Photovoltaic (PV) modules convert sunlight directly into electricity. This is achieved through the use of semiconducting materials. Common materials include Silicon (Si), Cadmium Telluride (CdTe) and Copper Indium Gallium Selenide (CIGS). As sunlight falls on these materials the energy is converted to electricity and because of the way the materials are structured within the panel it then flows out into the circuit formed by the rest of the system.
The sunlight that reaches the PV modules is composed of three different parts. These are:
- Direct Sunlight: The part of sunlight coming directly from the sun to the module
- Diffuse Sunlight: the part of sunlight that has been scattered by the atmosphere and arrives at any angle
- Reflected Sunlight: Sunlight that has reflected off the ground or other surfaces
It is not the case that PV solar modules need direct sunlight to work. They will generate electricity from any light that reaches them. This means that even in overcast conditions PV modules will continue to generate electricity. Other factors that can influence the output of a solar power plant include the orientation and angle of the modules and also any objects nearby that might cast shadows. As the sun moves through the sky its angle to the modules changes and this reduces the amount of available energy. Modules can be tilted and orientated to specific positions to maximise energy generated. Also shadows cast on the surface of the modules can produce large drops in output; if 10% of a module is shaded it can reduce output by over 50% depending on the way the system is wired. For this reason it is crucial to accurately identify the shading conditions of the desired site.
Electricity from the modules is gathered together via junction boxes and is converted from DC to AC by an inverter before either being used onsite or exported to the grid. All components such as cabling, mounts, inverter, transformer etc. are known as balance of system. PV modules are sold with a particular rating. This is known as their peak rating and is given in Watts peak (Wp). This rating is calculated under certain conditions which are never achieved in the real world but this measure is still useful as a way to describe the size of a system and how many modules are necessary. The actual energy generated by the system depends on many factors. It is usually called specific yield and is given units of (kWh/kWp). Two systems of the same size in Wp could have completely different specific yields.