Supercapacitors

SUPERCAPACITORS

Capacitors store electrical energy by accumulating charge on two parallel plates/ electrodes separated by a dielectric material. Aluminium Oxide is the dielectric material in the case of E caps. Capacitance value (C) of such a capacitor is given by the dielectric permittivity ꜫ, the distance d separating the electrodes, and the effective area A of the electrodes constituting the device, as per the following equation.

The energy stored in the device is given by the equation,

Where E is the energy stored in the Capacitor, C is the capacitance value of the capacitor and V is the capacitor terminal voltage.

As can be seen from the equation, the amount of energy stored in the capacitor can be increased in two ways; one is to increase the terminal voltage and the other way is to increase the capacitance value of the capacitor. The value of terminal voltage is limited by the voltage withstand strength of the dielectric material between the electrodes.

The capacitance value of the device can be increased in three different ways,

1. by increasing the effective area of the electrodes,

2. by increasing the permittivity of the dielectric separating the electrodes and

3. by decreasing the distance between the two electrodes.

Another important factor about capacitor technology is the ESR, or Equivalent Series Resistance of the capacitor having a very important effect on the capacitor’s turnaround efficiency when charging and discharging of the capacitor.

Supercapacitors (SCs) / Ultracapacitors or Electrical Double Layer Capacitors (EDLC) is the latest addition to the Electrical Energy Storage Devices (EESDs) comprising Electrolytic Capacitors (E caps.) and Batteries.

Batteries are one of the most cost-effective energy storage technologies available where the energy is stored electrochemically. All battery systems are made up of a set of low voltage/ power cells connected in series to achieve the desired terminal voltages and in parallel to provide the desired power ratings. Batteries can be charged and discharged in certain number of times given by the cycling capability of the electrode and the chemical used as electrode. Popular battery technologies are the Lithium – Ion and the Lead Acid Technologies. Batteries have fairly good energy density, power, life span and initial cost.

But they are of limited cycle life, higher maintenance cost, considerable weight and size. Also, there are some environmental concerns related to the use of batteries due to the hazardous material contained in them and the toxic gases generated during the charge – discharge cycles.

An ultracapacitor is constructed with symmetric carbon positive and negative electrodes separated by an insulating ion-permeable separator and packaged into a container filled with organic electrolyte (salt/solvent) designed to maximize ionic conductivity and electrode wetting. It is the combination of high surface-area activated carbon electrodes with extremely small charge separation that results in high capacitance. An electrical double layer capacitor/Supercapacitor is nothing but a high-capacity capacitor with capacitance values much higher than normal capacitors but lower voltage limits. Supercapacitors store charge electrostatically (non-Faradaic) by reversible adsorption of the electrolyte onto electrochemically stable high surface area carbon electrodes. Charge separation occurs on polarization at the electrode/electrolyte interface, producing a double layer. 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. The Supercapacitor offers excellent power handling characteristics when used by themselves or in combination with batteries provides extended back-up time.

Equation of Capacitance, C = ε0εr A/d, (C α A/d, where A – Surface Area, d- thickness of dielectric)

In the case of Supercapacitors, due to the porous nature of Activated Carbon the surface area will be much higher, resulting in higher capacitance.

FEATURES

Comparison of Supercapacitor with Energy Storage Devices