Non conventional energy sources

Energy generated by using wind, solar, small hydro, tides, geothermal heat and biomass is known a non-conventional energy. All these sources are renewable process of energy generation and do not cause environmental pollution.

NON CONVENTIONAL ENERGY SOURCES AND STORAGE DEVICES

 In 1939, Otto Hahn studied the effect of bombardment of fast neutron on uranium.

 The fission of U-235 nucleus by slow neutrons released enormous amount of energy. The energy released is called nuclear energy or fission energy.

 Nuclear fission is defined as the process of splitting of heavier nucleus into two or more smaller nuclei with simultaneous liberation of large amount of energy.

NUCLEAR ENERGY

 When U-235 is bombarded by slow moving neutron, unstable U-236 is formed.

 This nucleus disintegrates into two equal nuclei with the release of huge amount of energy and few neutrons.

MECHANISM OF NUCLEAR FISSION

 Heavy nucleus splits into two or more nuclei.

 Two or more neutrons are produced by fission of each nucleus.

 Large quantity of energy is produced .

 All the fission fragments are radioactive , giving off gamma radiations

 The fission products fall into two groups- one between mass no.80-100 and other 120-150

CHARACTERISTICS OF NUCLEAR

FISSION REACTION

 All the fission reaction are self propagating chain reaction because fission products contains neutrons which further cause fission in other nuclei.

 The nuclear reactions can be controlled ,by absorbing the neutrons using Cd, B or Hf.

 Every secondary neutron released in the fission reaction does not strike a nucleus , some escape into air. Hence a chain reaction cannot be maintained.

 The number of neutrons resulting from a single fission is known as multiplication factor. When it is less than 1 , no chain reaction take place. The process of combination of lighter nuclei to form heavier nuclei, with simultaneous liberation of huge amount of energy.

Example : Fusion reaction in sun

NUCLEAR FUSION

S.No	Nuclear fission	Nuclear fusion
1.	It is a process of breaking a heavy nucleus with projectiles into two or more light fragments with liberation of large amount of energy	It is a process of  fusion of lighter nuclei  to form heavier nuclei, with simultaneous liberation of huge amount of energy.
2.	This process occurs with the emission of radioactive rays.	It does not emit any radioactive rays.
3.	It takes place simultaneously at ordinary temperature.	It takes place at very high temperature(106K)
4	It gives rise to chain reaction	No chain reaction
5	Neutrons are emitted	Positrons are emitted
6	It can be controlled.	It cannot be controlled.

DIFFERENCES BETWEEN NUCLEAR FISSION AND NUCLEAR FUSION

 In the nuclear fission reaction , the secondary neutrons emitted from the fission of uranium may hit another uranium nuclei and cause fission producing more neutrons and so on. Thus a chain of self sustaining nuclear reaction is set up . This type of reaction is known as nuclear chain reaction.

 The secondary neutrons released may escape to the air and will result in the breaking of the chain and the amount of energy released will be less.

 For the chain reaction to continue, sufficient amount of U-235 must be present to capture the neutrons.

 The minimum amount of fissionable material requited to continue the nuclear chain reaction is called Critical mass. The critical mass of U-235 is between 1 to 100Kg.

 If the mass of the fissionable material is more than the critical mass, it is called super critical mass. And if it is lesser, it is called  sub critical mass.

 The super and sub critical mass may hinder the propagation of the chain reaction.

NUCLEAR CHAIN REACTION EXAMPLE OF NUCLEAR CHAIN REACTION

When U-235 is bombarded by thermal neutrons, it undergoes fission and releases three neutrons. These 3 neutrons strike another U-235 nucleus causing 3x3=9 neutrons. These nine further give rise to 27 reactions. This process is known as nuclear fission chain reaction.

 The energy released by the nuclear fission is called nuclear fission energy or nuclear energy. cause of the release of energy. The enormous amount of energy released during the nuclear fission isdue to the loss in some mass, when the reaction takes place. During nuclear fission, the sum of masses of the products formed is slightly less than thesum of masses of target species and bombarding neutron. The loss in mass get converted into energy according to Einstein equation. where c= velocity; m = loss in mass and E= energy

Total masses of reactants = 235.12 + 1.009 = 236.129 amu

Total masses of products = 140.91 +91.91 +3x1.009 + 235.847 amu

Loss in mass = 0.282 amu

The loss of 0.282 amu of U-235 liberates = 0.282x931.47 = 262.676meV

NUCLEAR ENERGY

NUCLEAR REACTOR OR PILE

 The arrangement or equipment used to carry out fission reaction under controlled conditions is called nuclear reactor.

 The energy released by the fission reaction in the nuclear reactor can be used to produce steam which can turn turbines and produce electricity.

COMPONENTS OF A NUCLEAR REACTOR

1. Fuel rods

2. Moderator

3. Control rods

4. Coolant

5. Protective screen

6. Heat exchanger / pressure vessel

7. Turbine

FUEL RODS: The fissionable material used in the nuclear reactor is enriched U-235. It is used in the form of rods or strips. It produces heat energy and neutrons , thus iniating nuclear chain reaction.

MODERATOR: It slows down fast fission neutrons . Ex., ordinary water, Heavy water , graphite, beryllium. The kinetic energy of fast neutron is reduced to slow neutrons(0.25 eV)

CONTROL RODS: To control the rate of fission of U-235 , these rods absorb the excess neutrons . So the fission reaction proceeds at steady rate. Movable rods made of Cd or B are suspended between fuel rods. These rods are lowered and raised as of need. If the rods are deeply inserted inside the reactor, they will absorb more neutrons and the reaction become very slow. If the rods are pushed outwards, they will absorb less neutrons and the reaction will be very fast. It controls the nuclear chain reaction and avoids the damage to the reactor.

COOLANT: Substance which cools the fuel core by removing heat produced by fission. The coolant liquid is circulated in the reactor core. It enters the base and leaves at the top. The heat carried by outgoing liquid is used to produce steam. Ex., Water ( act as coolant and moderator) Heavy water , liquid metal ( Na or K) , Air, organic compounds like poly phenyls.

PROTECTIVE SCREEN/ SHIELD: The moderator , control rods and fuel element are enclosed in a chamber which has a thick concrete shield(10m thick). These walls stop the nuclear radiations from moving out of the reactor. The environment and the operating persons are protected from destruction in case of leakage of radiation.

HEAT EXCHANGER : It transfers the heat liberated from the reactor core to boil water and produce steam at about 400Kg/cm2.

PRESSURE VESSEL : It encloses the core and also provides the entrance and as exit passages for coolant. It withstand the pressure as high as 200 atm.

TURBINE : The steam generated in the heat exchanger is used to operate a steam turbine, which drives a generator to produce electricity. The exhaust steam is condensed and sent back to the heat exchanger.

NUCLEAR REACTOR

LIGHT WATER NUCLEAR POWER PLANT

 It is the one in which U-235 fuel rods are submerged in water. Here water acts as coolant and moderator.

WORKING

 The fission reaction is controlled by inserting or removing the control rods of B-10 automatically from the spaces in between the fuel rods. The heat emitted is absorbed by the coolant (light water) .The heated coolant then goes to the heat exchanger containing sea water, which is converted to steam. The steam drives the turbines , generating electricity.

BREEDER REACTOR

 Breeder reactor is the one which converts non-fissionable material (U-238, Th-232 ) into fissionable material (U-235, Pu-239).

 In breeder reactor, of the three neutrons emitted in the fission of U-235, only one is used in propagating the fission of U-235.The other two are allowed to react with U-238. Thus two fissionableatoms Pu-239 are produced for each atom of U-235 consumed. The breeder reactor produces more fissionable material than it uses.

 The fissionable nucleides such as U-235 and Pu-239 are called fissile nucleides. The nonfissionable nucleides such as U-238 & Th-232 are called fertile nucleides. The most common breeding reaction is that of plutonium-239 from non-fissionable uranium-238. The term "fast breeder" refers to the types of configurations which can actually produce more fissionable fuel than they use, such as the LMFBR. This scenario is possible because the non-fissionable uranium-238 is 140 times more abundant than the fissionable U-235 and can be efficiently converted into Pu-239 by the neutrons from a fission chain reaction.

SOLAR ENERGY CONVERSION

It is the process of conversion of direct sunlight into more useful forms. It occurs by the following two mechanism.

(i) Thermal conversion (ii) Photo conversion

THERMAL CONVERSION

It involves absorption of thermal energy in the form of Ir radiation. Solar energy is an important source for low temperature heat which is useful for heating buildings, water and refrigeration.

Methods of thermal conversion

1. Solar heat collectors : It consists of natural materials like stones, bricks or materials like glass, which can absorb heat during day time and release it slowly at night. Used in cold places, where houses are kept in hot condition using 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. The black painted copper coil allows the cold water in and heats it up and flows out into a storage tank.

PHOTO CONVERSION

It involves conversion of light energy directly in to electrical energy. Ex. Photo galvanic cell or solar cell

PHOTOGALVANIC CELL

It is the one which converts the solar energy directly into electrical energy.

PRINCIPLE : The basic principle is based on the photovoltaic effect. When solar rays fall on a two layer of semiconductor devices, a potential difference between two layer is produced. This potential difference causes flow of electrons and produces electricity.

CONSTRUCTION : Solar cells consists of a p- type semiconductor ( Si doped with B) and n-type semiconductor(Si doped with P ) .

WORKING : When solar rays fall on 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. Thereby potential difference is produced which causes flow of electrons and hence current is generated.

APPLICATIONS OF SOLAR CELLS :

1. Lighting purpose

2. Solar pumps can be run by solar battery

3. Used in calculators, electronic watches, radios and TV.

4. Used to drive vehicles

5. Used in space craft and satellites

WIND ENERGY

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

o WIND MILLS : The strike of blowing wind on the blades of the wind mill make it rotating continuously. The rotational motion of the blade drives a number of machines like waterpump, flour mills and electric generators.

o WIND FARMS : When a number of wind mills areinstalled and joined together in a definite pattern it forms a wind farm. The wind farm produce alarge amount of electricity. The minimum speed required for satisfactory working of a wind generator is 15Km/hr.

Advantage: (i) no pollution (ii) cheap (iii) renewable

FUEL CELL

 A fuel cell is a device that converts a fuel and air directly into electricity, heat and water by means of electrochemical reactions.

 In a fuel cell, the electricity can be generated as long as the fuel and oxygen are supplied into the cell.

 It consists of an electrolyte and two electrodes.

 In a fuel cell , a fuel is sent through the anode and the oxygen is supplied through the cathode. The electrolyte carries the charged particles from anode and to cathode and vice versa.

 A catalyst such as Pt, Pd, Mg or Ni is often used to initiate and speed up the reactions at the electrodes.

 A fuel cell will also have a fuel reformer which can use hydrogen fuel from any hydrocarbon fuel like natural gas, methanol or even gasoline.

 A single fuel cell can generate a tiny number of direct current but a large number of such fuel cells in the form “stack” can provide the power output from few watt to mega watts. The cell consists of two porous carbon electrodes impregnated with a finely  divided platinum or nickel as catalyst with an electrolyte of 25-40% KOH .

 The cell develops an emf of 1.23V.

 The efficiency of hydrogen-oxygen fuel cell is 70% .

 The operating temperature is 60-70C .

 The cell power output is 300 watts to 5 kilowatts

 The cell may be represented as  C, Pt or Ni / KOH / C, Pt or Ni

WORKING :

At anode,

At cathode,

The net reaction is

HYDROGEN – OXYGEN FUEL CELL

 It is an electrochemical cell or many electrochemical cells connected in series, to be used as a source of direct current to at a constant voltage.

 Batteries are the store houses of electrical energy on demand.

 Batteries are classified as

1. Primary battery – It is the device in which the cell reaction is non reversible and it cannot be recharged. Ex. Leclanche cell, alkaline cell, button cell ( silver and mercury cell )_

2. Secondary battery – These cells are rechargeable and reusable. Its electrode reaction can proceed in either direction. During charging, electrical work is done on the cell to provide the free energy needed to force the reaction in the non-spontaneous reaction. Example – Lead acid cell , Nickel cadmium cell

3. Fuel cell – It is similar to a battery and produce electricity using chemicals. They do not run down like batteries. Ex. Hydrogen-oxygen cell, methanol fuel cell.

BATTERIES

 Alkaline battery is a more modern version of dry cell. It is introduced in 1949.

 It consists of KOH electrolyte and Zn powder act as anode. The carbon rod and manganese dioxide act as cathode.

 The cell may be represented as

At anode

At cathode

The overall net reaction is

The alkaline cell gives a voltage of 1 volt to 1.5 volt.

ADVANTAGES :

1. It can deliver higher current without severe voltage drop.

2. The shelf life of alkaline battery is 5-8 times more than leclanche cell.

3. Zn does not dissolve in alkaline medium readily.

USES : It is used in shavers, radios, tape recorders and electronic photographic flash units.

ALKALINE BATTERIES

SECONDARY BATTERIES

LEAD ACID STORAGE BATTERY OR ACCUMULATOR

It was invented by Gaston Plante in 1859. It acts both as voltaic cell and electrolytic cell. On supplying electrical energy, this acts as a voltaic cell . On recharging, the cell acts as an electrolytic cell.

CONSTRUCTION

1. The cell consists of a polypropylene container containing six lead-acid electrochemical cells connected in series.

2. The voltage of cell is always multiple of 2 V, it can be increased by dividing each cell with microporous polyethylene dividers.

3. All the cathode and anode plates within a single cell are linked together, the anode of one cell being linked with the anode of the next cell and the cathodes are also linked like wise.

4. The anodes are made of lead. Cathodes are made of solid lead (IV) oxide or Lead dioxide, mounted on a lead base.

5. Both poles of the battery are also made of lead.

6. The electrolyte is 80% solution of sulphuric acid saturated with Lead sulphate.

7. The cell is represented by

The electrode reactions are

Discharging : During discharge the lead plates (anode) act as voltaic cell, and the cell reaction are

At anode – At the negative pole (anode) oxidation of lead takes place 

Then the lead ions combine with sulphate ions to produce lead sulphate

Net reaction

At cathode – The electrons released at anode flow to the cathode, where reduction reaction takes place

 

Net reaction  

The overall cell reaction during discharging

When current is drawn , lead – acid battery becomes less efficient . So the battery has to be recharged.

RECHARGING

Charging is done by continuous application of potential from an outside power source. As long as the current is passed, Pb ions are reduced to lead metal while, at the lead dioxide electode, lead ions are oxidized . During charging, reverse reaction takes place

 LIMITATIONS

a. These batteries tend to slowly self discharge, so a car left idle for several weeks might be unable to start.

b. The sulphuric acid becomes viscous when the temperature is low, this inhibits the flow of ions between the plates and reducing the current. This makes it very difficult to start a car in cold weather.

 USES

a. The lead storage cells are mainly used in motor vehicle.

b. They also find applications in telephone exchanges , hospital , power stations, railway stations etc.,

 NICKEL CADMIUM BATTERIES

 LITHIUM BATTERY

OTHER TYPES OF SECONDARY LITHIUM BATTERIES

THE END