Effects of shading.

S

Scooby

Hello.

I'd appreciate your thoughts on my planned installation as I've received opinions from various installers which range from, literally, "No, your system is not viable." to "That's a shading factor of 1 - ie: little to no effect..."

My array will likely consist of 2 rows of 7 panels in portrait format, panels likely to be 190w Suntech or Sharp or similar. The problem I have is that for some winter months, a nearby telegraph pole will cause a moving shadow over both rows. The shadow will only begin to fall on the panels from 1pm onwards - up until then all is ok.

However, during the 3 peak summer months, the shadow won't hit the panels at all as it falls short of the roof sue to the high sun position. For the remaining 6 months, there will be partial shading - eg: only the lower row, and fractions in between!

Everything else is absolutely perfect - exactly south-facing, and roof at a 39o pitch.

I've even contacted the actual panel manufacturers, but they say 'ask the installers'. The installers, as I've said, have given every possible answer covering every possible range.

Thanks!
 
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If you can, get them to split the array L/R rather than top/bottom.

Any reason why 190W panels, cost? have a look at increasing to 250's

Make sure you have a dual tracker in the inverter to reduce shading issues on each half. with 14 panels, there will be a crossover point where the shading affects both simultaneously. As you said, there will be a reduction on output in the winter anyway, but this should make the best of an ackward roof.
 
Personally, I would say that the effect of shading can never be under estimated. We found that some installers said that it was not a problem and others said that it would kill the output of a whole section of the array on our house. A decent installer will offer an inclusive shading analysis and you can use that to make an assessment.

If shading proves to be a major issue you may mitigate some of the effects via the bypass diodes or, as suggested before, you could (and probably should) put some of the panels on a separate string with a separate tracker. The other approach is to use micro inverters so that each panel is controlled independently and the effect of shading will only drop that panel and not the whole string. That route is more expensive but you may find that the shade mitigation over the 9 off peak months pays for itself over the life of the FIT as every other panel will still be producing near peak and not be dragged down by the lowest performing panel on the string. The jury is out on micro inversion, some love them and some hate them but it cannot be denied that they can be helpful if shade is a problem.
 
If you number the top row of panels 1 to 7 and the bottom row 8 to 14 which numbers are affected in the winter and which numbers are affected in the spring and summer?
 
Interesting. We've just put a full 3D shading model and horizon into PV*Sol and run it through both on visualisations and output.

Final result: visualisation exactly as we suspected, looked spot on to what both I and the customer anticipated.

Factoring in the fact that most of the horizon tree shading would be in winter, PV*Sol's output figures were less than 10% different from no shading at all.

Watching the visuallisations time and time again, we're sure it's correct.

So you CAN overstimate the impact of shading.
 
Gents! Thanks a million for your prompt and thoughtful replies - most grateful & impressed!

(I'll outline more detail as requested by Liquidity further below.)

First, to answer Graeme Harrold, why 190w panels? Yes - it's to 'do' with cost, although that's not as simple as it seems. It's more to do with 'value for money'! For example, my two cheapest options are using either 185W Sharps or 190W cheapo Chinese. The cost difference between these two options is only £400 (the Sharps being cheaper) so my immediate reaction was to think "go for the 190s - it's only 400 squid more". However, a quick calculation showed me that the meagre 70kWh annual difference - woooo, that's £30 - wouldn't pay back the extra £400 outlay for waaay over 10 years - ergo, not worth it.

Similarly with Microinverters. These will add £thousands to the cost - for what gain? 20%? That will take decades to recoup! Worth it? Well, it just doesn't add up for me - unless I'm doing something wrong here.

And here's the other weird thing; I've been checking out Enecsys's website recently (because I thought micros would be the way to go) and followed their test example with interest; you know, the one that shows two identical PV arrays, both of which have dustbins (waaaay over the width of my 150mm post!) completely blocking one panel in each array. Yes, their micros increase overall output by 20%, but - hey - what really stood out for me was that the 'normal' array wasn't doing that badly either! And that was with a muckle great big bin in front of it. How much will that 20% gain cost you? Hmm, around £2-3k. Suddenly, the micro system ends up costing you pretty much what it'll return, if not more...

Also, for my situation, I need to take into account that the shading only begins at 1pm all year - so roughly half the day is absolutely fine in any case.

Worcester, does your software examine shading that falls over a whole panel like mine?

To get back to Liquidity, say top row is number 1 to 7 left to right, and bottom 8 to 14, also l to r. Typical winter scenario is that at 13.30, panels 1 and 8 will have complete vertical shadows on them, 150mm wide. At 15.30, the panels affected will likely be a lower corner of 3, and almost completely upper-half diagonal of 4, a complete lower diagonal of 9 and an upper (top left) diagonal corner of 10. (Phew...)

From mid-Nov to mid-Jan, the shadow will cover the full roof height. From mid-Feb to mid-Oct, only half the upper row will be covered at worst. Mid-March to Sept, only the lower row will be affected, and from mid-May through June, July to latish August they should all be in full sunshine.

Hence my thinking that a cheapo system will provide the same return on my outlay as an all-singing type.

Or am I more confused than I first thought...:angel_smile:?

Thanks again, folks - it's all helpful info.
 
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The "factor of 1" nonsense with shading calculations is one of my biggest issues with the SAP calculations. It is utterly nonsensical to suggest that a little bit of shading is going to have zero effect.
 
I started off by taking a short term view. I looked at a number of installers and began to make my decision based upon solar yield capacity vs price. I thought that the FIT was paid based upon the KWp output. A very helpful member on here then pointed out that the FIT is paid based upon generated electricity and not upon a SAP calculation. Had my roof been totally unshaded it would still have made sense for me to put as many cheap panels on the roof as I could because ROI would be fastest. However, my roof did suffer from some shade and this simple formula suddenly didn't stack up any more.

One of the three installers, who had expressed what I thought was excessive caution about shading on my roof suddenly began to make some sense. There really was no point in just throwing a load of panels up there if, for a fair bit of the time, one or more of them was killing the string. My cost would go up and my ROI time would increase.

So I started to think more about the longer term. One of the issues with this is that there are so many variables over which we have no control: how much sun there will be, if the FIT is killed or stunted through political intervention and so on. However, taking a pragmatic view and crossing fingers I took a look at our life style: I work from home (when I am not away on business) so we do consume power during the day. We have a number of garden features that pump water around and its a shame to turn them off. We could run the washing machine and dish washer during the day, etc, etc. So, with this in mind and the fact that the FIT is going to pay me for the energy that I produce at quite a decent rate and whatever I use during the day is free (and should get to be even better value as power prices increase over the next 25 years) and I will get something for what I export, maybe there is something in the idea of maximising my harvest over the longer term.

Now, in my case, and time will tell because it isn't installed as yet, my array would be affected by next doors chimney and aerial. A single string array would be knocked out until early afternoon for much of the day. A multi string tracker would mitigate some of this but, a dual string would certainly stunt half of the capacity. Interestingly, the chimney goes away quickly, but the aerial would shade a couple of panels for quite a while. It's amazing how a 2" diameter pipe has such a major effect.

Even this didn't convince me that micro inversion was worth the extra. One of the installers (the cautious one) put me in contact with a few of his customers and they, in turn, gave me access to their web pages so I could see real time energy production. One had little or no shading and the other had a tree obscuring a couple of panels later in the afternoon. The tree aside, it was a revelation to see how the panel production varied across the array. While the overall average yield per day was roughly the same for all of the panels (and be aware that maximum panel output varies - the worst will drag the string to some extent) , the instantaneous yield was going up and down almost randomly. In fairness, I haven't visited the sites but I am assuming that the variation is down to moving cloud and reflections. Theoretically at least, a micro inverter based system will track the variation and try to harvest the maximum yield on a per panel basis as opposed to a single string inverter that gets dragged down to the lowest common denominator. You have to offset this increased harvest against the slightly lower efficiency that micro inverters offer though.

I was still not convinced and this was mainly because of the increase MI cost vs a string inverter. They do cost more and there is more to go wrong. However, the Enecsys ones come with a 20 year warranty. Nobody knows if they will last 20 years but you will get a replacement in the event of a failure. True, there is an installation cost but, chosen wisely, better installers will offer an independent labour warranty to mitigate some of that risk. If one dies outside of that warranty, with care, they can be replaced without a huge labour and infrastructure cost. A string inverter on the other hand does not come with a 20 year warranty unless you pay extra. Statistically, a string inverter will not last 20 years so you have to build the operational cost into the equation (unless you intend to move house before it breaks). Considering this, the MI route, while still more expensive, is not as big a delta as it may first appear.

My post may come over as an advert for micro inversion. This is not the intention. I'm gambling on this route in my case because of the shading and I'm try to embrace an approach which is a longer term view that should give me an ROI of about 8 years and a healthy profit for the remainder of the FIT - somewhat under the assumption that the servicing costs of the installation will be much lower overall. Part of me also feels good about being even a little bit green. My feeling is that PV is a huge investment but the FIT, provided I get in early, pays a healthy return. Best then to invest a bit more (because of my shading) and hope to reap a reward overall. I wouldn't take the MI route with no shading, for sure. My plan may be full of holes, it may prove overkill. I have no idea. Only time and continuation of the FIT will tell.

Ian
 
Again, thanks so much - this is really helpful.

BiggsSolar, I have to agree - and I'm only using 'common sense', not any technical installer's knowledge. What is 'light shading' for heaven's sakes, and is a bit of soft shadow caused by a few dappled leaves the same as a stark, crisp shadow caused by a pole? It always struck me that the 'shading factor' used was too broad a brush. But, hey.

Ian, many thanks for your thoughtful post. It makes a lot of sense. No doubt in my mind that Microinverters will make a significant difference, but the jury in my head is still out on whether it would be cost effective in my case - taking into account that its benefit would only be needed for half of each day, and then for only roughly half the year.

I'm still trying to get my head around Enecsys's own test on their website - a black bin, which must be around 600mm wide, is completely blocking a panel (or two?) in an array. And yet the Micros only give a 20% increase in output? Hmmm.

It isn't that the Micros are poor performers, not at all; It's more that it appears that a fully blocked panel will only drop an array's output by around 20%! And I'd have thought that Enecsys would want the 'normal' array's performance to tumble as much as possible to make their system look good, so they'd have tried to get the worst possible situation with that bin... Surely? I really don't understand this.

If it's really only a ~20% improvement I can expect, then - taking a broad guesstimate - in my situation it would only amount to - ooh - 10%. Is this worth £2-3k extra outlay for me? It doesn't appear to.

Can I ask installers with software to answer this hopefully simple situation: A single straight array with 14 typical panels connected to a single Sunnyboy in full sun. Output is? Now, one panel in that array has full shading over its whole height - what drop in total output will occur. (Panel manufacturers cannot give me a straight answer to this :angel_smile:)

Thanks, all.
 
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A supplementary question, please! With, say, two rows of 7 panels, can each row be connected as usual - in series - and then both rows together in parallel to a Sunnyboy? So, that each row can work independently of the other? Or does this need a special inverter?
 
Scooby said:
Can I ask installers with software to answer this hopefully simple situation: A single straight array with 14 typical panels connected to a single Sunnyboy in full sun. Output is? Now, one panel in that array has full shading over its whole height - what drop in total output will occur. (Panel manufacturers cannot give me a straight answer to this :angel_smile:)
Click to expand...

I can't get a figure of how much a shaded panel will affect the overall output at a certain point in time but I could let you know how much of an annual effect it will have If you give me some dimensions, location, pitch of your roof and a photo I could try and give you an idea.
 
1pm.jpg

That's really kind, Biggs. This shows my garage roof at EX39, Devon. It is exactly south-facing, at around 39o pitch, and the max area is 8m x 3.9m (this includes eaves overlaps at each end, so usable space is more like <7m x 3.5m. This photo was taken in January, where the shadow height is at its worst. It's like this for around 3 months, but the shadow doesn't land on the roof at all for 3 to 4 months in summer - with every stage in between!

Not even panel manufacturers will tell me how much a thin, vertical, straight shadow will affect a panel and the array it's in! That's really the answer I need...
 
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Okay, estimating the size of the pole, I have run this through PV Sol Expert.

In the layout shown, it calculates an annual loss of 14% through shading.




Screenshot_Screenshot1.jpgScreenshot_Screenshot2.jpg
 
BiggsSolar, thanks very much for that info. Does your annual loss calc take into account the variation in shading height over the year - ie: the fact that there will be no shading at all over the 'highest' months?

Series530 (Gary?), it's good to see a real life example. Mind you, what I'd really love to see is your output performance with a series string! The professional answers you received mainly go to show how there still doesn't appear to be a definitive answer... Jeepers.

Is this info out there?!

Cheers.
 
Scooby said:
Not even panel manufacturers will tell me how much a thin, vertical, straight shadow will affect a panel and the array it's in! That's really the answer I need...
Click to expand...

Just to throw my bit into the debate - I've got 3.96kwp Sharp panels, 400tl Sunnyboy 51 degrees east on a 30 degree pitch. Telephone wire runs within about 300mm across the top of the array and we're in the North of England.

My point? We've generated 3100 kwh in 10 months!
 
series530 is Ian - not Gary.

Anyway, I think that you have hit the nail on the head in many respects: it's all but impossible to do a fair trail of one system against another because the trial period would need to encompass a controlled variation of all of the parameters that go to make up a typical energy harvest over an extended period. Unless those parameters were varied identically for each trial and the systems were assembled to reflect a real world environment it would not be an apples to apples comparison. Laboratory conditions would be the only way to do the trial and I am not aware of any such experiment.

We are left therefore with solar/shading predictions and anecdotal experiments that serve to give us an indication.
 
Ha ha - good result, SRE!


Sorry, Ian - I'd assumed you were Gary from 'GaryGary', the poster on the other thread!

It does appear to be as you say - no overall, unambiguous, clear picture. Which is crazy.

All I really want need know is, in an array of 'modern' panels connected in series, with their by-pass diodes and all that s***, if one panel has a full-height shadow, what effect does that make on the combined output of that array? I am actually staggered that this info isn't accessible. I don't blame the installers I've had round, and now fully understand where their complete range of opinions came from; some are clearly being cautious, others possibly delusional. And all because they don't have access to this actual info.

Jeepers.

Ah well, back to the thingy...

Thanks, all.
 
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Cheers, Worcester.

Yep, some good reading there. Some useful info too, but my 'simple' Q still unfortunately not answered; what effect in %-age terms will having a single panel with a full vertical shadow have in a string of 14?

I have, tho', pretty much decided how to go with this! I think...

I am taking into account two factors in my particular situation; 1, in the largest-producing months, the shadow just won't be problem at all, and, 2, for half the day, regardless of the time of year, the shadow also won't be a problem. Looking at this very crudely, if having a shadow on a panel reduces the overall output by, say, 25%, then taking into account my two factors above, I'd expect the total annual output to be only affected by - ooooh - a quarter of this amount. (I did say 'crudely').

So, continuing my crude way of looking at this, is it worth me forking out £1,000s extra in order to try and avert what is likely to be only a ~£50 annual loss? I can only say 'Non' to that.

I am thinking that a quality inverter such as the SunnyBoy 3000TL is worth having, as it seems to be able to optimise an array's performance under varying conditions. Is the 3000TL the 'standard' SunnyBoy model used, or will I need to specify the 'TL' (I have been quoted for a 'SunnyBoy', but don't know the model - is it likely to be a 'TL', or are there more basic SunnyBoys out there?)

I might as well ask if there's anyone out there wishing to quote for this job?! People on here seem to be more 'switched on' than many of those who've come round so far... I reckon I should make it a separate thread.

Thanks to everyone who replied to this thread - I'm very grateful.
 
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Scooby said:
Yep, some good reading there. Some useful info too, but my 'simple' Q still unfortunately not answered; what effect in %-age terms will having a single panel with a full vertical shadow have in a string of 14?
Click to expand...

I suppose the answer is that there is no straight answer. I'd imagine that on a dull day the percentage will be quite low as the light is diffused and therefore the effect of the shadow is minimal. On a very bright sunny summer's day then you will find that it will have a significant impact. When it comes to shading you really need to look at a yearly model.
 
Scooby said:
I am thinking that a quality inverter such as the SunnyBoy 3000TL is worth having, as it seems to be able to optimise an array's performance under varying conditions. Is the 3000TL the 'standard' SunnyBoy model used, or will I need to specify the 'TL' (I have been quoted for a 'SunnyBoy', but don't know the model - is it likely to be a 'TL', or are there more basic SunnyBoys out there?)
Click to expand...

I'd ask the installers whether they would recommend an inverter with two MPPTs so that the shading of the pole can be minimised.

Different inverters are good for different scenarios. There is no 'standard' when it comes to this kind of set up.

Shading, price, efficiency, sizing, even Ze readings are a major factor in selecting an inverter when you are considering yield alone. When you are considering connectivity for personal displays etc. as well as this then you will see that selecting the right inverter is very important.
 
Thanks, again, Biggs.

Yes, I see what you mean about the intensity of light. I guess what I'm asking about is when there is bright light with a defined shadow - all the panels in full light working at near max capacity. With diffused light, the shadow will be relatively insignificant, but then all the panels will be working 'less'.

In your helpful earlier post where you give me an idea of the annual figure for the reduction of output, did it take into account the 'unaffected' summer months and 'unaffected' day up until 1pm-ish?
 
Yes, the model that I used calculates the effects over a year - taking into consideration the suns path during the year and the fact that most of the time the array is unaffected.
 
Hello again.

I thought I should come back on here with some real-life results for those interested. PV system now beautifully installed and up & running courtesy of one of the contributors to this forum.

I took this photo today at around 2.30 pm. The array is split L & R, 8 panels each side. They are 200W Suntechs feeding an Aurora Power One (dual tracker) - a 3.2kW system.

PV shade.jpg

You can see that the telegraph pole is casting a full-height diagonal shadow over the bottom left panel, and around half way on the adjacent panel to its top-right. Bright, sunny day, although slightly hazy.

Outputs: RH array - 1170W and LH (shadowed) array 940W. (Peak output for the day - probably at around midday, was just over 2.5kW, which would have been split pretty evenly between the two as no shadow at that time.)

A couple of minutes later the sun went behind a cloud and both sub-arrays obviously dropped significantly in output as expected, and were nearly identical - 319 and 309W respectively.

I hope this is info is of interest. I can't analyse it too much myself as I am just a (happy!) customer not a PV installer. However, it strikes me as though the detrimental effect of the shadow isn't as great as I feared. Certainly the system as a whole is exceeding my expectations. Obviously the situation will become somewhat worse over the next few months as the shadow will cover both rows completely, before not becoming a problem at all over the main, shadow-free, summer months.

I am happy to provide updates if it's of interest. This kind of 'real-life' result is pretty much what I was looking for myself when I first came on here.

(I'm not sure of the forum etiquette as regards naming the installer. Certainly it is someone I am more than happy to recommend :winkiss:. I'm just aware that the purpose of the forum is to assist and not to advertise - certainly I received excellent help and advice from a number of people on here, so thanks to all. I guess, tho', that if asked by PM there should be no reason for me not to give this info - it is little different to a passer-by seeing my install and asking? Please advise if different.)
 
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@Scooby, keep the info coming please!
 
That's very interesting data Scooby and I'm really pleased to see it published. I don't know if this analysis is a reasonable one but I'll offer it and see if the logic makes sense to others:

If we split the array into two halves and assume that the RHS is the reference. On the left hand side one panel and about a third of a panel are struck off by the telegraph pole. If we assume that each panel has 3 bypass diodes that means that 4 diodes are operating out of a total of 24 in that half of the array. Thus, the left hand panel is running about 83% of the harvest of the right hand side. Your figures would suggest that it should be about 80%. Either way, pretty close.

Thus, the diodes are working. I guess that in low light the pole has a more fuzzy effect and doesn't come into play.

Good information. Please keep it coming.
 
Great to see the system working so well. It will be interesting to hear some updates so that we can analyse just how much impact the pole appears to have on the array. 1170w and 940w surprises me. I have always been surprised by how much a simple scaffold pole affects a system so the slight drop in yield is not what I'd expect.
 
Thanks for your replies - I'm pleased it's of interest and I'm happy to update it when conditions change. If there's any particular conditions which are of interest, Worcester, please let me know and I'll do my best to provide them.

Series530, I get the gist of what you're saying! My concern before having a system installed was whether by-pass diodes would be effective. My install suggest they are - which is a relief. It'll be interesting to see how having two panels in each sub-array completely zonked by the shadow. Also, when the shadow hits a panel on both arrays - that happens at around 15.30, I think.

Yes, very pleased with how they're going, Biggs!
 
I thought a bit more about this and something doesn't quite add up for me. If the shading is a hard shade (as seen by the pole) I can imagine it effectively reverse biasing all of the cells on the affected panel and the bypass diodes kicking in. Under these circumstances the panel will be bypassed and will contribute nothing to the harvest. If a number of cells on another panel also have shade and turn off, the associated bypass diodes on that section of the panel will also turn off but the whole panel may not. Each set of bypass diodes that kick in will allow current flow by with a 0.7V drop across each set. On a full string the drop is minimal so can be all but ignored.

We hear over and over that shading is bad (which it is) and that an affected panel will cause the string to drop to the output of the lowest performing panel. That would seem true provided the bypass diodes don't kick in due to a reverse biasing of the panel or, in the event of the panels in question not having bypass diodes. I read somewhere (I think Solaredge) that a 20% difference between light levels on any section supported by a single bypass diode is enough to trigger the bypass diode of that section. Looking at Scooby's pictures I would guess that there is a 20% difference between the light and dark areas and that this is enough to effectively reverse bias that section of the panel and trigger the bypass diodes.
No end of MPPT, micro inversion or anything else will deal with a bypass diode in play because the section of the panel serviced by the bypass diode has reverse biased and the bypass diode has kicked in to protect the panel.

So, this leads to me to think that there is a grey area between zero and this magic 20% number (if this number is right) where the section of the panel wont be performing optimally and, presumably the inverter will have to tune the MPPT to deal with this more resistive operating point. Perhaps here the whole of the string is affected as the MPPT is set by the string and not the individual panels. Once the bypass diodes kick in the sections of the string that are bypassed are no longer contributing anything to the energy harvest and the inverter will retune based upon the harvest available.

So, is it in this magic area between 0 and 20% shading the bit that everybody refers to where the string is effectively under performing as a whole and once the bypass diodes kick in that section of the string is simply switched out? Could somebody explain it to me please.

Many thanks
 
Good news and thanks for taking the time to post a valuable update, as Im about to double the size of my home system to 8kW (before the FIT is slaughtered), and I too would be plagued with a nice big pole on the last couple of panels up to about 12am, aster this Im clear.

Total yeild so far is 1959kWhr since 23rd May on a south east roof.....
 
@Series530

You've also got to consider "hard" shadows from dormers or other building edges that might knock out a few panels (not just the bypass diodes kicking in here) causing a significant drop in the string voltage, which could therefore drop below the inverter's cut in point, and where strings are in parrallel instead of into seperate MPP trackers (bad design you and I know) what effect that would have had.

Perhaps we should bear in mind that in the 'early' days of only a few years ago, panels didn't have bypass diodes and inverters were single string perhaps even without MPP technology.

I may be wrong, perhaps we are highlighting a problem that is now less true because of such developments.

Thoughts?
 
This may not be directly relevant, but I've also been monitoring the effect of a more diffuse shade caused by a silver birch tree on two right hand panels of one 8 panel string (of two strings). I knew the shading was there, but wanted to confirm that was why my new system was not performing as well as a nearby array (same orientation, v.similar size). The graph below shows the jump in output after 12pm when the 1 string that is in shade comes out into full sun (data collected on a clear sky day).

outofshadow.jpg

..and here's the results from that day comparing my array output with a nearby system:

LangstrothOther ArrayDelta
WhWhWh
8pm to 12pm285452172363
12 noon to 2pm5575560732
2pm to 4pm44754634159
4pm to 6pm17591186 -573




As you can see, my array when not shaded is producing the same output as it's unshaded near neighbour.
The morning shading though, even though "slight", results in a near halving of output.

(The tree is due to come down btw - it's also shading too much of our garden)
(For info: system size: 4Kw. Two strings of 8 on separate MPP trackers.)
 
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langstroth2 said:
This may not be directly relevant, but I've also been monitoring the effect of a more diffuse shade caused by a silver birch tree on two right hand panels of one 8 panel string (of two strings). I knew the shading was there, but wanted to confirm that was why my new system was not performing as well as a nearby array (same orientation, v.similar size). The graph below shows the jump in output after 12pm when the 1 string that is in shade comes out into full sun (data collected on a clear sky day).

View attachment 8313

..and here's the results from that day comparing my array output with a nearby system:

LangstrothOther ArrayDelta
WhWhWh
8pm to 12pm285452172363
12 noon to 2pm5575560732
2pm to 4pm44754634159
4pm to 6pm17591186 -573








As you can see, my array when not shaded is producing the same output as it's unshaded near neighbour.
The morning shading though, even though "slight", results in a near halving of output.

(The tree is due to come down btw - it's also shading too much of our garden)
(For info: system size: 4Kw. Two strings of 8 on separate MPP trackers.)
Click to expand...

To my thinking, this and what Worcester posted earlier is actually very relevant. What it says to me is that a hard shadow will knock out a set of bypass diodes, a panel or several panels and, if several panels suffer from hard shading, it may also take out a string which would lead to a partial or a complete inverter shut down.

Soft shading (due to diffused objects such as trees or structures which are some distance away) may not be enough to trigger the bypass diodes but will be enough to reduce the output of the string until a point where the bypass diodes trigger (about 10.50am).

The panel output doesn't increase then until about 11.30am when, I have to assume, the sun has moved enough to bypass the shading. At this point, we see almost a doubling of power output as the shaded array comes back on line. I don't believe that the string shut down though. Based upon 250W per panel, it seems like about 2 panels came on line at about 11.20 and the rest came back at about 11.30 (smoothed out on the graph).

Had the shading not taken place I suspect the gradual increase in harvest would have continued in a linear fashion until reaching the systems capability and we would have seen a smooth and gradual curve similar to what we saw before 10.50.

Something is also occurring between 10.50 and 11.30: the variation in power is pretty wild. I suspect that this is panels or sections of panels bypassing (or otherwise) as the shade takes more and more of a hold on the array. Smoothed out as it is, you can see jumps in power level as something like 4 panels are switching in and out and then the whole of the rest of the array suddenly comes on stream at about 11.30.

When my micro inverters finally arrive (something of a moving target) I will try to produce something similar. If nothing else, I should be able to track what is happening panel by panel as shade takes effect.

Thank for sharing this with us.

Highly interesting for me.... being an engineer, I like to understand what is happening.
 
Somewhat delayed further feedback from me on this - sorry. Due to pretty cloudy conditions recently with no shadows being cast...

Took these two readings yesterday, 22nd Oct. Bright sunshine, well-defined shadows, but not completely 'blue' skies.

15.00.jpg16.10.jpg

Left hand photo shows shading at 3pm, right hand at 4.10pm.
To recap, the array is split left & right, 8 panels each side.

At 3pm, LH array: 335W RH array: 1201W

At 4.10pm, LH array: 404W RH array: 510W

This info is for others to digest, and I have found the responses genuinely interesting.

The bottom line for me, of course, is that despite the lowish Autumn sun, the shadows in the afternoon, and the overwhelmingly cloudy & overcast weather since the install, it has still produced 192kWh in 25 days! So even under these 'worst-case' conditions (I'll have no shadow at all over the highest producing Summer months) , it is still on target! That, I find, is pretty darn impressive... :angel_smile:
 
Thanks for the info, Scooby.

These results are consistent with the tests we have done. I was surprised that the pole appeared to be having less of an impact on your previous posts but clearly the shadow cast by the pole is larger in these photos. The drop in production from 1201w to 510w as the shadow moves along it is dramatic and demonstrates the importance of getting the design right in the first place.

A quick question for the installers on this forum. What factor would you apply in your SAP calculations for this pole?

It seems that the system is actually slightly over performing. I calculate a yeild of over 195kWh over the whole of October in my software. It's been a pretty good month for solar production!
 
At first glance, (before the full story of this thread) I would probably have taken "Modest" as is clearly an obstacle, and therefore taken the factor of 0.8 (20%) drop.

For me to choose "None or very little" the pole would have to have been the other side of the road / track.
 
Those two pictures that you took Scooby are really interesting. For me it's another good illustration of the effects of shading. I'll assume for a moment that the power produced by the sun is constant at both times. I know it isn't, but for the sake of the analysis, I'll assume that it is. Your reference is the right hand array in the first picture which is producing 1.2KW. The left hand array, at that time has one panel where the bypass diodes have probably isolated the output and three more that are marginal. Worst case, we might expect 4 of the 8 panels turned off. The power produced on the left hand panel, at just 335W is about 28% of the output of the right hand panel.

Move forward to the later picture now. In this one the effect is almost symmetrical on both panels. Theoretically, with two panels of out 8 affected on each array, the power produced should be 6/8 * 1201 KW = 900W. However, what is produced is roughly half of that.

So, what is happening and why is it different to your pictures from some time back? My theory is this: these two pictures, at a time when the light levels are lower anyway, show situations where the bypass diodes are not triggering. Taking the RH array in the first picture as the reference, in all of the other situations the effect of shading is constraining the harvest to the performance of the worst panel because the difference in harvest between the light and shade is less than 20% of the panel capability (using the Solaredge numbers). As the diodes are not triggering, the whole array is being constrained to the performance capability of the worst panel.

So, where the irradiance is higher, the difference in power between best and worst sections exceeds the 20% value and the diodes trigger. So, a nice sunny day in the middle of summer would do the trick. However, early and late in the year or early or late in the day at other times, the diodes may not trigger and the power harvest will be reduced. Luckily though, this is at a time when the harvest is low anyway - so the loss is less of an issue.

Just a hypothesis... does this make sense or are there different factors at play here?
 
Hi Series. What you say makes sense to me, tho' I don't have the knowledge to confirm your hypothesis! Certainly, the LH array shows a much bigger reduction in output compared to my earlier - first - post on this.

I have to say I was pretty amazed at how little the original (last week's) effect of the shading was; it was slightly earlier - at 2.30 - and the shadow was affecting only 2 panels, one fully, and the other by less than a half, but still... The example above is half an hour later; as the picture shows, a total of 4 panels are now affected to some degree. However, the overall light levels between this one and last week's must have been pretty close in both cases as the RH array had outputs of 1201W and 1170W respectively.

I would suggest that comparing the 4.10 example with the others has to be treated with some caution as the light angle has changed a fair bit, but how much effect this has I don't know.
 

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