Swiss eBus Plus

This project aims to develop a new generation of electric urban buses to address the main challenges related to electric transport, i.e., the achievable driving range.

We approach the challenge from two directions. First, we focus on the charging infrastructure and develop a reusable software tool to facilitate the planning of electrification of transport systems. Second, we are reducing the energy consumption of on-board thermal systems by proposing intelligent thermal management algorithms to mitigate range problems, especially in cold climates.

Swiss eBus Plus Rendering — Rendering of the Swiss eBus Plus prototype bus. The fully electric bus will be 10.7 m long and offer space for up to 58 passengers. It will cover a full day of operation without recharging. Image courtesy by Carrosserie HESS AG

The project Swiss eBus Plus aims to launch a new generation of electric city buses. The advantage of electromobility is obvious: it allows considerable savings in fuel, pollutants and noise, from which passengers, residents and the environment benefit. However, compared to diesel-powered alternatives, electric buses still suffer from disadvantages in terms of necessary infrastructure (charging stations or overhead trolley lines) or achievable driving range (see research field A below). In this project, these challenges are tackled by incorporating the newest technology in core components such as generation 3a battery cells and a CO₂ heat pump, both of which allow a lighter build and more efficient operation of the vehicle. In addition, the vehicle structure is optimized for thermal performance, for instance, by employing double-glazed windows, improved insulation, and air curtains. Finally, the operation of the vehicle is to be optimized, mainly in terms of its thermal management (see research field B below).

One of the final goals of this project is to build a prototype of the next-generation electric city buses. The 10.7 meter long prototype bus will be charged in the depot overnight and should be able to cover a full day of regular operation (250-300 km) without additional charging opportunities, even in harsh winter conditions.

Due to generally poor insulation and doors that open frequently, the energy required to achieve a comfortable environment for the passengers is generally high in city buses. In the case of electric buses, this issue is even more severe as, on the one hand, there is no abundant heat source such as a diesel engine and, on the other hand, energy used for the thermal systems cannot be used for propulsion. Obviously, both these peculiarities are intrinsic to battery electric vehicles and limit the achievable driving range. Furthermore, the auxiliary power consumption experienced in modern electric buses (which also includes other consumers, such as lights, computing hardware, and the infotainment system) can be even higher than the traction power consumption, see the figure below.

Figure 1: Recorded energy consumption per driven distance averaged over one hour of operation for a trolley bus operated in Zurich. Depending on the ambient temperature, auxiliary consumers require more energy than the traction system.

In external page previous research we were able to show that by incorporating a thermal reservoir in the form of a hot water cycle into the traction energy management, the lifetime of the traction energy management can be significantly increased. Building on these findings, one of the goals of the project is to identify and utilize the various thermal reservoirs in the Swiss eBus Plus prototype in a similar manner, ideally in both summer and winter operation. In principle, the high complexity of the thermal system allows for such a consideration but it poses new challenges. The vapor compressor cycle (VCC) can operate in both cooling (“air conditioning”) and heating (“heat pump”) mode. As a result, it can either provide heating or cooling to both the cabin and the traction system cooling circuit. Due to this cross-coupling, waste heat in the traction system can be utilized to heat the cabin. Using classical control theory, it is difficult to account for all the influences present in this system. Therefore, an optimized thermal management strategy is to be devised that is based on a physical model of the entire system.

Enlarged view: Figure 2: Overview of the power flows present in the Swiss eBus Plus prototype. Filled arrows indicate the positive direction of power flows. If power flows in the reverse direction are possible, they are indicated with open arrows. Electric power flows are represented by blue arrows, mechanical power is represented by green arrows. Heat flows that can be directly influenced are shown as black arrows, heat flows that are driven by temperature differences are shown as orange arrows. Shaded blocks represent dynamic components (i.e., some form of a reservoir). Solid blocks without shading represent algebraic blocks (zero energy capacity). Dashed blocks represent system boundaries. — Figure 2: Overview of the power flows present in the Swiss eBus Plus prototype. Filled arrows indicate the positive direction of power flows. If power flows in the reverse direction are possible, they are indicated with open arrows. Electric power flows are represented by blue arrows, mechanical power is represented by green arrows. Heat flows that can be directly influenced are shown as black arrows, heat flows that are driven by temperature differences are shown as orange arrows. Shaded blocks represent dynamic components (i.e., some form of a reservoir). Solid blocks without shading represent algebraic blocks (zero energy capacity). Dashed blocks represent system boundaries.

The system under consideration is subject to an inherent trade-off between the energy consumption of the thermal systems and the achieved thermal comfort. For instance, thermal system energy consumption can be reduced in winter by lowering the cabin temperature set point, which, of course, is at the expense of thermal comfort. This trade-off has been quantified in a external page previous publication based on steady-state thermal models. As a next step, an intelligent thermal management strategy is to be developed, which will provide a tuning parameter to vary this trade-off according to the needs of the end user, i.e., the public transport operator. For them, such variations are of great interest to determine the amount of energy (and thus money) to “invest” in thermal comfort.

The research performed in this field builds upon the knowledge gained in the preceding research project called ISOTHERM.

Additional Information

Project Details

Start Date:

01.03.2021

Lead Researchers

Fabio Widmer
Mohammad Moradi

Partners

External Project Link

Open Positions

Currently available student projects are posted in the projects page. If you cannot find a project suitable for you, feel free to contact the responsible researchers.

HESS is always offering internship positions for students. Start and duration can be determined flexibly. Get your hands on real hardware! If you're interested, we can get you in touch.

Team

Fabio Widmer
Hitesh Sewani
Ilyas Seckin
Mohammad Moradi
Vicenz Münger

Former Participants

Arthur Jung
Luis Blunschi
Maurus Zwahlen
Noah Dall'Oglio