Prediction of the lifetime of urban electric bus traction batteries in the context of the overall system design

Autoren

Martin Ufert, Technische Universität  Dresden, Professur für Fahrzeugmechatronik

Jahr

2019

Zusammenfassung

Driven by the ongoing discussion about clean air in German cities, many municipal transport companies are pushing ahead with the conversion of their bus fleets from diesel buses to electric buses. In many cities, this is an elementary step in the package of measures to reduce emissions and comply with legal limits.
In recent years, various pilot projects have been launched and completed in various calls for proposals (e.g. Berlin, Hamburg, Dresden) [1]. In Dresden, for example, a 12 m electric bus with a conductive high power charging system was extensively tested as part of the "Electric Bus Line 79" project [2]. The aim of all of these projects was to test the different systems available on the market with regard to readiness for use and to gain initial experience in handling electric buses and their use in regular passenger service. An overview of the pilot projects in Germany is given in [1]. In many of these projects mainly vehicles were put into operation, which are then used on specially selected routes – often with low daily mileage in order to avoid the range problem. Only in a few projects was the number of vehicles procured sufficient to operate an entire line with electric buses.
In the coming years, the next step towards switching bus fleets to electric mobility is to be taken. Many public transport companies are planning to procure a larger number of vehicles with which entire lines can then be operated completely with electric buses. In order to maximise public awareness, there is often a desire to choose so-called volume lines. These are lines that have a high passenger volume and often a high daily mileage and require a corresponding range.
The vehicles currently available on the market have very different ranges. The manufacturer's specifications range from approx. 150 km for a 12 m vehicle [3] to over 300 km for an 18 m vehicle [4]. However, depending on the choice of line, this range may be too short to ensure safe operation under all circumstances. In such cases, charging the energy storage device during operation is required in order to be able to perform the daily driving performance. This charging can be carried out in different ways. The main degrees of freedom are the location, duration and power of the charging.
Switching from diesel buses to electric buses does not only mean simply replacing the vehicles, but is to be understood as a way of designing the system. Today, this design is often based on empirical values. For example, specific mean values (in kWh/km) are used to estimate the required energy content of the traction battery for a certain distance. For charging, known combinations of charging location and charging power are then used, which are often known from the pilot projects. However, in many cases such a system design has a rather heuristic character and often only a few design scenarios are compared with each other. This procedure is shown exemplarily in [5] and [6]. An optimal design, for example with regard to the required investment and operating costs, cannot be determined thereby.
This paper presents a methodology which automatically calculates and evaluates a multitude of technically possible configurations for a given route under given boundary conditions. The core of the methodology is a detailed, multi-stage battery model, since the traction battery is currently the most expensive single component of an electric vehicle [7] and its consideration therefore has a particularly high priority in system design.
Probably the most important differences between electric buses and diesel buses in daily operation are the limited range and the much more complex charging process (charging energy storage vs. refuelling in conventional diesel buses). Analogous to the refuelling process, charging the vehicles in the depot during the night break is the state of the art.  The required charging infrastructure confronts transport companies with great challenges, especially if a large number of vehicles are to be charged simultaneously in the depot. However, the charging process in the depot and the required hardware will not be examined in detail in this paper.
The focus of the methodology presented here is on the choice of a suitable energy storage device as well as a possibly necessary charging infrastructure along the route being considered. The following questions are to be answered:
● What is the power requirement of a vehicle to cover the considered distance?
● What is the required energy content of the traction battery?
● How many charging points are required along the line and where are they positioned?
● What charging power must be installed at the charging points?
● Is the stop time planned according to the timetable at the charging points sufficient?
● How does the intended operation affect the lifetime of the traction battery?

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