Glossary
Page 46
the measuring device. Disadvantage: every sensor has got to be calibrated separately: the calibration value is
the voltage the sensor delivers, when it is exposed to the Standard-Test-Conditions (STC).
17.15 Internal series resistance
This resistance results physically from the material being used for the production of the module as well as from
its cable connections. For crystalline modules it amounts about 0.5Ohm, for thin-film modules more than 2 Ohm.
The measurement of the internal series resistance is latterly possible with the measuring devices series PVPM.
For that purpose just one I-V-curve of the module has to be measured. With this data the device automatically
calculates the R
s
as well as the P
pk
and the parallel resistance R
p
.
The theoretically expected value of the internal series resistance R
s
can be calculated. The calculation can be
carried out with the software PVPMdisp, if the STC characteristic values for V
oc
, I
sc
, V
pmax
and I
pmax
of the module
are known.
The calculated Rs can be compared to the measured value which the PVPM measuring device displays after the
I-V-curve measurement. If the measured value is too high the wiring should be checked for burst, corrosion,
connection errors or wrong dimensions.
17.16 Solar Constant
Denotes the solar radiation that hits vertically on a surface outside the atmosphere. s=1,37 kW/m². Outer space
the solar radiation is almost constant; on earth it fluctuates in the course of daytime or time of the year and
varies depending on latitude and weather. The maximum value on earth lies between about 0,8 and 1,2 kW/m².
In annual means the irradiation in Germany amounts according to region between 850 and 1100 kWh/m².
17.17 Solar irradiation / insolation
The solar irradiation consists of the radiation which is directly from the sun and divers indirect proportions. Those
are radiation reflected by the environment, e.g. snowfields reflect very intensive the radiation of the blue sky,
and other diffuse radiation. For the exact calculation of the energy that hits a surface the angle between the
sunlight and the surface is decisive. It changes according to time of year and time of the day. Informations about
the exact calculation are offered at different internet addresses. The radiation is delimited by several factors;
even if the sky is beaming blue only around 90% of the total solar energy arrives on earth.
17.18 Standard Test Conditions
To make the power of PV modules comparable they always are measured at the same conditions: according to
IEC60904-3 the Standard Test Conditions (STC) consists of an irradiation of 1000W/m
2
, a light spectrum
according to AM=1,5 as well as a module temperature of 25°C. Since these requirements are hardly to find
under natural sunlight, methods were developed to calculate the STC values of PV modules from I-V-curves
measured under ambient conditions.
17.19 Current-Voltage-Characteristic
The Current-Voltage-Characteristic of a PV-Generator
shows different qualities and problems of the
generator and is the basis of a few substantial specific
values.
For example partially or hard shading will modify the
characteristic as well as high internal series resistance
and defective or missing bypass diodes.
Besides short circuit current I
sc
and open circuit
voltage V
oc
, voltage and current at the maximum
power point (MPP) I
pmax
and V
pmax
as a result you get
the fill factor FF:
FF = I
sc
*V
oc
/(I
pmax
*V
pmax
)
The I-V-curve is the basis for the calculation of the effective solar curve, which then allows the determination of
the peak power P
pk
and the internal series resistance R
s.
PVPM 1000X / 1040X
User Manual
PV-Engineering GmbH
High loss due to shading
0,0
0,5
1,0
1,5
2,0
2,5
3,0
0
10
20
30
40
50
V/Volt
I/
A
m
p
e
re
Characteristic without shading
Characteristic with 2 shaded regions
