Once the staple of brute-force power supplies and large industrial
and consumer power applications, ultracapacitors are
now finding their way into products of all sizes, particularly
portables. Also called supercapacitors, these components are
notable for capacitance values ranging into the thousands of
Farads and fast charge/discharge rates.
With the ability to store massive amounts of energy for
long periods of time, ultracapacitors behave more like a battery
than a standard capacitor. In fact, as things progress, they
will replace rechargeable batteries in a plethora of products,
from computers and digital cameras to cell phones and other
handheld devices.
WHAT'S AN ULTRACAPACITOR?
Basically put, the ultracapacitor or supercapacitor is a very
large, polarized electrolytic (electro-chemical) capacitor. In
describing these components, though, “large” refers to capacity,
not necessarily their physical size.
True, when it comes to generic electrolytic capacitors, the
greater the capacitance and/or voltage values, the bigger the
overall package. Electrolytic caps typically offer capacitance
values in the micro-Farad range from about 0.1 µF and topping
off at approximately 1 F with voltage ratings up to 1
kV dc. Usually, the higher the voltage rating, the lower the capacitance value, and the bigger the capacitor and the higher
the capacitance value, the bigger the package even as operating
voltage may decrease.
The same rules of thumb essentially apply to ultracapacitors.
These components come with capacitance values from 1 F
and up and operating voltages ranging from 1.5 to 160 V dc or
greater. As both values increase, so does capacitor size.
Early-generation ultracapacitors with values in the tens of
Farads were hefty clunkers, relegated to large power-supply
applications. Smaller ultracapacitors with miniscule voltagehandling
capabilities found employment as short-term power
backups in consumer electronics.
As capacitor technology evolved, new developments haven’t
only leveled the playing field, they’re also beginning to tip it
favorably for the ultracap market. Despite the vast similarities
between supercapacitors and their electrolytic counterparts,
there is a significant variation in their electrical and physical
proportions.
For example, a general-purpose 10-µF electrolytic with a 25-V
dc rating may measure only slightly smaller or even the same as
a 1- to 10-F, 2.7-V dc ultracap. And, with recent advancements,
boosting the operating voltage to 25 V dc may translate into a
size increase of less than double for the supercap, which may or
may not be significant depending on the application.
ANATOMY OF AN ULTRACAPACITOR
Basically, one can view an ultracapacitor
as a rechargeable battery. It stores a
charge proportional to its capacitance
and releases a charge when called upon
to do so. What sets the ultracap apart
from its electrolytic brethren is an electrical
double-layer architecture, which
enables higher capacities.
Standard capacitors sandwich a
dielectric substrate between two electrodes
attached to plates (Fig. 1).
Depending on the type of capacitor,
the dielectric can be aluminum oxide, tantalum
tetroxide, titanium oxide barium,
or polyester polypropylene, each of which
determines capacitance and voltage capabilities
(Fig. 2). The amount of dielectric
and the distance between the plates also
affects capacitance levels. However, the
maximum allowable distance between the
plates limits the amount of dielectric.
In this single-layer topology, increasing
the amount of dielectric to boost capacitance
is usually achievable in one of three
ways: widen the package and increase plate
size, lengthen the package and increase
plate distance, or a combination of both.
Any of the three solutions translates into a
physically larger capacitor as a tradeoff for
more micro-Farads.
Electric double-layer capacitors
(EDLCs), as the name implies, solves this
problem by adding a second dielectric
layer within the same package that works
in parallel across a separator with the first
layer (Fig. 3). EDLCs also employ nonporous
dielectrics such as activated carbon,
carbon nanotubes, carbon aero gels, and
select conductive polymers, which exhibit
higher storage capabilities than standard
electrolytic materials. This combination of
the extra layer and more efficient dielectric
material enables a capacitance boost in the
neighborhood of four orders of magnitude.
There is a tradeoff in terms of voltage
capability, traceable to the dielectric. In
EDLCs, the dielectric is extremely thin,
measured in nanometers, creating a large
surface area that’s responsible for higher
capacitance. However, these thin layers lose
some of the desirable insulating properties
of conventional dielectrics and therefore
require lower operating voltages.
SUPERCAP APPS
In relation to standard capacitors and
batteries, EDLCs have several advantages
that make them desirable alternatives.
These benefits include a greater number
of charge/discharge cycles than
rechargeable batteries, efficiencies
in the realm of 98%, a lower internal
resistance, high output power, better
thermal capabilities, and better safety
margins than both batteries and standard
capacitors.
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