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UNIT V ENERGY SOURCES AND STORAGE DEVICES SYLLABUS - controlled nuclear fission - - differences between nuclear fission and fusion - nuclear chain reactions - nuclear energy - light water plant - - solar energy conversion - solar cells - wind energy. Batteries, fuel cells and supercapacitors: Types of batteries – primary battery (dry cell) secondary battery (lead acid battery,

lithium-ion-battery) fuel cells – H2-O2 fuel cell. Nuclear fission: 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. Example:

92U235+ 0n1 → [92U236] → 56Ba141 + 36Kr92 + 30n1 + 200.5 MeV (Energy) Mechanism of nuclear fission: When U-235 is bombarded by slow moving , unstable U-236 is formed. This nucleus disintegrates into two equal nuclei with the release of huge amount of energy and few .

Characteristics of nuclear fission reaction: (Part-B) 1. Heavy nucleus splits into two or more nuclei. 2. Two or more neutrons are produced by fission of each nucleus. 3. Large quantity of energy is produced during the nuclear fission reaction.

4. All the fission fragments are radioactive in nature, giving off gamma 5. The atomic weights of ranges from 70 to 160. 6. All the fission reactions are self-propagating chain reaction because one of the fission products is neutron. 7. The nuclear reactions can be controlled by absorbing the neutrons using Cd, Boron. 8. Every secondary neutron released in the fission reaction does not strike the nucleus. Some neutrons escape into air. Hence a chain reaction cannot be maintained. 9. The number of neutrons resulting from a single fission is known as multiplication factor. When it is less than 1, nuclear chain reaction does not take place. Nuclear fusion: The process of combination of lighter nuclei to form heavier nuclei, with simultaneous liberation of huge amount of energy is called as nuclear energy. Example: Fusion reaction in sun (Thermo nuclear reactions) 1 4 0 41H 2He + 2+1e + Energy

Characteristics of nuclear fusion: 1. There is no limit on the amount of nuclear fusion that can occur. 2. It is possible only when the distance between the nuclei is of the order of one Fermi. 3. The amount of energy in fusion is four times more than that of fission. 4. Sufficient amount of must be provided to facilitate the fusion reaction. 5. Only lighter nuclei can undergo nuclear fusion reaction.

Differences between nuclear fission and nuclear fusion: (Part-B)

S.No. Nuclear fission Nuclear fusion It is the breaking of heavier It is the combination of lighter 1 nucleus. nuclei. 2 It emits radioactive rays. It does not emit radioactive rays. It takes place at very High 3 It occurs at ordinary temperature. temperature (>106K). The and atomic The mass number and atomic 4 number of the fission product is number of the fusion product is less than the heavier nucleus. higher than the starting elements. It does not give rise to Chain 5 It gives rise to chain reaction. reaction. 6 Neutrons are emitted. Positrons are emitted. 7 It can be controlled. It cannot be controlled. Example: Example: 8 92U235+ 0n1 → [92U236] → 41H1 2He4 + 2+1e0 + Energy 56Ba141 + 36Kr92 + 30n1 + Energy

Nuclear chain reaction: A fission reaction, where the neutrons from the previous step continue to propagate and repeat the reaction is called nuclear chain reaction.

Reason for less energy: Some neutrons, released in the fission of U235 may escape into the air and some absorbed by U239 present as impurity. This will result in breaking of the chain and the amount of energy released will be less than expected. How to improve the amount of energy? 1. For a nuclear chain reaction to continue, sufficient amount of U235 must be present to capture the neutrons. Critical mass 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 100 Kg. a) Super critical mass If the mass of the fissionable material is more than the critical mass, it is called super critical mass. b) Sub critical mass. If the mass of the fissionable material is less than the critical mass, it is called sub critical mass. 2. The super and sub-critical mass may hinder the propagation of the chain reaction. Nuclear energy: The energy released by the nuclear fission is called nuclear fission energy or nuclear energy. Cause of the release of energy: The sum of the masses of the products is slightly than the sum of the masses of the reactants. The loss in mass gets converted into energy according to the Einstein equation, E= mc2 Where m- loss in mass, c- velocity, E- energy Hazards of using nuclear energy: 1. The nuclear can damage the structure of cells in the human body.

2. It causes diseases like cancer and blindness. 3. It causes genetic disorder in a human body. 4. It causes sterility in young generation. Types of nuclear fission reaction 1. Uncontrolled nuclear fission reaction. Example- bomb 2. Controlled nuclear fission reaction. Example- . LIGHT WATER NUCLEAR POWER PLANT (Part-B) The arrangement or equipment used to carry out fission reaction under controlled conditions is called . 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. Control rods 3. Moderators 4. 5. Pressure vessel 6. Protective shield 7. Heat exchanger 8. Turbine 1. Fuel rods: The fissionable material used in the nuclear reactor is enriched U-235. It is used in the form of rods or strips.

Example: U235, Pu239 Function: It produces fuel and neutrons. This neutron starts nuclear chain reaction. 2. Control rods To control the rate of fission of U-235, movable rods made of cadmium (Cd) or boron (B) are suspended between fuel rods.

These rods absorb the excess neutrons. So the fission reaction proceeds at steady rate. 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 becomes very slow. If the rods are pushed outwards, they will absorb less neutrons and the reaction will be very fast. 113퐶푑 + 1푛 → 114퐶푑 + 훾 43 0 43 1 10 푛 → 11퐵 + 훾 5 퐵 + 0 5 Example: Cadmium, Boron Function: It controls the nuclear chain reaction and avoids the damage to the reactor. 3. Moderators The substances used to slow down the neutrons are called moderators. Example: Ordinary water, Heavy water, graphite, beryllium. Function: The kinetic energy of fast neutron (1meV) is reduced to slow neutrons (0.25 eV).

Fast neutron Slow

4. Coolant In order to absorb the heat produced during fission reaction, the coolant 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. Example: Water (act as coolant and moderator), Heavy water, liquid metal (Na or K) Function: It cools the fuel core. 5. Pressure vessel It encloses the core and also provides the entrance and exit passages for coolant. Function: It withstands the pressure as high as 200 atm.

6. Protective shield The moderator, control rods and fuel element are enclosed in a chamber which has a thick concrete shield (10m thick). Function: The environment and the operating persons are protected from destruction in case of leakage of radiation. 7. Heat exchanger It transfers the heat liberated from the reactor core to boil water and produce steam at about 400Kg/cm2. 8. Turbine The steam generated in the heat exchanger is used to operate a steam turbine, which drives a generator to produce electricity. Working of light water nuclear reactor: It is the one in which U-235 fuel rods are submerged in water. Here water acts as coolant and moderator.

Light water nuclear power plant

The fission reaction is controlled by inserting or removing control rods of B10 automatically from the space 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 (U238, Th232) into fissionable material (U235, Pu239). Thus the reactor produces or breeds more fissionable material than it consumes. Illustration: 푼ퟐퟑퟖ + 풏ퟏ → 푷풖ퟐퟑퟗ + ퟐ풆− ퟗퟐ ퟎ ퟗퟒ 푷풖ퟐퟑퟗ + 풏ퟏ → 푭풊풔풔풊풐풏 풑풓풐풅풖풄풕풔 + ퟑ풏ퟏ ퟗퟒ ퟎ ퟎ In breeder reactor, three neutrons are emitted during the fission of U239. Among the three neutrons, only one neutron is used for propagation step. The other two neutrons are allowed to react with U238. Thus two fissionable of Pu239 are produced from one fissionable U235. Therefore breeder reactor produces more fissionable material than it uses. Pu239 is known as secondary fuel or man-made fuel.

Significance: 1. The non-fissionable materials such as U238 and Th232 are called as fertile materials.

2. The fissionable materials such as U235 and Pu239 are called as fissile materials. 3. Efficiency of breeder reactor is more since regeneration of fissile materials takes place. Solar energy (Part-B) Solar energy conversion: It is the process of conversion of direct sunlight into more useful forms. It occurs by the following two mechanisms. (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. Uses: It 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.

Solar water heater

Photo conversion: It involves conversion of light energy directly in to electrical energy. Example: Photo galvanic cell or solar cell Photo galvanic cell or solar 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 layers is produced. This potential difference causes flow of electrons and produces electricity. Construction: Solar cell consists 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.

Working: When solar rays fall on n-type semiconductor the electrons from the valence band get promoted to the conduction band and cross p-n junction into p type semiconductor. Thereby potential difference is produced which causes flow of electrons and hence current is generated. 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.

2. Solar pumps can be run by solar battery.

Solar pump run by solar cells 3. Used in calculators, electronic watches, radios and TV. 4. Used to drive vehicles. 5. Used in space craft and . Advantages: 1. Solar cells can be used in remote areas and hilly regions. 2. Maintenance cost is low. 3. Solar cells are non-polluting and eco-friendly. 4. Their life time is long. 5. They need not be charged.

Disadvantages: 1. Capital cost is high. 2. Storage of solar energy is not possible. 3. It produces only DC current. 4. Solar energy is not available in night time. Wind energy (Part-B)  Moving air is called wind.  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 makes it rotating continuously. The rotational motion of the blade drives a number of machines like water pump, flour mills and electric generators.

Wind Energy

Nowadays windmill uses large sized propeller blades and connected to a generator through a shaft. Wind mills are capable of generating about100 kW electricity. Wind farms: When a large number of wind mills are installed and joined together in a definite pattern it forms a wind farm. The wind farm produces a large amount of electricity.

Condition: The minimum speed required for satisfactory working of a wind generator is 15Km/hr. Advantages: (i) It does not cause any . (ii) It is very cheap. (iii) It is renewable. Disadvantages: 1. Public resists for locating the wind forms in populated areas due to noise generated by the machines. 2. Wind forms located on the migratory routes of birds will cause hazards. 3. Wind mill interferes with electromagnetic signals. Batteries Definition A battery is an arrangement of several electrochemical cells connected in series that can be used as a source of direct electric current. Cell: It contains only one anode and one cathode. Battery: It contains several anodes and cathodes. Requirements of a battery: 1. It should be light and compact for easy . 2. It should have long life both when it is being used and when it is not used. 3. The voltage of the battery should not vary appreciably during its use. Types of battery: 1. Primary battery or Primary cell or Non – reversible battery 2. Secondary battery or Secondary cell or reversible battery 3. Fuel cell or Flow battery 1. Primary battery: It is the device in which the cell reaction is non-reversible and it cannot be recharged. Example: dry cell, cell

2. Secondary battery or storage cells. 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. Example: Hydrogen-oxygen cell, methanol fuel cell. Dry cell or Leclanche’s cell (Part-B) It is a primary cell which works without fluid component. Description:

Leclanche’s Cell

 A dry cell consists of cylinder which acts as anode.  This zinc cylinder is filled with ammonium chloride, zinc chloride and

MnO2 in the form of paste using starch and water.  A graphite (carbon) rod acts as cathode.  Cathode is immersed in the centre of the cell.  The zinc cylinder has an outer insulation of cardboard case.

Working:  When the cell is working, zinc loses electrons and Zn2+ ion gets dissolved in the electrolyte.  The electrons pass through the circuit and are consumed at the cathode.

 This causes discharge of NH4+ ions from the electrolyte.  In the cathode reaction Manganese is reduced from +4 oxidation state to +3 oxidation state.  The liberation of ammonia gas disrupts the current flow.

 This is prevented by the presence of ZnCl2

풁풏푪풍ퟐ + ퟐ푵푯ퟑ → [풁풏(푵푯ퟑ)ퟐ] 푪풍ퟐ (풔) 1. Anode – Zinc powder 2. Cathode – Carbon rod

3. Electrolyte – NH4Cl, ZnCl2 and MnO2

4. Cell representation- Zn(s) /NH4Cl/MnO2(s) 5. Anode reaction (oxidation) (loss of electron): 풁풏 → 풁풏ퟐ+ + ퟐ풆− 6. Cathode reaction (reduction) (gain of electron):

+ − − 푵푯ퟒ(풂풒 ) + 푴풏푶ퟐ(풔) + ퟐ풆 → 푴풏푶(푶푯) + 푵푯ퟑ

7. The net reaction is

+ ퟐ+ − 풁풏 + 푵푯ퟒ(풂 풒) + 푴풏푶ퟐ(풔) → 풁풏 + 푴풏푶(푶푯) + 푵푯ퟑ 8. The cell develops an e.m.f of 1.5 volt. 9. Disadvantages:

 Its lifetime is less. Since NH4Cl corrodes the zinc container easily.  Voltage drop occurs. 10. Uses: It is used in radios, tape recorders, torches and electronic photographic flash units. Lead acid battery or Lead storage accumulator (Part-B)  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. Description:  It consists of a 3-to 6 number of voltaic cells connected in series.

 In each cell, lead acts as a node and PbO2 acts as cathode.  Anode and cathodes are separated by insulators like rubber.  The entire arrangement is immersed in dilute sulphuric acid of 38% by mass with density 1.30 g/ml

Lead storage cell

1. Anode – Lead

2. Cathode – PbO2

3. Electrolyte - dil. H2SO4 (density- 1.38 g/ml) 4. Insulator- rubber or glass fiber.

5. Cell representation- Pb /PbSO4 // H2SO4(aq) // PbSO4/ PbO2 6. Anode reaction (oxidation) (loss of electron): − 0 푷풃(풔) + 푺푶ퟐ− → 푷풃푺푶ퟒ + ퟐ풆 E anode=-0.36V ퟒ (풔) 7. Cathode reaction (reduction) (gain of electron): 0 푷풃푶ퟐ + ퟒ푯+ + 푺푶ퟐ− + ퟐ풆− → 푷풃푺푶ퟒ + 푯ퟐ푶 E cathode=+1.69V (풔) ퟒ (풔) 8. The net reaction: 0 푷풃(풔) + 푷풃푶ퟐ + ퟒ푯+ + ퟐ푺푶ퟐ− → ퟐ푷풃푺푶ퟒ + 푯ퟐ푶 E cell=+2.05V (풔) ퟒ (풔)

E0cell = E0cathode − E0 anode

E0cell = 1.69 − (−0.36) = 2.05V

9. The cell develops an e.m.f of 2 volts. 10. Uses: 1. It is used to supply current mainly in automobiles such as cars, Buses, trucks etc. 2. It is also used in gas engine ignition, telephone exchanges, hospitals, power stations. 11. Advantages: 1. It is made easily. 2. It produces high current. 3. Self-discharging rate is low. 4. It acts effectively at low temperature. 12. Disadvantages: 1. Recycling of the battery causes environmental hazards. 2. Mechanical strain reduces the battery capacity. Lithium ion batteries (LIB) (Part –B) Lithium ion battery is a secondary battery. It has three components 1. Cathode – positive electrode – Layers of lithium cobalt oxide 2. Anode – negative electrode – Layers of porous carbon (graphite) 3. Electrolyte – Polymer gel Construction: Both anode and cathode are dipped in a polymer gel and are separated by perforated plastic separator. Working: During charging Li+ ion flows from the positive electrode (cathode) to negative electrode (anode) through the polymer electrolyte. Electrons also flow from the positive electrode to negative electrode through the wire. The electrons and Li+ ions combine at the negative electrode and deposit there as Li.

Charging reaction: 푳풊푪풐푶ퟐ + 푪 → 푳풊ퟏ−풙푪풐푶ퟐ + 푪푳풊풙

Discharging reaction:

푳풊ퟏ−풙푪풐푶ퟐ + 푪푳풊풙 → 푳풊푪풐푶ퟐ + 푪

During discharging reaction, reverse reaction occurs. That is Li+ ions flow from negative electrode to positive electrode through electrolyte. Similarly, electrons also flow from negative electrode to positive electrode through wire. Thus Li deposits at positive electrode. Uses: It is used in cell phone, note PC, portable LCD TV, power tools and electric vehicles. Lithium battery is the cell of future, why? 1. Its cell voltage is high, 3.0V 2. Since Li is a light weight metal, only 7g (1mole) material is required to produce 1 mole of electrons 3. Since it has the most negative E0 value, it generates a higher voltage than the other types of cells. 4. Since all the constituents of the battery are solids, there is no risk of leakage from the battery.

5. This battery can be made in variety of sizes and shapes. Fuel cells (Part –B) Fuel cell is a voltaic cell, which converts the chemical energy of the fuels directly into electricity without combustion. 푭풖풆풍 + 푶풙풚품풆풏 → 푶풙풊풅풂풕풊풐풏 풑풓풐풅풖풄풕풔 + 푬풍풆풄풕풓풊풄풊풕풚 Example: 1. Hydrogen – Oxygen fuel cell 2. Methyl alcohol –Oxygen fuel cell Description:  In a fuel cell, the electricity can be generated as long as the fuel and oxygen are supplied into the cell.  It consists of anode, cathode and electrolyte.  In a fuel cell, fuel is sent through the anode and the oxygen is supplied through the cathode.  The electrolyte carries the charged particles from anode to cathode and vice versa. Hydrogen – Oxygen fuel cell

1. Anode- Hydrogen gas 2. Cathode- Oxygen gas 3. Electrolyte- 25-40% KOH 4. Electrode- Two porous carbon electrode impregnated with a finely platinum or nickel as catalyst.

5. Cell representation: H2, C, Pt or Ni /KOH/ C, Pt or Ni, O2 6. Anode reaction:

− − 푯ퟐ + ퟐ푶푯 → ퟐ푯ퟐ푶 + ퟐ풆

7. Cathode reaction: ퟏ 푶 + 푯 푶 + ퟐ풆− → ퟐ푶푯− ퟐ ퟐ ퟐ 8. Net reaction: ퟏ 푯 + 푶 → 푯 푶 ퟐ ퟐ ퟐ ퟐ 9. The cell develops the emf of 1.23V. 10. The efficiency of hydrogen – oxygen fuel cell is 70 %. 11. The operating temperature is 60-70 °C. Advantages: 1. It is highly reliable. 2. It does not cause any pollution. 3. It produces portable water. 4. It is used in space vehicles, submarines. Disadvantages: 1. Hydrogen gas is expensive. 2. As hydrogen is a gas, it is difficult to compress in liquid form. 3. High pressure is needed when it is used in automobiles. Applications: 1. It is used in Apollo spacecraft to produce electricity and water. 2. It is used in military and other commercial vehicles of all types. Super capacitor (Electro chemical capacitor) (Part –B)  It is high capacity capacitor.  They store 10 to 100 times more energy per unit volume and deliver charge much faster than batteries.  Super capacitor use electrostatic double layer capacitance not conventional solid dielectric.

Design of the super capacitor

 It consists of two electrodes.  The electrodes are made of metal coated porous substance like powered activated carbon.  The two electrodes are separated by an ion permeable membrane and dipped in the electrolyte.  The electrolyte contains positive ions and negative ions and connects the electrodes.

Working:

 When the electrodes are connected to the power source, ions in the electrolyte forms electrical double layer.  This electrode/electrolyte interface creates an electric field between them. Thus electrical energy stores at an electrode/electrolyte interface.  Example: Positive electrode has a layer of negative ions at the electrode/electrolyte interface and vice versa.

Advantages:

1. It is highly safe. 2. Its life time is high.(10 to 20 years) 3. It can be charged in seconds. 4. It provides high power density and high load currents. 5. Its performance is good at low temperature. Disadvantages: 1. Cost is high. 2. It cannot be used as source for continuous power supply. 3. If higher voltage is required, the cells must be connected in series. 4. Self-discharge is high. Applications: 1. Voltage stabilization in start /stop system 2. Energy harvesting 3. Kitchen appliances 4. Consumer electronics 5. Wind energy 6. Utility meters 7. Remote power sensors, LEDs, switches