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Plug-in hybrid electric vehicles may have an unexpected value.
One car of tomorrow may not only get its energy from the grid but also may give "imaginary power" back to the grid. The plug-in hybrid electric vehicle envisioned by the Department of Energy would be plugged at night into a home wall outlet, connecting the car to a local electrical distribution system that would recharge the battery. The next day the car would travel using a combination of stored electric energy and fuel. According to a vision of Oak Ridge National Laboratory researchers, the car's charger would supply the grid with "reactive power," or non-active power, to help regulate local utility voltage. To convert the alternating current from the local electrical distribution system to the direct current needed by the car's battery, a rectifier, or charger, is required. Conversely, an inverter is needed to convert direct current to alternating current. The car would use alternating current to power the drive motor. The rectifier, or charger, could be located either in the car or at facilities designed specifically to recharge batteries of plug-in hybrid electric vehicles parked for extended periods at, say, apartment complexes, hotels and parking garages. Inverters have several uses, including the ability to inject reactive power to the grid or absorb this imaginary power from the grid. This helps regulate the voltage on distribution and transmission systems. Inverters can prevent "micro-voltage collapses" that frequently occur in the western United States. Such sudden voltage dips can cause dimming of lights, computer crashes, damage to equipment and destruction of semiconductor wafers during manufacture. DOE researchers are working with the U.S. automobile industry to optimize plug-in hybrid engine, motor and battery performance for efficient vehicle operation. ORNL and University of Tennessee power electronics experts at the National Transportation Research Center are seeking to improve inverter design to make the device smaller, lighter and less expensive. ORNL is a member of the Plug-in Hybrid Development Consortium. DOE also supports research on one way to reduce peak demands on the electric grid: deploy distributed energy resources—microturbines, fuel cells and photovoltaic panels—to provide electricity to both local buildings and the electric grid. The plug-in hybrid could be considered another distributed energy resource, but one that also stores energy. One group of ORNL researchers is studying the effect of plug-in hybrid electric vehicles on the electric grid. Another group is examining ways to make inverters and chargers smarter so they can help distributed energy resources provide voltage regulation as well as electricity to the grid. ORNL researchers led by Stan Hadley have found that the U.S. electric grid will operate more efficiently as more Americans charge the batteries in their plug-in hybrid vehicles after 10 p.m., when the electric load on the system has dropped to almost zero and the wholesale price for energy is least expensive. The researchers have analyzed the potential impacts of plug-in hybrid electric vehicles on electricity demand, supply, generation structure, prices and associated emission levels in 2020 and 2030 in 13 regions as specified by the North American Electric Reliability Council and DOE's Energy Information Administration. Their study assumed that by 2020 a mixture of sedans and SUV plug-in hybrids would make up one-quarter of the cars sold. They performed calculations using the Oak Ridge Competitive Electricity Dispatch model, which was developed at ORNL over the past 12 years to evaluate a wide variety of critical electricity sector issues.
The ORNL researchers ran seven scenarios for each region for 2020 and 2030. In each scenario they assumed these vehicles plugged in starting at either 5 p.m. (early evening) or at 10 p.m. (nighttime) and remained until fully charged. "We concluded that most regions must build additional electrical generating capacity or rely on demand response to meet the added demands from plug-in hybrid electric vehicles in the early evening charging scenarios," Hadley says. "This need will be critical by 2030 when plug-in hybrids will likely have a larger share of the installed vehicle base and thus exert a greater demand on the electrical system." Accommodating the peaks and valleys of electricity use is a major challenge for generators and transmission operators. Ideally, customers would reduce their consumption of electricity at peak load times in response to market prices or a utility's request. During hot summers, the demand for air conditioning between 2 and 6 p.m. can boost the peak load to the point that a utility must purchase power from another utility at a higher price. In sharp contrast, the grid on the same night may be so underutilized that energy is sometimes given away. Nighttime battery charging would greatly benefit generation and distribution companies, who normally see their facilities used efficiently for only a few hours each day. Local distribution grids would see a significant change in their electricity usage patterns. For example, early evening charging would probably use a higher proportion of natural gas to coal than nighttime charging, with consequent effects on carbon dioxide and other emissions. Limits have been placed nationally on nitrogen oxide emissions by power plants, but NOx caps do not exist for gasoline cars. Instead, each vehicle has a regulated emission rate per mile; more vehicles or more miles mean more emissions. The benefits of widespread usage of plug-in hybrids thus include reduced dependence on foreign oil and cleaner air. Another potential advantage of wide usage of plug-in hybrids is that charging stations can help regulate local voltage. Researchers at ORNL's Distributed Energy Communications and Control laboratory plan to test this concept. Under Tom King's direction, John Kueck, Tom Rizy and Fran Li have been experimenting with a control system for an inverter and testing how well a smart inverter or rectifier can regulate voltage to improve power quality. "Our goal is to show that distributed energy resources such as fuel cells, microturbines and arrays of solar cells can support the electric distribution system," Kueck says. "Our approach is a little different from the conventional electrical industry, which is calling for a massive retrofit of the communication and control system. "We think distributed energy resources can be controlled by local, independent, autonomous, adaptable controllers that function independently without causing problems to the others. Local voltage regulation from a fuel cell or microturbine is much cheaper and simpler than a massive, hierarchical control system. The local control would still, however, be under utility supervision." "We propose modifying the charger to control the local voltage as a service to the electrical system," Rizy says. "With this approach, utilities would not have to worry about voltages going too high or too low when numerous fuel cells, microturbines and photovoltaic arrays are connected to their distribution systems. The individual energy resource would control the voltage itself locally."
Smart chargers are needed to avoid negative impacts on the distribution system. For example, if several plug-in hybrid pickup trucks with 250-kilowatt batteries were recharged in 10 minutes on a feeder without a smart charger, the distribution system's reliability could be threatened. In one project, ORNL's Burak Ozpenici is examining possible rectifier designs that would perform rapid charging while providing reactive power compensation. Yan Xu, a postdoctoral researcher at UT, is devising intelligent controls to enable the inverter or distributed energy resource to inject dynamic reactive power into the ORNL distribution system to regulate the local voltage. She has developed a definition of non-active power that facilitates real-time control. In 2008 Kueck and Rizy will be testing ORNL's intelligent inverters with multiple distributed energy resources connected to a Southern California Edison distribution circuit. Both anticipate that grid-related tests down the road will involve plug-in hybrid electrical cars. For years electric cars have been an impractical fantasy. This persistent image is giving way to the reality that, with support from the electric grid, they will be an integral part of America's automobile future that can also give back to the grid.—Carolyn Krause Contact: John Kueck
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