Ultracapacitor
26 Aug

Introduction

Ultra-capacitor is also called a super-capacitor. It is an energy storing device and it stores the energy in the form of electrical field. The Ultracapacitor is electrochemical device where-in no chemical reactions are involved in the storing of its electrical energy. Ultra-capacitor is capable of holding hundreds of time more electrical charge quantity than the conventional capacitor.

A conventional capacitor contains two electrically conductive surfaces, separated by an insulating layer called a dielectric. The Capacitance of the capacitor depends on surface area of conductor. However, a practical limit exists as to the capacitance obtainable with this design. The structure of ultra-capacitor is different than the conventional capacitor. The ultra-capacitor employs a porous medium between the two electrodes. In ultra-capacitor the electrodes are made from graphite carbon in the form of activated conductive carbon, carbon nanotubes or carbon gels. The porous material passes the positive ions and blocks the electrons. The porous material and electrodes are saturated with the liquid electrolyte – aluminum.

The electrodes and porous material are very thin, but when coiled up together then their capacitance value goes up to hundreds of farads. In order to further increase the capacitance of an ultra-capacitor, we need to increase the contact surface area, A without increasing the capacitors physical size, or use a special type of electrolyte to increase the available positive ions to increase conductivity.

The typical capacitor cell is working on voltage range between 2 to 3 volt dependent upon the electrolyte used.  If we want to increase the cell terminal voltage of ultra-capacitor then cell must be connected in series and to increase capacitance value cell must be in parallel.

Charging/Discharging

At the time of charging/discharging of ultra-capacitor no chemical reaction is happening, so the charging/discharging of the ultra-capacitors can occur at the same rate. A number of methods are possible for charging of the ultra-capacitors. This may be either through constant current or constant power charging via a dc source or through ac charging methods. Ultra-capacitor is capable to take millions of charge/discharge cycle.

Cell Balancing

For most of the applications we need to connect the cells in series and parallel combinations. In the manufactured cells there are several factors like  tolerance in capacitance, resistance and leakage current that will imbalance the cell voltages of series stack. It is important to ensure that the individual cell voltage do not exceed with its maximum limit as this could result in electrolyte decomposition, ESR (Equivalent Series Resistance) increase and ultimately reduced life.

In the cell balancing the most important factor is cell capacitance between two cells and leakage current. The proper balancing can eliminate the imbalance. There are two types of balancing technique

  1. Passive Balancing – To dominate the total cell leakage current, we need to place the resistor in parallel with each cell. This will reduce the variation in of equivalent parallel resistance between the cells which is responsible for the leakage current.
  2. Active Balancing – If the energy sources are limited then the active balancing is preferred and it draws less current in steady state and requires large amount of current in case of cells imbalance.

Temperature Effect on Ultra-capacitor

The main advantage of ultra-capacitor is its wide temperature range. The typical temperature range of ultra-capacitor is from -40 deg C to 80 deg C. The life of ultra-capacitor reduces when it operates at higher temperature range. The internal resistance will increase reduces when it operates at lower temperature range.

Application

  1. Hybrid Buses Accelerate

Capture the Energy from Regenerative Braking System and release power to help hybrid buses accelerate.

  1. Provide cranking power to START/STOP system
  2. Burst power for lifting application

Advantage

  1. High Power Density
  2. Instant charging
  3. Very long lifetime
  4. Low ESR
  5. Low temperature performance

Disadvantage

  1. Low voltage per cell
  2. Cannot be used in high frequency application.

 

By – Santosh Ghorpade