Buses in the Shanghai pilot are made by Sunwin Bus, a Chinese joint venture company with Volvo of Sweden, and uses ultracapacitors manufactured by Shanghai Aowei. Foton America Bus, based in Germantown, TN, will be manufacturing the ultracapacitor buses that will be sold in the United States. The ultracapacitors are made of activated carbon and have an energy density of six watt-hours per kilogram. (For comparison, a high-performance lithium-ion battery can achieve 200 watt-hours per kilogram.) Clifford Clare, chief executive of Foton America, says another 60 buses will be delivered early next year with ultracapacitors that supply 10 watt-hours per kilogram.
“The ones in Shanghai right now have been on the road for three years without incident, without failure whatsoever, which in the bus industry is phenomenal,” says Clare, who adds that his company is in talks with New York City, Chicago, and some towns in Florida about trialing the buses. “It will end up being a third generation of the product, which will give 20 miles [of range per charge] or better.”
Sinautec is also in discussions with MIT’s Schindall about developing ultracapacitors of higher energy density using vertically aligned carbon nanotube structures that give the devices more surface area for holding a charge.
“So far we’re able to get twice the energy density of an existing ultracapacitor, but that’s not enough,” says Schindall. “We’re trying to get about five times.” Schindall says that this would create an ultracapacitor with one-quarter of the energy density of a lithium-ion battery.
“Right now the [Foton] buses can only go every other stop, a range of about 5 or 10 city blocks, and that’s okay for some routes, but here in the Boston area that would be too far [between charging spots],” Schindall adds. “If they could double that, or even quadruple that, it would increase by an order of magnitude the numbers of routes for which it could be a technical solution.”
There are some other important limitations. The 41-passenger buses, based on current technology, lose 35 percent of their range when air conditioning is turned on (from about 5 miles to about 3 miles), and have weak acceleration. But even under these conditions, they could still prove practical for municipal, campus, airport, and tourist buses.
“We want to replace a large portion of the diesel fleet in the United States,” says Ye. “We do need to have charging stations throughout various points of the network, but as energy density goes up, the number of stations will go down.”