Cell-String–Level Performance Modeling
Modeling PV system energy production is a critical step in the solar design process. Accurate energy predictions are required to understand the performance implications of different hardware components and to assess the financial returns of a proposed design. Multiple approaches and software tools can simulate solar energy production, ranging from simple array-level calculations to detailed component-level circuit models. In this article, I discuss taking the latter approach even further: to the sub-module level, which analyzes the shade impacts and electrical behavior of a design down to the level of cell strings and bypass diodes inside the solar modules.
Simulating solar designs to the cell-string level can have an appreciable impact on energy production estimates. Only by simulating at this level can you accurately assess the effects of bypass diodes, especially for commercial designs with interrow shading and residential designs in partial shade. Moreover, some integrated power electronics such as cell-string optimizers require a submodule simulation to accurately model their impact on energy production. Taking into account manufacturer-verified cell-string and bypass diode configurations helps ensure that a project’s predicted energy yield is as accurate as possible.
Module datasheets often include current voltage and power voltage curves that show how the module output power varies in relation to irradiance. When the irradiance on the module is very low—as is the case when the module is fully shaded—its power output is generally low. If this module is part of a string of modules connected to an inverter, it can cause the power of the entire string to drop because the current through the string can be only as high as the current through the most shaded module. Manufacturers integrate bypass diodes into their modules to mitigate this effect.
A bypass diode is a semiconductor device that, for the purpose of its application in solar modules, can be thought of as an on/off switch. When the diode is off, it is not conducting any current; but when it is on, it can conduct any amount of current. The diode typically turns on at a voltage of 0.6 V–0.7 V. Assuming a scenario with one bypass diode per module, when the diode is on, it effectively shorts out the module by routing the string current through the diode instead of through the shaded cells.
As an example, consider the case where nine out of 10 modules are capable of outputting 8 A of current at a voltage of 32.5 V, but one of the 10 modules is shaded and can produce only 1 A at about the same voltage, as shown in Figure 1. If current cannot bypass the weak module, then the total output power will be roughly 325 W (10 modules × 32.5 V × 1 A), because the entire string is forced to operate at the lowest module current. (This assumes the unshaded modules still operate at their rated Vmp; in reality, they operate closer to their Voc.) If, however, current can skip the shaded module because its bypass diode turns on, then the total output power becomes 2,340 W (9 modules × 32.5 V × 8 A), excluding some small power loss due to voltage drop across the diode. It is clearly preferable to bypass the shaded module, because the increase in output power from operating the string at the higher current level far outweighs the shaded module’s contribution to the total power.