IEEE Electrification Magazine - September 2016 - 32

TABLE 1. The main features of the different technologies.
Technologies/Features
Bad
Medium
Cost of
Infrastructure
Good

Life Cycle
Cost

Energy
Energy
Catenary Transmission Provider
Availability Safety Recovery Free
Efficiency
Dependency

Reversible substation

High

Medium

High

Low

Energy
storage

Off board

Medium

High

Low

On board

Low

Medium

High

Yes

High

Low

systems based on batteries [lithium-ion (Li-ion) in
most cases] or ultracapacitors. The batteries usually
have lower power density than the ultracapacitors in
charging and discharging modes even when the batteries present a much higher energy density. On the
other side, ultracapacitor life expectancy in terms of
cycles is much longer than the batteries (1 million
cycles versus 2,000). In Table 2, a comparison of the
on-board energy storage technologies is summarized. In Figure 1, different technologies are shown.

battery based on an on-board accumulation system with
ABB battery technology; their streetcars can be powered
from the catenaries or from the batteries. A similar
approach can be found in KinkiSharyo AmeriTram applications. Due to the different characteristics of the battery
systems and ultracapacitors systems, the trend right now
is the combination of both technologies. Thus, this solution is adopted by SIEMENS SITRAS system or CAF OESS.
Also, ALSTOM is proposing a new
system called ECOPAC based on this
hybrid technology to be installed in
the Nice Tram in the near future.
Today, a proper combination of batteries and ultracapacitors seems to be
the most feasible solution for highly
efficient catenary-free transportation,
but on-board accumulation could also
be an excellent solution for increasing
the whole system efficiency in conventional catenary systems.
Basically, all on-board accumulation systems use the energy regenerated during the braking process
to charge the on-board accumulation device. When the train demands energy for traction purposes, this energy can be extracted from the
accumulation device or from the catenary. In the cases
where no catenary exists, all the requested power is
provided by the accumulation system that is charged
at the train stations. In this section, a deeper description of the OESS focusing on the solution proposed by
CAF Group is given. This solution is branded Greentech [formerly (Acumulador de Carga Rápida (ACR)

The use of controlled
converters could
provide the ability
of returning the
braking energy
surplus into the ac
distribution network.

On-Board Accumulation
Systems

The most important manufacturers
already have their proposals for the
installation of on-board energy
storage systems (OESSs), and most
of them are based on batteries,
ultracapacitors, or a combination of
both. There are basically two main
applications of these kinds of technologies; the first one is the use of
accumulation systems for energysaving purposes, even when the train is connected to
the power supply system. The second one is the use of
energy storage for the autonomous train operation.
ALSTOM, for instance, installed batteries as a backup
system for its catenary-free aesthetic power supply system. Bombardier's modular integrated traction system
solution is based on ultracapacitors, and they can combine the proprietary PRIMOVE feeding solution with the
use of on-board batteries. Brookville also developed a

TABLE 2. The main features of the energy storage technologies.
Technologies/Features
Bad
Medium
Good

Life Cycle
Cost

Energy
Density

Power

Fast
Charging

Availability

Safety

Maturity

Fuel cell (Hydr.)

Low

High

Medium

Yes

Medium

Low

Batteries

Medium

High

Low

High

Flywheel

Medium

Low

High

Yes

Medium

High

Supercapacitors

Medium

High

Yes

High

I E E E E l e c t r i f i c ati o n M agaz ine / SEPTEMBER 2016

Table of Contents for the Digital Edition of IEEE Electrification Magazine - September 2016