Effects of shading.

[SRE]

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...

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]

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.

 

[Scooby]

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.

 

[Worcester]

Scooby, SMA labs have done a fair bit of work on this (visit my website!) or download the information for yourself frome here:

Made for shade.*SMA Solar Technology AG


http://files.sma.de/dl/3491/TECHOPTITRAC-AEN082412.pdf
and
http://files.sma.de/dl/7418/GlobalPeak-UEN101210.pdf

Not all the SMA inverters have 'Global Peak' - the 4000TL does, you and your customer can switch it on and off (summer / winter) using Sunny Explorer (free)

 

[Scooby]

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.

 

[SolarCity]

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 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.

 

[SolarCity]

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'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.

 

[Scooby]

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?

 

[SolarCity]

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.

 

[Scooby]

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.



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.)

 

[Worcester]

@Scooby, keep the info coming please!

 

[series530]

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.

 

[SolarCity]

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.

 

[Scooby]

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!

 

[series530]

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