Panel mounting, 12/24V and shade/snow considerations

[Tony Wright]

My planned off grid system will have to allow for snow and I have seen the advice that landscape panel orientation is best because snow build up at the bottom of the panels does not kill an entire panel. However I have also read that parallel wiring is better than series because shading only effects individual panels not the whole string.

So what is usually the best compromise for snowy Canadian winters?

In my particular case I am looking at about 400 watts total and would like 24V so could use 2X200W 24V panels (instead of 12V panels) in parallel, mounted in landscape and get the best of both worlds.

It also begs the question, why doesn't everyone use 24V panels. They seem to be much cheaper per watt and it simplifies the mounting and parallel/series issues.

Am I making sense?

Thanks
Tony

[Rob Beckers]

Hi Tony,

Before getting to your questions I'll take a step back and explain a few things that may not be all that clear to people (even those that install solar panels for a living!).

First number of cells: To reliably charge a 12V battery you need about 17 Volts (on a really cold day!). The solar pioneers figured out that takes at least 36 solar cells, with each producing about a half Volt (or about 18V total, but panel Voltage drops when they get warm). That's why you see all those 50 Watt and 100 Watt 36-cell panels out there, and for many years that was the standard.

Then residential/rooftop solar came along, for grid-tie. It was found that 36 cells took too many panels to get a reasonable Wattage, and eventually things settled on 60 cells for those panels. Given the half Volt per cell, a 60-cell panel runs at about 30 Volt. You can still charge a 12V battery from that, but it takes a more sophisticated charge controller that converts the Voltage down, those go under the name "MPPT" charge controllers (for Maximum Power Point Tracking, because they track the Voltage that causes the solar panels to produce the most power).

For industrial installs, that mostly get mounted on the ground, there was a need for even larger panels, and 72-cell modules became the standard there. Fewer panels to reach a Wattage means a cost-savings, that's why for the big solar farms you'll see 72-cell panels exclusively. As it happens 72 cells is exactly twice 36 cells and they run at about 36 Volt, so these panels can also be used to directly charge a 24V battery bank, without the need for Voltage conversion, so a cheap PWM charge controller can be used.

For a while we also had 54-cell panels, and 96-cell modules, but those have gone by the wayside, and right now it's 36- 60- and 72-cell.

Because of sheer volume 60-cell panels in particular have become a commodity and are cheap, heck, we retail them for 82 ct/W in Canadian dollars, about US$0.61/Watt. That never happened to the 36-cell modules, they are hardly used any more these days, production is limited, and prices are still in the multi-dollar per Watt range. The larger 72-cell followed the same path as 60-cell, thanks to the many very large installations and they too are cheap. Normally their pricing is very similar to 60-cell (per Watt).

In terms of sizes that are available: 36-cell is generally made in 50 - 120 Watt (there are larger and smaller ones, just not common and not sold much). For 60-cell we're at about 300 Watt per panel commonly available and cheap, while 72-cell is around 340 - 350 Watt (they actually are made up to 420 Watt, but those are not cheap!). On our end we mostly sell and stock 60-cell because those work best for residential use (if you ever tried to manhandle 72-cell modules on a rooftop during a stiff breeze you'll know why!).

Tony, so for your situation you could use a single or multiple 72-cell modules (and a regular/cheap size would be around 340 Watt per panel for those). You can directly charge a 24V battery bank with a cheap PWM controller with those. You would HAVE to connect them in parallel if you are going to use a PWM controller, since those don't do Voltage conversion and putting panels in series increases the Voltage coming out of the string!

If you are expecting snow cover and can't keep the snow off then 72-cell panels may not be the best solution though: You really need all those 72-cells to be working, and snow cover means that at the very least the Voltage is going to drop (if you're lucky that's all, if you're unlucky it will produce zero). Any Voltage drop means there won't be enough left to charge those batteries. For that multiple panels in series with an MPPT controller would be a better solution, it will give you some Voltage leeway.

Now to shading and landscape/portrait: I can't speak for 36-cell panels for this, they come in various forms and the effects of shading may vary. For 60-cell and 72-cell modules the panels are internally divided in three parts (that's changing, but for now let's keep it simple and this is still 99% true). If you stand a panel up in portrait (short side down) you'll see 6 columns of solar cells. The left two columns are one section, the middle two are a section, and the right two are a section. Each section has a bypass diode, and that's where the effects of shading come in.

The amount of current a solar cell can produce is largely proportional with the amount of light that hits it. Twice the light, twice the current. Compared to direct sunlight, shade will only contain a small percentage of the light intensity (literally just a few percent vs. sunlight). So, a shaded cell can only produce very little current! All the solar cells in a panel, all 60 or 72, are connected in series: The current going through one cell has to match that of all the other cells. That means the cell that produces the smallest current rules! If there is one shaded cell, being able to produce just 0.5 Amp, while all other cells have full sunlight and are able to produce 9 Amps, the full string of cells in the panel will still only be able to produce 0.5 Amp and no more! That is why shade is such poison for solar panels!

Not all is lost though, because of the bypass diodes. If we run the panel at a lower Voltage it may be possible to get full current out of the unshaded sections, while bypassing the shaded section. For example, a 60-cell panel can produce 9 Amp at 30 Volt in full sunlight, or 9 x 30 = 270 Watt. Now we shade the top-left cell (in portrait), and that cell drops down to 0.5 Amp (or just 0.5 x 30 = 15 Watt for the whole panel). If we bypass that left 1/3 of the panel, we can run it at 2/3 of the Voltage, or 20 Volt, and full current of 9 Amp, which makes 9 x 20 = 180 Watt. That is what an MPPT controller and the bypass diodes do. An MPPT charge controller 'sweeps' the Voltage range, and when the Voltage drops below 30 Volt it will cause bypass diodes to kick in if a section can't sustain the current that the rest of the panel can handle.

So, a single shaded cell, just 1.6% of the panel's surface, dropped the panel output by 33%! Still, that's better than without bypass diodes, it would have gone down to just 15 Watt or just 5.5%. The downside is that to get the higher output the panel needs to run at a lower Voltage, and you need an MPPT controller to make that happen (an MPPT controller will always look for the point in Voltage and current where the most power is produced). The take-home lesson is that no matter what, with shading you lose panel sections in thirds; even a little shade and 1/3 is gone (if you're lucky). That loss comes through a lower Voltage that the panel runs at.

That is where portrait vs. landscape mounting comes in: Because of the way the panel is divided in sections, and with snow often clinging to the bottom of a panel, it is better to mount in landscape. In landscape you can shade across the bottom of the panel and lose just 1/3 of the output. The same shade across the bottom on a portrait mounted panel will cut through all 3 sections, and result in near-zero panel output!

If you are going to charge batteries and have shading or snow to deal with it will take more than the minimum number of cells to keep things working. As explained, the trade-off to get higher output from a partially shaded panel is a drop in Voltage. So, you either need two 60-cell panels in series (or more), or multiple 72-cell modules, to get some Voltage leeway to deal with shading for a 24V battery bank.

When multiple panels or strings of panels are connected in parallel the effects of shading get more complicated: Yes, the shaded sections can be bypassed, but at the cost of Voltage of the string (or panel if there are no strings). Connecting strings together where one is running at 90 Volt (3 panels in series) and the other at 80 Volt (a shaded an bypassed section) will cause the whole system to run near the lower Voltage, or in this case 80 Volt. That is not entirely true, and the MPPT controller will try the whole Voltage range to see what delivers the highest output, but by-and-large this is what happens. So, shade a panel in one string and you essentially lose multiple times the same mirrored in the parallel strings.

That's the whole story!
I hope it makes things clearer.

-RoB-

[Tony Wright]

Wow Rob, great information and I like the history lesson.

The only bit I need help with is your paragraph.....

"If you are expecting snow cover and can't keep the snow off then 72-cell panels may not be the best solution though: You really need all those 72-cells to be working, and snow cover means that at the very least the Voltage is going to drop (if you're lucky that's all, if you're unlucky it will produce zero). Any Voltage drop means there won't be enough left to charge those batteries. For that multiple panels in series with an MPPT controller would be a better solution, it will give you some Voltage leeway."

I have seen several references elsewhere that say parallel panels are always better than series to combat shade, whether from snow or trees etc. My choice of two 72 cell 24V panels in parallel/landscape seemed to give me the best both worlds. Also I will certainly be using an MPPT controller and mounting the panels at about 80 deg because I am mainly interested in winter performance.

Tony