SOLAR ENERGY
LESSON 1
The Physics Of Sun:
The sun is a hydro dynamic sphere of intensely hot ionized gaseous matter (plasma) continuously generating energy by thermo nuclear fusion reactions. Most of the energy is generated by the fusion of hydrogen to helium. This energy produced in the core is transferred to the surface and subsequently radiated to space.
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The energy incident on the earth outside the earth is called extraterrestrial radiation .Extraterrestrial solar radiation is all of the beam radiation type also called solar radiation .Hence the beam radiation received on from sun on earth is almost parallel .The radiation coming from earth is equivalent to that coming from a black surface at 5762 k. This extra terrestrial solar radiation intensity that is nearly constant is called solar constant.The solar constant (Isc) is the energy from the sun, per unit time area , received on a unit surface area, perpendicular to the direction of propagation of the radiation and at the earths mean distance from the sun ,outside the earths temperature. The solar constant has a
universally accepted value 1367 ( W / m2 )
The distance between the earth and the sun varies a little through out the year. Because of this radiation the extraterrestrial flux(outside the earth’s temperature)also varies. The earth is the closest to the sun in the summer an farthest in the winter. This variation in distance produces a nearly sinusoidal variation in the intensity of solar radiation I that reaches the earth. This can be approximated by the equation
Where n is the day of the year
At the amount of energy is spread out over about 70,000 million square kilometers on the earth, this energy is obtained from the sun which is at a distance of 150 million kilometers from the earth . Due to this large distance the intensity of energy is reduced to 1.3 kilowatts per square meter at the top of the atmosphere (on surface directly facing the sun). On the surface of the earth (figure 10) this energy is further reduced to one kilowatt per hour by the atmosphere. This reduction is due to absorption and scattering by dust and water vapor present in the atmosphere. On the Clearest day at least 10 % of the Sun’s energy comes from scattered light. This energy cannot be focused using collectors, but can be absorbed by flat surfaces.
Atmospheric reduction of Sunlight.
The amount of radiation collected will depend on the angle of the surface
relative to the direction of the Sun
