The design of a photovoltaic module or system intended to provide safe conversion of the sun''s radiant energy into useful electricity must take into consideration the possibility of partial shadowing of the module(s) during operation. This test method describes a procedure for verifying that the design and construction of the module provides adequate protection against the potential harmful effects of hot spots during normal installation and use.
This test method describes a procedure for determining the ability of the module to provide protection from internal defects which could cause loss of electrical insulation or combustion hazards.
Hot-spot heating occurs in a module when its operating current exceeds the reduced short-circuit current (Isc) of a shadowed or faulty cell or group of cells. When such a condition occurs, the affected cell or group of cells is forced into reverse bias and must dissipate power, which can cause overheating.
Note 18212;The correct use of bypass diodes can prevent hot spot damage from occurring.
Fig. 1 illustrates the hot-spot effect in a module of a series string of cells, one of which, cell Y, is partially shadowed. The amount of electrical power dissipated in Y is equal to the product of the module current and the reverse voltage developed across Y. For any irradiance level, when the reverse voltage across Y is equal to the voltage generated by the remaining (s-1) cells in the module, power dissipation is at a maximum when the module is short-circuited. This is shown in Fig. 1 by the shaded rectangle constructed at the intersection of the reverse I-V characteristic of Y with the image of the forward I-V characteristic of the (s-1) cells.
By-pass diodes, if present, as shown in Fig. 2, begin conducting when a series-connected string in a module is in reverse bias, thereby limiting the power dissipation in the reduced-output cell.
Note 28212;If the module does not contain bypass diodes, check the manufacturer’s instructions to see if a maximum number of series modules is recommended before installing bypass diodes. If the maximum number of modules recommended is greater than one, the hot spot test should be preformed with that number of modules in series. For convenience, a constant current power supply may be substituted for the additional modules to maintain the specified current.
The reverse characteristics of solar cells can vary considerably. Cells can have either high shunt resistance where the reverse performance is voltage-limited or have low shunt resistance where the reverse performance is current-limited. Each of these types of cells can suffer hot spot problems, but in different ways.
Low-Shunt Resistance Cells:
The worst case shadowing conditions occur when the whole cell (or a large fraction) is shadowed.
Often low shunt resistance cells are this way because of localized shunts. In this case hot spot heating occurs because a large amount of current flows in a small area. Because this is a localized phenomenon, there is a great deal of scatter in performance of this type of cell. Cells with the lowest shunt resistance have a high likelihood of operating at excessively high temperatures when reverse biased.
Because the heating is localized, hot spot failures of low shunt resistance cells occur quickly.
High Shunt Resistance Cells:
The worst case shadowing co.........
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