Hybrid electric vehicles are becoming more practical.
Like American car companies and the consumers they seek, the power electronics group of Oak Ridge National Laboratory has taken a critical look at the Toyota Prius. After the Department of Energy purchased one of the Japanese gasoline-electric hybrid cars for the National Transportation Research Center (NTRC), technicians carefully disassembled the hybrid so ORNL and University of Tennessee engineers could determine how the Prius achieves roughly twice the mileage of the average American car.
"We have taken the Prius apart and run its electric motor and power electronics through a complete set of tests so we know what they will do under various conditions," says Mitch Olszewski, leader of ORNL's Power Electronics Integration Group. "We are doing the same with the Toyota Camry Hybrid and the Honda Accord Hybrid."
Laura Marlino, program technical manager, says, "The power electronics in the Prius is a ‘brute-force system' in which the most significant innovation lies in the electric motor." The Prius—the benchmark for ORNL's research on vehicle power management—is designed to maximize fuel efficiency and reduce emissions of regulated pollutants and unregulated greenhouse gas emissions. The computerized hybrid sedan can operate on only the downsized gasoline engine or the batterypowered electric motor or both. The car's nickel metal hydride batteries are recharged directly by the engine acting as a generator or by regenerative braking, in which kinetic energy is recovered when the driver depresses the brakes or coasts downhill.
One goal of DOE's FreedomCAR and Vehicle Technologies program is to aid the auto industry in designing a plug-in hybrid electric vehicle with a lithium ion battery that can be recharged by plugging the car into home electrical outlets. Ideally, the car could drive 40 miles on electricity only before relying on the gasoline engine. ORNL leads FreedomCAR research in power electronics and electric motors in support of the auto industry's development of hybrid vehicles and hydrogen fuel-cell cars as well as plug-in hybrids.
To meet DOE's cost targets for the plug-in hybrids, the ORNL group seeks to employ new technologies to redesign or minimize the vehicle's electric motor; the inverter, which converts direct current (DC) from the battery to alternating current to run the electric motor; the boost converter, which raises the battery's voltage to the level needed by the motor and the DC to DC converter, which manages the vehicle's power system in order to operate electric accessories. These accessories include CD players, seat warmers and electric drives—air conditioning compressors, power steering units and power brakes. The ORNL group calls the combination of the electric motor, inverter and converters the "electric traction drive system."
DOE's cost targets for tomorrow's hybrid vehicles are based on a payback of three years, roughly one-third that of the Prius at today's gasoline prices. Compared with the Prius, the electric traction drive system of DOE's envisioned hybrid cars must be 60% smaller, 55% to 65% lighter and 75% less costly to meet the payback cost targets.
ORNL has several strategies for reducing the volume, weight and cost of the drive system of the hybrid plug-in by 2020. Eliminating Toyota's $185 dedicated liquid coolant loop for cooling the power electronics is one approach. In its place, ORNL researchers propose using an innovative design in which the inverter is flooded with the electrically insulating refrigerant used for air conditioning, providing direct contact cooling of the electronic chips.
To meet the 2020 goal of reducing the size of the inverter by 60%, ORNL researchers are investigating the proposed strategy of replacing the inverter's silicon switches with silicon carbide chips, which are smaller, can operate closer together at higher temperatures and have the potential of using air cooling instead of liquid coolant. The tradeoff thus far has been daunting: silicon carbide devices currently cost 10 times more than silicon devices.
A University of Tennessee team is working with university partners to find packaging materials for silicon carbide devices that can withstand 200°C. Their counterparts at ORNL are gathering performance data from tests of manufacturers' silicon carbide diodes in inverters at high temperatures. Meanwhile, another team of ORNL researchers is redesigning the electric motor with the goal of making it more powerful, smaller and ultimately cheaper by eliminating the need for the current boost converter. A parallel approach is new electronic topologies that would eliminate bulky components in the boost converter. This strategy seeks to combine the functions of an inverter and boost converter into one multilevel converter and eliminate the magnetics, reducing both weight and cost.
The plug-in hybrid envisioned by DOE and auto manufacturers will have a lithium ion battery because the specific energy density—three times that of the Prius battery—allows designers to shrink lithium battery weight, increasing the fuel efficiency of the car when running on gasoline. Argonne National Laboratory is leading research on the lithium ion battery for electric vehicles.
To recharge a lithium battery 10 to 100 times faster, the battery must be coupled with an ultracapacitor, an electrochemical capacitor with two carbon electrode plates sandwiching an electrolyte fluid that can rapidly release considerable stored electrical energy each cycle. In April 2007 ORNL Associate Director Michelle Buchanan led a national workshop on electrical storage, including batteries and ultracapacitors, for DOE's Office of Basic Energy Sciences. A novel porous carbon material developed at ORNL is being tested as a candidate electrode for ultracapacitors. The reason: The nanostructured carbon particles have high accessible surface areas and high conductivity, are compatible with the electrolyte and show low degradation at higher cell voltages.
At present the cost of producing carbon-based materials with tailored energy-storage performance for ultracapacitors is $100 a kilogram, five times too high. A number of researchers get up each morning with the goal of bringing down the cost of vehicle components.—Carolyn Krause
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