3.3A short circuit current from a (damaged) 250W panel in winter?

[PeterW]

My friendly local PV installer let me have an Ulica UL-250M-60 panel that had come off worst in an encounter with an iron bar.  According to the data sheet, open circuit voltage is 37.6V and short circuit current is 8.7A.  The glass is cracked across the entire surface of the panel.  Where the bar hit, the glass, cell and backing are bashed right in and there is a small hole right through the panel.  Opening the junction box, there are 4 leads, W, X, Y and Z, which I assume connect the 3 strings in series.  A pair of bypass diodes D sits between each lead and the next one.  Lead W corresponds to the negative connector, Z to the positive.  I hope this illustrates the layout. 

W      X       Y       Z
I        I        I       I
I--D---I--D--I--D--I
I                         I
-                         +

I stood the panel facing bright sunshine, and tilted it as nearly perpendicular to the sun as I could make it, at about 13:00 last weekend.  Location is the middle of the UK.  Putting my multimeter clips onto the lead terminals, I measured as follows: W-X 12.7V; W-Y 25.3V; W-Z 37.9V, which corresponds nicely to the rated Voc.  I changed to the proper holes on the multimeter, set it to measure current and measured as follows: W-X 2.1A; W-Y 3.4A; W-Z 3.5A. 

Isc falls a long way short of the rated figure.  I haven't been able to trace the wiring of the panel, but as far as I can see, the W-X leads showing 2.1A Isc correspond to the string with the hole in it.  Strangely, when I measured the other way round (Z-Y, Z-X, Z-W), each of the 3 showed 3.3A. 

Please excuse the basic questions – this is my first venture into solar PV.  I've done a number of searches on this board and found answers to some of my questions.  These are the questions to which I couldn't find answers.  Should I expect weak winter sun to produce that much less current?  I know that you get less power on a winter day because there are fewer hours of sunshine, but I haven't been able to find any figures showing how much less than the 1000W/m2 test conditions you really get.  If the reduced current in the damaged string is in fact a result of the damage rather than weak sun, then shouldn't I expect the other two strings to show closer to the rated Isc?  Or could that be a result of the cracked glass blocking out a lot of the sunlight?  In either case, if standard Isc for the panel is 8.7A and standard current at Pmax is 8.2A, can I expect to get a similar proportion of my Isc – perhaps 3.1A - when the panel is loaded and properly running?  I had originally calculated that I needed a 40W panel for my project, so even if I get only 3A or so, that should give about 90W if I understand correctly.  Even after I've lost half of that through a charge controller charging a 12v battery, I think it should give me enough power.  Any suggestions on maximising the power and/or preventing further damage would be gratefully received, though.  For example, is it worth trying to test (how?) and then if necessary bypass the damaged cell, and if so, do I simply pick off the glass over the connecting ribbons and cut them, then solder on leads to bypass that cell and connect to the next in series?  And do I need to do so anyway in order to prevent further damage to the panel?  Presumably there's a risk that the hole in the cell has shorted it and/or neighbouring cells.  I'll seal the panel as a whole either with varnish as described on one of the threads here, or with a sheet of glass and some sealant.

Thanks very much.

[DamonHD]

I can tell you that in the UK there's a total of about 1 hour equivalent of full sun each mid-winter day on average vs ~5 each mid-summer day.  Most of that difference is because the day is 8h rather than 16h, and there's more overcast sky, but if you have a clear blue sky and an optimal tilt my *guess* is that instead of ~1kW/m^2 you might get 800W/m^2 due to the extra atmosphere that the sunlight is passing through.

Rgds

Damon

[PeterW]

Thanks Damon

All the sites I had found until now show total annual solar radiation, which doesn't really help in working out the figures at particular times, but I've just found an interesting site showing real-time solar radiation in Cambridge, which is only a little further north: http://www.ramseyweather.co.uk/wxsolar.php.  If the figures are correct, they suggest that even in mid-summer, there is only 600W/m2, and a lot less in winter.  The Ulica panel data sheet http://www.ulicasolar.com/modules04.asp shows I-V curves for a range of irradiance figures.  Approx 3A, which is what I observed last weekend, looks as though it would correspond to about 300W/m2.  From the Cambridge figures, this looks quite likely on a bright winter day.  So it may be just winter rather than the panel damage.

I would still be really interested in any thoughts on whether/how to test and isolate the damaged cell, though.

Thanks very much

Peter

[DamonHD]

I don't have good suggestions on dealing with the damage, but I suggest doing so sooner rather than later if possible since heating effects may well cause deterioration.

I'm not sure that I believe those irradiation figures, BTW, but I'll have a look!

Rgds

Damon

PS. Welcome to FL!

[bob golding]

not sure if it helps, but i have 3 damaged panels of the same rating as yours. at the moment they are producing around 7 amps in a 24 volt battery bank. they are connected in parallel. i only had one panel last summer and that produces around 6  amps on a bright summer day.
the temperature makes a big difference to the output. i would guess more than the cracked glass does, but i don't have any evidence for that yet. so going on the isolation figures that sounds about right. i am in the far south west so get the highest isolation in the UK.
i have forgone a  MPPT controller in favour of more panels. i have a 5 acre field to play in so space is not a problem. as i have yet to build a frame to hold the  2 new panels yet  they are just lying on the ground at the moment.
 i did have them propped up on some pallets but the wind blew them over. i haven't noticed a great drop in output having them flat on the ground. i will laminate them and fix them properly when the weather improves a bit. hope this helps.

[Larsmartinxt]

This should be pretty accurate, It also uses average weather for the area http://re.jrc.ec.europa.eu/pvgis/

[PeterW]

Thanks guys for the help, and thanks Damon for the welcome to this great board.  The JRC site shows around 600W/m2 for my location at 13:00 in Feb, which is higher than the Cambridge figures, and should be producing nearer 5A according to the data sheet.  Bob, am I right in thinking that panels in parallel add current, so you're getting just over 2A from each?  It's difficult to tell whether my lower readings are due to the damage or due to lower irradiance or a combination of both.  It certainly looks as though it can't all be blamed on lack of sunlight, so I guess the next step is to see if I can work out a way to test the dodgy cell and a couple of others for comparison and then if necessary bypass it.

[bob golding]

in theory  i  should be getting 2 amps per panel. i will check this when i get another clear day, bit rare down here in febuary. you can buy single cells on ebay. about £1 each i think, and try and remove the damaged one and replace it . i intend to do this sometime when i get some another 3 panels to play with. i am completely off grid and my wind turbine is broken at  the moment so cant afford to mess about with the panels yet. i was going to go in from the back of the panel and try and remove the damaged cell from there.