The voltage in solar cells wired in series is cumulative, which means that the cumulative voltage of all cells wired in series nZ is nZ times as high as the voltage in one cell. Owing to the power source characteristics of solar cells, the level of current in such series-wired configurations is determined by the weakest cell.
This scenario will now be discussed in terms of an example involving two solar cells wired in series (see Figure 4.17). The short-circuit current of cell B is only half that of cell A owing to partial shading via a leaf or bird droppings or the like. The I-V characteristics of the circuit arrangement as a whole, which arise from adding together the voltages from the two cells in the presence of equal current, approximately equate to the I-V characteristic curve of the weaker cell B extended by a factor of 2 along the voltage axis. The maximum power Pmax, at the MPP, of the overall I-V characteristic curve does not equate to the cumulative maximum power of cells A and B, but is instead only slightly more than twice the maximum power of the weaker cell.
Series Connection of two Different Solar Cells
Figure 4.17 Individual characteristic curve and overall I-V characteristics of two series-connected solar cells, where one of the cells is partly shaded
If this series connection is short-circuited, VB = — VA, and thus becomes negative, the power produced by the stronger cell A is converted to heat in cell B by virtue of the fact that the cell B current and voltage are both flowing in the same direction (operation in the third quadrant as shown in Figure 4.16). If only two cells are wired in series, cell B can withstand this scenario without any difficulty.
The scenario illustrated in Figure 4.17 is far less common for identically insolated series-connected solar cells since, owing to manufacturing tolerances, the characteristic curves of the two cells cannot be absolutely identical. Hence the maximum power of series-connected solar cells is always somewhat lower than the cumulative maximum power of the individual cells. Vendors try to minimize these so-called mismatch losses by incorporating into a module only cells whose IMPP current at the MPP is as similar as possible. To this end, each cell is measured after being manufactured and is assigned to the appropriate IMPP group (see cell characterization at the beginning of Figure 4.1).