Holistic Traction and Thermal Energy Management of Electric City Buses
OPEN ACCESS
Loading...
Author / Producer
Date
2024
Publication Type
Doctoral Thesis
ETH Bibliography
yes
Citations
Altmetric
OPEN ACCESS
Data
Rights / License
Abstract
While the electrification of city buses offers the potential to reduce lifecycle greenhouse gas (GHG) emissions as well as relieve city centers of pollutant and noise emissions, their widespread adoption faces two significant challenges. On the one hand, the heating, ventilation, and air-conditioning (HVAC) systems of such vehicles consume large amounts of energy, thus requiring large batteries to achieve a certain driving range. On the other hand, the batteries are still the heaviest drivetrain component, accounting for a large part of the total cost of ownership and the life-cycle environmental footprint of electric city buses. Hence, battery size should be minimized, leading to higher average load and thus accelerated degradation of the battery cells.
In this thesis, we propose holistic approaches to address these issues. We present five main contributions:
First, we introduce the publicly available Zürich Transit Bus (ZTBus) dataset, which consists of data recorded during driving missions of electric city buses in Zürich. The dataset consists of more than 1400 missions across all seasons, each of which containing detailed time-resolved information on the vehicle’s power demand, odometry, global position, number of passengers, etc. It serves as the empirical basis for the case studies presented in this thesis but may also prove valuable as a foundation for a variety of studies and analyses beyond the scope of this work.
Second, we investigate the joint optimization of the power split of a battery-assisted trolley bus along with its hot water system subject to a minimum battery lifetime requirement. As part of this investigation, we conduct a case study and show that, when using an optimized rather than a heuristic heating strategy, energy consumption can be reduced by up to 7% on some driving missions without sacrificing battery life time. If, in addition, the design of the thermal system is co-optimized, some further, yet smaller improvements are possible.
Third, we build upon these optimization results by developing an online controller for this hot water system. Applying Pontryagin’s minimum principle (PMP), we develop a simple controller that is able to approximate the optimal heating strategy, but does not require any predictive data. We implement this controller on a real prototype bus, thus allowing performance validation under real-life operating conditions. We demonstrate that real-life bus operation using this controller leads to a reduction in battery degradation of up to 10%.
Fourth, we introduce a novel way to address the trade-off between energy consumption and battery wear typically observed in hybrid electric vehicles (HEVs). The suggested approach involves tracking a battery lifetime target in a closed control loop by incorporating periodic measurements of the state of health (SOH). This approach enables the energy management system to reliably meet the target battery lifetime in the presence of disturbances and modeling errors. Furthermore, in order to facilitate interactive controller design, we devise an algorithm that is able to carry out simulations of a complete vehicle lifetime in less than a minute, which is about 70 000 times faster than a standard approach with only negligible concessions in terms of simulation accuracy. Thanks to this significant speed improvement, we can numerically optimize the battery health trajectory over the vehicle lifetime. Hence, we are able to show that, a linear trajectory results in only a small energy penalty of 0.05% over the vehicle lifetime in our case.
Fifth, we address the HVAC system in an electric city bus by analyzing the trade-off between the energy consumption and the thermal comfort of the passengers. We do this by developing a dynamic thermal model for the bus cabin, which we simplify by considering it to be in a steady state. We introduce a method that is able to quickly optimize the steady-state HVAC system inputs. We validate the steady-state approximation by comparing its results to dynamic simulations. We then present two case studies to demonstrate the practical relevance of the approach. In the first one, we show how the method can be used to compare different system designs based on an annual performance evaluation. In the second one, we show how the method can be used to extract setpoints for online controllers that achieve close-to-optimal performance without any predictive information.
Permanent link
Publication status
published
External links
Editor
Contributors
Book title
Journal / series
Volume
Pages / Article No.
Publisher
ETH Zurich
Event
Edition / version
Methods
Software
Geographic location
Date collected
Date created
Subject
Public transport; Energy and thermal management; Battery health; Electric mobility; Hybrid electric vehicle (HEV); HVAC system; Thermal comfort
Organisational unit
08840 - Onder, Christopher (Tit.-Prof.)