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.
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Advantages and Limitations of Supercapacitors
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Advantages
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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.
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Limitations
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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.
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Figure 2-10: Advantages and limitations of
supercapacitors.
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.
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