Solar PV Introduction
A module is made up of a series of solar cells. A solar PV cell is a semi-conductor device with boron and phosphorous additives. Unlike conductors (metals), semi-conductor materials require excitation before it can conduct electricity. This is called the photo-electric effect, where sunlight excites the electrons within the semi-conductor to generate useful electricity, a property enhanced with additives.
A semi-conductor has very unique characteristics, one of which is the ‘bandgap’ between the valence band and conduction band (essentially a gap within the electrons must travel to reach the conduction band where it is capable of generating useful electricity).
With increasing temperature the silicon semi-conductor becomes less effective as the atoms within the conductor become increasingly agitated and disrupts the flow of excited electrons (e-h pairs). At extremely low temperatures there is insufficient ionizing energy to create flow. However the extremes in temperatures are not reached in the day to day operations of solar PV (we’re talking 200°C).
In simplest terms; the hotter a solar cell gets, the less it is able to transport electricity and leads to a direct reduction in voltage.
Because power is a function of voltage and current (Power = Voltage X Current) a decrease in voltage will lead to a proportional decrease in power generation of the solar cell. Current stays relatively constant within the typical operating parameters of the solar PV module.
The normal operating cell temperature (NOCT) is often a better indicator of what the performance of the module will look like.
They are included in the module spec sheet and will have the performance output of your panel at more realistic operating conditions such as higher temperature, lower air mass and lower irradiance.
Solar PV modules are designed specifically to harness light. Each cell is manufactured with an etched surface that resembles small pyramids on a microscopic scale as well as up to 3 layers of anti-reflection coatings to minimize reflection for certain wavelengths of light. Combined with darker coloured modules in general, both light and heat have trouble escaping.
Essentially this means solar PV modules can be much hotter than the ambient temperature. On a summer day with temperature of 30°C, a PV module can have temperatures up to as high as 65°C.
Although more efficient in cold weather, the position of the sun is also further away and the days are much shorter. Because of this there is generally less available sunlight overall that can be harvested into electricity.
At higher temperatures, solar panels will suffer a power loss. However because summer days are much longer and the sun is closer, there is also more sunlight available.