A new analysis of a recent 12-vehicle Proterra electric bus demonstration (put on by Foothill Transit in California) by the National Renewable Energy Laboratory has found that these buses possess an average fuel economy roughly 4 times higher than that of baseline CNG buses.
The exact comparison shows an overall average efficiency of 2.15 kilowatt-hours (kWh) per mile for the electric buses, which equates to 17.48 miles per diesel gallon equivalent, and an average fuel economy of 4.04 miles per gasoline gallon equivalent for the compressed natural gas (CNG) buses, which equates to 4.51 miles per diesel gallon equivalent.
The National Renewable Energy Laboratory (NREL) analysis found that reliability was also quite high — with the number of miles between road calls averaging more than 9000. Only 3 of these road calls during the data period (April 2014 through July 2015) were pertaining to battery system issues. This is apparently “exceptional for an advanced technology bus in the early stage of commercialization.”
Here are some of the other findings of the analysis (via Green Car Congress):
- 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.
And an excerpt from the analysis:
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 (battery electric bus) 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.
A second report is expected to be released by NREL later this year, following the conclusion of another year of in-service performance evaluation