Local Shading and Hot Spot Formation

Due to local shading or failure, one or several solar cells can present a much smaller short-circuit current than the rest of devices in the series string. If the defective cells are forced to pass a current higher than their generation capabilities, they become reverse-biased, even entering the breakdown regime, and sinking power instead of sourcing it.

Figure 7.20 illustrates this behavior for an 18-cell string with one cell shaded so that its short-circuit current is half that of the remaining devices. String short-circuit is marked with an horizontal line, showing that in this condition the shaded cell is strongly reverse-biased, and dissipating the power produced by the unshaded cells. This effect of course severely degrades the efficiency of the module, but more important is the fact that it can get damaged.

Avalanche breakdown is characterized by a nonuniform distribution of current across the junction, breakdown occurring preferentially at localized regions, possibly correlated to damage during processing. Intense local heating can produce very high temperatures (a hot spot). If around 150 °C are reached, the lamination material becomes degraded and the module deterio­rates irreversibly [192, 193]. Due to the localized nature of the process, solar cells show large scattering in their reverse characteristics so that the module behavior under partial shading is not accurately predictable.

In order to devise the means of preventing hot spot failure, the worst case is considered. This occurs when the N-cell series string is short-circuited and a shaded solar cell is reverse-biased with the voltage of the remaining N — 1 good devices, as shown in Figure 7.20. The minimum N that will lead to hot spot formation (i. e. the maximum N for safe operation) depends on rather uncontrollable factors, as explained. For Si solar cells of standard technology it is around 15-20. Devices fabricated on upgraded metallurgical (UMG) silicon show a breakdown voltage of 10-12 V.

Current (A)

Figure 7.20 Computer simulation of the I – V curves of a 50% shaded cell, showing the typical “soft” reverse breakdown, and of 17 identical cells, unshaded, in series. When series-connected with the shaded cell, they curve labeled “series string” is obtained

Since larger series strings are generally used, the approach followed is to put a diode (bypass diode) in parallel, but in opposite polarity, with a group cells. The number of cells in the group is chosen so that hot spots cannot be formed. When one or several cells are shaded, they are reverse-biased only to the point where the diode across the group starts forward conduction. The diode carries away the necessary current to keep the group near short-circuit.

Figure 7.21 illustrates the operation of the bypass diodes. When the current forced through the shaded sub-string is such that the reverse bias equals the diode threshold voltage, the bypass diode sinks all necessary current to keep the string at this biasing point, hence preventing the power dissipated in the shaded cell from increasing. It is also apparent that the bypass diode leads to a significant increase of output power, allowing the module to keep delivering the power generated by the unaffected groups.

It is clear then that the smaller the number of cells per bypass diode, the lower the efficiency loss for a shading condition, but this means a higher cost and more complex fabrication. It has been proposed to integrate a bypass diode in each cell so that these effects will be minimized at the expense of a more complicated cell processing [194].

The practice is to take electrical terminals outside the encapsulation, not only for the extremes of the series string, but for intermediate points as well so that bypass diodes are connected in the junction box each 18-20 cells (Figure 7.22), or 12 cells for UMG-Si. Endurance to shading is a standard test for module qualification.

The influence of local shading on the module output depends on the details of the I – V curve of the cells as well. Under certain circumstances of partial shading, it is beneficial that the cells show some shunt resistance. However, tight control of leakage currents by processing is not easy.

Updated: August 23, 2015 — 10:57 pm