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Department of Electrical and Electronics Engineering ACADEMIC YEAR: 2017 (EVEN SEM)
Question Bank SEM: VIII EE 6801-ELECTRICAL ENERGY GENERATION UTILISATION&CONSERVATION UNIT-1 PART-A 1. List the Advantages of Electric Drives: Flexible control characteristics. Starting and braking is easy and simple Provides a wide range of torques over a wide range of speeds (both ac and dc motor) Availability of wide range of electric power Works to almost any type of environmental conditions No exhaust gases emitted Capable of operating in all 4 quadrants of torque –speed plane Can be started and accelerated at very short time 2. How will you make Choice of Electrical Drives? The choice of an electrical drive depends on a number of factors. Some important factors are: Steady state operation requirements: (nature of speed-torque characteristics, speed regulation, speed range, efficiency, duty cycle, quadrants of operation, speed fluctuations, rating etc) Transient operation requirement(values of acceleration and deceleration, starting, braking, speed reversing) Requirement of sources:(types of source, its capacity, magnitude of voltage, power factor, harmonics etc) Capital and running cost, maintenance needs, life periods Space and weight restrictions Environment and location Reliability
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Part-B 1. Briefly explain about various elements of the Electric Drive Systems with suitable diagrams.
A modern electric drive system has five main functional blocks as shown above a mechanical load, a motor, a power modulator, a power source and a controller. Power source: The power source provides the energy to the drive system. It may be dc or ac (single-phase or three-phase) Power Converter: The converter interfaces the motor with the power source and provides the motor with adjustable voltage, current and frequency. During transient period such as starting, braking and speed reversal, it restricts source and motor current within permissible limits Also the converter converts the electric waveform into required signal that requires the motor. Types of modulator: Controlled Rectifier(ac to dc) Inverter (dc to ac AC Voltage Regulator (ac to ac DC Chopper (dc to dc) Cyclo-converter (ac to ac) (Frequency converter) Controller: A well designed controller has several functions. The basic function is to monitor system variables, compare them with desire values, and then adjust the converter output until the system achives a desired performance. This feature is used in speed and position control. Electric motor: i) The basic criterion in selecting an electric motor for a given drive application is it meets power level and performance required by the load during steady state and dynamic operation. ii) Environmental factors: In industry such as in food processing, chemical industries and aviation where the environment must be clean and free from arc. Induction motors are used instead of DC motor. Mechanical Load:
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The mechanical load usually called as machinery such as flow rates in pump, fans, robots, machine tools, trains and drills are coupled with motor shaft. Classification of Load torque: Various load torques are broadly classified into two categori es. A) Active Load Torque
B) Passive Load Torque Load torques which have the potential to drive the motor under equilibrium conditions are called active load torques. Such load torques usually retain their sign when the direction of the drive rotation is changed. Torque due to the force of gravity, hoists, lifts or elevators and locomotive trains also torques due to tension, compression, and torsion undergone by an elastic body come under this category. Components of the Load Torque (TL) : The load torque TL can be further divided into the following components: 1.Friction torque TF: The friction will be present at the motor shaft and also in the various parts of the load. 2.Windage torque TW: When a motor runs, the wind generates a torque opposing the motion. This is known as the windage torque. 3.Torque required to do the useful mechanical work, TM: The nature of this torque depends on the type of load. It may be constant and independent of speed, it may be some function of speed, it may be time invariant or time variant, and its nature may also vary with the change in the load's mode of operation. The friction torque ‘TF’ can be resolved into three components as shown in figure
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Unit-II Part-A 3. Define luminous flux. It is defined as the total quantity of light energy emitted per second from a luminous body. It is represented by symbol F and is measured in lumens. The conception of luminous flux helps us to specify the output and efficiency of a given light source. 4. What is meant by candle power? It is defined as the number of lumens given out by the source in a unit solid angle in a given direction. It is denoted by CP. CP=lumens 5. Define MHCP. The mean of candle power in all directions in the horizontal plane containing the source of light is termed as Mean Horizontal Candle Power. 6. Define utilization factor. It is defined as the ratio of total lumens reaching the working plane to total lumens given out by the lamp. Utilisation factor= Total lumens reaching the working plane Total lumens given out by the lamp 7. What are the laws of illumination? Law of Inverse Squares: Illumination at appoint is inversely proportional to square of its distance from the point source and directly proportional to the luminous intensity (CP) of the source of light in that direction.
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If a source of light emits light equally in all directions be placed at the centre of a hollow sphere, the light will fall uniformly on the inner surface of the sphere. If the sphere be replaced by one of the larger radius, the same total amount of light is spread over a larger area proportional to the square of the radius. Lambert’s cosine law: The illumination at a point on a surface is proportional to cosine of the angle which ray makes with the normal to the surface at that point.
PART-B 2. State and explain the the law of illumination. (16) Laws of Illumination Inverse Square law Lambert’s Cosine law Law of Inverse Squares: Illumination at appoint is inversely proportional to square of its distance from the point source and directly proportional to the luminous intensity (CP) of the source of light in that direction. If a source of light emits light equally in all directions be placed at the centre of a hollow sphere, the light will fall uniformly on the inner surface of the sphere. If the sphere be replaced by one of the larger radius, the same total amount of light is spread over a larger area proportional to the square of the radius. Explanation The light from the Mini-Maglite™ spreads out equally in all directions. As the distance from the bulb to the graph paper increases, the same amount of light spreads over a larger and larger area, and the light reaching each square becomes correspondingly less bright. For example, adjust the distance from the bulb to the graph paper to 10 cm. At this distance, the graph paper touches the card. A 1 cm2 area will be illuminated. When the graph paper is moved 20 cm from the card, 4 cm2 will be illuminated on the graph paper. When the graph paper is moved 30 cm from the card, 9 cm2 will be illuminated, and so on. The area illuminated will increase as the square of the distance. The brightness of light is the power (energy per second) per area. Since the energy that comes through the hole you cut is constant but spreads out over a larger area, the brightness (or intensity) of light decreases. Since the area increases as the square of the distance, the brightness of the light must decrease as the inverse square of the distance. Thus, brightness follows the inverse-square law. If you had two light bulbs and knew that they both give off the same amount of light (same luminosity/power), then you could calculate the relative distance between the two of them simply by measuring their relative brightness. If you also knew what the luminosity/power of the bulbs was, you would then be able to determine the distance to both bulbs. Or, if you knew the distance to one of the bulbs you could determine the distance to the other one. This is how astronomers use the inverse square law of light to measure distances to stars or galaxies. They find stars that are the same kind (same size and temperature) and, therefore, have
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the same luminosity. They measure the brightness of the stars and can determined distances if they know either what the luminosity of the stars is or the actual distance to one of the stars by some other method. Lambert's cosine law: The illumination at a point on a surface is proportional to cosine of the angle which ray makes with the normal to the surface at that point. The inverse-square law generally applies when some force, energy, or other conserved quantity is evenly radiated outward from a point source in three-dimensional space. Since the surface area of a sphere (which is 4πr2 ) is proportional to the square of the radius, as the emitted radiation gets farther from the source, it is spread out over an area that is increasing in proportion to the square of the distance from the source. Hence, the intensity of radiation passing through any unit area (directly facing the point source) is inversely proportional to the square of the distance from the point source. Gauss's law applies to, and can be used with any physical quantity that acts in accord to, the inverse-square relationship. Let the total power radiated from a point source, for example, an unidirectional isotropic antenna, be P. At large distances from the source (compared to the size of the source), this power is distributed over larger and larger spherical surfaces as the distance from the source increases. Since the surface area of a sphere of radius r is A = 4πr 2 , then intensity I (power per unit area) of radiation at distance r is
The energy or intensity decreases (divided by 4) as the distance r is doubled; measured in dB it would decrease by 6.02 dB per doubling of distance The situation for a Lambertian surface (emitting or scattering) is illustrated and For conceptual clarity we will think in terms of photons rather than energy or luminous energy. The wedges in the circle each represent an equal angle dΩ, and for a Lambertian surface, the number of photons per second emitted into each wedge is proportional to the area of the wedge. It can be seen that the length of each wedge is the product of the diameter of the circle and cos(θ). It can also be seen that the maximum rate of photon emission per unit solid angle is along the normal and diminishes to zero for θ = 90°. In mathematical terms, the radiance along the normal is I photons/(s·cm2·sr) and the number of photons per second emitted into the vertical wedge is I dΩ dA. The number of photons per second emitted into the wedge at angle θ is I cos(θ) dΩ dA. It represents what an observer sees. The observer directly above the area element will be seeing the scene through an aperture of area dA0 and the area element dA will subtend a (solid) angle ofdΩ0. We can assume without loss of generality that the aperture happens to subtend solid angle dΩ when "viewed" from the emitting area element. This normal observer will then be recording I dΩ Da photons per second and so will be measuring a radiance of photons/(s·cm2·sr). The observer at angle θ to the normal will be seeing the scene through the same aperture of area dA0 and the area element dA will subtend a (solid) angle of dΩ0 cos(θ). This observer will be recordingI cos(θ) dΩ dA photons per second, and so will be measuring a radiance of
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photons/(s·cm2·sr), which is the same as the normal observer.
3. Explain the working of a sodium vapour lamp with in a neat sketch. Sodium vapor lamps are mainly used for street lighting. They have low luminosity hence require glass tubes of large lengths, which makes them quiet bulky. Construction: The lamp consists of a U shaped inner glass tube filled with neon gas at a pressure of 10mm. It also contains a small quantity of sodium and argon gas. The initial ionization voltage is reduced, as the ionization potential of argon is low. Two oxide coated tungsten electrodes are sealed into the tube at the ends. This tube is enclosed in an outer double walled vacuum enclosure to maintain the required temperature.
Low-pressure sodium (LPS) lamps have a borosilicate glass gas discharge tube (arc tube) containing solid sodium, a small amount of neon, and argon gas in a Penning mixture to start the gas discharge. The discharge tube may be linear (SLI lamp)or U-shaped. When the lamp is turned on it emits a dim red/pink light to warm the sodium metal and within a few minutes it turns into the common bright yellow as the sodium metal vaporizes. These lamps produce a virtually monochromatic light averaging a 589.3 nm wavelength (actually two dominant spectral lines very close together at 589.0 and 589.6 nm). As a result, the colors of illuminated objects are not easily distinguished because they are seen almost entirely by their reflection of this narrow bandwidth yellow light. LPS lamps have an outer glass vacuum envelope around the inner discharge tube for thermal insulation, which improves their efficiency. Earlier types of LPS lamps had a detachable
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dewar jacket (SO lamps). [5] Lamps with a permanent vacuum envelope (SOI lamps) were developed to improve thermal insulation. [6] Further improvement was attained by coating the glass envelope with an infrared reflecting layer of indium tin oxide, resulting in SOX lamps. LPS lamps are one of the most efficient electrically powered light source when measured for photopic lighting conditions up to 200 lm/W,[8] primarily because the output is light at a wavelength near the peak sensitivity of the human eye. As a result they are widely used for outdoor lighting such as street lights and security lighting where faithful color rendition was once considered unimportant. Recently, however, it has been found that under mesopic conditions typical of nighttime driving, whiter light can provide better results at l ower power LPS lamps are more closely related to fluorescent than high-intensity discharge lamps because they have a low–pressure, low–intensity discharge source and a linear lamp shape. Also like fluorescents they do not exhibit a bright arc as do other High-intensity discharge (HID) lamps; rather they emit a softer luminous glow, resulting in less glare. Unlike HID lamps, this can go out during a voltage dip, low-pressure sodium lamps restrike to full brightness rapidly. LPS lamps are available with power ratings from 10 W up to 180 W; however, longer bulb lengths create design and engineering problems. Another unique property of LPS lamps is that, unlike other lamp types, they do not decline in lumen output with age. As an example, mercury vapor HID lamps become very dull towards the end of their lives, to the point of being ineffective, while continuing to consume full rated electrical use. LPS lamps, however, do increase energy usage slightly (about 10%) towards their end of life, which is generally around 18,000 hours for modern lamps. An amalgam of metallic sodium and mercury lies at the coolest part of the lamp and provides the sodium and mercury vapor that is needed to draw an arc. The temperature of the amalgam is determined to a great extent by lamp power. The higher the lamp power, the higher will be the amalgam temperature. The higher the temperature of the amalgam, the higher will be the mercury and sodium vapor pressures in the lamp and the higher will be the terminal voltage. A s the temperature rises, the constant current and increasing voltage result in increased power until the nominal power is reached. For a given voltage, there are generally three modes of operation: 1.
The lamp is extinguished and no current flows.
2.
The lamp is operating with liquid amalgam in the tube.
3.
The lamp is operating with all amalgam evaporated.
The first and last states are stable, because the lamp resistance is weakly related to the voltage, but the second state is unstable. Any anomalous increase in current will cause an increase in power, causing an increase in amalgam temperature, which will cause a decrease in resistance, which will cause a further increase in current. This will create a runaway effect, and the lamp will
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jump to the high-current state (#3). Because actual lamps are not designed to handle this much power, this would result in catastrophic failure. Similarly, an anomalous drop in current will drive the lamp to extinction. It is the second state that is the desired operating state of the lamp, because a slow loss of the amalgam over time from a reservoir will have less effect on the characteristics of the lamp than a fully evaporated amalgam. The result is an average lamp life in excess of 20,000 hours. In practical use, the lamp is powered by an AC voltage source in series with an inductive "ballast" in order to supply a nearly constant current to the lamp, rather than a constant voltage, thus assuring stable operation. The ballast is usually inductive rather than simply being resistive to minimize resistive losses. Because the lamp effectively extinguishes at each zero-current point in the AC cycle, the inductive ballast assists in the reignition by providing a voltage spike at the zerocurrent point. The light from the lamp consists of atomic emission lines of mercury and sodium, but is dominated by the sodium D-line emission. This line is extremely pressure (resonance) broadened and is also self-reversed because of absorption in the cooler outer layers of the arc, giving the lamp its improved color rendering characteristics. In addition, the red wing of the D-line emission is further pressure broadened by the Vander Waals forces from the mercury atoms in the arc
UNIT-III HEATING AND WELDING PART-A 8. What are the advantages of electric heating? The main advantages of electric heating over other systems of heating such as coal, oil or gas heating are given below. Economical Cleanliness Absence of flue gases Ease of control or adaptation Automatic protection Upper limit of temperature Special heating features High efficiency of utilization Better working conditions Safety Heating of non-conducting materials 9. Classify the methods of electric heating. Kinds of electric heating A. Power frequency heating a. Resistance heating i) Direct resistance heating
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ii) Indirect resistance heating iii) Infrared or Radiant heating b. Arc heating i) Direct arc heating ii) Indirect arc heating B. High frequency heating a. Induction heating i) Direct induction heating ii) Indirect induction heating b. Dielectric heating 10. What is meant by indirect resistance heating? In this method, the current is passed through a high resistance wire known as heating element. The heat produced due to I2 R loss in the element is transmitted by radiation or convection to the body to be heated. Applications are room heaters, in bimetallic strip used in starters, immersion water heaters and in domestic and commercial cooking and salt bath furnace.
11. What are the properties of heating element material? The material of the heating elements should posses the following desirable properties for efficient operation and long life. High resistivity: It should have high specific resistance so that the overall length to produce a certain amount of heat may be smaller. High melting point: It should have high melting point so that high temperatures can be produced without jeopardizing the life of the element. Free from oxidation: It should be able to resist oxidation at high temperatures, otherwise its life will be shortened. Low temperature coefficient: It should have a low temperature coefficient so that resistance remains appreciably constant even with increases of temperature. This helps in accurate control of temperature. 12. What are the causes of failure of heating elements? Principle causes are Formation of hot spots General oxidation of the element and intermittency of operation Embrittlement caused by grain growth Contamination of element or corrosion 13. Write short note on infrared heating. In radiant heating, the elements are of tungsten operating about 2300°C as at this temperature a greater proportion of infra-red radiation is given off. Heating effect on the charge is greater since the temperature of the heating element is greater than in the case of resistance heating. Heat emission intensities up to 7500 watts/sq.m can be
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obtained leading to heat absorption up to 4300 watts/sq.m. This reduces the time taken by various drying process. 14. What is LASER welding? LASER (Light Amplification by Stimulated Emission of Radiation) welding is a welding process that uses the heat from a laser beam impinging on the joint. The process is without a shielding gas and pressure.
PART-B 4. What are the different methods of electric heating? Describe briefly the methods of direct and indirect resistance heating Electric heating is any process in which electrical energy is converted to heat. Common applications include space heating, cooking, water heating and industrial processes. An electric heater is an electrical device that converts electric current to heat by means of resistors which energy is transferred by conduction, convection or radiation. The heating element inside every electric heater is an electrical resistor, and works on the principle of Joule heating: an electric current passing through a resistor will convert that electrical energy into heat energy. Most modern electric heating devices use nichrome wire as the active element. The heating element, depicted on the right, uses nichrome wire supported by heat resistant, refractory, electrically insulating ceramic. Power frequency Heating a. Résistance Heating Direct Resistance Heating Indirect Resistance Heating Infrared or Radiant Heating b.Arc Heating Direct Arc Heating Indirect Arc Heating High Frequency Heating a. Induction Heating Direct Induction Heating Indirect Induction Heating b.Dielectric Heating Resistance Heating: This method is based upon the I2R loss. Whenever current is passed through a resistive Material heat is produced because of I2 R loss. There are two methods of resistance heating. They are i)Direct Resistance Heating
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In this method of heating the material or change to be heated is taken as a resistance and current is passed through it.The charge may be in the form of powder pieces or liquid. The two electrodes are immersed in the charge and connected to the supply. In case of D.C or single phase A.C two electrodes are required but there will be the electrodes in case of three phase supply. When metal pieces are to be heated a powder of high resistivity material is sprinkle over the surface of the charge to avoid direct short circuit.But itives uniform heat and high temperature. One of the major application of the process is salt bath urnaces having an operating temperature between 500˚C to 1400˚C. Indirect Resistance Heating Indirect resistance heating involves passing an electric current through a resistance heating element that transfers heat to the material by radiation, convection or conduction. Indirectresistance heating can be accomplished by placing material into a well -insulated furnace with resistance heating elements in the walls. The inside surface of the furnace is lined with heat resisting brick, ceramic, or fiber batting. The atmosphere may be air, inert gas, or a vacuum, depending on the requirements of the application.
Infrared or Radiant Heating Infrared (IR) or Radiant Heating Review Infrared (IR) or Radiant Heating is a form of heat energy transfer from an Infrared radiant energy source. Infrared radiant heating may be generated from an infrared lamp, sunlight, an electrical wire or element or a flame heat source. Radiant heating energy is in the form of electromagnetic waves. Infrared radiation (IR) has a wavelength that is longer than visible light, however shorter than microwaves. Infrared radiation wavelengths are between approximately 750 nm and 1 mm and spans over five orders of magnitude. The quantity of radiant energy is determined by the radiant power (or flux) with respect to time. The standard or SI unit for radiant energy is the joule. Another way to understand radiant infrared energy is that warming effect given by one standing in sunlight on a cold day. The reason one feels warmth is that the sun i s a infrared energy source. The infrared energy travels through space and converts to heat energy when absorbed by you and surrounding objects. Every object within the universe that has a temperature above absolute zero emits infrared energy. The hotter or higher an objects temperature above absolute zero, the more the object emits
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infrared energy. In industrial applications, such as heaters, the IR source typically operates at temperatures between 550 ° F 4250° F. The radiant energy output increases with the temperature of the IR source. Infrared is classified into and described in terms of three wavelengths: short wave, medium wave, and long wave. Keep in mind, that for a object to be heated by infrared energy, it must absorb the energy. An objects emissivity is factor from 0 to 1 that defines how well an object or material will absorb infrared energy. One of the greatest advantages of Infrared is the ability to heat materials faster than convection systems (heated air) Arc Heating An arc is produced between two electrodes develops high temperature about 3000 C to 3500 C Depending on the material
5. Draw and explain the induction and dielectric heating Types of Induction furnaces Core type a) Direct b) Indirect c) Vertical Coreless type Vertical type Induction furnace
The Ajax-Wyatt furnace is an improved version of the direct core type furnace and overcomes some difficulties mentioned .Since it is a vertical core type furnace the tendency of the current to interrupt the secondary circuit due to pinch effect is avoided due to weight of the charge in the main body of the crucible The circulation of a molten metal is kept up round the vee portion by convection currents and by electromagnetic forces between the currents in the two halves of the vee Vee must kept full of charge in order to maintain the continuity of the secondary circuit. For this reason this furnace id useful for continuous operation.
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Advantages Better power factor No changes of pinch effect due to heavy weight Highly efficient heat Accurate temperature control Indirect type Induction furnace
In this type an inductively heated element is employed to transmit the heat to charge by radiation The secondary forms the wall of the metal container and the iron core links the primary and secondary winding The AB part of the magnetic circuit is situated in the oven chamber and made from a special alloy which loses its magnetic properties at a particular temperature and regains then when cooled approximately to the same temperature The oven temperature is limited to the critical value without the use of external control equipment
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Coreless type Induction furnace
The furnace consist of ceramic crucible cylindrical in shape enclosed with in a coil with forms a primary of transformer and charge in the crucible forms the secondary of the transformer The flux produced by the primary winding setup eddy current in the charge which flow concentrically with those in the primary winding. These currents heat up the charge to the melting point and provide stirring action to the charge The charge to be in molten state at the start as was required in the case of direct type furnace .The crucible and coil are relatively light in construction and could be convenientl y titled for pouring Since the frequency of the supply is very high , the skin effect in the primary coil increases the effective resistance of the coil and hence copper loss is to be high and cooling arrangement necessary The coil is made up of hollow copper conductor through which the cooling water can be circulated ,the stray magnetic field due to the current in the primary coil may induce eddy current in the metal supporting structures and cause overheating Advantages Precise control of power Fast in operation No dust and smoke Erection and operation cost is low Intermittent operation is possible Applications Steel production Melting of non ferrous metal
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Unit-IV Part-A 15. Define Solar Constant SOLAR CONSTANT 1360 W/m² 16. How will you estimate the average solar radiation?
Sabbah Method. Paltridge method. 17. Define Sun declination angle. Sun declination angle, δ, is defined to be that angle made between a ray of the Sun, when extended to the center of the earth, O, and the equatorial plane. We take δ to be positively oriented whenever the Sun’s rays reach O by passing through the Northern hemisphere. 18. What is Vernal and Autumnal Equinox? There are two occasions throughout the year when the center of the Earth lies in the plane of the Sun. Since the Earth’s North – South axis of rotation is perpendicular to this plane, it follows that on these two days every location on the Earth receives 12 hours of sunshine. These two events are known as the vernal and autumnal equinoxes. 19. Define Solar Noon . It is that time of day at which the Sun’s rays are directed perpendicular to a given line of longitude. Thus, solar noon occurs at the same instant for all locations along any common line of longitude. 20. List the physical principles of the conversion of solar radiation into heat. Absorption Emission reflection transmission 21. List the Advantages of Concentrating Solar Collectors. The heat delivered by concentrating solar collectors is available at much higher temperatures. Higher temperatures allow the use of power generation equipment to produce both electricity and heat. Large economy-of-scale effects are observed when moving toward large concentration systems, rendering such technology very cost-effective (compared with PV for example)
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Part-B
6. Briefly discuss about solar radiation at the Earth’s surface.
The sun is the central energy producer of our solar system. It has the form of a ball and nuclear fusion take place continuously in its centre. A small fraction of the energy produced in the sun hits the earth and makes life possible on our planet. Solar radiation drives all natural cycles and process es such as rain, wind, photosynthesis, ocean currents and several others, which are important for life. The whole world energy need has been based from the very beginning on solar energy. All fossil fuels (oil, gas, coal) are converted solar energy. The radiation intensity of the ca 6000°C solar surface corresponds to 70 000 to 80 000 kW/m2 . Our planet receives only a very small portion of this energy. In spite of this, the incoming solar radiation energy in a year is some 200 000 000 billion kWh; this is more than 10 000 times the yearly energy need of the whole world. The solar radiation intensity outside the atmosphere is in average 1 360 W/m2 (solar constant). When the solar radiation penetrates through the atmosphere some of the radiation is lost so that on a clear sky sunny day in summer between 800 to 1 000 W/m2 (global radiation) can be obtained on the ground. SOLAR CONSTANT 1360 W/m². The duration of the sunshine as well as its intensity is dependent on the time of the year, weather conditions and naturally also on the geographical location. The amount of yearly global radiation on a horizontal surface may thus reach in the sun belt regions over 2 200 kWh/m2 . In north Europe, the maximum values are 1 100 kWh/m2 . The global radiation consits of direct and diffuse radiation. Direct solar radiation is the component, which comes from the direction of the sun. The diffuse radiation component is created when the direct solar rays are scattered from the different molecules and particles in the atmosphere into all directions, i.e. the radiation becomes unbeamed. The amount of diffuse radiation is dependent on the climatic and geographic conditions.
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7. With suitable diagrams explain about Solar radiationGeometry. The Earth’s daily rotation about the axis through its two celestial poles (North andSouth) is perpendicular to the equator,but it is not perpendicular to the plane ofthe Earth’s orbit. In fact, the measure of tilt or obliquity of the Earth’s axis to aline perpendicular to the plane of its orbitis currently about 23.5°.We call the plane parallel to the Earth’s celestial equator and through the centerof the sun the plane of the Sun. The Earth passes alternately above and below this plane making one completeelliptic cycle every year.
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Summer Solstice On the occasion of the summer solstice, the Sun shines down most directly on the Tropic of Cancer in the northern hemisphere, making an angle δ = +23.5° with the equatorial plane. In general, the Sun declination angle, δ, is defined to be that angle made between a ray of the Sun, when extended to the center of the earth, O, and the equatorial plane. We take δ to be positively oriented whenever the Sun’s rays reach O by passing through the Northern hemisphere. On the day of the summer solstice, the sun is above the horizon for the longest period of time in the northern hemisphere. Hence, it is the longest day for daylight there. Conversely, the Sun remains below the horizon at all points within the Antarctic Circle on this day.
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Winter Solstice On the day of the winter solstice, the smallest portion of the northern hemisphere is exposed to the Sun and the Sun is above the horizon for the shortest period of time there. In fact, the Sun remains below the horizon everywhere within the Arctic Circle on this day. The Sun shines down most directly on the tropic of Capricorn in the southern hemisphere on the occasion of the winter solstice. Vernal and Autumnal Equinox There are two occasions throughout the year when the center of the Earth lies in the plane of the Sun. Since the Earth’s North – South axis of rotation is perpendicular to this plane, it follows that on these two days every location on the Earth receives 12 hours of sunshine. These two events are known as the vernal and autumnal equinoxes. • The Earth is above the plane of the Sun during its motion from the autumnal equinox to winter solstice to vernal equinox. Hence, δ < 0 during the fall and winter. • The Earth is below the plane of the sun as it moves from vernal equinox to summer solstice and back to autumnal equinox (i.e. during spring and summer). So δ > 0 during these seasons. The latitude of a location on the Earth is the angle between the line joining that location to the center of the earth and the equatorial plane. For example, Chicago, Illinois is on the circle of latitude 41.8° N. All locations at the same latitude experience the same geometric relationship with the sun. The great semicircles along the surface of the Earthjoining the North to the South poles are called lines of longitude.
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UNIT-V Part-A 22. What is the cause of wind? Wind is caused by uneven heating and cooling of the earth’s surface and by the earth’s rotation. 23. Define Horizontal Axis Turbines (HAWT). Horizontal axis turbines are the most common turbine configuration used today. They consist of a tall tower, atop which sits a fan-like rotor that faces into or away from the wind, the generator, the controller, and other components. Most horizontal axis turbines built today are two or three-bladed, although some have fewer or more blades. 24. Define Vertical Axis Turbines (VAWT). Vertical axis turbines fall into two major categories: Savonius and Darrieus. Neither turbine type is in wide use today.
25. Define Lift. When the wind blows, a pocket of low-pressure air forms on the downwind side of the blade. The low-pressure air pocket then pulls the blade toward it, causing the rotor to turn. This is called lift.
Part-B 8.
Explain in brief about working of Wind energy system.
Wind energy is an abundant resource in comparison with other renewable resources. Moreover, unlike the solar energy, the utilization could not be affected by the climate and weather. Wind turbine invented by engineers in order to extract the energy from the wind. Because the energy in the wind is converted to electric energy, the machine also called wind generator. A wind turbine consists of several main parts, i.e., the rotor, generator, driven chain, control system and so on. The rotor is driven by the wind and rotates at predefined speed in terms of the wind speed, so that the generator can produce electric energy output under the regulation of the control system. In order to extract the maximum kinetic energy from wind, researcher put much effort on the design of effective blade geometry. In the early stage, the airfoil of helicopters were used in 2 wind turbine blade design, but now, many specialized airfoils have been invented and used for wind turbine blade design. working of a wind energy system Wind is the natural movement of air across the land or sea. Wind is caused by uneven heating and cooling of the earth’s surface and by the earth’s rotation. Land
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and water areas absorb and release different amount of heat received from the sun. As warm air rises, cooler air rushes in to take its place, causing local winds. The rotation of the earth changes the direction of the flow of air. This produces prevailing winds, including the Caribbean’s trade winds. Surface features such as mountai ns and valleys can change the direction and speed of prevailing winds. Wind energy uses the energy in the wind for practical purposes like generating electricity, charging batteries, pumping water, or grinding grain. Large, modern wind turbines operate together in wind farms to produce electricity for utilities. Homeowners and remote villages to help meet energy needs use small turbines. Throughout history people have harnessed the wind. Over 5,000 years ago, the ancient Egyptians used wind power to sail the their ships on the Nile River.
American colonists used windmills to grind wheat and corn, pump water, and cut wood. When power lines began to transport electricity to rural areas in the 1930s, the electric windmills were used less and less. Then in the early 1970s, oil shortages created an environment eager for alternative energy sources, paving the way for the re -entry of the electric windmill on the American landscape. WIND ENERGY BASICS
Wind turbines capture the wind's energy with two or three propeller-like blades, which are mounted on a rotor, to generate electricity. The turbines sit high atop towers, taking advantage of the stronger and less turbulent wind at 100 feet (30 meters) or more aboveground. A blade acts much like an airplane wing. When the wind blows, a pocket of lowpressure air forms on the downwind side of the blade. The low-pressure air pocket then pulls the blade toward it, causing the rotor to turn. This is called lift. The force of the lift is actually much stronger than the wind's force against the front side of the blade, which is called drag. The combination of lift and drag causes the rotor to spin like a propeller, and the turning shaft spins a generator to make electricity. Wind turbines can be used as stand-alone applications, or they can be connected to a utility power grid or even combined with a photovoltaic (solar cell) system. Stand-alone turbines are typically used for water pumping or communications. However, homeowners and farmers in windy areas can also use turbines to generate electricity. For utility-scale sources
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of wind energy, a large number of turbines are usually built close together to form a wind farm. Several electricity providers today use wind farms to supply power to their customers. 8.A. Explain with suitable diagram about the wind energy technologies and advantages of wind energy. Modern wind turbines are divided into two major categories: horizontal axis turbines and vertical axis turbines. Old-fashioned windmills are still seen in many rural areas. Horizontal Axis Turbines (HAWT) Horizontal axis turbines are the most common turbine configuration used today. They consist of a tall tower, atop which sits a fan-like rotor that faces into or away from the wind, the generator, the controller, and other components. Most horizontal axis turbines built today are two or three-bladed, although some have fewer or more blades. Vertical Axis Turbines (VAWT) Vertical axis turbines fall into two major categories: Savonius and Darrieus. Neither turbine type is in wide use today. Darrieus Turbines The Darrieus turbine was invented in France in the 1920s. Often described as looking like an eggbeater, this vertical axis turbine has vertical blades that rotate into and out of the wind. Using aerodynamic lift, these turbines can capture more energy than drag devices. The Giromill and cycloturbine are variants on the Darrieus turbine. Savonius Turbines First invented in Finland, the Savonius turbine is S-shaped if viewed from above. This dragtype VAWT turns relatively slowly, but yields a high torque. It is useful for grinding grain, pumping water, and many other tasks, but its slow rotational speeds are not good for generating electricity. The amount of energy produced by a wind machine depends upon the wind speed and the size of the blades in the machine. In general, when the wind speed doubles, the power produced increases eight times. Larger blades capture more wind. As the diameter of the circle formed by the blades doubles, the power increases four times.
Advantages And Disadvantages Of Wind Energy ADVANTAGES: Wind is a renewable energy resource. Wind patterns in the Caribbean provide strong, steady trade winds in specific areas throughout most of the year. Wind power can be used with battery storage or pumped hydro-energy storage systems to provide a steady flow of energy.
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Used as a “fuel,” wind is free and non-polluting, producing no emissions or chemical wastes. Use of wind power as a source of electricity will help reduce the Territory’s Complete dependence on fossil fuels. Wind farms can be combined with agricultural activities such as cattle grazing. DISADVANTAGES: Wind machines must be located where strong, dependable winds are available most of the time. Because winds do not blow strongly enough to produce power all the time, energy from wind machines are considered “intermittent,” that is, it comes and goes. Therefore, electricity from wind machines must have a back-up supply from another source. Wind towers and turbine blades are subject to damage from high winds and lightning. Rotating parts, which are located high off the ground, can be difficult and expensive to repair. The environmental drawback may be a wind farm’s effect on native bird populations, as well as its visual impact on the surrounding landscape. To some, the blades are an eyesore; to others, they’re a beautiful alternative to conventional power plants.
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