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Mount Electrolytic Capacitor

Faradigm Ultracapacitors

Ultracapacitors are being talked about a great deal in recent times. But what exactly are they? This article will clear all your basic doubts about that.

What are ultracapacitors?

Ultracapacitors, also called supercapacitors or electric double-layer capacitors, are capacitors that have been developed based on advanced developments in materials science and nanotechnology. These have capacitance values of several hundred Farads, unimaginable in conventional capacitors, enabling adequate energy storage for several short-duration, high-power applications.

How long has research on ultracapacitors been going on?

Ultracapacitors have been the subject of focused research and development efforts for the past two decades. All those efforts are coming to fruition recently. The price points are still such that they can't be readily used, but they have come to a level where they are very attractive for certain applications where reliability is far more important. This has been facilitated by remarkable advances in materials science and nanotechnology that allow the construction of stacks of thin interconnected electrochemical cells with very high surface area electrodes, and low Equivalent Series Resistance (ESR). This translates into the unimaginably high capacitance of ultracapacitors, which represent a new paradigm in energy storage, especially for short duration, high-power applications. Depending on their design and construction, the specific power capacity (watts per kg) of Ultracapacitors can be 10 to 100 times higher than that of most batteries!

Are ultracapacitors like standard capacitors?

Yes and no.

Ultracapacitors are like normal capacitors in the sense that once they are charged and energy is stored in them, a load can use this energy. But that's where the similarity ends.

Ultracapacitors are drastically different from normal capacitors in the way they are manufactured and in the way they store charge. Also, the amount of energy stored in ultracapacitors is very large compared to a standard capacitor because of the enormous surface area created by the porous carbon electrodes and the small charge separation created by the dielectric separator.

Can you give me a more technical description of ultracapacitors?

Ultracapacitors can be viewed as two non reactive porous plates, or collectors, suspended within an electrolyte, with a voltage potential applied across the collectors. In an individual ultra-capacitor 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 dielectric separator between the two electrodes prevents the charge from moving between the two electrodes.

Electrical energy storage devices, such as standard capacitors, store electrical charge on an electrode. Other devices, such as electrochemical cells or batteries, utilize the electrode to create an electrical charge at the electrodes by way of a chemical reaction. In both of these, the ability to store or create electrical charge is a function of the surface area of the electrode. For example, in standard capacitors, greater electrode surface area increases the capacitance or energy storage capability of the device.

As a storage device, the ultracapacitor, relies on the microscopic charge separation at an electrochemical interface to store energy. Since the capacitance of these devices is proportional to the active electrode area, increasing the electrode surface area will increase the capacitance, hence increasing the amount of energy that can be stored. This achievement of high surface area utilizes materials such as activated carbon or sintered metal powders. However, in both situations, there is an intrinsic limit to the porosity of these materials. In other words, there is an upper limit to the amount of surface area that can be attained simply by making smaller and smaller particles. An alternative method must be developed to increase the active electrode surface area without increasing the size of the device. A highly efficient electrode for electrical energy storage devices could be realized if the surface area could be significantly increased.

What are the advantages of ultracapacitors?

The advantages of ultracapacitors are:

Extremely low internal resistance or effective series resistance and consequently high efficiency (95% or more), with extremely low heating levels.
High specific power which can be up to 17 kW/kg at 95% efficiency.
Rapid charging. Ultracapacitors can charge fully in a few seconds (less than 10 seconds in most cases) which is a big advantage. You don't have to have the ultracapacitors permanently connected in parellel to your primary battery. You can have them in "standby" mode and connect to the battery just before using them.
Fast transient response (~ 20 micro seconds)
Wide operating temperature range (from - 45 Degrees Celcius to + 55 Degrees Celcius)
Simple charging method. No full-charge detection is needed since there is no danger of overcharge.
Deep charge and discharge. Ultracapacitors can be charged up to their full capacity and discharged all the down to a zero-charge state.
Very high rates of charge and discharge.
Easy installation and interface, with flexible mounting
Almost zero maintenance and long life, with little degradation over hundreds of thousands of cycles. While most commercially available rechargeable batteries can be charged 200 to 1000 times, ultracapacitors can be charged and discharged hundreds of thousands of times with no damage. However, in reality, they can be charged and discharged virtually unlimited number of times, and will last for the entire lifetime of most devices and applications they are used in, thus making them environmentally friendly. Battery lifetime can be optimised by only charging under favourable conditions, at an ideal rate and, for some chemistries, as infrequently as possible. Ultracapacitors can help in conjunction with batteries by acting as a charge conditioner, storing energy from other sources for load balancing purposes and then using any excess energy to charge the batteries at a suitable time.
Increased safety since they can handle short circuit and reverse polarity. Also, there is no fire and explosion hazard.
Improved environmental safety since there is no corrosive electrolyte and toxicity of materials used is low. Rechargeable batteries on the other hand wear out typically over a few years, and their highly reactive chemical electrolytes present a disposal and safety hazard.
Rugged since they have Epoxy Resin Sealed Case which is non corrosive.

What are the disadvantages of ultracapacitors?

The disadvantages of ultracapacitors are:

The amount of energy stored per unit weight in ultracapacitors (3-5 W-h/kg) is much less compared to electrochemical batteries (30-40 W-h/kg). So ultracapacitors can't be used as complete replecements for batteries.
As with standard capacitors, the voltage varies with the energy stored. Effective storage and recovery of energy requires complex electronic control and switching equipment, with consequent losses of energy.
Have the highest dielectric absorption of any type of capacitor.
Have high self-discharge rate, which is considerably higher than that of an electrochemical batteries.
Individual cells have low voltages, and so serial connections are needed to obtain higher voltages. Voltage balancing is required if more than three capacitors are connected in series.

Linear discharge voltage prevents use of the full energy spectrum.

What are the applications of ultracapacitors?

There are many potential applications of ultracapacitors:

Engine cranking
Short duration UPS
Adjustable speed drives
Forklifts
Lifts/elevators
Safe shutdown systems
Pulsed power applications
Power tools
Audio entertainment systems
Remote data acquisition
Static compensators
Peak power compensators
Voltage sag compensation
Voltage flicker mitigation
Wind turbine pitch control
Fuel cell assist systems
Regenerative braking
Rapid acceleration
Telecom power applications

About the Author

Amit Raje is the Managing Director of Aartech Solonics, a System Solution Oriented R&D Enterprise in the field of Specialised and Selected Energy Applications. More info on ultracapacitor products offered by Aartech can be found on faradigm.com.