When General Motors’ electric buses hit New York City’s streets for a test run a couple of years ago, the company found that the new hybrid vehicles—propelled by a diesel generator and electric motors—burned less than half the fuel of a conventional diesel and created 90 percent less pollution. One hitch: the roughly 5,000 dollars’ worth of batteries that mediated the flow of electricity between the generator and motor burned out after only a year. Now, GM plans to keep its hybrid buses rolling by replacing the batteries with a high-tech cousin of the capacitors that regulate power in electronic devices. The same technology may soon help make hybrid cars more efficient and affordable.
Unlike batteries, which store energy using chemical reactions, capacitors store power as static electricity—and so can charge and fire millions of times without wear. The ultracapacitors in GM’s new buses outpunch garden-variety capacitors by incorporating carbon electrodes riddled with pores and fissures. Each gram of carbon provides thousands of square meters of surface area; more surface area means more charge can be stored. Ultracapacitors also charge in a flash—which could boost fuel efficiency another 10 to 15 percent, says Andrew Burke, director of the University of California, Davis’s Electric Vehicle Power Systems Laboratory. That’s because a hybrid recovers short bursts of energy each time a driver brakes. Whereas batteries can capture only 60 percent of a vehicle’s braking energy, ultracapacitors grab over 95 percent. According to GM, swapping the new hybrids for the 13,000 transit buses that serve the top nine U.S. cities would save nearly 950,000 barrels of diesel fuel each year.