When a solar cell is operated in the first or third quadrants, electrical power loss and the heat resulting from solar radiation cause the cell to heat up. Assuming a maximum possible ambient temperature Tv of 40-50 °C and (as in the solar module datasheet) an allowable maximum cell operating temperature of 90-100 °C, under average cooling conditions a roughly 50 °C increase relative to ambient temperature is allowable. This allowable increase (around 20 W per cell for a 100 cm2 cell) will be reached if the entire module is exposed to the same thermal conditions in the presence of total area-specific loss of around 2kW/m2.
Maximum area-specific thermal loss pVTZ in the solar cell is given by
where: GZ is the solar cell irradiance (in practice, GZ — Gg); Az is the solar cell area; and Pvez is the electrical loss (V ■ I) in the solar cell during operation in the first or third power quadrant as shown in Figure 4.16.
In view of the relatively conservative nature of the suppositions that form the basis for Equation 4.2, in most cases applying it entails no risk. During operation in the first quadrant, i. e. the penetration area, the entire module is exposed to the same load. And in view of the relatively low voltage in this quadrant (VF ~ 0.8 V), and as noted in Section 188.8.131.52, reverse current IR-Mod rather than power loss can be the limiting factor in such cases.
The worst-case scenario for operation in quadrant 3 arises if, while a module is partially or completely shaded, only some of its cells are operating in the non-conducting direction and if the remaining, fully insolated cells are being heated by the module’s aggregate output. However, inasmuch as the immediately adjacent cells help to cool the module in this scenario, most vendors make somewhat less conservative suppositions for it and allow pVTZ values such as the following for individual partly shaded cells:
In principle, this approximatepVTZ value can be used to determine the number of bypass diodes needed per module. The higher thepVTZ limit value, the fewer bypass diodes per cell are needed (see Section 184.108.40.206).