Public Transit, Depot Charging

Electric buses used for public transit are often charged in two ways: on-route charging and depot charging. Depot charging can occur at a lower power rate using Level 1 or Level 2 charging, which makes it a slower process that is often done overnight.

Mountain line bus stop

Applicability, Benefits, and Tradeoffs

Providing charging for public transportation at transit depots allows the buses to be charged predictably based on known route schedules. The table below lists common benefits and tradeoffs of EVs and indicates how well this use case achieves these benefits. Green indicates the use case typically achieves the benefit. Yellow indicates the use case sometimes achieves the benefit. Red indicates the use case does not achieve the benefit.

Reduced Traffic Congestion: Public transit buses reduce traffic congestion by providing transportation for multiple community members with one vehicle, rather than if community members each drove personal vehicles.

Increased Equity and Accessibility: Investing in public transit can help remove economic and accessibility barriers to transportation for low-income households, seniors, those with disabilities, students, and other disadvantaged groups that may not have access to reliable personal vehicles. From an equity standpoint, electric busses also ensure that riders and those in proximity are not exposed to the harmful tailpipe emissions from diesel buses. These pollutants have health and environmental impacts that disproportionately affect sensitive populations such as children, pregnant women, seniors, and those with respiratory illnesses or cardiac conditions.

Reduced Emissions: Public transit buses offer significant opportunities for electrification. More than 340 electric buses are currently operating in the United States, with another 1,200 electric transit buses on order. Electrifying transportation creates the opportunity to energize transport with renewable energy, reducing overall emissions from the transportation sector. Public transit bussed also can help reduce vehicle miles travelled by personal light duty vehicles, which can result in significant reductions in criteria pollutants. One study in the Northeast and Mid-Atlantic regions demonstrated that electrifying heavy duty diesel vehicles, such as transit buses, resulted in 33 times greater reduction in nitrogen oxide and 7.5 times greater reduction in particulate matter per mile than electrifying gasoline powered light duty vehicles.

User Economic Benefits: The economic benefits to users often depend on the fare charged to public transit passengers and how this fare compares to the income of the community members. In Missoula, the local transportation system does not charge a fare to passengers, ensuring that all community members have access to consistent, reliable transport regardless of their income. Using public transportation, even when there is a fare, is often less expensive than purchasing, operating, and maintaining a personal vehicle.

Owner Economic Benefits: In general, the slower overnight depot charging from Level 1 or Level 2 charging equipment can allow electric buses to be effective substitutes for conventional buses without requiring faster, and more expensive, on-route charging. Battery charging can also be done at a lower power rate (below 100kW), which can help reduce the impact of demand charges. One tradeoff with overnight charging is that the bus battery must be sized so that the bus can operate all day without having to stop to charge. Larger battery sizes increase the cost of the bus and increase the bus’s curb weight, potentially decreasing overall operating efficiency. A key consideration for depot charging is that all electric buses will likely charge at the same time overnight, creating a spike in demand. If the overall load of the facility is higher than this demand, then it is unlikely that the depot charging would incur demand charges from the utility. However, if the load during depot charging is much higher than the facility’s normal load, then demand charges from the utility could be a large factor in the cost of charging. These two scenarios  were simulated by the National Renewable Energy Laboratory (NREL) using the base loads of the City of Missoula and the University of Montana and are illustrated in the figures below.

BEB ReOpt analysis for City of Missoula
Simulated load profile comparing bus depot baseload to bus charging load for battery electric buses in the City of Missoula. Analysis provided by NREL.
BEB reOpt analysis
Simulated load profile comparing university baseload to bus charging load for battery electric buses in the City of Missoula. Analysis provided by NREL.

Renewable Energy Co-Location: Depot charging with solar may prove challenging since depot charging is most commonly performed overnight, after most routes are done for the day and when the sun is not shining. Pairing the solar PV array with energy storage can help overcome this issue. In this scenario, the storage device would have all day to charge and would not need to discharge at the fast rate of Level 3, on-route charging. Storage can then be used overnight to mediate the large spikes in energy demand caused by charging multiple transit vehicles at once. The figure below was produced as part of an analysis conducted for this project to demonstrate how the load at a depot could be impacted by solar and energy storage technology. The storage component makes significant contributions to serving overnight depot charging loads which would help save financial resources by mitigating or avoiding demand charges. The cost savings will be greatly dependent upon the rate structure offered by the utility.