In order to understand a photovoltaic (PV) installation and its components we must be familiar with the following terms:
The output of a PV module is not stable it depends on irradiation, temperature etc., thus, we need fixed conditions in order to compare them, size and design our installation. These conditions are called as standard test conditions (STC):
a) Incident irradiance on the PV module surface: 1000W/m²
We consider that the output power of the module is the point of the IV curve which shapes the greatest area. As we can see the irradiance influences mostly the current.
b) Cell temperature: 25°C
We consider that the output power of the modules is the point of the IV curve which shapes the greatest area. As we notice the output voltage of a PV module depends mostly on its temperature.
c) Air Mass: 1.5
The Air Mass is the path length which light takes through the atmosphere normalized to the shortest possible path length (that is, when the sun is directly overhead AM=1).
The Air Mass is defined as:
AM = 1 / cos(Θ)
All the datasheets include their electrical characteristics data in STC. For instance:
|Nominal output (Pmpp)||W||280|
|Voltage at Pmax (Vmpp)||V||36.5|
|Current at Pmax (Impp)||A||7.7|
|Open circuit voltage (Voc)||V||44.5|
Is a series electrical connectivity of PV modules. An example of string is the following:
Assuming that our PV module’s output at STC is: Imp 7A, Vmp 30V, 210W If we will measure at points 1 and 2 we will find: 7A and 540V (18 PV modules X30V) and we get 3,78kW
An example of Array (parallel strings)
If we will measure at points 3 and 4 we will find: 14A and 540V (18 PV modules X30V) and we get 7,56kW. Taking in to consideration the above you can understand the attached Single Line Diagram (SLD) of a 2MW plant.Download SLD