Page 104 - Applied Photovoltaics

‘interconnect’
busbar
cracked
cell

Figure 5.25. Cracked cell indicating how ‘interconnect’ busbars can help prevent
open circuit failure.

6. Interconnect open circuits and series resistance—Fatigue owing to cyclic
thermal stress and wind loading leads to interconnect open circuit failures and
series resistances can gradually increase with age. As tin-lead alloy solder
bonds age, the solder becomes brittle and separates into grains of lead and tin
with cracks, causing increased resistance.

7. Module open circuits and series resistance—Open circuit failures and
ageing effects also occur in the module structure, typically in the bus wiring
or junction box.
8. Module short circuits—Although each module is tested before sale, module
short circuits are often the result of manufacturing defects. They occur
because of insulation degradation with weathering, resulting in delamination,
cracking or electrochemical corrosion.

9. Module glass breakage—Shattering of the top glass surface can occur
because of vandalism, thermal stress, handling, wind or hail. Roofing gravel
has been found to cause fractures in relatively low wind speeds after being
blown up the surfaces of tilted roof-mounted modules and then landing on the
next row of modules at near normal incidence (King et al., 2000).

10. Module delamination—This was a common failure mode in early
generations of modules, but is now less of a problem. It is usually caused by
reductions in bond strength, either environmentally-induced by moisture or
photothermal aging, or stress induced, by differential thermal and humidity
expansion. It has been more frequently observed in hot and humid climates.
Moisture migration through the encapsulant, sunlight and heat facilitate
chemical reactions leading to delamination.

11. Hot-spot failures—Mismatched, cracked or shaded cells can lead to hot-spot
failures, as discussed in Section 5.5.

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