Development of Energy Storage System for DC Electric Rolling Stock applying Electric Double Layer capacitor

Y.Sekijima1, Y.Kudo1, M.Inui1, Y.Monden2, S.Toda2, I.Aoyama2

1 Central Japan Railway Company, Aichi, Japan; 2 Toshiba Corporation, Tokyo, Japan

1 Introduction

Railway is said to be more environment-friendly than other transportation, especially on carbon dioxide emission (CO2) and energy consumption. To reduce them, Central Japan Railway Company (CJRC) is trying to lessen weight and pneumatic resistance of its rolling stocks and to apply a regenerative brake to its rolling stocks. As the result of these efforts, CJRC has been awarded by Ministry of the Environment in December, 2003. As for Kyoto Protocol executed on February 16th 2005 , CJRC also has reduced its consumption of transportation energy in 1995 by 10% in 2003. A regenerative brake can utilize brake energy generated in a rolling stock and is widely used for CJRC’s rolling stocks. In case of DC electric rolling stocks, which are used in Conventional Lines, a voltage of a wire rises by regenerated brake energy unless other trains are running nearby. This phenomenon leads “Regenerative Brake Failure”, which stops regeneration or lessen regenerated energy when a voltage of wire exceeds a certain value. In this condition, braking energy is compensated by an air brake due to shortage of regenerative brake energy, which increases mechanical wear and loss of electrical energy. Since more than 60% of CJRC’s DC electric rolling stocks have been equipped regenerative brake, Regenerative Brake Failure has been a serious problem. (Figure.1)

Figure.1 Regenerative Brake Failure On the other hand, in the automobile industry, hybrid cars are utilizing regenerated brake energy through secondary batteries. Thus, railway rolling stocks also could utilize regenerated brake energy by using energy storage devices such as secondary batteries. It can store regenerated energy which cannot be used for other trains in case of Regenerative Brake Failure as well as the stored energy can be used for acceleration. These procedures would lead an effective use of energy, so we have developed an onboard energy storage system. (Figure.2)

Regenerative brake of a current rolling stock Regenerative brake failure of a current rolling stock

Regenerated Energy

Motor Train A: Brake Train B: Acceleration

Little regenerated energy due to absence of other trains near by

Consumption of regenerated energy by mechanical brake

Figure.2 Energy Storage

2 Development of Energy Storage System

2.1 Selection of Energy Storage Device

As energy storage devices, secondary batteries such as nickel metal hydride battery (Ni-MH) or lithium-ion battery is frequently used for hybrid cars. On the other hand, some delivery trucks, which often run more than one million kilometers or for more than ten years, adopt an electric double layer capacitor (EDLC). The reason is that using EDLC could lead lower operational cost due to its long life span. We adopted EDLC, whose life span is longer than secondary batteries, as the energy storage device for CJRC’s rolling stocks because they are expected to be used for more than 30 years after production. EDLC is so durable that it can repeat charging and discharging rapidly without reducing its capacity or life time though capacity of EDLC is less than that of secondary batteries. Comparing with this, secondary batteries will end their life time after charging and discharging in several months when they are used in railway rolling stocks. Based on these characteristics, we have concluded that EDLC should be better than secondary batteries for energy storage device of rolling stocks.

2.2 Basic experiment using small-scale energy storage system

In the early phase of the development, we have produced a small-scale energy storage system which assumes a main circuit of rolling stocks as well as a wire, for developing the base of energy storage system and controlling method. The small-scale energy storage system uses 250V of voltage and 20A of current, while actual rolling stocks use 1500V and 400A. (Figure.3) We have experimented on this small-scale energy storage system, and verified that the basic control functions, such as charge of EDLC, acceleration using stored energy, and charging on Regenerative Brake Failure, are operated normally. We have also installed a control circuit chip used in CJRC’s DC electric rolling stock, Series 313, on an inverter of a small-scale energy storage system, and have verified that the developed control method was effective for actual rolling stocks.

Figure.3 Small-scale energy storage system

Regenerative brake of an EDLC installed rolling stock Acceleration of an EDLC installed rolling stock

EDLC EDLC Storage of regenerated energy in an EDLC when braking

Reuse of the stored energy for acceleration

2.3 Production of experimental full-scale energy storage system

In the next phase of development, we have constructed an experimental full-scale energy storage system, which can be installed on actual rolling stocks of CJRC’s Series 313. Regenerated energy generated in rolling stocks is normally supplied to a wire, so braking energy is generated from only a regenerative brake, that is, an electric brake (Figure. 4). On the contrary, in case of Regenerative Brake Failure, braking energy is generated from an air brake as well as an electric brake (Figure. 5). Thus, a capacity of onboard EDLC should be more than the amount of air brake energy, which cannot be supplied to a wire due to Regenerative Energy Failure. We have analyzed actual data of braking of Series 313 operated in Tokaido Line and Chuo Line in order to determine a capacity of an experimental EDLC, which is 0.6kWh. This value is an average of air brake energy in case of Regenerative Brake Failure. Regenerative Energy Failure actually happens in the high speed range of rolling stock, where air brake energy is so huge. Then EDLC could be used for reduction of a peak of an air brake energy, which helps to reduce mechanical wear of brake pads and wheel treads. Just to satisfy this purpose, the capacity of an EDLC could be less than the calculated one, which is 0.6kWh based on the amount of air brake energy. Then, we have determined the capacity of EDLC 0.28kWh, which is about half as much as the average air brake energy on Regenerative Energy Failure, considering installation on an actual rolling stock. Air brake energy on Regenerative Energy Failure sometimes exceeds 1kWh. Even in such a case, storing 0.28kW of brake energy in EDLC can reduce a peak of air brake energy in a high speed range, which is deducted by simulation shown on Figure.6.

Figure.4 Braking energy profile (Normal condition)

Current [A] Pressure [kPa] Speed [km/h]

Time [s]

Notch Voltage [V]

1500

1600

1700

1800

Motor Car: Brake Cylinder Pressure

Trailer Car: Brake Cylinder Pressure

Motor Current

Brake Notch

Speed

Wire Voltage

Figure.5 Braking energy profile (Regenerative Brake Failure) 0.28kWh of EDLC consists of 570 cells, which are connected in series. It charges and discharges between 700V and 1425V. Capacity and terminal voltage of each cell are respectively 800F and 2.5V. Table.1 shows the spec of the experimental onboard EDLC. The EDLC is controlled to discharge on acceleration of rolling stocks so that it can avoid Regenerative Energy Failure.

0

50

100

150

200

250

300

350

1500

1600

1700

1800

Voltage [V]

Time [s] 00'00 00'05 00'10 00'15 00'20 00'25 00'30 00'35 00'40 00'45 00'50 0

1

2

3

4

5

6

7 Notch

Current [A] Pressure [kPa] Speed [km/h]

Air Brake Force (Motor Cars)

Figure.6 Reduction of air brake energy peak (simulation)

Wire Voltage

Speed Motor Current

Motor Car: Brake Cylinder Pressure

Trailer Car: Brake Cylinder Pressure

Brake Notch

Air Brake Force (at Regenerative Energy Failure)

Air Brake Force (at Storage of Air Brake Energy)

Speed

Table.1 Spec of the experimental onboard EDLC EDLC Cell EDLC onboard unit Operating Voltage 2.5V 700V ~ 1,425V Cell Connection N.A Series (570 cells) Capacity 800F 1.4F Energy storage 0.69Wh (maximum),

0.5Wh (usable) 0.28kWh

Size[mm] 㱠35 × 135L 900W × 730H × 900D Weight 190g 430kg (including equipment box)

Prior to installing the experimental energy storage system including the EDLC, the converter, and the inverter on an actual rolling stock, we have checked its basic function such as insulation, control sequence, charging, discharging, and coordination of protection system. We have also verified that the experimental system was able to operate with other onboard electrical machine such as a motor and a VVVF control unit. We have judged it can be installed on an operational rolling stock without making a serious safety trouble.

3 Test Run

We have installed the experimental energy storage system on CJRC’s operational rolling stock of Series 313 and performed test runs between Nagoya and Jinryo (Located on Chuo Line) in the end of January, 2005. Some Series 313 rolling stocks have brake choppers and brake resistances for disposing brake energy in case of Regenerative Brake Failure in mountain lines. We have selected such rolling stocks as test cars, and replaced the experimental energy storage system with their brake chopper and brake resistance. We have installed not only a 0.28kWh of EDLC, a DC/DC converter, and a reactor under the floor of rolling stock (shown on Figure.7) but also an additional 0.28kWh of EDLC above the floor. Then the experimental equipment has been used for confirming its characteristics in case of 0.28kWh of EDLC as well as 0.56kWh of EDLC, which is about the average energy of air brake on Regenerative Energy Failure.

Figure.7 Photo of the test car

The experimental energy storage system has operated as we had planned, that is, 0.28kWh of EDLC installed under the floor has stored 8% of energy generated by motors, which corresponds to 1.6% of energy for acceleration. Figure.8 indicates that the air brake force decreases as EDLC begins to store energy. This phenomenon means that EDLC absorbs the braking energy, which can be at most 200kW and equal to one motor. Of course, the additional 0.28kW of EDLC in the cabin enables to store twice brake energy in EDLC. On the other hand, Figure.9 shows that the stored energy in the EDLC on brake has been used for acceleration.

Figure.8 Result of test run (on braking)

Figure.9 Result of test run (on acceleration)

To evaluate an effect of reducing peak of an air brake energy, we have measured temperature on wheel treads by an irradiation thermometer because temperature on wheel treads is thought to indicate an influence toward wheel treads when an EDLC is operating. Without storage of brake energy, the distribution centers of wheel tread temperature and brake cylinder pressure are respectively 150kPa and 100°C. On the contrary, these values are reduced to 120kPa and 50°C when an energy storage system is operating (Figure. 10). These data indicates a reduction of heat load on wheel treads, which could lead to a reduction of mechanical wear.

Figure.10 Temperature on wheel treads

4 Conclusion

Through a test run in the end of January, 2005, using Series 313, we have verified that the experimental energy storage system has stored some part of air brake energy and the stored energy has been reused for partial acceleration energy. We have also verified that an influence on mechanical wear could be reduced by measuring temperature on wheel treads. Thus the energy storage system, which is currently under development, could be an environment-friendly technology. EDLC is also one of environment-friendly technologies and is expected to be applied mainly to cars as well as be developed for large capacity, low cost, and so on. We are planning to install an EDLC of larger capacity on a rolling stock and to confirm its life span and durability through a long term endurance run in operation. Based on this result, we would like to improve performance, cost, and reliability of EDLC for its practical use.