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Solar Radiation (2)

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  Solar Radiation and its Measurement Introduction In general, the energy produced and radiated by the sun, more specifically the term refers to the sun‟s energy that reaches the earth. Solar energy, received in the form of radiation, can be converted directly or indirectly into other forms of energy, such as heat and electricity, which can be utilized by man. Since the sun is expected to radiate at an essentially constant rate for a few billion years, it may be regarded as an in-exhaustible source of useful energy. The major drawbacks to the extensive application of solar energy are: 1. The intermittent and variable manner in which it arrives at the earth‟s surface an d 2. The large area required to collect the energy at a useful rate. Experiments are underway to use this energy for power production, house heating, air conditioning, cooking and high temperature melting of metals. Energy is radiated by the sun as electromagnetic waves of which 99 per cent have wave lengths in the range of 0.2 to 4.0 micrometers (1 micrometer = 10 meter). Solar energy reachi ng the top of the earth‟s atmosphere consists of about 8 per cent ultraviolet radiation (short wave length, less than 0.39 micrometer), 46 per cent visible light (0.39 to 0.78 micrometer), and 46 per cent infrared radiation (long wave length more than 0.78 micrometer). SOLAR CONSTANT It is defined as the rate of solar energy arriving the top of the atmosphere, denoted by I SC . It is the amount of energy received in unit time area perpendicular to the sun‟s direction at the mean distance of the earth from the sun. Since sun‟s activity and distance vary throughout the year, solar energy also varies. I SC = 1.353 kW/m 2 INTENSITY OF SOLAR RADIATION Since the distance between sun and earth varies the extra-terrestrial flux also varies. Earth is closest to the sun in the summer and farthest away in the winter. This variation in distance  produces a nearly sinusoidal variation in the intensity of solar radiation that reaches the earth. I = I SC [1 + 0.033 cos (360n/365)] SOLAR RADIATION AT EARTH’S SURFACE   The so lar radiation that penetrates the earth‟s atmosphere and reaches the surface differs in  both amount and character from radiation at the top of the atmosphere. The radiation entering the atmosphere is partly absorbed by molecules, and a part of the radiation is reflected back into the space by clouds. Part of the solar radiation is scattered by droplets in clouds by atmospheric molecules and dust particles. Oxygen and ozone absorb nearly all the ultraviolet radiation whereas CO 2  and H 2 O vapour absorb some energy from infrared range.    (a)   Direct radiation or beam radiation Solar radiation that has not been absorbed or scattered and reaches the ground directly from the sun is called “direct radiation” or “beam radiation”.  It is the radiation which  produces a shadow when interrupted by an opaque object. (b)   Diffuse radiation It is the solar radiation received from the sun after its direction has been changed by reflection and scattering by the atmosphere. Since the solar radiation is scattered in all directions in the atmosphere, diffuse radiation comes to the earth from all parts of the sky. (c)   Insolation It is the total solar radiation received at any point on the earth‟s surface . It is the sum of the direct and diffuse radiation. More specifically insolation is defined as the total solar radiation energy received on a horizontal surface of unit area on the ground in unit time. The insolation at a given point or location on the earth‟s surface depends among other factors, on the altitude of the sun in the sky. As a result of absorption and scattering, the insolation is less when the sun is low in the sky than when it is higher. On a clear, cloudless day, about 10 to 20 percent of the insolation is from diffuse radiation, the proportion increases upto 100% when the sun is completely obscured by clouds. In the above figure, part of the radiation is reflected back into the space, especially by clouds. Furthermore, the radiation entering the atmosphere is partly absorbed by molecules in the air. Oxygen and ozone (O 3 ), formed from oxygen, absorb nearly all the ultraviolet radiation, and water vapour and carbon dioxide absorb some of the energy in the infrared range. In addition  part of the solar radiation is scattered by droplets in clouds by atmospheric molecules, and by dust particles. SOME DEFINITIONS 1.   Sun at zenith It is the position of the sun directly over head. 2.   Air mass It is the path length of radiation through the atmosphere, considering the vertical path at sea level as unity. Direct, diffuse and total radiation  The air mass (m) is the ratio of the path of sun‟s  rays through the atmosphere to the length of the path when sun is at zenith. At sea level, m=1 m=1, when sun is at zenith m=2 when zenith angle is 60 o   m= sec θ z  when m>3 Air mass = cos (altitude angle) except for very low solar altitude angles. 3.   SOLAR ANGLES Let   = Angle between an incident beam radiation I and the normal to the plane surface. Then, radiation intensity normal to the surface is I = I cos   Where   = Incident angle i)   Latitude,  l It is the angle made by the radial line joining the location to the centre of earth with the  projection of the line on the equatorial plane, denoted by  l . It is also given by the angular distance north or south of the equator measured from the centre of the earth. If P is the location on the earth‟s surface and O is the centre of the earth, the  l is given by the angle between the line OP and projection of OP on the equatorial plane. As a method of convention, the latitude will be measured as +ve for the northern hemisphere. ii ) Declination (  )   It is the angular distance of sun‟s rays north or south of the equator. It is the angle between the line extending from the centre of the sun to the centre of the earth and the projection of this line upon the earth‟s equatorial plane. Declination varies between 23.5 o on June 22 to 23.5 o on December 22. Fig. 2.4.1. Latitude , hour angle w, and sun’s  declination   Variation of sun‟s declination    The declination in degrees for any given day may be given by Cooper‟s equation.     (in degrees) = 23.45      where n is the day of the year e.g.: March 22 is the 31 + 29 + 21 = 81 st day   n = 81 iii) Hour angle (  ) It is the angle through which the earth must turn to bring the meridian of a point directly in line with the sun‟s rays. The hour angle is equivalent to 15 o  per hour. It is measured from noon based on the solar local time (LST) or local apparent time, being  positive in the morning and negative in the afternoon. It is the angle measured in the earth‟s equatorial plane, between the projection of OP and the projection of O line from the centre of the sun to the centre of the earth. Note: Local Solar Time(LST) - It is used to calculate the hour angle in all the equations above, which does not coincide with the local clock time.solar time can be obtained from the standard time observed by applying two corrections. The first correction arises because of the longitude between the location and the meridian on which the standard time is based. The correction has 4 minutes for every degree difference in longitude. The second correction is called equation of time correction . it is due to the fact that earths orbit and rate of rotation are subject to small perturbations. Thus, LST= Standard Time +/- 4(standard time longitude –   longitude of location) + (equation of time correction)  Negative sign is applicable for eastern hemisphere iv) Altitude angle (  ) It is the vertical angle between the projection of the sun‟s rays on the horizontal plane and the direction of sun‟s rays ( passing through the point).   v) Zenith angle (  Z ) It is the angle between the sun‟s rays and a line perpendicular to the horizontal plane through the point P. i.e., the angle between the beam from the sun and the vertical. Zenith angle is complimentary angle of sun‟s altitude angle.  Z =     

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Jul 23, 2017
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