Shading & bypass diodes

[DomB]

Chaps,

We're just installing a 6 panel array in which one of the panels gets shaded around 2pm. There's no room to move it out of the shade and we can't put in fewer panels without moving to micro-inverters (which we're not yet experienced with).

Just thinking through how much of an effect that one panel being shaded would have. Would the by-pass diodes kick in and reduce the array's output by just one panel or would it bring down the whole array? If it's just one panel that goes down then the system is probably worth installing, if it's the whole array then it's probably not!

Cheers

 

[Worcester]

Check out these articles from SMA:
Made for shade. SMA Solar Technology AG

and this excellent video explains the full effect:
OptiTrac Global Peak. SMA Solar Technology AG

Overall it's not actually so bad, just now got to find out if that inverter's got 'Opto-Trac Gloabl Peak' (Sorry I haven't got that list!)

 

[TedM]

The SMA inverters Sunny Boy 3000TL, 4000TL and 5000TL will be equipped as standard with OptiTrac Global Peak as of the second quarter in 2010. For older devices there will be a software update available.

from OptiTrac Global Peak. SMA Solar Technology AG

Also http://download.sma.de/smaprosa/dateien/3491/TECHOPTITRAC-AEN082412.pdf has more info on OptiTrac.

 

[claypole]

Bear in mind that the factory default setting is for the OptiTrac Global Peak to be "off". If you have no shading issues it does have a slight negative effect on generation. Easily switched on through Sunny Explorer when logged in as installer.

 

[DomB]

Thanks chaps. Nice vids. And nice technology.

I notice the shaded array without the OptiTrac still yields 80% - more than I expected. I guess this is the by-pass diodes kicking in on those shaded panels.

My understanding then is that the by-pass diodes allow the rest of the array to work at peak output while the Optitrac works to maximise the irregular power curve that is then produced, i.e. it's a combination of the two that keeps a partially shaded array's yield nice & high.

So I just need to confirm the SMA inverter I'm recommending has Optitrac - or look at how to install it if it hasn't.

Cheers

 

[edexlab]

Chaps,

We're just installing a 6 panel array in which one of the panels gets shaded around 2pm. There's no room to move it out of the shade and we can't put in fewer panels without moving to micro-inverters (which we're not yet experienced with).

Just thinking through how much of an effect that one panel being shaded would have. Would the by-pass diodes kick in and reduce the array's output by just one panel or would it bring down the whole array? If it's just one panel that goes down then the system is probably worth installing, if it's the whole array then it's probably not!

Cheers

when you say shaded by 2pm is that for the rest of the day?

anyway the bypass diodes are meant to allow the unaffected modules to carry on as normal and won't be brought down by the shaded module although if the shaded panel is part shaded then a reverse current will flow in that module ,it becomes a load rather than a generator resulting in the shaded cells heating up,this can reduce the life of the cells/modules affected
modules usually have a couple of diodes to stop voltages/currents high enough to damage individual cells from flowing in reverse
I know that sometimes for very visible jobs installers have put a dummy module in, for aesthetic reasons but i would'nt think this is'nt practical for a domestic job

you also need to be sure that losing one panel won't take the array out of the inverter parameters

 

[DomB]

Thanks Edexlab.

Yes, the shading hits the sixth panel about 2:15pm. All the other panels remain unshaded. The shade does go across the panel in line with the by-pass diode configuration.

Thanks re the headsup on the inverter parameters, I might just talk through my thought processes (prepare yourself!), see what you think:

It's a Sunnyboy 1200 with 6 Schuco 210 Poly's. The MPP voltage range of the inverter is 100V to 320V with a minimum start voltage of 100-120V. Each panel's Voc is 32.9.

So, with just 5 panels (say the sixth was in total shade) the voltage is 164.5V, enough to get the inverter started.

In terms of the MPP range: The 6 panels with the SB1200 have a Power Ratio of 105%. So for a good proportion of the day the system will work quite efficiently. When the shade does kick in the power ratio will start to creep up, heading towards a 5 panel configuration which gives a power ratio of 126%. I guestimate then that the ratio will go above 110% around 4:30 - 5pm. Which is 6pm in Summer Time. If 2/3 of the day's power is taken from between 9am and 3pm'ish I figure most of the day's juice is taken before the shadowing causes significant inefficiencies to start.

Phew.

Annyway, my conclusion was that the shading on the 6th panel wasn't enough to recommend not selling the system .

p.s. I did consider micro inverters but need to get on the training for them

 

[edexlab]

You don't have to look very far to see jobs which do have serious shading problems, but we don't want a reputation of fitting systems which are going to have problems and so we have told several customers that we would'nt consider their roofs for this reason ,
we are going down the micro inverter route not really for shading reasons but for a roof with multiple small elevations ,on which we could'nt use a string or multiple Mpp inverter but this has issues as they are limited for the type/ manufacturer of modules there is quite a small list that are compatible with voltage ratings etc

 

[DomB]

Agreed, we've turned down a few customers (and asked for references afterwards - thinking a reference saying 'they wouldn't install for us' is as good as one saying 'they did a good job')

Think I'll register for the training on the micro inverters just to keep the options open