Energy sources

ENERGY SOURCES

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ENERGY SOURCES
Battery
A battery is an arrangement of several electrochemical cells connected in series that can be used as a source of direct electric current. They may be considered as store houses for electrical energy on demand. There are many types of electrochemical cells, including galvanic cells, fuel cells, electrolytic cells and voltaic cells.
In a battery, electrons from an electron – rich chemical (donor) flow through the external circuit and return to the battery in another chemical which is an electron acceptor. The flow of electron is called electron discharge. The difference between the two voltages of the donor – acceptor pair is the cell voltage.
A Cell contains only one anode and cathode.
A Battery Contains several anodes and cathodes.

Requirements of a useful battery
1. It should be light and compact for easy transport.
2. The voltage of the battery should not vary appreciably during its use.
3. It should have long life both when it is being used and when it is not used.

TYPES OF BATTERY
Batteries are broadly classified into the following categories:
i. Primary Battery (or) Primary cells (or) Non-reversible Battery

In these cells, the electrode and the electrode reactions cannot be reversed by passing an external electrical energy. The reactions occur only once and after use they become dead. Therefore, they are not chargeable and they have to be discarded after the exhaustion of their reactants.
Ex. Dry cell, mercury cell.

ii. Secondary Battery (or)Secondary cells (or) Reversible cells
In these cells, the electrode reactions can be reversed by passing an external electrical energy. Therefore, they can be recharged by passing electric current and used again and again. These are also called storage cells (or) Accumulators.
Ex. Lead acid storage cell, Nickel-cadmium cell.

iii. Flow Battery (or)Fuel cells
In these cells, the reactants, products and electrolytes are continuously passing through the cell. In this chemical energy gets converted into electrical energy.
Ex.ydrogen-oxygen fuel cell.

iv) Reserve Battery
In these batteries, the active metals are kept separated by special arrangement. When it has to be already used, an activation device makes it ready. Such a battery is designed for long storage before use.

ALKALINE BATTERY

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ALKALINE BATTERY
It is an improved form of the dry cell. In this zinc act as anode and MnO2 act as cathode. The electrolyte in this cell is KOH (an alkali) and hence the name alkaline battery. This cell derives the power from the reduction of the cathode and the oxidation of the zinc anode.

Cell Reactions
At anode: Zn(s) + 2OH‾(aq) → Zn(OH)2(s) + 2e‾
At cathode: 2MnO2(s) + H2O(l) +2e‾ → Mn2O3 + 2OH‾(aq)

Overall Cell Reaction
Zn(s) +2MnO2(s) + H2O(l) → Zn(OH)2 (s) + Mn2O3 (s)

The emf of this cell is 1.5V. The zinc anode used in this cell is made porous to provide a large electrode area. This leads to the delivery of more current.
It sustains heavy use and has a longer shelf life. So it is called a heavy duty battery. It performs better in cold weather than the other type of batteries.

The main advantages of alkaline battery over dry battery are

(i) Zinc does not dissolve readily in a basic medium.
(ii) The life of alkaline battery is longer than the dry battery, because there is no corrosion on Zn. (iii) Alkaline battery maintains its voltage, as the current is drawn from it.

Uses:
It is used in calculators, watches.

LEAD STORAGE CELL OR LEAD ACCUMULATOR OR ACID STORAGE CELL

2009-11-16T23:09:00.000-08:00

LEAD STORAGE CELL OR LEAD ACCUMULATOR OR ACID STORAGE CELL

A lead acid storage is a secondary battery, which can operate both as a voltaic cell and as an electrolytic cell. When it acts as a voltaic cell, it supplies electrical energy and becomes “run down”. When it is recharged, the cell operates as an electrolytic cell.
A lead-acid storage battery consists of a number of (3 to 6) Voltaic cells connected in series to get 6 to 12V battery.
In each cell, the anode is made of lead. The cathode is made of lead dioxide PbO2 or a grid made of lead plates (anodes) are connected in parallel and a number of PbO2 plates (cathodes) are also connected in parallel. Various plates are separated from the adjacent ones by insulators like rubber or glass fibre. The entire combinations is then immersed in dil. H2SO4 (38% by mass) having a density of 1.30 gm/ml.

Cell representation:
Pb │ PbSO4 ││ H2SO4 (aq) │ PbO2 │ Pb

Working (Discharging)
When the lead-acid storage battery operates, the following reaction occurs.
At anode
Pb(s) + SO42‾ (aq) --->PbSO4(s) + 2e¯
(Oxidation of Pb to Pb2+ ions and formation of insoluble PbSO4)

At cathode
PbO2(s) + 4H+ + SO42¯ +2e¯ --->PbSO4(s) + 2H2O
(Reduction of PbO2 to Pb2+ ions, combination of insoluble PbSO4)

Overall cell reactions during use (discharging):
Pb(s) + PbO2(s) + 2H2SO4(aq) ---> 2PbSO4(s) + 2H2O + Energy

A single cell generates 2V and the six such cells in the battery produce a total of 12V.
From the above cell reactions it is clear that, PbSO4 is precipitated at both the electrodes and H2SO4 falls below 1.2 gm/ml. So the battery needs recharging.

Recharging the cell
The cell can be charged by passing electric current in the opposite direction. The electrode reaction gets reversed. As a result, Pb is deposited on anode and PbO2 on the cathode. The density of H2SO4 also increases.

The net reaction during charging is
2PbSO4 (s) + 2H2O +Energy --->Pb (s)+ PbO2(s) +2H2O(aq)

The recharging involves exactly the reverse process of the normal cell reaction. During the period of recharging, a secondary battery is converted from a galvanic cell to an electrolytic cell.
In the discharge of the battery, the sulphuric acid is replaced by water and hence the density of the electrolyte, H2SO4 decreases from the initial value of 1.2 g/cc to 1.0 g/cc. Decreasing density of H2SO4 is an indicative of the weakening of the battery.

Uses
1. Lead storage cell is used to supply current mainly in automobiles such as cars, buses, trucks
etc.
2. It is also used in gas engine ignition, telephone, exchange, hospitals, power stations, etc.

NICKEL-CADMIUM CELL (or) NICAD BATTERY

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NICKEL-CADMIUM CELL (or) NICAD BATTERY
This is also a rechargeable battery. It consists of a cadmium anode and a metal grid containing a paste of NiO2 cathode and an alkaline KOH.

Cell representation:
Cd │ Cd(OH)2 ││ KOH (aq)│ NiO2 │ Ni

Working (discharging)
When the NiCAD battery operates, cadmium is oxidised to Cd2+ ions and insoluble Cd(OH)2 is formed .

At anode
Cd(S)+2OH¯ --> Cd(OH)2 (s) + 2e¯
(Oxidation of Cd to Cd2+ ions and formation of Cd(OH)2)

At cathode
NiO2(s) + 2H2O + 2e¯---> Ni(OH)2 (s) + 2OH¯
(Reduction of NiO2 to Ni2+ ions and formation of Ni (OH)2)

Overall reaction during use (discharging)

Cd(s)+ NiO2 + 2H2O --> Cd(OH)2(s) + Ni(OH)2 (s) + Energy
From the above cell reactions it is clear that, there is no formation of gaseous products, the products Cd(OH)2 and Ni(OH)2 adhere well to the surfaces. This can be reconverted by recharging the cell.

Recharging the Battery
The recharging process is similar to lead storage battery. When the current is passed in the opposite direction, the electrode reaction gets reversed. As a result, Cd gets deposited on anode and NiO2 on the cathode.

The net reaction during charging is
Cd(OH)2 (s) + Ni(OH)2 + Energy--> Cd (s) + NiO2(s) + 2H2O

Advantages

1. It gives a constant voltage of 1.4V.
2. It is smaller and lighter.
3. It has longer life than lead storage cell.
4. Like a dry cell, it can be packed in a sealed container.

Disadvantages
It is more expensive than lead storage battery.

Uses
It is used in calculators, electronic flash units, transistors and cordless appliances.

LITHIUM BATTERY

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LITHIUM BATTERY
Lithium battery is a solid state battery because instead of using liquid or a paste electrolyte, solid electrolyte is used.

Construction
The lithium battery consists of a lithium anode and a TiS2 cathode. A solid electrolyte, generally a polymer, is packed in between the electrodes. The electrolyte (polymer) permits the passage of ions but not that of electrons.

Working (Discharging)
When the anode is connected to cathode, lithium ions move from anode to cathode. The anode is elemental lithium, which is the source of the lithium ions and electrons. The cathode is a material capable of receiving the lithium ions and electrons.
At anode: Li(s) → Li+ + e¯
At cathode: TiS2 (s) + e¯ →TiS2¯

Overall reactions
Li(s) + TiS2 (s) → Li+ + TiS2¯
Li+ + TiS2¯ → LiTiS2

Recharging the Battery
The lithium battery can be recharged by supplying an external current, which drives the lithium ions back to the anode. The overall reaction is

LiTiS2 → Li+ + TiS2¯
This cell is rechargeable and produces a cell voltage of 3.0V.

Other types of secondary Lithium Batteries
i. Li/MnO2
ii. Li/V2O5
iii. Li/MoO2
iv. Li/Cr3O8

Advantages of Lithium battery
It is considered to be cell of the future because
1. Its cell voltage is high, 3.0 V.
2. Li being a light-weight metal, only 7g (1 mole) material is required to produce 1 mole of
electrons.
3. Since Li has the most negative E° value, it generates a higher voltage than the other types
of cells.
4. All the constituents of the battery are solids and there is no risk of leakage from the battery.
5. This battery can be made in a variety of sizes and shapes.

Lithium - Sulphur

Lithium-sulphur battery is a rechargeable battery. Here the anode is Lithium and Sulphur is the electron acceptor. The electron from Li is conducted to S by a graphite cathode. β-Alumina (NaAl11O17) is used as the solid electrolyte, which separates anode and liquid sulphur.
This solid electrolyte allows the Li+ ions to migrate to equalize the charge, but will not allow the big poly sulphide product ions.
This battery is operated at high temperatures as Li and S should be in their molten states.

Various reactions
The Various electrode reactions are
At anode: 2Li → 2Li+ + 2e¯
At cathode: S + 2e¯ → S2¯
Over all reactions: 2Li + S→ 2Li+ S2¯
The S2¯ ions, formed, react with elemental sulphur to form the polysulphide ion.

S2¯+ nS → [Sn +1]2¯
The direct reaction between Li and S is prevented by the alumina present in the cell.

Advantages of Li-S battery

1. Li-S battery has light weight unlike the lead acid battery.
2. It possesses a high energy density.
3. It is used in electric cars.

SOLAR CELLS

2009-11-16T23:04:00.000-08:00

SOLAR ENERGY CONVERSION
Solar energy conversion is the process of conversion of direct sunlight into more useful forms. This solar energy conversion occurs by the following two mechanisms.

1. Thermal conversion
2. Photo conversion

Thermal conversion
Thermal conversion involves absorption of thermal energy in the form of IR radiation. Solar energy is an important source for low- temperature heat (temperature below 1000 C) which is useful for heating buildings, water and refrigeration.

Methods of thermal conversion
1. Solar heat collectors
2. Solar water heater

1. Solar heat collectors
Solar heat collectors consist of natural materials like stones, bricks (or) materials like glass, which can absorb heat during the day time and release it slowly at night.

Uses
It is generally used in cold places, where houses are kept in hot condition using solar heat collectors.

2. Solar water heater
It consists of an insulated box inside of which is painted with black paint. It is also provided with a glass lid to receive and store solar heat. Inside the box it has black painted copper coil, through which cold water is allowed to flow in, which gets heated up and flows out into a storage tank. From the storage tank water is then supplied through pipes.

PHOTOCONVERSION
Photoconversion involves conversion of light energy directly into electrical energy.

Methods of photoconversion
Photogalvanic cell or Solar cell

PHOTOGALVANIC CELL OR SOLAR CELL
Definition
Photogalvanic cell is the one, which converts the solar energy (energy obtained from the sun) directly into electrical energy.

Principle
The basic principle involved in the solar cells is based on the photovoltaic (PV) effect. When the solar rays fall on a two layer is produced. This potential difference causes flow of electrons and produces electricity.

Solar cells consist of a p-type Semiconductor (such as Si doped with B) and n-type semiconductor (such as Si doped with P). They are in close contact with each other.

Working
When the solar rays fall on the top layer of p-type semiconductor, the electrons from the valence band get promoted to the conduction band and cross the p-n junction into n-type semiconductor. There by potential difference between two layers is created which causes flow of electrons (i.e., an electric current). The potential difference and hence current increases as more solar rays falls on the surface of the top layer.
Thus when this p and n layers are connected to an external circuit, electrons flow from n-layer to p-layer, and hence current is generated.

Applications of Solar cells

1. Lighting purpose
Solar cells can be used for lighting purpose. Now a days electrical street lights are replaced by solar street lights.

2. Solar pumps run by solar battery
When a large number of solar cells are connected in series it form a solar battery. Solar battery produces more electricity which is enough to run, water pump, street light, etc.
3. They can be used to produce hydrogen by electrolysis of water. The liberated hydrogen can be used in H2 – O2 fuel cells.
4. Solar cells are used in boilers to produce hot water for domestic and industrial uses.
5. Solar cells are used in calculators, electronic watches, radios and TVs.
6. Solar cells are superior to other type of cells, because these are nonpolluting and eco-friendly.
7. Solar energy can be stored in Ni-Cd batteries and lead-acid batteries.
8. Solar cells can be used to drive vehicles.
9. Solar cells, made of silicon, are used as a source of electricity in space craft and satellites.

FUEL CELLS

2009-11-16T23:02:00.000-08:00

FUEL CELLS
Definition
Fuel cell is a voltaic cell, which converts the chemical energy of the fuels directly into electricity without combustion. It converts the energy of the fuel directly into electricity. In these cells, the reactants, products and electrolytes pass through the cell.

Fuel + Oxygen → Oxidation products + Electricity

Examples:
Hydrogen-oxygen fuel cell, Methyl alcohol-oxygen fuel cell, Propane – oxygen fuel cell.

1. Hydrogen-Oxygen fuel cell
Hydrogen-oxygen fuel cell is the simplest and most successful fuel cell, in which the fuel-hydrogen and the oxidiser-oxygen and the liquid electrolyte are continuously passed through the cell.
It consists of two porous electrodes anode and cathode. These porous electrodes are made of compressed carbon containing a small amount of catalyst (Pt,Pd,Ag). In between the two electrodes in electrolytic solution such as 25% KOH or NaOH is filled. The two electrodes are connected through the voltmeter.

Working
Hydrogen (the fuel) is bubbled through the anode compartment, where it is oxidised. The oxygen (oxidiser) is bubbled through the cathode compartment, where it is reduced.

At cathode
The electrons produced at the anode pass through the external wire to the cathode, where it is absorbed by oxygen and water to produce hydroxide ions.

O2 + 2H2O + 4e¯ → 4OH¯

At anode
Hydrogen molecules are oxidised at the anode with the liberation of electrons, which then combine with hydroxide ion to form water.

2H2+4OH¯ → 4H2O + 4e¯

Cell reaction
At anode: 2H2O + 4OH¯ → 4H2O + 4e¯ E = +0.83V
At cathode: O2 +2H2O+4e¯ → 4OH¯ E = +0.40V
Overall cell reaction: 2H2 + O2 → 2H2O E = +1.23V

Fuel Battery
When a large number of fuel cells are connected in series, it form a battery called fuel cell battery or fuel battery.

Advantages
1. H2-O2 fuel cells are used as auxiliary energy source in space vehicles, submarines or other
military-vehicles.
2. The energy conversion (from chemical to electrical) is highly efficient.
3. They have low maintenance cost.
4. H2-O2 fuel cell produces drinking water.
5. Noise and thermal pollution are low.
6. In case of H2-O2 fuel cells, the product of water is proved to be a valuable source of fresh
water by the astronauts.

Disadvantages
1. Their initial cost is high.
2. Life time of fuel cells is not known accurately.

Methanol Fuel cell
In this fuel cell, platinum black catalyst is deposited on a 10 sq. inch nickel plate which acts as fuel electrode. Silver catalyst is deposited on a nickel plate. This plate acts as oxygen electrode. Both of these electrodes are placed in 25% NaOH or KOH with air bubbling up the silver catalyst surface. Now 35 cc of pure methyl alcohol (methanol) is added and the fuel cell is activated. This fuel cell produces enough power to run a simple transistorized radio even without bubbling of air.

WIND ENERGY

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Wind energy
Energy recovered from the force of the wind is called wind energy. The wind energy is harnessed by making use of wind mills.

Wind mills
The strike of blowing wind on the blades of the wind mill make it rotating continuosly . The rotational motion of the blade drives a number of machines like water pump, flour mills and electric generators.
Now a days wind mill uses large sized propeller blades and are connected to a generator through a shaft. Wind mills are capable of generating about 100 KW electricity.

Wind Farms
When a large number of wind mills are installed and joined together in a definite pattern it farms a wind farm. The wind farms produce a large amount of electricity.
The minimum speed required for satisfactory working of a wind generator is 15 km/hr. The cost of power generation from wind farms has now become lower than diesel power and comparable to thermal power in several areas of our country.

In India –
The coastal regions of Gujarat
The hill tops in Madhya Pradesh.
The plains in Uttar Pradesh and Rajasthan.

Advantages of wind energy
1.It is a renewable energy source available free of cost.
2.It is more economic.
3.Wind mills are used to supply power to remote and rural areas.
4.It does not cause pollution during energy generation.

Disadvantages of wind energy
1. Low energy density
2. Use of energy storage batteries creates pollution.
3. Variable and irregular.
4. Wind turbines interfere with TV and other electromagnetic communication systems.
5. The wind farms produce unbearable mechanical noise from gear box.
6. Use of energy storage batteries creates pollution.