Electric double-layer capacitor (EDLC) is an electrochemical capacitor that has an unusually high energy density when compared to common capacitors, typically on the order of thousands of times greater than a high capacity electrolytic capacitor. It is also known as ultracapacitor, supercapacitor, supercondenser, pseudocapacitor.

Conventional capacitors have enormous power but store only tiny amounts of energy. Ultracapacitors offer a unique combination of high power and high energy. Ultracapacitors are capable of very fast charges and discharges, and are able to go through a large number of cycles without degradation.

Where high currents are involved and conventional batteries are deficient, ultracapacitors offer savings in size and a unique combination of high-power and high-energy performance parameters. Ultracapacitors fall between high-energy, low-power batteries and low-energy, high-power electrolytic capacitors.

Working Principle of Ultracapacitors

An ultracapacitor is a double-layer capacitor can be viewed as two nonreactive porous plates, or electrodes, immersed in an electrolyte, with a voltage potential applied across the collectors. In an individual ultracapacitor cell, the applied potential on the positive electrode attracts the negative ions in the electrolyte, while the potential on the negative electrode attracts the positive ions. A porous dielectric separator between the two electrodes prevents the charge from moving between the two electrodes.

The plates hold opposite charges which generates an electric field. Ultracapacitors store energy via electrostatic charges on opposite surfaces of the electric double layer, which is formed between each of the electrodes and the electrolyte ions.

Advantages of Ultracapacitors

  • Highest capacitance density of any capacitor technology
  • Faster charging and discharging
  • Low internal resistance in comparison with batteries
  • High power density due to high discharge currents
  • Ability to operate at temperatures as low as -40°C
  • Effective capacitance for specific pulse widths
  • Low equivalent series resistance (ESR)
  • Higher cycle life, making them suitable for automotive applications. Ultracapacitors can be charged and discharged almost an unlimited number of times
  • No chemical actions
  • Reliable operation in harsh environments
  • Do not release any thermal heat during discharge
  • Virtually maintenance free operation
  • Lowest cost per farad
  • Exceptional efficiency
  • Compact design
  • Extended operating life. Anecdotal evidence suggests that they lose about 80% of their storage capacity after 10 years, with a lifetime estimated to be 20 years
  • Extended temperature range vs. batteries
  • There is no danger of overcharging; when fully charged the ultracapacitor simply quits accepting a charge
  • They do not release any hazardous substances that can damage the environment. They are environmentally safe

Applications of Ultracapacitors

Consumer Electronics

Ultracapacitors find their widest use in the consumer electronics market where they serve as backup sources for memory, microcomputers, system boards and clocks. Another huge market for Ultracapacitors is the mobile phone market where they are increasingly used to reduce the size and performance of mobile phones. They are ideal for quick charge devices that require lesser time than batteries to charge such as moving toys. In such cases it is less expensive and need not be replaced during the device’s lifetime.

Transportation

The next largest market for ultracapacitors is in the transportation industry. They are widely known for their use in tandem with other energy sources particularly batteries. The primary source, however, is the engine while ultracapacitors serve as the secondary source. Here their main advantages lie in their ability to perform well in extremely low temperature up to -40°C along with their small size and lightweight design in comparison to batteries. Fuel cell powered automobiles are gaining momentum in the evolving ultracapacitor market. The ultracapacitor and fuel cell duo prove to be an ideal combination in stop-and-go traffic where ultracapacitors provide sudden bursts of energy during start up, while fuel cells provide the required sustained energy.

Uninterrupted Power Supply (UPS) Systems

Ultracapacitors are being used for temporary backup power in UPS systems. Ultracapacitors are being used as an alternative to batteries due to their large capacity (capacitance) and energy density. One of the main advantages of Ultracapacitors is its ability to provide instantaneous supply without any delays that can impair mission-critical applications. Another application of Ultracapacitors that is being considered is the short term bridge power applications for stationary UPS that are augmented with fuel cells. Ultracapacitors are best suited to provide start up as well as peak load buffering. Instead of additional battery storage, Ultracapacitors serve as the ideal way to reduce the size of the system as well as the costs.

Typical examples of Ultracapacitors are systems on the premises of critical-load utilities such as hospitals, banking centers, airport control towers, and cell phone towers. The critical time between a power outage and the onset of a generator is bridged effectively by Ultracapacitors, which provides power in the order of milliseconds to a few seconds.

Challenges of Ultracapacitors

  • The amount of energy stored per unit weight in first generation ultracapacitors is considerably lower than that of an electrochemical battery (3-9 Wh/kg for an ultracapacitor compared to 40-120 Wh/kg for a battery).
  • The voltage varies with the energy stored. To effectively store and recover energy requires sophisticated electronic control and switching equipment.
  • The foremost challenge is from traditional batteries such as the lead acid, lithium ion, nickel cadmium (NiCD), nickel metal hydride (NiMH) and others which existed in the market for more than hundred years
  • Equivalent Series Resistance values can be optimized only with efficient packaging of the ultracapacitor
  • Cost of raw materials are significantly high and plays an important role in the pricing of ultracapacitors
  • Adoption rates are only gradually increasing as end-users realize the benefits of ultracapacitors
  • Linear discharge voltage characteristic prevents use of all the available energy in some applications.
  • Power only available for a very short duration.
  • Low capacity compared to batteries
  • Cell balancing required for series chains.
  • High self discharge rate. Much higher than batteries.

For information on ultracapacitors, You can also read Batteries Vs EDLC Ultracapacitors, and Manufacturers List for EDLC Ultracapacitor Supercapacitor.