Study Says Proterra Electric Buses Are 8 Times More Efficient Than CNG
National Renewable Energy Laboratory (NREL) has released a fairly profound report about the use of 12 Proterra electric buses by Foothill Transit in California between August 2015 and December 2016.
There is a lot of interesting findings in the data, including that EVs in some cases are more than eight times more energy efficient than CNG bus alternatives.
NREL analysed the 35-ft buses used on Line 291, which employ two overhead fast charging stations. For comparison at the same time, eight 42-foot CNG buses were also used.
“The BEBs had an overall average efficiency of 2.17 kWh per mile, which equates to 17.35 miles per diesel gallon equivalent (mpdge). The CNG buses had an average fuel economy of 3.89 miles per gasoline gallon equivalent (mpgge), which equates to 4.34 mpdge. The operating duty cycle of a bus has a significant effect on fuel economy. Because Foothill Transit operates its BEB and CNG bus fleets differently, the efficiency results presented here are not considered an apples-to-apples comparison. In collaboration with a DOE-funded activity at NREL, the researchers had access to data collected on a selection of Foothill Transit CNG buses using data loggers. The data loggers recorded two days of CNG operation on Line 291 to provide a direct comparison to the BEBs in this evaluation. On Line 291, the average CNG fuel economy was 2.1 mpdge and the average driving speed (not including stops and idle time) was 18.1 mph. This is similar to the average driving speed (logged) for the BEBs (17.8 mph), providing a more accurate comparison for fuel economy. When comparing the logged data, the BEB fuel economy is more than 8 times higher than that of a CNG bus operating exclusively on Line 291.“
The average energy costs for the BEB fleet worked out to to $0.37 per mile. That is an adjusted $0.41 per mile after assuming a 10% loss during the charging process. CNG buses would be at about $0.50 per mile.
Here is some details about reliability and maintenance:
“NREL continued to track the bus reliability—measured as miles between roadcalls (MBRC)—during the second data period. The overall bus MBRC for the BEB fleet decreased from more than 9,000 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. The propulsion system-related MBRC was 16,405 for the BEBs compared to 56,710 for the CNG buses. There have been only three roadcalls for the energy storage system (ESS) during the evaluation (none in the latest data period). Thus, the ESS-related MBRC for the BEBs continues to climb, now surpassing 300,000.
NREL also analyzed work order data to provide a comparison of maintenance costs between the BEBs and the baseline CNG fleet. 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.
These combined totals include 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).
During this period, the BEBs experienced higher unscheduled maintenance costs ($0.14/mi) than the CNG buses ($0.10/mi). The cost for tire damage, which is not related to the technology, accounted for $0.07/mi to the overall cost of the BEBs. Foothill Transit reports that this is likely due to the differing use between the BEBs and CNG buses. The local routes tend to have more road damage, such as potholes and broken curbs. Since the BEBs are operated mainly on local routes, they incur more tire-related damage compared with the CNG buses that are often driven on freeways. The agency expects that the CNG buses would experience similar tire damage as the BEBs if they were only operated on the local routes. Average scheduled and unscheduled maintenance costs for the CNG buses were strongly impacted by a major preventive maintenance inspection (PMI) reached by many of the buses in the middle of the data period.
When excluding tire damage, the total maintenance cost is $0.14/mi for BEBs and $0.20/mi for CNG buses. The BEBs had a propulsion-system-only maintenance cost that was 70% lower than that of the CNG buses: $0.02/mi for the BEBs compared to $0.07/mi for the CNG buses.
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%); 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%).”
Achievements and findings:
- The current fleet of twelve 35-ft BEBs continues to operate well, accumulating more than 902,000 miles (through December 2016). Foothill Transit’s combined fleet of 17 Proterra BEBs (including three first-generation BEBs and two new 40-ft BEBs) has operated more than 1,134,000 miles.
- Bus MBRC for the entire evaluation period is more than 6,000, surpassing the target of 4,000 MBRC. Propulsion-related MBRC is more than 16,000.
- The on-route fast chargers operated reliably with minimal issues, none of which resulted in downtime for the buses. 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 report that the project continues to go well. The partners highlight the following key lessons learned 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.
- Review potential routes and consider the ones that best fit how BEBs operate based on driving range, duty cycle, and charging opportunities. An agency could benefit from conducting a route analysis and simulating how the BEBs would meet the range requirements.
- 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; Foothill Transit adjusts its summer schedule to account for more charging time.
- 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 consider the operational differences for BEBs and develop plans in achieving this goal.
Here is quick look at the cost of the buses over the years, which seems to be decreasing with every new order placed…which is another good sign.:
source: Green Car Congress