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M. Pravettoni
It is important here to distinguish between the different spectral irradiance that
the module “sees” depending on its orientation. First of all we call spectral Global
Horizontal Irradiance (spectral GHI), the irradiance measured pointing the detector
(a pyranometer for global irradiance; a spectrometer, for spectral irradiance: see
Sect. 10.5.2) vertically to the zenith of the particular location where measurement
takes place: this is the spectral irradiance “seen” by a module that lays horizontally
on the ground (see Fig. 10.6a). It is different from both the spectral Global Normal
Irradiance (GNI), which is measured pointing the detector always directly towards
the sun: this is the spectral irradiance “seen” by a module on a 2-axis sun tracker
(Fig. 10.6b). In general, when a module is not laying horizontally on the ground,
nor facing the direct sun rays on a 2-axis tracker, it “sees” the so-called in-plane
spectral irradiance, which is in general different from both spectral GHI and GNI.
(But of course, the in-plane spectral irradiance is equivalent to the spectral GHI if
the module lays horizontally on the ground and to the spectral GNI when the module
is on a 2-axis tracker, respectively).
Furthermore, clouds in the sky may limit the direct in-plane component B or
scatter additional light in favour of the diffuse component D: deviations from STC
can be even more significant in actual operating conditions.
These deviations in the spectral irradiance can cause relevant local differences
in the spectrum that may be detected differently by the PV modules and by the
pyranometers (due to their different spectral responses). As a consequence, under
a given level of irradiance G (recorded by the pyranometer), the PV module may
produce unexpectedly lower power than the power, which has been predicted based
on the above analysis.
This effect is referred to as spectral mismatch and should be corrected for the
correct energy rating. It can be shown that the spectral mismatch can be corrected
with the following spectral correction factor
(a)
(b)
Fig. 10.6 a Set of pyranometers measuring the Global Horizontal Irradiance (GHI); and b my friend
Wim Zaaiman while aligning the 2-axis tracker for testing at Global Normal Irradiance (GNI)
M. Pravettoni
It is important here to distinguish between the different spectral irradiance that
the module “sees” depending on its orientation. First of all we call spectral Global
Horizontal Irradiance (spectral GHI), the irradiance measured pointing the detector
(a pyranometer for global irradiance; a spectrometer, for spectral irradiance: see
Sect. 10.5.2) vertically to the zenith of the particular location where measurement
takes place: this is the spectral irradiance “seen” by a module that lays horizontally
on the ground (see Fig. 10.6a). It is different from both the spectral Global Normal
Irradiance (GNI), which is measured pointing the detector always directly towards
the sun: this is the spectral irradiance “seen” by a module on a 2-axis sun tracker
(Fig. 10.6b). In general, when a module is not laying horizontally on the ground,
nor facing the direct sun rays on a 2-axis tracker, it “sees” the so-called in-plane
spectral irradiance, which is in general different from both spectral GHI and GNI.
(But of course, the in-plane spectral irradiance is equivalent to the spectral GHI if
the module lays horizontally on the ground and to the spectral GNI when the module
is on a 2-axis tracker, respectively).
Furthermore, clouds in the sky may limit the direct in-plane component B or
scatter additional light in favour of the diffuse component D: deviations from STC
can be even more significant in actual operating conditions.
These deviations in the spectral irradiance can cause relevant local differences
in the spectrum that may be detected differently by the PV modules and by the
pyranometers (due to their different spectral responses). As a consequence, under
a given level of irradiance G (recorded by the pyranometer), the PV module may
produce unexpectedly lower power than the power, which has been predicted based
on the above analysis.
This effect is referred to as spectral mismatch and should be corrected for the
correct energy rating. It can be shown that the spectral mismatch can be corrected
with the following spectral correction factor
(a)
(b)
Fig. 10.6 a Set of pyranometers measuring the Global Horizontal Irradiance (GHI); and b my friend
Wim Zaaiman while aligning the 2-axis tracker for testing at Global Normal Irradiance (GNI)