260
M. Pravettoni
In theory this approach works well, but requires as an input information that may
be difficult to get, such as the in-plane
6 global irradiance G(t), the in-plane direct irradiances B(t) and the spectral irradiance G(λ, t), all functions of time t. Section 10.5
below illustrates the needed experimental equipment to get this information, but also,
in the absence of experimental data, where to find databases that contain modelled
values for G(t), B(t) and G(λ, t).
10.3 Three Relevant Exceptions
Before proceeding with the description of energy losses and failure mode mechanisms that limit the energy yield, as described in the previous section, three relevant
cases require our attention, where the approach presented so far needs to be somewhat
modified. These are the cases of module on trackers, of concentrating photovoltaic
(CPV) modules and of bifacial modules: they are briefly discussed in this section. In
all cases, the orientation of the modules has different characteristics from the orientation presented so far: therefore, the role played by the various in-plane irradiance
components is also quite different.
10.3.1 Energy Rating for Modules on Trackers
Sometimes modules in utility-scale installations are put on trackers, to optimize the
orientation and minimize the angle of incidence losses. Trackers move the modules
facing the Sun around one or two axis during the day and turn the module back
to their starting position at sunset: 1-axis tracking is simpler for many reasons, thus
cheaper, and easier to maintain; on the other hand 2-axis tracking has a higher energy
yield and is, in principle, easier to model. In this section we will refer for simplicity
only to 2-axis tracking.
When PV modules are on a 2-axis tracker, the orientation is always optimal by
definition and θ (t) = 0
◦ at all times: we have already encountered this condition
above and it has been defined with the acronym GNI, Global Normal Irradiance.
In this case, in clear-sky conditions the direct in-plane irradiance B(t) varies less
over the day than in fixed installations: this results in a much flatter G(t) profile all
over the day. Since it is now always θ (t) = 0
◦ , B(t) does not need a correction for
the angle of incidence, which is, thus, a less significant correction for modules on
trackers.
6 As introduced in the previous note, the in-plane irradiance is measured with the detector (a
pyranometer for total irradiance, a spectrometer for spectral irradiance) placed in-plane with the
PV modules. In most cases such equipment is not available on site, while meteorological data (or
modelled ones) can be found on GHI and GNI.
M. Pravettoni
In theory this approach works well, but requires as an input information that may
be difficult to get, such as the in-plane
6 global irradiance G(t), the in-plane direct irradiances B(t) and the spectral irradiance G(λ, t), all functions of time t. Section 10.5
below illustrates the needed experimental equipment to get this information, but also,
in the absence of experimental data, where to find databases that contain modelled
values for G(t), B(t) and G(λ, t).
10.3 Three Relevant Exceptions
Before proceeding with the description of energy losses and failure mode mechanisms that limit the energy yield, as described in the previous section, three relevant
cases require our attention, where the approach presented so far needs to be somewhat
modified. These are the cases of module on trackers, of concentrating photovoltaic
(CPV) modules and of bifacial modules: they are briefly discussed in this section. In
all cases, the orientation of the modules has different characteristics from the orientation presented so far: therefore, the role played by the various in-plane irradiance
components is also quite different.
10.3.1 Energy Rating for Modules on Trackers
Sometimes modules in utility-scale installations are put on trackers, to optimize the
orientation and minimize the angle of incidence losses. Trackers move the modules
facing the Sun around one or two axis during the day and turn the module back
to their starting position at sunset: 1-axis tracking is simpler for many reasons, thus
cheaper, and easier to maintain; on the other hand 2-axis tracking has a higher energy
yield and is, in principle, easier to model. In this section we will refer for simplicity
only to 2-axis tracking.
When PV modules are on a 2-axis tracker, the orientation is always optimal by
definition and θ (t) = 0
◦ at all times: we have already encountered this condition
above and it has been defined with the acronym GNI, Global Normal Irradiance.
In this case, in clear-sky conditions the direct in-plane irradiance B(t) varies less
over the day than in fixed installations: this results in a much flatter G(t) profile all
over the day. Since it is now always θ (t) = 0
◦ , B(t) does not need a correction for
the angle of incidence, which is, thus, a less significant correction for modules on
trackers.
6 As introduced in the previous note, the in-plane irradiance is measured with the detector (a
pyranometer for total irradiance, a spectrometer for spectral irradiance) placed in-plane with the
PV modules. In most cases such equipment is not available on site, while meteorological data (or
modelled ones) can be found on GHI and GNI.