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solar mod.pptx

  2. • Sun – largest member of solar system • Sun – is a sphere of extremely hot gaseous matter with a diameter of 1.39 X 109 m and it maintains a distance of 1.495 X 1011 m from earth. • Sun’s high energy is maintained by enormous nuclear energy, released by the continuous fusion reaction. • The fusion reaction involves four hydrogen atoms combining to form one helium atom (4 x 1H1  2He4 + 26.7 MeV). • The sun as other hot bodies radiates heat energy uniformly in all directions. The radiated heat energy moves out as electromagnetic waves. The radiated heat energy increases the temperature of a body on its interception and absorption. This radiated heat energy from the sun is called solar energy and it provides the energy needed to sustain life in our solar system.
  3. RADIATION SPECTRUM FROM SUN AND EARTH • Sun can be considered for all practical purposes as a blacCk body having high surface temperatureof 6000 K. • The radiation spectrum consists of emission at various wavelengths but more at shorter wavelengths as shown in Figure 2.2(a). • The maximum emissive power of the radiation takes place at wavelength of 0.48 m. • Similarly, the earth can also be considered a black bodyat temperature of 288°C. • The radiation spectrum from the earth consists of emission generally of longer wavelengths as shown in Figure 2.2(b). The maximum emission is taking place at wavelength of about 10 m.
  4. RADIATION SPECTRUM FROM SUN AND EARTH (CONT..) Irradiance • It is the rate at which radiant energy is incidenting on a unit surface area. • It is the measureof power density of sunlight falling per unit area and time. • It is measured in watt per square metre. • Heat energy is measured in joules and while watt or joules per second is unit of power. Irradiation • It is solar energy per unit surface area which is striking a body over a specified time. • Hence it is integration of solar illumination or irradiance over a specified time (usually an hour or kilowatt a day). • It is measured in kilowatt-hour or kilowatt day per square metre. • For example,if irradiance is 20 kW/m2 for 5 h, irradiation is 20 x 5 = 100 kWh/m2
  5. EXTRATERRESTRIAL RADIATION AND SOLAR CONSTANT • The earth revolves around the sun in an elliptical orbit as shown in Figure 2.3. • The earth is closest to the sun on 21 March and 23 September. • The earth is farthest from the sun on 21 June and 22 December. • The mean distance of the earth from the sun is 1.495 x 1011 m • The intensity of solar radiation outside the earth's atmosphere reduces with distance and itis dependent on the distance between the earth and the sun. • In fact, the intensity of solar radiation reaching outside the earth's atmospheric varies with the square of the distance between the centres of the earth and the sun. • This is the reason why earth receives 7% more radiation on 21 March and 23 September as compared to 21 June and 22 December.
  6. EXTRATERRESTRIAL RADIATION AND SOLAR CONSTANT (CONT..) • The intensity of solar radiation keeps on attenuating as earth propagates away from the surface of the sun, but the content of wavelengths in the radiation spectrum does not change. • The earth axis is tilted about 23.45° with respect to earth's orbit around the sun as shownin Figure 2.4. • Owing to this tilting of earth's axis, the northern hemisphere of the earth points towards the sun in the month of June and it points away from the sun in the month of December. • However, earth's axis remains perpendicular to the imaginary line drawn from the earth to the sun during the months of September and March. • The sun-earth's distance varies during earth's rotation around the sun, thereby varying the solar energy reaching its surface during revolution, which brings about seasonal changes. • The northern hemisphere has summer when the earth is tilting forwards the sun and winter when the earth is tilting away from the sun. • In the months of September and March, both the hemispheres are at the same distance from the sun and receive equal sunshine. During the summer, the sun is higher in the sky, while the sun is lower in the sky during winter for the northern hemisphere.
  7. EXTRATERRESTRIAL RADIATION AND SOLAR CONSTANT (CONT..) Extraterrestrial radiation • Solar radiation incident on the outer atmosphere of the earth is called extraterrestrial radiation. The extraterrestrial radiation varies based on the change in sun-earth's distance arising from earth's elliptical orbit of rotation. The extraterrestrial radiation is not affected by changes in atmospheric condition. Solar constant • It is defined as the energy received from the sun per unit time on a unit surface area perpendicular to the direction of propagation of solar radiation at the top of earth's atmosphere when earth is at its mean distance from the sun. The value of solar constant is taken as1367 W/m2. The extraterrestrial radiation can be determined by using solar constant as follows:
  8. EXTRATERRESTRIAL RADIATION AND SOLAR CONSTANT (CONT..) • Terrestrial radiation • When radiation passes through earth's atmosphere, it is subjected to the mechanism of atmospheric absorption and scattering depending on atmospheric conditions. • Earth's atmosphere contains various constituents, suspended dust and solid and liquid particles, such as air molecules, oxygen, nitrogen, carbon dioxide, carbon monoxide, ozone, water vapour and dust. • Therefore, solar radiation or intensity of radiation is depleted during its passage through the atmosphere. • The solar radiation that reaches earth's surface after passing through earth's atmosphere is called terrestrial radiation. • The propagation of solar radiation through earth's atmosphere isshown in Figure 2.5.
  9. EXTRATERRESTRIAL RADIATION AND SOLAR CONSTANT (CONT..) • From extraterrestrial region, the solar radiation reaches earth's surface in two ways: i. a part of sun's radiation travels through earth's atmosphere and its reaches directly, which is called direct or beam radiation, and ii. the remaining major part of the solar radiation is scattered, reflected back into the space or absorbed by earth's atmosphere. A part of this radiation may reach earth's surface. This radiation reaching earth's surface by the mechanism of scattering and reflecting, that is, reradiation, is called diffuse or sky radiation. • The diffuse radiation takes place uniformly in all directions and its intensity does not change with the orientation of the surface. • However, direct or beam radiation depends on the orientation ofthe surface. • The beam radiation depends on the angle of incident on the surface and its intensity is maximum when the solar radiation is falling normal to the surface. • The solar radiation propagating normal to its direction is specified by In .
  10. EXTRATERRESTRIAL RADIATION AND SOLAR CONSTANT (CONT..) Beam radiation • Solar radiation along the line joining the receiving point and the sun is called beam radiation. This is radiation has any unique direction. Diffuse radiation • It is the solar radiation which is scattered by the particles in earth's atmosphere and this radiation does not have any unique direction. Total or global radiation • Total or global radiation at any location on earth's surface is the sum of beam radiation and diffuse radiation. Air mass (m) • The radiation reaching earth's surface depends on (i) atmospheric conditions and depletionand (ii) solar attitude. Air mass is the ratio of the path length through the atmosphere which the solar beam actually traverses up to earth's surface to the vertical path length through the atmosphere (minimum height of terrestrial atmosphere).
  11. EXTRATERRESTRIAL RADIATION AND SOLAR CONSTANT (CONT..) • At sea level, the air mass is unity when the sun is vertically is in the sky (inclinationangle 90°).
  12. LATTITUDE & LONGITUDE • Any location on the earth can be described by two numbers-latitude and longitude. Latitude • On a globe of the earth, lines of latitude are circles of different sizes. The largest one is the circle at equator (circle at equator with centre at earth's centre) whose latitude is taken as zero. • The circles at the poles have latitude of 90° north and 90° south (or -90°) where these circles shrink to a point. • To specify the latitude of some point "P" on the earth's surface, draw the radius OP to the point P from centre O. • The elevation angle () of the point P above the equator is called latitude  as shown in the Figure 2.7(a). • There are 180 circles, of which 90 in each hemisphere specify the latitude of various points on earth's surface as shown inFigure 2.7(b). • Each degree of latitude is about 111 km apart. Latitude lines run horizontally and these are parallel.
  13. LATTITUDE & LONGITUDE Longitude • On the globe, vertical lines of constant longitude (meridians) extend from pole to pole similarto the segment boundaries on peeled orange. • Every meridian has to cross the equator and equator is a circle. Like any circle, it has 360 degrees or divisions. • Hence, longitude of a point is the marked value of that division where its meridian meets the equator circle. • The meridian passing through the Royal Astronomical Observatory at Greenwich, UK had been chosen as zero longitude. • The meridian passing through this location is called prime meridian. • The prime meridian or longitude is considered zero longitude and there are 180 longitude lines or degrees at cast (+180°) and 180 degrees at west (-180°) of Greenwich. • The longitude lines meet at poles and these have wide separation at the equator (about 111 km). • The longitudinal lines are shown in Figure 2.8. Solar noon is the time when the sun is at the longitude of the place
  14. BASIC SUN-EARTH ANGLES Latitude or angle of latitude () • The latitude of a location on earth's surface is the angle made by the radial line joining the specified location to the centre of earth with the projection of this line on the equatorial plane as shown in Figure 2.9. The latitude at equator is zero and and it is 90° at poles. Declination angle () • It is the angle made by the line joining the centres of sun and earth with the equatorial plane as shown in Figure 2.10 • The angle of declination varies when earth revolves around the sun. • It has maximum value of 23.45° when earth achieves a position in its orbit corresponding to 21 June and it has minimum value of -23.45° when earth is in orbital position corresponding to 22 December.
  15. BASIC SUN-EARTH ANGLES (CONT..) • The angle of declination is taken positive when it is measured above the equatorial plane in the northern hemisphere. • The angle of declination can be given by: Hour angle () • The hour angle at any instant is the angle through which the earth has to turn to bring the meridian of the observer directly in line with sun's rays. • It is an angular measure of time. • It is the angle in degree traced by the sun in 1 h with reference to 12 noon of the location. • The convention of measuring it is that the noon-based calculated local apparent time (LAT) is positive in afternoon and negative in forenoon as shown in Figure 2.11. • The earth completes one rotation (360°) in 24 h. • Hence, 1 h corresponds to 15° of earth's rotation. • As at solar noon the sun rays is in line with local meridian or longitude, the hour angle at that instant is zero. The hour angle can be given as follows:
  16. BASIC SUN-EARTH ANGLES (CONT..) Inclination or altitude angle () • It is the angle between sun's ray and its projection on horizontal surface as shown in Figure 2.12. Zenith angle (z) • It is the angle between sun's ray and normal to the horizontal plane as shown in Figure 2.12. Solar azimuth angle (s) • It is the angle between the projection of sun's ray to the point on the horizontal plane and line due south passing through that point. • The value of the azimuth angle is taken positivewhen it is measure form south towards west.
  17. BASIC SUN-EARTH ANGLES (CONT..) Angle of incidence () The angle of incidence for any surface is defined as the angle formed between the direction of the sun ray and the line normal to the surface as shown in the Figure 2.13. Tilt or slope angle () • The tilt angle is the angle between the inclined slope and the horizontal plane as shown in the Figure 2.13. Surface azimuth angle () • It is the angle in horizontal plane between the line due south and the horizontal projection of normal to the inclined plane surface. It is taken as positive when measured form southtowards west.