Bypass Diodes in Solar Modules

Parallel connection of a standard bypass diode to each solar cell obviates hot-spot formation. If a cell is shaded or defective, the bypass diode allows the current to bypass the remaining solar cells, in which case

Partial Shading of a Cell of a Module (Charging battery)

(partially) shaded cell

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Figure 4.21 Characteristic curves for a 36-cell module with Az ~ 102 cm2, cell temperature Tz — 25 0C and a partly shaded cell, while a12V battery bank is being charged; 35of the cells have 1 kW/m2 insolation (curve K35), while the partly shaded curve (in red) only has the insolation indicated. The partly shaded cell is subjected to the voltage (reduced by 12 V) of the 35 fully insolated cells (curve 35/12 V). The operatingpointisthe intersection of the characteristic curve and the barrier characteristic curves of the locally shaded cells

the negative voltage in the jeopardized cell is only around 0.6 to 0.9 V, and with Schottky diodes 0.3 to 0.5 V (bypass diode forward voltage; see Figure 4.18).

Use of a bypass diode for each cell is the optimal solution, but it is also very expensive and far from indispensable. Inasmuch as a solar cell operated in the third quadrant (i. e. in the non-conducting direction)

I-V-Characteristics of a Solar Module with a (partially) Shaded Cell

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Figure 4.22 I—V characteristic curves (as in Figure 4.19), without bypass diodes, with an Az « 102cm2 cell area, Tz — 25 0C cell temperature and one cell partly shaded. Although only 1 of the 36 cells is partly shaded (resulting in irradiance Gbz), the characteristic curves are drastically altered and power at the MPP decreases sharply. If bypass diodes are integrated into the module, the characteristic curves change somewhat, depending on circuit configuration

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Figure 4.23 Bypass diodes protect a group of series-connected solar cells against hot-spot formation. One bypass diode should be used for about 12-24 solar cells

can withstand up to a few volts without any difficulty, before the approximate value (from Equation 4.3) is reached for the maximum area-specific thermal power loss, depending on the vendor’s suppositions in this regard, one bypass diode for a group of 12-24 series-connected solar cells suffices (see Figure 4.23). In the case of a locally shaded cell in a group that is protected by a bypass diode, the current produced by the solar cells outside such a group is able to bypass the group via the bypass diode. The situation within such a bypass diode group is roughly the same as what would occur if the entire module were operated at short- circuit current, even if this is not actually the case. Bypass diodes need to be sized for somewhat higher than normal currents owing to possible short-term overload caused by cloud enhancements ([4.11] for >L25 ■ Isc-stc).

With conductive bypass diodes, the voltage in the shaded cells exceeds the voltage in the unshaded cells by the amount of the voltage drop at the diodes, i. e. it is about the same as the voltage generated by the bypass diode group under normal operating conditions and thus does not pose a problem. Figure 4.19 also displays a curve (for 11 cells plus one diode) representing the voltage at the partly shaded solar cell, for a scenario where one bypass diode is integrated for each group of 12 cells as shown in Figure 4.23. In this configuration, the maximum possible thermal power losspVTZ at a partly shaded cell is around 2.5 kW/m2, which means that no hot-spot damage will occur.

Proper use of bypass diodes for series-connected solar cells can thus reliably obviate hot-spot formation in individual shaded cells and the attendant damage. Under normal operating conditions, bypass diodes induce no power loss, and for reasons of safety should be integrated into all PV installations whose system voltage exceeds 12 V, except in cases where the solar cells exhibit controlled avalanche point character­istics or have pre-installed bypass diodes.

Many commercial solar cell modules integrate the requisite bypass diodes or such diodes can be integrated into the module junction boxes. In most cases one bypass diode for 10-24 solar cells suffices. The smaller a bypass diode group, the less sensitive the module is to partial shading and the higher the cost. It would be helpful if vendors’ module datasheets indicated the size of a bypass diode group for the module in question, or at least the number of bypass diodes per module.

If, as is the case with most modules, bypass diodes are integrated for groups of series-connected solar cells rather than for each individual cell, solar module power decreases disproportionately when any individual cell is shaded. Bypass diodes do not ameliorate this situation very much unless a large number of modules are series connected in a string, in which case the bypass diodes do help to prevent an unduly large power drop in the series string in the case of local shading of individual cells or an entire module.

Bypass diode sizing for maximum module operating temperature [4.11] is

Forward current IF > 1.25 ■ ISC_STC

(4.4)

Reverse voltage VR > 2 ■ VOC

(4.5)

Bypass diodes integrated into a module also need to be adequately cooled. The larger the solar cell area and the higher the solar cell voltage, the greater the need for cooling. Whereas for a module whose ISC is around 3.5 A, inexpensive non-cooled, plug-in 6 A diodes are sufficient, 12 A diodes, which need to be cooled, must be used for larger cells (e. g. 15 cm ■ 15 cm). This problem can be partly solved using Schottky diodes, whose forward-direction voltage drop is only around 0.3 to 0.5 V, although the dielectric strength of such diodes is considerably lower than for standard Si diodes. Bypass diodes in the non-conducting direction should exhibit about twice the open-circuit voltage of either the relevant module or (at a minimum) of all solar cells that are part of the module’s bypass diode group. That said, for reasons of lightning protection the highest possible inverse current should be used for bypass diodes (see Section 6.7.7).

Updated: August 5, 2015 — 2:54 am