Energy can neither be created nor destroyed. But it can be transformed from one form to another. Electrical energy is the form of energy that can be transmitted efficiently and easily transformed to other forms of energy. The main disadvantages with electrical energy involve storing it economically and efficiently. Electrical energy can be converted and stored in different forms:

  • Electrochemical Energy
  • Electrostatic Energy
  • Electromagnetic Energy
  • Electromechanical Energy



In this type of storage, electrical energy is converted and stored in the form of chemical energy. There are two main categories: batteries and fuel cells. Batteries use internal chemical components for energy conversion and storage whereas fuel cells use synthetic fuel (for example Hydrogen, methanol or hydrazine) supplied and stored externally. Both use two electrodes, an anode and a cathode, that exchange ions through an electrolyte internally and exchange electrons through an electric circuit externally. The Lead-acid battery, discovered by Plante in 1859, is the most widely used battery. The battery consists of pairs of lead electrode plates immersed in a dilute sulphuric acid that acts as an electrolyte. Every alternate lead plate is coated with lead dioxide. Discharging results in the conversion of both of the electrodes to lead sulphate. Charging restores the plates to lead and lead dioxide. The physical changes in electrodes during charging and discharging deteriorates the electrodes and hence reducing their life. The main advantages are they have a well-established technology.

The main drawbacks with batteries are:

  • Slow response during energy release
  • Limited number of charge discharge cycles
  • Relatively short life time
  • High internal resistance
  • Low energy density
  • Maintenance requirements for some types
  • Environmental hazards
  1. R. Grove demonstrated the first hydrogen-oxygen fuel cell in 1839. The byproduct of a Hydrogen fuel cell is water. By electrochemical decomposition of water into hydrogen and oxygen and holding them apart, hydrogen fuel cells store electrical energy. During discharge, the hydrogen is combined with oxygen, converting the chemical energy to electrical energy. The main advantages are environment friendly. The main drawbacks with fuel cells as energy storage elements are:
  • Slow response during energy release
  • Temperature dependence
  • Corrosion problems
  • Hydrogen storage
  • Inefficient transfer of electrical energy to chemical energy



Electric energy can be converted and stored in the form of electrostatic field between the parallel plates of a charged capacitor. The amount of energy stored is proportional to square of the voltage across the parallel plates and to its capacitance. For a fixed voltage, the volume energy density for a parallel plate capacitor is proportional to capacitance, which is proportional to the permittivity of the insulator between the parallel plates. Most of the insulators have relative permittivity in the range of 1 to 10. Due to the small capacitance, ordinary capacitors can store very limited amount of energy. Ultra capacitors use electrochemical material for improving permittivity and hence energy density. They require less maintenance and have much longer lifetimes compared to batteries. They have high energy density and does not having moving parts. The main drawbacks with capacitors are:

  • Cost
  • Temperature dependence
  • Not rugged



Electric energy can be converted and stored in the form of an electromagnetic field. A Superconducting magnetic energy storage (SMES) coil consists of a superconducting coil carrying large DC currents. The amount of energy stored is proportional to the square of the DC current flowing through the coil and to its inductance. The volume energy density is proportional to the permeability of the material used for the coil. In order to keep the temperature of the superconductor below its critical temperature, a cryogenic cooling system is required. Increasing the DC current increases the amount of energy stored. Once the current in the coil reaches its maximum value, the voltage across it is zero and the SMES is fully charged. This storage scheme has very low losses due to negligible resistance in the coil. Also SMES coils can be built for larger energy and power. The main drawbacks with SMES are:

  • Cost
  • Reliability in maintaining cryogenic cooling
  • Compensation of external stray fields
  • Electromagnetic forces on the conductors
  • Bulk/volume



Electrical energy can be converted and stored in the form of kinetic energy in a flywheel. Motor/generator sets, DC machines and induction machines are used for energy conversion. The amount of energy stored in a flywheel is proportional to the square of angular velocity and to its inertia for a given design stress. The energy storage technologies discussed above have their own advantages and disadvantages but the following advantages make flywheels a viable alternative to other energy storage systems:

  • Low cost
  • High power density
  • Ruggedness
  • Greater number of charge discharge cycles
  • Longer life
  • Less maintenance
  • Environmental friendly
  • Fast response during energy release

Flywheels can be designed for low speed or high-speed operation. A low speed flywheel has advantages of lower cost and the use of proven technologies when compared to a high-speed flywheel system. The main disadvantages are:

  • less energy stored per volume
  • higher losses
  • increased volume and mass


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