Page 101 - Applied Photovoltaics

Thermal expansion is another important temperature effect which must be taken into
account when modules are designed. Fig. 5.22 illustrates the expansion between cells
with temperature increases.
C

glass

interconnect

cell
EVA

Tedlar

D

Figure 5.22. Use of stress relief loops to accommodate expansion between cells
with increases in temperature.

Referring to Fig. 5.22, the spacing between cells tries to increase an amount į given
by
į CĮ Į D Tǻ (5.3)
g c
where Į g , Į c are the expansion coefficients of the glass and cell, respectively, C is the
centre-to-centre distance between cells and D is the cell length.
Typically, interconnections between cells are looped, as shown, to minimise cyclic
stress. Double interconnects are used to protect against the probabilistic nature of
fatigue failure by such stress.
In addition to interconnect stresses, all module interfaces are subject to temperature-
related cyclic stress, which can eventually lead to delamination.

5.9 ELECTRICAL INSULATION
The encapsulation system has to be able to withstand voltage differences at least as
large as the system voltages. Metal frames must also be earthed, except in particular
special circumstances, as internal and terminal potentials can be well above the earth
potential (Standards Australia, 2005). Conditions for which earth leakage safety
devices are required are specified by Standards Australia (2005):
1. Array V oc < 50 V—none required.

2. 50 V < Array V oc < 120 V—earth fault protection on DC side required if
system earthed and isolated, or DC-sensitive residual current device on AC
side if not isolated.

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