Electric Drive Technologies for Future Electric Vehicles Professor Daniel Costinett The University of Tennessee

Electric Drive Technologies for Future Electric Vehicles Professor Daniel Costinett The University of Tennessee November 8 th, 2017 With contributions from Burak Ozpineci, ORNL, and Dragan Maksimovic, CU

Motivation WHY ELECTRIFY VEHICLES?

Transportation Electrification Motivation Improve efficiency: reduce energy consumption Displace petroleum as primary energy source Reduce impact on environment Reduce cost EIA: Transportation accounts for 28% of total U.S. energy use Transportation accounts for 33% of CO2 emissions Petroleum comprises 93% of US transportation energy use

Vehicle Kinematics

Propulsion power [kw] Vehicle speed [mph] Example: US06 driving cycle 100 100 80 v [mph] v [mph] 60 80 40 60 20 40 10-min 8 miles 20 0 0 100 200 300 400 500 600 0 0 100 200 300 400 500 600 P v [kw] P v [kw] 80 60 80 40 60 20 40 0 20-20 0-40 -20-60 0 100 200 300-40 time [s] 400 500 600 Time [s] -60 0 100 200 300 400 500 600 time [s] Example: Prius-sized vehicle

Average power and energy 100 v [mph] Propulsion power [kw] Vehicle speed [mph] v [mph] 100 80 60 80 40 60 20 40 Vehicle speed [mph] P v [kw] P v [kw] 200 0 100 200 300 400 500 600 0 0 100 200 300 400 500 600 80 Propulsion power [kw] 60 80 40 60 20 40 0 20-20 0-40 -20-60 0 100 200 300-40 time [s] 400 500 600 Time [s] -60 0 100 200 300 400 500 600 time [s] Prius-sized vehicle Dissipative braking P vavg = 11.3 kw 235 Wh/mile Regenerative braking P vavg = 7.0 kw 146 Wh/mile

Gas Engine vs Electric Drive Max Efficiency 34% Max Efficiency > 95% Internal Combustion Engine (ICE) Electric Drive (ED) η ED,pk 95%; η ICE,pk 35% ED offers full torque at zero speed No need for multi-gear transmission

Conventional Vs. Electric Vehicle (Commuter Sedan comparison) Regenerative braking Tank to wheel efficiency Tank + Internal Combustion Engine NO 20% 1.2 kwh/mile, 28 mpg Electric Vehicle (EV) Battery + Inverter + AC machine YES 85% 0.17 kwh/mile, 200 mpg equiv. Cost 12 /mile [$3.50/gallon] 2 /mile [$0.12/kWh] CO 2 emissions (tailpipe, total) Energy Costs (10-yr, 15k mi/yr) 300, 350) g CO 2 /mile (0, 120) g CO 2 /mile [current U.S. electricity mix] $18,000 $3,000

Energy and Power Density of Storage 12000 2016 Camaro 6.2L V8 Mazda RX-8 1.3L Wankel

Conventional Vs. Electric Vehicle (Commuter Sedan comparison) Tank + Internal Combustion Engine (Ford Focus ST) Electric Vehicle (EV) Battery + Inverter + AC machine (Ford Focus Electric) Purchase Price $24,495 $39,995 Significant Maintenance $5,000 (Major Engine Repair) $13,500 (Battery Pack Replacement) Range > 350 mi < 100 mi Curb Weight 3,000 lb 3,700 lb Energystorage Refueling Gasoline energy content 12.3 kwh/kg, 36.4 kwh/gallon 5 gallons/minute 11 MW, 140 miles/minute LiFePO 4 battery 0.1 kwh/kg, 0.8 kwh/gallon Level I (120Vac): 1.5 kw, <8 miles/hour Level II (240Vac): 6 kw, <32 miles/hour Level III (DC): 100 kw, <9 miles/minute

CO 2 emissions and oil displacement study Well-to-Wheel Analysis of Energy Use and Greenhouse Gas Emissions of PHEVs (2010 report by Argonne National Lab)

The Impact of Policy Peter Savagian, Barriers to the Electrification of the Automobile, Plenary session, ECCE 2014

EV Everywhere Grand Challenge Today $12/kW 1.2 kw/kg 3.5 kw/l >93% efficiency

Power Electronics Electrification of transportation is an enabling technology for safe, reliable, and environmental future Issues: Battery energy storage and charging times inadequate for consumer expectations 33% of total cost of an HEV is due to power electronics Weight and volume miniaturization required; Thermal management and packaging increase weight and require overdesign Materials (semiconductor, inductor, capacitor) materials not cost-effective Intelligent and comprehensive design of power electronics is necessary to continue development of electric vehicles.

Background ROLE OF POWER ELECTRONICS

Power Electronics in Electric Vehicles Peter Savagian, Barriers to the Electrification of the Automobile, Plenary session, ECCE 2014

EV Drive Train Battery PHEV (~320VDC) HEV(~259VDC) Up to 700 VDC, depending on driving conditions 3-ф AC to motor Low Voltage modules (4-60VDC)

Cooling Systems Audi Q5 Example Engine coolant in gasoline vehicles typically 105 C Silicon power electronics begin to break down at 125 C Separate, dedicated cooling loop for electrical systems SOA: 1kW to cool 4kW of power losses

Design of Power Electronics

Powertrain Components Battery Charger (200 450 V DC) Bi-directional Converter Inverter 120 V AC/ 240V AC/ Fast Charger Torque to Drive Wheels

Example EV Power Electronics Module Component buildup of an EV Inverter built at ORNL

Substrate A ceramic piece sandwiched between two metal layers. Typically Direct Bonded Copper (DBC), Direct Bonded Aluminum (DBA) Function: Electrical conduction (horizontal plane) and insulation (vertical plane).

Power Device Chips The power device chips are soldered on the DBC. Function of Die Attach: Electrical connection and mechanical joints.

Interconnect Wire or Ribbon Bonds Typically aluminum or gold. Function: Electrical connection.

Baseplate (Base Metal) Typically copper or metal matrix composite (e.g. AlSiC). Function: Mechanical support and heat spreading.

Encapsulate Silicone Gel Function: Electrical insulation, mechanical protection (damps mechanical shock), and heat spreading.

Housing and Terminals The module is covered with a plastic package. The interconnects connect the power device chips to the copper terminal which protrudes from the package. Function: Mechanical protection and electrical connection.

Liquid Cooled Heatsink Typically, an aluminum liquid-cooled heatsink is used to cool inverters.

Power Modules The power modules are mounted on the heat sink with a thermal interface material (TIM) under the modules to increase the thermal conductivity. TIM is still responsible for around 40% of the thermal resistance from the junction to the ambient.

Gate Driver Boards The gate driver boards convert the computer signals into gate signals to switch the power devices on and off. The gate driver boards need to be as close to the power modules as possible to reduce the length of interconnections.

Sensors Current and voltage values are required as feedback for closed loop controllers to control torque (using current values) and speed (or flux using voltage values) of an electric machine.

Bus Bars Copper busbars are used for electrical connections between every power component of the inverter. For most inverters, bus bars must be designed to have minimum parasitic inductance.

Controller The controller boards are usually much more crowded than the one shown on the left and they include circuits for a microprocessor, typically a digital signal processor (DSP) analog and digital interfaces with the sensors and gate drivers

Capacitors Dc link capacitors are required in voltage source inverters (VSI). Capacitors generally occupy a third the volume of an inverter and weigh as much. They cost 15-20% of the inverter. In a VSI, they filter the dc link current and keep the dc link voltage constant. A current source inverter (CSI) would have an inductor instead of the dc link capacitor. The inverter shown on the left has significantly lower dc link capacitor requirements.

ORNL-built Segmented Inverter with Reduced Capacitance The advantage of this inverter is 60% reduction dc-link capacitor

Research HOW WE CAN IMPROVE POWER ELECTRONICS

Power Electronics for Electric Drives State of the Art (SOA): No WBG inverters commercially available Problems associated with power electronics for advanced vehicle applications include: Low efficiency at light load conditions for inverters and converters Low current density and device scaling issues for high power converters Lack of reliable higher junction temperature devices High cost of devices and power modules especially for WBG and advanced silicon devices Low power density for the low voltage electronics and cost of interconnects Lack of adequate protection for the devices High cost for low loss magnetics and high temperature films for capacitors

Research Areas Reduce size and weight of the inverters to meet the 2022 targets of 13.4 kw/l and 14.1 kw/kg WBG technology Increase the overall efficiency of the traction drive system Reduce the size of the passives with high frequency operation Reduced thermal requirements with high temperature operation Integrated topologies and passives Reduce cost and volume of the passives Integrate more functionality and reduce cost through component count Control algorithms and novel circuits Increase the safe operating area of the WBG devices using advanced gate driver circuits Increase the reliability of the system using protection circuits Design algorithms to optimize efficiency for light load conditions Novel system packaging and advanced manufacturing techniques Minimize parasitics and increase heat transfer using advanced packaging Reduce cost through novel interconnects Reduce cost through process optimization

Functional Integration Battery Charger (200 450 V DC) Bi-directional Converter Inverter 120 V AC/ 240V AC/ Fast Charger Torque to Drive Wheels

Example #1 BIDIRECTIONAL DC-DC CONVERTER

Drivetrain DC-DC Converter Oak Ridge National Lab, Benchmarking of Competitive Technologies η = 86% Battery Electric Drive η = 92% Bidirectional DC-DC Converter A DC-DC converter to boost battery voltage is included in many EVs Allows higher efficiency operation of ED Wider operating range Lower pack voltage V dc = 650 V

Drivetrain DC-DC Converter Oak Ridge National Lab, Benchmarking of Competitive Technologies η = 86% 88% Battery Electric Drive η = 92% 94% Bidirectional DC-DC Converter A DC-DC converter to boost battery voltage is included in many EVs Allows higher efficiency operation of ED Wider operating range Lower pack voltage V dc = 650 V V dc = 500 V

Drivetrain DC-DC Converter Oak Ridge National Lab, Benchmarking of Competitive Technologies η = 86% 88% 93% Battery Electric Drive η = 92% 94% Bidirectional DC-DC Converter A DC-DC converter to boost battery voltage is included in many EVs Allows higher efficiency operation of ED Wider operating range Lower pack voltage V dc = 650 V V dc = 500 V V dc = 225 V

Optimal Bus Voltage Bus voltage for maximum efficiency of electric drive varies with speed and torque Maximum ED efficiency only obtained when power electronics vary bus voltage dynamically Often, only high power operating points are considered in design, but under normal driving conditions, ED operates predominately at low power 100 80 US06 driving cycle v [mph] 60 40 20 0 0 100 200 300 400 500 600 80

Integrated DC/DC and Charger Designed combined drivetrain DC-DC and isolated battery charger Converter designed for high energy efficiency during normal drive cycles Highly efficient, low power isolation converter used as drivetrain converter at low power Traditional 2-stage drivetrain topology Charger Mode DC/DC Mode Combined isolated/non-isolated topology Prototype Measured Efficiency

Reducing Cooling Partnered with ORNL to integrate and test immersion cooled SiC power module SiC gate driver tested to 300 C 3D printed aluminum endcaps function as electrical, mechanical, and thermal components Complete 50 kw phase leg assembly

Example #2 BATTERY MANAGEMENT SYSTEM

Battery Management System Battery Management System EV battery consists of many (100 s) series battery cells (LFP, Li-ion, NiMH) Cells share a charging and discharging current, but may have mismatches in series resistance, capacity, operating temperature, health, or dynamics Cells binned by manufacturer to limit mismatch at beginning-of-life

Cell Balancing SOC max SOC min + V pack Small differences in cell characteristics are exacerbated over multiple charge and discharge cycles due to their series connection

Cell Balancing SOC max SOC min + V pack Discharge is limited by the first cell to reach the minimum allowable State-of- Charge (SOC).

Cell Balancing SOC max Inaccessible Capacity SOC min + V pack Discharge is limited by the first cell to reach the minimum allowable State-of- Charge (SOC). Effective pack capacity limited to the capacity of the lowest cell (in Amp-hours)

Cell Balancing SOC max Uncharged Capacity SOC min + V pack When recharged, charging is stopped when the first cell reaches maximum capacity, leading to incomplete charging of some cells and lower total pack capacity

Passive (Dissipative) Balancing SOC max SOC min + V pack Option 1: Dissipate, then recharge Repeat a number of cycles to balance all cells at full charge

Active Balancing SOC max SOC min + V pack Option 2: Redistribute Requires more advanced circuits Can be run continuously, during runtime

Incorporation of Functionality Combining the necessary HV-to-LV converter with balancing converters allows Effective (differential) efficiency of 100% Nearly zero effective cost R. Zane, M. Evzelman, D. Costinett, D. Maksimovic, R. Anderson, K. Smith, M. S. Trimboli, and G. Plett, Battery control, U.S. Patent 14/591,917, 2012.

Future Work WHERE POWER ELECTRONICS WILL TAKE US

Vehicle-Grid Interactions SAE Ground Vehicle Standards SmartGrid EV batteries used as energy storage for grid Intermittency mitigation coupled to renewables Load shifting Power compensation

Wireless Charging Wireless charging opportunity: Provides convenience to the customer. Plug-in and battery electric vehicles (PEVs) can be charged conveniently and safely without need for cable and plug. With WPT the charging process can be fully autonomous. Reduce on-board ESS size using dynamic on-road charging.

Wireless Charging Stationary Charging Opportunity/Quasi-Dynamic Charging In-motion/Dynamic Charging

Wireless Charging Stationary Charging Chevy Equinox and Toyota Prius Conversion Plugin In-motion/Dynamic Charging GEM Vehicle

World s First 3D Printed Car Makes Debut Layer by layer, inch my inch, the world's first 3-D printed vehicle seemingly emerged from thin air during the 2014 International Manufacturing Technology Show. In a matter of two days, history was made at Chicago's McCormick Place, as the world's first 3-D printed electric car -- named Strati, Italian for "layers"-- took its first test drive. September 2014

ORNL 3D-Printed Electric Shelby Cobra

3D Printed Liquid-Cooled 80 kw Inverter 80kW ORNL COMPACT Inverter Nissan LEAF Inverter (80kW)

Conclusions Power Electronics play a critical role in the future adoption of electric vehicles The Power Electronics engineer must holistically consider in-system performance, not solely efficiency in isolation Future technologies including wireless power transfer and 3D printing can revolutionize the industry

QUESTIONS: Daniel.Costinett@utk.edu MK502 curent.utk.edu potenntial.eecs.utk.edu ornl.gov