The use of batt ery-powered electric commuter trains instead of those which are propelled by internal combustion engines on only partially (or non) electrified railway lines can save considerable amounts of energy and can avoid exhaust emissi ons to a large extent with comparatively low investment costs. As path to these goals, the railway line , the operating and the trains are modeled for a particularly in teresting, real-life example. Then, for the generated models, it is shown how the energy demands of model -compliant (bat tery) electric commuter train s can be calculated and how their energy storage s can be designed. The models and the procedure can also be applied for a large number of comparable dimensioning cases when defining fu rther railway lines , operating and commuter train models, corresponding parameterization as well as the occasion ally necessary addition of slope and curvature resistances. T he calculation of energy demands and the design of energy storages , however, show that there are a large number of different alternatives for technical specifications as well as decisions relevant to energy eff iciency and business management, too. To determine the mo st suitable solutions , further measures such as the investig ation of the eff ects of intelligent modifications to the underlying models, the development and implementation of computer -aided simulations as well as the construction and testing of real (battery ) electric commuter trains make sense. Keywor ds: Battery, electric, battery electric, train, commuter train, recuperation, energy, Rh ine-Waal University, Clev es, Nijmegen, energy demand, energy storage, electrification, railway, rail road, railway line, railway track, battery electric multiple unit, BEMU 2 Schmetz, Struck 1 Introduction In discussions about a future, more sustainable energy supply, measures to increase energy efficiency do not always receive adequate attention. Energy, which is not needed at all, is the most environmentally friendly energy and, with a predicted share of two- thirds, almost the key to reach the German Federal Government's targets "Halving primary energy demand" and "Reducing electrici ty demand by one quarter" by 2050, which were defined in 2010. If the highly probable transition to e -mobility on the road by 2050 is excluded , which will have a favorable effect on primary energy demand as a result of improved conversion factors, and only moderate increases in demand are implied , the aforementioned electricity savi ngs target could already be achieved alone by the m easures listed in Table 1 . Table 1: Saving Potential s for Reducing Electric Power Demand by a Quarter Current Demand Saving Potential Archievable Savings Remaining Demand 591 TWh1 Energy -efficient Electric Motors (by 2020)2 27 TWh 564 TWh 564 TWh Energy -efficient Electric Motors (beyond 2020)3 11 TWh 553 TWh 553 TWh Complete Transition to LED -Lightni ng4 ~60 TWh 493 TWh 493 TWh Enforcement of a Power Factor of 0,953 48 TWh 445 TWh 445 TWh Use of High Voltage DC Transmission (HVDC )4 ~20 TWh 425 TWh 425 TWh Speed and Load Control of Drives ??? ??? Optimis ation of Power Semiconductors ??? ??? Use of Superconductors ??? ??? Further Saving Potentials, e.g . Energy -efficient Household Appliances ??? 1gross electricity demand (useable energy) 2014 according to the annual report of the German Arbeitsgemeinschaft Energiebilanzen e.V. 2according to EU directive 640/2009 3according to the German Zentralverband der Elektroindustrie (ZVEI) 4according to own extrapol ation (DC: direct current) 3 Schmetz, Struck From powertrain engineering, however, it is addit ionally known that the energy demand can often be significantly reduced by reducing friction and moving masses and, if possible, recuperating the energy required for unavoidable acceleration and lifting operations of the remaining (large) masses. In the transport sector, especiall y city buse s, trams and commuter trains have a great potential for recuperation of kinetic (but also potential) energy due to their typical, strongly intermittent load profiles. Th is is the reason why some public transport companies already use (battery) elect ric city buses, which can (sometimes additionally) be supplied from occasionally exi sting direct current (DC) electricity grids. Likewise, some (battery) electric trams with the same property are already being tested [1]. In contrast, for railway applications only trials o f diesel -electric hybrid commuter trains are known so far [2]. W ith regard to the currently fiercely focused emissions of internal combustion engines and the finite nature of fossil fuels, however, diesel -electric hybrid commuter trains do not repre sent a sustainable development goal anymore . In view of the increasing number of passengers and for protection of the environment, organizations such as the German Verkehrsverbund Rhein -Ruhr (VRR) are rather striving for the further expansion of the electrified railway network. Thus in its curre nt plan for regional transport [3], the VRR calls repeatingly for the electrification of the 65 km long, non- electrified section of the left Rhine railway line (Nijmegen/NL -) Cleves -Geldern -Krefeld ( -Dusse ldorf/Cologne) . This situation, the meanwhile usual tender for the operation of regional railway lines in rather smaller lots for periods of fifteen years, for wh ich often own, specific trains are procured, as well as a number of other particularly typical features which simultaneously simplify the necessary calculations, are motivation according to [4] and [5] to investigate for this railway line once the use of (battery ) electric commuter trains ( BECTs ) and carry out a design of the required battery storage s. 2 Present Situation and Modeling 2.1 Present Situation Meanwhile, the left Rhine railway belongs with its northern section Cleves -Geldern - Krefeld (the former connection from Nijmegen/NL to Cleves was cut in 1991) and the section Krefeld -Dusseldorf to the diesel network Niederrhein/Ruhr/Münste rland (also called Niers/Rhine/Emscher -network) according to the left part of Figure 1 , which contains yet some more strong frequented, but only partially electrified railway lines . 4 Schmetz, Struck Figure 1: Diesel -network Niederrhein/Ruhr/M ünsterland with the northern section of the left Rhine railway a nd a diesel commuter t rain (DCT) The operation of this diesel network is awarded since December 2 009 until November 2024 to the German railway company NordWestBahn GmbH based in Osnabrück (meanwhile except the railway line Dorsten -Herne -Dortmund, which is again operated by German r ailway company Deu tsche Bahn AG since 2014) . Espe cially on the railway line Cleves -Geldern -Krefeld -Dusseldorf quite high train and passenger kilometers are provided despite a s eries of infrastructure, train reliability and organizational problems. In addition, not least due to the founding of the Rhe in-Waal University of Applied Sciences in Cleves, this railway line has experienced a strong increase in passenger numbers and train travels (compression of the former hourly rythm to a half -hourly rythm on workdays since the annual timetable of December 2009, additional train travels in the morning hours since the annual timetable of December 2014 - despite strong capacity constraints due to the 35 km long missing second track in the se ction Cleves -Geldern and train shortage, additional late train tr avels on weekends and 24/7- operation at major events since the annual timetable of December 2016) . In regular service, the railway line is operated exclusively b y Alstom LINT 41 diesel commuter trains (DCTs), which op erate on average across all travel s in double traction (as shown in the lower part of Figure 1). Nevertheless, and despite additional increases to triple traction during peak hours standing passengers (even in the first class) are not the 5 Schmetz, Struck exception but more likely in the southern part of the non elec trified section of this railway line because of completely occupied seats . Therefore, s everal local authorities and political parties vehemently demand (in the meantime) the complete electrification (and the reconstruct ion of the second track in the sectio n Cleves -Geldern ). According to the German newspaper Rheinische Post of March 7, 2016, the netw ork branch of the German railway company Deutsche Bahn AG estimates the cost for the electrification and reconstruc tion of the second track to about EUR 500 million [6] . Upon request in June 2016, about EUR 155 million from this money are needed for the (double -track) electrification . This figure, consi sting of costs of EUR 2 million/ km (double -track) and additional cos ts of EU R 25 million for two transformer stations including traction power lines, is in principle also confirmed by the spokesman of the German passenger association Pro Bahn [6]. It also roughly corresponds to the cost of EUR 12 billion proposed by the German company Siemens AG in promoting its eHighway concept in 2012 for the electrification of a highway backbone network with a length of 6000 km, but including the direct DC converter station s for feeding. In contrast, only about EUR 1.25 million/km (in the case of obviously existing transformer stations) are necessary for the fully projected completion of the elect rification of the double track city line S 28 from Kaarst to Mettmann according to the standard Re 100, which is actually under construction [7]. Even with the use of this lower figure, the cost of electrification of the part Cleves -Geldern -Krefeld would still be in the order of EUR 100 million, a sum which, despite the rather dense traffi c of light passenger trains, cannot be justified with regard to the actual cost of fuel and the retu rn on investment, since the cancellation of freight and heavy passeng er trains two decades ago . In addition, there would be the need to carry out a so -called planning approval procedure, in which, according to experience, many objections can arise, which elimination can lead to further, substantial cost increases . 2.2 Modelin g Model of the Railway Line On the ra ilway line C leves-Geldern -Krefeld -Dusseldorf, including the start and end stations, there are thirteen stations in the distances according to the table in the right part of Figure 1. All trains stop at all stations. Since there are no significant topographical obstacles on the left lower Rhine, des pite the fact that the Rhine -Maas watershed has to be crossed twice, the railway line can be considered as s traight forward and level, so that no slope and curvature resistances must be taken into account in the following 6 Schmetz, Struck calculations. Although the railway line is designed for a maximum spe ed of 140 km/ h in the examined section, it is driven in normal operation only with a top speed of 120 km/h. For the sake of simplicity and estimation to the safe side, it is assumed for all travels between the stops that all trains accelerate as quickly as possible to the maximum speed of 120 km/h, which then is maintained as long as possible, before the trains brake down with a deceleration of -1.13 m/s². In reality, however, there are some shorter sections with speed restrictions, in particula r the use of the sidings in case of the regular train encounters at the stations Bed burg-Hau and Weeze as well as the direction tracks in the stations Kevelaer and Krefeld CS. Operating Mode l According to the December 2014 timetable published on http://www.bahn.de/kursbuch, and the offsetting of the reduced service on some public holidays with the additional morning partial rides, and the added weekend travels since the annual timetable of December 2016 there take place an average of 16.5 train travels per day exclusively on Mondays to Friday s, another train travel on Mondays to Saturdays and another seventeen trains travel s daily, all for both directions . This results in an operating model with 10790 train travel s per year and direction or a total of 21580 train travels of 92 km each per year. Therefore, almost 0.6 million train -kilometers out of the almost 2 million train-kilometers per year are done under the catenary and about 1.4 million train- kilometers take plac e on the non- electrified part of the line . Train Models Based on the technical data of the Alstom diesel trains LINT 41 used in regular operation, double tractions of two singl e units according to Figure 2 are defined here as train model s both for propulsion by internal combustion engines and for (battery) electric propulsion. These two train models are referr ed to DCT in case of propulsion by internal combustion engine s and to (Battery) Electric Commuter Train (BECT) in case of propulsion by (battery) electric drives . The BECT, for which the design of the energ y storage s is to be investigated here, should be an as much as possible equal built, virtual twin of the DCT, but with drive systems per single train unit according to Figure 3 consisting of one high voltage part, one cont rolled rectifier, one common DC -rail, four inverter d rives distributed to the four drive n axles and one or more energy storages instead of the two internal combustion engine s, the two gear transmissions and the two fuel tanks . This BECT, which consists of two single train units, is dimensi oned to provide the same performance as the DCT . Beyond these settings , its further specification takes place step by step in the sections 3 and 4. 7 Schmetz, Struck Figure 2: Technical Data of the Single Train Unit s of the Train Models Figure 3: Drive Scheme of a Single (Battery ) Electric Train Unit 3 Calculation of Energy Demands The calculation of energy demand s is based on the DCT. For this purpose, the driving resistance to be overcome must be determined first. In principle, the driving resistance is composed of the components rolling resistance, slope resistance, acceleration 8 Schmetz, Struck resistance and aerodynamic drag [8 ], if the breakaway resistance is neglected, which only occurs for a very short moment at start . In case of railways, the resistance when passing through track curves must also be taken into account. But here it can be excluded as well as the slope resistanc e due to the above -defined railway line model . This results in the formula (1) f or the driving resistance given in the upper left part of Figure 4 . With the data of the DCT defined above and excluding the acceleration resistance a ccording to formula (2), the speed-dependent driving resistance can be calculated, which is represented by the lower graph in the right part of this figure . Figure 4: Driving Resistances as a Functiont of Speed Because the formula (1) for the calcu lation of the driving resistanc e is rather complicated in praxis, in Ge rmany there are additionally Strahl's formulas for the calculation of the driving resistance of freight trains and the also quite complicated Sauthoff formula for the calculation of the driving resistance of passenger trains (formula (3) in the lower left part of figure 4), valid for calculations at constant (final) speeds [8]. In the Sauthoff formula, the rolling resist ance and the aerodynamic drag are combined by reference to the weight force to a speed -dependent driving resistance rL, 9 Schmetz, Struck which then only has to be multiplied by the mass of a train to obtain its overall driving resistance at a constant speed . Since the Sauthoff formula (3) considers in addition to the components due to the difference of the form ulas (1) and (2) also the increase of the rolling resistance coefficient due to the type of wheel bearings and the current speed, the effects of the length of a train, its induced aerodynamic undertow and a mean headwind on the aerodynamic drag, it deliver s a stronger speed -dependent graph for the driving resistance according to the upper graph in Figure 4 than the difference of the formulas (1) and (2 ). Therefore, for the purpose of an estimation to the safe side, here the Sauthoff formula is used f or the determination of the energy demand s for overcoming the driving resistances . The results according to S authoff’s formula, however, must be increased by the acceleration resistances due to formula (2), which have to be overcome after each stop for the accelaration from standstill to the maximum travel speed of 120 km/h . By analogy with the five -step transmissions of type Ecomat 5HP902 made by the German company Zahnradfabrik Friedrichshafen AG (ZF) which are actually installed in the LINT 41 trains and assumi ng a suitably matched combustion engine characteristic, it can be assumed for the DCT according to Figure 2, that the accelerations in the five steps are sectionally constant . Similar to normal operation, this leads to distances of 300 0 m for each acceleration process after depature and after each stop as well as to the respective distances that can be traveled at the maximum speed of 120 km/h, and to the related energy demands according to Table 2 . In contrast, the braking distances before each stop take o nly 500 m, and during each of the related braking phas es, another 0.9 kWh of energy are needed to overcome the driving resistance. These energy demands, however, support the necessary withdrawal of kinetic energy during the braking phas es. But they lead to a reduction in the recuperable energy, too. In addition, the idling demands during standstill and braking phases must be taken into account for the DCT, during which electrical drive systems can largely be switched off or used for recuperation. Under the usual assumption that 10% of the indicated power is needed to overcome the internal friction in the four internal combustion engines of the DCT with an effective power of 315 kW, thus 35 kW each, these energy demand s are at a total of 2.3 kWh per minute idle. Furthermore, it has to be taken into account, that for proper operation the internal combustion engines must be run in each case at least five minutes before departure from the starting station and after arrival at the destination 10 Schmetz, Struck station, and during t he standstill and braking phases within the travel distances. Therefor e, the related idle demand s are listed in Table 2, too. Table 2: Energy Demands for a T ravel from C leves to Du sseldorf in kWh km Station Driving1 Acceleration Braking Idling2 0 Cleves 0 0 0 11,5 4 Bedburg -Hau 7,0 24,5 0,9 12,65 13 Goch 21,3 24,5 0,9 3,45 20 Weeze 15,6 24,5 0,9 5,75 26 Kevelaer 12,7 24,5 0,9 3,45 35 Geldern 21,3 24,5 0,9 3,45 42 Nieukerk 15,6 24,5 0,9 3,45 46 Aldekerk 7,0 24,5 0,9 3,45 53 Kempen 15,6 24,5 0,9 3,45 65 Krefeld CS 29,8 24,5 0,9 5,75 68 Krefeld -Oppum 4,9 24,5 0,9 3,45 75 Meerbusch 21,3 24,5 0,9 3,45 92 Dusseldorf CS 44,0 24,5 0,9 12,65 Σ 216,1 kWh 294,0 kWh 10,8 kWh 75,9 kWh 1Energy demands to overcome the driving resistances during the acceleration and constant spe ed phases (without energy demands for the accelerations ) 2Energy demands during s tandstill and brake phases These energy demands for idling have to be increased by the requirement s of the auxiliary drives yet . In case of the DCT, these requirements are taken into account w ith an average power demand of 50 kW for driving of generators for the on- board electrical system, compressors for the air brake systems, coolant pumps and fans f or the waste heat of the internal combustion engines, and the lubrication of the gear transmission s. Another average power demand of 25 kW (corresponding to 50 kW only during the warmer half of the year) is added to drive the air conditioning compressors . Under the above operating conditions, this results in an additional energy demand of 125 kWh per train travel (corresponding to 1.25 kWh/ min) for the auxiliary drives . The same demand for the auxiliary drives is also assumed for the BECT. In the first instance, this may be surprising, since only for the DCT can be assumed that all the heat required for heating during the colder half of the year can be obtained from the c ooling circuits of the internal combustion engines, while in the BECT even in case of use of 11 Schmetz, Struck liquid -cooled electric motors, inverters and batter y storages including the ir management systems on ly the half of the heat is available, and therefore an additiona l electrically heating is necessary . But these assumptions are permissible with regard to the more efficient supply of energy to the on- board electrical system, the omission of the pumps for the lubrication of t he gear transmissions and th e same average po wer demand for driving the compressors of the air conditioning systems . Not listed in Table 2 is the energy that could theoretically be recuperated when braking a (modified) DCT, for example by means of flywheel energy storages or, in case of the BECT, by restoring it to the batteries. It results from reducing the kinetic energy of 24.5 kWh s upplied to the train models during each acceleration phase by the energy of 0.9 kWh required during each braking operation to overcome the driving resistance. T hus it is theoretically 23.6 kWh during each braking process or 283.2 kWh for a full train travel . But, since no recuperation is provided in the technical design of the DCT selected here, for the DCT it is not be examined further at this point . With the energy demands for the single operating phases determined in this way, a total energy demand of 838.5 kWh, calculated as shown in Figure 5 , is obtained, corresponding to 201.4 l iters of diesel fuel per train travel or 4.35 million liters of diesel fuel corresponding to 210 TJ of primar y energy per year for operation with DCTs versus 436.9 kWh per train travel respectively 9.43 GWh corresponding to 85 TJ primary energy per year for operation with BECTs . In addition, the maximum savings potential can be quantified t o 125 TJ of primary energy per year or 2.08 million liters of diesel fuel equivalents while avoiding the undesirable exhaust emissions . At the same time, the energy demand per train travel of a BECT represents the starting point for the electrotechnical design of its energy storage s in the next section . 4 Electrotechnical Design of the Energy Storages Determination of the capacity From Table 2 and Figure 5 can be calculated that the BECT requires for a travel on the considered railway line from the charging p oint in the starting station to reach the electrified part of the railway line model a total energy of 298.4 kWh. This amount of energy must be available in the batteries of the BECT at the begin of such a travel . In fact, the batteries must have a much higher nominal capacity as they lose capacity by aging, and should not be charged to their upper capacity limit or deeply discharged during normal operation . In addition, the energy demand of the battery management system must be taken into account, while here for simplification the rather low 12 Schmetz, Struck discharge losses are added to the charging losses . These facts are considered by surcharges of 25% (corresponding to 74.6 kWh) for a 20% capacity loss during the service life of the batteries and a further 53.9% surch arge (equivalent to 201 kWh) for duty cycle operation with a limit of 90% capacity for charging and a limit of 25% capacity for discharging . It is assumed tha t, with a remaining capacity of 25% in normal operation, there is also a sufficient reserve for an y emergencies. This means that the BECT must have a total nominal battery capacity of 574 kWh. Figure 5: Total Energy Demands and Savings Potential Configuration of the Energy Stora ges Before continuing the electrotechnical design of the energy storages , the electric drive system of the BECT must be specified more precisely. For this purpose, the requirements for the largely identical design s of the DCT and the BECT and same performances have to be taken into account. In addition, the efficiencies listed in Figure 13 Schmetz, Struck 5 and some common, but here not further explained rules for the dimensioning of electric drives must be considered. Then, for example, eight 500 V three -phase asynchronous traction motors with 100 kW continuous power each are sufficient to drive the eight axles of a BECT . Furthermore, there are eight at least two -fold overloadable traction inverters required which are highly suitable for operation in the field weakenin g range, and which require a DC -link voltage of at least 710 V on the input side under full load and under these conditions . Overall, this results in eight single electric drives , which are connected in each single train unit on the input s ide expediently via a common DC -rail and a common, controlled rectifier with a high voltage par t as shown in Figure 3, and regarding their control via a common data bus . Any opportunities for simplification of this basic stru cture should not be dicussed at this point . Furthermore, it is assumed that the batteries are connected via their own DC /DC- converters to the common DC -rail, with which the charge and discharge of the batteries can be controlled in both the battery electric and the pure electric operation . Any potenti al measures to save these DC/DC -converters should not be examined here either . With regard to the number of batteries, and for reasons of a good mass distribution and low line losses, it is assumed that four batteries with a capacity of 143.5 kWh each are provide d in the vicinity of the four pairs of axle drives of the BECT. These fo ur batteries, for example, can be installed at the locations of the internal combustion engines and gear transmissions of today's DCTs . Due to the relevant occupational saftey regulations like DGUV -I 8686, which apply in Germany, the four batteries should be designed as packs of fifteen stacks of 48 VDC each in series and with a sufficient number of stacks in parallel . Such battery stacks are usually constructed from single lithium- ion or lithium -iron phosphate battery cells . The characteristics and technical data of two suitable battery cells are shown in Figure 6 . Here lithium iron phosphate battery cel ls are chosen to maufacture the battery stacks because of their larger working temperature range, their suitability for higher currents, their nearly constant discharge voltage and their not shown, but commonly higher cycle stability, in spite of their lower capacity . Due to the increasingly large -scale production of high- performance energy storages for electric vehicles, there are now also other, fairly quic kly available, sufficiently safe and cost -effective alternatives [9], which require adequate attention , too. A favorable structure of the single battery stacks results, for example, from the arrangement of 240 battery cells of type YT26650FP according to f igure 6 in 16S15P configuration (thus sixteen battery cells in series and fifteen in parallel each), which, for example, can be monitored with the aid of chips like Infine on-Aurix of type SAK- TC 14 Schmetz, Struck 277 T . Then, a battery stack has an open circuit voltage of 51.2 VDC, a discharge current of 300 A at a voltage of 48 VDC and a capacity of 1.92 kWh. To obtain one single battery pack with a capacity of 143.5 kWh, fifteen of these battery stacks must be connected in series and each five in parallel, so that there ar e totals of 72,000 battery cells and 6000 management chips needed for the four required battery packs . In view of these figures, it becomes evident that in the real construction of the battery packs, additional effort is required to handle the heat released, above all during the charging and discharging processes, in the form of an efficient thermal management, which, however, can not be discussed here for reasons of scope. But a rough consideration of Figure 6: Characteristics and T echnical Data of Different Battery Cells the need for cables, circuit boards, cooling lines and mounting material in addition to the battery cells and management chips results in a n overall specific energy density of about 65 Wh/kg and a mass of about 2.2 t per battery pack. Even with the assumption of same masses of the drive systems of the DCTs and the BECTs, this leads to a mass increase of approximately 3 t per single train unit in spite the masses of the half -full fuel tanks are saved . But, when arranging the batte ry packs at the location of the internal combustion engines of the DCTs, the higher mass causes an increase in the axle loads of the driven axles, w hich is train -specific even beneficial . Only for reasons of 15 Schmetz, Struck simplification, a recalculation of the energy de mand of the BECT which is slightly increasing due to the larger masses may be just dispensed here. Check of the Charge and Discharge Cycles With the highest suggested charging current of 2.5 A at an average charging voltage of 3.4 V per battery cell, the net charging power can be assumed to 8 W due to charging losses within the batteries. Therefore, a chargin g time of 32 minutes is necessary for the required supply of each 74.6 kWh of energy to the four battery packs in regular operation after each travel, which can be shortened to a 16- minute quick charge in a few cases of heavy delays . Thus, a sufficient charging time is always ensured on the electrified part of the u nderlying railway line model even when t rains run each half hour or delay -related shortening s of the fifteen -minute reverse time in the destination station are necessary . In contrast, a sufficient charging time in the starting station, which is in the not elect rified section of the railway line model, can only be ensured if the reverse times per train are increased from fifteen to 45 minutes. In particular, when the trains have to run in an half -hourly rhythm due to the operating model, this requires the pr ocurement of an additional BECT . For this purpose, an adjustment of the operating model and the installa tion of two sufficiently robust charging stations, each as about 150 m long catenary sections, are required. Because of the favourable position of the s tarting station, the two charching stations can be connected with just 250 m long underground cables to a nearby substation of t he regional 110 kV power grid. Further modifi cations (or measures which harm the energy efficiency) of the BECT are necessary if the energy recovery during braking is investigated. Because for a deceler ation of a BECT with - 1,13 m/s² (both the railway -technical and the electro - technical admissibility of such a brake down in pure regenerative braking require separate checks) there is a braking power of almost 5900 kW required during the first second . After that, the braking power decreases by 200 kW with each additional second during each almost thirt y-seconds long braking process . Due to the su pportive driving resistance, the assum ed reverse efficiency of the drive system of 90% and total charging losses of about 1 W per battery cell, but without taking into account the damping effect of the inductances of the electric motors and any filters, the real initial braking power can be as sumed as 5000 kW, which decreases then by 170 kW per second. While the recovery of this braking power into the catenary on the electrified railway line section is not a particular problem, the reduced braking power during battery operation only falls after 26 seconds below the nominal net charging power of 540 kW, which is permitted 16 Schmetz, Struck for the four battery packs in normal operation ( Figure 7 ). Even with the permission of quick- charging, which reduces the lifetime of the battery packs, the then permissible maxi mum net cha rging power of 1080 kW is not enough to absorb the recuperable braking energy. To solve this problem at least the measures listed in Figure 7 come into question. Figure 7: Braking Performance and Measures to Absorb the E xcess E nergy The wide range of possible measures according to Figure 7 shows that this is the actual problem of the design of the energy storage s. Since the reason for the investigation is precisely the increase of energy efficiency (and the associated positive environmental effects ), the first two measures do not have to be further investigated. Although the third measure is still feasible under the premises set here, it permanently use s almost all of the time reserves built into the operating model and thus lead to a particula rly high susceptibility to delays (or to the need for procurement of another two additional BECTs). Likewise, only a slight acceptance of the longer travel times on behalf of the passengers as well as the res ulting passenger migration can be expected . But even the last of the measures listed in Figure 7, the coverage of the high storage requirements by means of special short -term storages such as supercapacitors (hereinafter referred to as supercaps), implies considerable disadvantages. For storing the exce ss energy of 16 kWh per braking process in regular operation, for example, at least 232 pieces of suitable Eaton supercaps of type XLM -62R1137- R are required, which can be arranged in four groups parallel to the four battery packs, and, for the best use of their capacity, can be connected via separate DC/DC -converters to the common DC -rails of the single train units . As a result, apart from an increased space requirement, the mass of a BECT 17 Schmetz, Struck increases by another 4 t and its price in the order of EUR 250,000.- . Therefore, all the measures listed in Figure 7 lead only to suboptimal solutions. Better solutions may be achieved if, in addition to minor adjustments to the operating and train models made so far, greater adjustments are allowed . This becomes evident in the r eview of the highest power demand s during the discharge cycles, which each occur at the end of the five single acceleration stages of the defined train model . These power dem ands can be calculated from the installed total power of the DCTs as 1120 kW, respectively 280 kW per battery pack, taking into account the efficiency assumptions shown in figure 2 and neglecting the needs of the auxiliary drives . The single battery packs, indeed, can provide a continuous discharge power of 1080 kW (as maximum e ven more than 2000 kW) when they are assembled from five parallel battery stacks . This means that the previously designed energy storage s are also suitable for the operation of BECTs with a much higher installed drive power, which either let accomplish shorter driving times due to faster accelerations or a waiver of the installation of additional supercaps by shifting the acceleration and braki ng profiles without travel time extensions . In any case, it is to be expected that larger -sized electric drives will be required in order to slow down the BECTs in normal operation as far as possible in a regenerative manner . 5 Conclusions In the previous se ctions, it was demonstrated for a real case, how the overall energy demand for a BECT can be calculated, its energy storages can be designed and its technical realization can be accomplished . It was also shown that in this manner great benefits in terms of energy demands can be achieved and exhaust emissions can be largely avoided . But i t has also become clear that a large number of different alternatives exists for technical specifications as well as for decisions regarding energy efficiency and economic aspects . For implementation, therefore, not only further, considerable effort is required to determine the best possible configuration of train units, energy storages and electr ic drives, but fully developed BECT s will need investments for additional trains and charging infrastructures . An additional, periodic investment requirement will also be created by the not yet inv estigated, but limited service life of the energy storages . Because in case of the underlying operating model already 2500 full load cycles occur per year. Nevertheless, in order to achieve an economic ally necessary minimum service life of energy storage s in the order of a half concession period, thus 7.5 years, a special emphasis has to be laid on the careful charging and 18 Schmetz, Struck discharging of the energy storages , especially for part load cycles, in conjunction with a proper design and adequate battery management . Although a number of key figures and real components have already been identified in the previous sections and further missing information has been determined by internet research, an investment calculation was not carried out here because of the still insufficient database. According to an evaluation of the advertising of a well -known manufacturer at the Innotrans -fair in the German capital Berlin in September 2016 [10], this manufacturer expects, under the assumptions and figures given there, as well as by offsetting the also advertised, lower maintenance costs against any interest rates, and from a purely monetary point of vi ew, at best an amortization within 28 years . In contrast, the tendency here is more likely to lead to a payback period in the order of ten years in the best case .