Transcript Electrical Energy Storage

BATTERY
Storage System Electric power is stored in batteries. Although various types of
batteries are available in the market Lead-Acid batteries are more common.
Lead-acid batteries: In 1859 French physicist Gaston Planté invented this
rechargeable battery. Despite having a very low energy-to-weight ratio and a low
energy-to-volume ratio, they have the ability to supply high surge current and relatively
large power-to-weight ratio. These features, along with their low cost, make them
attractive for use in motor vehicles and other applications..
Parts of a Lead-acid Battery:
A battery consists of a number of cells and each cell of the battery consists of (a)
positive and negative plates (b) separators (c) electrolyte and (d) the container.
Different parts of a lead-acid battery are described below:
(a) Plates. A plate consists of a lattice type of grid of cast antimonied lead alloy which
is covered with active material. The grid not only serves as a support for the fragile
active material but also conducts electric current. Grids for the positive and negative
plates are often of the same design although negative- plate grids are made somewhat
lighter.
Figure: Positive plate and negative plate
(b) Separators. These are thin sheets of a porous material placed between the
positive and negative plates for preventing contact between them and thus avoiding
internal short-circuiting of the battery. A separator must, however, be sufficiently porous
to allow diffusion or circulation of electrolyte between the plates. These are made of
especially-treated cedar wood, glass wool mat, microporous rubber (mipor),
microporous plastics (plastipore, miplast) and perforated p.v.c. as shown in Fig. 91. In addition to good porosity, a separator must possess high electrical resistance and
mechanical strength.
(a)
(b)
(c)
Figure 9-1: Separators: (a) and (b) Miplast type (c) Perforated type
(c) Electrolyte. It is dilute sulphuric acid which fills the cell compartment to immerse
the plates completely.
(d) Container. It may be made of vulcanized rubber or moulded hard rubber (ebonite),
moulded plastic, ceramics, glass or celluloid. The vulcanized rubber containers are
used for car service, while glass containers are superior for lighting plants and wireless
sets. Celluloid containers are mostly used for portable wireless set batteries.
Figure 9-2: Battery container
Full details of a Russian l2-CAM-28 lead-acid battery parts are shown in Fig 9-3.
Details of some of these parts are as follows :
1. negative plate 2.
separator 3. positive
plate. 4. positive group
5. negative group 6.
negative group grooved
support block 7. lug 8.
plate group 9. guard
screen 10. guard plate
11. cell cover 12. plug
washer 13. vent plug
14. monoblock jar 16.
supporting prisms of +
ve group 16. inter-cell
connector 17. terminal
lug 18. screw 19.
washer 20. nut 21.
rubber packing 22.
sealing compound.
9-5. Active Materials of a Lead-acid Cell
Those substances of the cell which take active part in chemical reaction and hence
absorb or produce electricity during charging or discharging, are known as active
materials of the cell.
The active materials of a lead-acid cell are :1. Lead Peroxide (PbO2) for + ve plate
2. Sponge Lead (Pb) for -ve plate
3. Dilute Sulphuric Acid (H2S04) as electrolyte
1. Lead Peroxide
It is a combination of lead and oxygen, is dark chocolate brown in colour and is quite
hard but brittle substance. It is made up of one atom of lead (Pb) and two atoms of
oxygen (O2) and its chemical formula is PbO2. As said earlier, it forms the positive
active material.
2. Sponge Lead
It is pure lead in soft sponge or porous condition. Its chemical formula is Pb and forms
the negative active material.
3. Dilute Sulphuric Acid
It is approximately 3 parts water and one part, sulphuric acid. The chemical
formula of the acid is H2S04. The positive and negative plates are immersed in this
solution which is known as electrolyte. It is this medium through which the current
produces chemical changes.
Hence, the lead-acid cell depends for its action on the presence of two plates
covered with PbO2 and Pb in a solution of dilute H2SO4 of specific gravity 1.21 or
near about.
Discharging:
When the cell is fully charged, its positive plate or anode is Pb02 and the negative plate
or cathode is Pb. When the cell discharges i.e. it sends current through the external
load, then H2SO2 is dissociated into positive H2 and negative S04 ions. As the current
within the cell is flowing from cathode to anode, H2 ions move to anode and SO4 ions
move to the cathode.
At anode (Pb02), H2 combines with the oxygen of Pb02and H2SO4 attacks lead to form
PbS04.
It will be noted that during discharging : —
Both anode and cathode become PbSO4 which is somewhat whitish in colour,
Due to formation of water, specific gravity of the acid decreases.
Voltage of the cell decreases.
The cell gives out energy
(ii) CHARGING (Fig. 9-5)
When the cell is recharged, then H2 ions move to cathode and S04 ions go to anode
and the following changes take place :
Hence, the anode and cathode again become PbO2 and Pb respectively.
It will be noticed that during charging :
The anode becomes dark chocolate brown in colour (Pb02) and cathode becomes
grey metallic lead (Pb)
Due to consumption of water, specific gravity of H2SO4. is increased
There is a rise in voltage,
Energy is absorbed by the cell.
Properties of the Lead-Acid Storage Battery
Efficiency:
Ideally, the charging and discharging processes of the lead-acid system should be
reversible. In reality, however, they are not. Some of the electrical energy intended for
charging is lost in the internal resistance and is converted to heat. When hydrogen is
lost, it also represents an energy loss. Typically, the charging process is about 95%
efficient. The discharge process also results in some losses due to internal resistance
of the battery, so only about 95% of the stored energy can be recovered. The overall
efficiency of charging and discharging a lead-acid battery is thus about 90%.
Since battery losses to internal resistance are proportional to the square of the current,
this means that high current charging or high current discharging will tend to result
in higher internal losses and less overall performance efficiency.
Amount of stored energy:
The amount of energy stored in a battery is commonly measured in ampere hours.
While ampere hours are technically not units of energy, but, rather, units of charge, the
amount of charge in a battery is approximately proportional to the energy stored in the
battery. If the battery voltage remains constant, then the energy stored is simply the
product of the charge and the voltage.
Charging and discharging rate:
The capacity of a battery is often referred to as C. Thus, if a load is connected to a
battery such that the battery will discharge in x hours, the discharge rate is referred to
as C/x. The charging of a battery is measured in a similar fashion. Figure 3.11 indicates
the effect of discharging rates on the relative amount of charge that can be obtained
from a lead-acid battery.
Figure 3.11 Effect of discharge rate on available energy from a lead-acid battery.
Note that higher discharge rates result in less charge being available as energy
to a load. At higher charging rates, a smaller fraction of the charging energy is
used for charging and a larger fraction is used to heat up the battery. The battery
can be fully charged at higher charging rates, but it takes more energy at higher
charging rates to obtain full charge.
Effect of temperature and discharge rate on available energy:
Figure 3.12 shows the effect of discharge rates and temperature on the relative
amount of charge that a battery can deliver. Again, slower discharge rates result in a
higher overall amount of charge being delivered by the battery.
Figure 3.12 Effect of temperature and discharge rate on available
energy from a lead-acid battery.
Effect of DOD on Life-Cycle:
Figure 3.13 shows how the depth of discharge affects the number of operating cycles
of a deep discharge battery. The PV system designer must carefully consider the tradeoff between using more batteries operating at shallower discharge rates to extend the
overall life of the batteries vs. using fewer batteries with deeper discharge rates and
the correspondingly lower initial cost.
Figure 3.13 Lead-acid battery lifetime in cycles vs. depth of discharge per cycle
Specific gravity of acid and terminal voltage:
Figure 3.14 shows how the electrolyte specific gravity and voltage vary during charging
and discharging of the battery. Note particularly the effect of the voltage drop across
the internal cell resistance during charging and discharging has not considered here.
For applications where maintenance of batteries is inconvenient, maintenance free,
sealed deep-cycle batteries exist, but their cost is higher than flooded battery.
Figure 3.14 Variation of cell electrolyte specific gravity and
cell voltage during charge and discharge at constant rate
Deep and Shallow Discharge Battery:
The shallow discharge batteries typically have a small amount of calcium combined
with the lead to impart greater strength to the otherwise pure lead. The plates can then
be made thinner with greater area to produce higher starting currents. These units
should not be discharged to less than 75% of their capacity. The shallow discharge
units have a smaller quantity of lead and are correspondingly less expensive.
Deep discharge lead-acid batteries use antimony to strengthen the lead and can be
cycled down to 20% of their initial capacity. The plates are thicker, with less area and
are hence designed for sustained lower level currents. These batteries are used in PV
system.
Vented and nonvented batteries:
In certain lead-calcium batteries, minimal hydrogen and oxygen are lost during
charging. This means minimal water is lost from the electrolyte. As a result, it is
possible to seal off the cells of these batteries, making them essentially maintenance
free. The trade-off, however, is that if these batteries are either purposely or
inadvertently discharged to less than 75% of their maximum charge rating, their
expected lifetime may be significantly shortened.
Lead-antimony electrodes, on the other hand, may be discharged to 20% of their
maximum rating. This means that a 100 Ah lead-calcium battery has only 25 Ah
available for use, while a 100 Ah lead-antimony battery has 80 Ah available for use, or
more than 3 times the availability of the lead-calcium unit. However, the lead-antimony
unit produces significantly more hydrogen and oxygen gas from dissociation of water in
the electrolyte, and thus water must be added to the battery relatively often to prevent
the electrolyte level from falling below the top of the electrodes. Water loss can be
reduced somewhat by the use of cell caps that catalyze the recombination of hydrogen
and oxygen back into water, which returns to the cell.
Chemistry of the Nickel Cadmium Storage Battery:
Ni-Cd batteries use nickel oxi-hydroxide for the anode plates and finely divided
cadmium for the cathode plates.
The electrolyte in the Ni-Cd system is potassium hydroxide (KOH).
The NiOOH anode is generally made of nickel fibers mixed with graphite- or nickelcoated plastic fibers. Small quantities of other materials such as barium and cobalt
compounds are also added to improve performance. The cathode is also frequently
made of a cadmium-coated plastic fiber. If the cathode is not a coated plastic, then it is
commonly mixed with iron or nickel. The fiber structures of anode and cathode
maximize the surface area while minimizing the amount of relatively expensive nickel
and cadmium required for the electrodes.
On discharge, the NiOOH of the positive plate is converted to Ni(OH)2 and the
cadmium metal of the negative plate is converted to Cd(OH)2.
The basic reactions are:
At the positive plate: NiOOH + H2O + e- = Ni(OH)2+OH
At the negative plate: Cd + 2OH- = Cd(OH)2 + 2eOverall: 2NiOOH + Cd + 2H20 = 2Ni(OH)+Cd(OH)
The voltage of the fully charged cell is 1.29 V. Unlike the lead-acid system where the
specific gravity of the electrolyte changes measurably during discharge or charge, the
KOH electrolyte of the Ni-Cd system changes very little during battery operation.
Properties of the Nickel Cadmium System
Ni-Cd batteries are more robust than lead-acid batteries. They can survive freezing
and high temperatures, they can be fully discharged and they are less affected by
overcharging.
Capacities range from small sizes up to more than 1200 Ah. Ni-Cd system can be
discharged at rates up to C over a wide temperature range, while still providing more
than 90% of its capacity to the load.
Very low internal resistance of the cells.
The lifetime of a Ni-Cd battery depends on how it is used, but is less dependent on
depth of discharge than that of lead-acid batteries.
A lifetime of at least 2000 cycles can be expected for a battery when it is not used
extensively at elevated temperatures.
Ni-Cd batteries may be charged at either constant current, constant voltage or
somewhere in between. This makes them especially useful in PV applications,
since a PV array tends to act as a constant current source, depending on the
cloud cover at the moment.
Disadvantages : Ni-Cd batteries has memory effect. If the battery is not fully
discharged prior to recharge, the battery will tend to discharge only to the point where it
was most recently discharged, and then lose cell voltage.
Disadvantages of Ni-Cd batteries include difficulty in determining the state of charge of
the batteries, they are heavier, and the toxicity of the cadmium, which creates an
environmental concern during production and disposal.
Ni-MH: (Nickel Metal Hydride)
`
Another technology that is becoming very popular, particularly in smaller applications
such as camcorders and laptop computers, is the nickel-metal hydride (NiMH)
battery. This battery replaces the cadmium cathode with an environmentally benign
metal hydride cathode, allowing for higher energy density at the cathode and a
correspondingly longer lifetime or higher capacity, depending on the design goal. The
anode is the same as in the Ni-Cd cell and KOH is used as the electrolyte. The overall
discharge reaction is
MH + NiOOH = M + Ni(OH)2 . (3.10)
The NiMH cell requires clever use of an oxygen recombination system to prevent loss
of oxygen during the charging cycle. Electrodes are also carefully sized to ensure that
the useful capacity of the battery.
Advantages: Increased energy density, lighter, environment friendly.
Disadvantages: Evolves more heat than Ni-Cd during charging, higher rate of self
discharge, more difficult to terminate at full charge during charging.
Lithium Ion (Li-Ion) Battery:
Lithium-Ion battery is an advanced battery. Lithium ion battery has high energy density,
low self discharge and no memory effect. These batteries are used where smaller
and light weight batteries are required such as cell phones and laptop computers.
However the main disadvantages of these batteries are the high price and fewer
charge/discharge cycles.
Advantages
Disadvantages
NiCd:
Inexpensive
Moderately heavy
High current draw
Pollute environment
Wide temp range (-20 to 60)
Memory effect
Fastest charge
More energy is available
NiMh
25 to 30% lighter and smaller
than NiCd
More expensive
Fewer environmental issue
Lower cycle life
Less memory effect
Limited current draw
2000 to 2800 mAh capacity
Limited temp. range
Greater self discharge
Li-ion:
Light and smaller
Expensive
Low self discharge
Sensitive to overcharge
Reacts with oxygen
Fewer charge/discharge cycle