Plug-in and Ride: The Promise and Potential Challenges of Electric Buses

The use of electric buses and other zero emission vehicles (ZEVs) holds great promise to help reduce vehicle emissions and promote a clearer, less polluting transportation sector.

Transit bus systems offer a great venue for deploying and testing the latest ZEV technologies. An estimated 40 U.S. transit systems now include electric-power buses as part of their fleet. To date, bus systems in California have been the greatest adopters of electric buses. The Santa Barbara Metropolitan Transit District began using electric buses in 2003 and currently has 14 in operation. Stanford University Transit presently has a fleet of 23 electric buses, which it launched in 2014. Foothill Transit in Northern California started using electric buses in 2010 and now has 30 in use. Foothill Transit has pledged to change all its buses over to electric power by 2030. Foothill Transit estimates that already, its annual electric buses eliminate the same amount of emissions as 2,424 gasoline-powered cars. A number of other California transit agencies have smaller fleets of electric buses.

Two UMTC Research Affiliates recently developed a comprehensive review of past and current electric bus deployments nationally. This research was led by Professor Eleni Christofa in Civil and Environmental Engineering and Professor Krystal Pollitt in Environmental Health Sciences. The review included discussions of the three main types of electric-power buses currently in use, and of different facets and impacts of transit agencies’ change to electric buses, including areas of challenge.

The primary type of electric bus in use today is the battery electric (BE) bus, and more than 20 U.S. transit agencies have incorporated BE buses into their operations, including the Worcester Regional Transit Authority (WRTA) and the Pioneer Valley Transit Authority (PVTA). BE buses contain an onboard electric battery, which provides all their power. These batteries are typically re-charged through plug-in stations; BE buses also capture and then use energy from regenerative braking. BE buses have no direct vehicle emissions, but there may be atmospheric pollutants associated with the generation of electricity used for charging their onboard batteries. One potential challenge with BE buses is the short driving range (30 to 130 miles) before needing to be recharged, and the impact of the need for recharging on route scheduling. These buses will typically be recharged at bus stop charging stations during their routes for quick charges (5 to 15 minutes). Some transit agencies also utilize slower charging stations at a central location such as a bus garage, for when BE buses are out of service. Even with the quick charges, it is important that bus schedules be adjusted to reflect the charging time.

BE buses are more expensive to purchase than traditional diesel-engine buses ($750,000 per bus compared to $435,000 per bus, respectively); however, they have a longer expected lifespan than diesel buses. BE buses also save fuel and maintenance costs. Proterra has stated that overall, the lifecycle costs of BE and diesel buses are similar. The PVTA estimates that each of its BE buses will save the agency $448,000 combined in fuel and maintenance costs. The PVTA also calculated that each of its BE buses will eliminate 244,000 pounds of carbon dioxide emissions compared to their diesel bus counterparts.

The second main type of zero-emissions buses are those powered by hydrogen fuel cell batteries. Fuel cell battery electric (FCBE) buses store hydrogen onboard in storage tanks and the hydrogen is then supplied to the fuel cells to generate electricity to power the vehicles. There are no emissions, as water is the only by-product for FCBEs. There are presently seven U.S. transit agencies operating FCBE buses; the electric bus at the Massachusetts Bay Transportation Authority (MBTA) uses FCBE technology.

With a typical purchase price of $1.2 million, an FCBE bus is much more expensive to purchase than a conventional diesel bus ($435,000) or a compressed natural gas bus ($500,000). FCBE buses also require special training for bus operators on using the technology and special hydrogen storing and fueling facilities; these are typically located at bus depots to allow vehicles to be refueled at day’s end. On the plus side, the fuel economy for FCBE buses has been reported to be double that for compressed natural gas or diesel buses.

The third main type of zero emission buses are fuel cell hybrid (FCH) plug-in buses which use a combination of both onboard batteries and hydrogen fuel cells. To date, only 7 U.S. transit agencies have used FCH buses, mainly in short-term demonstration projects. Transit agencies that have tried FCH buses have consistently reported significant downtime for the buses, due to issues with the batteries, the fuel cell systems, and the hybrid integrator, and to challenges in diagnosing specific problems.

Currently, BE buses seem to hold the most promise for wider deployment and use.

Written by:  Tracy Zafian, UMTC Research Fellow


Meet Our Affiliated Researcher: Dr. Amro Farid, Associate Professor at the Thayer School of Engineering, Dartmouth

Dr. Farid is an Associate Professor at the Thayer School of Engineering at Dartmouth and Director of the Laboratory for Intelligent Integrated Networks of Engineering Systems (LIINES). His research is devoted to enhancement of sustainability, and resilience in intelligent energy systems. His research team seeks to develop an internationally recognized, locally relevant and industrially-facing program of research that engineers intelligent & integrated control, automation, and information technology systems that support the operations and planning of large scale integrated energy systems. These activities encourage and facilitate technology policy that supports the achievement of energy, water, transportation & industrial policy objectives while eliminating barriers to sustainable and resilient automated solutions.

Within our electrified transportation systems research theme, the center has made two important achievements.

1.) Abu Dhabi Electric Vehicle Integration Study: In the first full scale study of its kind, the LIINES has studied the technical feasibility of electric vehicles with respect to three infrastructure systems: the road transportation system, the electrical grid, and the Abu Dhabi Department of Transportation’s Intelligent Transportation System. Acknowledgement: The LIINES is grateful to METI for its partial financial support of this research, Mitsubishi Heavy Industries for the use of its Clean Mobility Simulator software and the Abu Dhabi Department of Transport for providing data on its traffic patterns and intelligent transportation system.

2.) Hybrid Dynamics Modeling of the Transportation-Electricity Nexus: Building upon the previous research project and develops a hybrid dynamic system model of the full Energy-Transportation Nexus. By choosing to include the behavior of the transportation system in combination with the electrical system this model seeks to coordinate four types interdependent decisions: vehicle routing/dispatching, charging queue management, charging dispatch, and vehicle-2-grid stabilization.

Dr. Farid also maintains a research blog that highlights his latest conference proceedings, research and publications. His most recent publication is titled “A Hybrid Dynamic System Model for Multi-Modal Transportation Electrification,” published in the IEEE TRANSACTIONS ON CONTROL SYSTEMS TECHNOLOGY.

For more information on Dr. Farid you can click here.  We at the UMTC are pleased to welcome him to our Affiliate Researcher Network and look forward to working with him on interesting transportation research ideas in the future.

By: Melissa Paciulli, Manager of Research and Development at the UMTC