Will partial shading hurt the solar panels?

Kentill

Hi,
I have read some post on here about shade and diodes but I'm not sure if the shade burns up the diodes or what.
I have a situation where in the mornings I have partial sun on some of my panels until about 11:00 am.
My question is will that partial sun/shade destroy/degrade the panels. My panels are in series. I think but not sure
It poses a different problem if the panels are in series or parallel.
Thanks
Kent

vtmaps

Re: Will partial shading hurt the solar panels?

Kentill wrote: »

Hi,
I have read some post on here about shade and diodes but I'm not sure if the shade burns up the diodes or what.
I have a situation where in the mornings I have partial sun on some of my panels until about 11:00 am.
My question is will that partial sun/shade destroy/degrade the panels. My panels are in series. I think but not sure
It poses a different problem if the panels are in series or parallel.

For most modern crystalline silicon based panels, shading will not hurt the panel... but it does put some stress on the bypass diodes.

Your panel is itself composed of groups of cells that are in a series/parallel configuration. When a cell is shaded it block current flow from the other cells that are in its series string of cells (within the overall series/parallel arrangement of cells in the panel).

The bypass diodes are in parallel with a string of series cells in the panel. When that string is shaded, the current from other strings that are in series with the shaded string can bypass the shaded string through that string's bypass diode.

--vtMaps

DanS26

Re: Will partial shading hurt the solar panels?

I have a morning shade issue due to a line of trees to the east that clears by about 9:30am in the summer.

To mitigate the shading and increase production somewhat I arranged the strings in such a way that the shading does not affect all the strings all the time. When I first built the array I had the strings in parallel lines which meant that the shade moved down each string as the sun rose and exited each string at approximately the same time. Mistake.

I soon arranged the strings in blocks from west to east so that the shade would exit each string individually. Increase in power was seen immediately.

BB.

Re: Will partial shading hurt the solar panels?

I have not tested any solar panels in shade--But I do have concerns that the bypass diodes are used in a less than ideal environment. Electronics (including diodes) will last much longer if kept cool. Placing several diodes in a small plastic+glass box and running 8 amps through 1-3 diodes or so with 0.2 to 1.0 volts or so drop is less than ideal.

Older panels had J-boxes you could open and replace failed diodes. Many newer panels have sealed J-Boxes and the diodes cannot be replaced without performing surgery on the panels.

It would be interesting to take a good sized solar panel (125 watt or larger), put a lab power supply (or another panel outside in the sun) on the panel (covered or in a dark room), and run Imp current through the panel and measure the diode/j-box temperature.

I don't have the panels or supply to do that...

-Bill

Kentill

Re: Will partial shading hurt the solar panels?

Thanks for all the responses, in my setup I use about 1kwh out of my battery bank in 24 hrs. So on a normal sunny day around Noon of shortly after I have replenished the battery bank into absorb. It sounds like most new PV panels have bypass diodes in them. So it's not a big problem for me but I was just wondering if over time shading would harm the panels.

Is it possible to monitor the input voltage on the string to see even if the shading is causing a issue. My cc allows me to monitor the input voltage. What should I look for, loss of panel voltage. Now if I remember correctly even in full sun the string voltage drops from 55-59 to 48-49 in full sun with 3 - 18.5 volt panels.

mike95490

Re: Will partial shading hurt the solar panels?

Shading does not help the panels. When shaded and the bypass diodes kick in, the diodes heat up. There have been times the diodes fail, and need replacing.

inetdog

Re: Will partial shading hurt the solar panels?

mike95490 wrote: »

Shading does not help the panels. When shaded and the bypass diodes kick in, the diodes heat up. There have been times the diodes fail, and need replacing.

And while some diodes are failed and the panels are in partial shade, you do risk damaging the affected panel(s) from localized heating if you leave them hooked up to a load.

Kentill

Re: Will partial shading hurt the solar panels?

So can I assume that if only some panels are in partial shade and others are not, the diode current would be less if the panels were not in full high noon sun? Or is that a bad assumption?

vtmaps

Re: Will partial shading hurt the solar panels?

Kentill wrote: »

So can I assume that if only some panels are in partial shade and others are not, the diode current would be less if the panels were not in full high noon sun? Or is that a bad assumption?

Yes, that's correct. If it's early or late in the day, not as much current can be produced to pass through the bypass diodes.

--vtMaps

BB.

Re: Will partial shading hurt the solar panels?

Kentill wrote: »

So can I assume that if only some panels are in partial shade and others are not, the diode current would be less if the panels were not in full high noon sun? Or is that a bad assumption?

"It depends"...

If you have a single 18 volt Vmp panel and two diodes connected to a 12 volt battery. One diode is shorted, dropping Vmp to ~9 volt which will not drive current to a 12 VDC battery.

Put 5 of the same panels in series (Vmp~90 volts) driving a MPPT charge controller and a 12 VDC battery--Remove 9 volts (shorted diode) and you still have 81 volts Vmp and pretty much the same current.

Now two above panel with open diodes and shade 1/2 of panel. Again, the single panel string--No current flow--No damage (if open diode).

The 5 panel string, open diode across 1-18 cells in string with open diode--You have a potential of ~81 volts across "dark cell"--Diode is reverse biased and gets damaging voltage across one or more cells (more than ~12 volts or so), and Solar Cell is ruined. Even if you replace open diode--Panel with bad cell(s) will never generate full Vmp again.

So--It depends on configuration, shading, and type of diode failure. Some failures (shorted diode) could be repaired with a new diode (assuming you can get into J-Box).

Open diode will probably end up with damaged cell(s) in panel--Non-repairable because you would have to cut out and replace bad solar cells. Difficult, if not impossible, to repair in a cost effective manner--And still have a weather tight seal when repair is complete. Normally just replace with new panel.

If you have 10x 30v Vmp panel strings (common for GT inverter systems)--It may actually be difficult to determine if there is one failed panel (a few cells or shorted diodes). 2 parallel strings and 20 panels... Roughly 1/20th (5%) reduction in output power (I*V curves of solar panels have a relatively "soft peak" and a 10% reduction in Vmp will result in a ~10% reduction in power of one of the two strings)--5% to 10% reduction in power or string current is just about within panel mfg. specs and equipment accuracy.

-Bill

Kentill

Re: Will partial shading hurt the solar panels?

Getting back to post #5, can you tell from the input voltage if you have an issue like shorted or open diode?

BB.

Re: Will partial shading hurt the solar panels?

Are you looking for a "failed" panelinstalled in a working array? Or are you looking to debug and repair a bad diode on the repair bench?

An open diode is not really detectable if there is no shading--The bypass diodes never go into conduction on a well working array. If there is an open diode and shading, you will see the string current drop to near zero amps (if a 18 Vmp panel charging a 12 volt battery) or you will see 1/3 to 100% Vmp drop on a single string of panels with Vmp-array>>Vbatt (or Varray minimum for a GT type inverter). And the failing panel will need to be replaced (not repairable).

Shorted diodes and failed solar cells will reduce output voltage of a panel/string.... Again around 1/3 to 1/2 to 1/1 of Vmp-panel (shorting strings of solar cells--depending on how many diodes and how many of them have failed shorted). That can be repairable if the J-Box cover can be unbolted and the bypass diodes are replaced.

To a degree--I am still confused--You are focusing on a specific failure (bypass diodes). And I am really looking at the Working/Failed Panel level. When I have a system that is "failing", I just need to know that the panel has failed--I don't care why at that point.

If I have a panel that I want to repair, then I can open the J-Box and do a diode check (look for a shorted diode--string of cells). Replace all diodes (if one is bad, other(s) is close behind). Test, button up j-box, and put panel back in service/spares.

Single panel strings are easier to debug... Put current clamp on each string and the string with 1/2 the current vs the other parallel strings is bad (charge controller needs to be drawing near full current from array--If battery bank is in float with no loads--Low to zero current will be difficult to find a bad panel).

Multiple panel strings (5-10+ panels in series)--A current clamp will show bad string... But then need to test each panel in "bad string" individually for Voc, Isc, and, ideally, Vmp/Imp to find bad panel (could also be bad wiring/electrical connections too).

-Bill

Kentill

Re: Will partial shading hurt the solar panels?

Bill
To answer you first question, I don't know if I have a bad panel or diode, I just would like to know what to look for. What are the symptoms and could you tell from looking at the input voltage to the cc. From what you are saying I think you could look at the input voltage during shading and if the voltage is lower than during full sun you might have a problem.
Thanks for the responses from everyone. The information given on this Forum is Super.
Kent