64
A. Shah
– For a given absorber material, e.g. for c-Si, between different module
designs and different module implementations. Here, one notices an interesting relationship between the value of V oc and the magnitude of TC:
Modules that have—due to high recombination—low values of V oc have,
in general, also a strong temperature dependence, i.e. a high magnitude of
TC.
13
• The relative temperature coefficient TC is a parameter, which is difficult to
determine, both theoretically (many factors not mentioned here play a role)
and experimentally (costly equipment is needed for performing laboratory
measurements). Therefore, only field tests will reveal the “full truth”.
• The relative temperature coefficient TC is an extremely important parameter
for the application of solar modules on rooftops and in tropical countries.
In these cases, the operating temperature of the solar module easily reaches
75 °C, i.e. 50 °C more than under STC conditions. The conversion efficiency
of the solar module will thereby be reduced, with respect to the STC value
given in the datasheet: for amorphous silicon modules, the efficiency reduction will be around 10%, for CdTe modules it will be between 12 and 15%, for
heterojunction c-Si modules and for other recent c-Si modules (such as TOPCON modules) it will be around 15%, whereas for some other wafer-based
crystalline silicon (c-Si) modules, it can be up to approximately 20%.
3.5.3 Variation of Efficiency η in Function of Light Intensity
The variation of efficiency η in function of light intensity is governed directly by the
corresponding variation in V oc , and, indirectly, by the variation in FF, mainly through
V oc (see 3.13). On the other hand, J ph and, therefore, also J sc , can be considered to
remain directly proportional to light intensity. So, let us now look at the variation of
V oc (and η) with light intensity. There are two mechanisms here: 1. a fundamental
effect and 2. a practical effect:
1. A fundamental effect
According to (3.11), V oc increases logarithmically with J ph —i.e. with light intensity. By decreasing the incoming light by a factor of 10, i.e. by a factor of approximately e
2.3 , V oc will decrease roughly by 26 mV × 2.3 ≈ 60 mV. This is so,
13 There are two reasons for this: (1) If we have a higher value of V oc (and of η), we will be obtaining
a higher magnitude of TC, as TC is the relative temperature coefficient, and we have to divide by
V oc (or by η), in order to obtain it; (2) the difference in TC between “theoretical” and experimental
values (i.e. between Tables 3.3 and 3.4) is so large that it cannot be explained only by (1). We
therefore postulate that Surface Recombination, which is mainly responsible for the difference in
V oc obtained for c-Si modules, is strongly temperature dependent. This postulate remains to be
theoretically justified. Note that the slope of the red line in Fig. 3.21a is due to (2).
A. Shah
– For a given absorber material, e.g. for c-Si, between different module
designs and different module implementations. Here, one notices an interesting relationship between the value of V oc and the magnitude of TC:
Modules that have—due to high recombination—low values of V oc have,
in general, also a strong temperature dependence, i.e. a high magnitude of
TC.
13
• The relative temperature coefficient TC is a parameter, which is difficult to
determine, both theoretically (many factors not mentioned here play a role)
and experimentally (costly equipment is needed for performing laboratory
measurements). Therefore, only field tests will reveal the “full truth”.
• The relative temperature coefficient TC is an extremely important parameter
for the application of solar modules on rooftops and in tropical countries.
In these cases, the operating temperature of the solar module easily reaches
75 °C, i.e. 50 °C more than under STC conditions. The conversion efficiency
of the solar module will thereby be reduced, with respect to the STC value
given in the datasheet: for amorphous silicon modules, the efficiency reduction will be around 10%, for CdTe modules it will be between 12 and 15%, for
heterojunction c-Si modules and for other recent c-Si modules (such as TOPCON modules) it will be around 15%, whereas for some other wafer-based
crystalline silicon (c-Si) modules, it can be up to approximately 20%.
3.5.3 Variation of Efficiency η in Function of Light Intensity
The variation of efficiency η in function of light intensity is governed directly by the
corresponding variation in V oc , and, indirectly, by the variation in FF, mainly through
V oc (see 3.13). On the other hand, J ph and, therefore, also J sc , can be considered to
remain directly proportional to light intensity. So, let us now look at the variation of
V oc (and η) with light intensity. There are two mechanisms here: 1. a fundamental
effect and 2. a practical effect:
1. A fundamental effect
According to (3.11), V oc increases logarithmically with J ph —i.e. with light intensity. By decreasing the incoming light by a factor of 10, i.e. by a factor of approximately e
2.3 , V oc will decrease roughly by 26 mV × 2.3 ≈ 60 mV. This is so,
13 There are two reasons for this: (1) If we have a higher value of V oc (and of η), we will be obtaining
a higher magnitude of TC, as TC is the relative temperature coefficient, and we have to divide by
V oc (or by η), in order to obtain it; (2) the difference in TC between “theoretical” and experimental
values (i.e. between Tables 3.3 and 3.4) is so large that it cannot be explained only by (1). We
therefore postulate that Surface Recombination, which is mainly responsible for the difference in
V oc obtained for c-Si modules, is strongly temperature dependent. This postulate remains to be
theoretically justified. Note that the slope of the red line in Fig. 3.21a is due to (2).