Technologies: Battery-Powered and Hybrid-Electric Motors
Battery-Powered Motors
The number of battery-powered electric buses being manufactured is on the rise, though inherent limitations on battery technology make the electric bus an unlikely successor to the diesel engine for the majority of heavy-duty, urban applications. Where conditions are appropriate, however, electric, battery powered buses have proven themselves to be economical, reliable, and very popular with the public.
Electric buses emit nothing directly, but only indirectly, at the utilities from which they draw their power. The Santa Barbara Electric Transportation Institute estimates that, given the mix of fuel sources used to generate electric power in the Southern California region, Santa Barbara's electric buses cause approximately one third less CO2 to be emitted than would an equivalent diesel fleet.
Since power plants generate large amounts of electricity at a time, they produce the energy needed to drive a bus much more efficiently than would a single bus engine, and therefore generate proportionately fewer GHG emissions. Most electric fleets also recharge at night, when the more efficient 24-hour plants are on line, thereby avoiding the higher emissions of peak-hour power plants.
For geographical conditions of low relief and a temperate climate, with short distance routes and frequent-stop duty cycles, electric buses are an optimal technology. The nation's two largest operators of electric buses, Chattanooga and Santa Barbara, both made the decision to implement electric transit vehicles as part of larger projects to improve the livability of their central business districts.
In both cities, according to Santa Barbara analysts Paul Griffith, and Gary Gleason, "electric propulsion enabled quiet, exhaust free, odorless operation, and proved to be an immediate success with riders…Drivers reported that prospective riders would forego a ride on a diesel bus in order to wait for the next available electric bus." The first to adopt battery electric technology, Santa Barbara, put its first electric bus in operation in January 1991. It then captured 75 percent of Santa Barbara's 300 percent ridership increase for 1991.
Hybrid-Electric Motors
Hybrid-electric motors use an internal combustion engine in conjunction with an electric battery to increase the efficiency with which power is delivered to the drive train. Since they are not dependent exclusively on battery power, have shown a much greater range of performance capabilities in a variety of demonstration projects across the United States.
The advantage of hybrid technology is twofold: first, because the internal combustion engine only runs when the battery or drive system signals the need for more energy, it does not idle when the vehicle is coasting or at rest. This feature, currently available in personal automobiles, is not yet available in transit vehicles. Hybrids still consume less fuel while idling, however, and the stop-idle feature is expected soon to become standard in buses as well as cars.
The second and greater efficiency of hybrids comes from regenerative braking. In real-world operation, regenerative braking is estimated to recuperate 25 percent of a vehicle's kinetic energy at the moment of deceleration, converting the braking energy into electricity, which is then used to recharge the vehicle's battery. The result is increased fuel efficiency and, by extension, reduced GHG emissions.
"Hybrids," observes New York City Transit's Assistant Chief Maintenance Officer Dana Lowell, "are the only technology that reduces regulated and non-regulated emissions at the same time." Judging New York's ten-vehicle hybrid fleet to be "very successful," the city has now placed orders for 325 diesel hybrid-electric vehicles, and expects the technology to be fully commercially viable.
Hybrid-electric buses have demonstrated equal or superior performance to diesel-powered buses in almost all service situations. They have been operated in heavy-duty cycles in New York, Los Angeles, and Cedar Rapids, Iowa, where, according to a recent TCRP report (#59), Hybrid-Electric Transit Buses: Status Issues, and Benefits, they have shown "numerous…advantages [over diesel powered buses] such as smoother and quicker acceleration, more efficient braking, improved fuel economy, and reduced emissions." "The number of [hybrid-electric] vehicles," the report concludes, "is expected to quadruple in the U.S. alone during the next couple of years. In another several years, the worldwide hybrid bus fleet may well reach in to the thousands or even tens of thousands."
In the meantime, getting the most out of the batteries that serve both electric and hybrid-electric buses remains the greatest technical challenge, and one of the biggest research areas, for such groups as the Southern Coalition for Advanced Transportation, the Northeast Advanced Vehicle Consortium, and Chattanooga's own Electric Transit Vehicle Institute. "The biggest push in R&D," according to SCAT's Kevin Shannon, "is batteries, moving towards hybrids, complemented by natural gas or propane turbines."
Other agencies are watching New York's commitment to hybrids closely, and are ready to move ahead with the technology once they are confident that hybrids can survive heavy-duty service applications. According to the Santa Barbara Electric Transportation Institute's Zail Coffman, "Hybrid is really the coming thing. Fuel consumption on hybrids is 15-30% more efficient than a conventional diesel vehicle…Hybrids are going to make a big impact over the next decade."
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