2.8 The Supercapacitor
The supercapacitor resembles a regular capacitor with the exception that it offers very high capacitance in a small size. Energy storage is by means of static charge. Applying a voltage differential on the positive and negative plates charges the supercapacitor. This concept is similar to an electrical charge that builds up when walking on a carpet. Touching an object at ground potential releases the energy. The supercapacitor concept has been around for a number of years and has found many niche applications.
Whereas a regular capacitor consists of conductive foils and a dry separator, the supercapacitor is a cross between a capacitor and an electro-chemical battery. It uses special electrodes and some electrolyte. There are three kinds of electrode materials suitable for the supercapacitor, namely: high surface area activated carbons, metal oxide and conducting polymers. The one using high surface area activated carbons is the most economical to manufacture. This system is also called Double Layer Capacitor (DLC) because the energy is stored in the double layer formed near the carbon electrode surface.
The electrolyte may be aqueous or organic. The aqueous electrolyte offers low internal resistance but limits the voltage to one volt. In contrast, the organic electrolyte allows two and three volts of charge, but the internal resistance is higher.
To make the supercapacitor practical for use in electronic circuits, higher voltages are needed. Connecting the cells in series accomplishes this task. If more than three or four capacitors are connected in series, voltage balancing must be used to prevent any cell from reaching over-voltage.
The amount of energy a capacitor can hold is measured in microfarads or µF. (1µF = 0.000,001 farad). Small capacitors are measured in nanofarads (1000 times smaller than 1µF) and picofarads (1 million times smaller than 1µF). Supercapacitors are rated in units of 1 farad and higher. The gravimetric energy density is 1 to 10Wh/kg. This energy density is high in comparison to the electrolytic capacitor but lower than batteries. A relatively low internal resistance offers good conductivity.
The supercapacitor provides the energy of approximately one tenth that of the NiMH battery. Whereas the electro-chemical battery delivers a fairly steady voltage in the usable energy spectrum, the voltage of the supercapacitor is linear and drops from full voltage to zero volts without the customary flat voltage curve characterized by most chemical batteries. Because of this linear discharge, the supercapacitor is unable to deliver the full charge. The percentage of charge that is available depends on the voltage requirements of the application.
If, for example, a 6V battery is allowed to discharge to 4.5V before the equipment cuts off, the supercapacitor reaches that threshold within the first quarter of the discharge time. The remaining energy slips into an unusable voltage range. A DC-to-DC converter can be used to increase the voltage range but this option adds costs and introduces inefficiencies of 10 to 15 percent.
The most common supercapacitor applications are memory backup and standby power. In some special applications, the supercapacitor can be used as a direct replacement of the electrochemical battery. Additional uses are filtering and smoothing of pulsed load currents.
A supercapacitor can, for example, improve the current handling of a battery. During low load current, the battery charges the supercapacitor. The stored energy then kicks in when a high load current is requested. This enhances the battery's performance, prolongs the runtime and even extends the longevity of the battery. The supercapacitor will find a ready market for portable fuel cells to compensate for the sluggish performance of some systems and enhance peak performance.
If used as a battery enhancer, the supercapacitor can be placed inside the portable equipment or across the positive and negative terminals in the battery pack. If put into the equipment, provision must be made to limit the high influx of current when the equipment is turned on.
Low impedance supercapacitors can be charged in seconds. The charge characteristics are similar to those of an electro-chemical battery. The initial charge is fairly rapid; the topping charge takes some extra time. In terms of charging method, the supercapacitor resembles the lead acid cell. Full charge takes place when a set voltage limit is reached. Unlike the electro-chemical battery, the supercapacitor does not require a full-charge detection circuit. Supercapacitors can also be trickle charged.
Limitations Unable to use the full energy spectrum - depending on the application, not all energy is available. Low energy density - typically holds one-fifth to one-tenth the energy of an electrochemical battery. Cells have low voltages - serial connections are needed to obtain higher voltages. Voltage balancing is required if more than three capacitors are connected in series. High self-discharge - the self-discharge is considerably higher than that of an electrochemical battery.
Advantages and Limitations of Supercapacitors
Virtually unlimited cycle life - not subject to the wear and aging experienced by the electrochemical battery.
Low impedance - enhances pulse current handling by paralleling with an electrochemical battery.
Rapid charging - low-impedance supercapacitors charge in seconds.
Simple charge methods - voltage-limiting circuit compensates for self-discharge; no full-charge detection circuit needed.
Cost-effective energy storage - lower energy density is compensated by a very high cycle count.
Unable to use the full energy spectrum - depending on the application, not all energy is available.
Low energy density - typically holds one-fifth to one-tenth the energy of an electrochemical battery.
Cells have low voltages - serial connections are needed to obtain higher voltages.
Voltage balancing is required if more than three capacitors are connected in series.
High self-discharge - the self-discharge is considerably higher than that of an electrochemical battery.
By nature, the voltage limiting circuit compensates for the self-discharge. The supercapacitor can be recharged and discharged virtually an unlimited number of times. Unlike the electrochemical battery, there is very little wear and tear induced by cycling.
The self-discharge of the supercapacitor is substantially higher than that of the electro-chemical battery. Typically, the voltage of the supercapacitor with an organic electrolyte drops from full charge to the 30 percent level in as little as 10 hours.
Other supercapacitors can retain the charged energy longer. With these designs, the capacity drops from full charge to 85 percent in 10 days. In 30 days, the voltage drops to roughly 65 percent and to 40 percent after 60 days.