A Solution for Increasing the Efficiency of Diesel – Electric Locomotives with Supercapacitive Energy Storage
B. Destraz, P. Barrade, A. Rufer Laboratory of Industrial Electronics Swiss Federal Institute of Technology Lausanne CH – 1015 Lausanne EPFL Tel : +41 21 693 36 56 Fax : +41 21 693 26 00 [email protected] http://leiwww.epfl.ch/
1 – Acknowledgements Acknowledgements to Stadler Rail for providing data and for their cooperation.
2 – Keywords Supercapacitors, Diesel motors, locomotives, braking power, energetic losses, power assistance, efficiency, costs
3 – Abstract Diesel - electric traction is a well established technology in railways systems, mainly for lines with a low traffic potential. In those conditions, a diesel powered locomotive is chosen because the infrastructure costs are lower in comparison to a standard electric train. The main inconvenience of that technology is the primary energy source: oil resources are not infinite, prices are difficult to forecast and CO2 production increases global warming.
It’s therefore important to develop new strategies to increase the energy efficiency of diesel – electric trains. To reach that goal, a system with supercapacitive energy storage will be proposed in this paper. The aspects of production and exploitation costs are going to be presented in more details. The proposed solution will reduce diesel consumption and therefore also CO2 and other pollutant emissions while being economically viable.
4 – Introduction Supercapacitors are new components that can be used for short-duration energy storage [1]. The advantages of these components are a combination of those of batteries and conventional capacitors at the same time. They will find their place in a large range of industrial applications that need highly efficient energy storage system, as for example in the field of transportation. The power density (W/kg) is similar for classical capacitor and supercondensator, but the stored energy density (Wh/kg) is much higher for supercapacitors. The currently available supercapcitors are up to 2600 Farads (Maxwell Technologies – Switzerland). Their volume is 0.42 liters and their weight is 525 grams. In comparison to standard batteries, the energy density of supercapacitors is lower by an average factor of 10. However, their energy density is compatible with a large range of power applications that need high instantaneous power during short periods of time. The above characteristics of power demand are typically found in transportation systems. Another advantage in the use of supercapacitors rather than batteries is their life time. Other solutions for storing electrical energy (chemical, mechanical, etc.) are explained in [1] and in [2]. The aim of this paper is to present how supercapacitive storage can be used for increasing the energy efficiency in a diesel-electric railway system. Two different solutions will be presented to reach that goal : • Recuperation of braking energy • Changes in the diesel engine control
5 - Principle of a hybrid vehicle The basic principle of a hybrid vehicle with an energy recuperation system [3] is summarized in Fig. 1. That kind of vehicle contains at least one primary energy source such as a diesel, gas or other motor. These primary energy sources are connected to one or more generators, where mechanical energy is converted in electrical energy. It is then transmitted to the wheel motors to provide the needed traction power. All energy flows are controlled by dedicated circuits and recuperation of braking energy is therefore possible in that system. In comparison to classical diesel - electric vehicles, no such storage system exists and braking energy must be thermally dissipated.
In the case of the diesel – electric train proposed in that work, the type of primary motor (i.e. diesel generator) is unchanged, but will be associated to an electrical energy storage system to have the possibility to recuperate the braking energy..
Figure 1. Principle of a hybrid vehicle
6 – Typical itinerary In order to compare a standard diesel - electric train to the proposed solution and to dimension the main elements, a typical itinerary is going to be defined. The results presented will only be valid for a chosen itinerary but the same method can be applied to analyze any other railway lines.
Diesel - electric train propulsion is mainly chosen for peripheral areas transport, where the number of passengers is limited. A typical application would be in mountain areas where the absence of catenaries considerably limits the ”visual” impact of the line. An itinerary on the Merano - Malles railway line has been retained for that study. It’s situated in northern Italy, in a mountain area. In those conditions, energy constraints on the diesel motors are high (in comparison to a flatlands line) due to the high acceleration power needed on inclines and also to the high braking power during descents. The altitude curve of the typical itinerary (altitude versus time) is given in Fig. 2.
Altitude variation for a typical itinerary 1800
1700
1600
1500
1400
1300 Altitude (m) Altitude
1200
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1000 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 Time (s) Figure 2. Altitude variation for a typical itinerary That itinerary corresponds to a line where standard diesel - electric trains are used. The chosen trains are GTW systems built by Stadler Rail AG, Switzerland, whose main characteristics are: - Total weight (w/o load): 67 t - Total weight (fully loaded): 84 t - Diesel engine power: 2 x 380 kW - Max. power at the wheels: 620 kW - Max. speed: 140 km/h
Fig. 3 represents the GTW system that is planned to be built for the Merano - Malles line. It consists of two passengers coaches linked in the middle by a traction module. The solution for improving the locomotive overall efficiency must have the same dynamical characteristics (acceleration, maximal speed, ...) as the actual projected train.
Figure 3. GTW Train
7 – Standard GTW Train The energy flows in the actual GTW train are presented in Fig. 4 where all energy losses are represented with respect to where they occur. No percentage scales are given as the diagram is of qualitative nature. The primary energy source (the diesel motor in that case) is represented at the top. The energy (in - out) of the traction motors is represented at the bottom.
Mechanical energy provided by the diesel motor
Mechanical losses
Losses in the AC-DC converter
Losses in braking resistors Losses in the DC level Auxiliary equipments needs
Losses in the DC-DC converter Losses in the DC-AC converter
Energie moteurs de traction
Figure 4. Energy transfer in the GTW diesel - electric locomotive
8 – Proposed solution for increasing the overall energy efficiency The principle of a diesel - electric locomotive is given in Fig. 5. The principal energy loss is due to the dissipation of braking energy in the dedicated rooftop braking resistors. In a standard system, there is no energy buffer between the diesel generator and the traction motors. Therefore, the power corves of both motors are identical (with the disadvantage of a lower efficiency compared to a diesel motor directly driving the wheels) during positive power conditions. During braking phases, the diesel engine will be idling while the traction motors feed the braking energy to the braking resistors.
In order to improve the overall efficiency of a diesel - electric train, two different solutions will be presented:
Figure 5. Schematic of GTW power conversion chain
Figure 6. Schematic of the improved GTW power conversion chain with supercapacitors
A. Recuperation of Braking Energy The braking system in the GTW train is composed of an electrodynamic brake and a mechanical brake. The principle of electrodynamic brakes is to transform the braking energy into electrical energy that is, in standard trains, dissipated in dedicated braking resistors. For the proposed solution, all the braking energy will be transformed in electrical energy that will be recuperated with a suitable system.
As energy buffer, the chosen component for our application is the supercapacitor. This choice is motivated by the high power constraints during acceleration phases. Another reason lies in the much higher life time compared to a standard battery solution (the number of charge – discharge cyle is high). The proposed solution is given in the Fig. 6. In reality, it is not possible to remove the braking chopper and resistances from the train. This is due to security reasons. Braking power must be assured in all conditions - even if the (finite sized) supercapacitors are fully charged and cannot absorb any more energy. B. Changing Diesel Motor Control As described in paragraph A, a storage system is added in the locomotive. The added storage element does not only provide an energy buffer for storing braking energy, but it also allows to decouple diesel electricity generation from traction power requirements. In other words, two different energy sources can be used to provide the needed traction power: - The diesel generator - The supercapacitive storage bank
Each diesel motor has its own maximum efficiency point where the emissions are generally also at their lowest value. To increase the efficiency of the train, the control of the diesel engine is modified to either run the engine at its maximum efficiency or switch it off altogether. The mean traction power is therefore provided by the diesel motor and all variations around that mean power will be absorbed by the supercapacitive tank [6]. Fig 13 represent the calculation of the corresponding mean power for the chosen itinerary. Instantaneous Wheel power for typical itinerary Mean 1000
500
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Power (kW) Power -1000
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-2000 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 Time (s)
Figure 7. Mean power delivered by the diesel motor
Provided that there is enough available on-board energy storage, a 45 kW diesel motor is sufficient for powering the train without any change in the dynamic characteristics (speed, acceleration, ...) of a standard GTW train with two diesel generators of 380 kW each on the typical itinerary.
It should be seen that the use of a 45 kW diesel generator is an optimal choice for energy efficiency considerations. The trade-off however lies in the large number of required supercapacitors to provide sufficient energy storage. Because of the large number of supercapacitors required, such a solution is not economically viable. In order to find an optimal overall solution, other factors than energy efficiency have to be added in the model: cost, volume and weight have been added as optimization criteria.
9 – Sizing the supercapacitve bank The supercapacitor chosen for that application is BCAP0010 form Maxwell Technologies, Switzerland. Nominal capacitance is 2 600 Farads and the maximum voltage is 2.5 Volts. A discharge ratio of 0.5 has been retained for that application [7]; the higher the discharge ratio, the more energy can be stored in each cell, but the more the lifetime will be reduced.
Downsizing the maximal power of the diesel motor 700
600
500
400
300
200 Diesel motor motor needed power Diesel (kW)
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0 0 100 200 300 400 500 600 Stocked energy in the supercapcitive tank (MJ)
Figure 8. Reduction of required diesel generator power as a function of stored energy
An optimization routine has been developed [8] where the diesel generator is either working at its maximum efficiency point or is stopped completely. In that hypothesis, the needed power energy is directly provided by the supercapacitor tank and is decoupled from the diesel motor. The minimum required storage element is calculated as a function of available diesel generator power (working at its maximum efficiency). The results are given in Fig. 8. Note the exponential nature of the presented graph: the use of a low capacity storage element allow to reduce the diesel motor by a large factor. The corresponding number of supercapacitors (2 600 Farads) is given in Tab. II. In that conversion, the efficiency of the charge - discharge phase, the type of charging process (constant power or constant current) has been taken into account. The number of supercapacitors varies from 0 to 100 000.
As stated before, the ecologically best solution is found for the case of 100 000 supercapacitors. However, an optimal solution cannot be found without also accounting for economical criteria - i.e. finding a compromise between costs and efficiency/emissions considerations.
Disel motor Number of Volume Weight power (kW) supercapacitors (l) (kg) 45 100878 42369 52961 75 83772 35185 43981 100 81141 34079 42599 150 62574 26574 32851 200 44561 18716 23395 250 28919 12146 15183 300 17457 7332 9165 350 8202 3445 4306 400 5726 2405 3006 450 4457 1872 2340 500 3246 1365 1704 550 2410 1012 1265 600 976 410 513 640 51 22 27 680 0 0 0
Table II : Number of supercapacitors as a function of diesel generator power
10 – Simulations and results
To validate the different concepts explained in this paper, a complete train with supercapacitive storage has been simulated. For the chosen itinerary and train, Stadler Rail has calculated the required propulsion power which was chosen as a basis for our simulation work.
The complete energy conversion chain has been modeled and all control elements (energy flows between the energy sources - motors) have been included in the simulations. In order to compare the proposed solution to a standard GTW train, simulations were always run in both configurations. Detailed simulation results are given in [8]. Tab. III summarizes the results of fuel consumption reductions with respect to the standard GTW train for three different diesel generator sizes.
In order to determine the economically most viable solution, an assessment of lifetime cost (construction, exploitation) has been made for the different cases shown in Tab. III where the lifetime of a train is supposed to be 25 years.
Diesel motor power Reduction of fuel (kW) consumption (%) 760 0 470 29 380 44 300 47
Table III :Reduction of fuel consumption The economically best solution is found to be a 380 kW diesel motor. It corresponds to the half of the generating power currently installed in the GTW train and leads to a reduction in fuel consumption of 44 % (with no change in the train’s dynamic characteristics). Even though the cost of the supercapacitors are actually high, a diesel - electric train with supercapacitive energy storage proves profitable after 10 years of exploitation. With the announced further reduction of supercapacitor size and costs in the near future, the proposed solution will become even more interesting.
11 – Conclusion A novel solution to increase the energy efficiency of a diesel – electric train without impairing on its dynamic characteristics have been presented in that paper. To reach that goal, two different concept have been introduced : - Recuperation of braking energy - Changing diesel motor control.
For both solutions, an energy storage system must be added to the locomotive. For that application, supercapacitors have been chosen to act as energy buffer. Braking energy can therefore be recuperated in the locomotive and allows to decouple the diesel motor from the traction motors. The control of the diesel motor has been modified to run it either at its best efficiency or stop it altogether.
Different solutions for reducing the size of the diesel generator have been proposed and the best compromise between the reduction of fuel consumption and the supercapacitors’ cost is found with a storage bank composed of 15 000 supercapacitors (6.3 m3, 7.8 tonnes). The fuel reduction by 44 % is found in comparison to a standard solution without impairing on its dynamical characteristics (acceleration, maximal speed,…). Additionally, it was shown that despite the higher initial investment the solution proves cheaper than a traditional diesel - electrictrain after 10 years of operation.
The presented calculations are valid for the chosen itinerary, but the same method can be applied to any other railway line. An extension of the shown method can also be used for any other type of vehicle (like trams, trucks or cars).
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