Shading of PV array from trees

[bernard]

I have been reading the Solar pathfinder's manual, and interestingly they advice the following when it comes to shading of PV arrays,strings from coniferous(evergreen) and deciduous trees:

1. Do not count any half-hour periods shaded by evergreen trees, as they cast shadows year-round.

2. Do not count half-hour periods shaded by deciduous trees during the leaf-bearing months; for thermal collectors, count these half-hour periods at half their value during non-leaf-bearing months. For PV panels, these half-hour periods should be assigned a value of zero, unless the manufacturer can support a better figure.

If I understood correctly, Solar pathfinder is basically suggesting that the following percentage of solar radiation passes through trees:

for coniferous trees: 0%
for deciduous trees in summer: 0%
for deciduous trees in winter: 100%

I found that software's like PV Sol, use a bit different numbers:

for coniferous trees: 30%
for deciduous trees in summer: 20%
for deciduous trees in winter: 60%
I would like to hear some comments on this as Solar Pathfinder's percentages sounds a bit too conservative. And your experiences with tree shading.

Thank you.

[inetdog]

I have been reading the Solar pathfinder's manual, and interestingly they advice the following when it comes to shading of PV arrays,strings from coniferous(evergreen) and deciduous trees:


If I understood correctly, Solar pathfinder is basically suggesting that the following percentage of solar radiation passes through trees:

for coniferous trees: 0%
for deciduous trees in summer: 0%
for deciduous trees in winter: 100%

I found that software's like PV Sol, use a bit different numbers:

for coniferous trees: 30%
for deciduous trees in summer: 20%
for deciduous trees in winter: 60%
I would like to hear some comments on this as Solar Pathfinder's percentages sounds a bit too conservative. And your experiences with tree shading.

Thank you.

The exact results depend on the distance of the trees and the observed nature of the shade.
If the shadow stippling is smaller than the size of individual cells the higher percentage may be justified. But if an entire cell is covered the output of that bypass diode segment will be essentially zero.
For deciduous trees, if a trunk or large branch covers a single cell with shade, you will lose even if the leaves are gone.
Also some panels will produce significant output from indirect light.

[foo1bar]

I have been reading the Solar pathfinder's manual, and interestingly they advice the following when it comes to shading of PV arrays,strings from coniferous(evergreen) and deciduous trees:


If I understood correctly, Solar pathfinder is basically suggesting that the following percentage of solar radiation passes through trees:

for coniferous trees: 0%
for deciduous trees in summer: 0%
for deciduous trees in winter: 100%

I read that same passage as saying 50% for decidous trees in winter for thermal.
And 0% across the board for PV (unless the panel manuf. says it can do better.)

[inetdog]

I read that same passage as saying 50% for decidous trees in winter for thermal.
And 0% across the board for PV (unless the panel manuf. says it can do better.)

That is my reading too. The difference is that a thermal panel is not a "series circuit" and shading of 50% of the surface, regardless of how that shading is distributed, will cause only a proportional reduction in heating, while total shading of only three individual cells can wipe out the entire output of a crystalline panel. And amorphous panel might do better than that.

PS: I would guess that the Solar Pathfinder's manual will be conservative, since it is likely to be used by an installer who may need to guarantee output of the array at some level. While the PVSol program delves deeper into the details of the shading, based on a wide range of collected data instead of a theoretical analysis.

[russ]

I read that same passage as saying 50% for decidous trees in winter for thermal.
And 0% across the board for PV (unless the panel manuf. says it can do better.)

The 100% for decidious trees in the winter is not correct for PV.

[bernard]

Thank you inetdog, foo1bar, russ.

I deliberately did not mention the thermal collectors as they are not of my interest.

Then, what is the correct explanation of the upper Solar pathfinder's manual quote?

1. Do not count any half-hour periods shaded by evergreen trees, as they cast shadows year-round.

This means, that solar radiation passing through trees is:
for coniferous trees: 0%

2. Do not count half-hour periods shaded by deciduous trees during the leaf-bearing months;

This means:
for deciduous trees in summer: 0%

The third sentence is a bit weird:

or thermal collectors, count these half-hour periods at half their value during non-leaf-bearing months. For PV panels, these half-hour periods should be assigned a value of zero, unless the manufacturer can support a better figure.

for deciduous trees (PV panels not thermal collectors) in winter: 0% ??

[Mike90250]

shaded - even 5%, PV panels produce no useful output. Maybe in a string install, you could rely on the internal bypass diodes to keep the string alive, but that's a poor solution.

There are no mfgs of Shade Panels

[pleppik]

shaded - even 5%, PV panels produce no useful output. Maybe in a string install, you could rely on the internal bypass diodes to keep the string alive, but that's a poor solution.

There are no mfgs of Shade Panels

The company which makes the panels I installed claims that their modules are tolerant of partial shading. They have MPPT integrated into the modules and the cells are wired in a matrix configuration rather than in series. Here's their marketing explanation.

I can't personally attest to whether the modules are any more tolerant of partial shading than a more conventional (microinverter) design. But they did seem to do pretty well over the winter when I couldn't quite brush all the snow off.

[inetdog]

The company which makes the panels I installed claims that their modules are tolerant of partial shading. They have MPPT integrated into the modules and the cells are wired in a matrix configuration rather than in series. Here's their marketing explanation.

I can't personally attest to whether the modules are any more tolerant of partial shading than a more conventional (microinverter) design. But they did seem to do pretty well over the winter when I couldn't quite brush all the snow off.

If you go far enough into the white papers you reach stuff that is closer to an engineering explanation than a marketing explanation. It is intriguing.
The combination of parallel paths on the panel and integrated DC to DC converters looks interesting and the idea of using multiple fixed operating point inverters with varying turn on thresholds is an alternative to conventional MPPT. Without the DC-DC converters at the modules it would only work as well as a constant V (as a fraction of Voc) solution, but with them it seems to be a good approximation to conventional (searching) MPPT.

Time will tell.

[bcroe]

I have been reading the Solar pathfinder's manual, and interestingly they advice the following when it comes to shading of PV arrays,strings from coniferous(evergreen) and deciduous trees:

If I understood correctly, Solar pathfinder is basically suggesting that the following percentage of solar radiation passes through trees:

for coniferous trees: 0%
for deciduous trees in summer: 0%
for deciduous trees in winter: 100%

I found that software's like PV Sol, use a bit different numbers:

for coniferous trees: 30%
for deciduous trees in summer: 20%
for deciduous trees in winter: 60%
I would like to hear some comments on this as Solar Pathfinder's percentages sounds a bit too
conservative. And your experiences with tree shading. Thank you.

I don't like those 0% and 100% figures at all. Experience here is, it doesn't take much shading to knock
down a string output. Bypass diodes might allow unshaded areas to be active. BUT they will do so at a
reduced voltage. By the time the remaining string cell voltage rises high enough to work in parallel with
an unshaded string, they have slid up the curve to a very poor current level. If this is significant in your
ap, parallel strings are not the way to go. Bruce Roe