Ultracapacitors





	

Ultracapacitors


Ultracapacitors are electrical energy storage devices that have the ability
to store a large amount of electrical charge


Unlike the resistor, which dissipates energy in the form of heat, ideal
ultracapacitors do not loose its energy. We have also seen that the simplest
form of a capacitor is two parallel conducting metal plates which are
separated by an insulating material, such as air, mica, paper, ceramic, etc,
and called the dielectric through a distance, “d”.

Capacitors store energy as a result of their ability to store charge with
the amount of charge stored on a capacitor depending on the voltage, V
applied across its plates, and the greater the voltage, the more charge will
be stored by the capacitor as: Q ∞ V.





A capacitor has a constant of proportionality, called capacitance, symbol C,
which represents the capacitor’s ability or capacity to store an
electrical charge with the amount of charge depending on a capacitor
capacitance value as: Q ∞ C.





A Typical
Ultracapacitor

Then we can see that there is a relationship between the charge, Q, voltage
V and capacitance C, and the larger the capacitance, the higher is the
amount of charge stored on a capacitor for the same amount of voltage and we
can define this relationship for a capacitor as being:



Charge on a Capacitor
	
	capacitance and charge on a capacitor

Where: Q (Charge, in Coulombs) = C (Capacitance, in Farads) times V
(Voltage, in Volts)

The unit of capacitance is the coulomb/volt, which is also called the Farad
(F) [named after M. Faraday] with one farad being defined as the capacitance
of a capacitor, which requires a charge of 1 coulomb to establish a
potential difference of 1 volt between its two plates.

But a conventional one farad capacitor would be very large for most
practical electronic applications, hence much smaller units like the
microfarad (μF), nanofarad (nF) and picofarad (pF) are commonly used
where:
    Microfarad  (μF)   1μF = 1/1,000,000 = 0.000001 = 10-6 F
    Nanofarad  (nF)   1nF = 1/1,000,000,000 = 0.000000001 = 10-9 F
    Picofarad  (pF)   1pF = 1/1,000,000,000,000 = 0.000000000001 = 10-12 F
However, there is another type of capacitor available, called an
Ultracapacitor or Supercapacitor which can provide values from a few milli
-farads (mF) to ten’s of farads of capacitance in a very small size
allowing for much more electrical energy to be stored between their plates.

In our tutorial about Capacitance and Charge we saw that the energy stored
in a capacitor is given by the equation:
	
	energy stored in a capacitor

Where: E is the energy stored in the electric field in joules, V is the
potential difference across the plates and C is the capacitance of the
capacitor in farads and defined as:
	
	capacitance of a capacitor

Where: ε is the permittivity of the material between the plates, A is the
area of the plates, and d is the separation of the plates.

Ultracapacitors are another type of capacitor which is constructed to have a
large conductive plate, called an electrode, surface area (A) as well as a
very small distance (d) between them. Unlike conventional capacitors that
use a solid and dry dielectric material such as Teflon, Polyethylene, Paper,
etc, the ultracapacitor uses a liquid or wet electrolyte between its
electrodes making it more of an electrochemical device similar to an
electrolytic capacitor.

Although an ultracapacitor is a type of electrochemical device, no chemical
reactions are involved in the storing of its electrical energy. This means
that the ultra-capacitor remains effectively an electrostatic device storing
its electrical energy in the form of an electric field between its two
conducting electrodes as shown.



An Ultracapacitors Construction

	
	ultracapacitor construction

The double sided coated electrodes are made from graphite carbon in the form
of activated conductive carbon, carbon nanotubes or carbon gels. A porous
paper membrane called a separator keeps the electrodes apart but allows
positive ion to pass through while blocking the larger electrons. Both the
paper separator and carbon electrodes are impregnated with the liquid
electrolyte with an aluminium foil used in between the two to act as the
current collector making electrical connection to the ultracapacitors solder
tabs.

The double layer construction of the carbon electrodes and separator may be
very thin but their effective surface area into the thousands of meters
squared when coiled up together. Then in order to increase the capacitance
of an ultra-capacitor, it is obvious that we need to increase the contact
surface area, A (in m2) without increasing the capacitors physical size, or
use a special type of electrolyte to increase the available positive ions to
increase conductivity.

Then ultra-capacitors make excellent energy storage devices because of their
high values of capacitance up into the hundreds of farads, due to the very
small distance d or separation of their plates and the electrodes high
surface area A for the formation on the surface of a layer of electrolytic
ions forming a double layer. This construction effectively creates two
capacitors, one at each carbon electrode, giving the ultracapacitor the
secondary name of “double layer capacitor” forming two capacitors in
series.

However, the problem with this small size is that the voltage across the
capacitor can only be very low as the rated voltage of the ultra-capacitor
cell is determined mainly by the decomposition voltage of the electrolyte.
Then a typical capacitor cell has a working voltage of between 1 to 3 volts,
depending on the electrolyte used, which can limit the amount of electrical
energy it can store.

In order to store charge at a reasonable voltage ultracapacitors have to be
connected in series. Unlike electrolytic and electrostatic capacitors, ultra
-capacitors are characterized by there low terminal voltage. In order to
increase there rated terminal voltage to tens of volts, ultracapacitor cells
must be connected in series, or in parallel to achieve higher capacitance
values as shown.



Increasing An Ultracapacitors Value
	
	increasing an ultracapacitors value

Where: VCELL is the voltage of one cell, and CCELL is the capacitance of one
cell.

As the voltage of each capacitor cell is about 3.0 volts, connecting more
capacitor cells together in series will increase the voltage. While
connecting more capacitor cells in parallel will increase its capacitance.
Then we can define the total voltage and total capacitance of a
ultracapacitor bank as:
	
	ultracapacitors voltage and capacitance value

Where: M is the number of columns and N is the number of rows. Note also
that like batteries, ultracapacitor and supercapacitors have a defined
polarity with the positive terminal marked on the capacitor body.



Ultracapacitors Example No1
A 5.5 volt, 1.5 farad ultracapacitor is required as an energy storage backup
device for an electronic circuit. If the ultracapacitor is to be made from
individual 2.75v, 0.5F cells, calculate the number of cells required and the
layout of the array.
	
	ultracapacitors voltage

The array will therefore have two capacitor cells of 2.75v each connected in
series to provide the required 5.5v.
	
	ultracapacitors capacitance

Then the array will have a total of six individual columns, consisting of
two rows of six thereby forming an ultracapacitor with a 6 x 2 array as
shown.


6×2 Ultracapacitor Array

	
	ultracapacitors array



Ultracapacitor Energy

As with all capacitors, an ultracapacitor is a energy storage device.
Electrical energy is stored as charge in the electric field between its
plates and as a result of this stored energy, a potential difference, that
is a voltage, exists between the two plates. During charging (current
flowing through the ultracapacitor from the connected supply), electrical
energy is stored between its plates.

Once the ultracapacitor is charged, current stops flowing from the supply
and the ultracapacitors terminal voltage is equal to the voltage of the
supply. As a result, a charged ultracapacitor will store this electrical
energy even when removed from the voltage supply until it is needed acting
as an energy storage device.

When discharging (current flowing out), the ultracapacitor changes this
stored energy into electrical energy to supply the connected load. Then an
ultracapacitor does not consume any energy itself but instead will store and
release electrical energy as required with the amount of energy stored in
the ultracapacitor being in proportion to the capacitance value of the
capacitor.

As previously mentioned, the amount of energy stored is proportional to the
capacitance C and the square of the voltage V across its terminals giving.

	
	energy stored in ultracapacitors

Where: E is the energy stored in joules. Then for our ultracapacitor example
above, the amount of energy stored by the array is given as:
	
	electrical energy stored our ultracapacitors

Then the maximum amount of energy that can be stored by our ultracapacitor
is 22.7 joules, which was originally supplied by the 5.5 volt charging
supply. This stored energy remains available as charge in the electrolyte
dielectric and when connected to a load, the ultracapacitors entire 22.69
joules of energy is made available as an electric current. Obviously, when
the ultracapacitor is fully discharged, the stored energy is zero.

Then we can see that an ideal ultracapacitor would not consume or dissipate
energy, but instead take power from an external charging circuit to store
energy in its electrolyte field and then return this stored energy when
delivering power to a load.

In our simple example above, the energy stored by the ultracapacitor was
about 23 joules, but with large capacitance values and higher voltage
ratings, the energy density of ultracapacitors can be very large making them
ideal as energy storage devices.

In fact, ultracapacitors with ratings into the thousands of farads and
hundreds of volts are now being used in hybrid electric vehicles (including
Formula 1) as solid state energy storage devices for regenerative braking
systems as they can quickly giving out and receiving energy during braking
and accelerating afterwards. Ultra and super-capacitors are also used in
renewable energy systems to replace lead acid batteries.



Ultracapacitors Summary

We have seen that an ultracapacitor is an electrochemical device consisting
of two porous electrodes, usually made up of activated carbon immersed in an
electrolyte solution that stores charge electrostatically. This arrangement
effectively creates two capacitors, one at each carbon electrode, connected
in series.

Ultracapacitors are available with capacitances in the hundreds of farads
all within a very small physical size and can achieve much higher power
density than batteries. However, the voltage rating of an ultracapacitor is
usually less than about 3 volts so several capacitors have to be connected
in series and parallel combinations to provide any useful voltage.

Ultracapacitors can be used as energy storage devices similar to a battery,
and in fact are classed as an ultracapacitor battery. But unlike a battery,
ultracapacitors can achieve much higher power densities over a shorter time
duration.

Also, ultracapacitors are now used in many hybrid petrol vehicles as well as
fuel cell driven electric vehicles due to their ability to discharge high
voltages quickly and then be recharged once again ready for the next cycle.

By using ultracapacitors along with conventional fuel cells and automotive
batteries, peak power demands and transient variations in load conditions
can be controlled much more efectively.