How do you test a solar panel in winter?

[mike]

I found a solar panel locally at a betterthanaverage price.
It's winter, overcast, raining.

Short of taking along a 5KW light panel, are there any
tricks one can use to evaluate a used PV solar panel in winter?

For example, can I use a big photoflash and look at the output
transient on an oscilloscope and learn anything helpful?

It's not that I don't trust the seller...YES IT IS.... ;-)
mike

 

[mike]

I found a solar panel locally at a betterthanaverage price.
It's winter, overcast, raining.

Short of taking along a 5KW light panel, are there any
tricks one can use to evaluate a used PV solar panel in winter?

For example, can I use a big photoflash and look at the output
transient on an oscilloscope and learn anything helpful?

It's not that I don't trust the seller...YES IT IS.... ;-)
mike

I answered my own question.

I have a small 6" square solar panel.
I'd plotted its output in direct sunlight.
Maximum power was with 220 ohm load.
Put out about a watt at 14.1V.
I lashed it up with a 220 ohm load and hooked
it up to the scope.
Flashed with my tiny digital camera flash.
Got over 20V out of it for 40 microseconds.
So, does look like there might be some usefulness
in a flash test.
Any obvious flaws in this test?
mike

 

[Roland Mösl]

I found a solar panel locally at a betterthanaverage price.
It's winter, overcast, raining.

Short of taking along a 5KW light panel, are there any
tricks one can use to evaluate a used PV solar panel in winter?

For example, can I use a big photoflash and look at the output
transient on an oscilloscope and learn anything helpful?

It's not that I don't trust the seller...YES IT IS.... ;-)
mike

Winter is the best time to test a solar panel

Compere it with other PVs under cloudy weather.

http://live.pege.org/2005-house-plan/photovoltaic-compare.htm

 

[mike]

Winter is the best time to test a solar panel

Compere it with other PVs under cloudy weather.

http://live.pege.org/2005-house-plan/photovoltaic-compare.htm

Thanks for the input.
Most interesting thing about that page is that the
"super" panel is 2.5x the size and puts out 1.7x the amps.
Sure, it works better at low light. Good choice if you live
in dark climate, but it's not really an apples to apples comparision.

Wouldn't it be better and smaller to use two of the small panels
in series in winter and parallel in summer? Or just be done with it
and use electronics to maximize power output in any light?

When people pull "shady" comparisions like that, I wonder else
they're not telling me...

 

[mike]

I am glad you made this post! One of the questions I have relates to
this issue. I have heard that some panels do a lot better in cloudy
weather. With the inexpensive panel I am testing, I get zero output on
a heavily cloudy day and maybe 10% of panel rating on a moderately
cloudy day. If some panels had DOUBLE my 10% on the moderately cloudy
day, that still wouldn't amount to much. Can some panels do a LOT
better than others or are they all in the 10% area?
I know there are a million degrees of "cloudy" so please go easy on me.

Pete Stanaitis

I live near the 45th parallel. The published insolation numbers for
winter are a small fraction of the summer numbers on a SUNNY day.
On a cloudy winter day, forget it.

A typical solar cell puts out less than a volt maximum. The power curve
looks like a mountain. For each light level, there's a maximum power
point (voltage and current). That sucks for fixed battery voltages.
That's why people use controllers that can sense that peak point and
convert the voltage output to what the battery wants and maximize
the power at the current the solar cell wants to output at its peak.

You've got less than a volt, so you hook a bunch of them in series.
If you don't use an active controller, you want to adjust the number
in series so that the current is maximized at the battery voltage for
the light you have available. For low light, you want more cells in
series, but that gives you less current per unit area in bright light.
Then for more power, you replicate the whole shebang and put the
series-conected assembly in parallel with others until you run out
of space on a panel that you can manage easily. I suspect there's
some optimum size that minimizes the total cost of the structure.

You want the system to fail softly. So, the diodes isolate different
parts so that if one cell fails, it won't drag the others down.
Or if a tree blocks part of the panel, you don't want it to
drag the others down too. It's like car insurance. You pay a little,
the diode, to insure against an accident, dead solar cell.

As always, get a second opinion. By largest solar panel is 6" x 6".
I'm hardly an expert.


mike

 

[Al Forster]

I have read that solar panels that have individual cells do some

funny things if some of the cells in the panel are shaded.
Something about the total panel output dropping to the output of the
worst-case cell.
I guess that some panel mfrs attempt to minimize this effect by
using blocking diodes internal to each panel, but It would seem to
me that too many blocking diodes in the overall ckt would cost you
in power to the load.

You're mostly correct, but it's bypass diodes not blocking diodes. A
module with no bypass diodes with one shaded
and many unshaded cells will only output the current of the shaded
cell (under short-circuit conditions or at least, beyond the "knee
voltage"). In the worst case, the current generated by the other cells
can be dumped through the shaded cell, sometimes generating enough
heat to cause that cell to crack. Some manufacturers use bypass diodes
to alleviate this effect, but usually for strings of four or more
cells (i.e. not for each cell). Blocking diodes are most generally
used to prevent current flowing back from charged batteries through
the solar modules (eg at night), or for parallel-string systems where
current from a "good" string could dissipate in a "bad" string.

A little off topic, but I have seen ads where "they" talk about
using Schottky diodes for this application. I guess the assumption
is that they have lower forward voltages? I can't find data to
support this, though, so there must be some other reason for using
them?

Schottky diodes have a lower forward voltage drop than silicon power
diodes, so they're often used for (forward) bypass diodes. Schottky
diodes are not generally used for (reverse) blocking diodes due to
their high reverse leakage current (compared with silicon diodes).

Al

 

[Roland Mösl]

Thanks for the input.
Most interesting thing about that page is that the
"super" panel is 2.5x the size and puts out 1.7x the amps.
Sure, it works better at low light. Good choice if you live
in dark climate, but it's not really an apples to apples comparision.

Wouldn't it be better and smaller to use two of the small panels
in series in winter and parallel in summer? Or just be done with it
and use electronics to maximize power output in any light?

When people pull "shady" comparisions like that, I wonder else
they're not telling me...

Tell us where You live and what application You have.

When it comes to power an off grid application in winter,
I am only interested in the performance at cloudy weather
conditions.

 

[mike]

Tell us where You live and what application You have.

When it comes to power an off grid application in winter,
I am only interested in the performance at cloudy weather
conditions.

I don't have an application? Haven't seen anything that costs less
than the energy it replaces. But I'm always on the lookout for dirt
cheap solar panels.

I have done some consulting for radio amateurs with repeaters at
mountain sites.

 

[Roland Mösl]

I have done some consulting for radio amateurs with repeaters at
mountain sites.

And this is exactly the type of application,
where not the peak performance at 1000 W/m² counts.

Here counts the best performance at cloudy weather conditions
to come through some cloudy weeks in December