Among the findings are that the BEBs had an overall average efficiency of 2.17 kWh per mile—17.35 miles per diesel gallon equivalent (mpdge). The CNG buses had an average fuel economy of 3.89 mpgge, equating to 4.34 mpdge. However, because Foothill Transit operates its BEB and CNG bus fleets differently, the efficiency results presented are not an apples-to-apples comparison. Using data from two days of CNG bus operation on the route serviced by the BEBs line, the NREL team found that the BEB fuel economy was more than 8 times higher than that of a CNG bus operating exclusively on that route.

A team from the National Renewable Energy Laboratory (NREL) has released the second report summarizing the results of the battery-electric bus (BEB) demonstration underway at Foothill Transit in California. This latest report provides data and analysis on 12 Proterra BEBs and eight CNG baseline buses from August 2015 through December 2016.

Foothill Transit paid an average of $0.17/kWh for electricity for the BEBs and $0.96/gge for CNG. An apples-to-apples comparison of fuel cost—i.e., the cost for the CNG buses and the Proterra electric buses on the same route—found the average per mile cost for the electric buses was $0.41 per mile, including $0.04 per mile reflecting the expected energy loss of approximately 10% during charging. The estimated cost for the CNG buses on the same route was $0.50 per mile.



Monthly average energy cost per mile for the BEBs and CNG buses. Eudy and Jeffers (2017). Click to enlarge.

This does not reflect the credits from the Low Carbon Fuel Standard program, which results in a greater benefit for using electricity over CNG. CNG buses generate Low Carbon Fuel Standard credits, although not as many as electric vehicles do.

The overall bus miles between roadcalls (MBRC) for the electric fleet decreased from more than 9,000 during the period of the first report to just over 6,000. This is higher than the target of 4,000 MBRC—but much lower than that of the CNG buses, which achieved more than 29,000 MBRC.

NREL found that the propulsion system-related MBRC was 16,405 for the BEBs compared to 56,710 for the CNG buses. However, the energy storage system (ESS)-related MBRC for the BEBs continues to climb, now surpassing 300,000.

After removing accident- and warranty-related items for both fleets, the average per-mile maintenance cost for the data period was $0.21/mi for the BEBs and $0.22/mi for the CNG buses—including scheduled and unscheduled maintenance. Although the totals are very similar, the BEBs have lower scheduled maintenance costs ($0.07/mi) than the CNG buses ($0.11/mi).

The top three vehicle systems that accounted for the largest fraction of maintenance for the BEBs were (in order from highest to lowest) tires (34%); preventive maintenance inspection (PMI) (33%); and cab, body and accessories (17%). For the CNG buses, the top three vehicle systems for maintenance were propulsion-related (32%); PMI (24%); and cab, body and accessories (21%).

The on-route fast chargers have operated reliably with minimal issues; Foothill Transit’s combined BEB fleet (17 buses) has been charged more than 119,000 times since the fast chargers were installed. Availability of the two charging heads was 98% and 99%. Proterra reports that the high voltage batteries are showing little to no signs of capacity degradation to date, and current estimates show they may last for up to 12 years.

Foothill Transit and Proterra reported several key lessons learning since the beginning of the project:

Short-range, on-route-charged buses are inflexible and cannot be deployed at other service routes that do not connect to an on-route charging location.

Transit agencies bringing in electric buses should review potential routes and consider the ones that best fit how BEBs operate based on driving range, duty cycle, and charging opportunities.

Transit agencies should adjust route schedules to accommodate BEB charging time; this is part of the transition from conventional technology buses to electric buses. An agency may need to add deadhead miles prior to the start of the route depending on the location of the in-route charging station and availability of an in-depot charger.

The higher use of air conditioning lowers the effective range in hotter months; schedules need to be adjusted.

Charger availability is important for successful deployment. Foothill Transit installed two charger heads at its charging station to avoid downtime for charger unavailability.

Foothill Transit reports that there is still a lot of learning as the agency ramps up to a larger BEB fleet. The agency has a goal of transitioning its fleet to 100% electric by 2030. The team needs to develop plans in achieving this goal. As the technology improves to include longer range BEBs, Foothill Transit expects to manage more buses through overnight charging. To accomplish this, the agency is investigating options for charging a large number of buses in a limited space. —NREL report

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