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Solar Power Generation Mystery Solved

In a previous blog article, I had described a solar power generation installation that IEA has completed at one of its facilities. This installation has been in service for a little over a year now, and we have collected enough data to confidently assess its performance.

One peculiarity we have 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, we 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 had attended, we had 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 occurring, but having been careful during the original design to avoid objects from the South which might shade the array, weren’t quite sure where the shading could be coming from.

During a site visit, we looked suspiciously at nearby 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 about one third of the width of the module, while the module’s output is only about 1/3 of its neighbors.  (Click on the image for a larger view)

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Partial Shading by Disconnect Switch (Click for larger image)

By around 10:15 AM, the sun has risen sufficiently 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.

Here are screen shots of the array’s output power over the first few hours of the day.   (Click on the images for a larger view)

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8:30 AM–Only 21 Watts (Click for larger image)

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9:30 AM–Doing Better (Click for larger image)

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10:30 AM–Almost There (Click for larger image)

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11:00 AM–All Caught Up (Click for larger image)

A valuable lesson in solar array placement has been learned.  Even a small shadow from a nearby object can have a dramatic negative impact on the productivity of your solar power generation project.

Thanks for reading,
Ervin–Chief Engineer