Solar panels & shadows

[dinwood]

I was about to add another 200W of solar panels to our boat when I stopped to think about why I need it. We already have just over 200W which is sometimes plenty to provide all our needs at anchor for protracted periods (full-time live-aboards, mostly med based). However, the slightest shadow kills the output massively (eg from the backstay across our davit mounted panel), as does sailing heeled "the wrong way" relative to the sun. So I did some reading about shadow effects and why this problem hasn't been properly solved (most panels are for open field or roof-top installation where shadowed areas can most often be avoided). I came across just two commercial products which address the shadow problem; Sunpower Maxeon and Jinko Solar Maxim. Does anyone have any experience of these (or others I missed)? Anyone know where they can be bought from?

We could just add more panels, but finding unshaded places convenient for mounting and wiring is tricky.

Grateful as ever for any input to this from all you wise ones...

 

[whiteoaks7]

I haven't looked at the panels you're considering but here's a few points: 'normal' crystalline panels are constructed from a number of cells which are wired in series to produce the required output. Shading of a crystalline panel dramatically reduces output because a shaded cell sucks all the power from the unshaded ones and the output of the panel disappears. Amorphous panels suffer less - with these the output drop from shading is proportional to the area shaded. BUT the size of an amorphous panel per Watt is about twice the size of a crystalline one. So my advice: check the size of the panels you are considering and compare with the 'basic' crystalline types.

Let us know how you get on if you buy the types you're looking at...

 

[skyflyer]

are 'amorphous' the ones you get in flexible panels and crystalline in 'hard' panels or isnt it that simple?

 

[dinwood]

I didn't consider amorphous panels - the ones I mentioned are efficient; Sunpower lay claim to being over 20%. I'm looking for panels that are shadow tolerant and small for their output.

 

[skyflyer]

I didn't consider amorphous panels - the ones I mentioned are efficient; Sunpower lay claim to being over 20%. I'm looking for panels that are shadow tolerant and small for their output.

I think its one or the other isnt it?

 

[noelex]

The Sunpower Maxeon celłs are excellent. We have just fitted panels with this type of cell and I am very impressed with their performance so far.

The main advantage of these higher performance panels is that more watts can be fitted in the same area, but they do also have slightly better low light performance, good heat tolerance, and slightly better shadow performance.

But don't expect miracles. Shadows will still drastically cut performance. The only exception are amorphous panels. Unfortunately these are so inefficient that the overall output per unit area will generally be less even in a location prone to shadows.

The most important factor is location of the panels. It is impossible to find a shadow free location on a yacht, but often installations have items such as GPS aerials unnecessarily mounted where they can cast a shadow. Try and relocate these if possible. Wind and solar combine together very well, but it not unusual to see a wind generator mounted in such a way that it will greatly reduces the output of the solar array. If you are not careful, the overall positive contribution of the wind generator can be minor (although it will also help even out the electrical output, which is a help).

Wiring also plays an important role in shadow tolerance. There is some debate on this subject, but individual MPPT controllers for each panel are usually the best solution for improving shadow tolerance. Parallel connection is next best. Series connection is the worst option.

 

[dinwood]

Thanks for the input Noelex. According to a couple of papers I found, individual bypass diodes (1 BPD per cell, which is what Maxeon's are claimed to have) are massively more shade tolerant (eg losing c. 10% power rather than 66% for a typical small shadow). That got my hopes up. The Jinko panels have an inbuilt MPPT-like IC for each of their 3 strings of cells, which according to the tests I've read also produce much more shade tolerance. Not as good as 1 BPD per cell, but cheaper.
Where did you get your Maxeon panels from?

 

[JumbleDuck]

However, the slightest shadow kills the output massively (eg from the backstay across our davit mounted panel), as does sailing heeled "the wrong way" relative to the sun.

But don't expect miracles. Shadows will still drastically cut performance.

My experience is that a soft penumbral shadow has only a small effect while a hard umbral shadow only kills output if it completely covers a whole cell. I find it hard to imagine a situation where the shadow of a backstay could have a cataclysmic effect.

 

[Angele]

Wiring also plays an important role in shadow tolerance. There is some debate on this subject, but individual MPPT controllers for each panel are usually the best solution for improving shadow tolerance. Parallel connection is next best. Series connection is the worst option.

Hopefully RichardS will be along soon. He did some experiments in his garden with 4 panels - all in series or parallel pairs - and (from memory) his results confirmed this.

 

[noelex]

My panels are Sunpower X21 rigid panels. They do not have a bypass diode per cell. Sunpower sell the Maxion cells to a number of other solar panel manufacturers so it may be that someone is wiring the panels in this configuration. Do you have a link? I would be interested in reading it.

One MPPT controller per panel is becoming quite a popular option for sailboats. Several smaller controllers are often no more expensive than a single large controller. One MPPT controller per string is potentially better again, but the self consumption of MPPT circuitry, especially MPPT controllers that track well, is reasonably high so there is a danger that the extra self consumption can outweigh the improvement in output with shade this is especially true in low light.

 

[dinwood]

Hi all, OP here. Just to be clear, I'm interested in the effect on a single panel. With a 2nd panel and a 2nd MPPT controller I know I'll be much better off, but I'd like to get the most out of the additional panel as if it were stand-alone (and if it really was shadow tolerant I'd probably sell my original panel and keep just the one).
In my understanding, panels have their individual cells arranged in 3 "strings", each string is a series of cells (typically 20). If just one of the cells in a string is shadowed not only does its output drop, but its resistance goes through the roof, "blocking" the current from the entire string. This is what I frequently find in practice; my perfect 15A drops to around 10A or around 5A depending on where the shadow falls (and yes, I do sometimes get soft shadow effects that are less drastic). What I want is for the backstay or mast shadow (which are always "hard") to be a tolerable reduction, hence the interest in these specific panels.
PS I can tilt my panel on one axis so with the sun rising & setting in line with the boat I can (shadows apart) keep the output pretty high. Trade wind anchorages were pretty perfect!

 

[JumbleDuck]

If just one of the cells in a string is shadowed not only does its output drop, but its resistance goes through the roof, "blocking" the current from the entire string.

Yes, but it has to be a hard shadow which covers an entire cell to have a really drastic effect.

 

[Dukester52]

Just wondering - does the OP have more than 1 panel currently? If so, connecting them in parallel (rather than series) and installing diodes to prevent current flowing back to each panel if the other is shaded might help the situation?

 

[RobbieW]

Hi all, OP here. Just to be clear, I'm interested in the effect on a single panel....

Whilst youre interested in single panel performance, I suspect that single panel will be 100W ? I also have 200W of solar but deliberately installed 4x50W panels which, after some experimentation, are connected as diagonal pairs in series then paralelled. The series connection mitigates the longish cable runs to the controller, centre cockpit boat with most stuff in the middle & panels at the back, the diagonal connection mitigates the shadows from backstay etc

 

[noelex]

Hi all, OP here. Just to be clear, I'm interested in the effect on a single panel. With a 2nd panel and a 2nd MPPT controller I know I'll be much better off, but I'd like to get the most out of the additional panel as if it were stand-alone (and if it really was shadow tolerant I'd probably sell my original panel and keep just the one).
In my understanding, panels have their individual cells arranged in 3 "strings", each string is a series of cells (typically 20). If just one of the cells in a string is shadowed not only does its output drop, but its resistance goes through the roof, "blocking" the current from the entire string. This is what I frequently find in practice; my perfect 15A drops to around 10A or around 5A depending on where the shadow falls (and yes, I do sometimes get soft shadow effects that are less drastic). What I want is for the backstay or mast shadow (which are always "hard") to be a tolerable reduction, hence the interest in these specific panels.
PS I can tilt my panel on one axis so with the sun rising & setting in line with the boat I can (shadows apart) keep the output pretty high. Trade wind anchorages were pretty perfect!

20 cells is not enough to generate sufficient voltage to charge a 12v battery, so this is not an option I am afraid. Somewhere around 36 cells are needed. The voltage could be boosted but there are very few controllers that will do this as the process creates some inefficiencies.