PV module / array with a shaded cell

This tool visualises the behaviour of voltages and currents at the terminals of a shaded (or bad) cell, placed in a module or a group of modules. It evaluates the energy dissipation of this cell for various operating points, and its resultant temperature by making reasonable hypotheses for the thermal loss factor.

It has to be noted that these behaviours are studied in a marginal situation - representing the most unfavourable case - where only one single cell is disturbed. The user can adjust the irradiance, the module temperature in normal operation (i.e., without dissipation), and the shading fraction of the considered cell.

You can visualise three different cases, for a single module or arrays:

Module without protection diode

The graph shows the degraded performance curve of the whole module (grey), to be compared to the normal operation (dashed line). But it also shows the dangerous reverse bias applied on the shaded cell, for example at the V=0 point (short circuited module), or at the Maximum Power Point operation of the whole array. The reverse bias voltage occurs along with a current greater than the nominal photocurrent, resulting in a serious power dissipation of the shaded cell, resulting in its temperature increase.

This dangerous situation, named "Hot Spot", can be explained as each cell in the module is producing its rated current at bright sun, with its normal voltage, all of this power being dissipated in the shaded cell. (As they are connected in series, the current flow is identical in each cell, and the sum of voltages (shaded+non-shaded) is the module overall voltage, i.e., zero when short-circuited).

Module with protection by-pass diode

This graph also shows the regulating action of the protection diodes connected in antiparallel to a module or two half-modules. In this case the reverse voltage at the module's terminals is limited to the diode voltage (about –0.7V), the excess current being drawn by the diode. In this case, the power dissipation of the shaded cell is limited to the power production of the other cells inside the protected series. If too many cells in series, the module's manufacturer will distribute several diodes along the cells string.

These behaviours help understand that the effects of partial shadings on an array give rise to highly non-linear electrical behaviours, and that apparently negligible causes can lead to highly significant losses.

This graphical tool should lead the engineer to carefully study the wiring of the PV fields, in such a way that the foreseen shadings simultaneously affect the smallest number of series possible. For example, in shed mountings, he should place the module series longitudinally, in order that the mutual shadings (on the lower part of the back-shed) be limited to just one branch of cells or modules.