Energy Storage-a prerequisite for harnessing renewable energy Dr. P. C. Pant Solar Energy Centre Ministry of New and Renewable Energy [email_address]
Nature of renewable energy supplies and real challenge
Renewable energy supplies are continuing or repetitive current of energy occurring in natural environment
The initial input power from renewable energy supplies therefore is outside our control; and matching supply and demand in the time domain is a real challenge
Two ways to overcome this challenge are :
Matching the load to the availability of renewable energy supply or hybridize it with an auxiliary energy supply system
Storing the energy for future use i.e. keeping it available when it is wanted
Methods of energy storage
Chemical : hydrogen, ammonia
Heat : hot water, steam
Electric : Capacitors, electromegnets
Electro chemical : Batteries, fuel cell
Gravitational : water reservoir
Mechanical : Flywheel
An electrochemical apparatus-used for accumulating charge for use at later stage and that’s why called “charge accumulator”
Stores electrical energy in the form of chemical energy and then converts this chemical energy to electrical energy when needed: essentially an electrical energy storage device
A source of ‘portable' electrical power
Capable of delivering very large quantities of power for short period and being recharged at low rate for extended time
Reliable source of emergency power, instantaneously
A source of pure direct current for laboratory and other specific purposes
Features to study
Must be discussed from three points of view:
All three are interrelated and are necessary to understand the nature and performance of storage batteries
Cell and battery
Cell: basic building block that convert chemical energy into electrical energy; a cell consists of an assembly of two dissimilar electrodes (anode and cathode), seperated by seperators, immersed in an electrolyte in a suitable container having terminals coming out of electrodes for connecting to external circuit.
Battery: An electric battery consists of two or more cells connected in series or parallel
Classification of cells:
Primary cells: capable of converting chemical energy into electrical energy and in this process gets exhausted. The chemical reaction is irreversible.
Secondary cells: Also called storage cells. Convert chemical energy into electrical energy or vice versa by reactions that are essentially reversible
Various types of secondary cells:
Lithium ion (Li-ion)
Lead-Acid :Fundamental Principle
Fundamentally very simple to make
On hanging 2 metallic lead strips in and on opposite sides of a small glass jar filled with dilute sulphuric acid, connecting these strips called ‘plates’ with a source of direct current and allowing them to ‘charge’, the colour of one strip becomes increasingly dark brown , called positive plate; and other retains its original colour called negative plate
On removing the charging source, a voltage of about two volts is shown by a sensitive voltmeter across the two plates called electrodes.
A battery formed this way has no practical value because of low surface area of the strip to accumulate sufficient ‘active material’ i.e. brown lead peroxide of the positive plate and metallic ‘sponge’ lead of the negative.
The increase in effective area of plates for achieving greater and greater capacity for industrial use is one of the requirements for useful design of the battery.
This can be accomplished by:
Casting plates with complex ridges or grooves
Mechanically furrowed to obtain greater surface
Separate corrugated lead ribbons are rolled into spiral buttons and inserted in lead alloy frames.
Type of plate affects the characteristics and performance of battery.
All types have lead di oxide (PbO 2 )in the positive plate applied during manufacture and porous sponge lead (Pb) on negative plate.
The positive plates are welded to form a plate group and negative plates are assembled in the same manner.
The plates of different polarity are insulated from each other by separators.
The complete plate group assembly is immersed in dilute sulphuric acid in a container.
Thus an electrochemical couple of two different plates or electrodes (PbO 2 and Pb) in an acid electrolyte (H 2 SO 4 ) are all that is required to make a rechargeable, or storage battery.
Classification of battery according to +ve plate type
Flat plate type: different type of grid structure
Tubular plate type
-ve Plate is always flat plate of sponge lead
Classification of battery as per electrolyte type
Valve Regulated Lead Acid Battery
Absorbed Glass Mat type (AGM)
Classification of battery as per use
Ampere-Hour Rating The rating assigned to the cell shall be the capacity expressed in ampere-hours (after correction to 27 o C) stated by the manufacturer to be obtainable under specified condition. Generally for flooded battery it is taken at 10 hr rating and for VRLA it is at 20 hr rating.
Charge-discharge curve of a VRLA battery C20 rating