Development Of Control Oriented Electrical And Thermal Models Of An Electric Transit Bus Battery System

Open Access
Author:
Kunte, Harshad Sitaram
Graduate Program:
Mechanical Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 05, 2014
Committee Members:
  • Christopher Rahn, Thesis Advisor
Keywords:
  • Battery Systems
  • Thermal
  • Transit Buses
  • Modeling
  • Control
Abstract:
This thesis presents the insights derived from the empirical characterization, modeling, simulation, control-design, and verification tasks performed in developing energy storage system (ESS) controls for a plug-in electric transit-bus. The electrical and thermal behavior of a representative electric transit-bus ESS is analyzed through a system-level empirical characterization study. Experimental data and insights derived from the study are leveraged to develop a control-oriented electrical model of the battery-system, with a provision to incorporate thermal effects. Specifically, it is an equivalent circuit model with parameters dependent on both State of Charge (SOC) and temperature. Then, an Extended Kalman Filter based SOC estimator, implementable on battery monitoring hardware, is developed based on the aforementioned electrical model (albeit, for an isothermal case). Thus, the utility of experimental characterization in developing high-accuracy models and SOC estimators is shown. Further, challenges in migrating an SOC estimator from a simulation environment to a real system, are highlighted through SIL and HIL tests, and mitigation measures are suggested. Further, the thermal characterization study reveals strong thermal gradients, and sluggish heat-transfer dynamics within the ESS. These are found to be caused by the physical architecture of a typical transit-bus ESS. Thus, it cannot be treated as a single lumped thermal mass as is done for small and mid-size hybrid/electric vehicles. The need, process, and utility, of developing a high-fidelity thermal model for a transit-bus battery-system, is highlighted. This control-oriented model accurately predicts temperature evolution across the battery-system under external electrical and thermal load. Lastly, the electrical and thermal models developed in the work, are coupled and utility of the coupled models in system-level simulations and trade-off studies, is demonstrated.