Solar Panels and Shadows...

[Insequent]

I have finally got my new panels setup. Previously I had 7 x 260W Suniva 30V panels. Now I have 6 x 345W Sunpower 60V panels. As I have a 12 V system I have 2 x Outback FlexMax 80's. I really wish I had gone to a 24V house bank as I would only have needed 1 FlexMax. So 3 of the new panels feed each charge controller. In order to have both charge controllers synchronise (in terms of float charging etc) they are linked to a Mate 3 unit.

So far I have not been at anchor or off shore power long enough to get a daily Ah replenishment figure. But a few days ago just before midday I had peak readings of 124.8 A and 1740W. Not too bad for the low angle winter sun! So I think I should get a very healthy amount of Ah during the day. Over the next few weeks I hope to get some data on this.

My new configuration is in part to minimise shading. The 2 panels on the pilothouse roof are 2' further forward to much reduce the effect of shading from the flybridge hardtop. Part of that project necessitated re-positioning the Sat dome. On the hardtop I have just 4 panels located forward. The new mast config involved modifying the deck mounted mast. The lower 5' was discarded and the top half, well top two thirds, mounted on the hardtop. The mast now has the sat dome on the old radar platform with a new radar platform below it. I did not install panels at the aft end of the hardtop near the mast. There would have been shading of them for a lot of the time. As it is the mast only causes some shading when I'm south-facing, but in summer that will be less of an issue as well.

I can lower my VHP antennas to reduce panel shading at anchor. They hinge at the bottom, so its just a 5 second task to raise or lower each antenna. If I get motivated one day at anchor I will try and quantify the shading effect from the antenna, bearing in mind Twisted's warning to allow time for the charge controller to re-scan. Does anyone know how long I would need to wait?

Pic from dingy shows mast, but panels are hard to see as there is only a small airgap between them and the underlying structure. The antennas are lowered. I'll try and remember to take a pic from shore in the morning to actually show the panels themselves before departing on my cruise to points north.

 

[twistedtree]

The way an MPPT controller works is periodically it "scans" to find the maximum power point of the attached panels. The maximum power point is the voltage and current operating point for the panels that yields the maximum wattage. i.e. voltage multiplied by current.

Panels are a bit of an odd duck because they are what's known as a constant current source. As you vary the load on the panel, the current will remain nearly constant, and only the voltage will vary. So they will produce the same amount of current at any voltage, but only up to a point. Once the voltage gets too high, the current drops off, and it does so very fast - like a cliff. The result is that a panel's wattage output varies directly with the actual operating voltage. A panel's rated wattage is based on the highest voltage you can reach, just before the current plummets.

Now, go back to and MPPT controller. When it "scans" is varies the load on the panels while monitoring the voltage and current, and figures out where the optimal operating point is where it can extract the most watts (power). Then it settles in and starts running. An MPPT controller can do this because it is essentially a variable DC to DC converter. It takes whatever the best input voltage is, and converts it to the desired charger voltage. The only loss is in the DC to DC conversion and that's around 10%.

So what the video is really showing is the Outback's instantaneous response to shading, which is nearly meaningless. I haven't studied exactly how the outback or any other controller behaves and recovers in such a situation, but know that some experienced people wish the outback scanned more often. I suppose what you want in a controller really depends on the weather or other application factors. If you have stable weather without a lot of clouds, then less scanning and more producing is better. The outback leans this way. And if you have a lot of varying cloud cover or other variable shading, then you would prefer more scanning. But all MPPT controllers scan and adapt - it's the nature of the beast.

 

[twistedtree]

Insequent - I'm envious of your solar setup. I sure wish I could get that many panels on my boat.

You may have already done this, but you might be able to take advantage of having two charge controllers. If you have some panels that are more subject to shading than others, try grouping the panels subject to shading on one controller, and the panels not subject to shading on the other controller. Then shading will only confuse one controller while the other can keep cranking away with stable sunlight.

 

[dhays]

Thanks again TT. That was a simple enough explanation that even I understood it.

 

[Insequent]

Insequent - I'm envious of your solar setup. I sure wish I could get that many panels on my boat.

You may have already done this, but you might be able to take advantage of having two charge controllers. If you have some panels that are more subject to shading than others, try grouping the panels subject to shading on one controller, and the panels not subject to shading on the other controller. Then shading will only confuse one controller while the other can keep cranking away with stable sunlight.

With my old panels I did group them according to shade potential. I estimated that the ones that could get shading at times of the day produced about 60% of what the others did. It wasn't exact as I had 4 un-shaded panels on one controller and 3 potentially shaded panels on the other controller. I 'normalised' to a per panel output to get the 60% estimate. And they were not shaded all that much either.

Sorry I can't be more specific on how much shade and for how long. It was more of a problem in mornings and afternoon. In the most productive part of the day they were usually not shaded, but it did depend on what heading the bow had while at anchor as well.

 

[twistedtree]

You may have heard the saying... There is a simple answer to every complex problem, and it's wrong.

For most people building a solar system today, series wired panels will be most shade resistant. Shading of any kind will still be a big issue, but on boats it's nearly impossible to completely avoid. I think the key criteria for series wiring to be preferable are:

1) use of an MPPT controller

2) series wired panels with the highest voltage supported by the controller. The higher the voltage, the more shading the system can tolerate. As soon as the voltage drops below around 110% to 120% of the battery charge voltage, output will drop to zero.

If you don't use an MPPT controller, then parallel wiring is your only option. Older systems are pretty much all like this. But newer panels are generally much higher voltage so an MPPT controller is mandatory or you will end up wasting a huge amount of power because the panels will operate at battery voltage, not their max output voltage.