254
M. Pravettoni
Unfortunately, all these assumptions are not correct; therefore substantial corrections need to be applied to (10.2), to take into account the effects of temperature,
of the angle of incidence (for both the direct and the diffuse components) and of
the spectral mismatch between the spectral in-plane global irradiance and the STC
spectrum. These effects will be discussed in the next sections.
10.2.2 Effect of Temperature
The effect of temperature on module efficiency is by far the most important effect.
In clear-sky conditions, the temperature T m of PV modules depends on both the
irradiance G (the higher G, the higher T m ), the ambient temperature T a (the higher
T a , the higher T m ) and the wind speed v (the higher v, the lower T m ): the relation
that combines all these parameters to give T m is
T m = T a +
G
u 0 + u 1 v
,
(10.4)
where u 0 and u 1 are constants that can be determined experimentally and (10.4) is
referred to as the Faiman model from the name of the researcher who first proposed
it (see [1]).
With varying temperature, PV module efficiency decreases dramatically by several
percent, with respect to the efficiency at STC, as explained in Chap. 9. This effect is a
basic physical property of semiconductors. The theoretical details are far beyond the
scope of this book, but consequently the voltage of PV modules drops significantly
with temperature for practically all the PV technologies (thus the drop in efficiency),
whereas the current is less affected by temperature variations.
Combining the irradiance and temperature dependence of the module efficiency
η, the latter can be expressed as a function of two variables: the irradiance G and the
temperature T . We obtain the graph η(G, T ) which is shown in Fig. 10.3. From this
Figure the module efficiency η can be extracted for any value of irradiance G and
temperature T.
10.2.3 Effect of the Angle of Incidence
The cosine relation of (10.1) is an idealized, simplified approximation, as at wide
angles the incident light is, to a large part,
5 reflected by the surface of the modules.
Optical transmittance properties vary from glass to glass, encapsulant to encapsulant
and from technology to technology: in order to calculate the losses because of the
5 The shape and dimensions of the metal grid on top the component cells can also affect light
absorption at wide angles, where the metal grid can shade the active part of the cells.
M. Pravettoni
Unfortunately, all these assumptions are not correct; therefore substantial corrections need to be applied to (10.2), to take into account the effects of temperature,
of the angle of incidence (for both the direct and the diffuse components) and of
the spectral mismatch between the spectral in-plane global irradiance and the STC
spectrum. These effects will be discussed in the next sections.
10.2.2 Effect of Temperature
The effect of temperature on module efficiency is by far the most important effect.
In clear-sky conditions, the temperature T m of PV modules depends on both the
irradiance G (the higher G, the higher T m ), the ambient temperature T a (the higher
T a , the higher T m ) and the wind speed v (the higher v, the lower T m ): the relation
that combines all these parameters to give T m is
T m = T a +
G
u 0 + u 1 v
,
(10.4)
where u 0 and u 1 are constants that can be determined experimentally and (10.4) is
referred to as the Faiman model from the name of the researcher who first proposed
it (see [1]).
With varying temperature, PV module efficiency decreases dramatically by several
percent, with respect to the efficiency at STC, as explained in Chap. 9. This effect is a
basic physical property of semiconductors. The theoretical details are far beyond the
scope of this book, but consequently the voltage of PV modules drops significantly
with temperature for practically all the PV technologies (thus the drop in efficiency),
whereas the current is less affected by temperature variations.
Combining the irradiance and temperature dependence of the module efficiency
η, the latter can be expressed as a function of two variables: the irradiance G and the
temperature T . We obtain the graph η(G, T ) which is shown in Fig. 10.3. From this
Figure the module efficiency η can be extracted for any value of irradiance G and
temperature T.
10.2.3 Effect of the Angle of Incidence
The cosine relation of (10.1) is an idealized, simplified approximation, as at wide
angles the incident light is, to a large part,
5 reflected by the surface of the modules.
Optical transmittance properties vary from glass to glass, encapsulant to encapsulant
and from technology to technology: in order to calculate the losses because of the
5 The shape and dimensions of the metal grid on top the component cells can also affect light
absorption at wide angles, where the metal grid can shade the active part of the cells.