article - hybrid buses with energy from the tram … identified in order to retrieve the energy from...

10th ITS European Congress, Helsinki, Finland 16–19 June 2014 SP 0027

Hybrid buses with continuous energy transfer from tram network

Oscar Olsson1*, Ellen Alexandersson2, Stefan Pettersson2

1. Viktoria Swedish ICT, Lindholmspiren 3A, SE-417 56 Gothenburg, Sweden,

Phone: +46 70 489 3058, [email protected]

2. Viktoria Swedish ICT, Sweden

Abstract Expanding cities need to cope with significant challenges to provide good mobility, at the same time as congestion, emissions and noise should be minimized. Compared to combustion vehicles the electric vehicles have higher energy efficiency and reduced local emissions, and can therefore ease above challenges. Unfortunately, energy density in batteries is relatively low and batteries required to achieve a corresponding range of conventional vehicles can be both expensive and heavy. In this case study, the possibilities, ambiguities and requirements for utilizing Gothenburg’s existing tram network as continuous energy source for hybrid buses to allow increased electric range and reduce battery size is examined. A technical solution has furthermore been proposed to enable safe continuous charging. Recommended is also to implement supply based energy consumption on vehicles in order to optimize the network power utilization without additional infrastructure investments.

KEYWORDS: Hybrid bus, dynamic charging, tram infrastructure.

Introduction Expanding cities need to cope with major challenges for creating and providing sustainable mobility for all groups in the society. At the same time congestion, air emissions and noise should be minimized. Compared to conventional vehicles the electric vehicles have improved energy efficiency and greatly reduced local emissions. Thanks to these advantages, electric vehicles have high potential to help solving the urban challenges and contribute to the sustainable mobility in the cities.

Hybrid buses with continuous energy transfer from the tram network

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Unfortunately, the energy density of batteries is relatively low compared to for example diesel fuel and a battery required to provide a range comparable with a conventional vehicle’s range would be both expensive and heavy in weight, especially for public transport vehicles with long daily mileage. By frequently charging the battery the battery size could be reduced. However, static charging for example at bus stops requires the vehicle to stand still, which affects timetable, decreases value adding time and possibly also requires new and costly charging infrastructure. A potential solution is instead to use an existing tram network, like the one in Gothenburg, as a continuous source of energy also for public buses. In Gothenburg today, buses already share the same lanes as the trams during certain parts of the routs. If hybrid buses were charged, completely or partly, via the tram net the investment need for new charging infrastructure could potentially be reduced. At parts of routes where charging via the tram network is not possible or adequate, the vehicle continues on energy from the on-board battery or internal combustion engine, favourably supplemented with extra charging at bus terminus to minimize the need of fossil fuel and to reduce the battery size. Continuous power supply to buses with rubber wheels is proposed to be made possible by using a hybrid bus with a custom pantograph, called a current collector, to connect to the overhead line and a current returning arm to connect to the rail, see illustration in Figure 1. The principle is a mixture between solutions for trams and trolley buses. It is reminiscent of trams that normally are connected to the overhead line through a pantograph, but for the tram the current is fed back via the metal wheels. What distinguishes this hybrid bus to a trolley bus, apart from being capable of follow a tram network, is the flexibility added by not being bound to a certain route.

Figure 1: Illustration of a bus fitted with a tram pantograph


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The case study [1] described in this paper aims to explore the possibilities and as far as possible investigate ambiguities and requirements for utilizing the Gothenburg tram network as a continuous energy source, for hybrid buses, to increase the electric range while reducing the need for energy storage in batteries. The paper emphases on describing the devices needed to connect to the tram network, the hybrid vehicle prerequisites and the infrastructure constraints. In this paper, by a hybrid bus is meant a bus that uses two or more separated energy sources for propulsion of the vehicle and where at least one of the sources is electricity. Though, the technology to connect a bus to the tram network can be applied broadly, for example on pure electric buses. Methodology The study follows a problem-driven research methodology [2] with the goal of solving a real-world problem where the user’s need of the application is in focus. The methodology is close to the action research methodology, which is a process that brings together action and reflection, theory and practice, in participation with others, in the pursuit of practical solutions to issues of concern to people, individual persons and their communities [3]. The problem stated is to answer the question if it is possible to use the tram network to continuously transfer energy to hybrid buses and thereby electrify the public transport in a higher degree by using existing infrastructure. The word possible is in this case addressed to technology, economic and safety issues. By investigating the problem it can be found out if it is a reasonable way to follow to get a more sustainable transport solution in the city. This complex problem was investigated through collaboration between people from a broad range of disciplines with a high level of expertise in the diverse areas. The different disciplines includes engineers from heavy vehicle industry, institute researchers from within the electromobility field, people from the local public transport company, and representatives from the city of Gothenburg included in the planning of mobility in the city. This collaborative research methodology opens up for developing practical knowing by a participatory democratic process. Results Current collector and current return arm To connect the overhead line, a device, in this case study called a current collector, is required which could probably recall a tram pantograph but with lateral movability to compensate for the flexibility of the bus. The lateral movability could be achieved by for example adding additional two-way joints shown in Figure 2, suggested by [4], or mounted on a linear drive. The general pantograph technology has been known since a long time and is no longer


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protected by any patent.

Figure 2: Design of an active current collector, suggested by [4].

A bus does not have metal wheels that enable connection to the tram rails. A new design, in this report referred to as a current return arm, is therefore essential to be able to follow the rail and complete the electrical circuit. Similar techniques have been developed to demonstratively show how a truck conductively can retrieve power from a rail in the roadway while driving [5]. For safety reasons, the current return arm may not protrude from the vehicle side, as this could be dangerous for pedestrians. The current return arm should ensure a connection to the rails before contact with the overhead line is established. This provides additional protection against a potential difference in the vehicle towards the ground, which otherwise could be a risk while there is a connection on the overhead line and a failure in the insulation. The current collector should be hindered to rise, alternatively lowered automatically, if it is already in the upper position and if the current return arm cannot identify the tram rails. The positioning of the current return arm also gives knowledge of the rail position, which could be used to calculate the approximate position of the overhead line. The overhead line is positioned in a zigzag pattern from the imaginary centre above the rails to distribute the wear evenly on the contact strip of the current collector head. The centre on the pantograph should, regardless of the bus's position, always strive to be above an imaginary centre between the rails to avoid the risk of tearing down the overhead line, see Figure 3 below. A roof mounted current collector protruding from the side of the vehicle is estimated not to harm neither people nor anything in the surrounding environment, since it is the normal path of the tram pantographs. The solution is similar and accepted for trolleybuses.


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Figure 3: Current collector guidance based on current return arm position.

The current collector may be easily positioned with its centre in the middle of the approximately ± 400 millimetres zigzag pattern of the overhead line as long as the bus drives roughly in parallel with the rail. If the bus on the other hand turns in an angle compared to the overhead line, other than what is common for a tram, the current collector might eventually fail to follow the overhead line. In the Gothenburg tram network and with a standard 1250 mm contact strip as in Figure 4, this would occur when a bus turns approximately 50°, unless the collector head could rotate. To protect against overhead line accidently being teared down, for example when the bus makes a sharp turn and crosses the overhead line in the opposite direction, a maximum allowed angle should be implemented when the current collector must be retracted.

Figure 4: Angle between current collector head and overhead line. L is the length of the current collector head, l is the length of the contact strip attached to the head, and d is

the overhead line zigzag span. For given d and l, the maximum angle at which the contact strip can maintain contact with the overhead line is 50 degrees.


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On a rigid bus, the current collector and current return arm should preferably be placed aligned with each other near the rear axle of the bus. The rear of the bus follows an imaginary centre line between the tram rails better than the front part, particularly in curves, as the bus needs to take a wider turn to compensate for the vehicle length. One potential problem is that the rear axle load capacity and surrounding available space is already limited and additional components could be difficult to accommodate without affecting the number of permitted passengers. Vehicle requirements A Volvo 7900 Hybrid was used as reference vehicle in the case study. Two major challenges were identified in order to retrieve the energy from the tram network for continuous charging of the hybrid bus: 1. Ensure that the vehicle and the energy storage system receive accurate and continuous voltage and current. 2. Ensure safety requirements for proper grounding and insulation. A hybrid vehicle with a lot of electronics and batteries could be damaged if it is supplied with wrong voltage or current. Using the tram network as a continuous power source without any filter or converter would imply that the vehicle is subjected to a high degree of voltage fluctuations. The fluctuations in the tram network could for example occur when a tram accelerates, which results in a voltage drop, or the opposite when a tram brakes and feeds back energy to the net. High current or surge due to a short circuit or lightning strike could also affect the vehicle. Another problem that might appear when charging continuously throughout the route, is maintaining a continuous contact to the rails and overhead line, with the consequence of disturbing sparks and risk of deactivating the pantograph. This is also true to some extent for trams but would especially be the case for the more flexible bus. To assure isolation between chassis with 600 V components and the bus body, dual layer insulation or more is required. On trains and trolleybuses, all components are at least double insulated which will ensure that anyone who touches the vehicle is not exposed to an electrical shock. In practice this means that the high voltage components on the vehicle are contained within an insulation envelope surrounded by the component casing. The principal difference between today's hybrid bus and a bus that can connect to tram network is described in Figure 5 below. A voltage converter with galvanic separation on board will also be a necessary to manage and


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control the voltage levels and voltage fluctuations according to the electronic components requirement. The galvanic separation is required to isolate the bus chassis from the body and thus protect passengers and other road users in case of failure in the insulation. Filters and super capacitors need to be taken in consideration as well for this type of charging to compensate for any temporary connection losses.

~ =

AUX 600 V

+ Pantograph

+ 750V DC

0 V DC

= Chassis - Pantograph

AC/DC

= ~

DC/AC 3rd earth

ISM

ISM

~ =

AUX 600 V

= =

+ Pantograph

+ 750V DC

0 V DC

= Double insulation separating chassie

from drivetrain

- Current return

Supercap

DC/DC AC/DC

= ~

DC/AC

Figure 5: Left: Schematic diagram of a hybrid bus concept with double isolated components connected to the tram infrastructure. Right: A hybrid bus with the

appropriate power regulated at the charging station where any current leakage is also continuously measured and monitored.

Supply based energy supply to vehicle Of the approximately 60 rectifier stations in operation in Gothenburg today, there are several which have daily problems with temporary or thermal overload. The load on the 750 V grid increases heavily during spikes of up to 10 to 15 seconds when a large amount of power is required for acceleration of the heavy trams. A single tram can require up to 850 A during acceleration. The relay protection in the rectifier station for temporary overcurrent is usually between 1500 and 2000 A with an instantaneous protection usually around 2500 A. However the feeder cables become overloaded already at about 300 to 350 kW of continuous power output. A bus continuously charged with about 30 to 50 kW (about 70 % to 80 % of the route) is therefore preferable to intermittent charging of 100 kW to 200 kW at bus stops. Charge spikes of 100 kW to 200 kW will utilise too much of the available power, particularly in the feeder cables. Today there is no continuous communication between the tram infrastructure and the vehicles. By communicating a continuously monitored rectifier stations load, the vehicle’s power demand could be limited to the power capacity available. This would occasionally affect vehicle acceleration but prohibit network overload and shutdown. A tram network built based on the expected maximum current demand is with information and communication


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technology proposed to be optimised based on the total available capacity. Additional hybrid buses with increased vehicle intelligence could utilize the energy from the tram network when available, and internal energy storage when needed, without the need for infrastructure expansion. Measurement at the rectifier station could be temporarily with a current probe or with a more permanent shunt installed. Communication with each vehicle is preferably made through a common measurement server that collects all data from the rectifier stations in real time. In a small pilot study, direct communication between the rectifier station and vehicle might be desirable but real time overview visualisation of congested points or feeding from redundant rectifier stations will be more difficult to implement. The vehicles to be controlled will need a computer unit installed, which continuously receives data from the rectifier stations or from the measurement server. The communication between the vehicles and the rectifier station/measurement server is done with the appropriate wireless technologies such as 4G with prioritized traffic or guaranteed access time. Discussion In this paper we propose a novel solution for electrifying the public transport system in the city of Gothenburg to a higher degree, using already existing infrastructure. We also display what requirements that are requested to ensure a safe, functional technical operation. The benefit of using the existing tram network, also for other vehicles than trams, is that an increased number of electric vehicles can drive an increased distance while keeping the infrastructure cost low. To enable a continuous contact between the overhead line and the flexible hybrid bus, a current collector, similar to a tram pantograph but with lateral movement capabilities, is required. It is proposed to utilize information about the position of the current return arm in relation to the vehicle to be able to determining the position of the overhead line. With an inappropriate angle between overhead line and current collector head or loss of contact against the rail, the current collector shall automatically be lowered to not risk tearing down any overhead lines. Enabling a continuous conductive connection to the tram rail, that is electrically safe and without arcs, will be difficult to achieve without the several tons of contact pressure from a tram wheel axle. Dual independent current return arms are therefore suggested as a minimum to get sufficient contact. For a hybrid bus to be able to retrieve energy from the tram network, all components connected to the high voltage grid would have to be encapsulated with double insulation, which is normally the solution for public trams with metal wheels, as an additional 3rd earth


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connection for monitoring is not possible in the tram network. Retrofitting an existing Volvo hybrid bus with a voltage converter with the functionality similar to those currently implemented in the infrastructure would probably be a huge and costly intervention today. Looking ahead, there is a possibility that additional alternative solutions will be developed for both static and dynamic charging of buses in urban areas. If these solutions will demand additional safety insulation and voltage converters in the hybrid buses as mentioned above, the threshold for dynamically charging the buses from the tram network could be greatly reduced. One option for early tests is therefore to fit a trolleybus, where the vital insulation already exists, with the necessary equipment to connect to the tram network. However, these buses does not yet benefit from the economies of scale as regular hybrid buses. The Gothenburg tram network is by today's standards already peak overloaded at some locations, resulting in daily outages caused by a lack of power limitation of the trams. However, the average power consumption per rectifier station in Gothenburg, converted into the critical thermal load, is approximately 70 % of available capacity. With continuous monitoring, communication and smart control of tram’s and bus’s power utilization based on the rectifier stations momentary load, fewer outages due to overload could be achieved and the network could be utilized to a greater degree without the need of additional rectifier stations. This type of communication could be useful already in today's tram network, to reduce unnecessary stops due to overload, also without external energy supply to hybrid buses. Conclusions Preconditions and requirements for an environmentally friendly transport mode with energy supply from existing infrastructure have been presented. Technical solution has furthermore been proposed to enable safe continuous charging of a hybrid bus from the Gothenburg tram network also while driving. In the paper it is also described how to use the information about current return arm position to determine the correct current collector position towards the overhead line. Continuous monitoring and communication of the tram network load and smart control of the vehicles power utilization is furthermore proposed to utilize the existing network capacity to a higher degree. Future work A current collector and current return arm should be developed to demonstrate continuous current retrieving from the tram network. The current collector is initially proposed to mimic an existing pantograph for trams, which has a proven technology adapted to avoid damaging the tram network. The current collector is proposed to be controlled based on the current collector position against the rail and in relation to the vehicle.


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In order to increase the number of vehicles powered by the tram network, it is proposed to further examine the possibility to continuously measure, communicate and control the trams and buses power utilization based on available power at the rectifier stations. In this way, the network is expected to be utilized to a greater extent without adding additional rectifier stations and simultaneously get fewer outages due to overload. The further investigation is recommended regardless of an implementation of buses with power from the tram network.

Acknowledgements We would like to thank our partners included in the study; Volvo buses, Västtrafik and Trafikkontoret in Gothenburg. The study has been financed by Västra Götalandsregionen.

References 1. Olsson, O., Alexandersson. (2013) Hybridbussar med ström från spårvagnsnätet.

Retrieved from: https://www.viktoria.se/publications/hybridbussar-i-sparvagnsnatet Dec. 2013

2. Ellis, T., J. Levy, Y. (2008) Framework of problem-based research: a guide for novice researchers on the development of a research-worthy problem, Informing Science, vol.11, pp.17-33

3. Brydon-Miller, M. Greenwood, D. Maguire, P. (2003) Why action research?, Action Research, vol.1, pp.9-28

4. Ranch, P., Snygg, J. (2011) Utveckling av aktiv strömavtagare för tunga vägfordon, Stockholm. Retrieved from: http://www.elvag.se/blogg/wp-content/uploads/2011/12/Elvag-rapport-version-2011-12-02a.pdf Aug. 2013

5. Olsson, O. (2013) Slide-in electric road system, Conductive project report, Gothenburg, Retrieved from: https://www.viktoria.se/publications/Slide-in-ERS-Conductive-project-report-draft Jan. 2014

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