help please reduced output on hot days

[Fronius]

Your panels loose .53% for every degree above 25'c compared with a 235w ET panel which looses .43% so thats 1% differance for every 10 degrees above 25'c so if the panels are at 75'c the ET panels are 5% more efficient if all other factors are the same.

Any one confirm this is right.

I doubt that 75C has been hit on the average panel so far this year. Asphalt can melt at 60C. I have a temp sensor on the underside of my array ( 33deg S-facing ) and have not been past 47C.

 

[cbw]

Front faces were in excess of that (47) on testing this week

 

[billybee]

well going of 47'c thats a 2% diffrence in output. now going of pvoutput im seeing anywhere from 7.5% to 14% diffrence in performance. also no taking into account other factors to get an 8% diffrence due to temps if 10'c is 15 then we be looking at over 100'c to do that

 

[Gary K]

I doubt that 75C has been hit on the average panel so far this year. Asphalt can melt at 60C. I have a temp sensor on the underside of my array ( 33deg S-facing ) and have not been past 47C.

Fair enough. I just pulled some figures to work with. I have to get a 3 piece ladder out to get to my panels but will get the laser thermometer out at some point.

 

[Gavin A]

so can i ask what this NOTC 45°C +/-2% means. i do understand what you say about temps and take it on board but why would it cut down the out put on 6 days to neally exactly the same amount each day. wouldnt i expect to see some fluxuation in the peaks as 5'c change in temp would mean aprox 150w's change in peak output. there doesnt seem any middle ground its all or nothing so to speak. i'm getting the inverter changed next week and going off what your saying it shouldnt or wont make any diffrence. i'm a bit old fashoned and strongly believe that if you run something at 100% you shorten its life. so yes ive done the figures and i know i'll get a slight performance lose with the bigger inverter (could make things worse lol) maybe as much as 2% but it will confirm that its a panel problem if outputs stay the same. and if its down to amps i should see a higher peak on hot days.

i wish i could find some asun panels to compiar with on pvoutput.

you're rarely running it at 100% though.

its maximum rated output is 3300W, its nominal output is 3000W, and you're seeing it running at 240oW in long periods of full sun. If you get a bigger inverter the annual output will reduce for no benefit.

 

[billybee]

thanks for the help and i will report back after the inverter change. but taking in what you all have said that is possibly a couple of % on temps maybe a little on having 2 strings rather than 1, compairing 2 similer systems in the same area facing the same way with the smae pitch. maybe his 10% bigger system could out perform me by 13 to 14% (not factoring in that my panels do have out put peaks the same as his bigger system) but in 50 days the avarage diffrence is 20% today i got 17340w he got 21171 and i think thats around 21% diffrence.

 

[billybee]

you're rarely running it at 100% though.

its maximum rated output is 3300W, its nominal output is 3000W, and you're seeing it running at 240oW in long periods of full sun. If you get a bigger inverter the annual output will reduce for no benefit.

am i reading the technical data wrong the way i understand it is above 35% of rated Output Powerthe diffrence is 95.2 to 96.8 on perfromance anything below 35% i could pick a bit up as 3.0 starts inverting at 160 and the 3.6 at 120 and lets be hounest if thats all i'm getting were talking a few watts. if the bigger inverter increases peak out but on hot days (which im beginning to doubt now) i could pick up 400watts an hour. losing another 1% would be 180w on my best day and i could get some back of the slightly earlyer start up and shut down.

 

[SibertSolar]

Sounds to me like you have modules with shoddy junction boxes/bypass diodes fitted, either that or the cells in use are particularly low grade. Could it be possible that one or more bypass diodes are kicking into reverse bias because of individual sub-string cells getting too hot maybe? I've never come across a-sun modules and don't know what/whose cells they use, does anyone have any links/spec sheets?

NOCT = Normal Operating Cell Temperature and 45degreeC is within the typical range of most poly cells.

 

[billybee]

http://www.a-sun.co.uk/download/ASUN250-MB_EN.pdf

 

[SibertSolar]

Ok, thanks. Looks decent enough tbh. Well, if it's not something related to a defect diode then perhaps it is down to a combination of all the temperature related factors already outlined by others above. Would be interesting to see a thermal image of the array when the apparent losses occur!

 

[billybee]

so if its + or - 2% at 45'c should i expect to lose 10% thata 20x 0.53% ish over that temp range.

 

[billybee]

i carnt really call them losses just dont get above 2450w on hot days and because its already arrange to change the inverter, ive got my fingers crossed that its the panels kicking to much amp out for the inverter and by uping the inverter that it also ups the reduced peak output by the same %. i will post when i get some results either way. if nothing changes then its back to asun for an expernation of them on why ther claiming the temps we've had shouldnt effect the peak out put. as they told me there tested and used in italy with no reported power drops and its hotter there than here.

 

[sambotc]

They obviously know something that everybody else doesn't then, and bearing in mind they have temperature coefficients printed on their spec sheet, i'd say they are not telling the truth.

 

[FB.]

Today was a great day to see the temperature phenomenon.
It was about 20'C, no cooling benefit of any breeze and a mixture of sunny spells and cloud.

There were cloudy spells with generation being just a trickle, and when the sun came out power soared to 3.5kW. Then gradually the power output declined (as the panels got hotter) and settled around 3.0kW. Then a cloud would block the sun again, power would drop to 700W again, the panels cooled, the sun came out again and the power jumped back up to 3.5kW.

So I reckon that I was seeing a 15% loss of power output due to temperature.

The power-derating temperature coefficient of my panels is 0.43% per degree Celsius. So a rise of 35 degrees in panel temperature would explain the 15% drop in power output.

With ambient temperature around 20'C, the panels would appear to have been reaching 55'C after being in strong direct sun for many minutes.

 

[billybee]

well i agree there, the only other thing is on cold days i offtern get the same diffrence out of the last 21 days from 4 to 10kw production ive been out performed on 17 days by upto 23% but in saying that i have beaten him 4 times so even though were 10km apart weather could play some part may be cloudy here and raining there. thanks for the help anyway