Hot-spot formation and reduced solar generator output resulting from partial shading of individual solar cells are very real possibilities in PV systems. Such problems can be induced by bird droppings, fallen leaves, or unkempt natural vegetation. Figure 4.89 displays soiling on the individual cells of a solar generator resulting from bird droppings, which are observed relatively frequently in this installation owing to its having been installed below a high antenna mast and substantially reduce the output of the string affected. As a result of the 30° tilt angle, this type of module soiling is removed almost completely by heavy rain.
With the passing years, moss and lichen also often grow on module frames, substantially reducing the output of the modules affected. Figure 4.90 displays moss growing on a fouled module edge, while Figure 4.91 displays a relatively large expanse of lichen growth on a solar cell.