Jump to content

Hi and Hello from EL(plus a question or two)


Recommended Posts

Hi Guys,

Although a new poster to this forum, we have been lurking in the shadows for a while...

A (hopefully) quick and easy question for the expertise out there:

Just imagine.... 4kw of panels in 4 strings of 3 panels each, all north aligned in a lekker straight stripe. All strings are paralleled into a single mppt input on a good ole Axpert 5kva inverter and running sweetly... Now that the scenario is set, here is the twister....

During the course of the day, there is pretty much always a small section of one of the strings, sometimes more, depending on the hour, where a narrow band of shade from a small wind turbine mast is cast... The mast will be moved ultimately, but in the interim, how detrimental is the partial shading of one string to the performance of the remaining, unobstructed strings in the array? Yes, I am aware that a good patch of shade, although small, will hammer the output of a single string immensely. Will this underperforming string "suck some power" (for want of better technical term) from the active strings? If so, would it be feasible to drop a fat big overspecc'd power diode in series with the positive output of each string to prevent feedback to the string that is obstructed at any given time?

Thanx in advance...

E&G

 

Link to comment
Share on other sites

21 minutes ago, Joe Moer said:

how detrimental is the partial shading of one string to the performance of the remaining, unobstructed strings in the array? Yes, I am aware that a good patch of shade, although small, will hammer the output of a single string immensely.

Will this under-performing string "suck some power" (for want of better technical term) from the active strings?

Hy Jou Moer, skies, Joe Moer. :-) 

Other will pop around with more detail.

The simple answer: Very. Never ever let any shade ever fall on any part of the array whatsover.

No, it will not "suck some power", just lose a lot of power due to that patch of shade.

 

Link to comment
Share on other sites

8 hours ago, Joe Moer said:

......how detrimental is the partial shading of one string to the performance of the remaining, unobstructed strings in the array? Yes, I am aware that a good patch of shade, although small, will hammer the output of a single string immensely. Will this underperforming string "suck some power" (for want of better technical term) from the active strings?

Solar panels are divided into groups each witha bypass diode. If there is any shading the bypass diode "isolates" the affected group and the shading has less of and impact but still a small amount of shading can lead to loss of production to a far bigger area. This affected panel then acts as a "kink in the hosepipe" for the other panels in series with it.

https://www.e-education.psu.edu/ae868/node/875

Link to comment
Share on other sites

19 minutes ago, Chris Hobson said:

bypass diode "isolates" the affected group

True, but that pulls the Vmp (voltage where it makes max power) down for the whole array, so is still tends to affect parallel strings.

This is an old topic. Way back I tried to reason it out (the second shaded string acts like a large reverse-biased diode and should have little effect, in theory), and then I tested it with two 20W panels. Truth be told... I still have no idea exactly how it works. I know it causes a loss of power, but can't say how much and why :-)

Link to comment
Share on other sites

So in a nutshell: 

As we expect, all cells are wired in series to get to the desired voltage level of the PV module. However, this series connection raises an issue that when one cell is not generating power due to shading, for example, it will not be able to generate the same amount of current that other similar (unshaded) cells generate. And due to series connection, the total current of the module will be dictated by the weakest cell (shaded). As a result, that will create power loss due to current restriction. In addition, when the higher current generated by unshaded cells tries to pass through the shaded cell, the shaded cell might act like a load and the temperature will increase, and that might lead to a phenomenon known as “hot spot.” Hot spots cause physical damage within the module, like melted cells, cracked glass, or changing characteristics of cells.

Link to comment
Share on other sites

49 minutes ago, The Terrible Triplett said:

the shaded cell might act like a load and the temperature will increase

Reverse breakdown. Quick diversion into diode theory. Diodes block the flow of electricity when they are reverse biased, or at least, they do so up to a certain point. So your typical low-frequency rectifier bridge in just about every small wall-wart will likely be a 1N4007, which has a 1000V rating: It blocks reverse flow up to 1000V. Go over 1000V, and you get reverse breakdown. For most diodes, they get instantly destroyed when they break down. There is a kind of diode known as a Zener diode that is designed to be wired in reverse and to break down at a specific voltage and clamp it there. This is a useful device to have, but they too get hot and will be rated for maybe 1W or 2W max.

Fast forward to PV panels. A solar cell is a diode. Its a special kind of diode that puts out current when light shines on it, but in the dark it still acts like a diode. With sufficient voltage across it (ie enough cells in series) it breaks down as well, gets hot, and potentially destroys itself. But this is where the bypass diodes come in. They ensure that this cannot happen. The also do help a little bit with bypassing shaded cells, but their main purpose is to prevent damage to the cells, not to maintain power levels during shading (though they do help, or so I am told).

Link to comment
Share on other sites

Interesting reading....

My simple brain can deduce the  following, albeit a bit of a logic puzzle, and most probably flawed... If a portion of a panel is shaded, say one cell, and the panel as a whole is generating current, that one cell, because it falls between the ends of the protection diode, has the potential of being reverse biased, similar to a flat cell in a multi cell battery, and, as such, has a chance of being damaged... Enter a second fully lit identical panel in parallel with the partially shaded panel. Now, the overall voltage of the pos and neg buss that the panels are connected to, has a greater pd than the panel that has a single cell shaded, the partly shaded panel is now forward biased, and, as we know, cells act like diodes in the absence of light... The forward bias of the partly shaded panel is well within the breakdown limits as the 2 panels are identical... If it were a case that this forward bias were excessive, the  panel would certainly be damaged in short order if left exposed to light in an open circuit condition.... Certainly, should a full string of cells under full output be wired to a single cell in parallel, that single cell would depart this mortal coil in a puff of smoke...but this is not the case...

Feel free to correct me if my thinking is wrong...

Back to my original question...The answer that I can glean from all these fantastic replies is that a partially shaded panel will not influence the output of an unshaded panel in parallel with it.

Cheers

E

Edited by Joe Moer
spelling mistake
Link to comment
Share on other sites

4 minutes ago, Joe Moer said:

The forward bias of the partly shaded panel is well within the breakdown limits

Actually not. The breakdown voltage is at least an order higher. And the bypass diodes usually protect cells in groups of four, so you're well protected against this issue.

Link to comment
Share on other sites

9 hours ago, Joe Moer said:

Back to my original question...The answer that I can glean from all these fantastic replies is that a partially shaded panel will not influence the output of an unshaded panel in parallel with it.

..... and influence all the panels that are in series with it. Ah you say "These installers are nuts - I'm going to put all my panels in parallel". Now you potentially have an array that produces hundreds of Amps at 30V - the cabling and fusing is going to kill the project with higher costs. Easier just to chop the tree down or locate the panels where there is minimal shading.

Link to comment
Share on other sites

No Chris, I have 4 strings of 3 panels each in the array, 6mm cables running to a combiner, with a 5th set of cables pulled in for expansion. Voltages, currents and losses are within spec, shading losses will be taken care of soon... I am pulling around 4kw max from the panels when tested.... Reason for asking question....A  installer in this area insisted that, even in parallel, the slightly shaded string would load the buss.... I just wanted to confirm that I wasnt too far off my head...

As to relocating panels or similar, out of the question as there is the security risk that rears its head...

Cheers..E

Link to comment
Share on other sites

  • 4 weeks later...
1 hour ago, EdDee said:

You must share details, the dashboard looks good.

Link to comment
Share on other sites

1 minute ago, EdDee said:

You can get all the software stuff to do the install from here:

http://www.solpiplog.tk/

You can get the Pi from here:

https://www.pishop.co.za/store/

Then have fun setting it up and making up your dashboards...

http://www.digipoint.co.za:85/emoncms/dashboard/view?id=13

Are you "playing"a round with it now, because I just noticed your PV drop to ZERO and your AC IN jump to over 3000VA

Link to comment
Share on other sites

  • 2 weeks later...

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...