You are welcome LMDF,
The following lifted from here..
transportation.anl.gov
Excerpts: (Dynamic Brake= DB)
"Recovery of this DB electrical energy to do useful work would
provide considerable benefits."
"The primary hurdle to the recovery of DB energy is the lack of a suitable way to store the
energy until it is needed."
"However, since the electric traction motors
generate electricity during braking and the diesel engine generator set also generates electricity,
some type of electrical energy storage is preferred."
"Potential for Fuel Savings. The potential fuel savings from DB energy recovery are
tremendous if a suitable storage technology can be developed."
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All of the above excerpts point to the Vanadium Energy Storage System of VRB Power
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5.2 ENERGY RECOVERY
While the locomotive traction horsepower and downhill grades supply all the energy to
move a train, a significant amount of that energy is dissipated by braking action. Braking is
necessary to reduce speed and to keep the train from exceeding speed limits on downgrades. All
rail cars and locomotives have air brakes; in addition, diesel-electric locomotives usually also
have the capability for dynamic braking (DB), where the traction motors retard the train by
running as generators, producing excess electrical power that is dissipated in the locomotive as
heat in the resistance grids. Recovery of this DB electrical energy to do useful work would
provide considerable benefits. When DB energy is recovered and utilized, it reduces net energy
required from the diesel prime mover, thereby reducing fuel usage and emissions.
Both AC and DC locomotives have DB capability and reject the recovered energy in the
braking resistance grids. A small portion of that energy is used in fans to cool the braking grids,
but no other use is made of the energy.
A useful measure of the potential savings is the percentage of the train¡¦s kinetic energy
that is dissipated as heat in the dynamic brake resistance grids. This percentage varies
considerably with train and route characteristics. For freight operations, a slow (15 mph) train
traversing a mountainous route showed a 64% braking energy ratio, and a fast (60 mph) train on
a flat route showed a braking energy ratio below 10% (Addie and Concannon 1978). For
passenger and commuter operations on generally flat routes, the braking energy ratio ranged
from 8 to 60%, depending on the average speed and the frequency of stops.
The instantaneous power level of DB is relatively high, so the DB energy cannot be
effectively used by the locomotive at the time it is generated. For a significant portion of the
recovered DB energy to be utilized, it must be either stored during braking for later use over the
route or sent off the locomotive for use elsewhere. An electric locomotive can regenerate DB
energy back into the third rail or catenary, but this option is not available for the diesel-electric
locomotive.
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The primary hurdle to the recovery of DB energy is the lack of a suitable way to store the
energy until it is needed. Significant advances are being made in energy-storage technology for
road transportation. Many candidate energy-storage technologies for these applications are also
relevant to DB energy recovery, including electrochemical batteries (electrochemical storage),
superconducting magnetic energy storage, flywheels (rotating energy storage), capacitors
(electrostatic storage), regenerative fuel cells (electrochemical storage), and compressed gas (see
Balachandra et al. 2000 and INTELEC 1998). However, since the electric traction motors
generate electricity during braking and the diesel engine generator set also generates electricity,
some type of electrical energy storage is preferred.
Key technical parameters for DB energy storage need to be quantified. They include:
„FƒnVolumetric-specific energy density (kilowatt-hour per liter) and mass-specific
energy density (kilowatt-hour per kilogram),
„FƒnCharging and discharging volumetric specific power densities (kilowatt per
liter), and
„FƒnMass-specific power densities (kilowatt per kilogram).
Just as important are:
„FƒnRound-trip energy-storage efficiency (from locomotive system to storage
medium and back to locomotive system),
„FƒnEnergy-storage cycle life, and
„FƒnAbility to accept power at extremely high rates.
The energy-storage system must perform its function while installed on the locomotive
system and subject to the environmental variations experienced by the locomotive.
Electrochemical batteries. Electrochemical storage batteries are the predominant form of
energy storage for the few automotive hybrid and pure electric vehicles on the market. Battery
technologies include lead acid, nickel-cadmium, nickel-metal hydride, lithium ion, and lithium
polymer. Lead acid batteries are relatively inexpensive and have high energy density but are
quite heavy and have low power density and short calendar and cycle life. They have been
demonstrated in hybrid bus applications. Two production hybrid automobiles (Toyota Prius and
Honda Insight) use nickel-metal hydride batteries, which are lighter than lead acid but are very
expensive, run hot, and require cooling. Lithium ion batteries are used extensively in computers
but need more development before they find large-scale use in hybrid propulsion systems.
Lithium polymer batteries are very light and offer the prospect of high power and energy
densities, but they are extremely expensive and must be developed to reduce their cost before
they can be considered as candidates for hybrid propulsion systems. The ability of all these
battery types to withstand the high vibration and shock levels of a locomotive must be
demonstrated.
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Ultracapacitors. Ultracapacitors are very-high-capacitance devices that store electrical
energy in the form of an electrostatic charge between two electrodes. They may be a good choice
for the recovery of braking energy because they have the potential for higher specific power
density and much longer cycling life than batteries. On the other hand, ultracapacitors have lower
specific energy density than batteries. Ultracapacitors may play a role in a combined storage
system with electrochemical batteries.
Electric flywheels. Electric flywheels use an electric motor/generator to (1) spin up the
flywheel to store energy during braking or when excess power is available and (2) decelerate the
flywheel to recover energy for propulsion. Depending on motor size, electric flywheels are
capable of very high power operation, which is advantageous for regenerative braking; however,
today¡¦s flywheel systems have only moderate energy density. The very high rotational speed of
the rotor requires a high-performance bearing and appropriate containment. The tolerance of this
bearing to high vibration and shock levels must be demonstrated, along with the gyroscopic
effect of the flywheel in a locomotive environment.
Technical Barriers. The main technical barrier to recovering DB energy is the lack of an
affordable storage device with sufficient energy density and a high rate of energy storage.
Although energy-storage technologies in stationary and road-vehicle applications have met with
some success, the lack of on-the-rails experience is a major barrier to developing manufacturer
and railroad user credibility and confidence in the applicability, robustness, benefits, safety,
reliability, and lifespan of DB energy-recovery technology. The cost of DB energy-recovery
technology also is expected to be a barrier to its application. Electric flywheels have a unique
technical barrier in that their motor/generator is a variable-frequency machine. To overcome this
barrier, power electronics capable of efficient conversion of variable-frequency AC to DC to
fixed-frequency AC will be required.
Potential for Fuel Savings. The potential fuel savings from DB energy recovery are
tremendous if a suitable storage technology can be developed.
Suggested R&D. It must be recognized that other energy-storage applications (highway
vehicles, power quality) will serve broader markets than the freight locomotive and railroad
market ƒnand so will drive energy-storage technology R&D and commercialization. These
applications are rapidly maturing in the 2001¡V2010 time frame, and the time is now ripe for
studying DB energy recovery. Therefore, railroad-specific R&D should target adaptation to
railroad requirements of energy-storage technologies that are under development or have been
developed for highway vehicles.
Evaluation of the hardware of energy-storage technologies under railroad transportation
conditions, underpinned by an analysis of fuel benefit vs. energy-storage-system sizing
parameters, is required to select the appropriate energy-storage technology. Such a combined
program will yield insights into the adaptation of this technology to DB energy recovery and
identify energy-storage-technology performance gaps and the engineering requirements to
qualify the technology for railroad application.
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The results of the subscale evaluation should drive the enhancement of energy-storage
and storage-system technology features to meet identified railroad application performance gaps.
With these enhanced technology features in hand, a full locomotive-scale field demonstration of
DB energy recovery should be carried out with the participation of an end user, with accurate
evaluation of the user costs and benefits. The results of this full-scale field demonstration will
unequivocally establish the benefits of DB energy recovery and set the stage for its
commercialization.
Safety of energy storage devices needs to be addressed in a comprehensive R&D
program. Batteries will store electricity at about 1,000 volts, and flywheels will spin at high
speeds. In maintenance and accident situations, the stored energy must be dissipated without
endangering railroad personnel or the public.
To maximize the efficiency of the locomotive system, an optimized energy-management
and control strategy will be required. The control strategy must recognize the selected mode and
be flexible enough to maintain optimized operation. Some of the variables that must be
considered are train type (passenger, freight, switching), loading, route, energy-management
mode (e.g., regenerative braking or consist management), and energy-storage medium (battery,
ultracapacitor, flywheel, or combination). Electronic control hardware that is rugged enough for
locomotive use is readily available. Control strategies, however, must be developed to deal with
this new technology.
5.3 MOTORS AND DRIVES
Locomotive traction motors are very reliable and effective and are based on mature DC
and AC electric-motor technologies. Finding an electric drive mechanism that can handle the
tremendous power demanded by trains has always been a design challenge. Any improvement to
the efficiency of traction motor operation will need to be weighed against the demands of the
operational environment. The optimization of price vs. performance is highly competitive, and
specific design details are proprietary.
Traction motors are rebuilt typically upon failure. Rebuilt motors must not lose
efficiency, and if possible, should be retrofit with improvements. However, the traction motor
rebuild process introduces large variability into the efficiency values of the rebuilt motors, which
can have a big effect on performance. A small drop in efficiency can produce a large increase in
heat generated and cause the motor to burn out early.
Technical Barriers. The main technical barrier to improving the consistency of the
rebuild process is a lack of understanding of the causes of variability.
Suggested R&D. Research into the causes of inconsistency in the rebuild process would
help to identify ways to improve it.
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