Electromagnetic theory | Higher education » J. W. Kolar - Conceptualization and Multi Objective Optimization of the Electric System of an Airborne Wind Turbine

Source: http://www.doksinet 1/82 1/81 Conceptualization and Multi-Objective Optimization of the Electric System of an Airborne Wind Turbine J. W Kolar et al Swiss Federal Institute of Technology (ETH) Zurich Power Electronic Systems Laboratory www.peseeethzch Source: http://www.doksinet 2/82 2/81 Pareto-Optimal Design of Airborne Wind Turbine Power Electronics J. W Kolar, T Friedli, F Krismer, A Looser, M Schweizer, P Steimer, J Bevirt Swiss Federal Institute of Technology (ETH) Zurich Power Electronic Systems Laboratory www.peseeethzch Source: http://www.doksinet 3/82 3/81 Basics Electronic Power Processing Power Electronics Performance Trends Design Process Multi-Objective Optimization Source: http://www.doksinet 4/82 4/81 Basic Electronic Power Processing System - Electronic Switches / Power Semiconductors - Filter Circuits / Inductors, Capacitors - Heat Management / Heatsink - Sensor Circuits - Digital Signal Processing Source: http://www.doksinet 5/82 5/81 Basic

Electronic Power Processing System - Highest Efficiency - Highest Dynamics - Highest Compactness - Highest Compatibility - Highest Reliability Voltage Frequency Power Semiconductors Filter Circuits Interconnections Voltage Frequency EMC EMC Sensors Control Communication Source: http://www.doksinet 6/82 6/81 Basic Electronic Power Processing System - Highest Efficiency - Highest Dynamics - Highest Compactness - Highest Compatibility - Highest Reliability ► Example of a Three-Phase AC/AC Matrix Converter Source: http://www.doksinet 7/82 7/81 Three-Phase AC/AC Matrix Converter Prototype Output connectors Control boards Input filter Fans Heatsink 3 2.9 3 ~ kW/dm 2.9 kW/dm = 3 48 W/in Efficiency 95.5% Input RMS voltage Output Power Rectifier Switching Frequency Inverter Switching Frequency 400V 6.8 kVA 12.5 kHz 25 kHz Source: http://www.doksinet 8/82 8/81 Power Electronics Performance Trends ■ Performance Indices ─ ─ ─ ─ ─ Power Density [kW/dm3] Power

per Unit Weight [kW/kg] Relative Costs [kW/$] Relative Losses [%] Failure Rate [h-1] ► Understand the Mutual Coupling of Performance Indices Source: http://www.doksinet 9/82 9/81 Abstraction of Power Converter Design Process Performance Space Design Space ► Mapping of Design Space into System Performance Space Source: http://www.doksinet 10/82 10/81 Mathematical Modeling of Converter Design ► Multi-Objective Optimization Source: http://www.doksinet 11/82 11/81 Multi-Objective Design Optimization/ PARETO-Front ► Sensitivity to Technology Advancements ► Trade-off Analysis Source: http://www.doksinet 12/82 12/81 Example  Efficiency / Volume Trade-off of Inductors Operating Conditions and Parameters L, fP , I ■ Scaling of Core Losses  PCore  f P ( ) 2 V 1 A2 3 1 PCore  ( 2 ) l  l l ■ Scaling of Winding Losses l PWdg  I 2 R  I 2 Wdg  AWdg 1 PWdg  l  LI Source: http://www.doksinet 13/82 13/81 Converter Performance

Evaluation Based on η-ρ-σ-PARETO Surface ► σ: kW/$ Source: http://www.doksinet 14/82 14/81 Converter Performance Evaluation Based on η-ρ-σ-PARETO Surface ► ´ Technology Node´ ► Source: http://www.doksinet 15/82 15/81 “Out-of-the-Box” Wind Turbine Concepts Power Kite & Ground-Based EE-Generation Power Kite & On-Board EE-Generation Source: http://www.doksinet 16/82 16/81 Conventional 100kW Wind Turbine ► Characteristics - Tower - Rotor - Nacelle 35m/18 tons 21m / 2.3tons 4.4 tons ► ■ Large Fraction of Mechanically Supporting Parts / High Costs Source: http://www.doksinet 17/82 17/81 Air Rotor Wind Generator ► Helium or Hydrogen Inflated ► Magnus Effect - Additional Lift Source: http://www.doksinet 18/82 18/81 Revolutionize Wind Power Generation Using Kites / Tethered Airfoils [2] M. Loyd, 1980 ■ Wing Tips / Highest Speed Regions are the Main Power Generating Parts of a Wind Turbine Source: http://www.doksinet 19/82 19/81

Controlled Power Kites for Capturing Wind Power ► Replace Blades by Power Kites ► Minimum Base Foundation etc. Required ► Operative Height Adjustable to Wind Conditions M. Loyd, 1980 ■ Wing Tips / Highest Speed Regions are the Main Power Generating Parts of a Wind Turbine Source: http://www.doksinet 20/82 20/81 Controlled Power Kites for Capturing Wind Power ► ► Wind at High Altitudes is Faster and More Consistent Operate Kites at High Altitudes or Even in the Jet Stream 0 Source: 120m 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 . 0 1 . 2 1 . 4 1 . 6 1 . 8 2 kW/m2 2 . 0 700m Source: http://www.doksinet 21/82 21/81 Controlled Power Kites for Capturing Wind Power ► ► Wind at High Altitudes is Faster and More Consistent Operate Kites at High Altitudes or Even in the Jet Stream Source: http://www.doksinet 22/82 22/81 Pumping Power Kites Source: M. Diehl / K.U Leuven Ground-Based EE-Generation Source: http://www.doksinet 23/82 23/81 Basics of Power

Kites ► Kite´s Aerodynamic Surface Converts Wind Energy into Kite Motion Source: M. Diehl / K.U Leuven ■ Generated Force Could be Converted into Useful Power by Pulling a Load / Driving Turbines via a Tether ► ► Source: http://www.doksinet 24/82 24/81 Pumping Power Kites ► Maximum Power M. Loyd, 1980 Source: M. Diehl / K.U Leuven Source: http://www.doksinet 25/82 25/81 Pumping Power Kites for Capturing High Altitude Wind Power ► Lower Electricity Production Costs than Current Wind Farms ► Generate up to 250 MW/km2, vs. the Current 3 MW/km2 ► Research at the Source: http://www.doksinet 26/82 26/81 Pumping Power Kites for Capturing High Altitude Wind Power ► Lower Electricity Production Costs than Current Wind Farms ► Generate up to 250 MW/km2, vs. the Current 3 MW/km2 ► Research at the Carousel Configuration Source: http://www.doksinet 27/82 27/81 Airborne Wind Turbine Source: M. Diehl / K.U Leuven On-Board EE-Generation Source:

http://www.doksinet 28/82 28/81 Alternative Concept – Airborne Wind Turbine ► ► Power Kite Equipped with Turbine / Generator / Power Electronics Power Transmitted to Ground Electrically ► M. Loyd, 1980 Source: http://www.doksinet 29/82 29/81 Alternative Concept – Airborne Wind Turbine ► ► Power Kite Equipped with Turbine / Generator / Power Electronics Power Transmitted to Ground Electrically Source: M. Loyd, 1980 Source: http://www.doksinet 30/82 30/81 Basic Physics of Wind Turbines Maximum Achievable acc. to Lanchester / Betz High Crosswind Kite Speed  Very Small Turbine Area ► ► ► Source: http://www.doksinet 31/82 31/81 Comparison of Conventional / Airborne Wind Turbine ■ Numerical Values Given for 100kW Rated Power Source: http://www.doksinet 32/82 32/81 SkyWindPower AWT Concept ► Tethered Rotorcraft – Quadrupole Rotor Arrangement ► Inclined Rotors Generate Lift & Force Rotation / Electricity Generation Artist´s

Drawing of 240kW / 10m Rotor System ■ Named as One of the 50 Top Inventions in 2008 by TIME Magazine Source: http://www.doksinet 33/82 33/81 AWT Concept ► Reinforced Tether Transfers MV-Electricity to Ground ► Composite Tether also Provides Mechanical Connection to Ground Source: http://www.doksinet 34/82 34/81 AWT Concept Source: http://www.doksinet 35/82 35/81 Demonstration Plan Source: http://www.doksinet 36/82 36/81 Flight Modes / Parked Source: http://www.doksinet 37/82 37/81 Future Prospects Source: M. Diehl / K.U Leuven ■ Example for Thinking “Out-of-the-Box” ! Source: http://www.doksinet 38/82 38/81 Future Prospects Source: M. Diehl / K.U Leuven ■ Example for Thinking “Out-of-the-Box” ! Source: http://www.doksinet 39/82 39/81 Technical Feasibility of AWT Electrical System ► AWT Electrical System Structure ► Multi-Objective Optimization (Weight vs. Efficiency) ► Controls Aspects Source: http://www.doksinet 40/82 40/81

AWT Basic Electrical System Structure ► ► ► ► Rated Power 100kW Operating Height 8001000m Ambient Temp. 40°C Power Flow Motor & Generator 100kg ■ El. System Target Weight ■ Efficiency (incl. Tether) 90% ■ Turbine /Motor 2000/3000rpm Source: http://www.doksinet 41/82 41/81 Design of Electrical Power System ► Clarify Practical Feasibility of AWT Concept ► Clarify Weight/Efficiency Trade-off / Multi-Objective Optimization / PARETO-Front Source: http://www.doksinet 42/82 42/81 Tether Design DC Voltage Level η-γ-PARETO Front Source: http://www.doksinet 43/82 43/81 Tether DC Transmission Voltage Level ► ► ► ► Pth,1 = 100kW / lth = 1000m Strain Relief Core – Kevlar (Fth = 70kN, d=5mm) Cu or Al Helical Conductors - ½ Uth Isolated Outer Protection Jacket (3mm) Source: http://www.doksinet 44/82 44/81 Tether η-γ-PARETO Front ► Tether Voltage Vth,1 = 8kV ■ Total Weight of Tether: 320kg Source: http://www.doksinet 45/82 45/81 System

Overview  Source: http://www.doksinet 46/82 46/81 Possible AWT Electrical System Structures ► Low-Voltage or Medium-Voltage Generators / Power Electronics ► Decision Based on Weight/Efficiency/Complexity ► ►  Source: http://www.doksinet 47/82 47/81 Generator / Motor Design Dimensions Number of Pole Pairs η-γ-PARETO Front Source: http://www.doksinet 48/82 48/81 Generator / Motor η-γ-PARETO Front ► Medium Voltage vs. Low Voltage Machine Vth,1 = 8kV - PMSM – Radial Flux – Internal Rotor - Slotted Stator / Concentrated Windings – Air Cooling - Analytical EM and Thermal Models for Weight / Efficiency Optimization - P = 16kW / 2000rpm Thermal Model LV Machine HV Machine ■ LVG: Diameter 17cm (excl. Cooling Fins) / Width 60cm / p = 20 / η = 954% / Weight 51kg Source: http://www.doksinet 49/82 49/81 CAD Drawing of LV and MV Machine ► Fixed Parameters and Degrees of Freedom Source: http://www.doksinet 50/82 50/81 Generator / Motor

η-γ-PARETO Front ► Selected Design η = 95.4% γ = 3.1 kW/kg ■ Medium Voltage Machine Not Considered Further Source: http://www.doksinet 51/82 51/81 Comparison to Commercial Motors ► Motors Employed for Electric Propulsion of Glider Airplane Power Speed Cooling P = 10kW n = 2200rpm vL = 25m/s ■ Diameter 22cm Width 8.6cm Weight 12kg Pole Pairs 10 Efficiency 91% Source: http://www.doksinet 52/82 52/81 System Overview  Source: http://www.doksinet 53/82 53/81 Rectifier / Inverter Design Chip Area Heatsink Volume η-γ-PARETO Front Source: http://www.doksinet 54/82 54/81 Rectifier / Inverter Design ► 2-Level or 3-Level Bidirectional Voltage Source Rectifier - S = 19.3kVA - VDC = 750V - fS,min= 24kHz - TJ = 125°C - Foil Capacitor DC Link 1200V T&FS Si IGBT4s / 1200V SiC Diodes 600V T&FS Si IGBT3s / 600V Si EmCon3 Diodes ■ Maximization of Heatsink Thermal Conductance / Weight (Volume) - Max. CSPI Source: http://www.doksinet 55/82 55/81

Heatsink Optimization ► Maximize Thermal Conductance / Weight (Volume) pF,MAX k . pF pF,MAX k . pF operating point operating point n=5 d n=5 t t c c pF [N/m2] d PV pF [N/m2] PV vAir ≈ 5m/s s b/n b s b b/n ■ Highest Performance Fan ■ Fin Thickness / Channel Width Optimization pCHANNEL pCHANNEL VF [m3/s] VF [m3/s] VF,MAX VF,MAX Source: http://www.doksinet 56/82 56/81 Heatsink Optimization ► Maximize Thermal Conductance / Weight (Volume) PV /n PV pF,MAX Rth,d d n=5 Rth,a c/2 Rth,FIN c Rth,A t Rth,FIN Rth,A pF [N/m2] d TCHANNEL t s s b b/n ■ Highest Performance Fan ■ Fin Thickness / Channel Width Optimization k . pF operating point pCHANNEL VF [m3/s] VF,MAX Source: http://www.doksinet 57/82 57/81 Heatsink Optimization 1 n=34 Rth [K/W] 0.8 ► Optimum n=6 n=50 n=10 0.6 sub-optimum: n=16 / k=0.60 s=1.5mm / t=10mm Rth,sub=0.30 0.4 X L x b x c= 80x40x40mm3 0.2 Al with th= 210W/Km n =

[6, 10, 14, ., 42, 46, 50] X optimum: n=26 / k=0.65 s=1.0mm / t=054mm Rth,sub=0.26 0 0 ■ Highest Performance Fan ■ Fin Thickness / Channel Width Optimization 0.2 0.4 0.6 k = s/(b/n) 0.8 1 Source: http://www.doksinet 58/82 58/81 Rectifier / Inverter η-γ-PARETO Front - Switching Frequency Range 2470 kHz - Heatsink Temperature Range 55100 °C (Tamb = 40°C) ► Selected Design η = 98.5% γ = 19 kW/kg ■ 3-Level Topology Does Not Show a Benefit Source: http://www.doksinet 59/82 59/81 System Overview   Source: http://www.doksinet 60/82 60/81 8kVDC/750VDC DAB Converter Design Switches / Topology Transformer η-γ-PARETO Front Source: http://www.doksinet 61/82 61/81 DC/DC Converter Topology ► Bidirectional Energy Transfer - Dual Active Bridge - Weight ≤ 25kg - fS = 50125kHz  fS,m = 100kHz - Phase-Shift Control (φ = π/4) 0.8 kV ■ Implementation of Electronic Switches - SiC 8 kV Source: http://www.doksinet 62/82 62/81 DC/DC

Converter Topology ► Bidirectional Energy Transfer - Dual Active Bridge - Weight ≤ 25kg - fS = 50125kHz  fS,m = 100kHz - Phase-Shift Control (φ = π/4) 0.8 kV 8 kV ► ■ Implementation of Electronic Switches - SiC 10kV Si/SiC SuperCascode Switch Source: http://www.doksinet 63/82 63/81 Si/SiC Super Cascode Switch C/R Synchronous Switching  HV-Switch Controllable via Si-MOSFET * 1 LV Si MOSFET * 6 HV 1.7kV SiC JFETs * Avalanche Rated Diodes  Ultra Fast Switching  Low Losses  Parasitics * Passive Elements for Simultaneous Turn-on and Turn-off * Stabilization of Turn-off State Voltage Distribution JFETs MOSFET Source: http://www.doksinet 64/82 64/81 Si/SiC Super Cascode Switch C/R Synchronous Switching  HV-Switch Controllable via Si-MOSFET * 1 LV Si MOSFET * 6 HV 1.7kV SiC JFETs * Avalanche Rated Diodes  Ultra Fast Switching  Low Losses  Parasitics * Passive Elements for Simultaneous Turn-on and Turn-off * Stabilization of Turn-off State

Voltage Distribution JFETs MOSFET Source: http://www.doksinet 65/82 65/81 Selected Multi-Cell Converter Topology ► MV-Side Series-Connection / LV-Side Parallel-Connection Pi = 6.25kW Vth,1,i = 2kV ■ Winding Arrangement & Efficiency / Weight Optimization of Transformer Source: http://www.doksinet 66/82 66/81 Transformer Design ► ► ► MV-Winding Arranged Around Inductor Cores Cooling Provided by Heatpipes Stacked Cores - Scalable Arrangement ■ Optimization - Weight / Efficiency Trade-off Source: http://www.doksinet 67/82 67/81 Transformer Optimization ► Degrees of Freedom / Parameter Ranges Source: http://www.doksinet 68/82 68/81 Transformer η-γ-PARETO Front ► Selected Design η = 97% γ = 4.5 kW/kg ■ Transformer Volt-Second Balancing - Series Capacitor or “Magnetic Ear” Control Source: http://www.doksinet 69/82 69/81 Transformer Volt-Second Balancing – “Magnetic Ear” ► Magnetic Ear Magnetized with 50% Duty Cycle

Rectangular Voltage Winding ► Measured Aux. Current iaux / Voltage vm Indicates Flux Level ► Enables Closed-Loop Flux Control N27 E55 Ferrite Source: http://www.doksinet 70/82 70/81 System Overview    Source: http://www.doksinet 71/82 71/81 Overall System Consideration Total Weight Overall Efficiency η-γ-PARETO Front Source: http://www.doksinet 72/82 72/81 Determination of Overall System Performance ► Consideration of the η-γ-Characteristics of the Partial Systems ► Overall η-γ-Characteristic  Pout m ■ Efficiencies of the Partial Systems Need to be Taken into Account ■ PD/PR = Overrating Ratio (8x16kW/100kW) Source: http://www.doksinet 73/82 73/81 ► Overall System Performance ■ Final Step: System Control Consideration Source: http://www.doksinet 74/82 74/81 Electric System Control Stability Reference Response Disturbance Response Source: http://www.doksinet 75/82 75/81 System Control ► ► ► Control of

Flight Trajectory / Max. Energy Generation Generator (Motor) Speed / Torque Control etc. ► Control of DC Voltage Levels is Mandatory ! ■ Simplified Control-Oriented Block Diagram of the Electric System Source: http://www.doksinet 76/82 76/81 Control Block Diagram ► ► Ground Station Controls the Tether Voltage Control Objectives: LV DC Bus 650750V; MV (Tether) < 8kV ■ Only Tether Voltage at Ground Station is Measured (ITh Feedforward) ■ Motor AND Generator Operation Must be Considered Source: http://www.doksinet 77/82 77/81 Tether Voltage Control Plant ► Motor Operation (100kW) ► Source: http://www.doksinet 78/82 78/81 Voltage Control Reference Step Response ■ Overshoot Could be Avoided with Reference Form Filter Source: http://www.doksinet 79/82 79/81 Voltage Control Disturbance Response ■ Motor Operation 100kW  0 ■ Gen. Operation ─100kW  0 Source: http://www.doksinet 80/82 80/81 Conclusions ► AWTs are Basically

Technically Feasible ► AWTs Realization Combines Numerous Challenges - Aircraft Design - MVDC Transmission - MV/HF Power Electronics - etc. ► AWTs are a Highly Interesting Example for η-γ Trade-off Studies ► AWTs are Examples for Smart Pico Grids or MEA Power System Analysis ► AWTs is a Clear Example of Thinking “Out-of-the-Box” ! Source: http://www.doksinet 81/82 81/81 Source: http://www.doksinet 82/82 82/81 Questions ?