STORAGE SYSTEMS
Energy is useful only if available when and where it is wanted: - Carrying energy to
where it is wanted is called distribution keeping it available until when it is wanted is called
storage. The effective utilization of intermittent & variable energy storge Sources Such oy
solar energy & wind energy, often requires energy storge.
Energy storage Systems:
Methods for energy storage may be classified according to the form in which energy is
stoned; the following Categories appeal to be the most important:
1) Mechanical energy storage.
i) Pumped hydroelectric storge
ii) Compressed air Fly wheel.
2) Electrical storage the lead acid battery
3) chemical energy storage
i) Hydrogen
ii) Ammonia
iii) Reversible chemical reaction.
4) Electromagnetic energy storage
5) Thermal (heat) energy storage
i) Sensible heat
ii) Latent heat
iii) Chemical reactions
6) Biological storage
Electrical Storage Lead acid battery. Storage batteries:
Electricity is a high-grade form of energy & therefore great effort is made to find cheap &
efficient means for storing it A device that has electricity both as input and output is called an
(electrical) storage battery or (electrical) accumulator. Usually the combination of electrolyser & fuel
cell is not included as electrical storage. however.
Batteries form an essential Component of almost all photovoltaic & small wind system; &
there is steady development of battery powered vehicles.
Basic Battery Theory:
A battery is Combination of individual cells. A cell- is the essential combination of the
materials and electrolyte constituting the basic electro chemical energy Storer.
A battery can also be thought of a black box into which electrical energy is put, stored
electrochemically, and latter regained as electrical energy.
A generalized Cell Consisting of two electrodes Called the anode & cathode immersed in
suitable electrolyte. when an electrical load interface b/w one electrode & the electrolyte, freeing
both and electron & and an ion.
The electron flows throughout external load & the ion through the electrolyte. recombining
at the other electrode.
CONSTRUCTIONAL FEATURES OF LEAD-ACID BATTERIES:
A single cell of 2.12 to 2.15 V is the basic building block of a lead-acid battery. During
insufficient power supply from renewable energy resources, power is supplied by each
battery unit. These battery units are connected in series to get a 12-, 24, or 48-V strings,
which are then connected together in parallel to form a battery bank. The battery bank
supplies DC power to an inverter, which converts it to AC power for running appliances. The
inverter input, battery type and the required energy storages determine battery bank
voltage and current rating.
There are mainly two types of lead-acid batteries. They are,
1. Flooded lead-acid battery
2. Maintenance-free lead-acid battery
Flooded Lead-acid Battery:
Flooded-cell batteries consist of two sets chemical coated plates immersed in a
liquid electrolyte while using the battery. water present in the electrolyte gets evaporated
and the refilling for the same is done with distilled water.
Some of the applications of flooded lead-acid batteries are automobiles, forklifts and
uninterruptible power supply (UPS)systems.
Maintenance-free lead-acid battery:
In these batteries, liquid electrolyte is replaced by moistened separators and the
enclosure is sealed. Safety valves are provided to vent out oxygen and hydrogen gases
formed during charge-discharge. Because of this, they are known as Valve Regulated Leadacid (VRLA) batteries. There are primarily two types of VRLA batteries.
(i) Small Sealed Lead-acid (SLA) Battery:
It is also known as Gel Cell because of gel formation due to the addition of silica dust
to the electrolyte.
(ii) Absorbed glass mat battery:
It has a highly porous microfiber like sponge mat between the plates which absorbs
the liquid electrolyte and as a result, the battery does not contain free liquid electrolyte.
VRLA batteries are maintenance-free because no replenishment of water is required
for proper functioning of electrolyte. This is because the regulated valves facilitate the
recombination of hydrogen and oxygen to form water. Because of this, they are also known
as Recombinant Batteries
CHARGIND AND DISCHARGING LEAD-ACID BATTERY:
lead-acid battery consists of a cathode made of lead (Pb) and an anode made of lead dioxide (PbO2)
immersed in sulphuric acid electrolyte. The charge-discharge reaction is given as,
Pb+ 2H2 SO4+ Pbo2------------------> PbSO4+ 2H2O+ PbSo4
DISCHARGING MODE: At the anode, oxygen ions from the anode combine with sulphate
ions of the electrolyte to form lead sulphate. At the cathode, sulphate ions from the
electrolyte combine with lead ions from the cathode to form lead sulphate. Two electrons
enter the anode terminal and two electrons leave the cathode terminal by means of the
external circuit, for every two sulphate ions leaving the electrolyte. The whole process
corresponds to the current supplied by the battery to the external circuit. The departure of
two sulphate ions from the electrolyte reduces its acidity.
At cathode: Pb+SO4------------------------> PbSO4+2eAt anode: PbO2+SO4+4H++2e---------------------> PbS04+2H2O
CHARGING MODE:The charging of the battery is achieved by applying an external voltage at
battery terminals, greater than the voltage produced by the reactions at anode and
cathode. This results in the flow of current into the anode unlike during discharge, where it
flows out of the anode. The chemical processes are reversed and sulphate ions are released
into the solution thereby, increasing the concentration of sulphuric acid in the electrolyte.
At cathode: PbSO4+ 2H2O ------------------------> PbSO4+ SO4+4H++2eAt anode: PbSO4+2e---------------------> Pb+S04-2
The most critical thing during charging is to find the ideal charge voltage limit. A high voltage
limit (greater than 2.4 "V) per cell generates good battery performance but shortens service
life due to permanent grid corrosion on the positive plate. A low voltage (less than 2.4 V per
cell) is safe when charged at a higher temperature but it leads to sulphation on the negative
plate. Excessive formation of lead sulphate on electrodes decreases their effective surface
area thereby affecting the performance. Therefore, complete discharge of a lead-acid
battery should be avoided.
On the other hand, excessive charging such that no sulphate remains at the cathode for
maintaining the continuity of the charging current leads to liberation of hydrogen which is a
safety hazard. But, charging to the gassing stage with slight degree of bubbling is
occasionally done for cleaning up of electrodes and providing a mixing action on the
electrolyte. The freezing point of electrolyte depends on the state of the charge. Because of
this, only fully charged batteries can operate a low temperature whereas batteries in lower
stage of discharge can operate only in warmer environment.
Operational parameters of typical lead-acid batteries
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Specific energy: 20-35 Wh/kg
Energy density: 50-90 Wh/L
Specific power: Around 250 W/kg
Nominal cell voltage: 2V
Electrical efficiency: About 80% depending on recharge rate and temperature
Recharge rate: About 8 hours. They can be quick charged to 90%
Self-discharge: 1-2% per day
Lifetime: About 800 cycles, depending on cycle depth.
Quiescent (open circuit) voltage: 12.6 V
Discharging and voltage :11.8 V
Charge:13.2 to 14.4 V
Gassing voltage:14.4 V
Recommended floating voltage for charge preservation :13.2 V
After full charge, the terminal voltage drops quickly to 13.2 V and then slowly to
12.6 V.
ADVANTAGES AND DISADVANTAGES OF LEAD-ACID BATTERY:
DISADVANTAGES:
 Their application is limited to stationary and wheeled applications because of their
low energy density or poor weight-to energy ratio.
 They are mostly used for stand-by applications requiring periodic deep discharges
because they provide only a limited number of full discharge cycles.
 They are not eco-friendly because of lead content and acid electrolyte.
 Charging improperly leads to thermal runaway.
ADVANTAGES:
 They have low maintenance; no memory and no replenishment of electrolyte is
required in sealed lead-acid batteries.
 They have the lowest self-discharge among rechargeable battery systems.
 They have good service life and provide dependable service, if operated properly at
given operating limits and parameters.
 They have good reliability and working capability.
 Lead acid batteries can sustain high discharge rates.
They are relatively cheaper and are easy to manufacture.
Energy storage Via Flywheels: A flywheel is essentially a mechanical battery Consisting of a mass rotating around an axis
It stores energy in the form of kinetic energy and works by accelerating a rotor to very high
speed and maintaining. The energy in the System as rotational energy.
The Flywheel energy storage, sometimes referred to as a “super flywheel” is to accelerate a
suitable physical rotor Vacuum, via State achieved. an a "Super flywheel". a suitably designed to a
very high speed in an elective motor at which high energy storage densities
Conversely the energy stored in a Can be increased by increasing the velocity Energy stored
in a flywheel (E) is equal to the kinetic energy, given by
E=1/2 mv2
=1/2 m(2πRn)2
= 2π2mR2n2
Where,
V = Velocity of flywheel=2πTRn
E = Energy (Joule)
m = mass of the flywheel (kg)
R = Radius of gyration (m) (0.7071 R
̥
̥
R = Outer radius
n = Revolutions per second (revolutions/ min)/60
Applications of Flywheel Energy Storage (FES) Systems:
1. In transportation industry, both automotive and rail, they are used as an alternative to
the use of batteries in hybrid vehicles.
2. In the power industry, they are used for applications like uninterrupted power supply
(UPS), power grid stabilization, As an instant short-duration response for peak loads.
3. In experimental devices of nuclear fusion, FES systems are used for the required supply
of short bursts of high power.
4. In Inertial starters, FES systems are used where there are less severe operating
requirements.
5. Simple and cheap steel disc can be used for applications involving only a small quantity
of stored energy.
Advantages of Flywheel Energy Storage (FES) System:
1. They have a very high-power density.
2. They can charge-discharge at very high rates, thus delivering very high power, the only
limit being the transmission system and the torque the material of the flywheel can
withstand.
3. Unlike other energy storage systems, FES systems do not any pollution like chemical,
thermal or acoustical, when properly installed.
4. They have very high efficiency. For short-time storage, it reaches very near to 100% and
decrease with medium-or long-term storage.
5. FES systems have comparatively longer lifetime compared to that of lead-acid battery.
Disadvantages of FES Systems:
1. Flywheels can cause safety issues because of the catastrophic failure of the rotating
machine elements.
2. They have low energy density because of a large safety margin which is required for
safety concerns.
3. They generate noise but this can be overcome by proper design and installation.
4. They are subject to vibrations.
Pumped Hydroelectric Storage:
Rumped storage is an indirect method for temporarily storing substantial amounts of
electrical energy by pumping water from a lower to a higher level
In a pumped storage facility, the power generated in excess of the demand is used to pump
water from a lower reservoir (eg. lake, river or underground) to on upper reservoir.
During periods of peak demand, when the power demand exceeds the normal" generating
plant capacity, water from the upper level is allowed to flow through a hydraulic turbine at the lower
level. The turbine then drives a generator to produce in the usual way.
In most pumped storage plants, the turbine generator System is reversible & can serve to
pump water from the lower to the level, as well as to generate electricity. In pumping mode, the
generator belon a motor, chiven by electricity produced by a generator in the the main plant, and
the turbine then operates as a pump.
Schematic diagram of an underground pumped hydro Storage System
In underground pumped hydro system, the Upper reservoir may be at or near ground level.
The lower reservoir is placed Underground in natural Caverns, old mines or other underground
cavities.
In all system a reversible pump turbine motor-generator set is a principal piece of
equipment
The principle behind the pumped hydro is Simple & follows the law of potential energy, (PE)
that is, the raising of mass to an elevation, heigh or head H
It is given by PE=ρgQ
̥H
Where;
PE is Potential Energy (J)
g is gravitational acceleration 9.81 m/s2
Q
̥
is PE the natural flow rate of wate at the site, potential energy (T)
H is the vertical distance acceleration through which water falls in meters.
Also, PE=gmH
̥
m = mass (kg/sec) (ρ Q ).
Compressed Air storage:
Compressed air energy storage
This type of storage is analogous to Pumped hydro Storage whereas in pumped hydro
System excess energy generated by a base load Plant during periods of low demand is used to
increase the potential energy of hydrostatic Pressure of water, compressed- air energy storage
Compresses and stores air in reservoirs, aquifiers.
The stored energy is then release during periods of peak demand by expansion of the all
through an all turbine.
In general, the efficiency of Compressed air storage is comparable to that of pumped hydrostorage.
In a Conventional gas turbine, the compressor and turbine are connected. In a compressedair energy storage system, however, the turbine and Compression are uncoupled. So that they can
operate separately.
Furthermore, the electric generator, normally connected to the turbine, must also be
Capable of functioning as a electricity is supplied. motor when
Electric power in excess of the immediate demand is supplied to the motor / Generator
which drives the compressor, the at about 70atm is stored in a suitable reservoir.
The air is heated during Compression & may have to be Cooled prior to storage to may
Prevent damage to the reservoir walls. when additional power is needed to mee the demand, the
compressed air and heated using gas or oil fuel.
The hot compressed air is expanded in a gas turbine connected to the motor/generator unit
which now acts as a generator.
Ultra-Capacitor:
Construction:
 The dual side Covered electrodes of the ultracapacitor are created with the graphite
Carbon.
 A permeable paper film keeps apart both electrodes from each other.
 Paper Separator & Carbon conductors both au Permeated with the fluid electrolyte with
aluminium foil.
 For enhancement of Capacitance of ultracapacitor, we have to end enhance the operative
area, without the Capacitor Varying the dimensions of the capacitor.
 These Capacitor stores charges because of it is very large about fewer distances Plates also
has charges 6 huge amount of Capacitance value 100 of farad & among a Plates & its larger
area to accumulate charges.
 Then a typical, Capacitor Cell has a working voltage 1-3 volts, depending on the electrolyte
used. The amount of electrical energy it can store.
 For the storing of large amount charges, Capacitor should be attached in series.
Working:
 When the voltage provided at the +ve terminal it gets -ve ions towards them. And voltage at
the -ve terminal, attracts +ve ions towards them.
 The attraction makes a dual of coating of ions on both sides of the plate. It is named as
Double-layer Capacitor.
 Ultracapacitor stock energy by static charges on Contrary outsides of the electrical dual film.

They use the higher area of carbon for the energy storing substance, which causes higher
energy storage than the other normal Capacitors.
ADVANTAGES OF ULTRACAPACITORS:
1. Ultracapacitors have high specific power.
2. They operate conveniently during low-temperature charging and discharging.
3. They have a long life-time (virtually unlimited) and typically work for more than 1,00,000
cycles with an energy efficiency of more than 90%.
4. They are simple to charge with a charging time of just a few seconds and they do not
overcharge.
5. They are safe as they can survive harsh usage.
Limitations of Ultracapacitors:
1.
2.
3.
4.
They have low specific energy which is only a fraction of a regular battery.
Linear discharge voltage prevents them from using the full energy spectrum.
They have high self-discharge rate.
Ultracapacitors have high cost per watt.
They require series connection with voltage balancing as they have low cell voltage.
Applications of Ultra Capacitors:
The ultracapacitors are used in applications. where requirements are in between the
ranges of batteries and ordinary capacitors.
Some of the applications of Ultracapacitors are,
1. Ultracapacitors are perfect for power pulses, requiring a quick charge to compensate a
short-term power used.
2. They are most efficient to hold-up or bridge power g for a period ranging from a few seconds
to a few min and even to days during failure of main power source or battery or where the
battery is swapped out.
3. The combination of ion-impedance high-power capacitors with high-impedance high-energy
batteries results hybrid ultracapacitor with low impedance, high power and high energy. This
arrangement reduces stress on battery resulting in an increased lifetime of the battery.
4. In mobile computing, they are used for pulse power and hold-up in portable data terminals,
personal digital assistants, and all other portable devices using microprocessors.
5. In industrial applications like solenoid and valve activation, electronic door locks, and
interruptible power supplies.
6. In electric power trains, where ultra-rapid charging is required during regenerative breaking
and high current is required during acceleration.
7. In hybrid vehicles and fuel cell applications, they are ideal to supplement peak-loads.
8. In residential buildings, they are used to supplement for the peak-loads.
9. In wireless communications, they supply pulse power during transmission in GSM cell
phones, 1.5 and 2-way pagers, and other data communication devices.
Thermal (Heat) energy storage:
Energy Can be stored by heating melting or Vaporization of materials and the Energy
becomes available as heat when the process is reversed.
Thermal energy storage is divided 2 types
1. Sensible heat storage
2. Latent
→ storage by causing a material to rise in temperature is called "sensible heat storage”
→ storage by phase change, the transition from solid to liquid from liquid to Vapour is another
mode of thermal storage known as "Latent heat storage" in which no temperature change is
involved.
Thermal energy storage is essential for both domestic water & space heating applications
and for the high temperature storage systems needed for thermal power applications
1. Sensible Heat storage:
In general, thermal energy storage system can operate at many desired temperature levels
depending upon the use and choice of the system & material, ranging from refrigeration
temperatures to 1250°c
They have found wide use in many industrial applications, such as in the manufacture of
Cement, iron & Steel, glass, aluminium paper, plastic, rubber, & in food processing.
Sensible heat refers to thermal energy that results in an increase in temperature when
added to a material (or decrease of temperature when taken from it).
Example: water at temperatures above the freezing point & below its boiling point Can store
energy as sensible heat.
If the temperature too high for the use of water without pressurizations Special hydrocarbon
oils with high boiling points either alone or mixed with rock, can serve to store sensible heat.
Molten salt mixtures, sodium & potassium nitrates, Molten fluorides are used for heat
storage. at very high temperatures.
The ability of store thermal energy (or the storage density) in a given container of volume V is,
Qs/V = ρ Cp ΔT
Where:
Qs = Energy stored or thermal energy
ρ = Density of the storage medium
Cp = Specific heat of the material
ΔT= Difference in temperature/ temperature rise
Thus any of the following devices could be employed as sensible energy storage.
1) Pressurized water storage
2) Packed solid beds
3) Refractory materials (Mgo, Alo, sio)
4) Organic solid beds
The system Can have a nuclear or a fossil fuelled furnace as a primary heat Source. The base
load Portion of the plant is Capable of supplying more Steam than needed during periods of low
demand.
The excess steam is extracted at high pressure Via turbine extraction (as in feed water
heating) during these periods of low demand. The extracted steam is fed to Steam accumulators &
mixed with water thus producing saturated pressurised water.
The accumulators are later discharged through a Small peaking turbine during Periods of
high demand. Discharge Continue until a low specified pressure is reached in the accumulators. It is
observed that there is low Varying steam temperature entering the peaking turbine.
Latent Heat Storage:
Storage by phase change, the transition from solid to liquid or from? liquid thermal to
vapour is another mode of storage known Latent heat storage" in which no temperature charge, is
involved.
Latent heat thermal is energy that stored in (and can be moved from) or mixture when it
goes a is a substance change of phase (e.g. physical form). while the temperature remains
unchanged.
The heat that can be stored per unit mass (or volume) in this manner is Usually several times
greater than for Sensible heat storage.
The phase change from solid to liquid, taking place at the melting Point of the solid is
accompanied by the absorption of latent heat without a change in the temperature. The heat is
recovered when the process is reversed. (i.e liquid is Converted to solid) at the same temperature.
Hence the Can Same Store Saturated and freezing D Solid & liquid phases material when
present together can stored thermal energy as latent heal Steam (i.e steam & moisture) and freezing
water (i.e water & Ice) can be used for latent heat storage.
Advantages of phase change energy storage:
 It is greater than that is sensible heat storage because the latent heats are much
larger than the specific heats of the single phases of the materials.
 The system has the additional advantage of operating at essentially constant
temperature with low volume changes during phase changes.
 It also has the advantage of a wide choice of materials with different fusion. And
Evaporation temperatures, which allows a choice of operating temperatures. And
the ability to generate high temperature Steam for the peaking unit.
 Materials for phase Change energy storage These are Several materials that undergo
change of phase.
1. Glauber's Salt (Na2 SO4 10 H2O) or Sodium Sulphate decahydrate.
2. Water
3. Fe(NO3)2 6H20.
4. Organic Compounds.
5. Salt Eutectics.
Superconducting Magnetic Energy Storage System (SMES):
SMES is a novel technology that stores electricity from the grid, within the magnetic field of
a coil Comprised of superconducting wire with near - zero loss of energy.
SMES is a grid enabling devi that stores and discharges large quantity of power almost
instantaneously.
Superconducting Magnetic energy storage systems store energy in the magnetic field
created by the flow of direct current in a superconducting Coil, which has been Cryogenically cooled
to a temperature bel it's superconducting critical temperature.
A typical SMES System includes 3 parts. i.e Superconducting coil, power conditioning system
& cryogenically cooled refrigerator. Once the Superconducting coil is charged Current will not decay.
And the Magnetic Energy can be stored indefinitely.
The stared energy can be released back to the network by the discharging the coil. The
power Conditioning system uses an inverter/ rectifier to transform AC to DC power or Convert DC
back to AC power.
The inverter/ rectifier accounts for 2-3% energy loss in each direct SMES loses the least
amount of electricity in the energy storage process Compared to other methods of storing energy.
SMES system is highly efficient. The efficiency is greater than 95%.