Recent developments and applications of energy storage devices in electrified railways

Transcription

1 Published in IET Electrical Systems in Transportation Received on 6th June 2013 Revised on 7th August 2013 Accepted on 17th September 2013 Recent developments and applications of energy storage devices in electrified railways Tosaphol Ratniyomchai, Stuart Hillmansen, Pietro Tricoli School of Electronic, Electrical and Computer Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK ISSN Abstract: This study presents the recent application of energy storage devices in electrified railways, especially batteries, flywheels, electric double layer capacitors and hybrid energy storage devices. The storage and reuse of regenerative braking energy is managed by energy storage devices depending on the purpose of each system. The advantages resulting from the use of energy storage devices are presented by observing the results of both verification tests and practical applications in passenger services. Several real installations of energy storage for railways are shown and compared by using the Ragone plot. The effect of the use of energy storage devices on electrified railways of the future is discussed. Finally, a discussion on the recent applications and developments of energy storage devices is presented in this study. The effective use of energy storage devices is characterised on the basis of the specific applications and current trends of the research undertaken by public bodies and manufacturers. 1 Introduction Owing to the uncertain future state of energy resources and present concerns for environmental conservation, energy saving measures and clean energy sources have received significant interest for many electrified applications; public transport systems in particular have been the focus of efforts to conserve energy. Energy storage devices can be very helpful to solve the problem of energy management for electric vehicles. They have been in development from the Nineteenth Century up to the present day. There are many types of energy storage devices which are fully developed and are in use in electrified railways, such as batteries, flywheels, electric double layer capacitors (EDLCs) and hybrid energy storage (HES) devices, which are a combination of more than one energy storage technology. Their applications depend on the time of evolution and the purpose for which they were designed, which will be discussed in the following sections. The advantages of energy storage devices are not only reduction in total energy consumption, power peak and demand reduction, voltage regulation, energy and power compensation but also the possibility for light railway vehicles to run free of external power supply. Some locations on the railway cannot be electrified for aesthetic reasons, such as city centre squares or tracks passing historical buildings or where it is difficult to install overhead wire, such as on bridges, underground sections and in tunnels. In general, the main advantage of using energy storage in electrified railways is the reuse of regenerative energy from vehicle braking. If the power supply is designed with inverting substations, the braking energy is fed back to the AC grid; for standard DC railways with non-inverting substations, this energy is provided to another vehicle in the vicinity, if this vehicle is present and is accelerating. This situation is infrequent, however, and often most of the remaining braking energy is dissipated by a ballast rheostat or by mechanical braking. Onboard energy storage devices are sometimes quickly charged at the substation. Then, the energy that is stored in the storage devices would be supplied to support the vehicle for acceleration and running free of the catenary [1]. In general, batteries are characterised by high energy density, so they can store plenty of energy and support their load more than EDLCs and flywheels; however, they present recharge time higher than those of the EDLCs and flywheels. Another disadvantage of batteries is that they have a number of life cycles approximately equal to one hundredth of those of the EDLCs and flywheels [2]. The combination of batteries and EDLCs or flywheels, called Hybrid Energy Storage Device, has a better performance in comparison with the single energy storage device. This paper presents a literature review of the recent developments and applications of energy storage devices such as batteries, flywheels, EDLCs and HESs, used in electrified railways. The most important applications based on real practical tests in public transport are presented. The effect of the use of energy storage devices on future railways and the available types of installation are discussed. A comparison of each type of energy storage device is analysed and the rated capacities of the energy storage devices, in terms of power and energy density, are summarised by the Ragone plot. 9

2 2 Comparison of energy storage devices for railway applications 2.1 Advanced electrochemical batteries Conventional lead-acid batteries have been studied and designed since long time. Nowadays, lithium-ion and nickel-metal hydride (Ni-MH) batteries are the emerging technologies for transport application, because of an energy density higher than that of lead-acid batteries. Owing to their performance, these batteries are widely used to improve energy efficiency in public transportation, especially electrified railways such as tramways and metro trains. There have been many researches to study and develop the application of batteries by installing them aboard the vehicle or stationary at the substation. The results of the verification tests and practical uses show that the batteries could reduce the energy consumption of trains and the voltage fluctuations of the contact line and help vehicle running without external power supply in some areas Application of batteries for saving energy: The traditional batteries used in electrified railways in Japan were lead-acid type arranged in battery posts, which were installed in parallel with the power substation. The purposes of this application were a reduction of energy demand and a support of the power supply system for a short time in case of a fault. The lead-acid battery posts were installed in many substations on two lines, including the Maruyama and Yagasaki substations on the Shinetsu line in 1912, with a capacity of 1332 Ah; the Kawasaki and Oimachi substations on the Keihin line and Eiraku-cho and Harajuku substations on the Yamanote line in 1914, with a capacity of 1000 Ah [3, 4]. They were used for about 15 years before they were dismissed because of maintenance difficulties and the plans to build a new substation. In 1980, a lead-acid battery post was installed at Nakajima station on the Kabe line by Japan National Railway (JNR) for verification testing and was in continuous use for 3 years. The battery post capacity and maximum voltage were 792 cells and 1584 V, respectively. The objective of the testing was the analysis of the voltage drop compensation and the results were very effective for the investigation [3, 4]. More recently, lithium-ion batteries have been used several times for enhancing energy efficiency in Japanese electrified railways. The practical installations had two subsequent phases. In the first phase, a small temporary battery (TB) was installed for initial testing. Subsequently, the full-scale permanent battery (PB) was installed for the trains service. The rating capacities of the temporary and PBs are shown in Table 1 [3, 4]. In May 2005, the Kobe Municipal Transportation Bureau verified the performance of lithium-ion batteries by installing them in the Myodani substation of the Seishin-Yamate line, having an average slope of 2.9% over a distance of 4 km. They were then installed in double rated capacity for the previous verification test in the Itayado substation in February 2007 in order to save energy, thereby decreasing energy usage by over 300 MWh per year [3, 4]. In the Table 1 Specific capacity of the permanent and temporary batteries for saving energy Company Kobe Municipal Transportation Bureau West Japan Railway Company Kagoshima City Transport Bureau Nagoya Railroad Co., Ltd. Osaka Municipal Transportation Bureau Batteries lithium-ion battery (PB) lithium-ion battery (PB) lithium-ion battery (PB) lithium-ion battery (TB) Ni-MH battery (TB) Power rating, kw Energy rating, kwh autumn of 2006, lithium-ion batteries were installed in the Shin-Hikida substation of the West Japan Railway Company Hokuriku line for effective short-time voltage drop compensation and line voltage regulation [3, 4]. In March 2007, the Sakurajimasanbashidori station and the Nakasudori station of the Kagoshima City Transportation Bureau were installed with lithium-ion batteries to compensate for the voltage drop of the power line effectively [3, 4]. Furthermore, further verification tests of lithium-ion batteries were conducted in the Shin-Anjo station of the Nagoya line, which is a place very distant from the DC substations. The purposes of this verification test were voltage drop compensation and reduction of trains energy demand [3, 4]. The verification test with Ni-MH batteries was proved to reduce energy consumption by the Osaka Municipal Transportation Bureau at Komagawa substation on the Tanimachi line [3, 4] Batteries for catenary-free operation: Some areas in the city centre or urban zones need to preserve the characteristics of historical buildings or have cross sections such as bridges and tunnels. This means that trams running without an overhead line are a great solution which can benefit both the city councils and transportation bureaus [1]. A featured characteristic of lithium-ion batteries is their high energy density. There were many applications for lithium-ion batteries on trams for running catenary free. Lithium-ion batteries were installed on remodelled trams called Lithey-Tramy and Hi-tram by the Railway Technical Research Institute (RTRI) in 2005 and 2007, respectively. In April 2003, before the beginning of the service, the RTRI conducted tests for running the tram catenary free at a maximum speed of 40 km/h; the total distance was 17.4 km and the distance between the stations was 250 m. The electrified part of the line was operated with a voltage of 605 V. The onboard lithium-ion battery unit was composed of 168 cells in series and a rated capacity of 33 kwh, as shown in Table 2. This system had already operated in public service from August 2003 until January 2005; this vehicle had been redesigned for the new operating voltage of 750 or 1500 V of the electrified line [1, 5]. Secondly, a prototype vehicle Hi-tram was operated on Table 2 Specific capacity of batteries for running catenary free Vehicles Batteries Rated power, kw Rated energy, kwh Rated voltage, V Weight, kg Lithey-Tramy lithium-ion battery Hi-tram lithium-ion battery SWIMO Ni-MH battery

3 600 V of the electrified line of Sapporo Municipal Transport from November 2007 to March 2008 with passenger service. Catenary free running was performed on the total distance of 25.8 km at a maximum speed of 40 km/h. The onboard battery unit consisted of the series connection of 672 cells of lithium-ion batteries. Initially, the lithium-ion batteries were charged at a current of approximately 1000 A over a period of 60 s and then the train ran for 4 km catenary free. By November 2009, this vehicle was able to run with both operating voltage of 600 and 1500 V and a maximum speed of up to 80 km/h for a total length of 49.1 km within 60 min [1]. Lithium-ion batteries were also tested on the operation of the hybrid electric multiple units E995 by East Japan Railway and Mitsubishi Electric in The onboard battery unit consisted of 1512 cells of lithium-ion batteries (168 in series and 9 in parallel, 30 Ah per cell) with a stored energy of 163 kwh [1]. Ni-MH batteries were installed on a prototype vehicle called SWIMO (Smooth Win MOver) by Sapporo Municipal Transport and Kawasaki Heavy Industry with operating voltage of 600 V from December 2007 to March The onboard battery box consisted of 480 cells with rated voltage and capacities of 1.2 V and 274 Ah, respectively. The total rating capacity of the battery unit is shown in Table 2. This vehicle was able to run at a maximum speed of 40 km/h while running completely catenary free over a distance of 37.5 km. Another application in Europe, the Citadis tramway, with an Ni-MH battery, was chosen to operate for the first time in Nice, France, by Alstom transportation. This tram had a maximum speed of 30 km/h and was able to run catenary free over a length of 1 km [6]. Catenary free running was required in two historic squares, the Place Massena and Place Garibaldi, for a distance of about 500 m in each location. Twenty Citadis vehicles with Ni-MH batteries onboard were sent to service passengers and run without contact wire in these areas at low speed [7]. 2.2 Flywheels In 1988, flywheels were installed in the Keihin Electric Express Railway at Zushi post in Japan for storing regenerative energy. The report showed that flywheels could save up to 12% of total energy; this system is still operating. The rated energy and power of the flywheels are 25 kwh and 2000 kw, respectively [3]. In October 2000, flywheels were verified in tests at the London underground at a nominal voltage of 630 V DC. In rush hour, voltage drop of the electrified line was variable from 180 to 450 V. By using flywheels the average voltage increased by up to 530 V with the same load. The power of the flywheels was 300 kw and was designed mainly for testing. However, the actual regenerative braking capability of the trains was higher than 300 kw, and it was estimated that flywheels of at least 1 MW should be used to recover completely the braking energy. At that time, the London Underground substation annual electricity consumption was The 1 MW flywheels helped to reduce the power consumption by 26% or a year. The purchase and maintenance costs of the flywheel were and 2500 per year respectively, which would mean that the capital investment was recovered within 5 years [8]. The cooperation of three institutes of railways in Spain, the ADIF Railways infrastructure manger, CEDEX studies, the Experimentation Centre of Public Works and Transportation (Fomento) Ministry and the CIEMAT Centre for Energy, Environment and Technology Research designed, developed and tested a kinetic energy storage system by using flywheels. The first project, called ACE2, was launched in 2003 with the joint purpose of power consumption levelling and recovering braking energy. For this prototype, the flywheels store 200 MJ of energy and are able to transfer a power of 350 kw. The second project, called SA2VE, started in 2006 with the same objective and the flywheels store 3.2 MJ of energy and are able to transfer 5.6 MW of power within 9.5 min [9]. In addition, flywheels were installed on the roof of trams for catenary free operation in Rotterdam, the Netherlands. The energy and power capacity and speed of the flywheel were given as 4 kwh, 325 kw and rpm, respectively [7]. Further tests have been conducted with a similar flywheel system on Citadis trams. The weight of the Citadis was approximately 40 tons; the vehicle could run without external supply for nearly 2 km with a speed of 50 km/h. Catenary free running could continue over three stops before the flywheels need to be charged again. As the highlight of running catenary free, the Citadis could run catenary free across the Erasmus Bridge, covering a distance of 900 m and height of 15 m [7, 10]. In the second quarter of 2003, kw flywheel units were installed on the Lyon metro for regulating the third rail voltage between 850 and 860 V during train braking. As a result, for over 4 months of the metro operations, the track receptivity of the Lyon metro network was increased by these flywheel units. The use of mechanical brakes and their maintenance costs were also reduced [11]. A permanent magnet synchronous machine (PMSM) was used as electric motor/generator and it was connected to the railway power line with a bidirectional AC/DC switching power converter. The study used simplified calculations to design the flywheel systems and, hence, the evaluation of the optimal ratings for the machine and the converter has to take into account a real working cycle [12]. The cooling system is also very important for the development of flywheels with PMSMs and power converters. Flywheel systems are required to operate with variable loading conditions and, therefore thermal calculations have to refer to real cycle operations. To reduce friction losses, flywheels work in partial vacuum but this limits the heating dissipation of the PMSMs. Recent studies showed that three-level inverters can help to reduce the power losses of the electrical machine. Although three-level inverters have efficiencies lower than two-level inverters, they can be placed outside the vacuum cylinder with a more effective cooling of the flywheel system [13]. Flywheel systems for energy saving of light railway vehicles are still in development and a recent agreement between Alstom Transport and Williams group, including Williams F1 team, will lead to the installation of Williams hybrid power flywheels onboard Alstom s Citadis trams [14]. 2.3 Electric double layer capacitors EDLCs have good performances in terms of power density, charge and discharge time, long lifetime cycles, reduced maintenance cost, lower internal resistance and they are widely used for storing regenerative braking energy in public transportation. When vehicles have to frequently brake and cannot pump the regenerated electrical energy back to the power supply or to another vehicle running in the vicinity, EDLCs can store the regenerative braking energy. This energy is then reused by the vehicle to support the power coming from the main supply. EDLCs present 11

4 similar advantages of batteries, but they have some particular characteristics. EDLCs cannot only quickly recharge energy and have power densities higher than those of the batteries, but they also have a very large number of life cycles. There are many researches and applications which have studied the performance of the EDLCs from verification testing to practical use in passenger service. The evidence of studies of the EDLC can be shown in both academic articles and industrial applications. The academic articles have been published by researchers employed in public research institutes. On the other hand, the companies working in applications with EDLCs presented a range of different products to enhance the efficiency of electrified railways. Every company has conducted research and development by verifying tests on the real vehicle to confirm the performance of their product. Representative example of EDLC modules available on the market are MITRAC Energy Saver by Bombardier Transportation, SITRAS SES by Siemens Transportation Systems and Maximised Energy Efficiency Tramway System (STEEM) by Alstom Transport EDLC in academic articles: Two EDLCs were installed by Seibu Railway Co. Ltd at Agono and Shumaru substations in Japan, operating at 1500 V DC, in December 2007, with the same rated power and energy of MW and kwh, respectively. Owing to the distance of km between these two substations and the gradient of 2.5, regenerative braking energy was effectively used for this section. The measured result showed that with one train running between the two substations, the total energy recovered was 7.7 kwh and the storage systems were able to deliver back to the train 77% of the recovered energy [3, 4]. In addition, in Korea, EDLCs were installed in the Daedong substation of the Daejeon Metropolitan Rapid Transit Corporation to compensate for voltage drop and save energy. The rated power and energy of the EDLC modules were kw and kwh, respectively. Before they were installed in the substation, they had been tested in the laboratory, including insulation resistance tests, commercial frequency voltage withstand tests, control sequence tests, charge and discharge tests and protection tests. The results of the verification testing were satisfactory, and the EDLCs were then ready to be installed on the vehicles. In the real tests, the EDLCs could maintain the level of the catenary voltage around the nominal voltage of 1500 V DC [15]. For the onboard EDLCs, in January 2005, CJRC s rolling stock series 313, running between Nagoya and Jinryo on the Chuo line, had installed EDLCs to support the mechanical braking of rolling stock. The rated energy capacity of the storage device was calculated by considering the amount of energy dissipated by mechanical braking, supposing that regenerative braking was not available. They estimated that the average braking energy was 0.6 kwh and designed the EDLC system for this energy. The operation voltage was between 700 and 1425 V, whereas the maximum voltage, capacitance, rated power and weight of the EDLC module were 1200 V, 1.4 F, 180 kw and 430 kg, respectively. In the practical application, two EDLCs having a capacity of 0.28 kwh were installed under the floor of the coach. Both the storage devices were tested to reduce the need of mechanical braking when the train was not able to regenerate energy back to the power supply. The result showed that 0.28 kwh of EDLC could store 8% of regenerating energy from the motor, which was equal to 1.6% of energy for acceleration. Moreover, the EDLCs could effectively reduce the peak of the braking force and also the pressure of the brake cylinders and the temperature of the wheel treads [16]. In addition, there are simulation results based on a novel power electronic converter to confirm the performance of the EDLC in terms of energy saving on the Blackpool tram system in the UK. EDLC was modelled by a capacitor series with resistors as 17.8 F and 65 mω, respectively. EDLC with 50 kw converters can achieve an energy saving of kwh/km or 18% reduction for 100 passengers and 20% for an empty tram. For using the novel power electronic converter with EDLC, the energy saving is kwh/km or 23% reduction for 100 passengers and up to 28% for an empty tram. The energy saving can be achieved by implementation of the EDLC with the novel power electronic converter on that tram as a 0.09 kwh/km or up to 8%; an extra saving depending on passengers loading [17] EDLC in industrial application: MITRAC Energy Saver: Minimising environmental effect and reducing energy consumption costs are two of the main concerns of public transport. The MITRAC Energy Saver is based on the series connection of high performance EDLCs, which can quickly charge and discharge high power from train braking and acceleration. The schematic of the MITRAC energy saver is shown in Fig. 1 [18]. When the vehicle is braking, some of the regenerative energy is stored by the MITRAC Energy Saver and then for train acceleration this energy is distributed to support the power supply; this is one cycle of the MITRAC Energy Saver. The MITRAC Energy Saver is a product developed mainly for energy savings, power supply optimisation and reducing infrastructure investment, running free catenary and performance boosting [19]. Firstly, MITRAC Energy Saver was installed onboard a prototype of a light rail vehicle (LRV) for public transport by the German operator Rhein-Neckar-Verkehr Gmbh in Mannheim, Germany from September 2003 to 2008 [18]. The specific ratings of the MITRAC Energy Saver are shown in Table 3 [19]. The measured performance showed that MITRAC could reduce the consumption of the traction energy by 30%. The line Fig. 1 Schematic of MITRAC energy saver Modified from Steiner et al. [18] 12

5 Table 3 Main technical data of the MITRAC energy saver unit Application LRV 2003 LRV 2008 energy capacity, kwh 1 1 power capacity, kw weight, kg dimensions, mm (partly 550) typical installation two boxes for 30 m long LRV, 2 kwh, 600 kw two boxes for 30 m long LRV, 2 kwh, 600 kw voltage and current with and without the energy storage device were measured during the acceleration of the LRV up to the speed of 50 km/h. The result showed that MITRAC Energy Saver could reduce line current peak and voltage drop by 50%. For the metro systems, it is estimated that the braking energy available would be about 40% and the electrical energy available for the train would be 21% of the total energy needed for the acceleration [19]. Secondly, for the areas of historical buildings, city centre squares, tunnels or even transient faults in the grid where catenary free running is required, the additional aboard MITRAC Energy Saver energy storage device can support this requirement effectively. In the verification test, 1 kwh of MITRAC Energy Saver was installed onboard the prototype LRV in Mannheim; this vehicle could run without external power source at a speed of 26 km/h and a distance of 500 m. The storage device was recharged quickly by an overhead busbar feeder (instead of an overhead wire) at the substations. The feeder is capable of charging 3 kwh within 20 s with a maximum current of 1 ka. On the other hand, the main disadvantages of the MITRAC Energy Saver present are the higher train mass, increased by approximately 2%, and the further space required to be installed onboard the device. Lastly, the MITRAC Energy Saver has been used as an additional power supply for supporting train acceleration. This feature is of interest for weak power lines where the current must be limited to avoid unacceptable voltage drops. SITRAS SES: The SITRAS SES (stationary energy storage) system was one of the products from Siemens Transportation Systems designed for public transportation, such as metro trains and trams. This equipment was based on the 1344 Maxwell EDLCs and was installed trackside [20]. The schematic of SITRAS SES is shown in Fig. 2 [21]. The SITRAS SES could save nearly 30% of energy and regulate the voltage, which improved the reliability of the rapid transit systems and tramways. Normally, the benefits of EDLCs in the SITRAS SES are similar to the MITRAC energy saver from Bombardier Transportation. Another advantage gained by SITRAS SES is that the SITRAS SES could help the power supply system to avoid short periods of electrical failure and also reduce the effect on the voltage drop of many trains or trams consuming power simultaneously. The simulation and verification test showed the result that this energy storage device could reduce energy demand for 1 year by about 500 MWh, or in relation to CO 2 a 300 tonne reduction. The SITRAS SES rating 1 MW included BOOSTCAP capacitors with capacitance and terminal cell voltage of 2600 F and 2.3 V, respectively. The space for mounting this energy storage device for the wayside energy system was 3 m long and 2.7 m high. The SITRAS SES was able to support other trains running within a radius of up to 3 km. There are many applications of the SITRAS SES in public transportation, including Kölner Verkehrsbetriebe AG in Cologne, Germany between February 2001 and 2003; Madrid, Spain since July 2003 and Portland, Oregon, USA operating since 2002 [20]. The results showed that 320 MWh of energy consumption could be successfully saved per year and per station. For 400 stations and an approximate energy cost of $100 per MWh, this meant that the SITRAS SES could save energy cost by up to $12.8 million a year. Maximised Energy Efficiency Tramway System: The STEEM project has been established by cooperation between a public transport operator (RATP), Alstom transport (tram manufacturer) and a public research laboratory (INRETS) with the financial support of the French Agency for Environment and Energy Management (ADEME) under the French Framework Program on Research, Experimentation and Innovation in Land Transport (PREDIT) [22]. The traction and auxiliaries consumption, including the line characteristics (track profile and distance between stations) and the operational modes (number of unplanned stops between stations, commercial speed and comfort level) were taken into account for calculating the total energy demand and the energy storage capacity need. For the STEEM project, the modules of onboard energy storage devices Fig. 2 Schematic of SITRAS SES Modified from SIEMENS [21] 13

6 were based on EDLCs in series-parallel for about 48 modules, for which capacitance, voltage and weight for each module were 130 F, 54 V and 15 kg respectively. The chopper, a complete control traction tool and the EDLCs were packed in a box and installed on the roof of the trams. The circuit layout of the onboard EDLC modules of the STEEM project is similar to that of the MITRAC energy saver shown in Fig. 1. The objectives of this project were the enhancement of the energy efficiency of tramway systems and permitted vehicle running catenary free. The 21 Citadis 402 trams ran on the line T3 managed by the Autonomous Operator of Parisian Transports (RATP), Paris on a revenue passenger service since For the test result, the regenerative braking energy was stored by the EDLCs and then released to support the main power line, depending on the line receptivity. The T3 line was a short line (7.9 km) and a long tram car (40 m) running frequently every 4 min. There were 16 trams running for the rush hour, which meant a ratio of service which was more than 2 trams per km. As the T3 line had very high receptivity, the energy storage did not prove its efficacy much. Firstly, in terms of energy saving, the regenerative braking energy was effectively released to the auxiliary loads (mainly air conditioner and heaters). In the winter with cold weather, the results showed that there was no reduction of energy consumption because the trams had to supply more energy to the auxiliary devices than the traction system. On the other hand, in the spring with a milder climate, they could reduce the average daily energy consumption by 13%, with a minimum and maximum of 10 and 18%, respectively [22]. The measure of energy consumption depended on the driving style, elevation of the land and traffic restrictions. During the STEEM project, EDLCs were also used to test the capabilities of the trams to run catenary free. The first tram had operated on the T3 line between Porte d Italie and Porte de Choisy stations with a length of 300 m. The EDLC modules were quickly charged at the arrival station and then discharged to support the tram acceleration without another supply for running catenary free. The automatic control of this operation was based on the GPS localisation. For the future line, RATP will look at areas where it is very difficult to install an overhead line, such as crossing roads, tunnels, bridges, city centre squares and in front of some historical buildings. 2.4 HES devices The new HES device is the Sitras HES which consists of a nickel metal hydride battery and a Sitras MES (mobile energy storage) module based on EDLCs [5]. The idea behind this hybrid device is the integration of both EDLCs and batteries to obtain the same time high power and energy densities. The device has apparently better performances than the previous energy storage devices, especially for energy saving and running catenary free. The Sitras MES, having energy capacity between 1 and 2 kwh, can be charged quickly and then release the energy stored to the traction motors for the acceleration. On the other hand, a 18 kwh traction battery with high energy density is used to supply the tram for long distances between stations and power to air conditioning and heating required. The HES device is capable of recharging energy from regenerative braking and also from a dedicated quick charging unit at the substations. The specifications of the Sitras HES are shown in Table 4 [23]. In Table 4, generally the charging current is lower than the discharging current Table 4 Specification of an EDLC and a traction battery Parameters EDLC unit Battery unit nominal voltage, V current rating, A for discharging, 22 for charging power rating, kw energy capacity rating, kwh dimensions, mm weight, kg based on the C-rate, where C-rate is a measure of the batteries charged and discharged rate relative to its maximum capacity within 1 h. This means that the charging rate will also spent more time than the discharging rate to avoid damaging cells and to extend the batteries lifetimes. In November 2008, the Sitras HES was installed on the roof of a redesigned tram belonging to the Portuguese company Metro Transports do Sul S.A. (MTS), called Combino plus MTS, which serviced passengers between Almada and Seixal in the south of Lisbon. The in service operation for passengers with the application of the Sitras HES on the Combino plus MTS was certified and evaluated by the TÜV Süd GmbH according to the German Federal Regulations on the construction and operation of light rail transit systems (BOStrab) in terms of risk analysis, operation and protection concepts. This had the effect of reducing the CO 2 emissions by 80 metric tons per year. The typical catenary free length of the trams equipped with Sitras HES was approximately 2.5 km. In critical situations such as power outage of the train because of a failure of the pantograph or a fault within the substation or for short periods of maintenance works on the traction power supply, onboard traction batteries were able to power the tram over the next station. 2.5 Ragone plot The characteristics of storage devices can be easily visualised and compared with the Ragone plot, shown in Fig. 3 [24]. For each storage technology, this plot shows the typical ranges of power and energy densities on a log log scale. Although these characteristics are well established at single cell level, it is not clear how they are modified at module level and at storage device level. Moreover, an extensive comparative analysis of power and energy densities achieved by practical applications of energy storages for railways is apparently missing. The characteristic relationship between power and energy density of the recent and past development of energy storage devices is shown in Fig. 4. The unity slope line is related to the characteristic discharge time of the energy storage devices. The circle-mark and x-mark are used for the lithium-ion batteries for stationary and mobile applications, respectively. The figure shows that the discharging time of the lithium-ion batteries is between 2 and 8 min, with an average of 5 min. The stationary and onboard Ni-MH batteries are represented by a plus-mark and star-mark and the discharging time of approximately 6 and 30 min, respectively. The stationary flywheels are represented by a square-mark and have the discharging times of approximately 0.8 and 10 min, whereas the mobile flywheel is represented by a diamond-mark and has the discharging time of 0.8 min. EDLCs can be discharged very 14

7 Fig. 3 Typical Ragone plot showing the characteristics of different energy storage technologies Modified from Christen and Carlen [24] Fig. 4 Ragone plot of real energy storage devices deployed at electrified railways quickly with times between 3.6 and 36 s. MITRAC Energy Savers are represented by down-pointing triangles, with the discharging time of 12 s; the stationary SITRAS SES is represented by up-pointing triangles, having a time for discharging of 13 s; STEEM (ALSTOM) is represented by left-pointing triangles, with the discharging time of 11 s; the hybrid SITRAS HES has two energy storage devices; the Ni-MH battery, is represented by right-pointing triangles, with the discharging time of 12 min and EDLCs, also represented by right-pointing triangles, with the discharging time of 10 s. The total value of the energy and power densities for the hybrid storages is also represented by right-pointing triangles, having the discharging time of 3 min. Lastly, the practical applications of EDLCs proposed by academic researchers for stationary and onboard metro trains are represented by a pentagram-mark and a 15

8 hexagram-mark, respectively. The discharging time of a stationary EDLC is 9.5 s while the discharging times of the mobile metro trains are 5.5 and 19.5 s. It is clearly evident that the EDLCs have a quicker discharging time than the flywheels and the batteries, so they are suitable for use in LRVs for running catenary free and saving energy. Flywheels have an operating discharging time which falls between the batteries and the EDLCs; they are suitable for use in electrified railways, but the dimensions are typically bigger than those EDLCs and are not particularly suitable for onboard installation. Moreover, they require higher maintenance when compared with EDLCs for the presence of moving parts, unless magnetic bearings are used. The magnetic bearings support the moving parts without physical contact by using magnetic levitation. They are sometimes applied instead of mechanical bearings for reducing the friction in flywheels. The batteries are suitable for installing in substations and tracksides. Owing to the limited life cycle of the batteries, they have limited use in modern electrified railways. Fortunately, onboard HES devices are a high performance comparison against single use batteries or EDLCs. Therefore this is probably the new generation of energy storage devices used in electrified railways. 3 Types of installation As described in the previous sections, there are two main types of installation of energy storage devices: onboard and stationary or trackside. Both of them can store the recovery energy braking when a train brakes and release back to the power traction system when a vehicle accelerates. Batteries, flywheels and EDLCs were studied on both types of installation. Running catenary free is only possible with onboard energy storage, since obviously stationary energy storage is not capable of supporting a vehicle running without contact wire. In terms of energy consumption reduction, a Brussels metro line was studied as a comparison for the installation in terms of saving energy, which was involved in many criteria. Different scenarios were determined for the investigation, including the traffic conditions (high, moderate and low traffic volume), energy storage capacity, and a suitable place for the stationary device. Stationary energy storage seemed to be an easy method to support the electricity at the substation where it was installed. The optimisation of size and place were the significant factors which had to be appropriately considered. On the other hand, for an aboard energy storage device, the vehicle would require enough free space (normally on the roof) for accommodating the box of energy storage and the vehicle would have to carry approximately 2% more mass [18]. The result of the simulation showed that at high, moderate and low traffic volumes with six stationary energy storage devices having capacity of 4.53 kwh, optimistically the energy savings could be 16.1, 22.3 and 33.4% of the energy required by each train, respectively. In contrast, the onboard energy storage devices with a capacity of 1.46 kwh on each vehicle could produce energy savings of 26.3, 29.8 and 35.8% at the high, moderate, and low traffic volumes, respectively. From the results of a train with the same track, the onboard energy storage devices were more efficient than the stationary energy storage devices. If the whole system was considered, the total capacity of the onboard energy storage was 80.3 kwh (with 5 cars, 11 vehicles) while the total capacity of the stationary energy storage devices was kwh [25]. In facts, it is opinion of the authors that the comparison efficiency of these two types of energy storage devices installations should consider the cost of energy storage devices and their infrastructure parts, maintenance costs, energy costs and the years of payback. Therefore an economic assessment is one of the factors that must be taken into account to optimise the cost. The best installation method for energy storage is dependent on the agreement of the authority and the purpose of the energy storage devices. 4 Control strategy for energy storage devices Owing to the confidentiality of the product design, any publications and advertisements for the energy storage products available from the companies operating in this sector present mainly the advantages and only few drawbacks. They do not mention in detail how to design, control and connect the devices, or which power electronics interface is used to control the power flows with the electrified railway line. On the other hand, the control strategies of energy storage devices are published by independent researchers, who do not directly deal with the manufacturing companies. Many studies deal with the state of charge (SOC) control of EDLCs when a train follows an assigned speed profile. However, the amount of the regenerative braking energy that EDLCs can store and the methodology to select the most suitable capacity for an assigned rail vehicle are still questions that have not found an unique answer. In particular, the limitation of the state of charge of EDLCs is a critical trade-off between weight and volume of the storage elements and the power ratings of the converter. The optimal control charge and discharge of EDLCs with the speed profile of the train was verified by minimising the energy consumption and maintaining the terminal voltage of EDLCs and catenary within the upper and lower limits [26]. In addition, this optimisation was verified including sections where the train was running catenary free for planning the train timetable. The capacity of EDLCs can be designed by a novel analysis based on power flow calculation of the electrified network. This algorithm was simulated and studied on the Korean railroad lines which were still serviced. Substations, feeders, vehicles, rails and energy storages models were considered in terms of the admittance matrix. As a result, the power flow algorithm was capable of designing the optimal capacity of theedlcsforeachsubstationinbothonpeakandoffpeak headway scenario which the power and energy capacity range of the energy storages were MW and kwh, respectively. The energy storage specifications are shown in Table 5 [27]. The storage device was charged and discharged at least 500 times per day. The amount of energy saved by storage devices could be 28.74% at peak time (4 h a day), 39.79% at off peak time (15 h a day), with a daily average of %. EDLCs were then installed for testing modules in the Kyoung-san Railroad Experimental Centre with a power capacity of 1.76 kw and energy capacity of 7.23 kwh [27]. Moreover, the charge and discharge of EDLCs during braking and acceleration were controlled by a catenary current set-point (i* d ) and the boundary of DC bus voltage, keeping the voltage of EDLCs within the lower and upper voltage limits (v sc,min and v sc,max ). The working area of the control strategy based on the line current (i d ) and energy 16

9 Table 5 headway Specification of energy storages on peak and off peak Substation On peak headway (240 s) Off peak headway (480 s) Power, MW Energy, kwh Power, MW Energy, kwh Cheonho Seokchon Bokjeong Dandaeogeori Moran Base storage terminal voltage (v sc ) plane is shown in Fig. 5. These voltage limits could be selected by the total energy demand during acceleration and braking. The current set-point could be defined as the ratio of the total energy lost and the product of the average line voltage and cycle duration. The controller can automatically control the state of charge of energy storage devices within the working area, when the line current is lower and greater than the set-point or v sc < v sc,min and v sc > v sc,max. A high level of SOC had to be set up for the low speed train because the energy for train acceleration is more than the kinetic energy from train braking. On the other hand, the low level of SOC is suitable for high speed trains because regenerative braking had more influence than the energy for train acceleration [28, 29]. The fundamental control strategy of the energy management of the electrified railway can be divided into four main categories [30]. The criteria of these control strategies depend on the number and distance of gaps, speed profiles and amount of regenerative energy braking as detailed in the following sections. 4.1 Control strategy to maintain the maximum SOC The purpose of this control strategy is to keep the energy storage devices fully charged all the time to support the vehicle running through gaps of the power line. The controller scheme, shown in Fig. 6, is a relation between SOC and storage device power [30]. Positive power means that the storage releases energy to support the train motoring over a gap. On the other hand, negative power Fig. 6 Control strategy to maintain the maximum SOC Modified from RSSB research programme [30] means that the storage is recharged and this happens when the train is braking or is running under wire. The energy storage is recharged with constant power, which means approximately a constant recharge current charge, until the charging limit is reached. For example, the charging limit is the point where the battery voltage reaches 70% of the full charge value [31]. After the charging limit, the battery is recharged with reduced power until full charge is reached. This is because at the charging limit point, the battery voltage will approach the full charge value and the charging current must be reduced to avoid energy dissipation within the cells. The stored energy supports the train motoring through the gap until the discharging limit and down to the minimum SOC, preparing the storage devices to receive the braking energy for the next cycle. 4.2 Control strategy to maintain minimum SOC The purpose of this control strategy is to maintain the minimum SOC all the time to prepare the train to receive all the available regenerative braking energy. The controller scheme is shown in Fig. 7 [30]. The regenerative braking energy is charged in the energy storage devices when the train is only running under contact wires or braking within Fig. 5 Working area of control strategy Modified from Iannuzzi and Tricoli [28] Fig. 7 Control strategy to maintain the minimum SOC Modified from RSSB research programme [30] 17

10 gaps until the charging limit and, with reduced power, until full charge. The stored energy is released again to support the train motoring, preparing the storage devices to receive the braking energy for the next cycle. The power supplied by the storage device has two different levels, according to the motoring conditions. There are two different power levels because; for the train running under wire, the most available power of the traction systems obtain from the power supply, therefore energy storage devices should partially supply power to support the traction system less than the nominal value depended on the controller which helps the reduction of the currents coming from the substations; on the other hand, for the train running under gap, there is no power from the power supply to support the traction system, therefore only the nominal power of energy storage devices can support the train running through gaps because they are the sole source of power. In both cases, the storage supplies power until the discharging limit is reached and a reduced amount of power down to the minimum SOC. When the train is running under contact wires, the energy storage devices are regularly discharged following the characteristic line until the minimum SOC. 4.3 Speed-based dependent control strategy This control strategy for energy storage devices depends on the speed profile; therefore this strategy is suitable in both the minimum and maximum charge for recovering all the available braking energy and running through gaps, respectively. For a low speed train, the SOC of energy storage devices is maintained at maximum charge for powering the train to run through gaps. On the other hand, the SOC of energy storage devices is maintained at minimum charge so that they are capable of recovering the maximum regenerative braking energy. The controller scheme can be shown in Fig. 8 [30]. The regenerative braking energy will be charged in the energy storage devices when the train is running under contact wires or braking within gaps until the charging limit and eventually full charge (braking under wire or gap line). The stored energy is then discharged to support the train motoring through the gap until the discharging limit and down to the minimum SOC, prepared to receive the braking energy for the next cycle (motoring under gap line). When the train is running under contact wires (dashed-line zone: the power is less than when the train is running through gaps), the energy storage devices are regularly charged and discharged depending on the train speed profile following the characteristic lines. 4.4 Look ahead control strategy This control strategy is based on the route detail and a combination of the first two control techniques described in the previous sections. From the knowledge of the position of the gaps and the stop pattern, it is possible to decide in advance the SOC of the storage device, to maximise the energy recovery and to have enough energy to run across the gaps. Therefore the controller will switch to the maximum SOC every time the train is approaching a gap, while it will turn to the minimum SOC when the train is approaching a station. 5 Effect of energy storage devices for railway applications in the future There are many studies which demonstrate that the use of energy storage devices in electrified railways has provided many advantages over the last decade. In contrast, there are no specific reports on the failures of such devices. There is a clear consensus that energy storage devices can improve and enhance the performance of the electrified railway in ways such as energy saving, voltage levelling and power peak demand regulation. These results have a great effect on the costs associated to the energy consumption for traction, for example, 30% energy saving from the MITRAC energy saver and the SITRAS SES [18, 20]. Moreover, trains with energy storage devices produce less CO 2 emissions and are more environmentally friendly. In addition, the verification test showed that energy storage devices could store the regenerative braking energy up to approximately 21% of the whole energy system [18]. This means that there would not be more need of dissipating energy by means of the braking resistance. Improving energy efficiency is currently a very significant concern; therefore energy conservation and energy consumption cost reduction are the first priority of future energy plans [8]. Furthermore, the financial analysis is one of the factors which will encourage local power authorities to use energy storage devices. After the installation of energy storage devices, the requirement for new installations and maintenance of some parts of the infrastructure can be Fig. 8 Speed dependent control strategy Modified from RSSB research programme [30] 18