Shading on Solar Panels

[longjonsilver]

Let me see if i understand this correctly. Each solar cell produces 1/2volt. A solar panel often consists of 30-36 cells wired in series to produce 15-18volts. This means that if the panel gets even partial shade, its voltage drop to below the critical voltage needed to charge a 12volt battery. Is this not correct? So then why do we not wire two panels in series to give us 30-36volts to the charge controller? This way, if we get partial shade we would still have enough voltage to charge the 12volt batteries. This would allow us to have many more panels in areas of the sailboat that get partial shade, other than those on the bimini.

What am i missing here?

jon

[Paul Elliott]

The problem with series connection and shading of even one cell in the series string is this: Cells are essentially constant-current sources with a maximum voltage. A shaded cell has its current reduced, which restricts the current through the series string. You might consider the partially-shaded cell to be a resistor. If a cell is completely shaded, it looks like an open circuit.

So, the panel voltage and current don't just drop by the value of a single shaded cell's output.

[Celestialsailor]

Someone posted this link awhile ago here and I found it very useful and it may answer all of your questions. It's an experiment with shading.

https://www.youtube.com/watch?v=1qD3mN8VotQ&t=685s

[longjonsilver]

Originally Posted by Celestialsailor

Someone posted this link awhile ago here and I found it very useful and it may answer all of your questions. It's an experiment with shading.

Thanks. Yes, it answered a lot of questions, but it also raised more. When one panel of the two in parallel was partially shaded the total output in Amps was diminished significantly. (The voltage stayed constant due to the solar controller, no?) Diminished lower than if only one panel were being used, no?

So, is it possible to cut off the partially shaded panel (with some type of voltage or currant sensing switch) and only run the one panel in full sun?

jon

[TeddyDiver]

Originally Posted by longjonsilver

Thanks. Yes, it answered a lot of questions, but it also raised more. When one panel of the two in parallel was partially shaded the total output in Amps was diminished significantly. (The voltage stayed constant due to the solar controller, no?) Diminished lower than if only one panel were being used, no?

So, is it possible to cut off the partially shaded panel (with some type of voltage or currant sensing switch) and only run the one panel in full sun?

jon

It's possible with diodes. The draw back with diodes is the voltage drop. Shottky diodes are used in PV arrays as bypass diodes to overcome shading issues, but as all things, with a cost.

BR Teddy

[noelex 77]

Originally Posted by longjonsilver

When one panel of the two in parallel was partially shaded the total output in Amps was diminished significantly. (The voltage stayed constant due to the solar controller, no?) Diminished lower than if only one panel were being used, no?

No

In parallel the shaded panel will not reduce the output to less than the output of the other panels (other than a tiny fraction). There is no point disconnecting the shaded panel.

In series a shaded panel can diminish the output of the other panel. Bypass diodes can help reduce this effect, but not eliminate it.

I think parallel is better than series, but not as dramatically as the video suggests. I think with higher voltage panels and/or bypass diodes installed around each panel the results would have been closer. I dont think the results of the video are indicative of many series solar installations, but I still think parallel is the best option.

[Celestialsailor]

Originally Posted by longjonsilver

Thanks. Yes, it answered a lot of questions, but it also raised more. When one panel of the two in parallel was partially shaded the total output in Amps was diminished significantly. (The voltage stayed constant due to the solar controller, no?) Diminished lower than if only one panel were being used, no?

So, is it possible to cut off the partially shaded panel (with some type of voltage or currant sensing switch) and only run the one panel in full sun?

jon

Originally Posted by TeddyDiver

It's possible with diodes. The draw back with diodes is the voltage drop. Shottky diodes are used in PV arrays as bypass diodes to overcome shading issues, but as all things, with a cost.

BR Teddy

Most modern panels put out over 21 volts. Yes, diodes work but as stated with slight voltage drop. They are mainly a non feed-back mechanism for non-sun times.
Bottom line, we all live with partial shading,especially on a sailboat. I just compensate for it with articulating panel devices and averaging the input over a set time, then compensating with either more panels or another charging source. I have 330ah. of batteries and 300 watts of panels. In theory, that would give me close to 25 amps input but it's more like 10 amps max and over 7 hours, I average 6.5 amps, so I get around 45 amps a day. It almost keeps up with my demand and requires an hour or two of engine running which at the same time can run the watermaker when I chose.
I understand your concern but it will never be perfect...it's always a compromise.

[longjonsilver]

Originally Posted by noelex 77

No
I think with higher voltage panels and/or bypass diodes installed around each panel the results would have been closer.

So, with all other factors equal, the higher voltage panels are better?

jon

[noelex 77]

Originally Posted by longjonsilver

So, with all other factors equal, the higher voltage panels are better?

jon

No.

In general I think parallel connection is better for a shadow prone boat. This means the solar cells should ideally be arranged in a parallel connection of "low voltage" panels of around 36 cells.

Unfortunately, the solar market is geared towards household/industrial use. High efficiency 36 cell panels are difficult to get especially for rigid panels. So in practice if you want best performance the only option is often high voltage panels.

If you want to connect your panels in series, and I would not recommend this, it is important that the solar Vmp is always above the battery voltage. A shaded solar panel in series will reduce the system voltage.

In the video a single 36 cell panel connected to a completely shaded panel fails this criterion. Hence the very low output in series shown in the link. MPPT controllers (with very rare exceptions) cannot boost voltage. If the Vmp falls below the battery voltage the output will be very low.

So if the video had used higher voltage panels, the output in series would have been much better, but I would still predict the highest output (all other things being equal) would have been for parallel connection.

[Paul Elliott]

Originally Posted by longjonsilver

So, with all other factors equal, the higher voltage panels are better?

Not necessarily. Especially with an MPPT controller, slight differences in nominal panel voltage won't affect the output power.

Now some panels have all their cells connected in series, to deliver a high voltage. Others (such as mine) have the same number of cells, and so the same rated power, but the cells are connected as two series strings, and these strings are then connected in parallel. This gives a lower voltage and higher current, and should one corner of the panel become shaded it is likely that only one of the strings will be affected, so the panel delivers half the potential current. In a single-string high-voltage panel, there might be virtually zero power delivered in the same shading scenario.

On my boat I have three 100W panels connected in parallel, mounted on top of the dodger. This is possibly the worst location I could have chosen because of shading from the mast, boom, and mainsail. Knowing this, I still put the panels there, for aesthetic and practical reasons. I arranged the three panels side-by-side, port to starboard, with the long dimension oriented fore and aft. This improves the chance that a shadow won't fall across all three panels. Had I oriented the panels with the long dimension port to starboard, then the likely shadows would have shut down all three panels. Had I wired the panels in series, shading on any panel would have shut down the array.

If you can arrange your panels so that you don't have shadowing problems, then a series connection lets you use smaller wires for the same performance. Unfortunately, this is seldom the case on most sailboats.

[TeddyDiver]

Originally Posted by noelex 77

No.

In general I think parallel connection is better for a shadow prone boat. This means the solar cells should ideally be arranged in a parallel connection of "low voltage" panels of around 36 cells.

Unfortunately, the solar market is geared towards household/industrial use. High efficiency 36 cell panels are difficult to get especially for rigid panels. So in practice if you want best performance the only option is often high voltage panels.

If you want to connect your panels in series, and I would not recommend this, it is important that the solar Vmp is always above the battery voltage. A shaded solar panel in series will reduce the system voltage.

In the video a single 36 cell panel connected to a completely shaded panel fails this criterion. Hence the very low output in series shown in the link. MPPT controllers (with very rare exceptions) cannot boost voltage. If the Vmp falls below the battery voltage the output will be very low.

So if the video had used higher voltage panels, the output in series would have been much better, but I would still predict the highest output (all other things being equal) would have been for parallel connection.

One bypass diode for each panel and there's no such problem with series installation, if two per panel even better.
Ideally in limited panel size and partial shading installations like a boat the best output would demand a bypass diode for every single solar cell or pair at least.

[noelex 77]

Originally Posted by TeddyDiver

One bypass diode for each panel and there's no such problem with series installation, if two per panel even better.
Ideally in limited panel size and partial shading installations like a boat the best output would demand a bypass diode for every single solar cell or pair at least.

Most solar panels have between 3 and 8 bypass diodes. Unfortunately, when active they introduce a voltage drop.

I think if you want to use series connection, installing an additional bypass diode that will bypass the whole panel is worthwhile. However, on a boat with inevitable shading issues, I think the available evidence points to parallel connection as the better option (although I would still like to see more research).

[sailinglegend]

Originally Posted by noelex 77

...it is important that the solar Vmp is always above the battery voltage. A shaded solar panel in series will reduce the system voltage.....If the Vmp falls below the battery voltage the output will be very low....

Some potential confusion to readers here.

In a 32 cell panel the Vmp even under shading will always be about 16v. because solar cells are very nearly constant voltage sources, it is the current that is reduced with shade or lack of sun. Under shade each cell varies in voltage between 0.5v and 0.6v so there will always be a high enough voltage to charge a 14.4v battery. A 36 cell panel will be much better with a minimum Vmp of about 18v.

[noelex 77]

Originally Posted by sailinglegend

A 36 cell panel will be much better with a minimum Vmp of about 18v.

I think you are using the manufacturers' specifications at STC. They are measured with flash exposures and hence no chance for the solar cells to heat up. Solar cells have a strong negative temparture coefficient.

In practice, Vmp will be lower than 18v unless you are cruising artic areas.

Rather than a minimum, a Vmp of 18v for a 36 cell panel is more like a maximium that is unlikely to seen in practice

[TeddyDiver]

Originally Posted by noelex 77

Most solar panels have between 3 and 8 bypass diodes. Unfortunately, when active they introduce a voltage drop.

I think if you want to use series connection, installing an additional bypass diode that will bypass the whole panel is worthwhile. However, on a boat with inevitable shading issues, I think the available evidence points to parallel connection as the better option (although I would still like to see more research).

The voltage drop will occur only when one or more cells is shaded. Then of course the shaded cell or string of cells connected parallel with a bypass diode won't produce any current. The voltage drop would in such case be 0.5v/cell in a string plus 0.3v for the bypass diode.
In a case where all cells were installed in series with individual bypass diodes the voltage drop would be 0.8v for each shaded cell. In practice the whole PV array had to be 50% shaded before the output voltage dropping below 12v in a 230w array..