The capacitor is a two-terminal passive component,
which is widely used in electronics. Almost, every circuit we find in
electronics, use one or more capacitors for various usage. Capacitors
are the most used electronics component after resistors. They have a
special ability to store energy. There are different
types of capacitors available in the market, but one which is recently
getting popularity and promise a replacement or alternative of batteries in future, are supercapacitors or also known as ultracapacitors. A supercapacitor is
nothing but a high-capacity capacitor with capacitance values much
higher than normal capacitors but lower voltage limits.They can store 10
to 100 times more energy per unit volume or mass than electrolytic
capacitors, can receive and deliver charge much faster than batteries,
and tolerate more charging-discharging cycles than rechargeable
batteries.
Supercapacitors or Ultracapacitors are a new
energy storage technology which is developed heavily in modern times.
Supercapacitors are providing significant industrial and economic
benefits
The capacitance of a capacitor is measured in
Farad (F), like .1uF (microfarad), 1mF (millifarad). However, while the
lower value capacitors are quite common in electronics, very high-value
capacitors are also available, which store energy in much more high
density and available in very high capacitance value, ranged in Farad
likely.
In the above image, a locally available 2.7V,
1Farad super capacitor image is shown. The voltage rating is much lower
but the capacitance of the above capacitor is quite high.
Benefits of Super-Capacitor or Ultra-Capacitor
The demand of Supercapacitors is
rising day by day. The main reason for the rapid development and demand
is due to many other benefits of Supercapacitors, few of them are stated
below:
- It provides a very good life of approx 1million charge cycles.
- The operating temperature is -50 degrees to 70 degrees almost, which makes it suitable for use in consumer applications.
- A high power density up to 50times, which is achieved by batteries.
- Harmful materials, toxic metals are not the part of the Super Capacitors or Ultracapacitors manufacturing process which makes it certified as the disposable component.
- It is more efficient than batteries.
- Requires no maintenance compared with batteries.
Supercapacitors store energies in its electric
field, but in case of batteries, they use chemical compounds to store
energies. Also, because of its ability to quick charge and discharge,
the Supercapacitors are slowly entering in the battery market. Low
internal resistance with very high efficiency, no maintenance cost,
higher lifetimes are the main reason for its high demand in the modern
power source related market.
Energies in capacitor
A capacitor store energies in the form of Q = C x V.
Q stands for Charge in Coulombs, C for capacitance in Farads and V for
voltage in volts. So, if we increase the capacitance the stored energy Q
will also increase.
The unit of capacitance is Farad (F) which is
named after M. Faraday. Farad is the capacitance unit in respect of
coulomb/volt. If we say a capacitor with 1 Farad, then it will create a
1-volt potential difference between its plates depending on the
1-coulomb charge.
1 Farad is a very large value capacitor to use as a
general electronic component. In electronics, Generally, microfarad to
Pico farad capacitance is used. Microfarad is denoted as uF (1/1,000,000
Farad or 10-6F), nano farad as nF (1/1,000,000,000 or 10-9F) and Pico farad as pF (1/1,000,000,000,000 or10-12F)
If the value becomes much higher, like mF to few
Farads (Generally <10F), means the capacitor can holds much more
energies between its plates, that capacitor is called as Ultra capacitor or Supercapacitor.
The energies stored in a capacitor is E = ½ CV2 Joules. E is the stored energy in joules, C is the capacitance in Farad and V is the potential difference between the plates.
Construction of a Supercapacitor
Supercapacitor is an electrochemical device.
Interestingly, there are no chemical reactions are responsible to store
its electrical energies. They have a unique construction, with a large
conductive plate or electrode, which are closely situated with a very
small surface area. Its construction is the same as an electrolytic
capacitor with a liquid or wet electrolyte between its electrodes. You
can learn about different types of capacitors here.
Supercapacitor acts as an electrostatic device storing its electrical energy as the electric field between the conductive electrodes.
The electrodes, Red and blue, are coated double-sided.
They generally made of graphite carbon in the form of carbon nanotubes
or gels or a special type of conductive activated carbons.
To block the large electron flow between
electrodes and passing the positive ion, a porous paper membrane is
used. The paper membrane also separates the electrodes. As we can see in
the above image, the porous paper membrane is situated in the middle
which is green in color. The electrodes and paper separator are
impregnated with the liquid electrolyte. The aluminum foil is used as a
current collector which establishes the electrical connection.
The separation plate and the area of the plates
are responsible for the capacitance value of the capacitor. The relation
can be denoted as
Where, Ɛ is the permittivity of the material present between plates
A is the area of the plate
D is the separation between plates
So, in case of supercapacitor, the contact surface
is needed to be increased, but there is a limitation. We cannot
increase the physical shape or size of the capacitor. To overcome this
limitation special type of electrolytes are used to increase the
conductivity between plates thus increasing the capacitance.
The supercapacitors also called as double layer capacitor.
There is a reason behind it. Very small separation and large surface
area using special electrolyte, the surface layer of electrolytic ions
form a double layer. It creates two capacitor construction, one at each
carbon electrodes and named a double layer capacitor.
These constructions have a drawback. The voltage
across the capacitor became very low because of the decomposition
voltage of the electrolyte. The voltage is highly dependent on the
electrolyte material, the material can limit the electrical energy
storing capacity of the capacitor. So, due to the low terminal voltage,
an supercapacitor can be connected in series to store electrical charge
at a useful voltage level. Due to this, the supecapacitor in series
produce higher voltage than usual and in parallel, the capacitance
became larger. It can be clearly understood by the below Supercapacitor
Array Construction technique.
Supercapacitor Array construction
To store charge at a useful required voltage,
supercapacitors need to be connected in series. And for increasing the
capacitance they should be connected in parallel.
Let's see the array construction of the Supercapacitor.
In the above image, the cell voltage of a single
cell or capacitor is denoted as Cv, whereas the capacitance of a single
cell is denoted as Cc. The voltage range of a supercapacitor is from 1V
to 3V, the series connections increase the voltage and more capacitors in parallel increase the capacitance.
If we create the array, the voltage in series will be
Total voltage = Cell Voltage (Cv) x Number of rows
And the capacitance in parallel will be
Total capacitance =Cell Capacitance (Cc) x (Number of Column / Number of Row)
Example
We need to create a backup storage device, and for that a 2.5F super or supercapacitor is required with the 6V rating.
If we need to create the array using 1F
capacitors with the 3V rating, then what will be the array size and
capacitors quantities?
Total voltage = Cell Voltage x Row number Then, Row number = 6/3 Row number = 2
Means two capacitor in series will have a 6V potential difference.
Now, the capacitance,
Total capacitance = Cell capacitance x (Column Number / Row Number) Then, Coloumn number = (2.5 x 2) / 1
So, we need 2 rows and 5 column.
Let's construct the array,
The total energy stored in the array is
