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36 CHAPTER 1 Solar Cells and Arrays: Principles, Analysis, and Design

are the standard battery voltages. The load current is widely variable according to the
power consumption of the load.
In PV systems containing batteries, the batteries themselves are the loads on the
system; therefore, the current required is that of the nominal charging current of
the batteries. Basically, the solar cells can be combined to satisfy a wide range of
the load requirement concerning current, voltage, and power. A large solar cell array
is subdivided into smaller arrays called the solar cell panels, which are composed of
modules. Then a large array is built from modules. A module has conventionally 12-
V and 6-A current with 72-W power under standard test conditions with AM1.
Because each cell contributes 0.5 Voutput, one has to connect at least 24 cells in se-
ries to get 12 V. Because the solar cells must drive the load even at solar radiation
less than AM1, the reliable module contains 36 cells rather than 24 cells.
The current demand of 6 A can be satisfied by solar cells having an area of
2
6A/30 mA ¼ 200 cm and a diameter of 15 cm or 6 in cells.

5.1 THE IeV CHARACTERISTICS OF THE MODULE
The solar cell module is a unit array in the PV generator. It consists of solar cells
connected in series to build the driving force and in parallel to supply the required
current. A series-connected group of cells are called a solar cell string. Actually, the
strings are connected in parallel as shown in Fig. 1.31.
Assuming that the number of solar cells in the string is n e and the number of
strings in parallel are n s , then the total number of cells in the module is
n a ¼ n e .n s . Let us assume further to simplify the problem that the cells in the module
are identical having equal voltage drops ¼ V c and passing the same terminal current
I st , the string current. The module voltage is V st ¼ V m ¼ V a and the current of the
module I a ¼ I m , where m refers to module. To get I m ¼ f(V m ), we apply the Kirchh-
off’s circuit laws and replace each cell by its equivalent circuit as shown in Fig. 1.32.

FIGURE 1.31
Basic blocks of the solar cell module.