Page 341 - A Comprehensive Guide to Solar Energy Systems

346 A CoMPreheNSIVe GuIDe To SolAr eNerGy SySTeMS

FIGURE 16.11 (A) Series connected three PV modules with bypass diodes, and (B) the typical current–voltage and power
characteristics of series connected PV modules with moderate nonuniform shading [1].

curves of three series connected PV modules with moderate nonuniform shading. As it can
be observed in the figure, the impact of shading is reduction in operating current and P max at
higher voltages. Note that shading can produce multiple power peaks that are hard to differ-
entiate from each other with simple MPPT techniques. It was found that even a single shaded
cell could significantly reduce MPP extraction by 33% or more (at module level MPPT).
As stated earlier, shading causes a serious issue in PV modules, hot spots. By blocking
the flow of current and reverse biasing shaded cell, the system goes into thermal runaway or
accelerated degradation. Although bypass diode can protect cell, if bypass diode fails it can
generate severe heating (hot spots) and catastrophic failure. hence, the most effective solu-
tion is to avoid the shading effect at the cell level by integrating power electronics circuits.
Note that multiple cells used in a module will not stay matched and healthy over time
even when a same production batch is used and no components failed. This is due to vari-
ous reasons including differential shading (clouds, leaves, and obstructions) in different
sections, aging effects and direct damage for some reason. PV module defects and failures
increase the risk of reliable delivery of the solar capacity, which can result unplanned re-
placement costs, hence increasing the system’s levelized cost of energy (lCoe).
In PV systems there are a number of possible failure modes (observed characteristics of
failures) both in cell and module levels, which are briefly summarized below.

16.5.2 Hot Spots

hot spots occur due to a result of a structural defect in the solar cells or in a module or due
to poor soldering joints, which can all lead to an internal short-circuit, reduced perfor-
mance, partial or catastrophic failure, and burning.

16.5.3 Micro-cracks
Micro-cracks are virtually invisible microscopic level tears in the solar cells, which can oc-
cur during production of PV modules or during shipping or handling practices of panels.