Panels with more bypass diodes

[Ryban]

I noticed today that our new 175W Renogy panels (2 connected in parallel to a Victron MPPT charger), do not handle partial shading well at all. Even when one single cell on the panel was covered, the output went from 110W to 10W. I think there is a bypass diode in the junction box, but it's probably for bypassing the entire panel when used in series, rather than strings of cells within the panel.

Are there panels out there that people are using on their boats with more bypass diodes across the cells themslves to improve shade tolerance? We really need something better in these locations as they'll be succeptable to some degree of shading that we can't do much about. The alternative is something like 3 50W panels (each with a blocking diode) in series or something on each side, but that also just seems silly.

[GordMay]

Solar that works in the shade
Optivolt* says its technology can deliver up to 25 times more power in the shade than conventional panels.
More about ➥ https://www.pv-magazine.com/2022/05/...-in-the-shade/
* Optivolt ➥ https://optivolt.com/


“Photovoltaics in the shade: one bypass diode per solar cell revisited” ~ by Boudewijn B. Pannebakker et al
➥ https://onlinelibrary.wiley.com/doi/10.1002/pip.2898

[Ryban]

I wonder what they'll be charging though, because in theory, it's an extremely simple problem to solve that does not require technological advancement, only the addition of bypass diodes.

My take is that the market for panels that need to function while partially shaded is extremely niche, given most people buying solar panels are home owners, businesses, and utilities, who will place their panels such that they cannot be shaded, and if one or two do, there are many others to compensate.

[sailingharry]

No, because solar installations are massively subsidized by taxpayers, they are installed in very poor performing locations. North facing roofs as well as east and west, are all common. But shade is common as well, with 30% in shade common.

[s/v Jedi]

Originally Posted by Ryban

I noticed today that our new 175W Renogy panels (2 connected in parallel to a Victron MPPT charger), do not handle partial shading well at all. Even when one single cell on the panel was covered, the output went from 110W to 10W. I think there is a bypass diode in the junction box, but it's probably for bypassing the entire panel when used in series, rather than strings of cells within the panel.

Are there panels out there that people are using on their boats with more bypass diodes across the cells themslves to improve shade tolerance? We really need something better in these locations as they'll be succeptable to some degree of shading that we can't do much about. The alternative is something like 3 50W panels (each with a blocking diode) in series or something on each side, but that also just seems silly.

What you observe is normal for parallel operation. The only way to take advantage of the multiple strings in the panel, each with it’s own bypass diode, is by connecting the panels in series.

When you shade 1 cell, the string will go into bypass mode, which will drop the output voltage of the panel by a third, while the other panel is at full voltage connected in parallel. This is like connecting a 6V battery in parallel with a 12V battery, which doesn’t work at all. But in series, you get an 18V power source, so the full 6V is being utilized for power generation.

If you have a suitable MPPT controllers then it’s easy to rewire and compare with series connection.

[Ryban]

Originally Posted by sailingharry

No, because solar installations are massively subsidized by taxpayers, they are installed in very poor performing locations.

I'd be interested in seeing data on that -- even though there are subsidies in place, like the 30% federal tax credit, it's still expensive for the homeowner in most states to install solar, and they'll need to see a return on their investment eventually in the form of reduced utility costs/profits from selling back to the utility. So you'd think they'd do some optimization with their contractor.

That's a good point Jedi -- I don't know why I didn't think about that. I'll do some experimenting next time I'm on the boat, and the more I think about it, it probably makes sense for these panels to be in series.

[sailingharry]

Originally Posted by Ryban

I'd be interested in seeing data on that -- even though there are subsidies in place, like the 30% federal tax credit, it's still expensive for the homeowner in most states to install solar, and they'll need to see a return on their investment eventually in the form of reduced utility costs/profits from selling back to the utility. So you'd think they'd do some optimization with their contractor.

As a strong believer in capitalism, I agree with you. But anecdotally, as we walk through our neighborhood at home (and the many we walk through when walking our dog when out cruising) I am dismayed at the number of fundamentally flawed installations. I don't get it.

[Tupaia]

Originally Posted by sailingharry

As a strong believer in capitalism, I agree with you. But anecdotally, as we walk through our neighborhood at home (and the many we walk through when walking our dog when out cruising) I am dismayed at the number of fundamentally flawed installations. I don't get it.

Because they were installed by professionals. A professional is someone who gets paid and does not mean they have any knowledge. If you want a job doing properly do it yourself.

[smac999]

Originally Posted by Ryban

I noticed today that our new 175W Renogy panels (2 connected in parallel to a Victron MPPT charger), do not handle partial shading well at all. Even when one single cell on the panel was covered, the output went from 110W to 10W. I think there is a bypass diode in the junction box, but it's probably for bypassing the entire panel when used in series, rather than strings of cells within the panel.

Are there panels out there that people are using on their boats with more bypass diodes across the cells themslves to improve shade tolerance? We really need something better in these locations as they'll be succeptable to some degree of shading that we can't do much about. The alternative is something like 3 50W panels (each with a blocking diode) in series or something on each side, but that also just seems silly.

this is also why you want 2 mppts instead.

[team karst]

Seems to me you need a couple of “blocking” diodes. These will prevent reverse current. Bypass diodes have a similar function, but on series panels.

[s/v Jedi]

Earlier, I recommended to connect the panels in series. Someone else recommended to use a dedicated controller for each panel, which I have recommended often as well. Let me explain when to use which option:

First look at your panels: larger panels all have multiple strings of cells, most often three of them, each with a bypass diode in parallel and those strings are connected in series. This means you can consider them to be three smaller panels connected in series, example three strings each of 12V making a panel of 36V.

When one or more cells in a string is shaded, the bypass diode triggers and the current flows through the diode instead of through the string of cells. This means you loose production of that string. In the example you lose 12V so panel output becomes 24V instead of 36, but current stays the same. In reality you have some losses from the diode too, costing you another 0.5V.

Now we connect two panels in series. We simply get 6 strings in series and the voltage increases to 72V. You can see that series connection is the logical thing to do, in line with the way the panels are built.

In parallel you get into all sorts of problems when one panel is shaded, even if one cell is shaded. Theoretically a MPPT controller can lower it’s input impedance enough to increase current and lower voltage enough that the shaded panel can contribute again, but now you are so far off the optimum operating voltage and current that the whole thing has become silly, so it simply means you lose production from shaded panels. You also need blocking diodes or cells that resist back feeding because the panel with higher voltage will try to dump it’s energy into the shaded panel. Parallel is a problematic setup.

The third option is a dedicated controller for each panel. This seems like an ideal option and it is when comparing to parallel connection. But compared to series connection there are significant drawbacks because each controller gets an array input voltage that is only half of what a single controller with two panels in series gets. This means that you get less energy out of the panels.
But the big advantage of dedicated controllers is that they find the best MPPT setting for that panel in that location with that shading.

So what to do? If your panels are mounted as if it’s one large panel, like on an arch on the stern, then series connection is the best option. If they are mounted in different positions, like on opposite sides of the boom which can shade one panel without shading the other, then dedicated controllers are better.

So when is parallel better? When you use old tech PWM controllers that can’t handle array voltages too high for the battery voltage, you are forced to use parallel connection.

This leaves the old wives tales of panels in series loosing all output when one single cell in one panel is shaded. It’s hard to believe that first hand accounts of this happening are all hoaxes so we have to assume this has happened. The only way this can happen is if there are no bypass diodes (all panels manufactured in the past 30 years or so have them) or they have been removed or they have blown up or are faulty. I suspect most accounts come from cheap panels that may have diodes missing or such cheap diodes that they don’t work.

I recommend to wire each panel individually to an electrical box so that you can choose and change configuration right there inside the box. Also very helpful when a panel gets damaged etc. When you use a cheap DIN rail box with DIN rail terminals, you can also add DIN rail mounted surge suppressors when you have a proper bonding system with underwater plate.

[noelex 77]

It would be great to see more real world experimentation on this issue, but my view is that on a sailboat the systems that generally produce the highest yield from best to worst are:

One controller per panel
Parallel connection
Series connection

Unfortunately, the ease and cost of installation follows the reverse order. Series connection is generally the cheapest and easiest and one controller per panel is the most involved and expensive. So series connection is sometimes sensible, but the solar yield will be less. If using series connection I would install an extra bypass diode around each panel.

There are exceptions to this order such as with very small or low voltage panels.

These systems can also be mixed. For example two controllers can be used for four panels.

[Pete7]

Other than the fact that its cold outside and we won't see the sun for another 4 months I think I will try this out. The two panels on an arch are identical the the MPPT sized so I could wire in parallel or series. Also the junction connectors are all MC4 connectors, again easy to switch.

So ear marked as a science experiment next Spring

Originally Posted by noelex 77

If using series connection I would install an extra bypass diode around each panel.

I can see that for a whole panel being badly shaded, but if its just a string because the mast is casting a shadow, that ought to be only one string. Downside is its another bunch of connections exposed to the atmosphere. A plain series installation means I don't have the extra connections with the parallel blocks. Needs real time research I think recording daily MPPT data.

Pete

[sailingharry]

Jedi, very clear and helpful post.... until I try and apply it. LOL.
I have a pair of 400W bifacial ready to put in my arch when it gets replaced this winter. They operate around 35V.
You seem to recommend series. But then you say individual controllers are good for a dodger, for instance, where one gets shaded. My new arch has been designed to minimize shadows to the maximum extent possible, but I still have things like a backstay that can shade them.
Given an operating voltage in the mid 30s, I should still be able to trigger the mppt with at least one string shaded in a single panel. But putting in series makes that even more tolerant.
To add a bit to the decision, 2 50A controllers run around $360, a single 75a controller is $100 more (arguably, a 30A controller would be sufficient, saving another $50 each -- but I am about convinced that $50 is well spent). And of course reduces redundancy.
So does series add any benefit over individual controllers? And does it add $100 worth of benefit?

[Pete7]

Originally Posted by sailingharry

My new arch has been designed to minimize shadows to the maximum extent possible, but I still have things like a backstay that can shade them.

During the summer even in strong sunshine I couldn't really detect an obvious shadow from the back stay. However, the mast in very strong mid day sunshine did cast an obvious shadow as expected, and yes it did reduce the power of two panels in parallel. This needs to be taken in context though, because even in calm water at anchor the boat moves around and sways from side to side. The effect is the MPPT is constantly seeing different levels of power coming in, so having to adjust the maximum power point every few seconds.

So any slight shade from the back stay isn't worth worrying about as the angle of the panel to sun is constantly changing anyway.

Pete