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NREL analysis of Proterra electric bus demo finds average fuel economy nearly 4x that of CNG baseline buses

Proterra battery-electric buses in service in a 12- vehicle demonstration by Foothill Transit in California offered significant fuel savings compared to similar conventional vehicles, according to a recently published analysis of the results of the demonstration by the National Renewable Energy Laboratory (NREL).
The NREL team found, inter alia, that the battery-electric buses (BEBs)had an overall average efficiency of 2.15 kWh per mile on the service route, which equated to 17.48 miles per diesel gallon equivalent (DGE). The baseline CNG buses had an average fuel economy of 4.04 miles per gasoline gallon equivalent (GGE), which equates to 4.51 miles per DGE. These results indicated that the BEBs have an average fuel economy that is nearly 4 times higher than that of the CNG buses.
The NREL team also found that reliability, measured as miles between roadcalls (MBRC), was high during the data period (April 2014 through July 2015). The overall bus MBRC for the BEB fleet was more than 9,000. During the data period, there were only three roadcalls that were attributed to issues with the battery system, the primary power system for the buses.
This resulted in an ESS-related MBRC of more than 133,000. The NREL team called this “exceptional for an advanced technology bus in the early stage of commercialization.” The CNG baseline buses achieved an overall bus MBRC that was more than 45,000, which is expected of a new, fully commercial product.
Other findings included:

  • The average energy efficiency was 2.15 kWh per mile over 399,663 miles of use.
  • The average battery pack SOC is 75.4%, indicating a possibility for a usage window shift.
  • The average runtime per day is 13.2 hours with an average of 13 charges per day. Each charge averages 20 kWh energy delivered.
  • Accessory loads contribute to the overall range capability, as more than 50% of “system on” time is spent at a speed of 0 mph where lighting and HVAC loads are still required.

Advanced technology demonstrations typically experience challenges and issues that need to be resolved. The challenges and lessons learned from the demonstration included bus-related problems as well as programmatic issues. One major challenge is addressing demand charges and time of use charges that affect electricity cost. This will be a major challenge for any fleet looking to deploy electric buses that charge during peak times.
The industry needs to work on a permanent solution for all BEB adopters to keep costs reasonable in the future. Another challenge is training operators and maintenance staff in the differences between BEBs and conventional buses. Foothill reports that there is still a lot of learning as the agency ramps up to a larger BEB fleet. The team needs to understand how service can transition to a higher number of buses.

Background. Foothill Transit began a demonstration of three Proterra battery electric buses (BEBs) in October 2010. That first demonstration went well, and Foothill moved forward with an order of 12 next-generation BEBs.
Foothill’s project goal for these buses was to fully electrify one route in its service area and to investigate the feasibility of the technology for other routes. The buses are charged on route at a charging station built at a transit center mid-way along the route. In March 2014, Foothill Transit began operating the new fleet in its service area.

Foothill Transit is collaborating with the California Air Resources Board (CARB) and the US Department of Energy’s (DOE’s) National Renewable Energy Laboratory (NREL) to evaluate the buses in revenue service. CARB staff has been gathering data on zero-emission buses (ZBuses) to assess the status of the technology; the majority of ZBus data collected and reported to date are for fuel cell electric buses. CARB would like to have similar analysis and reporting for the other primary ZBus technology being adopted in the state of California: BEBs. CARB enlisted NREL to conduct a third-party evaluation of the Foothill Transit fleet.
Foothill does not operate conventional buses that are similar in size, weight, and year to the BEBs; for the primary baseline buses, NREL selected eight new NABI compressed natural gas (CNG) buses of the same model year as the BEBs.

Futures.Foothill Transit will continue operating the 12 BEBs on the same line and is evaluating other routes that might be well suited for electric buses. The agency has orders for Proterra’s new Catalyst 40-foot BEB. The first two will be fast-charge buses, followed by 13 of the Catalyst XR extended range model BEBs.
NREL plans to continue the in-service performance evaluation for another year and expects to publish a second report in late 2016.
NREL is also very interested in performing chassis dynamometer testing of the BEBs and the CNG buses and hopes to precisely quantify both energy and emissions savings in a controlled environment on a variety of drive cycles.
Foothill Transit serves a 327-square-mile area covering the San Gabriel and Pomona Valley region of Los Angeles County.