Environmental Engineering Reference
In-Depth Information
Figure 4.21
I-V and P-V Solar Cell Characteristics with Maximum
Power Point (MPP)
Since the temperature coefficient of the voltage is higher than that of the
current, the temperature coefficient
α
PMPP
of the MPP power is negative. For
α
PMPP
=-6
•
10
-3
/°C. A
temperature rise of 25°C causes a power drop of about 10 per cent.
In order to make possible comparisons between solar cells and modules , MPP
power is measured under
standard test conditions
(STC) (
E
= 1000 W/m
2
,
=-3
•
10
-3
/°C and
silicon solar cells it is between
α
PMPP
ϑ
= 25°C, AM 1.5). The generated power of the solar modules under real
weather conditions is usually lower. Hence STC power has the unit W
p
(Watt-
peak).
Considering the irradiance dependence, the current dominates the device
behaviour, so that the MPP power is nearly proportional to the irradiance
E
.
Another parameter is the
fill factor
(FF) with the definition:
(4.45)
The fill factor is a quality criterion of solar cells that describes how well the
I
-
V
curve fits into the rectangle of
V
OC
and
I
SC
. The value is always smaller
than 1 and is usually between 0.75 and 0.85.
Together, the MPP power
P
MPP
, the irradiance
E
and the solar cell area
A
provide the solar cell
efficiency
η
:
(4.46)
The efficiency is usually determined under standard test conditions. Table 4.4
summarizes the various solar cell parameters.
