A recent announcement has set off a small flurry of articles about an electric city bus design presented by Sinautec Automobile Technologies. They demonstrated a electric city bus design incorporating ultracapacitors for energy storage. This fits into efforts by multiple transit agencies to reduce the carbon footprint of bus fleets using a range of options over the years, from biofuels to hydrogen to batteries to overhead electric lines and hybrid-electric diesel. Those options have seen varying rates of adoption in different cities.
The bus demonstrated by Sinautec involved a system design in which some bus stops serve a double duty as charging stations. Several cities have electric buses in their transit fleets today. Those buses are quieter than the noisy roar of traditional diesel buses, but existing electric buses utilize overhead wires to deliver electricity. Overhead wires have been used for over a hundred years to power electric trains and buses, and while it’s a simple way to power electric buses it does require building and maintaining the system and the wires detract from neighborhood esthetics.
In the Sinautec design instead of overhead wires along every foot of the bus route, overhead wires are only installed at those bus stops which double as charging stations. Their design relies on the fast charging characteristics of ultracapacitors. A quick recharge allows passengers to load and unload, and for the driver to take a quick break.
They have spent the last three years demonstrating and testing a fleet of 41-seat buses in an area of Shanghai. Their 41 seat bus has a 30-35 miles/hr top speed, a 5.9 kilowatt-hour ultracapacitor pack, uses 1.5 kilowatt-hours per mile, giving approximately 4 miles range depending on air conditioner usage. Obviously at this speed this bus will only be usable in areas with limited speed such corporate or college campuses, city centers, or ground transit around airports.
The ultracapacitors in the demonstration fleet have an energy density of six watt-hours per kilogram. For comparison lithium-ion battery packs can store 200 watt-hours per kilogram. The ultracapacitors bring other advantages such as longer lifetime, better reliability, greater charging efficiency, faster charge times. To charge the 5.9 kilowatt-hour pack in one hour at 240 volts would require a 25 amp circuit.
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