Renewable Energy Sources, Exercises of Environmental Science

Download Renewable Energy Sources and more Exercises Environmental Science in PDF only on Docsity! RENEWABLE ENERGY SOURCES by Dr. Banamali Dalai Second Semester, Heat Power & Thermal Engineering. Subject Code: P2HTCC12 Module: I, II & III DEPARTMENT OF MECHANICAL ENGINEERING CAPGS, Biju Patnaik University of Technology, Odisha Chhend, Rourkela, Pin-769015 1 2 M. Tech (Mechanical Engineering) Syllabus for Admission Batch 2016-18 2ndSemester Page13 ………………………………………………………………………………………………………………….. Renewable Energy Sources Module I Energy scenario and renewable energy sources: global and Indian situation. Potential of nonconventional energy sources, economics. Solar Radiation: Solar thermal process, heat transfer devices, solar radiation measurement, estimation of average solar radiation. Solar energy storage: stratified storage, well mixed storage, comparison. Module II Hot water system, Practical consideration, solar ponds, Non-convective solar pond, extraction of thermal energy and application of solar ponds. Wind energy: The nature of wind. Wind energy resources and modeling. Geothermal energy: Origin and types of geothermal energy and utilization. Module III OTEC: Ocean temperature differences. OTEC systems. Recent OTEC developments. Wave energy: Fundamentals. Availability Wave-energy conversion systems. Tidal energy: Fundamentals. Availability Tidal-energy conversion systems; Energy from biomass: Photosynthesis; Biomass resource; Utilization of biomass. Books S. P. Sukhatme, Solar Energy Principle of Thermal Collection and Storage‟, Tata McGraw Hill, 1990. G. L. Johnson, Wind energy systems, Prentice Hall Inc. New Jersey. J. M. Kriender, Principles of Solar Engineering‟, McGraw Hill, 1987. Reference V. S. Mangal, Solar Engineering‟, Tata McGraw Hill, 1992. N. K. Bansal, Renewable Energy Source and Conversion Technology‟, Tata McGraw Hill, 1989. P. J. Lunde, Solar Thermal Engineering‟, John Willey & Sons, New York, 1988. J. A. Duffie and W. A. Beckman, Solar Engineering of Thermal Processes‟, Wiley & Sons, 1990. (b) Secondary energy sources: Burning of fuel produces heat which is utilized to produce steam in power plant and the electricity is generated. So heat and electricity are termed as secondary energy sources. The table presented below for major primary and secondary energy sources in Global situation. 5 Source Extraction Processing Primary Energy Secondary Energy Preparation Power Station 6 Coal Steam, Thermal Hydro Nuclear Natural Gas Natural Gas Petroleum LPG, Petrol, Diesel Thermal Petrochemical Steam Solar thermal, Heat, Elect Solar PV Renewable Wind Generator, Electricity Energy Cook Stores Gasifiers, Biofuels Producer Gas, Bio Gas, Electricity. Open or Deep mines Coal CokePurification Mining Enrichment Gas Well Treatment Oil Well Cracking & Refining Solar, Wind, Biomass Energy Conversion Device Electricity Non-Commercial and Commercial Energy Sources: All energy sources which are available in nature like wind, sun, hydro etc. are non-commercial energy sources. Biomass like cattle dung, agricultural waste, fire woods etc are used by rural people for burning purposes and solar energy for drying purposes. These are also called as non-commercial energy sources because for production of heat or energy technology is not essential. Applications of solar energy, wind energy, hydro energy for electricity and lifting water from the ground require technology are termed as commercial energy sources. 7 10 India’s Energy Reserves:- The Ministry of statistics and program Implementation, Govt. of India, 2012 has produced the following data. Coal Main fossil energy reserves in India at 286 billion tons and 41 billion tons of lignite. These are available in eastern and southern belts of the country. Crude Oil Limited to 757 million tons m3. Natural Gas Limited to 1241 billion tons m3. Nuclear Energy Uranium can fuel only 10,000 MW pressurized heavy water reactors (PHWR). *India depends primarily on Uranium to run the reactors. Thorium is also another source for nuclear energy which runs in fast breeder reactor. The fast breeder reactor has been rejected by Europe and USA due to safety concerns. 11 Renewable Energy Sources:- The capacity addition in renewable energy was about 27,300 MW in 2012. Technology Capacity Installed in MW by 2012. Coal 11,202 Hydro 38,990 Renewable 27,300 Gas 18,381 Nuclear 4,780 Total 201,473 Table: India’s Installed power generation capacity. Thermal 54.4% Hydro 21.60% Renewable 10.90% Gas 10.10% nuclear 2.7% 12 So, total renewable energy’s contribution becomes almost 33% (includes Hydro power), plan wise grid connected renewable energy contribution is given in Table below. Table: Power densities of renewable energy sources and the conventional energy forms. Renewable Energy Sources in KW/m2 Conventional Energy in KWh/m2 Wave < 100 Hot Plate 100 Extra terrestrial solar radiation < 1.35 Coal 500 Wind < 3 Nuclear 650 Solar radiation 0.2 Power Cable 1000,000 Tidal 0.002 Biomass Production 0.002 Geothermal heat 0.00006 15 India has launched a solar mission with an aim to install 20,000MW grid solar power, 2000MW off grid system, 20 million solar lights and 20 million m2 solar thermal collector by 2020. 1.3 Origin of Renewable Energy Sources:- All available energy sources in the world that come from three different primary energy sources. (i) Isotropic dissociation in the core of the earth. (ii) Movements of the planets (iii) Thermonuclear reactions in the earth. * The largest energy flow comes from solar radiation, which is also responsible for the development of fossil energy sources, namely oil, coal and gas due to bio conversion which has occurred million years ago. All available natural renewable energy sources are presented in the diagram and their conversion is also shown below. 16 Geothermal Power Station Geothermal station Hydel Power station Glacier Power station Wind Energy Convertor Wave Power Station Sea Current Station Thermal Ocean Power Plant Heat Pump Station Power Station Conversion System Photosynthesis Solar Cells Thermal Collector Tidal Power Station Primary Energy Sources Thermal Energy Conversion Energy Conversion Process Secondary Energy Isotropic dissociation Geothermal at the core of the earth Condensation Rain Melting Atmospheric Movement Wave Movement Sea Currents Heating of earth’s surface and the atmosphere Bio production Gravitational pull Tides of planets Fig: Use of renewable energy sources directly or indirectly through energy conversion process. Electri cal Energy Therm al Energy Chemi cal Energy 17 * Another source of energy is the geothermal energy originates from the earth’s surface itself . The theoretical potential of geothermal energy is much lesser (less than by an order of 4) than the solar radiation. * The third source of renewable energy is the movement of the planets . The force of attraction between planets and gravitational pull creates tide in the sea. This energy source magnitude is very less compared to geothermal energy. Limitations:- a) The real difficulty with the renewable energy sources are that the power density of those energies are very less in comparison to conventional energy sources. b) Since the solar and wind energies fluctuate with respect to day and season; the surface area requirement will be large and so also storage device for heat and electricity. The thermal energy storage system (sensible heat storage systems) have low efficiency, while the phase change storage systems suffer density variations in two phases and stability over several cycles. Electrical storage device like batteries are heavy and not environment friendly. 20 Depth (km) 2000 4000 6000 4000 5000 Temp 0C Fig: (a) Temperature distribution along depth of earth and (b) different layers of earth along depth (a) Crust Iron Core Magma Mantle (b) The core has two layers: (i) Solid iron core and (ii) Magma (outer core made of very hot melted rock). Mantle consists of rock and magma, spreads over depth of 2600km. Crust is the outer most layer of the earth whose depth is 5-8 km under ocean or 25-60 km on the continents. 21 When there are cracks on the crust, the lavas (partly magmas) comes out to the surface through the gap is called volcanic eruption. All the under ground constituents absorbs heat from magmas. 1.4.1.2 Geothermal resources estimation:- Let us assume a large mass close to the earth surface and spreads along the depth with density ρr , specific heat capacity Cr and a cross-sectional area A. Assuming uniform matrixial composition and no convection, the temperature variation is linear with depth y, which is expressed as: where T0 is the temperature at y = 0 and the temperature gradient inside the earth’s surface. So, at )1.......(..........0 y dy dT TT == dy dT )2..(.......... , 1011 y dy dT TTyy +== 22 )3....(..........01 1 dydT TT y − = After rearrangement, Consider an element dy on the earth’s crust where the temperature is greater than T1, then the heat content dE can be written as: And also The total useful heat in the rock is obtained by integration from y1 to y2 as: Assuming ; the energy of the rock is given by Where ( ) ( ), 1TTCAdydE rr −=  ( ) ( ) )4(.................... 1yy dy dT CAdydE rr −=  ( ) ( ) )5(.................... 1 2 1 yy dy dT CAdydE y y rr −=   (slope) Sconstant == dy dT ( ) ( ) 2 1 2 1 2 2 1 1 y y rrr y y rr yy sACdyyyACsdEE       − =−==    ( ) )6.......(.......... 2 1 2 12 yyACsE rrr −=  21 and between yy dy dT s = 25 1.4.3 Power Generation Technology: The different types technologies are used to generate electricity depending the type of resource at site. 1.4.3.1 Direct Steam Geothermal Plant: If the steam is available from the geothermal site, then it is directly fed to the turbine to produce electricity. The stone and dust particles are removed from the steam before entering into the turbine. 1.4.3.2 Flash Steam Power Plant: This type of power plant is used when high temperature hot water and a mixture of steam and water is available at the geothermal site The hot water is directly supplied to the flash tank where the water is separated in the flash chamber and some amount of water is converted into steam. The steam is directly fed into the turbine to produce electricity. Steam after passing through the turbine is brought into the condenser and then enter into the cooling tower. The condensed and cooled water is blow down to the well. 26 Flash steam water power plants for geothermal resources are available in the range from 5MW to 100 MW. Turbine Generator Water Separator Steam Condenser Cooling Tower Injection WellProduction Well Fig: Schematic diagram of a single flash geothermal power plant. Steam Direct use Water Condensate 27 1.4.3.3 Binary Cycle Power Plants: The schematic diagram of the binary geothermal power plant is shown here. The secondary fluid normally organic working fluid (like CFC, HFC etc.) is converted into vapor in a boiler by the exchange of heat with the geothermal fluid or it may be pre-heated before entering into the boiler. The vaporized organic working fluid is fed into the organic Rankine cycle turbine that produce mechanical work to produce electric generator. Organic fluid condenses in a condenser and cooling tower to produce the liquid which is again fed to the boiler in a closed loop cycle. Binary plant’s size varies between 500 MW and 10 MW. 30 1.5 DIRECT USE TECHNOLOGY:- The geothermal energy is directly used at the site of extraction by installation of plants because these are non-transportable to long distances. Some cases the available geothermal energy in the form of steam and hot water is not utilized directly because these are contaminated with chemicals, dirty water, stones etc. Heat exchangers are sometimes used to transfer of heat. 1.5.1 Tidal Energy:- The tidal energy is developed due to rise and fall of water level in the sea. There is a force of attraction between the earth and sun. The water level in the sea is balanced by the gravitational force of attraction by earth. The tide in the sea is developed due to rotation of earth about itself which creates imbalance on the force of attraction due to variation of the distance. * So, the main period of tide is diurnal at about 24 hours and semidiurnal at 12 hours 25 minutes. 31 * The tidal range, R = Change in height between two successive high and low tides. * The value of R in an open sea is 1 meter and near to coastal region 20 meter. * The theoretical potential of tidal energy is estimated to be 3X106 MW or 3.3 billion t COE/a. COE/a → Cost of Energy per annum. 1.5.2 Tidal Generating Force (Gjevik, 2011):- From the figure in the next slide, consider the moon is located at M, O is the centre of the earth and P is a point on the surface of the earth. R is the distance connected between centres of the earth and moon; and d is the distance between the point P on the earth’s surface to moon centre. Let r be the magnitude of radius of the earth. Then in vector form we can write, )1.....(....................Rdr  =+ 32 ha  M o P r  m A B R  d  ra  Fig: Moon and Earth system The gravitational force between earth and moon is: Where G is the gravitational constant. The acceleration at the center of the earth is: )2....(............................... 2 R R R MM G Me  )3..(............................... 20 R R R M Ga M   = 35 The vector has two components i.e. radial, ar and horizontal, ah. The direction of ah is along the circular arc APB. Again, The following data are given: Mass of the earth, Me = 5.974 x 1024 kg Mass of the sun , Ms = 1.991 x 1030 kg Mass of the moon, Mm = 7.347 x 1022 kg Mean distance earth-moon, R = 3.844 x 105 km Mean distance earth-sun, R = 1.496 x 108 km Radius of the earth r = 6.370 x 103 km   ( ) )12........(cos. 1cos3 r 2 3  RrrR R r M M g r aa e m r =−      ==  ( ) )13.........(sin 2sin 2 3 3 mrm e m h RrR R r M M g r r aa  =            ==  36 o AB to Moon Fig: The equilibrium tide 2   =m 37 T gSA Pt 2 2 = Tidal Power:- Consider a small volume of water in a basin within tidal range is shown in the diagram. Let the area of the basin be S, density ρ and range A has a mass ρSA at a centre of gravity of A/2. Fig: Principle of power generation from tide If water falls through height A/2, the potential energy per tide is given by (ρSA)g(A/2). If T is the time period then the average power of one is, The range of a tide, A = A s – An. Where As = Maximum height in for spring tide (the pull of the moon and sun which are aligned). An = Minimum height for neap tides (the pull of the moon and the sun are at right angle to each other) . The sinusoidal oscillation of tide is shown in the figure in the next slide. Barrier with turbine High tide level Low tide level Surface area, As 40 Energy of Ocean Tides:- Energy in the ocean is available in the form of tidal generating forces. The tidal energy varies from one geographical region to the other. Some part of the tidal energy is lost due to: (i) Dissipation which results from friction between layers of flow, (ii) Power interchange between the earth and its atmosphere, and (iii) Change of energy from kinetic to potential form or vice-versa in the course of motion. * The above factors can be taken into account while calculating the total tidal energy. * The quantitative estimates of the energy flow are equal to 2.4 TW. * Dynamic Tidal Power (DTP) is a theoretical generation technology which interacts between kinetic and potential energies in the tidal flows. According to this technology, a very long dam (30-35km) will be built from coasts to the inner side of the sea or ocean without an enclosing area. Tidal phase differences are allowed to 41 enter across the dam. * Tidal power can generate energy for 10 hours per day when tide move in and out. * The tidal power is economic when the mean tidal range 7m or more. 42 WIND ENERGY 45 Origin of Wind: The flow of air starts when there is pressure difference between two places. The region where solar radiation is less the atmospheric air gets low temperature and hence low pressure region. On the contrary where the solar radiation is high the atmospheric air gets heated and pressure is high. These differences in atmospheric air pressure (pressure gradient) cause acceleration of the air particles which is called wind. The rotation of earth about its own axis creates Coriolis force which superimposes on the pressure gradient. The direction of wind motion is affected by this Coriolis force. In the Northern hemisphere, the moving object turns towards right due to the effect of the Coriolis force if the observer moves in the direction of wind movement. Similarly, the moving object turns towards left in the southern hemisphere. * In a friction free, rectilinear and stationary wind movement, the force due to pressure gradient and Coriolis force are of same magnitude but in opposite direction. The wind motion due to Coriolis force is known as geostropic wind. 46 High Pressure Low Pressure Fp Fc Fp p Fp (Pressure force) Fc (Coriolis force) 990 mb 1000 mb 1020 mb Fig: Geostropic wind on the northern hemisphere As a result of pressure difference, the air first moves towards low pressure region. It then follows inclined movement towards right due to Coriolis force. This inclination towards right continues till the magnitude of Coriolis force is exactly equal to the pressure gradient force. At this point the wind moves in the direction of isobars whose motion is in the same direction as that of geotropic winds. ΔX =1000 km ISO bar 47 Consider a small air element whose Coriolis force is equal to the product of the Coriolis acceleration and mass of the air, i.e. Where Fc = Coriolis force in newton ω sin ϕ = angular velocity of earth at the latitude ϕ (1/sec) ϕ = latitude ΔXΔYΔZ = Volume of the considered small air element in (m3) ρa = density of air (m/sec) vg = geostropic wind velocity (m/sec) The pressure force (Fp) on the air element can be written as: Where Δp = pressure difference on the air element (N/m2) ΔYΔZ = area of air element (m2). By equating, * It is seen that the pressure gradient is directly proportional to the velocity of the geostropic wind. ( ) agc ZYXvF  = sin2 ZYpFp = a gagpc X p vpXvFF   )sin( 2 1 sin2   === 50 The operating power of Hadley circulation is the strong solar radiation at the equator. The air gets heated, rises high and moves towards north and south, where it is deviated towards east as result of Coriolis force. The air gets cooled and sinks down in the latitude region ±300 (+ North, – South) and flows back towards the equator, where it is deviated towards west due to the Coriolis force. These are the regions where local storms overlap and wind-flows are not always predictable. In the northern and southern region around latitudes ±600 the westerly winds of Rossby circulation dominate the region. These winds have wave-form character and vary strongly in the flow patterns. Wind Flow and Wind Direction:- Wind speed is classified on representative scale of 12. The order of wind classification is in m/sec or knots (1 nautical miles = 1.852km/hr). The direction of wind are normally divided into eight segments: North, North-East, East, South-East, South, South-West, West and North-West. 51 Power Density of the Wind:- Power density of the wind is calculated based on the normal area (A) to the direction of flow of wind stream. The kinetic energy (dE) contained within the mass of the element (dM) is: Where dE = Kinetic energy (joule) dm = Elemental mass (kg) v = dx/dt = wind velocity (m/sec). (Here dx is the path travelled in the direction of wind in time dt). If the density of air is ρa and dv is the elemental volume in m3 then dv = A.dx and dm = ρa dv …………..(ii) V V dx A dM x z y Fig: Derivation of power density )..(.................... 2 1 2 idmvdE = 52 The mass element dm can be expressed as: dm = Aρa .v.dt (kg)………....(iii) So the K.E. is: dE = (1/2) ρa Av3dt The power, P is: and power density in (w/m2 ) is: It is seen that the wind power density (Pressure) depends upon the cube of wind velocity. Wind Measurement: - Wind pressure Measurement:- Applying Bernoulli’s equation the total pressure (Pt ) can be calculated as: and the velocity can be calculated as: The velocity can be calculated if both the pressures are known. The Prandtl tube is used for pressure measurement. ( ) ( ) )........(2/1Pressure Static 2 ivvPP ast += ( ) )........(.......... 2 v PP v a st  − = dt dE P = 3 2 1 v A P P a== 55 Altitude Dependence of Wind Speed: The maximum velocity of jet stream occurs at a height of 10 km. The velocity there is nearly five times more than its magnitude at a height of 10 m. In the boundary layer the velocity of flow varies linearly on a log-log representation. It indicates the variation of wind velocity is exponential. The wind velocity at a height H is obtained as: Where = average annual velocity (in m/sec) at a height H (in m). = annual average velocity (in m/sec) at a height of 10 m. H = height (m) g* = exponent. The above equation is accurate up to height of 200m. The values of the exponent are given in table below. m/sec. 10 * 10 __ g H H vv       = Hv _ 10 _ v 56 Description of Land Exponent Open land with a few obstacles i.e. grass and field land with very few trees, coasts, deserts, islands etc. 0.16 Land with uniformly distributed obstacles upto the height of 15m such as building complexes, small cities, forests, bushes, trees and hatches. 0.28 Land with big and non-uniform obstacles such as centres of big cities, high obstacles like trees 0.40 Table: Boundary Layer Exponent for Different Ground Obstacles 57 Recording of wind data: The wind speed is measured by an anemometer and wind direction is measured by a wind vane attached to a direction indicator. Anemometer works on one of the following principles. (i) The oldest and simplest anemometer is a swinging plate hung vertically and hinged along its top edge. Wind speed is indicated by the angle of deflection of the plate with respect to the vertical. (ii) A cup anemometer consists of three or four cups mounted symmetrically about a vertical axis. The speed of rotation indicates wind speed. (iii) A hot-wire anemometer measures the wind speed by recording cooling effect of the wind on a hot-wire. The heat is produced by passing an electric current through the wire. (iv) An anemometer can also be on sonic effect. Sound travels through still air at a known speed. However, if the air is moving, the speed decreases or increases accordingly. 60 Wind Energy Converters:- The wind energy converters convert wind energy to electrical and mechanical energies. Maximum Power Coefficient:- The maximum power coefficient of the wind energy can be defined as the ratio of the convertible power to the theoretically maximum power from the available wind energy. V1 V2 A1 A2A0 Fig: Wind stream profile in an external wind turbine Blade Surface 61 The incompressible, friction free and one dimensional wave is shown here. The flow is called Rankine-Froude momentum theory. The flow velocity (V1 m/sec) and cross sectional area (A1 m2) enters into the surface of the wind blade and leaves out with velocity (V2 m/sec ) at a cross sectional area (A2 m2). According to the equation of continuity: At the surface of the rotor: Where ρa = air density (kg/m3 ) v0 = Wind velocity at the surface of the rotor (m/sec) A0 = rotor disc area (m2 ) So v0 can be written as: We know that the power density, The power of the rotor is: ......(vi)/sec......m 3 2211 . vAvAm == ....(vii)Avm a kg/sec.... 00 . = ( ) ..(viii)..........vvv m/sec... 2 1 210 += ( ) ).........(W 2 1 and (W) 2 1 2 3 221 3 11 ixAvPAvP aa  == ( ) )(....(W)....... 2 1 or (W) 2 3 2 3 1121 xAvvAPPPP a −=−=  62 The maximum power is obtained when the wind speed (v2) is zero. The ideal power coefficient (Cp ) of a wind machine is the ratio of the power P of the rotor to the maximum wind power, i.e. ( ) ( )( ) ( ) ( ) )....(....................W11 4 1 Or 2 1 4 1 Or 2 1 Or (W) 2 - 2 Or 2 1 2 2 1 23 10 210 2 2 2 1210 . 2 2 2 1 . 2 2 22 2 1 11 xi v v v v vAP vvvvvvvAP AvmvvmP v Av v AvP a a aa       −      +=       +=−+=       =−= =       (xii)..........AvP a (W).... 4 1 0 3 1max = )....(11 2 1 2 1 2 1 2 max xiii v v v v P P Cp               −      +== 65 ( ) )........()1( )2/1( )2/1( 2 2 3 2 max xx v u v u C Av uAuvC P P C R a Ra PR −= − ==   The maximum CPR is obtained by setting By solving we will get u/v = 1/3. So the maximum value: CPR max = (4/27) CR ….(xxii) )....(..........0 xxi v u CPR =         2.5 5.0 7.5 1.0 0 0.3 0.5 0.1 0.7 CR = 2.3: C –Profile CR = 2.3: Rectangular CR = 1.33: Hemispherical Cp v2/v1 Fig: Comparison of ideal power coefficient with maximum values of power coefficient of resistive rotors. 66 Wind Stream Profiles:- Blade Profile S H bp V ( ) )...(.......... 0 xxiiiLdxPPF pb uLA  −=The lift force (FA) is given by: PL = Pressure at the lower side of the profile (N/m2) , Pu = Pressure at the upper side of the profile (N/m2), L = Length (m), bp = width of the profile (m). * The pressure is lower on the upper side than the lower side. 67 bp FA FR FRS αA Plane of Profile W Buoyancy Coefficient and the Drag Coefficient:- For an asymmetrical profile, there exists two forces: (1) The lift force (FA ) perpendicular to the direction of flow, and (2) the drag force (FR) parallel in the direction of flow. Let us assume: αA = incident angle or angle of attack (angle between the profile and the flow direction ). 70 αA α u V0 w β Profile Chord Rotor axis FA FR FRS FS FT R dR RE RN L Hub U UE Velocities and Forces at the Rotor Blade:- γ u = Circumferential velocity of rotor blade (m/sec), v0 = Velocity of wind (m/sec), w = Approach velocity of wind (m/sec), β = Blade angle (Angle between profile plane and rotor plane), FRS = Resultant of the FA and FR . Fs = Axial component of FRS , RE= Outer rotor radius, RN = radius of the hub, uE = Peripheral velocity at the edge of the blade, αA = angle of attack, γ = angle between wind velocity v0 and relative 71 approach velocity, L = Rotor blade length, For an element which delivered power dP along the length dR of the rotor blade, one can write: Where dFT is an elemental tangential force. It is expressed as: So the power produced is given by: Using the expression for dFA we can write; and for the area element: Where NR is the number of rotor blades and dR is the length of the elemental rotor. By substitution of dA in dP we can get power of an element, newton co sdFdF AT = (W) TdFudP = (W) cos AdFudP = 2 0 2 00 2A cos Again, (W) 2 C cos vu v w v dAwudP a + == =   dRNbdA Rp= (W) 2 C 2 0 2 02A dRNb vu v uwdP Rpa         + =  72 The total power of the rotor can be calculated by integration of dP from RN to RE . So, Similarly, thrust force can be calculated on the rotor blades along the vertical axis: (W) 2 2 0 2 02 dRNb vu v uw C P Rpa R R a E N + =   (N) )( 2 or (N) )( 2 or (N) 2 sin 2 0 2 2 2 0 2 2 2 0 2 2  + = + = + == E N R R Rpa a s Rpa a s a a As dR vu u Nbw C F vu u dRNbw C dF vu u dAw C dFdF    75 Electric Generators:- Electrical generators convert the rotational energy into mechanical energy then to electrical energy. Commercially available generators with slight modification are used for converters with gear box. Specially designed three phase generators are used for gearless converters. Synchronous Generator:- These generators are equipped with a fixed stator at the outside and a rotor at the inside located on a pivoting shaft. Normally DC is supplied to the rotor to create a magnetic field. When the shaft drives the voltage is created in the stator whose frequency matches exactly the rotational speed of the rotor. This type of generators are used most of the places but the disadvantage is that it runs with constant speed of the rotor and fixed frequency. It is therefore not suitable for variable speed operations in the wind plants. 76 Asynchronous Generator:- The asynchronous generator is electromagnetic generator. The stator of this generator is made of numerous coils with three groups and is supplied with three- phase current. The three coils are spread around the stator periphery and carry currents, which are not in phase with each other. This combination produces a rotating magnetic field, which is the key feature of the asynchronous generator. The angular speed of the rotating magnetic field is called the synchronous magnetic field and is given by: Where f = frequency of the stator excitation, p = number of magnetic pole pairs. The stator coils are embedded in slots of high permeability magnetic core to produce a required magnetic fields intensity with low exciting currents. The rotor in this generator is squirrel cage rotor with conducting bars embedded in the slots of the magnetic core. The bars are connected at ends by a conducting ring. The stator magnetic field rotates at the synchronous speed given above. The relative speed rpm 60 p f Ns = 77 between the stator and the rotor induces a voltage in each rotor turn linking the stator flux V = (-dΦ/dt), Φ being the magnetic flux linking the rotor turn. Foundations:- The type of foundations required to anchor towers and thus wind energy converters, into the ground depends upon the plant size, meteorological and operational stress and local soil conditions. Erection of wind converters on a coastal line is much more costly. Depending on the soil conditions, there types of foundations namely gravity foundation, monopole foundation and tripod foundations are used . All these foundations are discussed in the beginning. Turbine Rating:- The normal rating of a wind turbine has no standard global rating. The power output of a turbine is proportional to the square of the rotor diameter and also to the cube of the wind speed. * The rotor of a given diameter will generate different power at different wind speed (like 300 KW at 7m/sec and 450 KW at 8 m/sec). 80 WAVE ENERGY & OCEAN ENERGY 81   2 D Fig: Water particles from a sphere of diameter ‘2r’ in a sea wave. fP 2A  D Wave Energy:- The power in the wave (P) is proportional to the square of the amplitude (A) and to the period (T = 1/f, f = frequency) of the waves. Where A is the amplitude of the wave in meter and f the wave frequency . Ex:- the long period (~10 sec) and large amplitude (~2 m) waves found in deep sea area where the power generation with energy fluxes of 50-70 KW per metre width of incoming wave. * Deep water waves are found when the mean depth of sea bed (D) is more than about half the wavelength, λ. * Deep water waves are available at a mean depth of 50m or more. Fig:(b) 82 * The Fig:(b) in the previous page shows the motion of the water particles in the deep water wave. The motion of water particle is circular with an amplitude that decreases exponentially along the depth and becomes negligible for D > (λ/2). * In shallow water, the movement of water takes place elliptically and the water movement occurs against bottom of the sea, including friction and dissipation. * The wave height is determined by wind speed, the duration of which the wind is blown, the distance over which the wind excites and the depth and topography of sea floor. * When the wave speed reaches the maximum practical limit depending upon the time or distance, the wave is said to be fully developed. * Oscillatory motion is highest at the surface and diminishes with depth exponentially. * For standing waves near a reflecting coast, the wave energy is also present at great depth due to pressure fluctuations (very slow amount of wave but makes it count for wave power). 85 Assuming the wave as sine nature, the velocity of propagation is: Vw = ωλ. So, Power of Waves:- The centre of the gravity of the peak from the figure can be calculated as If ν is the frequency of the wave then the wave power is given by: P = m.g.2ys.ν   2 g Vw =  2ys 2yc y x Fig: The centre of gravity of the peak falls down by 2ys yielding work       === ydx dxy dxdy ydxdy dm ydm yc 2 2 1 86 Technology of Wave Power Plants:- Several working models have been developed by the researchers and laboratory and numerical tests are also conducted to test the viability for the commercial purposes. The literature presents some of the commercial viable wave energy devices by their energy extraction method, size etc. According to the device location, it is classified as: * Shoreline devices -----→ (oscillating water columns (OWC), tapered channel (TAPCHAN) ), * Bottom fixed near –shore devices -----→ pendulor, * Off-shore devices. Shoreline Devices:- These type of devices require less maintenance and installation is also easy. The main type of shoreline devices are the OWC and TAPCHAN. 87 Oscillating Water Columns:- The shoreline devices are partly submerged in structure . The air is trapped inside the open below free water surface as shown in the diagram. The incident water waves cause the height of the water surface to oscillate and the air is channeled through a turbine to drive the electric generator. Air Waves Air turbine Valve Water Column Air Fig: Oscillating water column device 90 Gravity base Mono pile Piled Jacket Floating Fig: Example of system configuration Offshore Devices:- The extraction of energy from the wave is possible if the devices are installed at or near water surface of the sea. In most of the cases, the main element is the oscillating body that either floats or submerged near the surface. The conversion of oscillating motion of body into mechanical energy has been done by mechanical devices like hydraulic pumps or rams which are incorporated into the floating body. Some of the examples are Swedish Hose Pump. 91 Fuel Hose pump Reaction Plate Anchor Figure: Hose Pump High pressure sea water to generator 92 E v ap o rato r C o n d en ser Engine Hot water inlet 250C Hot water outlet 230C G Cold water inlet 50C Cold water outlet 70C T u rb in e Evaporator Pump 250C 50C 18m 23 0C T0C Fig: A closed Rankine cycle power plant in an ocean thermal energy power plant 120m Ocean Thermal Energy Conversion (OTEC):- * A concept first proposed by the French engineer Jacques Arsene d’ Arsonval in 1880s. 95 Limitations:- OTEC plant set up for power generation is relatively expensive . Therefore it seems little chance for replacement of power generation in future. Photovoltaics: * The use of solar power in the form of electricity is called phototvoltaics. The photovoltaic process converts solar radiation into electricity through a solar cell. * Photovoltaic systems are available in the range of milliwatts to megawatts. * Photovoltaic modules may include mono-crystalline silicon, Polycrystalline silicon, amorphous silicon and cadmium telluride and copper indium gallium selenide/ sulfide; out of these commonly used modules are polycrystalline silicon solar cells or thin film amorphous silicon cells. * Mono-crystalline silicon is used for space applications or automobile vehicles. * Photovoltaic has gathered third position among the renewable energy sources of their global use after hydro and wind power. 96 Solar Thermal Energy(STE):- * The solar thermal energy produces power from thermal devices (collector) by convection process. There are three types of collectors based on temperatures i.e. low, medium and high temperatures. * Low temperature collectors are used for residential heating purposes like swimming pools and for drying. * Medium temperature collectors are advance flat plate collectors where temperature range is around 1000C or more for residential or commercial heating. * High temperature collectors are usually employed for solar power generation. STE is much more efficient than photovoltaics especially for heat applications. 97 BIOMASS ENERGY & PHOTOSYNTHESIS