Introduction to Electric Energy Conversion
and Renewable Energy Sources
ELEC-A8001 Johdatus sähköenergiajärjestelmiin
Marko Hinkkanen
Aalto University
School of Electrical Engineering
15 September 2015
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http://climate.nasa.gov
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Outline
Energy Trends in European Union
Energy Conversion
Electric Drives
Renewable Energy
Wind Energy (Incl. Some More Exotic Concepts)
Utility-Scale Solar
Summary
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EU 2050 Roadmap: Reduction of Green House Gas Emissions
100%
100%
80%
Power Sector
80%
Current policy
60%
Residential & Services
60%
Industry
40%
40%
Transport
20%
20%
Non CO2 Agriculture
Non CO2 Other Sectors
0%
1990
2000
2010
2020
2030
2040
0%
2050
A roadmap for moving to a competitive low carbon economy in 2050, European Commission, COM(2011) 112, 2011
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EU Final Energy Consumption by Sector and Share of Electricity
Impact assessment, European Commission, SWD(2014) 15 final, 2014
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EU Net Power Generation Capacity
With Further Breakdown of Renewable Energy Source Capacities in Right Columns
Impact assessment, European Commission, SWD(2014) 15 final, 2014
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Outline
Energy Trends in European Union
Energy Conversion
Electric Drives
Renewable Energy
Wind Energy (Incl. Some More Exotic Concepts)
Utility-Scale Solar
Summary
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Electric Energy Is an Energy Carrier
I
Examples of energy sources
Non-renewable: Crude oil, coal, natural gas, natural uranium
Renewable: Solar, wind, biomass
Secondary: Hydrogen, electric energy
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Secondary energy sources are also referred to as energy carriers
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Energy has to be converted to a useful form
I
Electric energy can be flexibly and efficiently converted to other forms
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But, storing of electric energy is a problem!
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Conversion Efficiency
I
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Power is the rate at which energy is
converted
dE
P=
dt
Power balance
Pin = Pout + Pd
I
System
Useful
output
power
Pout
Conversion efficiency
η=
Pout
Pin
is between 0 and 1
I
Input
power
Pin
Losses Pd
In steady state, η = Eout /Ein
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Examples of Energy Conversion
Energy conversion
device
Automobile engine
Power plant boiler
Steam turbine
Electric generator
Electric motor (large)
Incandescent lamp
Electric heater
Silicon solar cell
Battery
Input
energy
Chemical
Chemical
Thermal
Mechanical
Electric
Electric
Electric
Solar
Chemical
Useful output
energy
Mechanical
Thermal
Mechanical
Electric
Mechanical
Light
Thermal
Electric
Electric
Typical
efficiency (%)
25
85
45
95
90
5
100
15
90
Efficiency values: http://www.ems.psu.edu/∼radovic/Chapter4.pdf. Note that these values are just examples and that efficiencies depend also on
the operating point.
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Importance of Energy Efficiency
Figure: ABB
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Outline
Energy Trends in European Union
Energy Conversion
Electric Drives
Renewable Energy
Wind Energy (Incl. Some More Exotic Concepts)
Utility-Scale Solar
Summary
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Electric Drive
Frequency converter
(electric energy converter)
I
Electric drive is a system, which
converts electric energy into
mechanical work
I
Direction of the energy flow can
also be opposite
I
Energy flow can be controlled by
means of an electric energy
converter
ABC
Grid
Electric motor
(electromechanical
energy converter)
Mechanical
load
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Two-Level Converter
a
b
Udc
c
a
b
c
Udc
Main circuit
Equivalent circuit
I
2-level converters typical in low-voltage drives (below 1 kV)
I
3-level converters in medium-voltage drives
I
Modular multi-level converters are becoming an interesting option in grid
applications (> 10 kV)
I
Frequency converter may consist one or two this kind of converters
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Voltage (p.u.)
Example: AC-Side Waveforms of a Two-Level Converter
2
1
0
−1
−2
Phase current (p.u.)
0
2
4
6
8
10
2
4
6
Time (ms)
8
10
2
1
0
−1
−2
0
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Energy Consumption of Electric Motors in the EU
78%
other
22% electric
energy use
other
60%
electric
motors
40%
electric energy use
I
Share of electricity in final energy use is estimated to rise up to 28% by 2050
I
Electric drives and electric energy converters are important for sustainable
use of energy
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Water Pumps at Google Data Center, Hamina, Finland
Figure: Google
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Heavy-Duty Industrial Centrifugal Fan
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1.35-MW 690-V induction motors (most commonly used machine type)
Figure: www.pollrichdlk.com
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Pump and Fan Drives
I
Fluid flow is controlled by adjusting the speed ωM with the frequency converter
I
If the fluid flow reduces to 50%, load power PL may drop down to ∼30%
I
Significant energy savings are possible (in comparison with fixed-speed
drives, where the flow is controlled with a valve)
Grid
Frequency converter Induction motor
PL
3
PL ∝ ω M
ωM
Power PL
ωM
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Wind Turbine
Gearbox
Generator
Frequency
converter
Figure: ABB (modified)
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Wind Turbine: Permanent-Magnet Synchronous Generator
I
Power levels up to 8 MW
I
Typical gear ratios around 100:1 (but direct drives also exist)
I
Optimal blade speed (which yields the maximum power) is controlled by the
frequency converter
Gearbox
PMSG
Frequency converter
690 V / 20 kV
Grid
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Battery Electric Vehicle Powertrain
I
Nissan Leaf (2014)
I
Maximum power 80 kW
I
Maximum torque 280 Nm
I
Gear ratio 8:1
I
Li-ion battery pack 24 kWh
Charger
DC-DC converter
Junction box
Inverter
PMSM
Gearbox
Figures: Nissan (modified)
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Battery Electric Vehicle Powertrain
I
Most common motor types in electric vehicles
I
I
I
Permanent-magnet synchronous motor (PMSM)
Induction motor (IM)
Permanent-magnet-assisted synchronous reluctance motor (PM-SyRM)
I
When braking, the power flows from wheels to the battery pack
I
Electric machines are capable to operate both as a motor and a generator
Battery pack
Inverter
PMSM
Wheels
Gearbox
Motoring
Generating (braking)
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Goliath Shipyard Crane
I
Modern cranes can be
semi- or fully-automated
I
Constant load torque due
to the gravity
I
When lowering the load,
the hoist motor operates
as a generator
I
How much power is
needed to hoist mass m
at speed v ?
Figure: Konecranes
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Why Controlled Electric Drives Are Needed?
1. Enabling fast and accurate motion control
I
Robotics, elevators, cranes, process automation. . .
2. Improving energy efficiency
I
I
Process flow is controlled by means of the motor speed
Pumps, fans, compressors. . .
3. Conserving braking energy
I
Transportation, cranes. . .
Similar technologies are applied in grid integration of renewable energy sources
(wind, solar, etc.)
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Outline
Energy Trends in European Union
Energy Conversion
Electric Drives
Renewable Energy
Wind Energy (Incl. Some More Exotic Concepts)
Utility-Scale Solar
Summary
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Electricity From Renewable Sources in EU
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Outline
Energy Trends in European Union
Energy Conversion
Electric Drives
Renewable Energy
Wind Energy (Incl. Some More Exotic Concepts)
Utility-Scale Solar
Summary
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Power From the Wind
I
Wind energy Ew and power Pw flowing through an area A
Ew =
1
ρAvw t v 2
2 | {z } w
⇒
Pw =
1
ρAvw3
2
m
where ρ is the air density and vw is the wind speed
I
Power extracted by the turbine
1
P = Cp · ρAb vw3
2
where Ab = πrb2 is the area swept by the blades, rb is the blade length, and Cp
is the power coefficient
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Power Coefficient Cp
0.5
Power coefficient depends on
I
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Blade design
Pitch angle
Tip speed ratio λ = rb Ωb /vw , where
Ωb is the angular speed of the rotor
blades
I
Theoretical limit: Cp < 59%
I
In variable speed turbines, Cp can
be kept at its maximum (if the power
and rotor speed are below their
rated values)
Power coefficient Cp
I
0.4
0.3
0.2
0.1
0
0
5
10
15
20
Tip speed ratio λ
Example power coefficient curve
Figure (modified): Petersson, “Analysis, modeling and control of doubly-fed induction generators for wind turbines,” Ph.D. dissertation, KTH,
Stockholm, Sweden, 2005
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Torque of the Blade Rotor
I
Blade length for a given rated power
1
P = Cp · ρπrb2 vw3
2
I
⇒
1
2
Blade tip speed vb = rb Ωb limited to avoid excessive mechanical forces, wear,
and audible noise
P
r P
Tb =
= b =
Ωb
vb
I
rb =
2P
Cp ρπvw3
2
Cp ρπvw3 vb2
!1
2
3
P2
Rated torque Tb ∝ P 3/2 increases more than proportional to the power level
More information: Polinder et al., “Trends in wind turbine generator systems,” IEEE J. Emerg. Sel. Topics Power Electron., 2013
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Squirrel-Cage Induction Generator (SCIG)
I
Old concept, manufactured mainly during the last century
I
Operates at (almost) constant speed
I
Simple, robust, and cheap
I
Power levels up to 1.5 MW
I
Typically three-stage gearbox
I
Power above the rated wind speed limited using the stall principle
Gearbox
SCIG
Transformer
e.g.
Grid
690 V / 20 kV
Capacitor
bank
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Doubly-Fed Induction Generator (DFIG)
I
I
I
I
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Most popular concept nowadays (power levels up to 5 MW)
Rotor winding is connected via slip rings to a converter
Power rating of the converter is about 25% of the rated power (corresponds to
the speed range 60%. . . 110%)
Optimised tip speed ratio λ ⇒ improved energy yield
Less power fluctuations and audible noise
Gearbox
DFIG
Grid
Frequency converter
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Permanent-Magnet Synchronous Generator (PMSG)
I
Power levels up to 8 MW
I
Fully rated frequency converter (expensive, more losses)
I
Better grid-fault ride-through capability
I
No brushes ⇒ less maintenance
I
Is the gearbox necessary?
Gearbox
PMSG
Frequency converter
690 V / 20 kV
Grid
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4-MW 690-V Frequency Converter
Grid-Side Converter Unit is Visible in the Figure
Figure: ABB
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Conceptual PMSG: Operating Principle
β
q
I
Three-phase stator winding
I
Permanent magnets (e.g. NdFeB or
SmCo) in the rotor
I
Very high efficiency
I
No magnetizing supply needed
I
Equal (but opposite) reaction forces
affect the rotor
d
F
α
F
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Number of Poles
q
q
d
2 poles (p = 1)
I
d
4 poles (p = 2)
Angular rotor speed Ω = ω/p, where ω is the angular supply frequency
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Generator Scaling
I
Shear stress of active air-gap surface area
τ=
1
B̂g Âs cos γ
2
[N/m2 ]
I
Air-gap flux density B̂g is limited due to saturation
I
Stator current loading Âs is limited due to dissipation
I
Generator size roughly proportional to the (rated) torque
T = rr (2πrr ` τ ) = 2τ Vr
since Vr = πrr2 ` is the rotor volume
I
Price of the generator depends on its size and materials
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Generator Scaling: Effect of the Gear Ratio
I
Rated mechanical power
P = T Ω = 2τ Vr Ω
where Ω is the generator speed
I
Assumption: lossless gearbox
P = Tb Ωb = T Ω
I
Generator size depends on the gear
ratio R = Ω/Ωb
T = Tb /R
Wind-turbine gearbox (R = 78 . . . 136)
Figure: GE
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4-MW 10-rpm Direct-Drive PMSG
Rated Speed Around 10 rpm, Very High Number of Poles
Figures: http://www.eal.ei.tum.de/fileadmin/tueieal/www/courses/EAGUA/lecture/2013-S/x Handout Alan Jack new developments.pdf
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Future Concepts: Superconducting Direct-Drive Generators
High Temperature Superconductor (HTS), Operated at 30. . . 50 K
Generator
Gearbox
Pc=c4.5/6MWc
Pc=c5cMW
13m
13m
Pc=c8/10MW
13m
5m
12m
Shaft
Hub
DirectcDrive,cSync,c
HTS
AAMSC8
DirectcDrive,c
Sync,cCU
AEnercon8
6m
Conventional
GearedcwithcDFIG
ARepower8
Mass2of2Nacelle2
+2Hub
+2Blades
Extrapolated2for282MW
mTop~410t
mTop~500t
mTop~750t
mTop~800t
135m
138m
Tower
>55m
Nacelle
100m
Blade
mTop~480t
Figure (modified): D. McGahn, “Drivetrains: direct drive generators and high temperature superconductor based machines,” MIT Windweek, 2009,
http://web.mit.edu/windenergy/windweek/Presentations/P7%20-%20McGahn.pdf
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Growth in Turbine Size
Figure (modified): D. McGahn, “Drivetrains: direct drive generators and high temperature superconductor based machines,” MIT Windweek, 2009,
http://web.mit.edu/windenergy/windweek/Presentations/P7%20-%20McGahn.pdf
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Airborne Wind Turbine: Power Kite by Joby Energy
Figures: Kolar et al., “Conceptualization and multiobjective optimization of the electric system of an airborne wind turbine,” IEEE J. Emerg. Sel.
Topics Power Electron., 2013
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Size Comparison: 100-kW Conventional and Airborne Wind Turbines
56 rpm
2000 rpm
1.2 m2
v = 33 m/s
30 m
800. . . 1000 m
370 m2
v = 10 m/s
Figures (modified): Kolar et al., “Conceptualization and multiobjective optimization of the electric system of an airborne wind turbine,” IEEE J.
Emerg. Sel. Topics Power Electron., 2013
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30-kW Buoyant Airborne Turbine
Figure: http://www.altaerosenergies.com/
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Outline
Energy Trends in European Union
Energy Conversion
Electric Drives
Renewable Energy
Wind Energy (Incl. Some More Exotic Concepts)
Utility-Scale Solar
Summary
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Utility-Scale Solar
I
Solar generator produces the DC voltage
I
DC-bus voltage is adjusted for maximum power-point tracking (600. . . 850 V)
I
Additional DC/DC converter between the generator and the inverter is often
used in residential-scale solar (for better maximum power-point tracking)
Solar
generator
Solar
inverter
LCL filter
400 / 20 kV
Grid
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Utility-Scale Solar Plant
Finsterwalde I-III Solar Power Plant Cluster (Peak Capacity 83 MW)
Inverter stations
Figure: http://www.q-cells.com/consumer/power-plants/references.html
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2-MW Inverter Station With Two 1-MW Central Inverters
Figure: ABB
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Central Inverter and Solar Generator
3
1
5
1
Solar module (photovoltaic module)
2
Solar string
3
Solar array
4
Solar generator
5
Solar array junction box
6
Inverter
4
2
6
PV
S8
0
0-5
7
1…20
Figure: ABB
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Outline
Energy Trends in European Union
Energy Conversion
Electric Drives
Renewable Energy
Wind Energy (Incl. Some More Exotic Concepts)
Utility-Scale Solar
Summary
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Electrification Is a Key Trend
Just a Few Selected Examples Were Considered in This Lecture
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Electrified transport sector
I
Renewable and distributed energy production
Smart grids
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DC grids (buildings, communities, HVDC)
Energy storages (batteries, pumped hydro)
Electricity, heat, transport, water, gas
Energy and resource efficiency
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Everywhere (from devices to systems)
Wide-bandgap power semiconductors (SiC, GaN)
Internet of things, industrial internet
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Concept of a DC Grid: Less Conversion Stages ⇒ Less Losses
AC grid
Wind
Solar
Fuel cell
DC bus
Motor loads
Battery pack
Digital electronics
LED lights
H2
Steam
O2
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Methods for Mitigating the Climate Change Problems
I
Eliminate coal-fired power generation (or develop CCS)
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Increase nuclear power?
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Generation of environmentally clean energy
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Electrified (mass) transport
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Smart grids (efficient generation, transmission, distribution, and utilization of
electricity)
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Preserve rain forests and promote forestation
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Control human and animal population?
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Simpler lifestyle (33% of energy in the world can be saved)
Source: Bose, “Global energy scenario and impact of power electronics in 21st century,” IEEE Trans. Ind. Electron., 2013
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