Quick one: unused solar panel on the roof, better short or open circuit?

[Zucca]

Hi,

my life is a mess. I just installed solar panels on the roof but the rest of the system is still work in progress....
It's a long story...

According to what I know solar panels act like current pumps, so it would be better for them to short + and - together to let the current flow....
otherwise the current is flowing to the internal resistance...

Now, to minimize performance degradation I would assume a solar panel is happier with the output shorted rather than open...

If you know more than me about solar panels please leave a comment below, I would like to know if I am missing something.

Should I short the + - strings coming down the roof if I do not use the solar panels on the roof (yet)?

thanks in advance!

[nctnico]

Open (ofcourse!)

[Jeroen3]

I don't know of the degradation mechanism open vs shorted.
But I do know eventual unshorting can be a problem with big sparks. Unless done at night.

Better to terminate them into the isolator with the isolator opened.

[rteodor]

I've seen in some solar park a small part of the panels covered with black material. They kept it there for ~2 years.

I had mine for an entire summer (March till October) open and uncovered. With cloud lensing I got at some point 15% more power than their Wp maximum. Spec says 2% loss of performance in the first year and 1% for the rest. Does not mention the loading conditions.

My guess is it will not make much difference but out of caution better to let them open (and cover them if possible and those few percent of performance cut matter in your case).

One thing I know for sure: protect the connectors. Mine started to corrode.

[Zucca]

Thanks for all the replies, I designed my system to have about 10-20% more than what I need, so I am not interested to save a few percents of efficiency.

Open (ofcourse!)

May I ask why if it is so obvious? Just curious...

[DavidAlfa]

Solar panels slowly degrade, so leave it open and put a cover to preserve it.

May I ask why if it is so obvious? Just curious...

Isn't it obvious? A short circuit would dissipate all the generated power in the panel itself, accelerating aging even more.

[zilp]

Isn't it obvious? A short circuit would dissipate all the generated power in the panel itself, accelerating aging even more.

And when you leave it open, the sun stops shining? Like ... what do you think where the energy goes with the circuit open?

[DavidAlfa]

Not good either because unloaded panels can reach high voltages, will also degrade them faster.
That's why the best option is to cover them so they're not exposed to sunlight.

Removing all the technical argon, this gives a pretty good idea:
https://www.sciencedirect.com/science/article/pii/S2211379716301280

[fourtytwo42]

Not good either because unloaded panels can reach high voltages, will also degrade them faster.
That's why the best option is to cover them so they're not exposed to sunlight.

Removing all the technical argon, this gives a pretty good idea:
https://www.sciencedirect.com/science/article/pii/S2211379716301280

They can reach RATED VOC and that's it, no higher. No damage occurs to panels operating at VOC.

[f4eru]

Open is probably thermally the worst case.
Short is a little bit better, a part of the electrical energy gets dissipated into the wires.
Best is to put a load on them (light bulb or so), so they run really cooler at peak power.

[Siwastaja]

Open is probably thermally the worst case.
Short is a little bit better, a part of the electrical energy gets dissipated into the wires.
Best is to put a load on them (light bulb or so), so they run really cooler at peak power.

And here we have the right answer. It's surprising to see how people think their first instinct must be "obviously" correct even when they have absolutely no idea about the actual physics involved.

[Zucca]

Open is probably thermally the worst case.
Short is a little bit better, a part of the electrical energy gets dissipated into the wires.
Best is to put a load on them (light bulb or so), so they run really cooler at peak power.

I still think not all the sun energy collected MUST go somewhere. I mean the solar panel current MUST go somewhere NOT the energy (which depends by the voltage)

After some cog spinning actions in my brain I can definitely say short circuit is the best for the panels, but worst for the cables.

A solar panel is a device that converts the sun rays into a current. It is basically a current pump....
This is why a solar panel in short circuit is perfectly happy because it can push the current somewhere.

Symmetrical to your power supply on your bench, in open circuit it is happy because he can keep the voltage on the output terminals.

More details in the immense
https://www.pveducation.org/
Fun fact, the short circuit current is proportional with the sun intensity... and it makes sense!

FUU I need to finish up my system ASAP!

PS: I scrap all my light bulbs.... I have only led. A bucket of water with a thin wire in it seems too much McGyver 

[nctnico]

Open is probably thermally the worst case.
Short is a little bit better, a part of the electrical energy gets dissipated into the wires.
Best is to put a load on them (light bulb or so), so they run really cooler at peak power.

I still think not all the sun energy collected MUST go somewhere. I mean the solar panel current MUST go somewhere NOT the energy (which depends by the voltage)

After some cog spinning actions in my brain I can definitely say short circuit is the best for the panels, but worst for the cables.

A solar panel is a device that converts the sun rays into a current. It is basically a current pump....
This is why a solar panel in short circuit is perfectly happy because it can push the current somewhere.

Wanting to short a solar panel is idiotic. A solar panel consists of cells in series and you'll be pushing full short circuit current through each cell which kicks the crap out of the weakest cells and degrading them quicker. You'll also be dissipating energy inside the solar panel heating it even further while it is already baking in the sun. More degradation.

Just leave it open and cover the panel if possible.

[gf]

You'll also be dissipating energy inside the solar panel heating it even further while it is already baking in the sun.

Why should a shorted panel heat up more than an open one?
In both cases there is no energy supply besides the sun, which heats them up both.
Where should the energy come from to heat it up even further?

Only when you withdraw (electrical) energy from the panel, you cool it down a little bit.
But in order to withdraw energy, I*V must be > 0, i.e. neither open nor shorted.

[thm_w]

Instead of arguing just read the link DavidAlfa posted, it has actual data instead of assumptions.

Not good either because unloaded panels can reach high voltages, will also degrade them faster.
That's why the best option is to cover them so they're not exposed to sunlight.

Removing all the technical argon, this gives a pretty good idea:
https://www.sciencedirect.com/science/article/pii/S2211379716301280

If OP can cover them, its going to be the best end result, but may not be an option, or too much effort, etc.

[fourfathom]

Wanting to short a solar panel is idiotic. A solar panel consists of cells in series and you'll be pushing full short circuit current through each cell which kicks the crap out of the weakest cells and degrading them quicker. You'll also be dissipating energy inside the solar panel heating it even further while it is already baking in the sun. More degradation.

The current still flows in an open-circuit (unloaded) solar cell, it just flows through the same intrinsic solar cell diode that establishes the V_OC. 

[nctnico]

You'll also be dissipating energy inside the solar panel heating it even further while it is already baking in the sun.

Why should a shorted panel heat up more than an open one?
In both cases there is no energy supply besides the sun, which heats them up both.
Where should the energy come from to heat it up even further?

Only when you withdraw (electrical) energy from the panel, you cool it down a little bit.
But in order to withdraw energy, I*V must be > 0, i.e. neither open nor shorted.

You are assuming the voltage across the cells inside the panel is zero but this won't be the case. Unless you have super conducting solar cells.

[Zucca]

You are assuming the voltage across the cells inside the panel is zero but this won't be the case. Unless you have super conducting solar cells.

this is the IV curve of a solar panel
https://www.pveducation.org/pvcdrom/solar-cell-operation/iv-curve


tell me at what output voltage the panel will work when the output are shorted....

[Zucca]

Instead of arguing just read the link DavidAlfa posted, it has actual data instead of assumptions.

Of course covered is the best, but I can't as you guessed.
My question was between shorted or open.

the link of DavidAlfa compared load vs open condition, and it does not take account about a shorted condition.
Why they did not put a third set of panels shorted in the comparison is beyond my comprehension.

[Zucca]

Anyway my panels have a short circuit current of 11.41A at STC.
the cable are 8AWG MTW TWN75.... so I think they will get hot but do not burn my home down.

I am seriously considering to short my solar strings up.

[nctnico]

You are assuming the voltage across the cells inside the panel is zero but this won't be the case. Unless you have super conducting solar cells.

this is the IV curve of a solar panel
https://www.pveducation.org/pvcdrom/solar-cell-operation/iv-curve


tell me at what output voltage the panel will work when the output are shorted....

Again: ask yourself what the internal voltages are at short circuit current and whether the current is distributed evenly with the panel operating outside it's intended use case scenario. You don't know. If you Google around a bit then you'll see that various people recommend to NOT short PV panels for a prolonged period of time.

Either way, if you put a solar panel out in the sun, it will degrade so covering it is the best option. Then again, the panels will be on your roof for decades out in the sun so likely it is not worth the work compared to having the panels sitting uncovered for a couple of weeks.

Also, the connectors for solar panels are not made to be connected / disconnected under load. So you'd have to make sure it is absolutely dark before connecting / disconnecting the strings to prevent damage to the connectors in case you'd short the panels. From my own solar panels I can see there is always a bit of residual voltage during the night. That would be another very good reason not to short the panels.

[Siwastaja]

You don't have to understand the internals of solar panels, although that is helpful because it's nice when things agree.

Basic understanding of conservation of energy is enough. Clearly shorting/opening solar panels do not affect the emissivity of the panels much (this would be visible to eye), so the heating of the panel has to be similar in both cases (open and shorted), where no external work is done. Only when significant output power is generated into external load, only then the panel can run cooler, because energy is extracted from the system.

Now if you understand how the panel works, and the equivalent circuit of the diode turning on, clamping the cell voltage to certain open-circuit voltage and internally dissipating generated power, you can see these two mental approaches agree, which is nice.

The only thing which does not agree with this is nctnico's intuition, but this is no news, we can see every day this is something not to be trusted.

For the OP, the best solution, by far, is to install an inverter and start putting the investment into use. Second best solution is to try to use the generated power to heat water with a resistive heater or do something mildly useful like that.

[CatalinaWOW]

Not good either because unloaded panels can reach high voltages, will also degrade them faster.
That's why the best option is to cover them so they're not exposed to sunlight.

Removing all the technical argon, this gives a pretty good idea:
https://www.sciencedirect.com/science/article/pii/S2211379716301280

They can reach RATED VOC and that's it, no higher. No damage occurs to panels operating at VOC.

This paper doesn't directly address the OPs question.  The paper proposes that heat is the damage mechanism, and that in the open circuit condition more heat is dissipated in the open circuit panel than in the panel fed to a charge controller.  All makes sense.  The 15-18% of the input solar energy that is convert to electric current is sent off to a box distant from the panel.  But the OPs question is about shorting the panel, with no information about how the short will be accomplished.  The only energy that will be exported from the panel will be that dissipated in wiring that is thermally distant from the panel.  It could be almost as much as when connected to a charge controller, or possibly almost nothing.

Prior suggestions to route the panel to a dummy load make more sense.

Covering the panel is a good alternative, but requires attention to detail.  Best practice would be to provide a ventilation gap under the cover, and make the cover reflective.  A black cover lying directly on the panel will result in heating nearly the same as the open circuit condition, and possibly even more.

[MF-jockey]

I don't understand at all how you can have unused solar panels on the roof. There are various small inverters that can be used to feed electricity directly into the home network, or battery charging controls and used car batteries that are available almost free of charge. Fans to cool the solar panels would also be a useful use of the excess energy.

[bdunham7]

But the OPs question is about shorting the panel, with no information about how the short will be accomplished.  The only energy that will be exported from the panel will be that dissipated in wiring that is thermally distant from the panel.  It could be almost as much as when connected to a charge controller, or possibly almost nothing.

Why a short at all?  If this theory that energy taken from the panel reduces the overall heating of the panel and thus extends its life is correct, then wouldn't you want a load that comes as close as possible to maximum power at maximum insolation?

I propose he divide his array into three sections--leave one section open, short another and put a suitable resistor load across the third.  Then when he eventually gets the system operating (and if he has per-panel monitoring) he can then compare how the three groups of panel perform long-term.