Mismatch losses are caused by the interconnection of solar cells or modules which do not have identical properties or which experience different conditions from one another. Mismatch losses are a serious problem in PV modules and arrays under some conditions because the output of the entire PV module under worst case conditions is determined by the solar cell with the lowest output. For example, when one solar cell is shaded while the remainder in the module are not, the power being generated by the "good" solar cells can be dissipated by the lower performance cell rather than powering the load. This in turn can lead to highly localised power dissipation and the resultant local heating may cause irreversible damage to the module.
Mismatch in PV modules occurs when the electrical parameters of one solar cell are significantly altered from those of the remaining devices. The impact and power loss due to mismatch depend on:
- the operating point of the PV module;
- the circuit configuration; and
- the parameter (or parameters) which are different from the remainder of the solar cells.
Differences in any part of the IV curve between one solar cell and another may lead to mismatch losses at some operating point. A non-ideal IV curve and the operating regime of the solar cell is shown below. Although mismatch may occur in any of the cell parameters shown below, large mismatches are most commonly caused by differences in either the short-circuit current or open-circuit voltage. The impact of the mismatch depends on both the circuit configuration and on the type of mismatch, and is demonstrated in more detail in the following pages.