NREL Study Compares Costs Of Operating CNG Buses Versus Electric Buses
If you operate a public transportation fleet, cost is a critical factor. Over millions of miles, a difference of a penny or two per mile can add up to serious money. All electric buses tend to cost more to buy but are expected to cost less to operate over their anticipated 12 year useful life. Is that assumption accurate?
A new study by the National Renewable Energy Lab tabulated data collected by Proterra electric buses operated by Foothill Transit in southern California and other buses in the Foothill fleet that were powered by compressed natural gas. The results show the Proterra buses were up to 8 time more efficient than their CNG powered cousins.
When operated on the same route, average fuel economy for the CNG buses came to 2.1 miles per diesel gallon equivalent. By contrast, the Proterra electric buses had an observed MPDGe of 17.35 — more than 8 time higher. Cost of fuel per mile worked out to be 41 cents for the electric buses and 50 cents for the CNG powered vehicles.
There are other costs associated with operating a transportation fleet that include such items as how often repair crews have to be dispatched to deal with breakdowns away from the repair facilities located at the terminal. Here, the Proterra fared less well against the competition, averaging 16,405 miles between road calls versus 56,710 for the CNG buses. Still, overall maintenance costs came out slightly in favor of the electric buses — 21 cents per mile versus 22 cents per mile.
Cost per mile for unscheduled maintenance again showed an advantage for the CNG buses, $0.14 versus $0.10 — but Foothill Transportation attributes half that difference to replacing damaged tires more frequently on the Proterra buses. The operate exclusively on city streets with potholes and broken curbs as opposed to the CNG buses in its fleet, which spend most of their time on smooth, well maintained highways.
Foothill Transportation found virtually no degradation in battery power during the 6 month observation period, suggesting that the batteries should not need replacing during the 12 year projected useful lives of the electric buses. It also found the charging equipment used by Foothill Transportation to keep the buses charged while away from the terminal was almost entirely trouble free during the test period.
On balance, the Proterra electric buses are operating well in commercial service and equal or exceed the expectations of the transportation authority, which plans to transition to 100% electric buses by 2030. Another entry in the good news column is that the cost of the Proterra buses has declined by more than one third since Foothill Transportation bought its first Proterra buses in 2009. They cost $1,200,000 a piece back then versus $789,000 each today.