Going Solar Chapter 10 : Know the effect of partial shading
Solar PV Panels are build to last very long, hence it is important to anticipate the changes around the panels. Things that is easy to be overlooked such as a growing foliage will soon grow tall after few years and causing shade over your panels. Other possible changes includes new construction of new tall buildings nearby.
Other sources of partial shading will be shadow from a protruding pipe on the roof, roof ornaments, chimneys, flag pole or even shadow from the neighboring panels.
Partial shade can significantly reduce the power generation from your panels, hence look hard for all potential objects that can cause partial shade. A growing tree beside the house is often overlooked.
One shall not under estimate the effect of partial shading of a solar PV panel as it will severely impede the performance of a panel, even a small area of a solar PV cell. If the panel is part of a bigger string array in series, it can even cause a total shutdown of the string.
Never underestimate effects of partial shading
A partially shaded solar PV cells will cause an increase of internal resistance, which in turn reduce the amount of current that is able to pass through the semiconducting cell. This have great impact on solar PV panel power output as cells on a module is mostly arranged to be in series. A drop in current flow of a single cell will affect the whole string.
When there is a partial shading, even a single solar PV cell, the whole module output is affected. It is usually improved with placing a bypass diode that channel the current from the section with all unshaded cell away from the section with partial shaded cell.
A 25% partial shading of a single cell can reduce the whole module efficiency by 25%, a 75% partial shading of a single will be worst affected than three cells with 25% partial shading.
The effects of partial shading of a solar panel. It is depend on how the shade is fall on the cells, but it will be worst affected if the most shaded cell is cover a significantly large area of the cell. Above is the rough estimates of reduction cause by partial shade.
As unshaded cells try to pass more current than the shaded cell is capable of handling (partial shading cause drops in short-circuit current of that cell – which means less current can pass through that cell), and excess power will be dissipated as heat.
The enormous power dissipation occurring in a small area results in local overheating, or “hot-spots”, which in turn leads to destructive effects, such as cell or glass cracking, melting of solder or degradation of the solar cell. It can be seen via a infrared thermal imaging where hot spots can be heated up to 100 degree Celsius. There is other causes of hot spots such as defective or broken cells and defective solder joints.
The hot-spots on a solar PV panel should not be ignored.
