When the photovoltaic cells of the pv panels are illuminated while they are connected to an external load, they worka as a generator of energy and generate current and voltage values that are variable depending on the temperature and irradiance (magnitude that describes the radiation or intensity of solar illumination that reaches us as an instantaneous power per unit area, W / m2 or equivalent units) at that moment and from the point of work imposed by the load (this means that, for example, if our panel is connected to a 12V battery, will limit the output voltage of the panel, to 12V).
Let’s see, then, graphically, the characteristic parameters in the following graph, to define them later:
The basic parameters:
Thus, the fundamental parameters of a solar module are:
The short circuit current (ISC) is the value of the current flowing through the panel when the voltage at its terminals is zero, V = 0 Volts, and is the maximum current that could be obtained (ideally) from the panel when he works as a generator.
The open circuit voltage (VOC) is the highest voltage that can polarize the device when it works as a generator.
The point of maximum power (PM) is a point of work in which the power delivered by the solar panel to the external load is maximum. The power P delivered by the panel is: P = I x V, which is also represented in the previous figure. The corresponding work point, (VMP, IMP), defines the nominal values of the voltage and current at the maximum power point (not to be confused with maximum voltage and current), being: PM = IMP x VMP.
The form factor (FF, from English, Fill Factor): is the ratio between the maximum power (or the product of the current and the voltage at the point of maximum power) and the product of ISC x VOC. Its value is higher as the better quality the cell is. In general, a low value of FF is associated with the stock of efficiency losses in the device, while a good quality cell usually has FF values greater than 0.70. For example, SolarWorld solar panels have an FF> 0.75, which confirms that the higher the FF, the more quality the panel has.
Efficiency: is usually expressed as a percentage and is the ratio between the electrical power delivered by the panel and the power of the radiation that falls on it.
These are the fundamental parameters of a solar panel, which must always be measured under a series of internationally accepted working conditions, known as Standard Measurement Conditions (STC – Standard Test Conditions), which are defined by 1000 W / m2 of irradiance, with a spectral distribution AM1.5G and 25 ° C of temperature.
These are the conditions that are taken into account when expressing the nominal values of a solar panel in its technical specifications.
It can be checked, for example, if we download the technical sheet of the solar panel SolarWorld SW 260 Poly, as it is indicated that the values of the sheet are under the STP conditions:
This was the today article about how solar panels works according with the basic parameters that you will need for any calculation related with pv systems.