Ultracapacitors are like superbatteries: super-efficient, quick to charge and discharge, long-lasting — and holding very, very little energy. Now a Chinese company and its U.S. partner have found a clever way to deploy ultracapacitor buses:
There’s just one catch: the best ultracapacitors can only store about 5 percent of the energy that lithium-ion batteries hold, limiting them to a couple of miles per charge. This makes them ineffective as an energy storage medium for passenger vehicles. But what ultracapacitors lack in range they make up in their ability to rapidly charge and discharge. So in vehicles that have to stop frequently and predictably as part of normal operation, energy storage based exclusively on ultracapacitors begins to make sense.
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The trick is to turn some bus stops along the route into charge stations, says Dan Ye, executive director of Sinautec. Unlike a conventional trolley bus that has to continually touch an overhead power line, Sinautec’s ultracapacitor buses take big sips of electricity every two or three miles at designated charging stations, which double as bus stops. When at these stations, a collector on the top of the bus rises a few feet and touches an overhead charging line. Within a couple of minutes, the ultracapacitor banks stored under the bus seats are fully charged.
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The buses can also capture energy from braking, and the company says that recharging stations can be equipped with solar panels (although this is mainly to further the perception that the vehicles have a lower carbon footprint). Ye says the buses use 40 percent less electricity compared to an electric trolley bus, mainly because they’re lighter and have the regenerative braking benefits. They’re also competitive with conventional buses based on fuel savings over the vehicle’s 12-year life, based on current oil and electricity prices. Sinautec estimates that one of its buses has one-tenth the energy cost of a diesel bus and can achieve lifetime fuel savings of $200,000.“The ultracapacitor bus is also cheaper than lithium-ion battery buses,” says Ye. “We used the Olympics (lithium-ion) bus as a model and found ours about 40 percent less expensive with a far superior reliability rating.”
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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.)
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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, and have weak acceleration. But even under these conditions, they could still prove practical for municipal, campus, airport, and tourist buses.