8 Translation of I-V Data to Standard Test Conditions
8-2
reference condition, I
sco
. Using these (%/°C) units makes the same value applicable for
both individual modules and for parallel strings of modules.
α
Imp
= Normalized temperature coefficient for I
mp
, (%/°C). Normalized in the same
manner as α
Isc
.
β
Voc
, (%/°C) = Temperature coefficient for module open-circuit-voltage.
γ
mpp
, (%/°C) = Temperature coefficient for module maximum power point voltage.
T
c
= Cell temperature inside module, °C. Obtained by taking the back-surface module
temperature from the sensor and adding the temperature differential (typically 3 degrees)
Translation equations
The basic translation model used here makes the following approximations:
1. P
mp
is proportional to E, the irradiance
2. I
mp
does not depend on temperature
3. V
mp
is independent of E
4. P
mp
varies with temperature according to γ
mpp.
(taken from the datasheet)
5. V
oc
is independent of E
In light of these approximations we make these assumptions:
I
sc
scales directly with E and with temperature.
V
oc
varies linearly with temperature according to β
Voc
.
P
mp
scales with E and varies with temperature according to γ
mpp
V
mp
changes with temperature depending on γ
mpp
only, since α
Imp
is much smaller than
α
Isc
and is assumed to be zero.
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