Development of ESS for Regenerative Energy of Electric Vehicle

Development of ESS for Regenerative Energy of Electric Vehicle Hanmin Lee, Gildong Kim, Changmu Lee Korea Railroad Research Institute, Uiwang-City, Gyeonggi-Do, Korea Abstract The energy storage system is considered to be one of the useful devices for energy storing and recycling. The energy storage system reduces primary energy consumption without affecting transport capacity and punctuality. Also the energy storage system can stabilize the system voltage. This paper presents the development of the energy storage system. Namely, effect of the energy storage system for energy saving, applying EDLC as the storage unit of the energy storage system, the energy storage system design, installation and field tests. Introduction The electric railway is a clean and energy saving system, because it requires relatively less energy than automobiles by transporting the same passengers or goods. Six thousands of vehicles are operated on Korean urban transit system. This system is 95% of regeneration system. Especially, the VVVF-Inverter vehicle has a merit of the highest regeneration rate. Energy consumption is 90% for traction and 10% for auxiliary supply. Braking energy is about 30% of energy consumption. Up to 30% of the tractive power of vehicles capable of returning energy to the power supply can be regenerated during braking and that this energy can be used to feed vehicles which are accelerating at the same time. The energy generated by braking vehicle would simply be converted into waste heat by its braking resistors if no other vehicle is accelerating at exactly the same time. Such synchronized braking and accelerating can not be coordinated, the ESS(energy storage system, here after) stores the energy generated during braking and discharges it again when a vehicle accelerates. The ESS creates optimum conditions for energy regeneration in urban transit system. The ESS is able to store and discharge energy extremely quickly, consequently enabling a complete exchange of energy between vehicles, even if they are not braking and accelerating at precisely the same time, as is most frequently the case in everyday service. The ESS reduces primary energy consumption without affecting transport capacity and punctuality. In addition, the ESS can stabilize the system voltage. The ESS is considered to be one of the useful devices for energy storing and recycling. This paper presents the development of the ESS. Namely, effect of the ESS for energy saving, applying the electric double layer capacitor(edlc, here after) as the storage unit of the ESS, The ESS design, installation and field tests. ESS(Energy Storage System) The recent environmental protection trend requires more strict energy saving, therefore every transportation system should reduce energy consumption to the minimum value. High-efficiency operation system, energy saving and CO 2 emissions shall be addressed as important issue in railway system. These issues are the most essential factors of railway, major public transportation system. Recently, saving energy in the electric railway system is studied. For such new energy saving, the ESS is considered for storing energy. Energy saving is possible by efficient use of regenerated energy. Regenerated energy is recycled amongst vehicles by mean of charge and discharge corresponding to powering and braking of electric vehicle operations. This energy saving contributes to cut CO 2 to reduce greenhouse gas emissions. Recycling regenerated energy demonstrate significant effect on peak cut of consumption energy in railway substation. Absorption of excess energy avoids regeneration failure due to high traction voltage. EDLC(Electric Double Layer Capacitor) In urban transit system, about 40% of the traction power fed for powering vehicle is converted to braking energy and dissipated as heat. The ESS provides ideal energy-saving for railway system, by means of storage and recycle of braking energy. For such new energy saving, the

EDLC of huge capacitance is considered for storing energy. And then the EDLC is considered to be one of the useful devices for energy storing and recycling. The ESS is developed by applying EDLC, which is attracting widespread attention as a large-capacity energy storage medium. The EDLC, which does not rely on chemical reactions, suit to frequent rapid charge/discharge, and everlasting in principle. The EDLC contains no heavy-metal parts and so is environment-friendly. If the electrodes of an electrolytic capacitor are dipped into an electrolyte, thin insulated layers form in the electrolyte that is in contact with the electrodes. As a result, two pairs of what are known as electric double layers are formed between the electrode and the insulated layer. If the allied voltage is lower than the minimum voltage that electrolyzes the electrolyte, the layers are maintained and the gap between them is extremely thin, so that a large amount of energy can be stored. The structure of an electric double-layer capacitor is shown in Figure 1. The electrolyte constitutes the two layers and a middle layer. Porous carbon is used so that the electrodes offer a large surface area, and organic liquid often used as an electrolyte. The general equivalent circuit of an electric double-layer capacitor is indicated with a capacitance(c) and a resistance(r) connected in series. This is a good approximation of any real capacitor if long-term processes are excluded. The terminal voltage of the capacitors v and the charge/discharge current of the capacitors I can be expressed by the following equation. The equation illustrates the fundamental fact that increases in the charge/discharge current mean increases in the value of Ri. This situation causes not only increased energy loss but also a small usable amount of terminal voltage. When electric double-layer capacitors are applied to high power energy storage systems, therefore, techniques to increase the capacitance and reduce the inner resistance are required. (1) Fig. 1 Structure of EDLC EDLC is applied in the ESS. EDLC is ideal not only from the point of view of its price and performance. The ESS is able to store and discharge energy extremely quickly. It also offers a long service life compared with the flywheel energy storage unit and maintenance-free. It enables the energy content to to be scaled or cascaded without any problem. Design of ESS In lines in which regenerative vehicles are used, regenerated energy is recycled amongst vehicles for efficient use. However, about 40% of excess energy generated by braking vehicles will be wasted when no powering vehicle is existed within effective distance. This dissipation constitutes resistance heat at electrical brake and frictional heat at mechanical brake. The ESS is simple solution to store the braking energy in an EDLC and recycle it to powering vehicle. The ESS as one of railway power traction system will be installed in the substation.

Fig. 2 Energy flow in urban transit system When the ESS is installed in substation, figure 2 shows energy flow. The ESS stores excess energy from braking vehicle. Powering vehicle receives traction power fed from a substation and the ESS. And exchange of excess energy amongst powering vehicle and braking vehicle. The entire ready-to-connect the ESS is laid out in substation. Figure 3 shows the configuration of the ESS. Fig. 3 Configuration of ESS The system can be connected either directly to the traction power supply system or the busbar in the substation by means of the connection unit, which comprises the disconnector, the DC high-speed circuit-breaker and the precharging unit. The connection between the connection unit and the actual storage unit is made by the standard vehicle converter, which functions as a step-up/step-down converter. The specification of the ESS is shown in Table 1. EDLC Chopper Configuration Elec. Characteristic Table. 1 Specification of ESS Mass Size Max. Volt. Max. Amp. Capacity Total energy Inter. Resist. Technical Data 250Kg 1.75M(H) X 1.2M(W) X 0.7M(D) 583 V 250 A 13.75 F 2.34MJ 102 mω Configuration Size 1.9M(H) X 1.4M(W) X 1.0M (D) Feeder Volt. EDLC DC high-speed circuit-breaker Discharging resistance Charging Volt. Discharging Volt. Max. Amp. Volt. Max. Amp. Size Rated Volt. Braking Amp. Resistance Max. Amp 750 V 735 V 250 A 230 V 500 V 250 A 500mm X 170mm X 540mm 750V 400A 10 Ω 100 A

Installation of ESS Since the ESS is design to directly fit to DC system, no high voltage equipments are required and allowed to located minimum space. The ESS is very compact, its main components being the capacitor bank(edlc) and the converter(shopper) with connection unit(feeder circuit braker). The ESS is installed inside substation of LRT test track. Figure 4 shows the substation and the station. The LRT test track is located in Gyeongsan City. The length of line is 2.37km. There is 4 stations. The feeder voltage is 750V. Figure 5 shows powerelectronics and EDLC installed in substation. The size of powerelectronics and EDLC are represented. Fig. 4 LRT test track Test of ESS Fig. 5 Installation of ESS Factory test Before the ESS is installed in LRT substation, the ESS is test regarding charge/discharge in a factory. Figure 6 shows that at the moment(time T1) the feeder voltage falls a little bit by traction power of presumed acceleration vehicle. The ESS discharges(negative) energy. The feeder voltage is stable at 750V. The ESS discharges continuously until time T2. As soon as accelerating for traction comes to an end, the feeder voltage starts to rise again and the ESS is recharged until time T3 by the energy regenerated by braking vehicles. Fig. 6 Factory test of ESS Field test The field test is performed in LRT test track. The figure 7 shows that a vehicle accelerates at time T1. At this moment, The ESS supplies energy to the railway system. The EDLC voltage is

reduced and the EDLC current is negative. AT time T2, A vehicle brakes. The ESS recharges. The EDLC voltage increases and the EDLC current is positive. At time T3, a vehicle accelerates. The ESS supplies energy to the railway system again. A vehicle accelerates from time T4 to T5. The ESS is recharged. When the ESS is on/off, the feeder voltage is observed. As soon as the ESS stops at time T5, the feeder voltage is enormously fluctuated. Fig. 7 Field test of ESS (Ⅰ) Above comprehensive same result is shown in figure 8. Fig. 8 Field test OF ESS (Ⅱ,Ⅲ) This paper does not show how much energy is saved, because there was no time to get enough data regarding the saved energy. In the future, we will obtain data for the energy saving from LRT test track. Conclusion The ESS is considered to be one of the useful devices for energy storing and recycling. The ESS reduces primary energy consumption without affecting transport capacity and punctuality. Also the ESS can stabilize the system voltage. This paper presents the development of the ESS. Namely, effect of the ESS for energy saving, applying EDLC as the storage unit of the ESS, the ESS design, installation and field tests. The result of tests for LRT test track indicates effect of the ESS. The ESS is charged by energy regenerated by braking vehicle. The ESS discharges energy for traction power of accelerating vehicle. Therefore, the feeder voltage is stable at

specified voltage. In the future, we will obtain data for the energy saving from LRT test track. Application of the ESS and efficient use of regenerating energy cut CO 2 emissions and affects environmental preservation. References [1] SIEMENS AG, "SITRAS SES, Energy storage system for mass transit systems", 2005. [2] MEIDEN brochure, "CAPAPOST Regenerative Power Storage System", 2006. [3] Takechi KONISHI, Shin-ichi HASE, "Energy Storage System for DC Electrified Railway Using EDLC", QR of RTRI, Vol. 45, No. 2, May. 2004. [4] RRR, "Power recycle vehicle", 2005. [5] Hase, S., Konishi, T., Okui, A., Nakamichi, Y., Nara, H.,andUemura, T. : PCC-Osaka 2002, Proc. Fundamental Study on Energy Storage System for DC Electric Railway System., 1456-1459, 2002. [6] Joyelle Harris Jones, "Global Markets for Capacitors, Flywheels and SMES Systems : Emerging and Mature Technologies", Business Communications Company, Inc., 2005. [7] A. Schneuwly, M. Bartschi, V. Hermann, G. Sartorelli, R. Gallay,R. Koetz, "Boostcap Double-layer capacitors for peak power automotive applications", Proceedings of the Second International ADVANCED AUTOMOTIVE BATTERY Conference(AABC), Feb. 2002, Las Vegas, Nevada. Author s Biography Hanmin Lee received the M.S. and Ph.D. degree from Korea University, Seoul, Korea, in 2006. Currently, he is with the Advanced Electric Multiple Units Research Team of Korea Railroad Research Institute, Gyeonggi-do, Korea. He was part of the Electric Power Research Team with the Korea Railroad Research Institute from 2000 to 2003. His research interests include power quality, energy storage system and Harmonics. Gildong Kim received the M.S. and Ph.D. degree from Myongji University, Seoul, Korea, in 2003. He was a researcher of Shinkansen Inverter lab. In Toshiba, Japan.. Currently, he is a head of team and with the Advanced Electric Multiple Units Research Team of Korea Railroad Research Institute, Gyeonggi-do, Korea. His research interests include VVVF propulsion system, advanced electric train design and energy storage system. Changmu Lee is coursing for a doctorate at Korea University from 2003. He is presently a senior researcher of the Advanced Electric Multiple Units Research Team at Korea Railroad Research Institute. His research interests include power quality and power system control of electric railway.