SwissTrolley plus

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The project «SwissTrolley plus» aims to replace the diesel-powered auxiliary power unit of standard trolley buses with a high-performance traction battery. This replacement offers the following advantages over state-of-the-art technology:

  • Existing trolley bus lines can be extended without having extend expensive infrastructure, i.e., bus stops outside the grid can be served by electric buses.
  • Reduced noise and pollutant emissions during off-grid operation.
  • Around 15% reduced energy demand due to regenerative braking.
  • Active peak load balancing in the electricity grid.

This project is a cooperation of Carrosserie HESS AG (Bellach, Solothurn), Verkehrsbetriebe Zürich (VBZ), Berner Fachhochschule (BFH), and ETH Zürich. It is sponsored by the Bundesamt für Energie (BFE) and the Swiss Competence Center for Energy Research (SCCER).

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Project Description

New Vehicle Concept

State-of-the-art trolley buses need a diesel-powered auxiliary power unit (APU) to ensure maneuverability during a shortage of electricity. The engine, despite being rarely used, leads to increased vehicle weight and thus increases the energy consumption. When eventually in use, the undersized APU runs mainly in cold-start mode, leading to extreme noise and pollutant emissions.

The goal of «SwissTrolley plus» is to avoid those drawbacks by replacing the APU with a high-performance traction battery. The traction battery enables regenerative braking, which allows to reduce the energy demand of the vehicle by about 15% compared to a state-of-the-art trolley bus. Furthermore, the battery allows purely battery-electric driving outside the overhead wire network for several kilometers. This feature is used to extend existing bus lines and remove parts of the overhead wire network, thus in both situations saving infrastructure building maintenance costs. Finally, the battery capacity can be used to stabilize the electricity grid by balancing the unsteady power demand, which is a major factor for energy pricing in trolley networks.

Cooperation of universtities and industry

This project is a follow-on project of the very successful and trusting cooperation between ETH and Carrosserie HESS AG during the AHEAD project. As a new partner, Verkehrsbetriebe Zürich (VBZ) provides the needed know-how and experience from the view of a public transportation operator in order to achieve the ambituous goals. The Berner Fachhochschule (BFH) investigates the differences of currently available battery types and tests their operating characteristics in various load scenarios. One aspect of this research is the process of battery aging. Accurate aging models are especially of interest for financial aspects and the electrochemical degradation due to fast load transients.

Advantages for commercial and private users

The outcome of this project is a demonstration vehicle that is ready for market introduction.

The industrial partners HESS and VBZ befenefit in several ways from the achievements of this project:

  • The project leads to a novel trolley bus that HESS can offer in its product line.
  • The ability to drive the bus without overhead wires enables VBZ to extend the trolley bus range and the reduced energy demand of the new vehicles lead to reduced operational costs.
  • The new technology attracts public attention and thus gains popularity of both media and investors.

Private users benefit while using the public transportation. Trolley buses with the new technology allow to reach areas where previously only conventional diesel buses were able to serve the bus stops. The pure-electric vehicles minimize energy consumption and CO2 emissions and reduce noise and pollutant emissions.

Goals & Challenges

On-line energy management with optimal performance

The «SwissTrolley plus» bus has two sources of power: the electricity grid, and the traction battery. The energy demand of the propulsion system during operation is defined by the driver. This setup provides a degree of freedom: In each moment of time, the energy management system of the vehicle needs to decide whether to draw electric power from the grid, or from the battery. The associated decision of how to "split" the power demand between the two power sources has a huge impact on the energy efficiency of the vehicle and turns out to be not trivial. Therefore, a proper understanding of the vehicle and its surrounding conditions is crucial for the development of an energy management algorithm.

Based on the expertise in control systems theory and hybrid vehicle propulsion systems, an optimal energy management strategy is developed at the Institute for Dynamic Systems and Control both in terms of theoretical studies and practical tests in close collaboration with HESS and VBZ.

The goal of the energy management strategy is to maximize the energy efficiency of the vehicle, while ensuring that the driver’s power request is always met. At the same time, all operational constraints of all components are monitored and maintained. This problem belongs to the class of dynamic optimal control problems that can only be solved using dedicated tools from the theory of optimal control.

Position-based charging strategy for wire-free zones

When the «SwissTrolley plus» bus is operated on a bus line that features sections without overhead wires, the energy management system needs to guarantee that the battery contains enough energy to travel through these sections in battery electric mode. Thus, a simple constrol structure would be to constantly charge the battery during regular operation in order to have a completely charged battery when the bus enters the wire-free section. Although this solution is very robust against uncertainties within the wire-free sections, it is not optimal. Only if the characteristics of the travel route are taken into account it can be ensured that the recuperation energy from braking maneuvers is entirely stored in the battery. Such a position-dependent energy management strategy requires the integration of positioning data from a global satellite navigation system combined with a road map that provides the appropriate data.

Battery-health conscious operating strategy for maximum battery lifetime

Battery lifetime is a crucial issue for operators of trolley buses. Usually, trolley buses are designed for a lifetime of more than 20 years. With this project, the aim is to achieve a battery lifetime of at least 10 years, which implies only one battery replacement over the full lifetime of the bus. Battery health is influenced by many operational factors, such as peak and average charging and discharging power, highest and lowest operating temperature, depth of discharge, etc. The energy management therefore needs to be carefully programmed in order to avoid damaging the battery.

Peak-load shaving for cheaper electricity

Caused by the tight timetable in public transportation, the bus drivers tend to maximize the acceleration and deceleration in the range of the passenger’s comfort zone. Such driving behavior results in a strongly fluctuating power demand that is hardly predictable. The operator of the electricity grid needs therefore to apply a large amount of expensive stabilization energy to balance the demand side. While the driver demands peak power (ca. 200 kW) only during the short acceleration phases, the average power demand is much lower (ca. 35 kW). With the traction battery, the peak power demand can be reduced and the grid load is smoothened – eventually leading to less stabilization energy and thus to a lower price of electricity.

Reduction of the energy demand for HVAC systems

The energy demand of systems related to heating, ventilation and air-conditioning (HVAC) of public transportation buses are responsible for a substantial part of the total energy demand (ca. 30-40%). This project specifically investigates how the HVAC energy demand can be reduced without affecting the passenger comfort, and how the HVAC systems can be operated more efficiently. 



Ritter A., Elbert P., Onder C., Poster SwissTrolley+, 2015, SCCER Mobility Annual Conference 2015, Zurich, Switzerland

Andreas Ritter, Philipp Elbert, and Christopher Onder, Energy Saving Potential of a Battery-Assisted Fleet of Trolley Buses, IFAC-PapersOnLine, (2016) New York, NY: Elsevier.

Ritter A., Elbert P., Onder C., Poster SwissTrolley+, 2015, SCCER Mobility Annual Conference 2016, Zurich, Switzerland


This project is a cooperation with the industrial partners HESS AG and VBZ.

Research related to battery technology is conducted at Bern University of Applied Sciences (Berner Fachhochschule, BFH)

It is sponsored by the Swiss Federal Office of Energy SFOE (Bundesamt für Energie, BFE) and the Swiss Competence Center for Energy Research (SCCER-mobility).

More information can also found on the official project website, which will be updated continuously throughout the project.


23 September 2015
vbzonline announces the replacement of the Diesel-Generator power units with traction batteries within 35 trolley buses of their fleet.

9 October 2015
Tages-Anzeiger reports that VBZ has cleared Zurich's trolley bus and streetcar stop "Albisriederplatz" of overhead wires.

8 September 2016
Zürichsee-Zeitung and Lokalinfo AG discuss the new electric mobility startegy pursued by VBZ. Next to the new battery buses, a new generation of trolley buses will contribute a valuable part to a reliable public transportation system of the future.

17 and 18 January 2017
20 Minuten, Aargauer ZeitungBlick, Der Landbote, Luzerner Zeitung, NZZOltner Tagblatt, Solothurner Zeitung, SRF, Tages-Anzeiger, Züricher Unterländer, and Zürichsee-Zeitung present the project «SwissTrolley plus» on their websites according to the media conference that was held at the Zentralwerkstatt of VBZ in Altstetten.

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