Civil Engineering Reference
In-Depth Information
FIGURE 8-4
Construction of pv module: 1) frame, 2) weatherproof junction box, 3) rating plate, 4) weather
protection for 30-year life, 5) pv cell, 6) tempered high transmissivity coverglass, 7) outside
electrical bus, 8) frame clearance. (Source: Solarex Corporation, Frederick, Maryland, With
permission.)
The output-terminal current I is equal to the light-generated current I
, less
L
the diode-current I
and the shunt-leakage current I
. The series resistance
d
sh
represents the internal resistance to the current flow, and depends on the
p-n junction depth, the impurities and the contact resistance. The shunt
resistance R
R
s
is inversely related with leakage current to the ground. In an
ideal pv cell, R
sh
(no leakage to ground). In
a typical high quality one square inch silicon cell, R
= 0 (no series loss), and R
=
∞
s
sh
= 0.05 to 0.10 ohm and
s
R
= 200 to 300 ohms. The pv conversion efficiency is sensitive to small
variations in R
sh
, but is insensitive to variations in R
. A small increase in R
s
sh
s
can decrease the pv output significantly.
In the equivalent circuit, the current delivered to the external load equals
the current I
generated by the illumination, less the diode current I
and
L
d
the ground-shunt current I
of the cell is
obtained when the load current is zero, i.e., when I = 0, and is given by the
following:
. The open circuit voltage V
sh
oc
