IEEE Electrification Magazine - September 2014 - 42

1) An SMES can store energy for a lengthy period with an
efficiency higher than 95%.
2) An SMES can offer a large storage capacity and fast
response time.
3) An SMES is easy to maintain and produces pollutionfree energy.
4) An SMES is easy to manipulate and has a long lifetime
as it has no rotating parts except for vacuuming and
refrigeration parts.
However, SMES has not been used widely for large-scale
station applications because:
1) There is a limited production of superconducting
materials.

Grid
Battery
Control Unit

R
R1

-
+ Third Rail
Figure 4. The battery storage system of a commuter train.

Grid
SMES
Control Unit

-
+ Third Rail
Figure 5. The SMES system of a commuter train.

Grid

2) There is a high cost for the refrigeration and operation
of superconductive materials.
3) The quenching protection of superconducting and the
ecological influence of high-intensity magnetic field is
at the stage of research.

Ultracapacitor Storage System
Figure 6 shows the ultracapacitor energy storage system
for regenerative energy applications, which includes the
ultracapacitor bank, insulated-gate bipolar transistor (IGBT)
chopper, dc high-speed circuit breaker, disconnecting switch,
sensors, and microcomputer control unit. An ultracapacitor
offers a low-voltage capacity, and a large number of ultracapacitors are often connected in series-parallel modules. However, any major wavering in ultracapacitor parameters, which
could be due to ambient conditions, would result in ultracapacitor voltage fluctuations. Such changes would shorten the
service life of ultracapacitors and significantly reduce their
operation reliability for regenerative energy applications.
Thus, ultracapacitor storage systems are often designed to be
relatively small and could not fully absorb the regenerative
energy generated by several trains on high traffic railways. At
present, the ultracapacitor storage system is used in railway
transit systems located in Frankfurt, Germany, Madrid, Spain,
and the Beijing commuter subway in China.

Flywheel Energy Storage System
Figure 7 shows the flywheel energy storage for regenerative energy. A flywheel energy storage system includes a
rotating component, electric machine, bidirection converter, and vacuum suspension chamber, which would make
use of high-speed rotating elements. At the energy storage
stage, the bidirection converter drives the electric motor in
flywheel to accelerate to a certain constant speed. At the
energy release stage, the flywheel drives the electric
machine as a generator when the mechanical energy is
converted to electric energy. Flywheel energy storage
offers several advantages including high energy density,
high reliability, easy maintenance, a charge-discharge
cycle, which is independent of the depth of charge-discharge, and a nonpolluting device with a low environmental impact. However, the flywheel energy storage system is
suitable for a short discharge time and high-power applications as it cannot store energy for a long time. Currently,
the flywheel energy storage system is used in the
New York City Subway and in the tramway in Hong Kong.

Ultracapacitor Bank
Control Unit

-
+ Third Rail
Figure 6. The ultracapacitor storage system of a commuter train.

I E E E E l e c t r i f i c ati o n M agaz ine / september 2014

regenerative energy for supplying the
Distribution Grid at train stations
In the "Regenerative Energy Braking for Storage at Train Stations" section, we discussed the use of energy storage
devices for regenerative energy, which would increase the
energy investment cost in train stations. In this section, we
propose a regenerative energy scheme, shown in Figure 8,
that could directly feed the regenerative energy back to the
distribution grid at train stations for lighting, ventilation, air

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2014