Now that we have introduced the solar cell, it is time to introduce the source of the energy - the sun. The sun has many properties that could be discussed at length. For example, the color temperature of the light, the nuclear (fusion) processes that occur within the sun, or the geometry of Earth and the sun that establishes the size of the solar disk as viewed from Earth. However, for the purpose of solar cell studies, two parameters are most important: the irradianceâ - that is, the amount of power incident on a surface per unit area - and the spectral characteristics of the light. The irradiance value outside Earth's atmosphere is called the solar constant, and is 1365W/m 2 . After being filtered through Earth's atmosphere, several portions of the solar spectrum diminish, and peak solar irradiance is lowered to approximately 1000 W/m 2 . This is the typical irradiance on a surface, or plane, perpendicular to the sun's rays on a sunny day. If one were to track the sun for eight hours, the average daily solar irradiance would be approximately 1000(8/24)=333 W/m 2 . On a fixed (nontracking) surface, the typical values in sunny locations range between 180-270 W/m 2 .
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