Sun releases energy continuously by a fusion reaction that produces 3.94 X 1023 kW of power. This is the main source astronomically available in the sun, but it has to cover about 150 X 106 km for the sun radiation to reach the earth. Besides, only a very small fraction of this energy becomes incident on the earth’s surface. The amount of power received by the earth is 1.73 X 1014 kW, which yields to 340 W/m2 if considered uniformly distributed over the earth’s surface.
There are three astronomical effects in the forms of geometrical factors, which determine the seasonal variation of solar radiation incident on the earth and these are shown in Fig. 12. The earth revolves around the sun in an elliptical orbit being closest to the sun, i. e. at perihelion, about 3rd January and farthest from the sun, i. e. aphelion, on July 5. The eccentricity of the present-day orbit is E0 = 0.017.
Fig. 13. Incident solar radiation on earth’s surface.
know the power density, i. e. Watt per meter per minute on the earth’s outer atmosphere and at right angle to the incident radiation. The density defined in this manner is referred to as the solar constant.
In order to appreciate the coming of solar irradiation on the earth’s surface, it is very helpful to simplify the situation as shown in Fig. 13 where the earth is represented as a sphere. This implies that at the equator a horizontal surface at that point immediately under the sun would receive 1360 W/m2. Along the same longitude but at different latitudes the horizontal surface receives smaller solar radiation from the equator towards the polar region. If the earth rotates around the vertical axis to the earth-sun plane, then any point on the earth surface receives the same amount of radiation throughout the year. However, earth rotates around an axis which is inclined with the earth-sun plane, and therefore, the same point receives different amounts of solar irradiation in different days and times in a day throughout the year. Hence, seasons start to play role in the incident solar radiation variation. Additionally, diurnal variations are also effective due to day and night succession. As a result of earth’s rotation around an inclined axis surprisingly polar region receives more radiation in the summer than at the equator. An important feature is the absence of seasons at the tropics and the extremes of six - month summer and six-month winter at the poles . The theoretical and natural occurrence aspects of the solar radiation and its energy have already been explained in the previous sections. However, the practical applications and beneficial use of the solar radiation require consideration of practical and engineering aspects. Here, the efficient and sustainable use of the solar energy comes into view. For instance, in any design of solar energy powered device, it is necessary to know how the power density will vary during the day, from season to season, and also the effect of tilting a collector surface at some angle to the horizontal. From the practical applications point of view, for most purposes solar energy applications can be divided into two components, namely, direct (beam) radiation, and scattered or diffuse radiation.
Direct radiation as the name implies is the amount of solar radiation received at any place on the earth directly from the sun without any disturbances. In practical terms, this is the radiation, which creates sharp shadows of the subjects. There is no interference of dust, gas and cloud or any other intermediate material on the direct solar radiation. However, diffuse radiation is first intercepted by the constituents of the air like water vapor, carbon dioxide, dust, aerosols, clouds, etc., and then released as a scattered radiation in many directions. In other words, direct radiation is practically adsorbed by some inter-mediator, which radiates electromagnetic waves similar to the main source, which is the sun. This is the main reason why diffuse radiation scatters in all directions and close to the earth’s surface as a source does not give rise to sharp shadows. It is obvious than that the relative proportions of direct to diffuse radiation depends on the location, season of the year, elevation from the mean sea level and time of day. On a clear day, the diffuse component will be about 10-20% of the total radiation, but during an overcast day it may reach up to 100%. This point implies practically that in the solar radiation and energy calculations, weather and meteorological conditions must be taken into consideration in addition to the astronomical situation. On the other hand, throughout the year the diffuse solar radiation amount is smaller in the equatorial and tropical regions than the sub-polar and polar regions of the world. The instantaneous total radiation can vary considerably throughout the day as shown in Fig. 14.