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   Batteries (storage technology)  evaluated  
Batteries (or accumulators) are electrochemical energy storage devices used for a wide variety of purposes. Compared to other storage devices batteries have very high energy densities, but low power density and therefore high charging times. Some modern high performance batteries do however reach power densities that are promising for braking energy storage in automotive and (to a smaller extent) railway applications.
Technology field: Regenerative braking and energy management
close main section General information
  close sub-section Description

Batteries (or accumulators) are electrochemical energy storage devices used for a wide variety of purposes ranging from watches and cell phones to cars and industrial applications.

As can be seen from the Ragone diagram in Figure 1, batteries have very high energy densities compared to other storage devices, but suffer from low power density and resulting high charging times. The reason lies in the conversion from electric to electro-chemical energy and back. Some modern high performance batteries especially the Nickel-metal hydride type do however reach power densities that are promising for braking energy storage at least in automotive applications.

Figure 1: Ragone diagram


Source: IZT

The most relevant requirements posed on high-power batteries for transport applications are high power and energy densities. A high charge acceptance to maximize regenerative braking utilization, and long calendar and cycle life, electric and thermal balance and recycleability are additional technological challenges.

In the following some of the more relevant battery types are briefly presented.

Lead acid batteries can be designed to be high power and are favoured by low price, high safety and reliability. A recycling infrastructure for lead acid batteries is in place. Drawbacks are low energy densities, poor performance at low temperatures, and short calendar and cycle lifes. Lead acid batteries are currently used in many electric vehicles. Advanced high-power lead acid batteries are presently being developed for application in hybrid-electric vehicles.

Nickel-cadmium batteries are used in many electronic consumer products. They have higher specific energy and a better cycle life than lead acid batteries, but do not deliver sufficient power and are therefore not promising for braking energy storage.

Nickel metal hydride batteries, used in computer and medical equipment, have good energy and power densities. Recyclability is satisfactory, but a recycling infrastructure does not exist yet. Nickel metal hydride batteries have a much longer life cycle than lead acid batteries and are safe. They are used successfully in electric cars and recently in hybrid-electric cars. Challenges of nickel metal hydride batteries are high prices, high self-discharge and heat generation at high temperatures, the need to control losses of hydrogen, and low cell efficiency.

The lithium ion batteries are characterised by high energy density and are therefore an attractive option for laptops and cell-phones. Further benefits are high specific power, high energy efficiency, good performance at high temperatures, and low self-discharge. Recycleability is also acceptable. These characteristics make lithium ion batteries suitable for braking energy storage. A commercial use in transportation is however still impeded by high costs and calendar life is quite high but could still be improved.

Lithium polymer batteries have the potential to provide the high specific power needed for braking energy storage. In addition, they are safe and have good cycle and calendar life. For a commercial use in transport cost has to drop and higher specific power batteries have to be developed.

close main section General criteria
  close sub-section Status of development: concept
    (no details available)
  Time horizon for broad application: (no data)
    When it comes to railway applications, today’s batteries fall behind flywheels and supercapacitors. Therefore broad application in railways seems doubtful but not impossible if future batteries improve substantially in performance.
  Expected technological development: highly dynamic
    Although batteries have existed for a long time, there is potential both for further advances of existing battery types and for the development of entirely new concepts.

Energy saving

  Benefits (other than environmental): not applicable

Depends on application context.

Cf. Stationary energy storage in DC systems, Diesel-electric vehicles with energy storage and On-board energy storage in DC systems.

  Barriers: high
    Mainly performance characteristics of today's batteries and high costs.
    Success factors:
    (no details available)
  Applicability for railway segments: medium
    Type of traction:  electric - DC, electric - AC, diesel
    Type of transportation:  passenger - regional lines, passenger - suburban lines
    Applicability depends on specific application context. Whereas present power densities of batteries are sufficient for a use in hybrid electric cars, for railway applications the required battery capacity would add too much weight to the vehicle and need too much space. An energy storage based on batteries is therefore presently not cost-efficient in rail vehicles. Further developments in battery technology could however facilitate a railway application in long-term perspective.
    Grade of diffusion into railway markets:
  Diffusion into relevant segment of fleet: 0 %
  Share of newly purchased stock: 0 %
    (no details available)
  Market potential (railways): (no data)
    Applicability to railways is presently too doubtful to assess the corresponding market potential.


close main section Environmental criteria
  close sub-section Impacts on energy efficiency:
  Energy efficiency potential for single vehicle: > 10%
  Energy efficiency potential throughout fleet: 1 - 2%
    Cf. Stationary energy storage in DC systems, Diesel-electric vehicles with energy storage and On-board energy storage in DC systems.
  Other environmental impacts: ambivalent
    Many battery types contain toxic substances (lead, cadmium etc.). The environmental impact depends on specific chemical composition and recycling infrastructure.
close main section Economic criteria
  close sub-section Vehicle - fix costs: not applicable
    Investment costs for energy storage systems depend on application context but are generally high.
  Vehicle - running costs: significant reduction
    (no details available)
  Infrastructure - fix costs: low
    Recycling requires certain additional efforts in workshops.
  Infrastructure - running costs: unchanged
    (no details available)
  Scale effects: medium
    In general scale effects are limited since battery technology is a mature technology. However for new developments in battery technology, scale effects could be achieved.
  Amortisation: (no data)
    Depends on application context. Cf. Stationary energy storage in DC systems, Diesel-electric vehicles with energy storage and On-board energy storage in DC systems.
close main section Application outside railway sector
  close sub-section Status of development outside railway sector: in use
    Nickel metal hydride batteries are already used for braking energy storage in hybrid-electric cars.
  Time horizon for broad application outside railway sector: in 5 - 10 years
    The first hybrid-electric cars are on the road but even if the concept proves attractive for customers, wide-spread diffusion will take some time.
  Expected technological development outside railway sector: highly dynamic
    Cf. technological potential in railways
  Market potential outside railway sector: high
    (no details available)
close main section Overall rating
  close sub-section Overall potential: interesting
  Time horizon: long-term
    Batteries will only become an option for brake energy storage on rail vehicles if clear performance advances are achieved. Presently flywheels and double-layer capacitors are the more promising options.
References / Links:  Caputo 2000;  Hennig, Stephanblome 2000;  Hentschel et al. 2000;  US Department of Energy
Related projects:  Studies performed on energy storage systems
Contact persons:
 date created: 2002-10-09
© UIC - International Union of Railways 2003
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