Design for partial shade?

[humanappliance]

Hello, I'm new to solar and learning a ton here! We recently bought a small off-grid a-frame cabin and would like to add solar to support a modest electrical load while we're there visiting. In the summer, the panels should get a few hours of direct sun, but the rest of the day and most of the year the sun will be coming through a line of trees and direct sun will be dappled.

Are there special considerations to think about when designing for dappled sun conditions? Is this situation even viable for solar?

Thank you!

 

[Q-Dog]

Sunlight has to hit the panels. Dappled is just another word for shadows and panels don't do much in shadow.

 

[humanappliance]

Some sun will hit the panels. Let's say each panel will have around 40% sun, unevenly distributed.

 

[zanydroid]

You will get maybe 5-10% of the power so it will be pretty terrible. Even if you got large patches of light and installed the fanciest shade compensation hardware (eg microinverters) you will likely get that amount since there is a theoretical limit of power from the way the solar panels are wired internally.

I use half cut cells (which have superior internal wiring for shade) and microinverters with my bad shading situation and I only get th equivalent production of .5-1 sun hours on the worst days of the year.

 

[zanydroid]

Some sun will hit the panels. Let's say each panel will have around 40% sun, unevenly distributed.

Still terrible.

The distribution of sun on the panel will affect how much power you get. Since the pattern of sun on the panel will vary throughout the year and as the trees change it is very difficult to model. In my experience after a year of watching shade production you should just assume the worst and be happy if any improvement comes from having optimizers or microinverters. The math and empirical measurements have been done many times on the forum.

The only reliable annual production modeling is for simple unobstructed light hitting the whole panel.

Regarding difficulty of modeling, if a hard shadow is rotated 90 degrees there will be quite different output power because the solar panel is not wired symmetrically.

 

[zanydroid]

The inconvenient truth is that the default behavior of solar cells is that shadows or decreased light of any sort block or throttle power from the cells getting good light.

With the wiring of bypass diodes and parallel current paths within the module being some ways to claw a bit back

 

[Hedges]

Or zero.
If 40% of the panels get full sun, then you can design something that works.
If 100% of the panels get some shade all the time, no panels ever gets full sun, you won't get much of anything.

The "few hours of direct sun" could be great. Try to quantify that in terms of time of day, month, and panel orientation, then use an insolation calculator to evaluate.

Maybe you will get half the effective sun hours that would be available without the trees, and that should be perfectly viable. But only during those summer months.

Any running surface water in the winter?

 

[bhbaker220]

My solar generation has fallen off significantly due to tree shading at different parts of the day. I have very tall pine trees which don’t shed much needles in the fall/winter. PVWatts was very accurate in August, but September was a little lower by 10%, October 25% lower, and November was about 40% lower. Some of this reduction was due I think in more cloudy days than the 30 year average that PVWatts uses but I don’t have any multiple years of experience to try and segment this out from shading.
I used the phone app SunQuest to gain a better perspective on the impact of shading. It shows the path of the sun during the various times of the day and allows you to use your phones camera to pan the horizon and follow the sun position at various parts of the day and see which trees are going to impact shading. While it does not quantify the impact of shading on power impact, it helps you determining what parts of the day are going to have lower power output.
On a sunny day in November, my 400 watt panels peak at about 325 watts/panel with no tree shading. Once the sun goes behind the pine trees, that drops to 100-150 watts/panel on the panels on a higher pitch section of my roof, but drop to about 25 watts/panel on a lower pitch section of the roof. The peaks and valleys line up very well with what you see on SunQuest.
If I would have used SunQuest before I put my system in, I probably would have put more panels on the higher pitch section of the roof and less on the lower pitch. Now that I know this, I might move some panels around to generate better performance.