Power Electronics

Future Electric Vehicle Success Will Depend on Its Energy Source

The electric vehicle is one potential answer to rising fuel prices and the U.S. dependence on foreign oil. However, the performance of today’s electric vehicles is limited by its on-board rechargeable battery, which is not yet optimum.

A MAJOR GOAL FOR ELECTRIC VEHICLES IS THE ABILITY TO USE A single battery charge to provide a range near that of current internal combustion engines, which is about 300 miles. In the long term, the fuel cell or some other exotic device may provide the desired capability. For the near term, the battery is the only choice for an on-board energy source.

Electric vehicle (EV) batteries are intended to give power over sustained periods so they must have a high ampere-hour capacity. These so-called traction batteries must have a relatively high specific energy and power-to-weight ratio. Ideally, the traction battery should weigh very little because it adds to the weight the vehicle must carry, which will limit its range. And, the battery should provide all the power necessary to achieve maximum range. Compared to liquid fuels, all current battery technologies have much lower specific energy, which impacts the maximum all-electric vehicle range.

On an energy basis, the price of electricity to run an EV is a fraction of the cost of liquid fuel needed to produce an equivalent amount of energy (energy efficiency). Industry estimates note that over the life of a battery, the cost of buying and recharging a battery is still usually lower than the cost of fossil fuels suitable for propelling a vehicle. However, the replacement cost of a spent battery dominates the vehicle's operating costs.

Although the electric vehicle doesn't pollute, the electricity used to recharge the battery usually comes from an electric utility employing fossil fuels that can produce pollutants. However, this will change as more non-polluting renewable energy sources like wind and sun produce the electricity.

Of all the available battery technologies, lithium-ion (Li-ion) types offer several advantages over other types of secondary batteries, including lighter weight and higher energy density. Now, battery developers are working to create larger, higher power lithium-ion traction batteries.

Batteries for EVs may have to produce hundreds of volts to power the traction motor. A single lithium-ion battery has a typical output of 3V to 4V. Therefore, battery packs require multiple individual batteries.

BATTERY DEVELOPMENT

  1. Argonne National Laboratory is addressing the development of individual batteries through a closely integrated R&D effort that addresses the barriers to industry acceptance. Today, four constraints prevent broad acceptance of Li-ion batteries with cells larger than 10Ah for electric vehicles.
  2. Safety associated with graphite anodes and volatile, flammable electrolytes.
  3. Power fade due to increased cell impedance attributed to a metal oxide cathode.
  4. Capacity fade associated with the loss of lithium through parasitic side reactions because power and/or capacity fade control battery life.
  5. The cost of components, particularly the cathode and cell packaging. Argonne has focused on the safety and life issues with new electrolytes and additives that promote the formation of stable passivation films at the electrodes, as well as more stable cathode materials. Argonne is also developing less expensive cathode materials and innovative packaging that greatly reduces costs.

Existing Li-ion cell designs use liquid organic electrolytes that, while successful in small electronic devices, have safety limitations that preclude their use in larger-battery applications. Many applications, particularly military, seek maximum energy density beyond even today's notable performance. Argonne Laboratory has developed new electrolytes and rechargeable chemistries that can improve capacities from 120 Wh/kg to 200 Wh/kg (watt-hours/kilogram).

BATTERY MANUFACTURE

A report by IDTechEx says that Toyota is the world's largest manufacturer of electric vehicles by value and the leader by a big margin in hybrid cars. Toyota obtained its hybrid batteries from Panasonic EV Energy of Japan, a joint venture with the world's largest battery maker, Panasonic of Japan, formerly Matsushita.

Now the traction battery power game has been completely turned on its head by Panasonic buying Sanyo of Japan, which supplies lithium-ion traction batteries to a huge variety of pure and hybrid electric vehicles of all sorts. Antitrust legislation required Panasonic to hand control of Panasonic EV Energy to partner Toyota in June 2010 to head off a monopoly forming in traction batteries. After purchase of these shares, Toyota now has 80.5% of the battery maker, now renamed Primearth EV Energy, a company that has made over three million traction batteries albeit only NiMH, a chemistry that will peak in usage within the decade. Indeed, observers have predicted that, even as Primearth starts to make lithium-ion traction batteries, there will be few beyond General Motors (its current customer) that will buy such a critical component from a subsidiary of a competing car manufacturer.

CURRENT ELECTRIC VEHICLES

Among the electric vehicles now employing lithium-ion batteries are the Tesla Roadster (Fig. 1), Toyota Prius (Fig. 2), Chevrolet Volt (Fig. 3), and Nissan LEAF (Fig. 4). The Prius is a hybrid that uses a small gasoline engine to recharge its battery while driving. The Volt also has a small gasoline engine for recharging while driving. The Tesla Roadster, Volt, and LEAF have provisions for plug-in recharging. Of these, one of the first all-electric, plug-in recharging vehicles was the Tesla Roadster and is a good example of electric vehicle design.

Tesla Motors' Roadster electric sports car has a range up to 250 miles on one battery charge, depending on driving conditions. And while it's cruising, it doesn't release any emissions.

The weights of the body and batteries alike are key design factors in electric cars. Tesla based its chassis design on the Lotus Motors Elise, which is similar in size to the Roadster. Roadster designers had a choice between aluminum, fiberglass, or carbon fiber for body panels. They opted for carbon fiber, whereas the Elise's body is fiberglass. Carbon fiber is more expensive, but it saves about 200 lb.

One of the initial design decisions was to select the optimum battery among lead-acid, nickel-metal hydride, and Li-ion technologies. Li-ion won because, for the same power, it's lighter and has greater energy density than the others.

Another important initial design decision was the choice of motor, either ac or dc. Tesla opted for a three-phase, four-pole ac induction motor, rated at 248 HP with a nearly flat torque curve from 0 to 13,500 rpm. The motor provides direct drive to the car's rear wheels, shown in Fig. 5. It can provide negative torque, otherwise known as regenerative braking to help charge the batteries, which extends its range.

TWO-SPEED TRANSMISSION

A unique two-speed electrically actuated manual transmission provides optimum efficiency. The gear shift has an electromechanical transmission lock (park position), and there's no clutch. This transmission maximizes acceleration while also allowing a reasonably high top speed. First gear gets you from zero to 60 in about 4 seconds, while second gear can take you to over 130 mph.

The Roadster's gearing works together with the gear shift and internal power electronics to provide familiar driving behavior. First gear has strong regenerative braking that recharges the battery during deceleration and feels like an engine braking in low gear. Second gear has less regenerative braking and feels like third or fourth gear. The Roadster needs only two gears because the motor delivers high torque over a much wider range of rpm than a gasoline engine.

The Tesla's brain is the Power Electronics Module (PEM), an air-cooled, computer-controlled system for the motor (Fig. 6). It has an integrated power inverter and high-rate charging system. It also is an inverter, charger, and motor controller. This configuration lets regenerative braking flow through the charging system into the PEM, which employs an IGBT output stage.

BATTERY POWER

The Energy Storage System (ESS) contains Li-ion batteries in a durable, tamper-resistant enclosure behind the seats, very close to the center of the car. It provides a mechanical structure to mount the batteries, electrical interconnection between the cells, interconnection to the power electronics unit, a network of microprocessors for maintaining charge balance and temperature monitoring, and a cooling system. Its independent safety system isolates high voltage outside the enclosure under a variety of detectable safety situations.

To keep temperatures under control, the electronically controlled liquid cooling system uses a secondary loop in the cabin air conditioning system to provide chilled coolant circulated throughout the ESS. A resistive heater can heat the batteries in extreme cold conditions.

Battery packs consist of 11 identical groups of 621 lithium-ion cells (called sheets) connected in series and parallel. Each cell is 18 mm in diameter and 650 mm long with a nominal 3.6 V, 2.2 A/hr rating. Each sheet produces 32.4 V (nominal). One sheet supplies voltage for the car's accessories, such as lights and power windows. The remaining ten sheets provide 324 V for the motor.

The Roadster comes with the Electric Vehicle Service Equipment (EVSE), a home-based charging system. It operates from a 220-V ac power line that lets it recharge a battery in 3.5 hours. During charging, the maximum input power is 240 Vac at 70 A, 45 to 65 Hz, and about 17 kW. Projected battery life is more than 100,000 miles and five years. Battery replacement must be performed at a Tesla Service Center at a cost yet to be determined, as battery prices continue to drop.

The Battery Safety Monitoring system checks battery temperature, current, and voltage, as well as vehicle factors, such as high acceleration (indicating a crash), smoke, and temperature. The monitor includes overcharging protection, three layers of fuses, and sensors that trigger the batteries to disconnect in the case of high temperatures, sudden impact, or rollover. It disconnects the high voltage if any of these factors are out of range.

Determining energy efficiency requires a computation of well-to-wheel efficiency from the source of the fuel (usually the oil well or natural gas) to the wheels. Tesla points out that the Honda Civic CX is the most efficient traditional gasoline car with an efficiency of 0.52 km/MJ. The hybrid Toyota Prius has an energy efficiency of 0.56km/MJ. A PEM fuel-cell car has a calculated efficiency of 0.77km/MJ, though this technology is in its infancy and could conceivably increase over time. In contrast, the Roadster has a computed efficiency of 1.14 km/MJ.

MODEL S

Tesla is currently developing the Model S, an all-electric family sedan. Tesla unveiled the car March 26, 2009 with an anticipated base price of US$57,400 (or US$49,900http://en.wikipedia.org/wiki/Tesla_Motors - cite_note-TeslaPrices-12 after a US federal tax credit). The Model S will have three battery pack options for a range of up to 300 miles (480 km) per charge. As of January 2011, Tesla has taken about 3,500 reservations for the Model S and expects to begin delivering cars to customers in 2012. Tesla plans to build the Model S in 2012 in a Fremont, California, plant formerly operated by NUMMI, a defunct joint venture of Toyota and General Motors.

RECHARGING

AeroVironment is now shipping and installing its UL-listed single-family and multi-unit residential Smart Charging Dock (model EVSE-RS+). Designed with an open architecture to ensure interoperability with the thousands of individual utilities nationwide, the grid-connected system charges electric and plug-in electric hybrid cars by turning drivers' homes into convenient, smart refueling hubs.

AeroVironment designed its Smart Charging Dock and supporting software to integrate easily with a utility's IT network over various communication methods, including GPRS (cellular), Ethernet, WiFi and ZigBee. The charging system enables communication with the utility to allow for easy monitoring of energy use, troubleshooting and data analysis to help optimize the grid. Utilities can then assess charging patterns and proactively manage the needs of electric vehicles on the utility's generation and distribution systems. Utilities will ultimately be able to aggregate collected data for a global view of EV energy usage and user habits.

The company recently announced that it would be providing and installing the charging systems and data network for the '“nations first privately-funded EV charging ecosystem” in Houston for New Jersey-based NRG Energy, an example of how AeroVironment collaboratively supports an energy company and its EV-driving customer base. AeroVironment is working closely with NRG Energy to ensure that its charging hardware and software work seamlessly with the companyís management system, allowing for grid optimization, CO2 footprint data and energy consumption reports.

AeroVironment's Smart Charging Dock can charge an electric or plug-in hybrid car automatically, or be pre-programmed by a utility or driver for optimal power draw so that the car charges when energy costs are lowest. EVSE-RS+ is designed to be compatible with all SAE J1772-compliant electric car models from major automakers.

RECHARGING #2

Leviton announced that its Evr-Green™ 160 Level 2 Home Charging Station has received Underwriters Laboratories Inc.® (UL) Listing for Electric Vehicle Supply Equipment (EVSE) in the United States and Canada.

The charger is part of Leviton's Evr-Green line of residential, commercial and public charging equipment. Evr-Green chargers comply with all industry standards, including the current National Electrical Code®. Assembled in the United States, they also are backed by a three-year warranty, and are compatible with all major automakers vehicles (SAE J1772™compliant).

The Evr-Green 160 Level 2 Home Charging Station provides up to 16 A at 240 VAC (3.8kW output). It will be available in a patent-pending plug-in design and a hardwired version. The Evr-Green 160 is compatible with Leviton's Evr-green Home Charging Station Pre-Wire System. The installation kit was UL Listed in 2010.

The Evr-Green 160 has enhanced safety features, outdoor ratings, environmentally friendly components and a modular design for upgradeability. The Evr-Green products are planned to be available through multiple channels - electrical distributors, retail, online and through automakers in 2011.

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