ABSTRACT
The amount of sunshine falling on a photovoltaic array depends not only on the strength of the sunlight, but on the orientation of the array, especially in relation to the position of the sun. Before discussing the solar resource, therefore, some basic terms should be defined. As the sun moves across the sky, light rays strike the array from different directions. Sunlight is scattered in the atmosphere and reflected by the ground; the light striking a photovoltaic array comes not just from the direction of the sun, but from other directions as well. Like the length of day, the sun’s trajectory across the sky changes from season to season, with the change being more pronounced at higher latitudes. As sunlight passes through the earth’s atmosphere, absorption of certain wavelengths modifies the spectrum. Sunshine, the fuel of every Photovoltaic system, is different at the high altitudes and high latitudes where cold climates occur.
