. Outside the Earth's atmosphere, the incident power on a unit surface, perpendicular to the solar rays, assumes a value of about 1360W / m² (variability of ± 3% due to the altitude of the orbit of the earth), this value is called Solar Constant. On the Earth's surface, at sea level, under optimum weather conditions and sun at noon, the power density is approximately 1000 W / m². Global solar radiation that reaches the surface of the ground is given by the sum of three components:
- Direct radiation affects any surface with a single and well-defined incidence angle.
- The diffused radiation affects this surface with various angles (PV devices can also operate in the presence of diffuse radiation only)
- An inclined surface may also receive reflected radiation from the ground or from mirrors of water or other surfaces (e.g. adjacent buildings walls). This contribution is called ALBEDO and must be carefully evaluated.
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