Partial shading estimation...

[Dave]

Picture a large fully grown tree in the middle of summer with all of its
leaves in full array.

Now picture the same tree in winter with all of its leaves blown off and
only the branches remaining.

The question is what is the net shading effect on solar panels in the winter
when the sun is low and the tree is bare vs the summer when the tree is full
(essentially opaque to the panels) but the sun is higher in the sky so there
is no shade on the panels?

A solar pathfinder analysis assumes the shading is complete and does not
account for the no leaves in winter condition which allows a lot of sun
light to penetrate the tree structure and fall on the panels.

Is there any data regarding this situation or how to properly take it into
account?

Dave

 

[Anthony Matonak]

Is there any data regarding this situation or how to properly take it into
account?

I'm not an expert and have no data to share. That said, I believe
that the design of the array and how the power it's generated is
used will vastly change it's performance under partial shading
conditions. In the worst case, I believe that even a tiny bit of
shade on just the wrong part of an array could reduce the output
of the entire thing to zero.

Anthony

 

[Bill Kaszeta / Photovoltaic Resources]

Picture a large fully grown tree in the middle of summer with all of its
leaves in full array.

Now picture the same tree in winter with all of its leaves blown off and
only the branches remaining.

The question is what is the net shading effect on solar panels in the winter
when the sun is low and the tree is bare vs the summer when the tree is full
(essentially opaque to the panels) but the sun is higher in the sky so there
is no shade on the panels?

A solar pathfinder analysis assumes the shading is complete and does not
account for the no leaves in winter condition which allows a lot of sun
light to penetrate the tree structure and fall on the panels.

Is there any data regarding this situation or how to properly take it into
account?

Dave

This is a case where the operating voltage of the array has a large effect.

The basic fact is that the current generated by a PV module is determined
by the cell with the lowest irradiance on it. Totally shade a single cell and
lacking any bypass diodes, the output of a module would be zero. Bypass
diodes are used across sections of PV modules to provide a current path
around a shaded cell, but the voltage of the bypassed part of the module
is lost and may result in no output from the remainder of the module if the
resulting voltage is too low to use for charging a battery or feeding an
inverter (to list a few possible designs).

Shade from a branch that is spaced even a few meters or yards away
from the PV module is not total, there is the diffuse light from clouds
and/or blue sky and this amounts to some 30% of the total irradiance.
Worst case is a 70-80% loss, not 100%.

If you have an array for charging a 12-volt battery system, each PV module
will have an output determined by the cell with the lowest irradiance on it.
Modules without shading will have full output for the irradiance level.
If you have a higher voltage array with PV modules in series, each series
string will have a current determined by the cell with the lowest irradiance
on it anywhere in the series string. If there are bypass diodes, the output
voltage of the series string will be lowered by the voltage of the bypassed
section of the module.

It is not generally possible to calculate the effect of a tree on a PV array,
but you can measure the current once the situation develops. If you have
not yet built the array, you can use a single module as a sensor and move
it across the intended mounting area while measuring the output current
into a battery (or less accurately the short circuit current).


Bill Kaszeta
Photovoltaic Resources Int'l
Tempe Arizona USA
[email protected]

 

[Bill Shuler]

Is there any data regarding this situation or how to properly take it into
account?

Dave,

From Joel Davidson's "The New Solar Electric Home" book, he states that
winter branch shading will sacrifice at least ten percent of the potential
power
harnessed with a pv module or array. There is a definate reduction in power,
for when I first began experimenting with a solar panel, a Solec s-050, I
had it on my back deck, aimed to the South with some oak trees that way.
Using an amp meter, I watched the deviation of current as the trees became
obsticles of the sun light. As a result, I now have the pv array elevated
and placed
further North from the woods to overcome the loss during the period from
October through February.

Hope this helps, and I hope you don't have to fell many trees to get
good results, though now, the burning logs feel nice in the woodstove!
Bill (South Carolina)