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   Double-layer capacitors (storage technology)  evaluated  
Double-layer capacitors (also termed "supercapacitors" or "ultracapacitors") store energy in the electric field of an electrochemical double-layer. The use of high surface-area electrodes results in an extremely large capacitance. The power and energy densities make capacitors an option for brake energy storage in rail vehicles.
Technology field: Regenerative braking and energy management
close main section General information
  close sub-section Description
   

Double-layer capacitors also called ‘supercapacitors’ or ‘ultracapacitors’ store energy in the electric field of an electrochemical double-layer. The use of high surface-area electrodes result in an extremely large capacitance.

Principle

Modern double-layer capacitors consist of two activated carbon electrodes, which are immersed into an electrolyte. The electronic contact between the electrodes is inhibited by a separating membrane which at the same time permits the mobility of the charged ions. If a voltage is applied between the two electrodes, the organic electrolyte supplies and conducts the ions from one electrode to the other. When the capacitor is charged, anions and cations are located close to the electrodes and balance the excess charge in the activated carbon. This way, across the boundary between activated carbon and electrolyte two charged layers of opposed polarity are formed.

Figure 1: Double-layer capacitor

Caps_pic.jpg

Quelle: Schneuwly et al. 2002

 

Characteristics of modern double-layer capacitors

Currently-marketed ultracapacitors reach values of over 3000 F in the high power range. These high values can be achieved with electrodes covered with specific active coal whose surface density per weight unit can reach 3000 m2/g.

Current ultracapacitors made by Montena obtain energy densities of up to 5 Wh/kg and power densities of up to 10 kW/kg.

Typical charging times range from 300 to 5 sec. For braking energy storage on rail vehicles, the high energy supercaps type with charging time of about 300 sec., and the high power supercaps type with charging time of about 10 sec are of special interest.

Figure 2: Technical data of some Montena double-layer capacitors

Caps_Table.gif

Quelle: Schneuwly et al. 2002

 

Comparison with other storage technologies

Figure 3: Ragone diagram

Ragone-diagram_caps.gif

 

Source: IZT, data from Schneuwly et al. 2002

 

Fields of application

Transportation applications: railways (on-board and stationary storage of braking energy in DC systems, diesel-electric vehicles, catenary-free operation of city light rail, starting system for diesel engines), hybrid-electric cars.

Industrial applications: Uninterruptible power supplies, elevators, pallet trucks etc.

Manufacturer:

Alcatel Alsthom, Cooper Electronic Technologies, EPCOS, Montena Components SA and others.

close main section General criteria
  close sub-section Status of development: test series
    Several projects are actually running in the field of transportation applications, e.g. tram supply without catenary and voltage drop compensation for weak distribution networks.
  Time horizon for broad application: 5 - 10 years
    (no details available)
  Expected technological development: highly dynamic
   

The main development goals for double-layer capacitor are

  • long lifetime
  • increase of the rated voltage
  • improvement of the range of operating temperature
  • increase of the energy and power densities.

According to Montena, the rated voltage will be increased up to 3 V within the next years. To easily access automotive applications a temperatures range from –35 up to 105 °C seems realistic. Montena fixes energy and power density goals for double-layer technology at 10 Wh/kg and 10 kW/kg.

From on-going research and development, Montena expects an increase of the electrolyte decomposition voltage and ionic conductance, an increase of the electrode accessible surface, chemical and mechanical stability as well as electronic conductance and the separator electronic insulation level and ionic conductance. A main activity lies in the development of new electrolytes based on the combination of novel organic solvents and improved conduction salts, permitting not only a higher rated voltage and a higher conductivity but also a larger operating temperature range.

    Motivation:
    If used as a storage technology for braking energy, the motivation is saving energy. Other possible applications include the catenary-free operation of city trams.
  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: medium
   

State of development

Double-layer capacitors are only starting to become a mature and reliable technology.

    Success factors:
    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.
  Applicability for railway segments: medium
    Type of traction:  electric - DC
    Type of transportation:  passenger - regional lines, passenger - suburban lines
   

As can be seen from Figure 4 the performance of double-layer capacitors is fitted for the range from light rail vehicles to regional trains.

Figure 4: Ragone diagram and charging times (corresponding to braking times of different trains)

Ragone-diagram_trains.gif

Source: IZT, data mainly from: Hentschel et al. 2000

    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): low
    (no details available)
    Example:
    Stationary storage system in urban transport network Cologne
close main section Environmental criteria
  close sub-section Impacts on energy efficiency:
  Energy efficiency potential for single vehicle: > 10%
  Energy efficiency potential throughout fleet: not applicable
    Cf. Stationary energy storage in DC systems, Diesel-electric vehicles with energy storage and On-board energy storage in DC systems.
  Other environmental impacts: neutral
   

 

close main section Economic criteria
  close sub-section Vehicle - fix costs: high
   

According to Montena, during the next years the costs of double-layer capacitors will decrease significantly. Several reasons are given:

  • First, the nominal voltage of Montena BOOSTCAPs®, currently rated at 2.5 V, will go up to 3 V within the next years. This way, the costs of a module built up from several capacitors units will be reduced by about 25 %.
  • Second, an important cost reduction will come from the automation of the whole production process. Especially, the winding technology used for the production of double-layer capacitors offers a big potential for cost reduction.
  • Third, cf. Scale effects .

Montena estimates that, thanks to these improvements, future costs per energy content will come down to 10 US$ per 1000 F. In addition, module costs will decrease due to higher production volumes and new low-cost voltage sharing technologies.

  Vehicle - running costs: significant reduction
    Cf. Stationary energy storage in DC systems, Diesel-electric vehicles with energy storage and On-board energy storage in DC systems.
  Infrastructure - fix costs: not applicable
    For on-board applications no additional infrastructure investment is needed. In the case of Stationary energy storage in DC systems, infrastructure cost is high.
  Infrastructure - running costs: not applicable
    cf. Infrastructure fix costs.
  Scale effects: high
    A substantial cost reduction can be expected from increased production quantities. Given a high market potential for double-layer capacitors outside railways, high production volumes will allow to purchase the capacitor materials at higher volumes and therefore lower prices. Especially the electrode price as the key material in a double-layer capacitor depends strongly on the ordered quantity.
  Amortisation: 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.
close main section Application outside railway sector
  close sub-section Status of development outside railway sector: prototype
    Montena is realizing in collaboration with the University of Applied Science of Central Switzerland and other partners, a minibus based on the combination of a combustion engine with an electric power train involving a BOOSTCAP, a double-layer capacitor by Montena.
  Time horizon for broad application outside railway sector: in 5 - 10 years
    (no details available)
  Expected technological development outside railway sector: highly dynamic
    Cf. Technological potential outside railway sector
  Market potential outside railway sector: medium
    According to the Paumanok Supercapacitor Market Survey 1999/2000, the world market for double-layer capacitors was estimated to increase up to 500 Mio US$ within the next 5 years (i.e. by 2004/2005).
close main section Overall rating
  close sub-section Overall potential: very promising
  Time horizon: mid-term
    Double-layer capacitors are becoming a promising solution for brake energy storage in rail vehicles. Competition mainly comes from fly-wheels which at present still offer certain advantages over capacitors. However capacitor technology has developed quite dynamically in the recent past and has been attributed great future potential by some experts.
References / Links:  Avart, Chabas 2001;  Caputo 2000;  Charvet 2001;  Hennig, Stephanblome 2000;  Kötz, Carlen 2000;  Schneuwly et al. 2002
Attachments:
Related projects:  Studies performed on energy storage systems;  THALES: hybrid tram train with on-board ultracapacitors
Contact persons:
 date created: 2002-10-09
 
 
© UIC - International Union of Railways 2003
 
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