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Conventional and non Conventional sources of energy

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The document contains information on various sources of conventional and non-conventional sources of energy. It also tells how electricity is generated from those sources with proper block diagrams and video animations.

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Conventional and non Conventional sources of energy

  1. 1. Conventional and Non-Conventional Sources of Energy 1. Electricity generation from Conventional Sources of Energy Conventional energy sources are the sources which are abundantly used for power generation. The sources mainly include coal, petroleum, natural gas, water and nuclear sources. This energy sources are exhaustible except water which is renewable source of energy. Electricity generation from some important conventional energy sources are discussed below: 1.1 Thermal Sources (Coal) Thermal energy is the energy released on combustion of fossil fuels (e.g. coal, natural gas). This energy is used to turn water to steam and rotates steam turbines which drives an electric generator. Figure 1.1.1: General Layout of Coal-fired Station Figure 1.1.1 shows layout of a typical coal-fired power station. Continuous source of coal is a prerequisite for thermal power generation. Coal is fed to the boiler through coal handling plant where impurities are removed and pure coal is pulverised to fine powder. This is done to increase efficiency of boiler as pulverised coal undergoes complete combustion. The mixture of pulverised coal and preheated air is burnt in combustion zone in boiler. [1] The ash produced as a result of combustion is removed to ash storage through coal handling plant. The heat energy released on combustion is utilised to convert water into steam at high temperature and pressure. Steel tubes run along the boiler walls in which water is converted to steam (figure 1.1.2). The flue gases exchange their heat in superheater, economiser and air preheater as they get exhausted to the atmosphere. High pressure superheated steam (through superheater) is expanded in series of turbines namely high pressure, intermediate pressure and low pressure turbines coupled to an alternator. As a result alternator rotates and electrical energy is generated which is stepped up with the help of a transformer for transmission. The expanded low pressure steam is exhausted in condenser to be condensed to water and reused.
  2. 2. Figure 1.1.2: Schematic of Coal-Fired Thermal Power Plant 1.2 Nuclear Sources Nuclear energy is a result of nuclear reaction viz. fusion and fission reactions. The nuclear fission reaction involves breaking down heavy nucleus to stable lighter nuclei while fusion reaction combines lighter nuclei to heavy stable nucleus towards Iron (Fe) (most stable element). The figure 1.2.1 shows the stability of various nuclei. Both type of nuclear reactions are accompanied by mass deficit and is directly proportional to the amount of energy released. [1] Figure 1.2.1: Binding Energy per Nucleon vs Mass Number Curve
  3. 3. Figure 1.2.2: Schematic Arrangement of Nuclear Power Plant [2] Nuclear power plant uses the same technology as that of conventional steam power generation with nuclear reactor for heat generation instead of coal furnace and boiler. [2] Figure 1.2.2 shows the schematic arrangement of nuclear power plant. Most nuclear power stations use Uranium as fuel. When bombarded with neutrons, Uranium undergoes fission reaction in 35 ways one of them is as follows: Each of the neutrons produced in the reaction strikes with another Uranium nucleus, thus causing nine subsequent reactions and so on to form a chain reaction as shown in figure 1.2.3. The tremendous amount of heat energy
  4. 4. Figure 1.2.3: Chain Reaction of Uranium-235 produced in fission of Uranium or other heavy elements in nuclear reactor is extracted by pumping fluid or molten metal like liquid sodium or gas through the pile. The heated metal or gas is then allowed to exchange its heat to the heat exchanger by circulation. [2] Atomic power plants usually employ a double folded heat removal scheme shown in figure 1.2.4. First, heat from reactor(R) is transferred from the fission reaction by the coolant. In next stage, in heat exchanger (H), coolant transfers its heat to water. This produces steam in heat exchanger. Steam passes through turbine (T) connected mechanically with the generator. The turbine rotates due to expansion of high pressure steam and electrical power is generated from generator coupled to the turbine. After losing its energy in the turbine, the steam is converted into water in the condenser (C) which is again delivered to the heat exchanger. [1] Figure 1.2.4: Heat circuit in an atomic power plant [1] This video explains functions of different parts of nuclear power plant. 1.3 Hydel Sources The energy of water utilised for hydro-power generation can be kinetic or potential energy. Kinetic energy of water is its energy in motion and is a function of mass and velocity while potential energy is a function of difference in level of water between two points called the head. The hydro energy that can be generated is dependent on the quantity of water available, the rate at which it is available and the head available. [1] The power generated is directly proportional to the head available as greater head implies greater pressure to drive turbines. [1]
  5. 5. Figure 1.3.1: General Layout of Hydroelectric Plant The essential requirement for hydro-electric power generation is the availability of continuous source of water. Figure 1.3.1 shows the general layout of hydro-electric plant with an artificial storage reservoir. Artificial reservoir like dam is built where natural lake or reservoir is not available. Dam ensures that water is available from wet season to the next dry season. [1] Water from the storage reservoir is carried through penstocks or canals to the powerhouse. Water after passing through the turbine is discharged to the stream. [1] [2] The powerhouse is located at an optimal position from reservoir considering the head loss due to friction in penstock. The powerhouse contains prime movers, generators and transformers. The prime movers convert kinetic energy of water into mechanical energy. The alternator or generator coupled to turbine converts mechanical energy to electrical energy. The prime movers can be reaction or impulse turbines classified on the basis of forces used. [2] Figure 1.3.2 shows different types of turbines employed for power generation. Reaction turbines make use of lift forces while impulse turbines use drag forces. The reaction turbines are employed when head available is low and high flow rate while it is converse for the impulse turbines. Figure 1.3.2: Different types of Hydraulic turbines This video explains how hydro energy is employed for power generation. 2. Electricity generation from Non-Conventional Sources of Energy Non-conventional energy is considered the energy of the future. Solar energy, tidal energy, geo-thermal energy, wind energy, biomass energy are some the non-conventional energy sources and can be used effectively to generate electricity. This sources can be either reused or abundantly available in nature. Hence this are often termed as renewable sources of energy. Power exploitation from some of these sources is explained below: 2.1 Solar Energy Solar energy is regarded as an in-exhaustible (hence renewable) source of energy as the sun is expected to radiate at constant rate for a few billion years. [3] Solar energy reaches the top of the atmosphere at the rate of 1.353 kW/m2 . Part of this energy is reflected back to space and part is absorbed by atmosphere. In full sunlight, the solar energy reach the ground at the rate of roughly 1 kW/m2 . [3] More energy form sun strikes the earth in one hour than all the energy consumed on the planet in a year. [4] Solar energy, received in form of radiation, can be converted directly or indirectly into other forms of energy such as heat and electricity.
  6. 6. Solar energy can be converted into electricity in two ways: (1) Concentrated Solar Thermal Energy (CSTE) systems and (2) Photovoltaic systems. CSTE systems consist of reflecting surfaces which concentrate solar radiations onto boilers to produce high pressure steam required for power generation. Figure 2.1.1 shows schematic of solar thermal electrical power plant. Solar thermal collectors used for concentration can be either trough (or parabolic) concentrators or tower systems or dish systems (figure 2.1.2) differentiated by the temperatures that can be achieved on concentration. [4] The heat is transferred via heat transfer fluids (oil, molten salt) to produce steam in heat exchanger which then drives turbines coupled to electric generator. Photovoltaics or solar cells are devices which convert incident solar radiation to electrical energy.Solar cells in form of thin cells are semiconductor devices that convert 3% to less than 30% of incident solar energy into DC electricity with Figure 2.1.1: Solar Thermal Electrical Power Plant Figure 2.1.2: Different types of Solar Thermal Collectors
  7. 7. efficiencies depending on illumination intensity, solar cell design and temperature. [4] Electricity generation is a result of photovoltaic effect shown in figure 2.1.3. Since a solar cell gives low voltage hence such cells are connected in Figure 2.1.3: Photoelectric Effect parallel-series configurations to permit high currents and voltages of the order of kilovolts, known as a solar module. The electrical power output of photovoltaic cell is roughly proportional to the rate at which solar radiation falls on its surface. Hence insolation is increased by tracking compound parabolic (non-focussing) concentrators. The basic photovoltaic system integrated with the utility grid is shown in figure 2.1.4. [3] Solar array converts solar insolation to DC electrical power which is then converted to AC signal in DC, AC inverter to integrate with the utility grid. [3] Battery storage enables electrical energy to be stored from utility feeder through rectifier or directly from solar array to be used during no insolation times. Blocking diodes ensures the battery does not discharge to array during times of no insolation. [3] Figure 2.1.4: Basic Photovoltaic System integrated with power grid [3]
  8. 8. 2.2 Wind Power Wind energy is harnessed via windmills which convert kinetic energy of wind to mechanical energy. [3] A generator further converts it to electrical energy or is used to run the machine such as for water pumping, mill grain etc. Wind energy conversion systems uses two types of turbines classified as Horizontal Axis Wind Turbines (HAWT) and Vertical Axis Wind Turbines (VAWT) on the basis of axis of rotation relative to wind stream. (Figure 2.2.1) Figure 2.2.1: Wind Turbine Types, VAWT- (a),(b),(c) and (e) HAWT- (d) HAWT uses vane or wind sensors to point in direction of the wind. [3] HAWT requires gearbox shown in figure 2.2.2 which turns slower rotation of blades to faster rotation suitable to drive an electric generator. VAWT does not need to be pointed in the wind direction to be effective as it is independent of wind direction. [3] [4] HAWT and VAWT rotors are either lift or drag devices. Lift devices have higher rotational speeds and more output power can be developed than by drag devices. The ratio of power extracted by a lift device to that of drag device is usually greater than 3:1 for the same swept area. Figure 2.2.2: Wind Turbine showing gearbox
  9. 9. Figure 2.2.3: Basic Components of Wind Electric System The figure 2.2.3 shows the basic components of wind-electric conversion system. Wind turbines convert wind energy into rotary mechanical energy. A mechanical interface, consisting of a step-up gear and a suitable coupling transmits the energy to an electrical generator. The controller senses the wind direction, wind speed, power output of the generator and necessary performance quantities of the system and initiates appropriate control signals to take corrective actions. The system also takes care of electrical faults, extra wind energy and protects from excessive temperature rise of the generator. [3] Video shows applications of wind energy, working principle of wind power plant and different parts of wind turbine. 2.3 Geothermal Energy Geothermal energy is an inexhaustible source of energy present as heat in the earth’s crust. [3] Geothermal energy of the Earth’s crust originates from the original formation of the planet (20%) and from radioactive decay of minerals (80%). The geothermal gradient, the difference in temperature between the core of the planet and surface, drives a continuous conduction of thermal energy in form of heat from the core to the surface. Figure 2.3.1 shows a typical geothermal field. The hot magma (molten mass) near the surface (A) solidifies into igneous rock (B). Groundwater coming through fissures gets heated up by heat transfer from the rock or hot gases and steam emanating from the magma [3] [4] and gets collected in permeable reservoir (C). The hot water then escapes through fissures (E) in solid impermeable rock (D) in form of fumarols (F) or hot springs (G). [3] [4]
  10. 10. Figure 2.3.1: A Typical Geothermal Field The hot water escaped through fissures is found at surface in following forms: 1) Dry steam at about 200 °C and 8 bar pressure [3] emanating from vapour dominated reservoir. It is passed to low pressure turbine via centrifugal separator to generate electricity as shown in the figure 2.3.2. Figure 3: Schematic of Vapour-Dominated Power Plant 2) Wet steam, mixture of hot water and steam which is formed due to flashing of liquid water as it rises to the low pressure surface (ground) in liquid dominated reservoir with water temperature above 100 °C. Hence it is passed through flash separator to separate steam and then passed to turbine to generate electricity as shown in figure 2.3.3.
  11. 11. Geothermal resources is also found in form of Hot Dry Rock (HDR) system but no underground water shown in figure 2.3.4. In such cases, a cavity is created by detonating an explosive at the bottom of a well drilled into the rock. Water is circulated through the cavity to extract heat from the rock. Water-steam mixture thus obtained is utilised for generation of electricity with binary liquid system using Freon as turbine working fluid. [3] Figure 2.3.4: Heat Extraction from Hot Dry Rocks Geothermal resources having temperatures low enough to produce steam are used for heating applications in industries and households. The video shows different forms through which geothermal energy rises to earth’s surface in Iceland and gives an idea to generate power utilising geothermal energy. Figure 2.3.3: Schematic of Liquid-Dominated Power Plant
  12. 12. The video explains the occurrence of geothermal energy, animates different ways to utilise it for power generation and benefits of geothermal energy and its waste products. 2.4 Biomass energy Biomass is an organic matter produced by plants, both terrestrial and aquatic, and their derivatives. It includes forest crops and residues, crops grown for their energy content on ‘energy farms’ and animal manure. [3] Biomass is indirectly obtained from solar energy through photosynthesis by plants. One of the advantages of biomass fuel is that it is often a by-product, residue or waste-product of other processes such as farming, animal husbandry and forestry. Video gives basic idea on power generation from different biomass sources. Biomass resources can be found in any of the three forms [3] : (1) Solid mass in form of wood and agriculture residue (2) Liquid fuels by converting biomass to methanol and ethanol (3) Biogas obtained on Bio digestion Table 2.4.1: Biomass Conversion Technologies [3] Conversion Process Principle Products Further Treatment Premium FuelsSolids Liquids gases Wet Anaerobic Digestion Methane b and Carbondioxide Carbondioxide Removal Methane Fermentation Ethanol Distillation Ethanol Chemical Reduction Mixture of oils Fractional distillation Hydrocarbon liquids Thermal processes Liquefaction Char Pyroligneou s acid oils and tars Fuel gas b Steam reforming and/or shift reaction Methane Methanol or Higher AlcoholsGasification Char Fuel gas a,c Steam-gasification Char Methanec Hydrogenation Mixture of oils Fractional distillation Hydrocarbon liquids Oil Extraction Vegetable oil Esterification Diesel substitute Key: a = low calorific value (5-10 MJ/m3 ) b = medium calorific value (10-25 MJ/m3 ) c = low calorific value (30-45 MJ/m3 ) The table 2.4.1 shows various conversion technologies used to obtain fuel from biomass. Biogas can be produced by digestion, pyrolysis or hydrogasification and is a mixture containing 55-65% Methane, 30-40% Carbon dioxide and rest being the impurities (H2, H2S and N2). Biogas is a slow burning gas having calorific value between 5000 to 5500 kcal/kg. [3]
  13. 13. Figure 2.4.1: Fixed Dome type Biogas Plant The figure 2.4.1 shows fixed dome biogas digester. Slurry is subjected to anaerobic digestion/fermentation using different types of microorganisms (bacteria, fungi, virus etc.).[3] The gas is collected at the top and can be used for cooking, water heating and industries. Fuels obtained on biomass conversion technologies having good enough calorific value can be burnt to produce steam and run the turbines to generate electricity. 2.5 Tidal Energy Tides are generated by action of gravitational forces of the sun and moon on the water of the earth. [3] [4] The tides are periodic vertical rise and fall of water. The tidal rise and fall of water is accompanied by periodic horizontal to and fro motion of water called tidal currents. [4] The amplitude of tides covers a wide range from 25 cm to 10 m because of the changing positions of the moon and sun relative to earth. The speed of tidal currents is in the range of 1.8 km/h to 18 km/h. [4] Figure 2.5.1: Principle of Tidal Power Generation Figure 2.5.1 shows the principle of tidal power generation. The principle is to utilise differential head during high and low tides in operating a hydraulic turbine. The idea is to hold water in a basin during high tide and then let water back to sea through a turbine, thus producing power. [3] The power can also be generated by utilising both high and low tides by double cycle system shown in figure 2.5.2.
  14. 14. Figure 2.5.2: Double cycle system Tidal power extraction involves construction of a long barrier across a bay or an estuary to create a large basin on the landward side. The barrier includes dykes, gate-controlled sluices and the power house. [4] The different tidal power schemes employed for power generation are Single Basin Tidal Power Scheme and Linked Basin Tidal Power Scheme (figures 2.5.3(a) and 2.5.3(b)). In former type, the power is generated from both tides using double cycle system but involves only one basin. In the latter type, there are two basins on the landward side with the power house located in barrier between the two basins. Power is generated by water flowing from high basin to low basin during high tide when sluice gates of low basin are closed so that sufficient head is available. The water is then let to sea during the low tide. [3] [4] The typical hydraulic turbine employed for tidal power generation is shown in figure 2.5.4. Figure 2.5.3: Different Tidal Power Schemes Figure 2.5.4: Layout of Tidal Power Plant
  15. 15. References 1. Abhijit Chakrabarti, M. L. Soni, P.V. Gupta and U. S. Bhatnagar, A Textbook on Power System Engineering (Second Edition), DhanpatRai and Sons, New Delhi, 2010 2. JB Gupta, A Course in Electrical Power (Fifteenth Edition), S. K. Kataria and Sons, New Delhi, 2013 3. G.D. Rai, Non-Conventional Energy Sources (Fifth Edition), Khanna Publishers, New Delhi, 2007 4. Tasneem Abbasi and S.A. Abbasi, Renewable Energy Sources Their Impact on Global Warming and Pollution, Prentice Hall India Learning Private Limited, 2010