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There are three critical factors which affect the instantaneous output of a PV cell or module:
The output power of a PV cell or PV module directly depends on the solar irradiance on its surface. As irradiance “G” increases, the current “I” increases due to an increase in the level of the photoelectric effect. Voltage output V, on the other hand, varies only slightly with changing irradiance (see figure). This means that as soon as the sun illuminates the surface of the cell/module, the voltage rises to a value that is close to VOC. Regardless of a change in solar irradiance, such as shading from passing clouds, the voltage will fluctuate only slightly below that VOC range. The current, however, will increase in direct proportion to the irradiance, only reaching the module’s full current under strong irradiance conditions, such as 1 kW/m2. For this reason, current produced by the photoelectric effect in a PV cell is referred to as photocurrent. Passing clouds or people, which cast shadows on the PV modules, will cause a drop in current output. Since current is directly proportional to power, shading has a significant effect on power output.To summarise, an increase/decrease in solar irradiance causes:
and therefore an almost linear increase/decrease in power output.
The performance of PV cells also varies with temperature. Since PV cells and modules cannot convert 100% of the absorbed light into usable electricity, some of this energy is lost in the form of heat, which causes the temperature of the cell to increase. This is a normal part of the operation of a PV cell and cannot be avoided. As the cell gets hot, its power output drops. This is because the open circuit voltage VOC decreases significantly when the internal cell temperature rises (see figure). The short circuit current ISC increases slightly with increasing temperature but not enough to compensate for the large drop in VOC. Under normal conditions, the PV cell will always operate at a temperature higher than the ambient temperature. The ideal operating state for maximising power output is high irradiance with low temperatures. Such conditions are rare, e.g. high up in the mountains in the winter on a clear day. To summarise, an increase in cell temperature causes:
and therefore, a significant decrease in power output.
In the next document, we will name the STC (Standard Test Condition) parameters and will help you understand why such STC is required.
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