We often hear system designers complain that they can’t reduce their shading below ~1 to 2% on their ground-mount solar array, regardless of how widely they space the rows. This shade loss shows up in arrays with steeply tilted modules (30° or greater). The designer tries increasing the row spacing, and beyond a point it has practically no effect on the shade losses. In essence, this is “zombie” shading that is practically impossible to kill.
This phenomenon was a mystery—and the reason why it was occurring surprised even us, forcing us to reconsider what shade losses even are.
An incorrect hypothesis
This “zombie” shading shows up for our users all the time. It seems to come up most often on projects in northern states, such as Minnesota or Massachusetts. And it isn’t just a user error—this is something that anyone can replicate with a steeply tilted array.
Given the geographic footprint, we thought we knew what the cause was: latitude. At high latitudes, much of the sun’s energy comes in from lower angles in the sky. With lower sun angles, you have longer shadows, and therefore greater row-to-row shading, even with wide row spacing. In fact, we already looked into this in our earlier analysis on wall-mounted solar, and showed that the weighted average elevation angle of sunlight is 32° in Alaska vs. 47° in Florida.
Back to the high-tilted designs: the plot thickens when we look at this same design in Charlotte. With an array much further to the south, the shade losses at very wide row spacing are practically the same! This pretty much proves that this loss isn’t from the sun angles at northern latitudes.
The surprising answer
We then decided to dig deeper into the numbers behind the shade loss—specifically, the components of sunlight. There are two main parts of sunlight that we talk about all the time: the direct sunlight (the beams from the sun) and diffuse sunlight (the brightness of the blue sky). Typically, direct sunlight accounts for 60 to 70% of total sunlight, with diffuse accounting for 30 to 40%. But there is also a third one that rarely matters: Albedo (the reflection from the ground). Albedo never matters for standard arrays, for two reasons. First, the ground makes up a tiny proportion of the modules’ plane-of-array view, since modules that are tilted up are mostly “looking at” the sky, with the ground a tiny slice of the view. Second, the reflection from the ground only strikes the first row of the array, and the first row then blocks the ground reflection from the second row of modules. As a result, the overall albedo number is always really small, typically 0.1% to 0.3% of total sunlight.
Yet once we looked into the details, it turns out that the “albedo is tiny” mindset changes once your tilt is up to 30° tilt. When you tilt the array up at 30°, the plane-of-array albedo contribution to sunlight actually goes up to over 2%. Not huge, but now it is something you will notice. As the plane-of-array albedo value grows, you still have the phenomenon that the first row of modules blocks the view of the second/third/etc. As a result, the increased plane-of-array albedo sunlight is immediately lost and counted as shade loss.
It is this “ground shading” that ends up accounting for the majority of the “zombie” shade losses in these high-tilt arrays. Depending on the tilt angle, the lost albedo accounts for between 60-90% of the shade losses in these designs. For example, in the example below, note that 1.8% of the 2.9% shade loss is from albedo shading (and most of the rest comes from diffuse, not direct).
So the reason this phenomenon was coming up in northern locations wasn’t due to the sun angles, it just happens that designers were designing high-tilt systems in northern latitudes, and the tilt made the albedo a more significant share of sunlight.
A philosophical question: What is shade loss, really?
This ultimately brings up a broader question: When is shading loss actually shading loss? We typically think of shade as something blocking sunlight from striking the surface of a module. But in this case, blocking the reflection from the ground feels different. After all, most arrays don’t even get much albedo/reflection sunlight to begin with. High-tilt arrays have a much bigger plane-of-array albedo because of their tilt, but then immediately lose it from row-to-row shade. It’s arguably just an accounting technicality.
If a tree falls in the woods, and there’s nobody there to hear it, does it make a sound? If my high-tilt array gets more ground reflection, but then immediately blocks it from itself, is it really shade loss?