Module Circuit Design

A bulk silicon PV module consists of multiple individual solar cells connected, nearly always in series, to increase the power and voltage above that from a single solar cell. The voltage of a PV module is usually chosen to be compatible with a 12V battery. An individual silicon solar cell has a voltage of just under 0.6V under 25 °C and AM1.5 illumination. Taking into account an expected reduction in PV module voltage due to temperature and the fact that a battery may require voltages of 15V or more to charge, most modules contain 36 solar cells in series. This gives an open-circuit voltage of about 21V under standard test conditions, and an operating voltage at maximum power and operating temperature of about 17 or 18V. The remaining excess voltage is included to account for voltage drops caused by other elements of the PV system, including operation away from maximum power point and reductions in light intensity.

In a typical module, 36 cells are connected in series to produce a voltage sufficient to charge a 12V battery.

While the voltage from the PV module is determined by the number of solar cells, the current from the module depends primarily on the size of the solar cells and also on their efficiency. At AM1.5 and under optimum tilt conditions, the current density from a commercial solar cell is approximately between 30 mA/cm2 to 36 mA/cm2. Single crystal solar cells are often 100cm2, giving a total current of about 3.5 A from a module. Multicrystalline modules have larger individual solar cells but a lower current density and hence the short-circuit current from these modules is often approximately 4A. However, there is a large variation on the size of multicrystalline silicon solar cells, and therefore this current may vary. The current from a module is not affected by temperature in the same way that the voltage is, but instead depends heavily on the tilt angle of the module.

If all the solar cells in a module have identical electrical characteristics, and they all experience the same insolation and temperature, then all the cells will be operating at exactly the same current and voltage. In this case, the IV curve of the PV module has the same shape as that of the individual cells, except that the voltage and current are increased. The equation for the circuit becomes:

where:
N is the number of cells in series;
M is the number of cells in parallel;
IT is the total current from the circuit;
VT is the total voltage from the circuit;
I0 is the saturation current from a single solar cell;
IL is the short-circuit current from a single solar cell;
n is the ideality factor of a single solar cell;
and q, k, and T are constants as given in the constants page.

The overall IV curve of a set of identical connected solar cells is shown below. The total current is simply the current of an individual cell multiplied by the number of cells in parrallel. Such that: ISC total = ISC × M. The total voltage is the voltage of an individual cell multiplied but the number of cells in series. Such that:

ISC total = ISC × M

IMP total = IMP × M

VOC total = VOC × N

VMP total = VMP × N

I-V curve for N cells in series x M cells in parallel.

 
1 cell

 

 
1 cell

 

1 cell