Charging Infrastructure for Battery-Electric Buses

Charging Infrastructure for Battery-Electric Buses

This piece was originally published in the May/June 2020 issue of electroindustry.

by David Warren, Director of Sustainable Transportation, New Flyer of America Inc.

Mr. Warren is a preeminent industry leader in the battery- electric and clean propulsion space. He leads New Flyer’s sustainable technology strategy, with a specific focus on zero emission technologies.

Buses and motor coaches currently incorporate a wide range of propulsion systems: clean diesel, natural gas, diesel-electric hybrid, and zero-emission electric (trolley, battery, and fuel cell). With the advancement of lithium-ion battery technology, major cities and states are implementing policies or regulations to transition transit from low emissions to zero emissions.

In the United States alone, the transportation sector represents 27 percent of total greenhouse gas (GHG) emissions nationally.1 Every zero-emission bus (ZEB) can eliminate 1,690 tons of CO2 over its 12-year lifespan. ZEBs may also eliminate 10 tons of nitrogen oxides and 350 pounds of diesel particulate matter, improving air quality in the communities they serve.

In California, the Innovative Clean Transit regulation was established to transition all transit buses to zero emissions by 2040. Major cities throughout North America have also put in place policies to transition to zero emissions.

Figure 1: Charging begins when the pantograph makes conductive contact with a cross-rail system on the bus.

To accomplish these objectives, charging infrastructure is a crucial part of the zero-emission system that must be factored into the transition of transit fleets to ZEB propulsion.

Charger Standards for Transit— The Foundation for Interoperability

There are currently two Standards from SAE International, previously known as the Society of Automotive Engineers, that define the general physical, electrical, functional, and performance requirements for direct current (dc) conductive charging of electric buses:

  • Plug-In Charging (SAE J1772)
  • On-Route Charging (SAE J3105)

Plug-in charging transit buses typically utilize SAE recommended practice J1772 with a Combined Charging System Type 1 connector. This permits a transit bus to charge using the same electric vehicle supply equipment (EVSE) as electric vehicles such as a Chevy Bolt. Typical EVSE power for overnight charging ranges from 50 to 175 kW.

On-route charging utilizes SAE J3105 with a power transfer system based on Conductive Automated Connection Devices. This system typically involves an overhead pantograph system mounted on an overhead gantry that automatically lowers onto the roof of the bus when initiated by the driver. Charging begins when the pantograph makes conductive contact with  a cross-rail system on the bus as shown in Figure 1. This charging method allows for high-power ratings of up to 600 kW.

Inductive charging for transit (also known as wireless or cordless charging) is being deployed in limited transit applications. SAE J2954/2 is currently under development to define criteria for interoperability and electromagnetic compatibility.

Working closely with leading electric vehicle supply equipment (EVSE) firms such as Siemens, ABB, ChargePoint, Heliox, and others, transit bus original equipment manufacturers collaborated closely with the Federal Transit Administration, the American Public Transportation Association, the Canadian Urban Transit Research & Innovation Consortium, and the Electric Power Research Institute to ensure complete interoperability between vehicles and EVSE.

SCALABILITY CHALLENGES OF FLEET CONVERSION TO ELECTRIC

Transit buses in major cities are utilized up to 16 hours per day, with ranges exceeding 200 miles and consuming up to 500 kWh of energy stored in the lithium-ion batteries. For reference, a Nissan Leaf Plus car is equipped with a 62 kWh battery and may operate two to three hours on a charge.

Large metropolitan transit agencies have fleet sizes that can amass 300 buses at a single site. Over an eight-hour nighttime period, 150 MWh of energy is needed to recharge the buses for next-day operations. Constant power of 20 MW over an eight-hour duration is needed to support daily operations—the equivalent power output of a small hydroelectric dam.

Power requirements for transitioning transit to fully electric requires critical planning with utility and power providers to ensure the entire infrastructure system can provide power to a single site in an economical and practical manner.

ON-ROUTE CHARGING

Cities currently utilizing or deploying on-route opportunity charging of transit buses include New York, Portland, Salt Lake City, Vancouver, Minneapolis, and Los Angeles.

On-route charging (utilizing SAE J3105) is an approach that allows a transit bus to be charged throughout the entire day in-service and away from the bus depot. This charging method negates the need for overnight charging and requires that the bus remain on a designated route with chargers typically at one or more turnaround endpoints.

The typical process for on-route charging is shown  in Figure 2. A bus configured with on-route charging often requires 6-8 minutes of charging at 450 kW for every hour of operation. Continuous operation requires that charging happen periodically throughout the day. Buses that utilize on-route charging can be equipped with only 150-200 kWh of batteries compared to the long-range and depot-charged bus (500 kWh typical); this results in a lower vehicle cost that is offset by higher EVSE costs, the complexity of off-site construction, and real estate costs.

Arrival: 0 MinutesChargingDeparture: 6 Minutes
  • Driver stops at parking reference
  • Parking brake is activated
  • Pantograph is lowered onto bus rails
  • If the bus is in the correct position, wireless communication between the bus and the charging station begins
  • Bus initiates charging
  • Driver is notified that charging session
  • has started
  • Main circuit of charger is switched on
  • Bus batteries are connected to charger
  • Continuous monitoring of charging begins
  • At the end of the charging session, the drive releases the parking brake to stop power transfer
  • The pantograph is raised and the charger notifies the bus when complete
  • The bus is able to depart the parking zone

Figure 2: Typical process for on-route charging

Energy and Power—Using Analytics to Optimize Performance

Energy and power analytics are an opportunity to enhance efficiency, reduce energy consumption, and optimize performance of the EVSE and buses using the Internet of Things and GPS technology.

ANALYTICS FOR ASSESSING BUS PERFORMANCE

Figure 3 represents an array of analytics used for assessing the transit bus performance on each day and any given route.

The “Energy Wheel” (top left portion) provides the ability to assess the energy consumption in kWh by major subsystem—the motor, electric heating, and low-voltage and high-voltage accessories. Based on the total energy consumption of these systems, the range of the bus in terms of mileage and duration can be determined for actual and full-charge capability.

The state-of-charge (SOC) percentage (top right portion) provides a graph of the SOC by time of day and mileage accumulation to assess whether the bus is operating as designed with EVSE.

The power consumption graph (lower right) shows the amount of power consumed during any given time throughout the  day, as well as documenting the regenerative power that is created during deceleration to recharge the batteries and extend the range capability of the bus.

ANALYTICS FOR ASSESSING CHARGING PERFORMANCE

Figure 4 represents an example of on-route charging throughout the day for a bus using SAE J3105 rapid charging. The graph displays the SOC percentage of the bus while in motion, idle, and charging on-route. In this example, the bus starts the day with one short- duration depot charge (noted in green), followed by eight on-route charging sessions (noted in dark blue) over a nearly 16-hour period while in operational service. In this example, the SOC of the batteries operates between 60 and 90 percent SOC. This type of information is useful for battery engineers and researchers to optimize the charging patterns and enhance the duty cycle for extended battery life.

Key Takeaways

  • The electrification of transit buses is progressing rapidly as lithium-ion battery mass production costs decrease through the automotive consumer market. As this technology becomes more affordable, government agencies and policy makers are mandating zero-emission technology for clean air opportunities for pollutant and GHG emissions reduction.
  • SAE charging Standards are encouraging wide deployment of interoperable EVSE charging equipment to ensure infrastructure services all brands and types of vehicles.
  • Scalability of battery-electric transit fleets, from pilot projects to full fleet conversion, is complex and will require significant public and private investment.
  • On-route charging of electric buses provides an option for distributing power throughout the city and recharging within normal transit operations.
  • Performance analytics of the bus and EVSE are key to optimizing power and energy efficiency to reduce costs and optimize asset utilization. ei

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1 CALSTART, Race to Zero Emissions (R2ZE)

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