system. The other is created by the aerodynamics
of the rotating blade, which emits a noise when passing the tower, known as the tower thump
or simply the aerodynamic noise. The first may be at a somewhat low level, generally uniform
over the year. The other (the tower thump) can be loud. It varies with the speed of blade
rotation and may cause most of the problems and complaints. Some residents describe the
tower thump noise as being like a boot in a tumble dryer. A large wind turbine can produce an
aggregate noise level of up to 100 dB(A), which weakens to a normal level within a 1.5-km
distance. The worst conditions are when the wind is blowing lightly and the back-ground
noise is minimal. Residents up to 1-km radius have complained to the Environmental Health
Department about noise from such turbines.[3]
2.2.5.2 Electromagnetic Interference (EMI)
Any stationary or moving structure in the proximity of a radio or TV tower interferes
with the signals. The wind turbine tower, being a large structure, can cause objectionable EMI
in the performance of a nearby transmitter or a receiver. Additionally, the rotating blades of
an operating wind turbine may reflect impinging signals so that the electromagnetic signals in
the neighbourhood may experience interference at the blade passage frequency. The exact
nature and magnitude of such EMIs depend on a number of parameters. The primary
parameters are the location of the wind turbine tower relative to the radio or TV tower,
physical and electrical properties of the rotor blades, the signal frequency modulation scheme,
and the high-frequency electromagnetic wave propagation characteristics in the local
atmosphere. EMI may be a serious issue with wind farm planning. For example, 5 of the 18
offshore wind farms planned around the U.K. coasts were blocked by the U.K. Ministry of
Defense due to concerns that they may interfere with radar and flight paths to airfields close
to the proposed sites. Detailed studies on the precise effects of wind turbines on radar and
possible modifications in radar software may mitigate the concerns. The potential cost of such
studies and legal appeals should be factored into the initial planning of large wind farms.[3]
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2.2.5.3 Effect on Birds
The effect of wind farms on wild life and avian population that includes endangered
species protected by federal laws has created controversy and confusion within the
mainstream environmental community. The breeding and feeding patterns of some birds may
be disturbed. The wind turbine blade can weigh up to 1.5 t and the blade tips can travel at up
to 200 mph, a lethal weapon against any airborne creature. The birds may be killed or at least
injured if they collide with a blade. Often the suction draft created by the wind flowing to a
turbine draws the birds into the airstream headed for the blades. Although less usual, birds are
attracted by the tower hum and simply fly into the towers. On the other hand, studies at an
inshore site near Denmark have determined that birds alter their flight paths 200 m around the
turbine. Thus, a wind farm can significantly alter the flight paths of large avian populations.
In another study, the population of water fowl declined 75 to 90% within 3 yr after installing
an offshore wind farm in Denmark. Such a large decline could have a massive impact on the
ecosystem of the surrounding area.
The initial high bird-kill rate of the 1980s has significantly declined with larger
turbines having longer slower-moving blades, which are easier for the birds to see and avoid.
Tubular towers have a lower bird-hit rate compared to lattice towers, which attract birds to
nest. The turbines are now mounted on either solid tubular towers or towers with diagonal
bracing, eliminating the horizontal supports that attracted the birds for nesting. New wind
farms are also sited away from avian flight paths.
It is generally agreed that migration paths and nesting grounds of rare species of birds
should be protected against the threat of wind farms. Under these concerns, obtaining
permission from the local planning authorities can take considerable time and effort.[3]
2.2.5.4 Other Impacts
The visual impact of the wind farm may be unpleasant to the property owners around
the wind farm. Wind farm designers can minimize aesthetic complaints by installing identical
turbines and spacing them uniformly.
Because wind is a major transporter of energy across the globe, the impact of the
energy removed by many large wind farms on a grand scale may impact the climate.[3]
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2.2.6 Potential Catastrophes
A fire or an earthquake can be a major catastrophe for a wind power plant.
2.2.6.1 Fire
Fire damage amounts to 10 to 20% of the wind plant insurance claims. A fire on a
wind turbine is rare but difficult to fight. Reaching the hub height is slow, and water pressure
is always insufficient to extinguish the fire. This generally leads to a total loss of the turbine,
leading to 9 to 12 months of downtime and lost revenue. The cost of replacing a single 30-m
blade can exceed $100,000, and that of replacing the whole 3-MW turbine can exceed $2
million.
The following are some causes of fire in wind turbines:
1) Lightning Strike
Lightning arresters are used to protect the turbine blades, nacelle, and tower assembly.
However, if lightning is not properly snubbed, it can lead to local damage or total damage if it
leads to sparks and subsequent turbine fire. Lightning occurrences depend on the location.
Offshore turbines are more prone to lightning than land turbines. On land, lightning is rare in
Denmark, whereas it is frequent in northern Germany and the Alps regions, and even more
frequent in parts of Japan and the U.S., particularly in Florida and Texas. The growing trends
of using electrically conducting carbon fiber-epoxy composites for their high strength and low
weight in the blade construction make the blades more vulnerable to lightning.
For this reason, some manufacturers avoid carbon fibers in their blades, more so in
large, tall turbines for offshore installations.[3]
2) Internal Fault
Any electrical or mechanical fault leading to a spark with the transmission fluids or other
lubricants is a major risk. The flammable plastic used in the construction, such as the nacelle
covers, is also a risk.
147
Typical internal faults that can cause excessive heat leading to a fire are as follows:
• Bearings running dry and failing
• Failing cooling system
• Brakes becoming hot under sustained braking
• Oil and grease spills
• Short circuit in the battery pack of the pitch-control system
• Cables running against rotating or vibrating components
Frequent physical checks of the entire installation, servicing and maintenance, and a
condition-monitoring system, accessed remotely by computers used in modern installations,
can detect potential fire hazards and avoid fires.[3]
2.2.6.2 Earthquake
Lateral loads resulting from an earthquake are important data to consider in designing
a tall structure in many parts of the world. The wind tower, being always tall, is especially
vulnerable to seismic events. The seismic energy is concentrated in the 1- to 10-Hz frequency
band. A dynamic analysis is required, as a dynamic response amplification is expected.
However, because of the complexities in modeling and performing such analyses, it has been
standard practice to represent seismic loads with equivalent static loads. The severity of the
seismic loads, the potential failure modes, and the resulting effects require that design
engineers make reasonable tradeoffs between potential safety concerns and economics during
the design phase. To alleviate this difficulty, the recent trend in the U.S. has been to require
dynamic analyses to estimate seismic stresses. For example, all primary components of
nuclear power plants in the U.S. must be dynamically analyzed for the specified seismic
loadings. This is required even for plants located in seismically inactive areas.[3]
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2.2.7 Potential Technology Development and Recent Trends
2.2.7.1 Potential Technology Development
Figure 2.35 World Total Installed Capacity in 2001-2010 (Ref: 9)
According to the World Wind Energy Report 2009:
Worldwide capacity reached 159213 MW, out of which 38312 MW were added.
(Figure 2.35)
Wind power showed a growth rate of 31,7 %, the highest rate since 2001.
The trend continued that wind capacity doubles every three years.
All wind turbines installed by the end of 2009 worldwide are generating 340 TW per
annum, equivalent to the total electricity demand of Italy, the seventh largest economy of the
world, and equalling 2 % of global electricity consumption.
The wind sector in 2009 had a turnover of 50 billion.
The wind sector employed 550000 persons worldwide. In the year 2012, the wind
industry is expected for the first time to offer 1 million jobs.
China continued its role as the locomotive of the international wind industry and
added 13800 MW within one year – as the biggest market for new turbines –, more than
doubling the installations for the fourth year in a row.
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The USA maintained its number one position in terms of total installed capacity and
China became number two in total capacity, only slightly ahead of Germany, both of them
with around 26000 Megawatt of wind capacity installed.
Asia accounted for the largest share of new installations (40,4 %), followed by North
America (28,4 %) and Europe fell back to the third place (27,3 %).
Latin America showed encouraging growth and more than doubled it installations,
mainly due to Brazil and Mexico.
A total wind capacity of 200000 Megawatt will be exceeded within the year 2010.
Based on accelerated development and further improved policies, WWEA increases its
predictions and sees a global capacity of 1900000 Megawatt as possible by the year 2020. [9]
Figure 2.36 Top 10 Countries by Growth Rate in 2008 and 2009 (Ref:9)
The growth rate is the relation between the new installed wind power capacity and the
installed capacity of the previous year. The highest growth rates of the year 2009 with more
than 100 % could be found in Mexico which quadrupled its installed capacity, once again in
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Turkey (132 %) which had the highest rate in the previous year, in China (113 %) as well as
in Morocco (104 %).(Figure 2.36)[9]
In the year 2009, altogether 82 countries used wind energy on a commercial basis, out
of which 49 countries increased their installed capacity.[9]
China and the USA established themselves as the by far largest markets for new wind
capacity, together accounting for 61,9 % of the additional capacity, a share which was
substantially bigger than in the previous year (53,7 %).[9]
Figure 2.37 Top 10 Countries by Total Capacities in 2008 and 2009 (Ref: 9)
Nine further countries could be seen as major markets, with turbine sales in a range
between 0,5 and 2,5 Gigawatt: Spain, Germany, India, France, Italy, the United Kingdom,
Canada, Portugal, and Sweden. [9]
Country Share of New Capacity 2009:
Twelve markets for new turbines had a medium size between 100 and 500 Megawatt: Turkey,
Australia, Denmark, Mexico, Brazil Ireland, Poland, Japan, New Zealand, Belgium, South
Korea, and Greece. [9]
Offshore Wind Turbines:
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Offshore wind capacity continued to grow in the year 2009. By the end of the year,
wind farms installed in the sea could be found in twelve countries, ten of them in Europe and
some minor installations in China and Japan. Total installed capacity amounted to almost two
Gigawatt, 1,2 % of the total wind capacity worldwide. Wind turbines with a capacity of 454
Megawatt were added in 2009, with major new offshore wind farms in Denmark, the United
Kingdom, Germany, Sweden and China. The growth rate of offshore wind is with 30 %
slightly below the general growth rate of wind power. In Denmark, so far the largest offshore
wind farm was inaugurated in the North Sea: Horns Rev II, 209 Megawatt. China installed the
first major offshore wind farm outside of Europe – a 21 Megawatt, near Shanghai.[9]
Position
2009
Country Total
Offshore
Capacity
[MW] End
2009
New
Offshore
Capacity
[MW]
Installed in
2009
Total
Offshore
Capacity
[MW] End
2008
Rate of
Growth [%]
1 United Kingdom 688,0 104,0 574,0 18,1
2 Denmark 663,6 237,0 426,6 55,6
3 Netherlands 247,0 0,0 247,0 0,0
4 Sweden 164,0 30,0 134,0 22,4
5 Germany 72,0 60,0 12,0 500,0
6 Belgium 30,0 0,0 30,0 0,0
7 Finland 30,0 0,0 30,0 0,0
8 Ireland 25,0 0,0 25,0 0,0
9 China 23,0 21,0 2,0 1050,0
10 Spain 10,0 0,0 10,0 0,0
11 Norway 2,3 2,3 0,0 /
12 Japan 1,0 0,0 1,0 0,0
TOTAL 1955,9 454,3 1491,6 30,5
Table 2.6 Offshore Overall Capacity in 2009 (Ref: 9)
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Figure 2.38 Top 5 Countries in Offshore Wind (Ref: 9)
Continental Distribution:
Figure 2.39 Continental Distribution 2007-2009(Ref: 9)
The most dynamic progress of the wind industry took place in Asia, followed by North
America and the focus of the global wind sector moved further away from Europe.[9]
2.2.7.2 Recent Trends
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1) Small Wind Systems
The vast majority of wind power is generated by large wind turbines feeding into the
electricity grid, while small wind turbines generally provide electricity directly to customers.
The United States is the leading world producer of small wind turbines. These
residential turbines are erected and connected directly to the customer‘s facility or to the
electricity distribution system at the customer‘s site. The manufacture and marketing of wind-
powered electric systems sized for residential homes, farms, and small businesses have
experienced major growth in the past decade. These small wind turbines, defined as 100 kW
or less in capacity, have seen significant market growth, and the industry has set ambitious
growth targets: growth at 18 to 20 percent through 2010.[5]
2) System-Design Trends
Significant research and development work is underway at the NREL and the National
Wind Technology Center in Golden, CO. The main areas of applied research conducted at the
NWTC are as follows:
• Aerodynamics to increase energy capture and reduce acoustic impacts.
• Inflow and turbulence to understand the nature of wind.
• Structural dynamics models to minimize the need of prototypes.
• Controls to enhance energy capture, reduce loads, and maintain stable closed-loop behavior
of these flexible systems are an important design goal.
• The wind turbine design progress includes larger turbines on taller towers to capture higher
wind speed; the design difficulty increases as these machines become larger and the towers
become taller.[3]
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2.2.8 Future Expectations
2.2.8.1 Short Term: Present to 2020
The key technological issues for wind power focus on continuing to develop better
turbine components and to improve the integration of wind power into the electricity system,
including operations and maintenance, evaluation, and forecasting. Goals appear relatively
straightforward: taller towers; larger rotors; power electronics; reducing the weight of
equipment at the top and cables coming from top to bottom; and ongoing progress through the
design and manufacturing learning curve. Figure 2.40 summarizes the incremental
improvements under consideration.
Although no big breakthroughs are anticipated, continuous improvement of existing
components is anticipated, and many are already being actively developed. For example, there
are advanced rotors that use new airfoil shapes specifically designed for wind turbines,
instead of those based on the design of helicopter blades. These rotors are thicker at points of
highest stress and reduce loads during turbulent winds by flying the blades using turbine
control systems. Other improvements include the use of composite materials and advanced
drive trains. In particular, gear boxes are a major area of concern for reliability. Approaches
for improving of this component include direct drive generators; greater use of rare-earth
permanent magnets in generator design; possibility of single-stage drives using low-speed
generators; and distributed drive trains using the rotor to drive several parallel generators.
Advanced towers are a major focus for innovation, given the current need for large cranes and
transport of large tower and blade sections. Concepts under investigation include self-erecting
towers, blade manufacturing on site, vibration damping, and tower drive train interactions.
There is certain to be some development of offshore wind in the United States in the
near term, but it is not expected that this will have a significant impact before 2020.
Nonetheless, there is a near-term opportunity to learn from offshore projects in Europe and
the United States, if offshore wind is going to have an impact in the medium term.
Other near-term opportunities will lie in improving the integration of existing wind
power plants into the transmission and distribution system, which includes using improved
computational models for simulating and optimizing system integration.[5]
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Figure 2.40 Areas of Potential Wind Power Technology Improvements(Ref:5)
2.2.8.2 Medium Term: 2020 to 2035
Mid-term wind technology development will have two thrusts: the movement toward
offshore, and its implications for turbine design; and the development of efficient low-wind
speed turbines. Development of offshore wind power plants has already begun in Europe
(approximately 1200 MW of installed capacity), but progress has been slower in the United
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States. Nine projects are in various stages of development in state and federal waters. In
addition to technical risks and higher costs, these projects have been slowed by social and
regulatory challenges.
In the mid-term, offshore turbines will have a larger size and generating capacity than
onshore turbines, but, due primarily to technical and cost concerns, development will likely
lag behind onshore machines. Transmission siting issues with offshore wind power plants will
be simplified because of fewer siting impediments. However, underwater cables must be
carefully constructed, and there will likely be a move to develop microgrids with HVDC to
integrate the offshore resources. Offshore wind technologies face several transition problems
as they move from near-shore, land-based sites to offshore sites of various depths, and finally
floating designs. Assessment tools for sensitive marine areas, wind loads, and system design
are not now ready for offshore development. Offshore projects must be built to handle both
wind and wave loads, and components must be able to endure marine moisture and extreme
weather. Offshore wind projects have a higher balance of station cost (approximately 2/3 of
total costs) than onshore projects, and thus will rely on cost reductions across the system in
order to become more competitive. All of these developments pose both technological and
organizational problems and will require continuous research and development in order to be
feasible. It should be noted that challenges posed by the greater technical difficulties of
offshore wind power development are being addressed by other countries. However, political,
organizational, social, and economic obstacles may continue to inhibit investment in offshore
wind power development, given the higher risk compared to onshore wind energy
development.
In terms of onshore development, as the higher wind speed sites are used, wind power
development will move to lower wind speed sites, which will require turbines that are
relatively efficient at lower wind speeds, necessitating larger rotors with lighter, stronger
materials, and increased tower height.[5]
2.2.8.3 Long Term: After 2035
At present, no revolutionary technology to extract energy from wind has been
proposed, but several designs, e.g., vertical wind turbines or eggbeaters, are again under
consideration. There have been conceptual proposals to access high-altitude winds using
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balloons or kites. Component improvements will continue, with additional emphasis on
offshore turbine installation. Floating offshore platforms may gain interest, but first must
come experience from anchored offshore wind facilities.[5]
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2.3 Geothermal Energy and Electric Generation
2.3.1 Introduction
Geothermal comes from the Greek words thermal which means heat and geo which
means earth. Georthemal is the thermal energy contained in the rock and fluid in the earth‘s
crust. It is almost 4,000 miles from the surface of the earth to its center; the deeper it is the
hotter it gets. The outer layer of the earth, the crust, is 35 miles thick and insulates the surface
from the hot interior.
After the Second World War many countries started using geothermal energy,
considering it to be economically competitive with other forms of energy. Geothermal energy
did not have to be imported and, in some cases, it was the only energy source available
locally.[10]
Over the past 20–25 years, worldwide electricity production based on geothermal
sources has increased significantly; the installed generating capacity has grown from
1300MWe in 1975 to almost 10,000MWe in 2007. About 75% of this nearly 9000MWe
increase comes from about 20 sites that produce in excess of 100MWe. These geothermal
power projects convert the energy contained in hot rock into electricity by using water to
adsorb heat from the rock and transport it to the earth‘s surface, where it is converted to
electrical energy through turbine-generators. Moreover, direct applications of geothermal heat
offsetting the need for electricity production and burning of fossil fuels has also gained
importance over the years; the estimated installed thermal capacity of direct-use projects was
more than 28,000MWt in 2005.
It is estimated that more than 97% of current geothermal reservoir production is
frommagmatically driven reservoirs. Geothermal reservoirs may also develop outside regions
of recent volcanic activity, where deeply penetrating faults allow groundwater to circulate to
depths of several kilometers and become heated by the geothermal gradient.
More than 90% of exploited fields are ―liquid-dominated‖ under pre-exploitation
conditions with reservoir pressures increasing with depth in response to liquid-phase density.
―Vapour-dominated‖ systems, such as The Geysers in California (USA) and Larderello (Italy)
have vertical pressure gradients controlled by the density of steam. In the vapor-dominated
systems, steam is cleaned and then passed directly into low-pressure turbines. Typically,
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water from high-temperature (>240 ◦C) reservoirs is partially flashed to steam. Heat is
converted to mechanical energy by passing steam through low-pressure steam turbines. A
small fraction of geothermal generationworldwide is generated using a heat exchanger and
secondary working fluid to drive turbines.[11]
2.3.2 Geothermal Resources
Geothermal energy (earth heat) can be found anywhere in the world. But the high-
temperature energy that is needed to drive electric generation stations is found in relatively
few places.
Figure 3.1 A Geothermal Reservoir [Ref:12]
2.3.2.1 Model of a Hydrothermal Geothermal Resource
There appear to be five features that are essential to making a hydrothermal (i.e., hot
water) geothermal resource commercially viable. They are:
(a) a large heat source
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(b) a permeable reservoir
(c) a supply of water
(d) an overlying layer of impervious rock
(e) a reliable recharge mechanism.
A highly schematic depiction of such a system is shown in Fig. 3.2.
Cold recharge water is seen arriving as rain (point A) and percolating through faults
and fractures deep into the formation where it comes in contact with heated rocks. The
permeable layer offers a path of lower resistance (point B) and as the liquid heats it becomes
less dense and tends to rise within the formation. If it encounters a major fault (point C) it will
ascend toward the surface, losing pressure as it rises until it reaches the boiling point for its
temperature (point D). There it flashes into steam which emerges as a fumarole, a hot spring,
a mud pot, or a steam-heated pool (point E). The boiling curve is the locus of saturation
temperatures that correspond to the local fluid hydrostatic pressure.
Figure 3.2 Schematic Model of a Hydrothermal Geothermal System(Ref:13)
The intent of a geothermal development project is to locate such systems and produce
them by means of strategically drilled wells. As might be presumed, most (but not all)
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hydrothermal systems give away their general location through surface thermal manifestations
such as the ones described above.
If any one of the five features listed as needed for a viable hydrothermal resource is
lacking, the field generally will not be worth exploiting. For example, without a large heat
source geofluid temperatures will be relatively low, i.e., the thermal energy of the system will
be insufficient to support exploitation long enough to make it economic. Without sufficient
permeability in the formation, the fluid will not be able to move readily through it, i.e., it will
not be able to remove much of the stored thermal energy in the rock. Furthermore, low
permeability will cause poor well flow or, even worse, may prevent any production from the
reservoir. Without fluid in the system there is no heat transfer medium and the thermal energy
of the formation will remain in the reservoir. Without an impermeable cap rock, the geofluids
will easily escape to the surface appearing as numerous thermal manifestations and the
pressure in the formation will quickly dissipate. And lastly, without a reliable and ample
recharge to the reservoir, the geofluid will eventually become depleted when it supplies a
power plant.
With the exception of requirements (a) and (d), deficiencies in the others have been
addressed through research and field practice. Insufficient permeability can sometimes be
remedied by artificial means such as hydraulic fracturing (called "hydrofracing") in which
high-pressure liquid is injected from the surface through wells to open fractures by means of
stress cracking. However, unless the newly created widened fractures are held open with
"proppants" they will re-close when the injection ceases. If little water is present in the
formation or recharge is meager, all untlsed geofluid from the plant can be reinjected.
Furthermore, external fluids can be brought to the site by some means and injected into the
formation. Such a process exists at The Geysers field in northern California in the United
States in which treated municipal waste water from nearby communities is sent to the field via
pipeline to assist in the maintenance of an inventory of fluid in the reservoir.[13]
2.3.2.2 Other Types of Geothermal Resources
As of 2004, hydrothermal resources are the only geothermal systems that have been
developed commercially for electric power generation. However, there are three other forms
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of geothermal energy that someday may reach the commercial stage. They are: hot dry rock,
HDR (or enhanced geothermal systems, EGS): geopressure; and magma energy.
1) Hot Dry Rock (HDR)
There are many geothermal prospects that have high temperature but are lacking fluid
in the formation or the permeability is too low to support commercial development. These
systems can be "enhanced" by engineering the reservoirs through hydraulic fracturing. An
injection well is drilled into the hot formation to a depth corresponding to the promising zone.
Cold water is injected under high pressure to open existing fractures or create new ones.
Once the formation reaches a state of sufficient volume and permeability, another well (or
wells) is drilled to intercept the newly formed "reservoir". Ideally, a closed loop is thus
created whereby cold water is pumped down the injection well and returned to the surface
through the production well after passing through the hot, artificially-fractured formation. The
ideal HDR concept is illustrated in Fig. 3.3.
Figure 3.3 Ideal Hot Dry Rock Production Scheme(Ref:13)
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Considerable research has gone into development of the HDR concept and a good deal
continues today. Table 3.1 summarizes these projects.
Country Location Dates
United States Fenton Hill, New Mexico 1973-1996
United Kingdom Rosemanowes 1977-1991
Germany Bad Urach 1977-1990
Japan Hijiori 1985-present
Ogachi 1986-present
France Soultz 1987-present
Switzerland Basel 1996-present
Australia Hunter Valley 2001-present
Cooper Basin 2002-present
Table 3.1 HDR Projects Worldwide(Ref:13)
There are many practical problems in developing a HDR system. It is difficult to
control very deep, directional, geothermal wells. Drilling techniques in the oil industry now
permit wells to be turned 90 ~ while being drilled, allowing the well to drain several vertical
pockets of petroleum. However, oil wells tend to be shallower than the ones envisioned for
HDR, the temperatures encountered are far lower, and the rocks are not as hard as those found
in geothermal regions. Furthermore, the HDR wells must be precisely aimed to hit the deep
target in order to form a closed fluid circuit. Lastly. if some of the engineered fractures are not
connected to the production well, injected fluid may be lost to the formation. This would
require continuous makeup water to maintain the power plant in operation. Some of these
difficulties appear to have been at least partially solved in the on-going research, particularly
at the Japanese sites.[13]
2) Geopressure
Along the western and northern coastline of the Gulf of Mexico, there is a potent
energy resource called "geopressure". During the drilling for oil and gas in the sedimentary
coastal areas of Texas and Louisiana, fluids have been encountered with pressures greater
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than hydrostatic and approaching lithostatic. Hydrostatic pressure increases with depth in
proportion to the weight of water, i.e., at about 0.465 lbf/in2 per ft. However in formations
where the fluid plays a supportive role in maintaining the structure of the reservoir, the weight
of the solid overburden roughly doubles the gradient to approach the lithostatic value of 1.0
lbf/in2 per ft.
Geopressured reservoirs were formed along the Gulf Coast through the steady
deposition of sediments that created an overburden on the underlying strata. Figure 3.4 is a
simplified cross-section through a geopressured reservoir. Periodically, subsidence occurred
causing compaction of the rock layers. Subsidence also resulted in steeply dipping faults that
can isolate elements of the formation. With the heavy overburden and no way to dissipate the
load, the pressure within these lenses of sand grows to levels in excess of hydrostatic.
In the geopressured reservoirs of the Gulf Coast, the pressures were sufficiently high
to prevent drilling for oil and gas. With improved understanding of these zones and better
drilling techniques, these reservoirs can now be safely drilled.
Geopressured reservoirs are characterized by three important properties that make
them potentially attractive for geothermal exploitation: (1) very high pressure, (2) high
temperature, and (3) dissolved methane.
Figure 3.4 Cross-section Schematic of a Geopressured Reservoir(Ref:13)
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The first property allows the use of a hydraulic turbine to extract the mechanical
energy stored in the form of high pressure; the second property allows the use of a heat engine
of some kind to extract the thermal energy; and the last property allows for either the
combustion of the gas on site for power generation or for sale to enhance the economics of a
development project.
However, there are six criteria that must be satisfied before geopressured reservoirs
can be commercially developed; these are:
Is the fluid hot enough, say >230°C?
Is there sufficient methane dissolved in the fluid?
Is the high-pressure sand sufficiently permeable?
Is the high-pressure sand sufficiently thick?
Is the sand formation fault-bounded but not too fractured?
Can we guarantee that no subsidence will occur?
The economic viability of a geopressured geothermal project requires a "yes" answer to all of
these questions.[13]
3) Magma Energy
The last of the geothermal resources is one that goes directly to the source of the heat,
namely, a magma body relatively close to the surface of the earth. The concept is to drill a
well into the magma, insert an injection pipe and pump cold water down the well under great
pressure. The cold fluid will solidify the molten magma into a glassy substance that should
crack under the thermal stress imposed on it. If the water can be made to return to the surface
by passing upward through the cracked, extremely hot glassy material, it would reach the
surface hot and ready for use in a Rankine-type power plant.
As simple as it is to describe the concept, it is not as easy to carry out such a plan. The
U.S. Dept. of Energy conducted two research projects aimed at understanding the magma
environment in the 1970s and 1980s. The first one was carried out at the lava lake within the
crater of Kilauea Iki on the island of Hawaii. This effort succeeded in drilling through the
solidified crust of the lake into the still-molten lava that had a temperature of about 1000°C
(1800°F). In fact 105m of core were obtained from the melt zone and several experiments
were run to understand the mechanism of energy extraction from a lava body.
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The second research program, the Magma Energy Program, was directed at obtaining
a better scientific understanding of the existence and behavior of large magma bodies within
calderas. The one selected for study in the mid- 1980s was the Long Valley caldera in central
California and the research was performed by the Sandia National Laboratory of
Albuquerque, New Mexico. The caldera is an oval-shaped region about 18x32 km with a
prominent resurgent dome. At the time, the dome had risen some 235 mm over the period
from 1980-1985, making it both scientifically interesting and practically important to gain a
clearer understanding of the phenomenon.
The original goals of the program were to:
Demonstrate the existence of crustal magma bodies at depths less than25,000 ft:
Develop and test new drilling technology for hostile environments:
Better understand the creation and evolution of the Long Valley caldera:
and
Better define the hydrothermal system related to the caldera.
An ambitious exploration well was planned, targeted for a final depth of 20,000 ft (6000 m).
The conceptual design of the well is shown in Fig. 3.5 (to scale in vertical direction). Since an
existing 40-in diameter mud riser was in place to a depth of 39 ft from an earlier aborted well.
this was used instead of the planned 40-in surface casing. The well was to be drilled in four
phases: Phase I- to 2500 ft, Phase II- to 7500 ft, Phase III- to 14,000 ft or 300°C (600°F)
whichever came first, and Phase IV- to a total depth of 20,000 ft and 500°C (900°F). In 1989,
Phase I was successful in reaching 2568 ft with the 20-in casing, alter encountering massive
lost circulation at the shallowest depths. Phase II was completed to 7588 ft in November
1991. Core samples were taken at the 2568 ft and the 7588 ft points by drilling ahead some
100-200 ft. The well was not continued beyond Phase II owing to a shift in DOE policy away
from fundamental research and more toward applied research. In 1996 the well was handed
over to the U.S. Geological Survey for use as a monitoring well.
Since the well only reached depths that were routinely achieved at other geothermal
fields, it failed to produce much new drilling technology. For example, it had been planned to
develop insulated drill pipe to maintain the drilling muds at reasonable temperatures in
extremely high temperature formations: this was not done. It did produce some scientific
information that led to a better understanding of the nature of the Long Valley caldera, but no
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further projects have appeared to try to tap the vast amount of thermal energy contained in
near-surface magma bodies.[13]
Figure 3.5 Conceptual Design of Long Valley Magma Energy Exploratory Well(Ref:13)
2.3.3 Geothermal Power Plants
Geothermal power plants use the natural hot water and steam from the earth to turn
turbine generators for producing electricity. Unlike fossil fuel power plants, no fuel is burned
168
in these plants. Geothermal power plants give off water vapours but have no smoky
emissions. Geothermal electricity is for the base load power as well as the peak load demand.
Geothermal electricity has become competitive with conventional energy sources in many
parts of the world.[10]
Figure 3.6 A Geothermal System(Ref:12)
Figure 3.7 Turbine Generator(Ref:12)
169
Natural steam from the production wells power the turbine generator. The steam is
condensed by evaporation in the cooling tower and pumped down an injection well to sustain
production.
Like all steam turbine generators, the force of steam is used to spin the turbine blades
which spin the generator, producing electricity. But with geothermal energy, no fuels are
burned.[12]
2.3.3.1 Dry (Direct) Steam Power Plants
Dry-steam plants were the first type of geothermal power plant to achieve commercial
status. Their history goes back 100 years to 1904 when Prince Piero Ginori Conti built and
operated a tiny steam engine using the natural steam jets that issued from the ground at
Larderello in the Tuscany region of Italy. Since the geofluid consisted solely of steam, it was
fairly easy to hook up a mechanical device to take advantage of the available energy.
Although the Prince's engine only generated enough electrical power to illuminate five light
bulbs in his factory, it was the springboard for larger plants.[13]
It is worthy to notice that the presented structure is also considered in the field of
aeronautics, the hydraulic turbine is then replaced by a jet engine.[31]
2.6.5 Hydropower’s Future in a Fluid Energy World
To predict the future is comparable to purchasing a lottery number and then mortgage
the expected winnings to a dwelling anticipating to win the pot. Time parameters need to be
clearly identified to impart meaning to the prediction of hydropower‘s future. Hydropower in
different countries is in varied phases of utilization of the respective states‘ hydroelectric
potential. Myanmar in 2000 had 365MW of installed hydropower, but plans call for installing
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39,600MW in the next two decades. Hydropower as a domestic electricity resource has and
continues to serve as an incubator energy source to change the standard of life of the state that
turns to harnessing it. Hydropower‘s past is instructive to evaluate its future while the
installation process has been significantly changed.
Data are like guardrails in uncertain mountain terrain. Data serve to project future
electricity demand and the construction response is in the form of hydroelectric project
construction schedules to meet the anticipated electricity need. Hydroelectric projects are
capital demanding and schedule sensitive. Projected construction values provide a road map
how different systems anticipate to serve future electricity needs within a given time frame.
The world‘s hydroelectric systems will add 157.8 GW in 2008, and nearly 83% of this
expansion is placed in Asia. Of the 130 GW in Asia, China builds 80 GW, or 61%. These data
serve to illustrate unevenness in distribution globally and regional electricity planning policy
differences on how to foster energy autonomy. The dominance of the hydroelectric sector in
Asia and South America points to the introduction of energy availability and the
industrialization process in these two regions since WWII. Power and change gravitate
towards each other.
Norway and Switzerland turned early to hydropower as coal-poor states and turned
this energy source into a major agent to change the standard of life for their citizens. In 2000,
Norway‘s reported per capita annual electricity use was 27,600 kWh, Switzerland‘s 8500
kWh. Currently in Africa, the per capita electricity consumption per year in 2000 was 524
kWh while the world averaged 2475 kWh/p/y. These few selected values as reference markers
illustrate the impending force of pressure to use local electricity sources—hydropower—to
further socioeconomic change. Additional pressure in this process will come out of the
urbanization process in Africa. Increased fossil fuel prices contribute their influence to
enhance hydropower‘s rising role in the electricity generating sector. Africa in 2000 had
22,104MW of installed hydropower, comprising 21.7% of the continents‘ electricity supply,
or 112.2 kWh out of 524 kWh/p/y were hydropower generated. Economic pressures to change
this condition can be identified in Ethiopia, which in 2000 had 378MW of installed
hydropower. Two percent of Ethiopia‘s hydropower potential is actually in use. In 2006,
791MWwere on line, by 2010/11 this is to reach 4000MW. In 2005 the per capita
consumptionwas 28 kWh/p/y, and 80% of the country‘s population had no access to
electricity. The exploitable potential is 30,000MW, 4461MW are under construction or
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‗‗committed‘‘ for construction. The future of hydropower opens the path to change consistent
with local conditions and possibly including options similar to those observed in Norway and
Switzerland when they got the light.
Hydropower‘s future in Asia parallels its current economic change. China, India, Iran,
and Turkey are turning into major hydropower states. Hydropower development originated in
the US and Europe, hence to expect contemporary parallel applications obliges to include the
varied time frames of project construction periods. While the Chinese system will be the
world‘s largest, the Indian, Turkish, and Iranian systems are impressive for their respective
magnitudes. Also to be included are the S.E. Asian states with significant hydropower
resources. It is useful to refer once again to available water volumes in km3 and m
3/s to
identify local hydropower resources.
Hydropower‘s future is inseparable from economic evaluations. Cash flow goes into
construction without return for the time until a certain quantity ofMWs enter service. Cash
and interests rates act as constraints upon profligate spending schedules. However, projects
today are projected to generate for 100 years+ and the repayment schedule is somewhere from
12 to 25 years. Not included in this assessment projection are transmission and transforming
installations. In the economic sphere, it is not only project costs, but also money market
conditions within each economic system, hence prediction ismade in an unstable economic
climate. This leaves the question, how can an economic system function without an effective
electricity generating system?
While hydropower has its limitations, there are two options to enhance this energy
source: one by turning to run-of-the-river bulbar generation; and two by pumped-storage. This
second system is a practical approach to enhance the ‗‗hydropower energy bridge.‘‘ It needs
to be noted, the electricity system as in place in 2009 will be notably different in 2050 as
technology introduces changed electricity systems in most likelihood phasing out the fossil
fuel era in the electricity sector. The run-of-the-river bulbar units can be placed without any
dam, notably in very large rivers like the Amazon, Yangtze, Orinoco, Parana, Congo, Lower
Mekong. The pumped river projects are already in use and serve as stand-by for peak load
demand.
As the world has turned multi-energy source dependent, the need for energy has
reduced states‘ energy autarchy and source options. Additional energy sources have become
the norm in the 1930–2009 period. This will foster the search for a more universal energy-
272
electricity source replacing the energy system familiar in 2009. Hydropower will outlast most
of the currently known energy sources because of its favorable economics. It also may be
helped by change water management, notably urban–industrial water supply systems, and
significantly the dams needed for irrigation projects. Irrigation currently provides 40% of the
worlds‘ food production. To start a hydropower project is expensive, to operate it, it out
competes all comers.
Hydropower has played a prominent part in the electrification phase of the
industrialization process. As the less industrialized states of the world expand their secondary
sector, low cost electricity will be sought to further this phase of domestic change. Economy
in investment strategy and the inherent advantage of long term low cost electricity supply and
rising urbanization rates use hydropowered electricity to service local energy needs. India
plans to integrate the national fluvial system by ‗‗interconnecting‘‘ its key rivers to enhance
hydrological management and enlarge its hydropower system by 55,000MW by 2012. In
China hydropower serves as a key link in its evolving energy matrix. Chinese plans call for
158 GW installed in 2010 and 270 GW by 2020. If each kW installed averages $1200, that
bill by 2020 will be $134,400,000,000, and this is for 112 GW, not 270 GW. Iran and Turkey
pursue a comparable course of action. Brazil has to plan on 4500 MW/year to avoid
brownouts or blackouts. The options are limited. China currently (2009) burns annually 43%
of the world‘s coal production, this may illuminate the future for hydropower in China.
Current limitations for clean bulk low cost electricity make hydropower the ‗‗electricity
bridge‘‘ to that electricity source four to five decades hence. Electricity‘s future is in the
oceans and the sky not in cane sugar or corn fields. Hydropower for its part contributes to
ease newcomers to the industrial world into functional electricity depending energy
systems.[32]
273
2.7 Hydrogen Energy and Electric Generation
2.7.1 Introduction
Fossil fuels (i.e., petroleum, natural gas and coal), which meet most of the world‘s
energy demand today, are being depleted fast. Also, their combustion products are causing the
global problems, such as the greenhouse effect, ozone layer depletion, acid rains and
pollution, which are posing great danger for our environment and eventually for the life in our
planet. Many engineers and scientists agree that the solution to these global problems would
be to replace the existing fossil fuel system by the hydrogen energy system. Hydrogen is a
very efficient and clean fuel. Its combustion will produce no greenhouse gases, no ozone layer
depleting chemicals, little or no acid rain ingredients and pollution. Hydrogen, produced from
renewable energy (e.g., solar) sources, would result in a permanent energy system, which we
would never have to change.[33]
2.7.2 Electrical Energy Storage
An energy system dominated by electrical power has to be as flexible as the present
fossil fuel system (Fig. 7.1). The required new storage and transportation system has to meet
the requirements of the customers. The energy carrier hydrogen fits into this system;
hydrogen can be used in almost all paths of our energy system (Fig. 7.2).
Following the generation of electricity the production of hydrogen by electrolysis
offers some benefits to power utilities: Electrolysers as a load in the electrical network can be
used for frequency control or load management, thereby saving reserve capacity, and power
plants can be operated at rated power with the best efficiency and highest revenues. Hydrogen
can be utilised, for example, in fuel cells with a high efficiency to produce power.
In island networks or areas with a weak grid and/or growing power demand, a
hydrogen system with electrolyser, storage and fuel cell or customers with a reliable power
supply. Hydrogen can be produced at the supply site via electrolysis during low load periods
and reconverted to power in peak load periods. Especially for the use of renewable energies
like wind or solar power the temporal discord between production and demand of energy can
be compensated for.[34]
274
Figure 7.1 Energy Supply Structure(Ref:34)
Figure 7.2 Paths for Hydrogen(Ref:34)
275
2.7.3 Electrolyser
Technically, electrolyser is a vital component that electrolysis the hydrogen, and the
developments in the electrolysers improve total hydrogen energy system concept.
The GHW (Gesellschaft fur Hochleistungselektrolyseure zur Wasserstofferzeugung
mbH), a joint enterprise of the German companies HEW, Norsk Hydro Electrolyser and
Motoren-und Turbinen-Union Friedrichshafen GmbH (MTU), has developed an electrolyser
with superior features adapted to the production of hydrogen from electricity sources which
have strongly variable power generation.
The electrolyser has a high degree of efficiency, excellent part-load performance and
high gas puritywith fast intermittent operation. The technology of the GHW electrolyser is:
low-cost PSU electrode diaphragm
operating pressure: 30 bar
efficiency: 80% rated load up to 90% (20% load)
load range: 20% - 110%
typical power units: 0.5 to 2.5 MW and more.
A high-performance electrolyser is a key component in future hydrogen energy
systems. Projects and studies with the new electrolyser cover the following fields at GHW,
HEW or CONSULECTRA:
Harvest of PV-solar energy in the 'Solar- Wasserstoff Bayern Project'.
Harvest of fluctuating wind energy in 'weak grid systems with a relatively large
amount of wind power generation.
Power grid load control (Fig. 7.3). Because of its extremely fast regulating behaviour
the electrolyser can be used as a variable load which draws power anticyclic to the
power production, regulating the needs of a utility and thus replacing some of the
necessary regulating and reserve power plant capacity. If such systems are installed on
a large scale for future commercial hydrogen production, the following additional
advantages could be envisaged for the utility: (i) fewer losses in power plant operation
(ii) less wear on power plant regulating components (iii) improvement of regulating
response time and grid stability.[34]
276
Figure 7.3 Hydrogen Filling Station Network with Electrolyser as Controllable Load(Ref:34)
2.7.4 Hybrid Systems
2.7.4.1 Solar-hydrogen Energy Systems
If solar energy, in its direct and/or indirect forms (e.g., hydro, wind, etc.), is used to
manufacture hydrogen, then the resulting system is called the ‗‗solar-hydrogen energy
system‖. In this system, both the primary and secondary energy sources are renewable and
environmental1y compatible, resulting in a dean and permanent energy system. Fig. 7.4
presents a schematic of the solar-hydrogen energy system.
In this case, it is assumed that the conversion to the hydrogen energy will take place,
and one-third of hydrogen needed will be produced from hydropower (and/or wind power)
and two-thirds by direct and indirect (other than hydropower) solar energy forms. The same
percentage of energy demands by sectors as the above systems will be assumed. It will further
be assumed that one half of the thermal energy will be achieved by flame combustion, one-
quarter by steam generation with hydrogen/oxygen steam generation and the last quarter by
catalytic combustion; electric power will be generated by fuel cells; one-half of the surface
277
transportation will use gaseous hydrogen burning internal combustion engines and the other
half will use fuel cells. In air transportation, both subsonic and supersonic, liquid hydrogen
will be used.
Figure 7.4 Solar-hydrogen Energy System(Ref:33)
A hybrid system schematic from the system in Denizli is shown in Figure 7.5 and Figure
7.6.[36]
Figure 7.5 System without Hydrogen (Ref:36)
278
Figure 7.6 System with Hydrogen(Ref:36)
2.7.4.2 Wind to Hydrogen System
The project carried out in Minnesota is expecting to realize an integrated wind-
hydrogen system in order to produce hydrogen via electrolysis.
Figure 7.7 Construction of the 1.65 MW Wind Turbine at the Morris Research Center(Ref:35)
The wind turbine is a Vestas NM 82 with a rated capacity of 1.65 MW that is expected
to produce 5.6 million kWh of electricity annually at this site. The turbine was installed in
early 2005 and is now supplying power to the University of Minnesota.
279
This phase will incorporate a 400 kW electrolyzer, hydrogen storage tanks, and an
internal combustion engine that will use the hydrogen for ―on-demand‖ electricity.[35]
280
3 FINANCIAL AND ECONOMIC VIEW OF RENEWABLE ENERGY
3.1 Cost of Renewable Energy Systems
Below tables illustrate the current cost assumptions for renewable technologies in 2007
and 2020 cost projections and comparisons:
Technology Overnight
Capital
Cost ($
per KW)
Capacity
Factor
Variable
O&M
(+ Fuel
Costs) ($
per MWh)
Fixed
O&M
($ per
KW)
Levelized
Cost of
Energy
($ per
KWh)
Biopower
Biopower-
IGCC
3,766 83% 6.71(+$15)^
64.45 $0.080
Biopower-
Stoker
3,520 85% 3.74(+35)#
91.79 $0.0977&
Biopower-50
MW
Fluidized Bed
3,629 85% 4.26(+35)#
94.49 $0.101&
Biopower 2,596 85% 7.27(+28)#
166.13 $0.090
CSP
CS 3,645 65% 8.10 0.00 $0.071**
CS 5,021 31% 0.00 56.7 $0.200
CS 34%
$0.170
CS-Trough 3.271 34% 0.00 60.2@
$0.130
CS 4,153 43% 31.20 34.3 $0.170
CS-Trough $0.160-
$0.190
PV
PV 4,050 21% 0.00 10.4 $0.220**
PV- $0.260
281
Distributed
PV Flat Plate 5,487 25% 0.00 19.5 $0.251&
PV 2-Axis 8,876 32% 0.00 46.6 $0.330&
PV-
Distributed
$0.150
PV-
Distributed
$0.080
PV-Central 6,038 22% 0.00 11.7 $0.320
Wind
Onshore
Wind
1.923 36% 0.00 30.3 $0.069+
Onshore
Wind
1,052 45% 0.00 26.2 $0.033**
Onshore
Wind
927 46% 0.00 25.3 $0.029**
Onshore
Wind
32.5% $0.100
Onshore
Wind
1,820 42% 0.00 72.7 $0.068&
Onshore
Wind
1,765 33% 0.00 26.0 $0.073+
Onshore
Wind
1,713 35% to
50%Φ
5.70 11.9 $0.064 to
$0.047Φ
Onshore
Wind
1,983 34% 0.00 16.5 $0.071+
Onshore
Wind
3,851 34% 0.00 89.5 $0.157+
Onshore
Wind
2,388 37% to
52%Φ
15.60 18.7 $0.094 to
$0.071Φ,+
Conventional
Pulverized 2,058 85% 4.64(+16.7)^
27.53 $0.050
282
Coal
Conventional
Gas
Combined
Cycle
962 87% 2.09(+$45.1) 12.48 $0.060
Conventional
Combustion
Turbine
670 30% 3.60(+$69.3) 12.11 $0.100
Table 8.1 Current Cost Assumptions for Renewable Technologies (2007)(Ref:5) &Calculated from inputs based on 20 year economic life and real cost of capital of 7.5%
+Levelized costs here are generic and do not include site specific development costs or cost of
facilitating delivery #Fuel cost per MWh imputed from EPRI summer study levelized cost and TAG specifications for CFB
biomass plant *Fuel cost per MWh reported by source
^Fuel cost imputed from AEO2009 Early Release model solution. AEO2009 Energy Prices (2007$/mmBtu) in 2012 are $1.91 for coal, $6.63 for natural gas, and $1.96 for biomass
**EERE numbers are for 2010 @
This estimate comes from a personal communication with Steve Gehl of EPRI ΦDepending on wind class
Technology Overnight
Cost (per
kW)*
Capacity
Factor
Total
Capital
Cost
(per
MWh)
Transmission
Cost
(per MWh)
Levelized
Cost of
Energy
($ per
kWh)+
Conventional
Sources
Pulverized
Coal
1,985 85% 52.30 3.61 $0.083
[$0.079]
IGCC 2,233 85% 60.64 3.61 $0.088
[$0.084]
IGCC with
Sequestration
3,171 85% 69.54 4.01 $0.103
[$0.099]
283
Combined
Cycle
928 87% 18.63
3.88
$0.083
[$0.079]
Advanced
Combined
Cycle
892 87% 17.98 3.88 $0.079
[$0.075]
Advanced
Combined
Cycle with
Sequestration
1,729 87% 34.64 3.93 $0.110
[$0.106]
Combustion
Turbine
647 30% 33.55 11.41 $0.138
[$0.127]
Advanced
Combustion
Turbine
587 30% 30.71 11.41 $0.121
[$0.110]
Renewables
Biopower
Biopower 3,390 83% 61.62 4.14 $0.097
[$0.093]
Biopower-
Stoker
85%
$0.096
Biopower-
Stoker
85% $0.101
Biopower 90% ~$0.080**
Geothermal
Geothermal 1,585 90% 75.44 5.00 $0.103
[$0.098]
CS
CS 2,860 72% 4.47 $0.050
CS 4,130 31% 180.02 11.00 $0.212
[$0.201]
CS 34% $0.170
284
CS 34% <$0.083
PV
PV 2,547 21% 135.81 $0.141
PV $0.220
PV $0.260
PV 5,185 22% 292.84 13.69 $0.313
[$0.299]
PV-
Distributed
$0.110
PV-
Distributed
$0.050
PV-
Distributed
$2.50/Wp
installed
cost
$0.075-
$0.010&
Wind
Onshore
Wind
1,896 35% 81.38 8.66 $0.100
[$0.091]
Onshore
Wind
1,076 46% $0.033
Onshore
Wind
916 49% $0.027
Onshore
Wind
42% $0.078
Onshore
Wind
33% $0.097
Onshore
Wind
1,630 38%-52%
(depending
on wind
class)
$0.05-
$0.043
Offshore
Wind
3,552 33% 154.36 9.31 $0.191
[$0.181]
285
Offshore
Wind
2,232 38%-52%
(depending
on wind
class)
$0.074-
$0.053
Table 8.2 2020 Cost Projections and Comparisons(Ref:5) +[] contain AEO estimates of busbar levelized cost of energy, i.e., without transmission related costs *The overnight cost includes the effects of technological learning but does not include other project
costs, which are reflected in the levelized cost estimate **Cost estimate is for 2015
&Interpolate between reported targets for 2015 and 2030
Estimates of the cost of energy from new generating facilities indicate that the
levelized costs of wind and other renewables are typically greater than the levelized cost of
energy from generators fueled by coal or natural gas. Table 8.3 shows estimates of the
national average levelized cost per MWh of new generation facilities constructed in 2012 in
the AEO2009 from the EIA by technology type.
Technology Capacity
Factor
Capital
Costs
Fixed
O&M
Variable
O&M /
Fuel
Costs
Transmission
Costs
Totala
Pulverized
Coal
85% 56.9 3.7 23.0 3.5 87.1
(58.1)
Conventional
Gas
Combined
Cycle
87% 20.0 1.6 55.2 3.8 80.7
(72.7)
Conventional
Combustion
Turbine
30% 36.0 4.6 80.1 11.0 131.7
(121.5)
CSP 31% 218.9 21.3 0.0 10.6 250.8
(166.1)
Wind 36% 73.0 9.8 0.0 8.3 91.1
(84.9)
286
Offshore
Wind
33% 171.3 29.2 0.0 9.0 209.5
(164.9)
PV 22% 342.7 6.2 0.0 13.2 362.2
(308.1)
Geothermal 90% 76.7 21.6 0.0 4.9 103.3
(66.8)
Biopower 83% 61.1 8.9 24.7 3.9 106.6
(84.0)
Table 8.3 Levelized Cost of Energy (in 2007 per MWh) for New Plants Coming Online in 2012(Ref:5) aNumbers for total LCOE from AEO2008 in parentheses
NOTES: Fuel cost imputed from AEO2009 Early Release model solution. AEO2009 Energy Prices (2007$/mmBtu) in 2012 are $1.91 for coal, $6.63 for natural gas, and $1.96 for biomass. O&M,
operating and maintenance
The levelized costs reported in the last column of this table include capital and finance
costs (including the cost of site development), variable O&M (including fuel), fixed O&M,
and the cost of transmission necessary to connect the new facility to the grid. The costs for
renewables do not reflect the renewable PTC. However, they do reflect the effects of state
RPS policies on the mix of wind resources and other renewables that are expected to come
on-line in response to those policies.
Table 8.3 shows that the three renewable technologies with the lowest cost of energy
are geothermal, biopower, and wind. Pulverized coal and conventional gas combined cycle
are less costly than all of the renewable technologies. According to the AEO2009 results, the
present $20 per MWh level of the PTC would basically close the gap between the levelized
costs of new wind and the LCOE of new coal plants, ignoring issues of relative
dispatchability. However, the costs of other technologies, particularly solar PV, concentrating
solar power, and offshore wind would remain higher than the other renewables, and additional
subsidies or set-asides in RPS policies would be necessary for these technologies to penetrate
the markets given existing costs.
Table 8.1 shows the levelized costs of renewable sources of generation from EIA
compared to those from the EERE Office at DOE; a recent report from Standard and Poor‘s
(S&P); the inputs to the American Wind Energy Association (AWEA) and NREL wind study;
and the Solar Energy Industry Association (SEIA). While the estimates in Table 8.2 include
the costs of installation and construction of transmission necessary to facilitate power
287
delivery, Table 8.1 contains more generic estimates of costs relevant for today or for 2010, the
first year reported by many sources.
The snapshot of costs presented in this table does not reveal a number of important
factors that affect the estimates of levelized costs.
3.2 Wind Power Cost
Table 8.1 estimates of levelized cost of energy for onshore wind in 2010 range from
$0.029 to $0.10 per kWh, with EIA estimates falling in the middle at $0.069 per kWh. Most
estimates of the capital cost of new wind facilities are in the $1,750 per kW range, close to 10
percent lower than the EIA estimates of nearly $1,900 per kW.17 In addition, EIA estimates
that average capacity factors are somewhat lower than recent forecasts from EPRI.
A single national average estimate of the levelized cost of wind would not
communicate how wind costs depend on the capacity factor of new wind turbines, which in
turn depends on wind class. Figure 8.1 shows estimates from DOE of the amount of wind
capacity available at different levelized costs of energy, after netting out the PTC, and how
the cost of energy increases when moving from higher wind classes to lower wind classes and
from onshore sites to offshore sites.
Capacity factors differ across the country, as shown in the regional differences for
existing facilities in Figure 8.2. Capacity factors for wind have been improving over time due
to improvements in equipment performance, although this improvement may be offset as the
lower cost sites are taken.
The costs of offshore wind are likely much more uncertain as currently there are no
operating offshore facilities in the United States. As a result, we are several years from a point
where we can be more certain about what offshore wind generation costs would look like in
the future and how they would compare to the costs of other renewables.[5]
288
Figure 8.1 Projected 2010 costs of wind with production tax credit, $1,600/MW-mile
transmission, without integration costs of various wind classes(Ref:5)
Figure 8.2 Wind capacity factor in 2006 by region and vintage of wind facility (Ref:5)
289
3.3 Solar Power Cost
3.3.1 Solar Photovoltaic Cost
The cost of energy produced using solar PV technology is a function of the efficiency
of the cell in producing electricity, which is typically 15 percent or less depending on the
material system and the total cost of installation. The capital cost of a PV cell module is
typically expressed as dollars per peak watt of production ($/Wp) and is determined by the
ratio of the module cost per unit of area ($/m2), divided by the maximum amount of electric
power delivered per unit of area (module efficiency multiplied by 1,000 W/m2, the standard
insolation rate at 25°C). In Figure 8.3, this cost per peak watt ($/Wp) is indicated by a series
of dashed straight lines having different slopes. Any combination of area cost and efficiency
on a given dashed line produces the same cost per peak watt indicated by the line labels. For
example, present singlecrystalline Si PV cells, with an efficiency of 10 percent and a cost of
$350/m2, have a module cost of $3.50/Wp. The area labeled I in Figure 8.3 represents the first
generation (Generation I) of solar cells and covers the range of module costs for these cells.
Areas labeled II and III in Figure 8.3 present the target module costs for Generation II
(thinfilm PV) and Generation III PV cells (advanced future structures) that are still in
development.
Table 8.3 PV Power Costs as Function of Module Efficiency and Cost(Ref:5) For PV or PEC to provide full level of C-free energy required for electricity and fuel-solar power cost
needs to be 2 cents/kWh($0.40/Wp)
290
In addition to module costs, a PV system also has costs associated with the
nonphotoactive parts of the system, called balance of system (BOS) costs, which are in the
range of $250/m2 for Generation I cells. The total cost of present PV systems is about $6/Wp.
Taking into account the cost of capital, interest rates, depreciation, system lifetime, and the
available annual solar irradiance integrated over the year (i.e., considering the diurnal cycle
and cloud cover, which reduce the peak power by a factor of about 1/5), the $/Wp figure of
merit can be converted to $/kWh by the following simple relationship: $1/Wp ~ $0.05/kWh.
This calculation leads to a present cost for grid-connected PV electricity of about $0.30/kWh.
The estimates of levelized energy costs for PV generally are distributed around the 30 cents
per kWh level, as shown in Table 8.2. The one exception was a 2004 SEIA study of levelized
costs that predicted the cost of energy from PV would fall to about $0.14/kWh by 2010, in the
absence of aggressive policies to promote the technology and to $0.08/kWh with such policies
in place.
Table 8.4 Fractional energy PV rooftop supply curves for three U.S. interconnections(Ref:5)
The costs of supplying electricity from rooftop PV installations will vary across
different locations and depend on factors such as the cost of land, options for orienting the
291
installation (particularly on roof tops), and amount of energy produced in a particular location.
A study of the factors affecting supply curves for solar PV from rooftops used data on
building stock, roof top orientation, solar insolation, and other factors to construct relative
supply functions for solar PV for three U.S. electric interconnections as shown in Figure 8.4.
These supply curves relate to the system with the greatest yield, which results from the best
orientation in the most productive location. The supply curves show the higher costs of
producing electricity using solar PV in the east compared to the west, and the resource limits
in different locations.
Table 8.5 Price, customer cost after subsidy, and number of PV installations per year in
California under California Energy Commission incentive programs(Ref:5)
Largely as a result of state-level policies to promote the use of solar PV, the number of
installations is growing. As shown in Figure 8.5, in California, about 130 MW of the
cumulative PV capacity installed by 2007 was under incentive programs administered by the
California Energy Commission (CEC), more than double the total amount installed under
these programs as of 2004. This increase in capacity coincided with the 2006 launch of the
California Solar Initiative with a funding level of about $3.3 billion for subsidy payments
292
available to new solar PV installations. Data from the CEC on total costs and costs to
customers of PV installations suggest that costs per kW for consumers rose slightly over this
period, a period of only slight increases in consumer costs per Watt of PV installations, due in
part to the subsidies afforded by the California policy. The CEC PV database contains
information on about one-third of the total amount of PV capacity installed in California.
Most of the solar PV installations appear to be taking place in regions that have
aggressive pro-solar policies. According to solarbuzz.com, in 2006 California accounted for
63 percent of the grid-connected PV market, and New Jersey, which also has an aggressive
policy to promote PV, accounted for 19 percent. In general, achieving grid parity, the point at
which electricity from PV is equal to or cheaper than power from the electricity grid, would
require a two to three times improvement for costs per kWh for the whole system (PV
modules, batteries, inverters and other system components) as well as for installation and
O&M costs.[5]
3.3.2 Concentrating Solar Power
According to the EIA AEO2009 model runs, the levelized cost of generating
electricity using concentrating solar power is higher than the cost of wind, but lower than the
cost of solar PV as shown in Table 8.3. If technological learning for CSP is a function of
aggregate investment, as assumed by the EIA, then the economics of concentrating solar
power may be improved by policies that promote investment in this technology and provide
incentives for using it to generate electricity. Twelve states have set asides for solar
technologies in their RPS policies, and in nine of those states, which include Arizona, New
Mexico, and Nevada, solar thermal generating technologies qualify for the set aside. The set
aside typically requires that a specific portion of the RPS target must be met with a solar
technology. Some policies also include a credit multiplier for generation from solar such that
solar-produced electricity creates RECs at a ratio of greater than 1 to 1.
Estimates of the levelized cost of central station concentrating solar power have
typically been around 16 cents per kWh at the busbar. EIA reported a much higher levelized
cost of 25 cents per kWh in the AEO2009 forecast, reflecting increases in 2007-2008 in raw
materials costs. With the 16 cent cost as a starting point, the supply curve for concentrating
solar power in the southwestern United States shown in Figure 8.6 displays costs at the
293
busbar. The total supply curve in this graph is the horizontal sum of the individual supply
curves for different levels of solar resource intensity. This cost curve is very flat at levels of
around 16 cents per kWh.
Table 8.6 Supply curves describe the potential capacity and current busbar costs in terms of nominal levelized cost of energy (LCOE) of concentrating solar power(Ref:5)
Colored lines indicate different amounts of insolation measured in kilowatt-hours per square meter per day.
Figure 8.7 shows a supply curve that goes beyond the busbar and takes into account
the costs of incremental transmission necessary to deliver power to load. This curve is based
on assumptions about the portion of local load that could be served by solar power, the
availability of transmission to move power from generation sites to load centers, and the cost
of expanding transmission at $1,000 per MW-mile, lower than the $1,600 per MW-mile used
in DOE wind study. As shown in Figure 8.7, the resulting aggregate supply curve for this
region has a bit of slope to it, rising to approximately 18 cents per kWh at or near 180 GW of
generation. The basic message from the fairly flat slope of this supply curve is that at this time
the constraining factor for concentrating solar power supply is not the amount of the resource,
which is widely distributed and available abundantly in the southwest, but the costs of
developing that resource.
294
Table 8.7 Concentrating solar power supply curve based on 20 percent availability of city peak demand and 20 percent availability of transmission capacity(Ref:5)
Colored lines indicate different amounts of insolation measured in kilowatt-hours per square meter per day
How this cost picture might change over time depends on future adoption of
renewable technologies. According to the WGA study, technology learning and economies of
scale in manufacturing and installation indicate that the levelized cost of energy in 2015 for a
parabolic trough technology would decrease by 50 percent with an increase of 4 GW of
installed capacity. The American Solar Energy Society (2007) anticipates further decreases in
levelized cost of another 25 percent between 2015 and 2030. Research and development is
also expected to have an important effect on costs. DOE‘s Office of EERE anticipates that
both capital costs and capacity factors for concentrating solar power could improve
dramatically through its R&D program for concentrating solar power, including storage
capacity and location of new systems in the most productive sites. Levelized costs of energy
at the busbar could decrease by 50 percent as soon as 2010, as shown in Table 8.1, though this
sounds quite optimistic.[5]
295
3.4 Geothermal Power
Most of the economic U.S. hydrothermal resources are located in the western states.
Recent studies sponsored by the WGA identify approximately 13,600 MW of geothermal
potential in the west that could be developed economically, at busbar costs of up to 20 cents
per kWh in $2005, and 5,600 MW that reasonably could be developed by 2015 at costs of less
than 10 cents per kWh in $2005. Both cost estimates omit the renewable PTC that would
reduce the costs of developing these resources.
The WGA report and one conducted by the CEC were used to update the geothermal
supply curves in NEMS. The supply curves are limited to the 80 most likely sites to be
developed and extend to include 8 GW of new capacity. The NEMS geothermal supply curve,
shown in Figure 8.8, is similar to the supply curves found in the WGA report. According to
EIA, this supply curve, added to the NEMS model with the development of AEO2007, would
not capture all potentially economic geothermal resources, but it is an important start and
likely does capture the most economic resources available (Smith, 2006). Enhanced
geothermal systems (EGS) may offer greater opportunity in the future, but this technology is
too early in its development to reliably estimate its cost.
Figure 8.8 Geothermal Supply Curve (Ref:5)
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Existing geothermal generating capacity is closer to 2.5 GW. One hurdle to the
development of geothermal resources is that, like wind, they may be located far from load and
require new transmission lines to facilitate delivery. However, geothermal energy provides
constant, baseload power, which is an advantage over solar and wind.[5]
3.5 Biopower Cost
The costs of new biopower generation will depend on two important factors: the
generation technology and the cost of the fuel. In its NEMS model, EIA assumed that any
new biopower generation would use gasification with a combined cycle technology. These
generators have high capital costs and lower heat rates than a conventional boiler. However,
none of these types of biopower generators are now in commercial operation in the United
States, so it is difficult to know how the predicted costs would compare to actual experience.
In its Technology Assessment Guide, EPRI reported costs for both stoker and circulating
fluidized bed boilers, technologies that are well suited to the small scale of most biomass
plants and that can handle the fuel well EPRI. Capital costs, including interest during
construction and project specific costs, would be on the order of $3,400 per kW for each
technology with capacity factors of 85 percent. The levelized cost of energy would depend on
fuel costs, but the EPRI summer study reports a cost of approximately 9.6 cents per kWh for a
fluidized bed generator in 2010, which, assuming similar fuel costs of $34 per MWh (about
$2.70 per MMBtu of high heat value), would yield a levelized cost estimate for a stoker of 9.4
cents per kWh.
The costs of biomass fuels are also subject to uncertainty and potential volatility.
Much of the existing biopower generation occurs as self-generation at facilities that have a
ready source of fuel (such as pulping operations, paper mills, or forest products plants).
Expanding capabilities beyond these generators could involve shipping fuel, which can get
quite costly, which suggests that future biopower generation capability would be located close
to fuel sources and use more economical biomass fuels that are concentrated locally and do
not face substantial competition for their use.
This uncertainty about fuel costs is reflected in the different estimates of levelized
costs of biopower reported in Table 8.1. The fuel cost assumptions in the recent EIA forecasts
are substantially lower than those assumed by other sources, including EPRI and S&P. These
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lower costs are a major factor in the substantially lower levelized cost of energy in the EIA
numbers, which are about 85 percent lower than those provided by other sources.
One option for greater use of biomass fuel is co-firing the fuel with coal. Biomass co-
firing of up to 10 to 15 percent of fuel on a heat-input basis is a potential way of reducing the
CO2 emissions associated with coal-fired generation. The costs of making a coal-fired
generation facility available for co-firing could be substantial and involve large investments in
new fuel-handling equipment. Certain types of boiler configurations are more amenable than
others. Even though co-firing counts as renewable generation under many state RPS policies,
co-firing has not increased much in response to state RPS policies. Placing a cap on CO2
emissions may be necessary to drive coal plants to start making the investment necessary to
co-fire, and then only when the facility can identify an economic source of biomass fuel.[5]
3.6 Costs in 2020
Table 8.2 provides estimates from a few different sources of the levelized costs in
2020 of a range of different renewable technologies.
Table 8.2 also includes levelized cost of energy projections for a number of fossil
generation technologies based on the AEO2009 forecasts. These forecasts all include the
effect of learning on reducing capital costs, where the potential cost reductions from learning
vary across technologies. The projections from EIA also include the effect of moving along
the supply curve, such as when less accessible or lower quality wind resources are tapped for
wind electricity generation.
Table 8.2 shows a wide range of forecasts on the future of renewables costs. Most of
the forecasts envision renewables as continuing to be more costly than the EIA forecasts of
generation using conventional coal and gas technologies. The exceptions are the EERE
forecasts that envision substantial improvements in costs for concentrating solar power and
wind, and the SEIA forecast for solar PV. The differences between the program scenarios and
the baseline scenarios for the EERE forecasts show how full funding of renewable energy
research at DOE is expected to affect the future costs of renewable generation.
The different forecasting groups and scenarios also envision different rates of change
in levelized costs of energy over the next decade as shown in Figure 8.9 and Figure 8.10,
which compares forecasts of costs for 2020 and 2010 for several sources for four of the
298
technologies. This graph shows that EERE and EPRI Summer Study forecasts envision large
decreases in costs of wind generation between 2010 and 2020 while the levelized costs in EIA
increase as a result of the cost increases inherent in tapping increasingly difficult sites, which
are not reflected in the estimates reported by the other studies.
Figure 8.9 Levelized Cost Estimate for Biomass and Solar PV Systems in 2010 and 2020(Ref:5)
299
Differences in cost projections for wind turbines appear to be at least partly due to
differences in assumptions about capacity factors. The predictions from EERE are largely the
result of improvements in engineering resulting from research and development in this
technology and greater deployment.
Figure 8.10 Levelized Cost Estimate for Wind and Solar Thermal Systems in 2010 and 2020(Ref:5)
300
In the AEO projections, capacity factor predictions for 2020 are based on where the
wind resource would be developed in that year. The model presumably would have used up
the better sites for the least-cost development of resources in earlier years. Incorporating
resources found in higher wind class regions, as suggested in Figure 8.1, would likely lead to
lower capacity factors at new facilities after the better wind sites are taken.
Wind study assumed that, as a result of technology improvements, capacity factors
would improve between 2005 and 2030 by 10 to 18.7 percent, with faster rates of
improvement anticipated in the lower wind resource regions. Most of this improvement is
expected by 2020. This study also assumed that capital costs of new onshore wind generators
would fall by 5 percent between 2005 and 2020, and that new offshore wind generators would
see capital cost decreases of just over 10 percent during the same period. This study also
anticipated a marked decline in variable and fixed O&M costs between 2005 and 2020,
particularly for offshore installations.
Figure 8.11 Learning curve for PV production (Ref:5)
Concentrating solar power (CSP) and photovoltaics (PV) also have a wide range of
future cost predictions, representing the large degree of uncertainty and differing opinions
about how solar costs are likely to evolve over this decade. Solar PV is expected to remain
more expensive than CSP, although the SEIA forecasts dramatic improvement in the cost of
distributed PV, and EERE anticipates decreases in PV costs, too. EERE also projects potential
301
cost improvements for solar thermal projects. But unlike other forecasters, EERE predicts
substantially lower costs in the near term, suggesting differences in what goes into their cost
measures. The DOE Solar Energy Technologies Program report envisions declines in CSP
costs of about 50 percent from present levels, similar to the aggressive technology case from
the EPRI Summer study. Analysis of the evolution of PV costs suggests that the prices of PV
modules have followed a historical trend along the ―80 percent learning curve.‖ That is, for
every doubling of the total cumulative production of PV modules worldwide, the price has
dropped by approximately 20 percent. This trend is illustrated in Figure 8.11. The final data
point for 2003 corresponds to about $3.50/Wp and a cumulative PV capacity of 3 GW. A
major reduction in the projected future cost of PV modules depends on the introduction of
thin films, concentrator systems, and new technologies. The graph projects the path of future
costs under historical learning rates as well as with slower and faster rates of learning.[5]
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