Wednesday, June 24, 2009

Shade Will Degrade

Here's something that I've been meaning to check out. Although our main roof is fully loaded with panels, there is still some open roof space on the garage roof:

For a few weeks I've been thinking "When everything settles down I'm going to see about adding a few more panels here so we can generate even more power! "

The groSolar people told us that they didn't want to put panels in that space because late in the day (especially during winter) it will be shadowed by the higher main roof. I took that to mean that we'd get a lot less power out of panels that received shade during part of the day but I didn't fully understand. I thought it was analogous to putting buckets out in the yard to collect rain water: if some of the buckets were blocked for part of the day, well sure, they wouldn't end up with as much water as the fully exposed ones, but at least they'd contribute some water.

^ =

That's a horrible analogy, as it turns out. I've researched this (I surfed the internet) and learned that the shaded panels would actually degrade effectiveness of the entire system. Panels and buckets don't operate the same way.

EcoDepotUSA says:

"Once a solar cell or a portion of a cell is shaded it becomes a load and draws power instead of producing it. Watch the amp meter of your system when a hand is passed over a module and you will see a substantial drop in output."

WholeSolar says this:

"If even one full cell is hard shaded, the voltage of that module will drop to half of its unshaded value in order to protect itself. If enough cells are hard shaded, the module will not convert any energy and will, in fact, become a drain of energy on the entire system... Because all cells are connected in a series string, the weakest cell will bring the others down to its reduced power level."

It turns out that there are some types of panels out there that are "shadow protected" but these are more expensive and don't (yet) have the track record of reliability that the basic poly-crystalline panels have.

We're better off with just the panels we already have and no more. I'm sure the installers explained this but I didn't catch on. Just one more reason why at least in our case it's a good thing we left this job to the professionals.

1 comment:

The Cashmere Bunny said...

Thank you for your blog. I will be watching it for updates. We have installed solar heating for our pool and Solar Hot Water tank. Both save us money. I would love to install panels for electricity.