Super Capacitor

SUPER CAPACITOR

Super capacitor is an electronic device that store large amount of electric charge. They store 10 to 100 times more energy per unit volume and deliver charge much faster than batteries.

Unlike ordinary capacitors, super capacitors, do not use the conventional solid dielectric, but rather they use electrostatic double-layer capacitance.

Design of super capacitor

Super capacitor (Electro-chemical capacitor) consists of two electrodes (made from metal coated with a porous substance like powdery activated carbon) separated by an ion-permeable membrane (separator) and dipped in an electrolyte, containing positive and negative ions, connecting both the electrodes.

Working (or) Storage Principle

When the electrodes are connected to the power source, ions in the electrolyte form electric double layers (Helmholtz electrical double layer) of opposite polarity to the electrodes polarity, creating an electric field between them.

For example, positively polarized electrodes will have a layer of negative ions at the electrode/electrolyte interface. Similarly negatively polarised electrodes will have a layer of positive ions at the electrode/electrolyte interface.

Fig. 8.7 Typical construction of a super capacitor

1. Power source

2. Collector

3. Polarized electrode

4. Helmholtz double layer

5. Electrolyte having positive and negative ions

6. Separator

This electric field polarizes the dielectric, so its molecules lineup in the opposite direction to the field and reduce its strength. It means that it stores more electrical energy at an electrode-electrolyte interface.

1. Type of super capacitors

Supercapacitors are mainly classified into three types

1. Double layer capacitors

2. Pseudo capacitors

3. Hybrid capacitors

1. Double layer capacitors

It consists of two electrodes separator and electrolyte. The electrolyte is the mixture of positive ions and negative ions dissolved in water. The two electrodes are separated by a separator.

2. Pseudo capacitors

It starts electrical energy by electron change transfer between electrode and electrolyte. This can be done by redox reaction.

3. Hybrid capacitors

It is developed by using techniques of doubld layer capacitors and pseudo capacitors. In hybrid capacitor both double layer capacitance and pseudo capacitance is achieved.

Table 8.1: Differences between battery, capacitor and super capacitor

2. Advantages and disadvantages of super capacitors

Advantages

1. It is highly safe

2. Its life time is very high (10 to 20 years)

3. It can be cycled millions of time

4. It can be charged in seconds

5. It provides high power density and high load currents

6. Its performance is excellent even at low temperature (-40°C)

Disadvantages

1. Cost per watt is high

2. It cannot be used as source for continuous power supply

3. If higher voltage is required, the cells must be connected in series.

4. High self-discharge, it is higher than most batteries.

3. Applications

Super capacitors are used in many power management applications like,

1. Voltage stabilization in start/stop system

2. Energy harvesting

3. Kitchen appliances

4. Regenerative braking system

5. Consumer electronics

6. Wind energy

7. Utility meters

8. Remote power for sensor LEDs, switches

9. Energy efficiency and frequency regulation