Page 92 - Applied Photovoltaics

5.5 HOT-SPOT HEATING
Mismatched cells within a module can result in some cells generating and some
dissipating power. In the worst case, the whole output of the ‘good’ cells can be
dissipated in the ‘bad’ cell when the module or string of modules is short-circuited.
Fig. 5.8 shows a string of cells with one bad cell and Fig. 5.9 shows the impact of the
bad cell on module output. The bad cell becomes reverse biased to the voltage-
generating capability of the good cells, as shown in Fig. 5.10.

n ‘good’ cells 1 ‘bad’ cell
(cracked, shaded,
bird droppings etc.)
Figure 5.8. One ‘bad’ cell in a string, which reduces the current through the ‘good’
cells, leading to them producing higher voltages that can often reverse-bias the
bad cell.

Figure 5.9. The impact on module output of a ‘bad’ cell in a string of ‘good’ cells
(after Ross & Smokler, 1986).

+ –– +

Figure 5.10. Reverse biasing of the ‘worst’ cell in a string. This occurs as the
‘good’ cells try to drive a higher current through the ‘bad’ cell than it can
accommodate, even when short-circuited.

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