The Sun is a sphere of intensely hot gaseous matter with a diameter of 1.39 × 109 m and is, on the average, 1.5 × 1011 m from the earth. As seen from the earth, the sun rotates on its axis about once in every 4 weeks. However, it does not rotate as a solid body; the equator takes about 27 days and the polar regions take about 30 days for each rotation. The sun has an effective black body temperature of 5777 K.
Solar energy is the result of electromagnetic radiation released from the sun by the thermonuclear reactions occurring inside its core. All of the energy resources on earth originate from the sun (directly or indirectly), except for nuclear, tidal, and geothermal energy.
The sun radiates considerable energy onto the earth. Solar radiation intensity, rarely over 950 W/m2 has led to the creation of many types of devices to convert this energy into useful forms, mainly heat and electricity. Radiant light and heat from the sun is harnessed using a range of ever-evolving technologies such as solar heating, PV, CSP, solar architecture, and artificial photosynthesis.
Fig. shows schematically the geometry of the sun–earth relationships. The eccentricity of the earth’s orbit is such that the distance between the sun and the earth varies by 1.7%. At a mean earth–sun distance L = 1.495 × 1011 m, the sun subtends an angle of 32 degree. The radiation emitted by the sun and its spatial relationship to the earth result in a nearly fixed intensity of solar radiation outside of the earth’s atmosphere.
The solar constant ISC signifies the energy from the sun per unit time received on a unit area of surface perpendicular to the direction of propagation of the radiation at mean earth–sun distance outside the atmosphere. The mean value of solar constant is equal to 1368 W/m2. Therefore, considering a global plane area of 1.275 × 1014 m2 and the mean radius of the earth being approximately 6371 km, the total solar radiation transmitted to the earth is 1.74 × 1017 W, whereas the overall energy consumption of the world is approximately 1.84 × 1013 W.
