Measuring the Effect of Cell Mismatch on Module Output

Photovoltaic modules are commonly built using solar cells with similar IV characteristics to ensure that mismatch is minimized. Mismatch occurs because the IV characteristics of cells within a module are not identical. When connected in series, cells do not perform at their individual maximum power point. Instead, the cells perform at a combined maximum, which is less than the sum of the individual maximums. Various cell-sorting methods are used by PV module manufacturers. These different sorting strategies, including bin tolerances, have an effect on mismatch in the module. This work explains how cell mismatch in a module can be measured on a finished product using a module IV flash tester, by comparing a Suns-Voc pseudo IV curve to a measured IV curve. Further, this work presents a methodology for accurately measuring mismatch in a module, demonstrates the validity of the method using simulations, and compares the simulations with experimental data. This approach can be used by module manufacturers to characterize the module losses caused by cell mismatch, select a cell-sorting strategy, and optimize the cell-binning tolerance