8 CdTe and CuInGaSe 2 Thin-Film Solar Cells
213
the capacity of higher performance at conditions which are not optimal, compared
to crystalline silicon devices.
8.6.1 Low-Light Conditions
CdTe as well as CIGS are direct-band-gap materials with high absorption coefficients.
In diffused light conditions, the spectrum changes its range of wavelengths towards
the blue region, so the distribution of light power is more concentrated in the 300–
500 nm range. This is very important if we consider that different absorber materials
work at different wavelength ranges. For diffused light solar cell materials, like CdTe,
that have their working range centred in the UV and the visible part of the spectrum
are preferable, in these conditions.
It has been demonstrated [10] that mono-crystalline and multi-crystalline silicon
technologies when installed in Southern Europe (Cyprus) have very large seasonal
variations in module efficiency, with higher efficiencies reached in winter periods
and lower ones in summer time. Also CIGS and CdTe thin film technologies have
demonstrated highest efficiencies in winter time but with a reduced seasonal variation
in comparison to crystalline silicon.
8.6.2 High Temperature Conditions
The operating temperature of a PV module can strongly affect its performance.
As mentioned before, PV modules are measured at STC, but their actual operating
temperatures are generally higher.
For this reason a Nominal Operating Cell Temperature (NOCT) was introduced
to give an indication of the PV device operating temperature; it is therefore a useful
parameter for PV system designers. According to IEC 61215 and 61646 regulations,
NOCT is measured in open circuit at 45° inclination, 800 W m
−2 with a wind speed
of 1 m s
−1 and at an air temperature of 20 °C. This results in typical NOCT values
around 50 °C. However, still real PV operating temperatures exceed NOCT values
especially in summer days.
Special attention to the operating temperature must be given in the case of BIPV,
as the operating temperature of the PV module is given by the heat exchange mechanisms at the back of the modules. If the PV installation is designed and assembled
without proper ventilation, high module operating temperatures will result. During
summer days thin film glass/glass modules can reach temperatures up to 70 °C, 79 °C,
and 92 °C respectively for open-rack mounted, for building added photovoltaics and
for building integrated photovoltaics.
Now due to the different temperature coefficient of each absorber material, the loss
of power due to higher operating temperatures—compared to STC conditions—will
be more pronounced for some technologies and less for others.
213
the capacity of higher performance at conditions which are not optimal, compared
to crystalline silicon devices.
8.6.1 Low-Light Conditions
CdTe as well as CIGS are direct-band-gap materials with high absorption coefficients.
In diffused light conditions, the spectrum changes its range of wavelengths towards
the blue region, so the distribution of light power is more concentrated in the 300–
500 nm range. This is very important if we consider that different absorber materials
work at different wavelength ranges. For diffused light solar cell materials, like CdTe,
that have their working range centred in the UV and the visible part of the spectrum
are preferable, in these conditions.
It has been demonstrated [10] that mono-crystalline and multi-crystalline silicon
technologies when installed in Southern Europe (Cyprus) have very large seasonal
variations in module efficiency, with higher efficiencies reached in winter periods
and lower ones in summer time. Also CIGS and CdTe thin film technologies have
demonstrated highest efficiencies in winter time but with a reduced seasonal variation
in comparison to crystalline silicon.
8.6.2 High Temperature Conditions
The operating temperature of a PV module can strongly affect its performance.
As mentioned before, PV modules are measured at STC, but their actual operating
temperatures are generally higher.
For this reason a Nominal Operating Cell Temperature (NOCT) was introduced
to give an indication of the PV device operating temperature; it is therefore a useful
parameter for PV system designers. According to IEC 61215 and 61646 regulations,
NOCT is measured in open circuit at 45° inclination, 800 W m
−2 with a wind speed
of 1 m s
−1 and at an air temperature of 20 °C. This results in typical NOCT values
around 50 °C. However, still real PV operating temperatures exceed NOCT values
especially in summer days.
Special attention to the operating temperature must be given in the case of BIPV,
as the operating temperature of the PV module is given by the heat exchange mechanisms at the back of the modules. If the PV installation is designed and assembled
without proper ventilation, high module operating temperatures will result. During
summer days thin film glass/glass modules can reach temperatures up to 70 °C, 79 °C,
and 92 °C respectively for open-rack mounted, for building added photovoltaics and
for building integrated photovoltaics.
Now due to the different temperature coefficient of each absorber material, the loss
of power due to higher operating temperatures—compared to STC conditions—will
be more pronounced for some technologies and less for others.