Page 95 - Applied Photovoltaics

For silicon solar cells, the maximum group size per diode, without causing damage, is
about 10–15 cells per bypass diode. Hence for a normal 36 cell module, three bypass
diodes are needed to ensure the module will not be vulnerable to hot spot damage.
Not all commercial modules include bypass diodes. If they do not, care must be taken
to ensure the modules are not short-circuited for long periods and that parts of the
module are not shaded by surrounding structures or adjacent arrays. It is possible for
a diode to be integrated into each cell, creating a lower cost means of protecting
individual cells (Green, 1980).

For modules in parallel, as shown in Fig. 5.15, an additional problem, thermal
runaway, can occur when bypass diodes are used—one string of bypass diodes
becomes hotter than the rest, therefore taking up a larger share of the current, hence
becoming even hotter and so on. Diodes should therefore be rated to handle the
parallel current of the module combination.

Figure 5.15. Bypass diodes in paralleled modules.

Standards Australia (2005) gives specifications for non-embedded bypass diodes,
when used. They should be rated to survive twice the open circuit voltage of the
protected module(s) or cell(s) and 1.3 times their short circuit current.
Some modules also include a blocking diode, as shown in Fig. 5.16, to ensure current
only flows out of the module. This prevents, for instance, a battery from discharging
through the module at night.

Blocking diodes in strings are not universally recommended or used because they
waste some of the collected energy (Albers, 2004). When blocking diodes are used, as
for bypass diodes, they should have a voltage rating at least twice the open circuit
array voltage and a current rating 1.3 times the short circuit current at STC of the
protected circuit (Standards Australia, 2005).

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