Towards Robust Electric City Bus Fleets: Uncertainty Quantification of Electric Bus Energy Consumption

Research output: ThesisDoctoral ThesisCollection of Articles

Abstract

Electric vehicles have emerged as a promising alternative to replace traditional internalcombustion powered vehicles. The use of electric vehicles not only reduces local greenhouse andnoise emissions, but capitalizes on their superior energy efficiency. However, expensive lithiumionbatteries dictate the investment costs of electric vehicles. These costs can be compensatedfor by high utilization of the vehicles, such as in public transportation. Electric buses offer a viable option to tackle climate change, since they are utilized more thanpassenger vehicles and have a greater passenger capacity. Popular research foci of electricbuses have been on kinematics, charging, heating and cooling, driving style, route, payload, batteries, and predicting energy consumption. However, the uncertainty quantification of theenergy consumption of electric city buses has received comparably less attention. The energy consumption of six electric buses was studied in the Helsinki metropolitan areain Finland with simulations and measurements of their commercial operation. This experimentaldata was used for an uncertainty quantification process. Nonlinear and linear surrogatemodels were fitted to the experimental data. Model validation showed that the generatednumerical simulation and surrogate models adequately describe the experimental data. Thesemodels were utilized for a sensitivity analysis of energy consumption. Ambient temperature and rolling resistance variation were identified as the important environmentalfactors, which caused one fourth of the energy consumption variation. Comparable impact was contributed by the variation in stops and driving aggressiveness, which were the most important route dependent factors. Stops and driving aggressiveness were surrogate metrics that were used to portray the effects of variation in traffic and passenger flow. Other effects had a minor impact on the energy consumption, which include tolerance of component specifications and battery resistance variation. Furthermore, consumption on Helsinki city routes was found to be 15% higher on average than on suburban Espoo routes. Generalization of the results is limited, since the variation of the uncertain factors is relatedto the geological location and bus type. For example, abundant heating is linked to the coldregion of the Nordic countries. Considerably more energy has to be sacrificed to heating inelectric buses, since there is less waste heat generated by the electric powertrain than that ofa diesel motor. The effect of ambient temperature variation could be reduced with enhancedinsulation, alternative door designs, and using a heat pump instead of diesel heater. Althoughthe results are limited to the case at hand, the proposed approach can also be implemented inother engineering systems.
Translated title of the contributionKohti Robusteja Sähköbussilaivastoja
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
Supervisors/Advisors
  • Tammi, Kari, Supervising Professor
Publisher
Print ISBNs978-952-60-8816-7
Electronic ISBNs978-952-60-8817-4
Publication statusPublished - 2019
MoE publication typeG5 Doctoral dissertation (article)

Keywords

  • electric bus
  • energy consumption
  • uncertainty quantification
  • sensitivity analysis

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