The shading is the most important causes of vulnerability for the power output of PV module. 5% (for example) of shading on the PV module leads to a decrease in the ability of the module may reach more than 60 or 70% of the module power. This is due to, the connected cells in a string, where the cell with the lowest illumination is the main reason for the reduction of the current series in full. On the other hand, it is possible that the cells become shaded reverse biased, and power dissipation by the rest of the cells of the string, this creates a “hot spots”, which have the thermal potential to destroy the module. That is why on PV modules it should avoid all causes of shading. Shading due to the buildings, chimneys, trees, towers, or even by other parts of the same building.
2. 1. Previous Researches
Shading, which is located on a cell of a group of cells connected in series string, leads to the reverse bias in these cells, reverse bias and the microplasma breakdown have been explained and described physically by Bishop (1988). a comprehensive analysis have been made of the reverse biased solar cells, and applied to Bishop's model by Kovach (1995), in order to obtain conclusions regarding a hot spot formation and the yield reduction of PV panels. Alonso and Chenlo (1988) gave measurements for crystalline cells and amorphous cells led to a commercially made available. Kovach (1995) again found that the poor PV panel lay-out and under the same conditions of shading leads to a significant energy losses, and even small shadows can significantly affect the return of energy. The researchers also observed that all the solar cell I-U characteristics in reverse bias shows more variation from bias forward, and this result was verified by Laukamp et al. (1999).
To protect cells that have been shades of exposure to the breakdown, by adding a bypass diodes. Participated in the early eighties a number of authors in the development of improvements to the design of PV module, calculates the maximum number of solar cells required for each bypass diode, in order to avoid the formation of hot spots (Arnett and Gonzales, 1981; Bhattachary and Neogy, 1991; Gupta and Milnes, 1981; Shepard and Sugimura, 1984). Based on these calculations, the hot spot test become an essential part of the approval and confirmation for crystalline silicon modules according to IEC 61215 (1993). As a general rule, the series of solar cells in string that contain a number n of a cell, and fitted with one bypass diode, the absolute value of the breakdown voltage of reverse biased solar cells must be greater than n to n +1 times 0.5 volts. This value is almost equivalent to the MPP (Maximum Power Point) voltage of the (n -1) non-shaded crystalline silicon cells in the string plus the transmission voltage of a silicon bypass diode, i.e.from 0.5 to 1 volt. The weakest link in the chain of cells are the solar cell exposed to the maximum breakdown...