System variants for operation of trams without a catenary

Dr. Karsten Rechenberg, Markus Gaudenz Siemens AG TS GT E 1 Werner-von-Siemens-Straße 67 91052 Erlangen, Germany Phone: +49 (9131) 7-23834 / Fax: +49 (9131) 7-23362 [email protected] [email protected] URL: www.siemens.com

Keywords Hybrid vehicles, Traction, Electric vehicles

Abstract This paper deals with possible tram operations without a catenary, taking advantage of an on-board electrical energy storage. A system like this is very interesting in certain cases of operation, e.g. driving through historical city centers without a catenary, using energy stored in SuperCapacitor- modules instead. In order to compare different kinds of feasible solutions technical and investment costs have to be considered. Another objective to operate without a catenary is to increase energy efficiency in comparison to conventional trams, particularly by transferring the braking energy into the electrical energy storage of SuperCapacitors.

Motivation Many efforts were made within the last few years to minimize the power consumption of rail vehicles. The efficiency of the traction system was increased by reducing electrical and mechanical losses, the aerodynamic design of vehicles was modified in order to minimize the air resistance and many other actions were taken. One approach has not been considered thoroughly so far: Storing the kinetic energy during the time of braking in order to reduce the amount of energy needed for the next acceleration. Why was this approach outside the field of R&D interest? Because no suitable energy storage was available for the high requirements of railway vehicles. Recently SuperCaps with sufficient energy content had been developed.

BELFORT_2004.DOC/1 von 6 SuperCap or Flywheel? What kind of energy storage became reasonable within the last years for railway applications? We have to discuss the advantages and disadvantages of the electrical storage SuperCap and the mechanical storage Flywheel. It is true that the Flywheel is able to store a high amount of energy, but the power to charge and uncharge is limited (250 kW @ 5 kWh). This would lead to an unreasonable relation between the size of the energy storage and the needed rate of power. Also, the high kinetic energy means a high potential risk of violation for passengers and bystanders in case of vehicle crash. On the other hand the SuperCap is an electrical energy storage that easily can be integrated in the conventional electrical drive system. In opposite to the flywheel this system can provide the whole energy content within a few seconds. The effect of accidents or damage of the SuperCaps can be reduced by adequate means. Beyond that we are convinced that the possibilities of SuperCaps are much higher than the ones of a flywheel.

Saving energy The recent development of storage technology on the base of SuperCaps allows us today to use them efficiently as traction energy storage in trams. Therefore the total energy consumption can be reduced by storing the kinetic energy during the braking phase of the tram. The following figure shows a typical speed profile with its separate phases of a urban vehicle traveling from one stop to the next.

P [kW] F Ztraction,max F [kN] traction v v [km/h] max P max

Edrive, max

P aux

phase 1 phase 2 phase 3 phase 4 phase 5 time [s] accelerating constant speed coasting braking stop

Ebrake, max

Fig. 1: Typical speed profile of a metro vehicle

In order to get rid of the catenary, the power to accelerate has to be provided by a source or storage inside the vehicle. As indicated in figure 1 it is not sufficient to use only the energy from the phase of deceleration due to losses. The difference of the areas above and below the time-axes must be provided from an energy source. Here an on board diesel engine or a fast charging equipment at every stop of the vehicle can provide the needed amount of energy. So only the losses during a cycle must be provided, the main part will be used from the energy storage.

BELFORT_2004.DOC/2 von 6

Integration of the storage into the vehicle After having found the best type of energy storage this device has to be integrated into the traction system of the vehicle. The figure below shows a possible integration of a combustion engine and SuperCap energy storage into the a conventional drive system.

Pantograph DC 750 V Cooling

SAR

M 3 3 Auxiliaries converter 3 M DC 750-V container PWMI 3 Cooling

G 3 DM 3 M 3 3 3 M DC 750-V container PWMI 3

Fig. 2: Vehicle concept for integration of on energy storage and an independent on-board energy source into a conventional drive system

On the left side are the energy sources: the 750 V voltage from the pantograph and, in order to replace a conventional catenary system, a diesel engine. The outlined 750 V connection necessarily has not to be an ordinary catenery system, a charging unit installed at the stops might be used instead. We call this voltage the “supply voltage”. The energy storage of the SuperCaps are linked to a DC/DC converter. This converter is controlling the charging and uncharging operations of the storage. The converter is needed because the voltage of the DC link capacitor is approximately as high as the suppling voltage. On the other side of the converter the voltage of the SuperCaps depends on the energy content of the storage. This way of connecting the storage can be realized easily with the possibilities of Siemens standard traction converters (SIBAC) and ensures an efficient operation of a tram. It results that the advantage of the SuperCap as a electrical storage for the integration in comparison to the flywheel is evident.

Vehicle operation without catenary – a survey of different systems In conclusion of the previous sections we decide to use SuperCaps in storage applications. As explained above, we have to recover the losses of the propulsion system, driving resistance, potential energy and auxiliaries. The necessary energy might be taken from a combusting engine or from a charging unit at the stops. The following systems are configurations for operating without a catenary:

System 1: − Stationary charging stations with SuperCaps integrated at the stop.

BELFORT_2004.DOC/3 von 6 − Tram travels from one charging station to the next. − Mobile storage is charged during the time (about 15 sec) at the stop. System 2: − SuperCap storage is charged by an energy source that is permanent on board (diesel; in future: fuel cells)

System 3: − Combination of both systems

The systems 1 is only applicable for theoretical reviews. In practice it is not suitable, because a tram often runs within the individual traffic of cars and buses. So stops and their duration are unpredictable. In case of traffic jams the time between two charging stops might be to long to provide enough power to the auxiliary system (e.g. air condition, lights etc.). In that case the storage runs empty and it is impossible to reach the next stop for charging. To avoid this case it is necessary to carry a small energy source (exceptionally battery). The choice for a specific project depends on various parameters like number of vehicles, length of the tracks, environmental regulations, noise emission and other requirements. In simulations many possibilities were compared. An optimum was found for an example of a 16 km track (double) with typical distances between the stops. The results are shown in Table I. The system 2 is chosen that only the kinetic energy during braking is fed into the SuperCap.

Design strategy Number of Design of the Number and stationary mobile energy size of diesel annotation charging storage engines per stations (16 vehicle km/double tracked) Conventional track electrification 0 0 0 catenary

SYSTEM 1: 33 6.7 kWh 0 Pure energy storage operation

SYSTEM 2: 0 2.2 kWh 2 x 180 kW

Diesel-electric operation with energy storage without SYSTEM 3/1: 27 5.6 kWh 1 x 50 kW catenary Energy storage operation with auxiliary diesel engine and charging at the stops SYSTEM 3/2: 24 3.3 kWh 1 x 180 kW Diesel-electric operation with energy storage and charging at the stops

Table I: Values for storages, the charging units and the energy source of different systems

The following diagram presents the results of simulations on the base of the four systems. The most important value is the energy consumption. It can be seen easily that a reduction of more than 20% in comparison to an already optimistic conventional system with catenary (30% of the braking energy

BELFORT_2004.DOC/4 von 6 can be fed into the grid during braking) can be reached. The higher the rating of the diesel engine the lower the value of the energy, that has to be stored in the SuperCaps. So the optimum in combining system 1 and system 2 strongly depends on the current prices of the SuperCaps, energy costs and the aleady above mentioned operating datas. The difference in energy consumption between the systems can be explained by having a look at the losses of the system. Due to the high currents during the short charging period at the stops we have a lower efficiency in comparison to the other systems. In system 2 the size of the energy storage is calculated on the base of the needed energy for acceleration. Therefore it is not able to store the whole kinetic energy of the vehicle. The excessive energy has to be converted in thermical losses in the braking resistor. The variants of system 3 overcome these disadvantages by combining the advantages of system 1 and 2. In result this combination leads to a higher efficiency and less losses.

360

180

0 100 0

0 50 86,2 0 25,2 65,3 55,96 55,32 53,79 "diesel power [kW]" 50,65 0 "energy ratio of diesel [%]" 6,73 2 5,58 "energy consumption [kWh]" conventional 3,3 system 1 "usable storage energy content [kWh]" system 2 system 3/1 system 3/2

Fig. 3: Simulation results of the various systems

Costs of the system A comparison (without LCC) presents figure 4. The lowest costs can be archieved with system 2. Here no fast charging units are required. The costs also do not depend on the length of tracks, because the whole equipment needed to guarantee a cruise without catenery is emplemented in the vehicle.

BELFORT_2004.DOC/5 von 6 400 system 1 system 2 system 3/1 system 3/2 300

200 ve costs [%] ti a rel

100

0 0 5 10 15 20 25 30 35 40 railway line (double tracked) [km]

Fig. 4: Comparison of relative investment costs of the systems depending on track length (each with 20 vehicles)

The disadvantage of a combustion engine (exhaust and noise) makes the second variant of system 3 interesting. In this design the size of the engine and therefore its exhaust and noise could be reduced. On the other hand costs increase due to the necessary charging units along the track. If a diesel engine is only accepted for exceptional operation the investment costs are the highest. As the prices of SuperCaps are expected to fall significantly within the next years, all three presented systems are attractive in comparison to the conventional system with catenary. To implement a SuperCap storage (up to about 3..4 kWh) is yet an economical promising solution. In few years many rail vehicles are supposed to have SuperCap energy storages.

Summary The results above show that the use of SuperCaps in trams is technically possible and economically reasonable. Especially the energy consumption decreases significantly in comparison to conventional systems. In the future energy storage become more and more attractive because the prices of SuperCaps are expected to fall continuously within the next years.

BELFORT_2004.DOC/6 von 6