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
1. ELECTROCHEMICAL ENERGY STORAGE
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
W.
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
2. ELECTROSTATIC ENERGY STORAGE
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
3. ELECTROMAGNETIC ENERGY STORAGE
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
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Compensation of external stray fields
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Electromagnetic forces on the conductors
•
Bulk/volume
4. ELECTROMECHANICAL ENERGY STORAGE
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
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Ruggedness
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Greater number of charge discharge cycles
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Longer life
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Less maintenance
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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
