What’s Inside Chicago’s Electrified Buses?

Chicago's twenty electric buses will save the city $13M in operating expenses and reduce pollution at the same time.

Chicago, the city that electrified the blues, is now plugging in its public transportation system. The Chicago Transit Authority is adding twenty battery-powered electric buses to its fleet. That’s a small fraction of the 1800 buses that the CTA already operates, but it’s the start of a gradual conversion from gas guzzlers to green machines.

CTA’s pilot study with two battery-powered buses purchased in 2014 demonstrated that electric buses are feasible in Chicago’s sweltering summers and bone-chilling winters, so the city decided that the time is right to make the transition to a fleet that’s more eco-friendly and less costly to operate.

Image courtesy of Proterra

Image courtesy of Proterra

Vehicle Specs

CTA is purchasing vehicles from Proterra, a world leader in electrified mass transit. Each bus is made of a composite material – balsa wood, carbon fiber, and resin – to improve its power-to-weight ratio and reduce its impact on the road surface. The company claims that the bus, whose curb weight is less than 15 tons (13.6 kg), can accelerate from zero to 20 mph in under seven seconds. (Having driven in downtown Chicago many times, I’d say that’s more than adequate for the traffic conditions.) Incidentally, the curb weight is comparable to that of a diesel bus; Proterra’s lighter exterior is negated by the extra weight of the batteries.

The drivetrain, which features a pair of 220 kW electric motors along with a two-speed automatic transmission, cranks out a respectable 510 hp, allowing the bus to climb a 26% grade at 40 mph. By comparison, a typical diesel bus of the same size delivers under 300 hp. Who says EVs are underpowered?

The vehicle’s 300 kWh Li-ion battery pack, complete with charge monitoring and thermal management software, provides a range of about 200 miles. Regenerative braking, combined with a few strategically placed 500 kW quick-charging stations along the route, keeps the electric fuel tank topped off while the bus is on duty. When it returns to the depot at the end of the day, it can fully recharge in under three hours using the 125 kW chargers. The depot chargers, by the way, are bidirectional, letting them participate in vehicle-to-grid (V2G) energy management systems.

Lower Fuel and Maintenance Costs, Lower TCO

The electric buses cost $900k apiece, about $300k more than the diesel buses that the city most recently purchased, but like most EVs, their total cost of ownership (TCO) is lower in the long run. The battery-powered buses purchased in 2014 proved to be less expensive to fuel and maintain than the diesel units they replaced, providing an annual savings of $54k per bus. Over its expected twelve-year lifespan, each bus will save the city nearly $650k, resulting in a pretty nice return on investment.

Less Pollution (Two Kinds)

Diesels tend to be loud and smelly, so switching to EVs reduces both noise levels and air pollution. A typical diesel bus generates 72 dB of noise – a little louder than a vacuum cleaner. Proterra’s buses are quieter than a normal conversation, coming in at just 57 dB. On the air quality front, each electric bus will prevent 1452 tons of CO2 and other pollutants from entering the atmosphere over its twelve-year lifespan. For twenty buses, that’s a reduction of over 29,000 tons of CO2. If the city replaced all 1800 of its diesel buses with EVs, they would reduce carbon emissions by 2.6 million tons.

Chicago’s moniker, “The Windy City,” has nothing to do with meteorology; it refers to the excessively warm breath emanating from the oral cavities of certain elected officials, which, some may argue, approaches the same toxicity as the tailpipe emissions from a fleet of diesels. Green technologies like electric buses can’t solve the first issue, but they are helping to trade some of the city’s hot air for a cooler breeze, and that provides plenty of benefits further on down the road.


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