Supercapacitors: New Generation Energy Storage

What is Supercapacitors? How do Supercapacitors work?

Depending on the developing technology and the increase in the usage areas of the technology, electrical energy storage needs are gaining importance day by day. While energy storage studies continue at full speed in the scientific world, the purpose of these studies is; to store energy more easily and quickly, to store more capacity, and to use stored energy for a long time. At the same time, the longevity of the units where energy is stored is one of the most important issues studied.

Supercapacitors

Supercapacitor, also called two-layer or ultracapacitor. It is very different from normal capacitors as it has a very high capacitance. Capacitors store energy through a static charge in response to an electrochemical reaction. Applying a voltage to the positive and negative plates charges the capacitor. Capacitors are divided into 3 types according to their energy storage levels. The first of these are electrostatic capacitors with a dry separating layer at the base.

This classic capacitor has a very low capacity and is mainly used for tuning radio frequencies and filtering. Energy storage sizes range from a few picofarads (pf) to several microfarads (uF). The second group includes electrolytic capacitors. Electrolytic capacitors provide higher capacitance than electrostatic capacitors and can store energy up to a microfarad (uF) degree, which is a million times more than a picofarad. These capacitors use a moist separating layer and are used for filtering, buffering and signal to bind. The third group is; They are supercapacitors that can store energy at a farad degree.

The supercapacitor is used for energy storage that enters charge and discharge cycles with high currents and short and frequent periods. Farad is a unit of capacity named after the British physicist Michael Faraday. A farad stores one coulomb of electrical energy when one volt is applied. A microfarad is one million times smaller than a farad, and a picofarad is also a million times smaller than a microfarad.

SEE ALSO: What is a Capacitor? How Capacitor Works?

Features of Super Capacitors

All capacitors have voltage limits. While the electrostatic capacitor can be made resistant to high voltage, the supercapacitor is limited to 2.5-2.7 volts. Voltages of 2.8 volts and higher are possible, but the life of the capacitor is shortened. Several supercapacitors are connected in series to achieve higher voltage.

Serial connection lowers the total capacity and increases internal resistance. Wires consisting of more than three capacitors require voltage stabilization to prevent any cells from entering overvoltage. A similar protection circuit is used in lithium-ion batteries. The specific energy of the supercapacitor ranges from 1 Wh / kg to 30 Wh / kg, 10-50 times less than Li-ion batteries. As electrochemical batteries produce a usable constant voltage, the voltage of the supercapacitor decreases linearly.

When the supercapacitors are in the charging state, the voltage increases linearly and when the capacitor is full, they do not need a full charge detection circuit and the current drawn is completely reset. In the state of discharge, the voltage drops linearly. The charging time of a supercapacitor is 1-10 seconds. The charging characteristic is similar to that of an electrochemical battery, and the charging current is largely limited by the current carrying capacity of the charger. The initial charging can be done very quickly and the maximum charge takes extra time. There is no problem with overcharging the supercapacitor. Therefore, it does not require a full charge detection circuit and the current is reset when the charge is finished.

Supercapacitors can be charged and discharged almost unlimitedly. Unlike an electrochemical battery that has a lifetime, supercapacitors have little wear and tear as a result of long-term use. When higher voltages than specified are applied, its life is shortened. Supercapacitors work stably in hot and cold environments. However, batteries drift far from steady-state in the same environment.

Usage Areas of Supercapacitors

When a fast charge is required to meet a short term power requirement, the use of supercapacitors is ideal. Batteries are chosen to provide long term energy. Combining the two with a hybrid battery satisfies both needs, which means longer service life. These types of batteries are available today in lead-acid batteries. The use of supercapacitors is most effective in extending power gaps from a few seconds to a few minutes. This kind of application is used on LIRR, one of the busiest railways in North America. To prevent voltage sag during acceleration of a train and to reduce peak power usage, a 2MW supercapacitor bank is being tested in New York against flywheels supplying 2.5MW of power. Both systems must provide continuous power for 30 seconds at the respective MW capacity and be fully charged at the same time. Testing of both systems should take at least 20 years.

Authorities consider the flywheels to be more robust and more energy-efficient than this application. Large supercapacitors are also used in Japan. 4MW systems are used in commercial buildings to reduce network consumption and facilitate loading during peak demand times. Another area of application is to start back up generators during power outages and provide power until the power returns. With the development of supercapacitors, their usage areas are also diversified. Today, targets such as the use of electric cars in power supplies and the use of cell phones in batteries are a sign that supercapacitors can be used in many areas in the future.

Reference: https://www.sciencedirect.com/topics/chemistry/supercapacitors

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