For up to 12 V PV plants, where a module short circuit cannot occur, modules without bypass diodes can normally be used. However, modules for higher-voltage installations should always be protected by bypass diodes, or the cells used need to exhibit highly specific cut-off region characteristics. Bypass diodes in larger-scale solar modules and solar cells are prohibitively expensive owing to the cooling capacity needed for these devices. If cell barrier capacity is strategically decreased, controlled avalanche point characteristics can be achieved.
It would be preferable here for each solar cell to integrate a bypass diode, but of course in such a way that the other characteristics of the cells are not degraded. Figure 4.24 displays the complete characteristic curves in all power quadrants for this putative solar cell, which would not need bypass diodes.
Figure 4.25 displays the characteristic curves at short-circuit current for the 36-cell solar module in Figure 4.19. Inasmuch as the voltage in partly shaded cells remains low at all insolation levels, power loss is likewise low, which means that the absence of bypass diodes would pose no risk for the cells. Various vendors have tried to make such solar cells, but none have been commercialized as yet as their guide values go downhill under normal operation.
Bypass diodes should only be dispensed with if the module vendor guarantees that the solar cells used in the modules integrate bypass diodes or exhibit controlled shutdown behaviour. However, dispensing with diodes means that the key tasks performed by these devices in larger systems, such as obviating excessive power drops in partly shaded cell series strings, will not be performed as efficiently as would otherwise be the case. Hence a configuration of one bypass diode per module should be adopted for systems that are prone to considerable shading at certain times of the day or year.
I-V-Characteristics of a Solar Cell with Controlled Breakdown
Figure 4.24 I—V characteristic curves for a (putative and not commercially available) monocrystalline solar cell (AZ « 102 cm2) with controlled avalanche point characteristics in all quadrants, with and without insolation, and with a cell temperature of 25 °C (usage metering system)
(partially) shaded cell
Figure 4.25 Characteristic curves of a short-circuited 36-cell module whose cell temperature is Tz — 25 0C, one of whose cells is partly shaded, and whose avalanche point characteristics are shown in Figure 4.24; 5 of the cells have 1 kW/m2 insolation (curve K35), while the partly shaded curve (in red) only has the irradiance Gbz indicated. The operating point is the intersection of curve K35 and the barrier characteristic curves of the locally shaded cells. Such a module would not require bypass diodes