A. Insolation and Its Characteristics
The energy radiated by the naturally existing fusion reactor we call the Sun is the input to PV systems. Even though the rate of energy radiated by the sun is 389 septillion (389 x 1024) W, only a fraction of this is intercepted by the earth-atmosphere system, with an average value of 1370 W/m2 at the outer atmosphere. This is called the solar constant and it is equivalent to 1.963 Langleys/minute or 1.963 calories/cm2/minute. After accounting for reflection and transmission losses through the atmosphere, the insolation (incident solar radiation) reaching the earth’s surface amounts roughly to one kW/m2 on a clear afternoon near the equator. Insolation has been studied for many years and investigations are still being pursued worldwide. Several factors influence the level of insolation reaching the earth’s surface at a specific location. The major ones are: geographic location of the site, orientation, time of day, season of the year, sun-earth relative motion, and atmospheric conditions. Insolation consists of two components: direct or beam radiation and diffuse or scattered radiation. Total or global radiation refers to the sum of these two components.