In a previous article, we spoke about a solar power generation installation that was completed at a facility was described. This installation has been in service for a little over a year now, and enough data has been collected to confidently assess its performance.
One peculiarity that was noticed, is that one of the modules appeared to consistently generate less electricity that the other 11 modules in the array. Upon further review of the data, it was determined that the under-generation occurred in the morning hours and by 10:30 AM or so, that particular module had “caught up” with the other modules in its production. What could be going on?
During an installation training course we learned that even small amounts of shading of a solar module can “short out” the cells within the module and cause dramatic reductions in output power. We suspected this could be happening, but having been careful during the original design to avoid objects from the South which might shade the array, were not quite sure where the shading could be coming from.
During a site visit, we looked suspiciously at near high voltage utility lines as a culprit. Upon further review, we eliminated that possibility because of their height and relatively small diameter.
It was not until we propped a ladder on the roof, and climbed up and took a look, that we solved the mystery. Along the side of the solar array, the installer had mounted a small electrical disconnect switch, required by the local Utility, for safety reasons. The switch was located about one foot east of the end of the array and was mounted on strut, sticking up about two feet into the air.
In the accompanying photograph, you can see a shadow from the switch on an end solar module. As the sun rises in morning, the shadow begins to fall across the module from east to west. At around 9:00 AM, the shadow cuts across one third of the width of the module, while the module's output is only about 1/3 of its neighbors.
Partial Shading by Disconnect Switch
By around 10:15 AM, the sun has caused sufficient that the shadow only cuts across 10% of the module's width. Output is reduced by only about 25% when compared to the others. By 10: 30-11: 00 AM, the sun has risen high enough that the shadow is no longer touching the module and output is similar to the other modules in the array.
8:30 AM-Only 21 Watts
9:30 AM-Doing Better
10:30 AM-Almost There
11:00 AM-All Caught Up
A valuable lesson in solar array placement has been learned. Even a small shadow from a nearby object can have a dramatic negative negative impact on the productivity of your solar power generation project.