Both inverter quality and the long-term performance of the DC side of a PV installation are crucial factors for a PV system’s ability to deliver high energy yields in a sustained fashion. In order to obtain optimal baseline conditions for the long-term degeneration to which PV installations are unavoidably subject, it is essential that modules with the lowest possible tolerances be used (e. g. minus tolerance 0% or ±3%, instead of the ±10% or even higher values obtained in the past). Low power tolerances also help to reduce solar generator mismatch loss (see Section 4.4).
Figure 4.89 Soiling (from bird droppings) of a framed horizontal Siemens M55 module with a 30° tilt angle. By looking carefully at the bottom edge of the module, it is possible also to see the striations of grit that typically form over time on installations of this type with flat tilt angles
The following are the main long-term problems that have been observed on the DC side:
• Soiling, notably on the edges of framed modules.
• Increasing local shading induced by tree growth.
• Ageing-induced degradation.
• String failure resulting from faulty string diodes, fuses or the like.
• Faults in modules or in generator junction boxes.
• Snow covering, and possible mechanical damage from snow pressure.