Zero-Emission Electric Bus Modeling and Validation for Public Transportation Electrification

In 2015, the US EPA listed electricity generation as producing slightly more CO2 than transportation.  Since then the adoption of natural gas, the growth of renewable sources, and the retirement of coal-fired power plants has significantly reduced the carbon produced by electricity generation.  The next obvious step is to reduce vehicle transportation emissions.

A key technical step is the electrification of transportation technologies. Electric vehicles (EVs) can significantly reduce the emission and the noise as compared with conventional diesel or gasoline vehicles and can fully utilize emerging green energy resources, such solar and wind, from the utility grid. The energy efficiency of electric buses is much higher than diesel buses and compressed natural gas (CNG) buses resulting in a much lower operation cost for public transportation. When an electric bus is connected to the utility grid, the onboard battery energy storage system (ESS) can be made available to support the reliable and stable operation for the utility grid.

The opportunities offered by this program don’t stop in the exciting application of engineering tools and principles.  A program of this scope with so many contributors requires a high degree of collaboration and coordination, and participating in the weekly meetings alone offered valuable insight into the scope of the project, as well the complexities in integrating the hydrogen and electric systems.  During a day-long hazards and safety analysis for the new vehicles, the theme of engineering ethics took center stage as we moved through potential hazardous events during daily operation of the truck. It was inspiring to see so many man-hours dedicated to exhausting the possible scenarios that could cause harm to the operator or public. To leave no stone unturned, nor concern unanswered, required a concerted effort.  This occasionally lead to disagreements, though when resolved, yielded the best solutions.  The hazard and failure mode analysis not only gave the project a new dimension, but it added a unique insight into a facet of engineering seldom touched on in school.

The early part of the new year will see the completion of the prototype truck.  Successful testing and optimization of the various algorithms and integrated systems will lead to phase two of the program, where 15 additional trucks will undergo a similar retrofit.  Regardless of the future scope of this program, the lessons it has offered the student interns through these last few months will be enduring.  I am confident that the experience gained while working alongside talented engineers, participating in advancing drivetrain and propulsion systems for the vehicles of the future, and being involved in the complexities of project management will be huge boons to our careers, particularly in the face of a changing transportation sector.

smartgrid vision
Fig. 1. Vision: Smart Grid + Transportation Electrification
 

While the power industry has successfully mitigated the additional loads caused by smaller electric vehicles to the legacy utility grid, bus electrification could be a game-changer.  The peak power of a typical on-route fast charging station is in the range of 350 to 500 kW. This power requirement is equivalent to the power demand of approximately 100 houses. Fast chargers draw high currents from the distribution system, which may cause a system overload if not managed properly. 

To study the performance impact on a legacy power grid of both on-route and depot bus charging, CEM researchers developed two zero-emission electric bus simulation models in the Powertrain System Analysis Toolkit (PSAT).  A route profile captured by GPS data logger on a diesel bus, including elevation, speed, latitude, and longitude, are processed and used as the input for the PSAT bus models. It is reasonable to assume that an electric bus has the same route profile as a diesel bus. The efficiency and driving range of the two bus platforms are evaluated and compared using the actual GPS route data.

 powertrain system
Fig. 2. Powertrain system configuration of a zero-emission electric bus

 

simulation study

Fig. 3. Simulation study results.
 

In addition to numerical simulation, CEM researchers work closely with bus vendors to get field-measured data on zero-emission buses, as a means of improving model fidelity in PSAT. Key model parameters, including motor efficiency, converter efficiency, and accessory load, are tuned to improve the bus model accuracy. The proposed modeling approach is a promising method for transit bus energy efficiency studies. The developed numerical models can be used to plan fast charging stations and fleet schedules. Validated bus simulation models are a valuable resource to insure the successful transition of electric bus technology to market.  It is an essential method for of studying the grid impact of the public transportation electrification. In June 2017, the detailed study results were presented at IEEE International Transportation Electrification Conference and Expo (ITEC) at Chicago IL.

simulated and measured batter

Fig. 4. Simulated and measured battery SOCs of a typical route.
 

Reference:
X. Feng, M. Lewis, and C. Hearn, “Modeling and validation for zero emission buses,” Proc. IEEEInternational Transportation Electrification Conference and Expo (ITEC), Chicago, IL, June 2017.