Interactive Operations Research

Transit Fleet Electrification

Many transit agencies are pursuing fleet electrification, recognizing its benefits related to the environment, cost-effectiveness, and social image. Electric buses, which emit 62% fewer greenhouse gases than their diesel counterparts, can enhance air quality and improve overall quality of life. E-buses require less maintenance for their simpler motor structures and have lower long-term costs in fuel and upkeep. Financial incentives such as tax credits further add to the appeal of fleet electrification. This comprehensive range of benefits is the driving force for transit agencies to actively electrify their transit fleets, which requires choosing a fleet composition and size, deploying chargers at specific locations with certain outlet capacities, and configuring an optimal charger scheduling plan that satisfies operational constraints.

Complexities of Charger Scheduling Optimization

Bus charging can be broadly divided into overnight and en-route charging. Overnight charging is the process of moving chargers between E-buses during their long overnight layover so their State of Charge (SOC) reaches its full battery capacity for the next day’s operations. In contrast, en-route charging is the process of charging E-buses during their shorter layovers throughout the course of the day. The main advantage of en-route charging is that it extends the daily driving range of the e-buses, allowing an operator to acquire smaller battery-size fleets that can recharge en-route to complete their assigned bus block. En-route charging schedule optimization is a complex problem, independently and compared to overnight charging, as it distributes a limited resource – chargers with certain capacities- among E-buses that compete for charging time during layovers to finish their block.

Electrification: A Three-Sided Problem

The many benefits of fleet electrification are the driving force for transit agencies to actively electrify their transit fleets, which requires choosing a fleet composition and size, deploying chargers at specific locations with certain outlet capacities, and configuring an optimal charging schedule that adheres to operational and regulatory constraints. The main decisions of transit fleet electrification are related to the following three:

E-Bus: Bus type specifications, including the battery size and the minimum safety charge level. The battery size determines the range - how long a bus can operate before recharging.

Chargers: The location of chargers, the number of charging outlets at each location, and their charging rates directly impact the SOC profiles and the ability to run their route on charge.

Station Restrictions: En-route charging must adhere to bus schedule restrictions. For example, short layovers may not be sufficiently long enough for charging due to the time operators need to plug a charger, which might not justify the minimal charge gained.

E-Bus Pro: Interactive Transit Electrification Tool

E-bus Pro is an interactive tool for transit planners to design electrification plans by choosing charger locations, charger outlet capacity, charger type, and electric bus specifications like the range and minimum required safety charge. The tool takes schedule data from the GTFS and generates an optimal en-route charging plan that adheres to scheduling constraints and other regulatory conditions. Compared to conventional dashboards used for capturing trends, the proposed tool allows "what-if" scenario analysis in pursuit of an electrification objective (e.g., the number of electrified buses or the total distance travelled by the electric fleet). The tool is interactive as it allows for testing different electrification plans, intuitive as it does not require any technical training (except some tutorial videos), accessible as it is cloud-based, data-driven as it uses General Transit Feed Specification (GTFS) data, scalable as it can be applied on any transit system, optimized as it uses advanced Operations Research to find the best charging schedule subject to operational constraints, and customizable as it can adhere to agency-specific regulations and operational restrictions.

Behind the Engine

In the background of E-bus Pro is an optimization model that takes from GTFS data the schedule of each bus - defined formally as a “bus block” in Transit terminology. The optimization seeks to maximize a specific objective subject to operational constraints, such as:

Charge continuity: Each E-bus maintains a charge level within the limits of the minimum required safety charge and the battery size.

Layover time restrictions: Each E-bus can only charge during a portion and within the boundaries of its layover time.

Charger outlet capacity: The number of E-buses charging at any given station should be smaller than the number of outlets.

Energy consumption prediction: The optimization can integrate alternative energy consumption models that consider route distance and time, temperature, load profiles, and height profiles.

SOC-dependent charging: The optimization can integrate non-linear SOC-dependent charging rates to replicate faster charging at lower SOC.

Overview of the Tool

The figure below shows an overview of E-bus Pro, which consists of an Input Panel, Output Summary Panel, Charger Profile Panel, SOC Profile Panel, Charger Outlets Input Panel, and the Network Map.

Input Panel: This panel takes the E-bus specifications and the other charging operations features. The E-bus characteristics include the battery size and the safety charge, which is the minimum battery level a bus must maintain to handle unexpected operational demands. This safety buffer deals with unforeseen challenges like detours, traffic delays, or sudden increases in energy usage (e.g., due to severe weather conditions). Cut-off time is the overlay time under which charging is infeasible to avoid short charging sessions, which are not economical. Timing efficiency is the portion of available time that can be effectively used for charging, maximizing the use of resources, and minimizing operational downtime. Other operational constraints can also be added depending on the operators' preferences.

Charger Outlets: This panel pops up when a charger is clicked, allowing for changes in its outlet capacity. Setting the outlet capacity to zero implies that the charger does not exist.

Output Summary: This panel summarizes the system performance after executing the charger scheduling optimization model by clicking “Solve.” The Output Summary panel specifically shows how many buses are electrifiable, given the features of the input panel, the location and outlet size of each station, and the optimal charging schedule proposed by the model.

Charger Profile: This panel shows the charging profile of each charger in the system, depicting specifically which bus charges at which station and during what time period. The table underneath the profile shows the charging schedule of each bus, which is color-coded depending on the bus status. For example, if a bus (block) is electrifiable, it would appear as a green row.

SOC Profile: Clicking on each E-bus in the Charger Profile table creates that bus’s SOC profile along its path, showing the energy drops due to traveling and energy boosts due to charging.

Network Map: The map shows the intensity of E-bus movements in the different parts of the network. The render shows whether all parts of the network adequately and equitably benefit from electrification and highlights areas of improvement for deploying new charger stations or increasing existing charging station capacities.

Adjust Bus Features

As the E-bus features are adjusted, the network map shows the extent of electrification in the system, and the output summary panel shows the number of electrified buses.

Adjust Charger Outlet Capacity

Click on chargers to view their outlet capacity. Adjust their capacities and run the optimization to find a new charging schedule. Explore areas of improvement where the network is less green and investigate alternative distribution of outlets to those locations.

Interpret Charging and SOC Profiles

Flip through the charging profile of different chargers and visualize their utilization. Navigate the details of the charging schedule in the table below. The buses are color-coded with green for E-buses. Clicking on an E-bus shows its SOC profile.

Mehdi Nourinejad, PhD.

Team Lead

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