The performance of PV-modules is usually characterized by the
nominal power at standard test conditions. However more relevant for
the end-user is the energy production. To arrive at a so-called energy
rating procedure the P(Gi,Tm) matrix is defined,
which makes it possible to calculate the expected annual energy production
of a PV-module at any given location with known frequency distribution
of horizontal irradiation and ambient temperature.
In order to make the de P(Gi,Tm) independent
of the time and location of the tests the effective irradiation (Gi)
is measured with a device with the same characteristics as the module
under test. Such a device can be a suitable reference cell or, even
better, the module under test itself. The latter, however, is only possible
in case the module under test is stable in time.
Measurements were performed to test the applicability of the
so-called self-reference for a-Si modules. Furthermore the difference
between the effective irradiance and the real irradiance (measured with
a pyranometer) was quantified for the test location Petten, the Netherlands.
The conclusions are:
·
Self-reference
is not applicable for a-Si modules because of the time instability,
even after 1 full year of degradation.
·
Self-reference
signals (the short circuit current of the module under test) can be
calibrated outdoor by comparison with a pyranometer at irradiance levels
above 800 W/m2. The uncertainty of such a calibration is
within 3%. The difference between the effective irradiance
and the real irradiance on annual basis at Petten is virtually zero
for x-Si modules and about 1% for a-Si modules.