Module Circuit design refers to the parameters selected for producing the appropriate solar module. Those parameters are mainly the number of cells, total voltage and total current. The silicon PV module consists of a number of single solar cells connected, typically in a series connection, so as to increase the power and voltage produced to greater values than that obtained from a single solar cell.
PV module voltage is usually selected so as to be compatible with the battery voltage of 12V. The single silicon solar cell has a voltage which is slightly under 0.6V under standard conditions of 25 °C and AM1.5 illumination. Taking into consideration the anticipated reduction in the PV module voltage because of temperature and also that a battery may require voltages that can be equal to 15V or more to be charged, most of the modules contain 36 solar cells connected in series. This generates an open circuit voltage approximately equal to 21V under standard conditions, and an operating voltage at maximum power and operating temperature nearly equal to 17 or 18V. The excess remaining voltage is included so as to account for any voltage drops caused by other elements of the solar system, as operation away from maximum power point and any reductions in intensity of incident light.
In a common module, 36 cells are connected in series to produce enough voltage for charging a 12V battery.
Voltage and Current of PV module
Although PV module’s voltage is determined by the number of individual solar cells, the current produced from the module mainly relies on the size of the solar cells and also on the efficiency of the solar cell. At standard conditions of AM1.5 and under optimum tilt conditions, the commercial solar cell’s current density is usually between 30 mA/cm2 and 36 mA/cm2. The single crystal solar cell is regularly 100cm2, producing a total current nearly equal to 3.5 A from the solar module.
Multicrystalline modules are made of larger individual solar cells but have a lower current density and thus the short circuit current from these modules is usually equal to 4A. Yet, there is a significant variation on the size of multicrystalline silicon solar cells and therefore the short circuit current may change. The module’s current is not affected by temperature in the same manner that the voltage is affected, but on the other hand heavily depends on the tilt angle of the module.
If all the module’s individual solar cells have similar electrical characteristics, and they all exposed to the same insolation and temperature conditions, then all the cells will operate at the same exact voltage and current. Thus, the IV curve of the PV module will have the same shape as the curve of the individual cells, except for the factor of increase of the voltage and current. The equation for the circuit then becomes:
N : number of cells in series
M : number of cells in parallel
IT : the total current from the circuit
VT : the total voltage from the circuit
I0 : the saturation current produced from a single solar cell
IL : the short circuit current from a single solar cell
n : the ideality factor of a single solar cell
q, k, and T : are constants.
The total current is basically the individual cell’s current multiplied by the number of cells in parallel, while the total voltage is the individual cell’s voltage multiplied by the number of cells in series as below: