CY 6251- UNIT- III

ENERGY SOURCES

Nuclear energy – fission and fusion reaction and light water nuclear reactor for power generation (block diagram only) – breeder reactor – solar energy conversion – solar cells – wind energy – fuel cells – hydrogen-oxygen fuel cell – batteries – alkaline batteries – lead-acid, nickel-cadmium and lithium batteries.

Introduction:Mankind is searching continuously for a new, cheap, pollution free and abundant power source because of the limited availability of conventional sources like coal and petroleum. Moreover, with rapid increase of population, industries and living standard of the people, it is estimated that the present conventional energy sources will not available for the next 100 years to meet out the demands of electric power.Otto Hahn discovered the nuclear fission of 92U235, which releases large amount of heat energy, which is many million times that of in conventional energy sources. So it becomes very important. For example, the complete fission of 1 kg of 92U235 produces around 2 x 107 kilowatt hour energy; where as the same amount of energy can be produced by the complete combustion of 3 x 106 kg of high grade coal.

NUCLEAR REACTION:

In nuclear reactions, the nucleus of an atom undergoes a change to forms new atom with release of large amount of energy. The large amount of energy released in a nuclear reaction is due to the conversion of some of the mass into energy. A nuclear reaction is not affected by the changes in temperature (T) and pressure (P). It is an irreversible process. The energy changes associated with the nuclear reactions are about many million times more than that of in chemical reactions. A nuclear reaction equation is balanced by making atomic numbers (Lower figure on the symbol) equal on both sides and also by making the mass numbers (Upper figure on the symbol) equal on both the sides of equation, taken separately.

Example: 7N14 + 2He4 8O17 + 1H1

α – Particle

S. No / NUCLEAR REACTION / CHEMICAL REACTION
1. / It involves a change in the
composition of the nucleus. / It involves the rearrangement
of outer electrons.
2. / New element is produced. / No production of new element.
3. / Energy change is enormous. / Energy change is less.
4. / The rate of the reaction is
not affected by the external
factors like concentration,
T, P and catalyst. / The rate of the reaction is
affected by the external
factors like concentration,
T, P and catalyst.

Energy released in nuclear reactions:

The energy released in nuclear reactions is million times more than that of in chemical reactions. It is generally determined by using Einstein’s mass-energy relation E = m x c2 , where E – Energy equivalent of mass m and c–velocity of light. For a change of atomic mass unit (amu), the corresponding energy change,

E = 1.6605 x 10-27 kg x (2.9979 x 108 m/s)2

= 1.4924 x 10-10 kg m2 / s2

= 1.4924 x 10-10 J

= 1.4924 x 10-10 J x ( 1 eV / 1.602 x 10-19 J )

= 931 x106 eV

= 931 MeV

That is a mass of 1 amu is equivalent to 931 MeV of energy.

Generally in nuclear reactions the mass of the products is less than that of the reactants. The mass difference is converted into energy.

For example, 3Li7 + 1H1 2 2He4 + energy

7.160 amu 1.0078 amu 2(4.0026) amu

The mass difference = 8.1678 – 8.0052 = 0.1626 amu

Then the amount of energy released = 931 X 0.1626 MeV = 151.4 MeV.

MASS DEFECT:It is defined as the difference between the mass of an atom and sum of the free masses of its nucleons (p & n).For example, the mass defect of 2He4 atom can be calculated as follows:

Total expected mass of He = ( 2 x p ) + ( 2 x n )

= (2 x1.00815) + (2 x 1.00899) = 4.03428 amu.

Measured mass of He = 4.0026 amu

Hence, mass defect of He = 4.03428 – 4.0026 = 0.03168 amu.

The mass defect always represents the amount of disappearance of matter during the formation of a nucleus by packing together the p & n.

BINDING ENERGY(EB):The energy that binds the nucleons (proton & neutron) together in a nucleus is called binding energy. It is expressed in terms of MeV. It is a measure of the force that holds the nucleons together. Consequently, the greater is the binding energy, the more stable is the nucleus.The binding energy (EB)and mass defect ( m) are related as EB = 931 X m

TYPES OF NUCLEAR REACTION:

NUCLEAR REACTION

NUCLEAR FISSION NUCLEAR FUSION

NUCLEAR FISSION:The process in which a heavier nucleus splits up into two or more approximately equal weight nuclei, with simultaneous release of large amount of energy. Example: Fission of 92U235 by neutron (0n1) as follows,

92U235 + 0n1 56Ba140 + 36Kr93 + 3 0n1 + Energy

A large amount of energy is released during nuclear fission, because of a loss in mass, which is converted into equivalent amount of energy in accordance with Einstein’s mass-energy relation, E = m.c2 .A heavier nucleus can undergoes fission in many ways andgives variety of products. For example, the fission of 92U235 occurs around 35 ways.Three of them are given asfollows:

1. 92U235 + 0n1 [92U236] 56Ba140 + 36Kr93 + 3 0n1 + Energy

2. 92U235 + 0n1 [92U235] 54Xe144 + 38Sr90 + 20n1 + Energy

3. 92U235+ 0n1 [92U235] 53Cs144 + 37Kr90 + 20n1 + Energy

NUCLEAR FUSION: The process in which two or more lighter nuclei combine to give a stable heavier nucleus, with simultaneous release of large amount of energy. Example: Fusion reaction takes place in the sun is given as follows:

1H2 + 1H2 2He4 + Energy (23 x 108 kJ/mol)

1H2 + 1H3 2He4 + 0n1 + Energy (17.2 x 108 kJ/mol)

4 1H1 2He4 + 2 +1e0 + Energy (26 x 108 kJ/mol)

As the fusion process involves combination of two positively charged nuclei, which repel each other, it occurs at extremely high temperatures (4x106oC). Because at such very high temperatures nuclei have sufficiently high kinetic energies to overcome the repulsion and they undergo fusion to form a bigger nucleus. Such high temperatures can be attained by using the heat released in a fission reaction. There is a greater loss of mass in a fusion reaction than in a fission reaction, and hence the amount of energy released in a fusion reaction is much more than that of in fission reaction.

S.No / NUCLEAR FISSION / NUCLEAR FUSION
1. / The process in which a heaviernucleus
splits up into two or moreapproximately equal
weight nuclei,with simultaneous release of
largeamount of energy. / The process in which two or morelighter
nuclei combine to giveastable heavier
nucleus, withsimultaneous release of
largeamount of energy.
2. / It is a chain reaction. / It is not a chain reaction.
3. / It is carried out by bombarding theheavy
nuclei with projectile particlelike neutron. / It is carried out by heating the lighter
nuclei to very high temperatures.
4. / It involves emission of radioactiverays. / It doesn’t involve emission of radioactive
rays.
5. / During this process, neutrons areemitted. / During this process, positrons are emitted.
6. / It can be controlled. / It cannot be controlled.
7. / It occurs at an ordinary temperature. / It occurs at very high temperature.
8. / The mass number and atomicnumber of the daughter elements are considerably lower than that of
the parent nucleus. / The mass number and atomic numberof
the product is higher than that of the starting elements.

CHARACTERISTICS:

  1. 92U235 bombarded by slow moving neutrons, it splits into two or more smaller nuclei along with release of two/more neutrons.
  2. Fission products are all radioactive and their atomic masses about 70 – 160.
  3. Fission reaction is a self propagating chain reaction
  4. The chain reaction can be controlled by the materials like Cd, B, Steel or Hf. This process can be done in a device is called nuclear reactor.
  5. The number of neutrons, resulting from a single fission is called multiplication factor. When this is less than 1, a chain reaction does not take place.

Fission of 92U235

CHAIN REACTION:

Propagation of 92U235 fission reaction

When 92U235 nucleus, is bombarded by a thermal / slow neutron, the following fission process occurs with the release of three neutrons.

92U235 + 0n1 56Ba140 + 36Kr93 + 3 0n1 + Energy

Each of these 3 neutrons produced in the above reaction, hits another 92U235 nucleus to produces 9 neutrons. These 9 neutrons further cause fission in nine92U235 nucleus to give 27 neutrons and so on.This process of propagation of the reaction by multiplication in 3’s at each fission is called chain reaction.

NUCLEAR REACTOR / NUCLEAR PILE:It is a device in which the nuclear fission reaction is carried out at a controlled rate, so that the released energy can be utilized for constructive purposes like generation of electricity.It has six main parts, reactor core, reflector, pressure vessel, protective shield, heat exchanger and turbine.

REACTOR CORE:It is an essential part of a nuclear reactor, where controlled fission reaction is made to occur and where heat energy is released. It consists of an assemblage of fuel elements, control rods, coolant and moderator.It has a shape of circular cylinder of diameter 5 to 10 m.

  1. Fuel element: 92U235 is used as a fuel element.
  2. Control rod: The chain reactions are controlled by using movable control rods. These control rods possess high absorption of neutrons.Examples. B and Cd.
  3. Coolant: The main function of coolant is to remove the heat energy produced in the reactor and to bring it out for utilization. Examples. H2O, D2O and liquid Na.
  4. Moderator: The main function of moderator is reducing the kinetic energy of fast neutrons produced in the fission reaction. Graphite, Be & D2O are used as moderators.

Nuclear reactor

REFLECTOR: It is placed around the reactor core to reflect back some of the neutrons that leak out from the core. Usually reflectors made of the same materials as the moderators.

PRESSURE VESSEL:It consists of the reactor core and reflector. It provides the entry and exit for coolant. It can withstand a pressure of upto 200 atm. It has holes at the top in order to insert or pull out the control rods.

PROTECTIVE SHIELD: It is made of concrete and steel, which absorbs neutrons, gamma rays and also other radiations; thereby it protects the human-beings and environment from harmful radiations.

HEAT EXCHANGER: It transfer the heat released from the reactor core to boil water and get steam at about 400 kg/sq.cm.

TURBINE:The steam coming out from the heat exchanger is used to operate the turbine, which drives a generator to produce electricity.

BREEDER REACTOR:The device which converts the non-fissionable material into fissionable material is called breeder reactor.

Consider the fission of 92U235 by slow neutrons,

92U235 + 0n1 56Ba140 + 36Kr93 + 3 0n1 + Energy

Out of three neutrons per fission, 0.5 neutrons escape out & 2.5 neutrons are ejected. Among these 2.5 neutrons, one neutron is used to maintain steady-state chain reaction, while out of the remaining 1.5 neutrons, 0.6 neutron is absorbed by coolant, structural material, moderator, etc. The remaining 0.9 neutrons are captured by 92U238 isotope & convert it into Pu239, which is a fissionable material, which is also known as secondary fuel or man-made nuclear fuel. Thus, the

consumption of primary fuel, 92U238 may be compensated to some extent by the production of secondary fuel Pu239. The extent of compensation is measured by “conversion factor” which is the ratio of no. of secondary fuel atoms produced to that of no. of primary fuel atoms consumed.

There are three cases:

(a)Conversion factor < 1: The quantity of secondary fuel atoms produced is less than that on primary fuel atoms consumed. Thus, the number of primary fuel material will steadily decrease until exhaustion.

(b)Conversion factor = 1: The maximum rate of energy production will depend on the available fuel material and no increase will be possible.

(c)Conversion factor > 1: It is possible to design a reactor in which more fissile material can be produced than that of needed for operation. Such reactors are called breeder reactors and the regeneration process is called breeding.

Breeding of Pu239 is possible in fast reactors where as that of U233 from Th232 is favoured in thermal reactors. The Pu239 is expected to meet out the need of nuclear power due to the more abundance of U238 than that of Th232.

Breeding cycle of conversion of non-fissile U238 into fissionable Pu239.

SOLAR CELLS: A device which converts the solar energy into electrical energy is called solar cell. It can be formed by connecting alarge number of photogalvanic cells in a series pattern. It is also called photovoltaic cell.

Solar cells

PRINCIPLE: Solar cells consist of a p-type semiconductor (Si doped with B) and n-type semiconductor (Si doped with P). They are in close contact with each other so that a limited extent of electrons (from n-type semiconductor) and positive holes (from p-type semiconductor) can cross the junction between the two types of semiconductors.

WORKING:When the solar rays fall on the outer layer of p-type semiconductor, the electrons in the valence band absorbing light energy and promoted to the conduction band. Since the conduction electrons can easily cross the p-n junction into the n-type semiconductor, a potential difference between two layers is created, which cause flow of electrons. When p and n layers are connected to an external circuit, electrons flow from n-layer to p-layer, thus the current is generated.In an actual practice, a large number of solar cells are joined together in a definite pattern to get solar cell panel, which is capable of providing sufficient electric power to lift water from deep well / light in a house / operate radio & television, etc.

Solar panel for producing electricity.

APPLICATIONS:

  • It is used in boilers to produce hot water for domestic and industrial uses.
  • It is used in electric street light and motor pumps.
  • It is used in calculators, TV, electronic watches & radio and as a source of electricity in space craft and satellites.

ADVANTAGES:

  • It can produce about 0.7 W of electricity on exposure to the sun.
  • It is non-polluting and eco-friendly source of electricity.
  • It needs little maintenance only.

DISADVANTAGES:

  • It can produce electricity in the day hours only; it fails during night hours and rainy seasons.
  • It is expensive.

WIND ENERGY:The energy obtained from the movement of air is called wind energy. It can be recovered by using wind mill.It consists of large blades, mounted on the shaft on vertical poles. The shaft is connected to a crank, whose free end is of U-shape, which is connected to the rod of water pump. When wind strikes against the blades, then these start rotating. The rotating blades rotate U-shaped portion of the crank up, and down, thereby the pump works, and lifts water from the ground or flooded mines. A wind mill can also be used to grind cereals.

A wind mill can also used to generate electricity. This is done with the help of wind mills. The crank of the wind mill is connected to a dynamo (generator). When the blades of wind mill rotate, they turn the coil of generator, thereby generating electricity.

Wind mill

The amount of electricity produced by a single wind mill is very small, and hence, it cannot be used for commercial purposes. Consequently, a number of wind mills are erected side-by-side over a large area of land, which is called wind farms. The output energy of each wind mill in a wind energy farm is coupled together to generate electricity on a commercial purposes.

ADVANTAGES:

  • Wind energy available free of cost.
  • It is pollution free and eco-friendly project.
  • It is an inexhaustible source.

DISADVANTAGES:

  • Installation and maintenance of wind mill is expensive.
  • Wind flowing with sufficient speed is not available every where.
  • The speed of wind at a place varies with time and seasons.
  • The wind is not predictable.

FUEL CELL: A device which converts the chemical energy of the fuels directly to electricity is called fuel cell.Example: H2-O2 fuel cell.Generally all combustion reactions are redox reactions. However, when a fuel is burned, the electron exchange occurs between an oxidizing agent (air) and the fuel. Thus, the heat energy is released. But in a fuel cell, electrical energy is obtained without combustion from oxygen and a gas that can be oxidized.

i.e. Fuel + Oxygen Oxidized products + Electricity

H2-O2 FUEL CELL:

It is the simplest and most successful one. It consists of two inert porous electrodes (Graphite impregnated with finely divided Pt) and 25 % KOH solution as an electrolyte.

H2 and O2 gases are bubbled through the anode and cathode compartments respectively.

CELL REACTIONS:

Anode: 2 H2(g) + 4 OH-(aq) 4 H2O(l) + 4 e-

Cathode: O2(g) + 2 H2O(l) + 4 e- 4 OH-(aq)

Net cell reaction: 2 H2(g) + O2(g) 2 H2O(l)

The standard emf (Eo) of the cell is 1.23 V, but in actual practice, it is 0.8 to 1.0 V.Usually, a large number of these cells are combining together in series to give a fuel battery.

CONDITIONS FOR ELECTRODES:

  • It should be good conductor
  • It should be good electron source
  • It should not deteriorated by an electrolyte
  • It should be an excellent catalyst for the reactions take place on their surfaces.

APPLICATIONS:

  • It is used as an auxiliary energy source in space vehicles, sub-marines, military vehicles, etc.
  • It produces fresh water as the product during the process.

BATTERY: Batteries are considered as store houses of electrical energy on demand. An electrochemical power source or battery is a device which converts chemical energy derived from the chemical reactions (redox) into electrical energy. It consists of two or more cells inter connected together in a series pattern. It acts as a portable source of electrical energy.Now a day’s portability of electronic equipments in the form of hand sets has been made possibly by batteries. A variety of electronic goods have been made, more useful and popular with the introduction of rechargeable storage batteries having reliability, better shelf life and tolerance to service.

Batteries are classified into three categories, such as primary battery, secondary battery and flow battery.

PRIMARY BATTERY / PRIMARY CELL: These are the cells in which the electrodes and electrode reactions cannot be reversed by passing an external electrical energy. The cell reactions are irreversible and occur only once. They are not rechargeable. They have to be discarded after the exhaustion of their reactants. Example. Alkaline battery, Dry cell (Leclanche’s cell), Mercury cell and Weston- Cd cell, etc.

SECONDARY BATTERY / SECONDARY CELL: These are the cells in which the electrodes and electrode reactions can be reversed by passing an external electrical energy. The cell reactions are reversible. They are rechargeable. Hence they can be used again and again. Example. Pb acid battery, Ni-Cd battery, Solar cell and Edison cell (Fe-NiO cell), etc.

FLOW BATTERY/ FUEL CELL: A device which converts the chemical energy of the fuels directly to electricity is called fuel cell. Example. H2-O2 fuel cell, Propane-O2 fuel cell, Methanol-O2 fuel cell, etc

Sl.No / PRIMARY CELLS / SECONDARY CELLS
1 / The cell reactions cannot be
reversed by passing an external
electricity. / The cell reactions can be reversed
by passing an external electricity.
2 / The cell reactions are irreversible. / The cell reactions are reversible.
3 / They are not rechargeable. / They are rechargeable.
4 / They have to be discarded after
The exhaustion of their reactants. / They can be recharged and reused
again and again.

ALKALINE BATTERY: It is an improved form of the dry cell. It contains an alkali as electrolyte, it is called alkaline battery. This cell derives its power from the oxidation of Zn at anode and reduction of MnO2 at cathode.

Anode : Zn,

Cathode : Graphite rod surrounded by the paste MnO2