PV Cell Interconnection and Module Fabrication

A bare single cell cannot be used for outdoor energy generation by itself. It is because (i) the output of a single cell is very small and (ii) it requires protection (encapsulation) against dust, moisture, mechanical shocks and outdoor harsh conditions. Workable voltage and reasonable power are obtained by interconnecting an appropriate number of cells. Cells from same batch are used to make PV module. This is done to ensure that mismatch losses are minimal in the module.

The electrically connected cells are encapsulated, typically by using two sheets of ethylene vinyl acetate (EVA) at either side. EVA is a good electrical insulator, transparent material and has very low water absorption. The encapsulant cannot provide rigidity to the module, for which glass is provided at the front side of the module. At the rear side of the module a hard polymer material, typically, polyvinyl fluoride (PVF, also known as tedlar) is used. Theses layers are arranged as shown in Fig. 1.9 and hermetically sealed to make it suitable for outside applications for 20-30 years without environmental degradation. This assembly is known as solar module – a basic building block of a PV system. Most common commercial modules have a series connection of 32 or 36 silicon cells to make it capable of charging a 12-V storage battery. However, larger and smaller capacity modules are also available in international market.

Fig. 1.9: PV Module Details

Cell Mismatch in a Module: In a module, several cells are interconnected, it is very important that these cells should match as closely as possible. That means V_oc, I_sc, V_m and I_m (or fill factor) for all cells must be exactly same. Any mismatch in the characteristics of these cells leads to additional mismatch loss. Therefore, peak power of the combination is always less than the sum of individual peak power of the cells. Only under ideal case when all cells are identical that the resultant peak power would be equal to arithmetic sum of that of its constituents.

When two cells with mismatched characteristics are connected in series and load is applied, both cells are bound to carry same current. The composite characteristics of the combination can be obtained by adding the individual output voltage of the cell corresponding to a common current, for all operating points. At a particular operating point, while one cell may be operating at peak power, the other may not. Thus, peak power of the combination is always less than the sum of individual peak power of each cell. Also, if such a combination is short circuited, equal and opposite voltages are produced by individual cells and therefore, one cell will be generating power while the other will be dissipating it. Had the two cells been perfectly matched no power would be generated or dissipated.

Similar conclusion may be drawn by considering a parallel combination of two mismatched cells. Here the voltages of the cells are bound to be equal, but the currents will be different and hence the maximum power points. It can also be shown that larger the number of cells in a module more would be the possibility and quantum of mismatch loss.

To reduce mismatch losses, modules are fabricated from cells belonging to same batch. Also, cell sorting is carried out to categorize cells having matched parameters with specified tolerance.

Effect of Shadowing: Partial shadowing may have serious consequences and may completely damage a module due to creation of hot spot. Let us examine the operation of a module under the conditions of: (i) partial shadowing of a cell in an open circuited, series string of cells and (ii) complete shadowing of one cell in a short circuited, series string of cells.

When a cell is partially shadowed, the shadowed portion will not produce any power, but the remaining portion will remain active and produce power. The generated voltage by illuminated portion will forward bias the parallel rectifier corresponding to shadowed portion as shown in Fig. 1.10. If shadowed area is relatively small, the large circulating current through it will result in excessive heating of the shadowed portion. The phenomenon is known as hot spot effect and may completely damage the module for prolonged partial shadowing.

Fig. 1.10: Partial Shadowing of a Cell

A short-circuited, series string of (n + 1) cells with one cell completely shadowed is shown in Fig. 1.11. Here the voltages produced by n illuminated cells add up and appears as reverse bias voltage of nV volts across the shadowed cell. If peak inverse voltage (PIV) of the shadowed cell is more than the reverse bias, no current will flow. If, however, the PIV is less than total reverse voltage appearing across the shadowed cell, current will flow through the string, dissipating large power in the shadowed cell, leading to possible damage of the module. The chances of damage to the shadowed cell, due to excessive heating, increase with the number of cells in the string. If the string supplies a load instead of being short-circuited, the chances of damage persist through to a lesser extent. The damage due to shadowing can be avoided by connecting a bypass diode across the affected cell as shown in Fig. 1.11.

Fig. 1.11: Shadowed Cell and Bypass Diode Connection

This bypass diode would allow an alternative path for the load current. During healthy operation, the bypass diode has no role as the cell voltage would keep it reverse biased. Even so, its use would result some loss because of finite reverse leakage current through it. It is neither practical, nor required to incorporate a bypass diode across each cell in a module. It has been the international practice to provide a bypass diode for every 18 crystalline silicon solar cells in a series string. Thus, the internationally standard module with 34–36 cells would contain two bypass diodes placed inside its terminal box.