Wireless Charging of Electric Vehicles Using
Strongly‐Coupled Resonance
Morris Kesler
WiTricity Corporation
National Electric Transportation Infrastructure Working Council
Outline
• Motivations
• Introduction to WiTricity’s Technology
– Resonance
– Coupling and Quality Factor
• Application to EV/PHEV Charging
– System description
– Performance
– Issues
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In the Middle of the Night…
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From the Wall Plug to the Device
Approaches to Wireless Energy Transfer
Radiative techniques
Induction
Omni-directional
Directed
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Add Resonance to the Picture
• Resonator:
– Stores Energy
– Energy oscillates between two modes (spatial, temporal, form, etc.)
– Examples: Pendulum, Quartz crystal, LC Circuit
• Coupled Resonators:
– Coupling mediates energy exchange between resonators
– Efficient and selective energy transfer can be achieved
– Examples: Coupled pendulums, coupled waveguides (filters, switches)
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Coupled Resonators
• Described using coupled‐mode theory
– Parameters: Coupling rate (κ), loss rate (Γ), resonant frequency
• Conditions for efficient energy transfer
– “Similar” resonant frequencies
– Coupling rate greater than loss rate
• Figure of Merit for system
– U = κ/sqrt(Γ1Γ2) = k*sqrt(Q1Q2)
– Optimum efficiency only a function of U
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Efficiency of Energy Transfer
90% at U=20
Optimum efficiency only a
function of the figure-of-merit U
η=
50% at U=3
(1 +
U2
1+U
2
)
2
where
U=
κ
Γ1Γ 2
= k Q1Q2
Coupling and Q are
important factors
Resonators with High Quality factor enable efficient energy transfer over distance.
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Using Magnetic Resonance
Coupled Magnetic Resonators
• Magnetic resonator
– Simple example is a loop and capacitor
B
E
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A Multitude of Applications
Consumer Electronics
Medical Devices
Electric Vehicles
Robotics
Solar Power
Lighting
New applications are limited only by one’s imagination
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Residential Use Case
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Requirements for Wireless Charging of EV
• Power levels up to 3.3 kW (initially)
• High‐efficiency (90% end‐to‐end)
• Tolerant to parking variations
• Tolerant to variations in vehicle ground clearance (vehicle loading, tire pressure, etc.)
• Safely operate with people in and around vehicle
• Satisfy EMC/EMI requirements
• Safe, unattended operation
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Resonators designed for
high Q and coupling,
efficient power electronics
Efficiently operate over a
range of magnetic coupling
EM fields below ICNIRP
limits where accessible
Low radiated EM fields,
Choice of frequency
Detection of foreign objects,
Built-in temperature sensing
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System Components for Wireless EV Charging
Device
Electronics
Device
Resonator
AC/DC
(Rect.)
Battery
Source Electronics
Mains
Power
BMS
AC/DC
(PFC)
RF AMP
(DC‐RF)
Source
Resonator
Control
Source Efficiency
> 95%
Wireless Efficiency
90 – 98.5%
Rectifier Efficiency
> 99%
AC Mains to Battery Efficiency of greater than 90% possible
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Tolerance to Offsets
Parking Tolerance
Direction of Travel
Air-Gap Variations
Source
Resonator
Δz
Device Resonator
Source Resonator
Δy
Typical ranges:
Δx
Δx up to +/- 20 cm
Device
Resonator
Δy up to +/- 10 cm
Δz up to +/- 2.5 cm
Systems must operate at high efficiency over this range of offset.
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Magnetic Field Strengths
•
Zone 1: Energy Transfer Region
– Largest B field
– No prolonged human exposure
•
Zone 2: Under Vehicle Region
– B rapidly decreasing
– No prolonged human exposure
•
Zone 3: Exterior Region
– B < ICNIRP MPE
– Unlimited human exposure
•
Zone 4: Vehicle Interior
– B < ICNIRP MPE
– Unlimited human exposure
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Foreign Object Debris
• Magnetic field in energy transfer region (between coils) is large
– Maximum field depends on coil design and size
– Can cause heating of some metallic objects
• Examples of likely debris
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Detection of Foreign Object Debris
• Two Basic Approaches
• Passive techniques:
– Reduce likelihood of FOD interacting hazardously with high magnetic fields.
– Large coils to reduce peak B field
– Shaped structures
• Active techniques:
– Detect and react to the presence of FOD.
– Reduce power or interupt charging
– Scales to higher power • WiTricity prototype FOD detection system demonstrated
– http://www.youtube.com/watch?v=my5fvOh15kg
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WiTricity Prototype System
Standard Capture Resonator
Rectifier
Standard Source Resonator
Integrated Power Supply
(Level 2)
(1)
Standard Resonator Configuration
(10-15cm or 15-20cm offset)
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WiTricity 3.3 kW Prototype
On‐Vehicle Installation , June 2010
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Electric Smart Car Demonstration
Device Coil mounted on Car
Source Coil on Floor
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Wireless Charging for EV/PHEV is Coming
High efficiency ( > 90%)
High power rates (3.3 kW and greater)
Power transfer over several tens of cm
“Robust” to: misalignment, weather, vehicle materials, building materials
• Safe operation in residential, commercial, and municipal configurations
•
•
•
•
Availability of wireless charging will increase adoption rates for EV/PHEV
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