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

[thm_w]

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.
...
The only energy that will be exported from the panel will be that dissipated in wiring that is thermally distant from the panel.

So it does address the question then, as you've deduced, it just does not tell you how much the difference will be.
If you wanted to, you could go up and measure the temperature of the panel and compare various states.

covered (with appropriate material) > connected to load > shorted > open circuit

[nctnico]

Again, this question is really simple to answer by applying some common sense and logic. Are solar panel manufacturers recommending to short solar panels when not in use? I have not been able to find any that say the panels should be shorted when not connected to an inverter. Secondly, are solar panels / arrays of solar panels shorted while an installation is not in use? I have not been able to find any recommendation to do so. Even though in a larger install (say hundreds of panels) it is likely solar panels are fitted long before they are hooked up to the inverters. So where does that leave manufacturers of solar panels to optimise their panels for in case there is no load connected? Short or open? The obvious answer is 'open' because in the end that involves the least amount of work.

And there is another electrical reason to keep panels open as well. When shorted: In case of partial shading you'll push the highest current possible through the bypass diodes which prevent cells from becoming reverse biased.

[bdunham7]

And there is another electrical reason to keep panels open as well. When shorted: In case of partial shading you'll push the highest current possible through the bypass diodes which prevent cells from becoming reverse biased.

While true, I'd point out that for most of the issues raised in this discussion, the important but seemingly not mentioned issue is that in normal optimal operation (MPPT) the current is typically fairly close to the short circuit current.  So then you have to ask, in the case of a short circuit of a fully insolated panel or perhaps a single cell shorted with a near zero resistance (say a 20mm silver bar) where does that energy go?

[thm_w]

We've already established that common sense logic was incorrect.

Any solar panel manual will state for storage to keep in a clean and dry environment away from sunlight. Manufacturers expect the panel to be in storage, being installed, or to be in place and in use. With the "being installed" part lasting maybe a number of days at most. Installers will often cover the panels as well, although that is more for electrical safety reasons AFAIK.

https://www.canadiansolar.com/wp-content/uploads/2020/08/Installation_Manual_of_Standard_Solar_Modules_en.pdf
https://static.trinasolar.com/sites/default/files/UM_M_0002_D_TrinaSolarVertexSeriesUserManual_EN_DEG21C.20_2021A_EN_20210512.pdf

[bdunham7]

So it does address the question then, as you've deduced, it just does not tell you how much the difference will be.
If you wanted to, you could go up and measure the temperature of the panel and compare various states.

covered (with appropriate material) > connected to load > shorted > open circuit

I think that if you were shorting the array using the same wiring that you would normally use in service, then the voltage across the panels and cells will be really, really low.  Not zero, but low enough that there might be secondary effects (such as variations in cell performance, hotspots, etc) sufficient to make it not clear whether open or shorted was the better choice given only those two options. 

If you can accept some guesstimates for numbers (better ones with citations welcome), lets say you have a solar panel of 1.5m², a solar incidence of 1kW/m², an emissivity of 0.78 (which I'm presuming will not change for open, loaded or shorted states--again if that's wrong please show me), an ambient temperature of 40C and an maximum output (at these conditions) of 30V @ 10A for 300W with a cell temp of 80C.  Let's say the wiring from the panel back to the load (perhaps a microinverter) is 0.2R.

The total energy incident on the panel is 1.5kW, the amount absorbed is 1170W.  If the panel is open circuit, it as to thermally dissipate 1170W.  If it is loaded to 300W output, then it only has to dissipate 870W.  If it is shorted, there are 20W dissipated in the wires (10A * 0.2R = 2V, 2V * 10A...) and the panel has to dissipate 1150W.  So there is only a miniscule reduction in the power dissipated in the shorted configuration and then you have to worry about hotspots and whatever else I haven't thought of. 

I'll leave it up to someone else go estimate how much hotter a panel that runs at 80C (40C over ambient) when it is dissipating 870W will get when dissipating 1170W.

[nctnico]

We've already established that common sense logic was incorrect.

Any solar panel manual will state for storage to keep in a clean and dry environment away from sunlight. Manufacturers expect the panel to be in storage, being installed, or to be in place and in use. With the "being installed" part lasting maybe a number of days at most. Installers will often cover the panels as well, although that is more for electrical safety reasons AFAIK.

https://www.canadiansolar.com/wp-content/uploads/2020/08/Installation_Manual_of_Standard_Solar_Modules_en.pdf
https://static.trinasolar.com/sites/default/files/UM_M_0002_D_TrinaSolarVertexSeriesUserManual_EN_DEG21C.20_2021A_EN_20210512.pdf

Common sense logic is correct by definition. Nowhere in your documents it says panels should be shorted. And it is only logical manufacturers recommend to store panels in a dark place because they degrade in sunlight (which could eat into the warranty period which typically starts from the moment of installation).

So please do everyone a favour and come up with a document from a solar panel manufacturer that recommends shorting solar panels when not connected to an inverter.

The total energy incident on the panel is 1.5kW, the amount absorbed is 1170W.  If the panel is open circuit, it as to thermally dissipate 1170W.  If it is loaded to 300W output, then it only has to dissipate 870W.  If it is shorted, there are 20W dissipated in the wires (10A * 0.2R = 2V, 2V * 10A...) and the panel has to dissipate 1150W.  So there is only a miniscule reduction in the power dissipated in the shorted configuration and then you have to worry about hotspots and whatever else I haven't thought of. 

Where this goes wrong is that solar panels don't convert infrared spectrum (which transfers heat) but mostly the visible light spectrum (400nm to 700nm). So the panels don't transfer any of the heat they receive from the sun into electricity. And hence, there is no cooling effect by loading the panels.

[Someone]

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.

If you are concerned that the cabling won't safely carry the short circuit current continuously then you've got a seriously dangerous setup....   the IV curve is very flat and those currents are "normal"

[thm_w]

Common sense logic is correct by definition. Nowhere in your documents it says panels should be shorted. And it is only logical manufacturers recommend to store panels in a dark place because they degrade in sunlight (which could eat into the warranty period which typically starts from the moment of installation).

So please do everyone a favour and come up with a document from a solar panel manufacturer that recommends shorting solar panels when not connected to an inverter.

Why would they have instructions for when someone stores the panels in sunlight, which I've already pointed out they say to not do?

[Someone]

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.

DIY/mixed installations are not legal in all regions, so there isn't always an easy way to utilise excess solar panels. Note that permanently connected inverters have situations of curtailment/export limit, if shorting panels increased their lifespan then I'd expected an inverter to feature this mode during those zero load conditions.

[thm_w]

DIY/mixed installations are not legal in all regions, so there isn't always an easy way to utilise excess solar panels. Note that permanently connected inverters have situations of curtailment/export limit, if shorting panels increased their lifespan then I'd expected an inverter to feature this mode during those zero load conditions.

This would cost more for the inverter to implement. And as discussed above its not going to make a huge difference compared to a proper load. Especially when probably 95%+ of the energy is dumped into the grid in most scenarios.

Diversion/dump loads on battery systems are a thing though: https://thesolarstore.com/diversion-load-controllers-c-40_44.html

[Someone]

DIY/mixed installations are not legal in all regions, so there isn't always an easy way to utilise excess solar panels. Note that permanently connected inverters have situations of curtailment/export limit, if shorting panels increased their lifespan then I'd expected an inverter to feature this mode during those zero load conditions.

This would cost more for the inverter to implement. And as discussed above its not going to make a huge difference compared to a proper load. Especially when probably 95%+ of the energy is dumped into the grid in most scenarios.

Diversion/dump loads on battery systems are a thing though: https://thesolarstore.com/diversion-load-controllers-c-40_44.html

I framed that carefully as the no load situation. It's not uncommon for people to oversize the panels relative to the inverter or have curtailment. Going further and having some excess panels mounted but not grid/load connected isn't wildly unheard of.

[Zucca]

While true, I'd point out that for most of the issues raised in this discussion, the important but seemingly not mentioned issue is that in normal optimal operation (MPPT) the current is typically fairly close to the short circuit current.  So then you have to ask, in the case of a short circuit of a fully insolated panel or perhaps a single cell shorted with a near zero resistance (say a 20mm silver bar) where does that energy go?

This is so true, this is why I still believe that a short circuit condition is less harming than an open circuit one.

Common sense logic is correct by definition. Nowhere in your documents it says panels should be shorted. And it is only logical manufacturers recommend to store panels in a dark place because they degrade in sunlight (which could eat into the warranty period which typically starts from the moment of installation).

So please do everyone a favour and come up with a document from a solar panel manufacturer that recommends shorting solar panels when not connected to an inverter.

My common sense tells me that there is no, and will never be, a document from the solar panel manufacturer telling to short the panels. Unused panels on the roof it is just such a crazy situation that it should not be considered in the specs.
Also wrong common sense logic is wrong by definition.... we are running in circles here.

Also I do not agree with the theory that as soon the sun hits the panels there is some energy that needs to go somewhere. This is wrong IMHO.
According to my investigation the cell physics only tells that if there is sun radiation, the panels generate a current proportional with the sun radiation intensity.
The energy factor starts to play a role when we force a voltage on the panel output.

Two symmetrical theoretical examples:
A) A VDC power supply keeps the output voltage constant. When open there is no power generated, even if there is voltage.
B) A solar panels exposed in the a constant sun radiation keeps the current constant. When shorted there is no power generated, even if there is current.

Here comes my bold statement: in a exposed open circuit panel, the current is forced to run internally creating the VOC at the terminals... and this is worst than letting the current flowing outside the panels in the shorted condition. 

Me silly thinking it was a quick question....

[floobydust]

Counter intuitive but PV modules run hotter open-circuit, apparently. I remember IR thermal imaging showing they run cooler into a short-circuit.
https://www.sciencedirect.com/science/article/pii/S2211379716301280
"These results are in agreement with the already reported results in reference [9] that has shown that amorphous silicon (a-Si: H) solar cells operating under open circuit conditions can degrade more when compared to similar cells operating under maximum power conditions after 13 days of field exposure."

[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.

My bad they are 10AWG from the solar panels strings.
Quick disconnect are QO2L30S, rated 30A.

It looks like 10AWG is also rated for 30A which makes sense.

As stated before my current short circuit is 11A and change at STC.

So in most of the cases I will have an current of short circuit of <12A far away from the max 30A rated.

This weekend I will short the panels strings at night after the quick disconnect and monitor the situation during the day.

[bdunham7]

Where this goes wrong is that solar panels don't convert infrared spectrum (which transfers heat) but mostly the visible light spectrum (400nm to 700nm). So the panels don't transfer any of the heat they receive from the sun into electricity. And hence, there is no cooling effect by loading the panels.

The amount of power that comes from near infrared is non-negligible, but that's irrelevant in any case.  The total incidence of 1kW/m² includes the entire spectrum, visible/IR/UV and FM radio if you like.  All of that is available to heat the panel--there's no distinction between the amount of heat you get from 1kW of infrared and 1kW of green light (provided the emissivity is the same for both, which should not be taken for granted).  If 300W of power is taken out of the equation, that's 300W less that is available for heating.  So if I have a 10-panel array out there cooking in the sun and connected only to my 3kW kettle, when the kettle is off all of the energy incident on the panels and not reflected is absorbed as heat, since there is no other option available.  As soon as I flip on my kettle, there's 3kW of power that has to come from somewhere, and the only possible place is from the absorbed portion of the energy incident on the panels.  What the internal process is within the solar cells for absorbing that energy is not a question you need to answer to know that outcome.

[bdunham7]

Counter intuitive but PV modules run hotter open-circuit, apparently. I remember IR thermal imaging showing they run cooler into a short-circuit.
https://www.sciencedirect.com/science/article/pii/S2211379716301280
"These results are in agreement with the already reported results in reference [9] that has shown that amorphous silicon (a-Si: H) solar cells operating under open circuit conditions can degrade more when compared to similar cells operating under maximum power conditions after 13 days of field exposure."

Into a short circuit or into maximum power?  I haven't seen anything regarding a short circuit and I'd repeat my question "where does the energy go?"

[Zucca]

I could not resist...

https://diysolarforum.com/threads/quick-one-unused-solar-panel-on-the-roof-better-short-or-open-circuit.68310/

[Zucca]

I haven't seen anything regarding a short circuit and I'd repeat my question "where does the energy go?"

Energy generated = Energy used

At short circuit the energy used is the heat generated by the current flowing into the cables due to the cable resistance.
The panel is simply spitting out the current and having a small output voltage.
In this condition the panel is hardly generating any power.

 

[bdunham7]

In this condition the panel is hardly generating any power.

Yes, the 'generated' power is the output.  If the input power (solar incidence * E) is the same in each case, then if there is little or no output power the rest is converted to heat.  Since the output power of an open panel is zero and the output power of a shorted panel is very small, then the result is that the amount converted to heat is almost the same.  Unless there is something else that could happen to that power, of course, but I don't know what it would be.

[Zucca]

If the input power (solar incidence * E) is the same in each case

This is where I tend to disagree. Let's keep the solar incidence/radiation constant.
The efficiency E (if I understood you well) only tells the max power the panels COULD generate in best conditions.

In other words in my mind there is no a constant power that needs to go somewhere, but there is a constant CURRENT that needs to go somewhere.

[fourfathom]

I haven't seen anything regarding a short circuit and I'd repeat my question "where does the energy go?"

Energy generated = Energy used

At short circuit the energy used is the heat generated by the current flowing into the cables due to the cable resistance.
The panel is simply spitting out the current and having a small output voltage.
In this condition the panel is hardly generating any power.

Let's put a fat near-zero-Ohm jumper across the panel connections.  (In full sun) the I_SC short-circuit current will be flowing through that jumper.  Zero Ohms, so zero power there.

But current is flowing, and something has to push that current.  This is the photovoltaic diode, which in effect converts photons to electrons (I'm dodgy on the actual physics, but that's probably close enough for now).  The photodiode has an intrinsic series resistance, and *that's* where the power is dissipated, not the cables (unless you have very lossy cables.)

Looking at a panels rated V_OS and I_SC, and the V/I curve, you can figure out pretty closely what the diode characteristics are.  The published simulation models also show these effects.

[Kleinstein]

Counter intuitive but PV modules run hotter open-circuit, apparently. I remember IR thermal imaging showing they run cooler into a short-circuit.
https://www.sciencedirect.com/science/article/pii/S2211379716301280
"These results are in agreement with the already reported results in reference [9] that has shown that amorphous silicon (a-Si: H) solar cells operating under open circuit conditions can degrade more when compared to similar cells operating under maximum power conditions after 13 days of field exposure."

The panels may emit mre IR radiation when open circuit than under closed circuit conditions, but this does not mean they are actually hotter. More to the contray more IR emitted means lower temperature.
Still the difference should be small.
The idea of using an IR camera to check the panels is from the voltage having an effect on the IR emissions. So one can see cells that are significant different from others.

Amorphous Si cells are a thing of the past and may indeed prefer a short.

For normal Si cells there should be very little (if any at all) advantage from a short - so why to the effort and take the small risk from overheating of a poorly made temporay contact.

[bdunham7]

The efficiency E (if I understood you well) only tells the max power the panels COULD generate in best conditions.

No, by 'E' I mean emissivity as I referred to earlier.  I'm assuming (without justifying that assumption) that the solar panel has an overall E of 0.78 so that 78% of the incident energy is absorbed by it (one way or another) and 22% is reflected.  That means that in my example there is 1170W of input power that has to be accounted for.

[thm_w]

I framed that carefully as the no load situation. It's not uncommon for people to oversize the panels relative to the inverter or have curtailment.

It happens sure but its not common. Again, probably 95%+ of the time the energy from the panels is being used, then there is a small section of time it is not (like Dave with his undersized microinverters). This is different from 100% of the time the panel sitting unused, which is what OP is talking about.

Going further and having some excess panels mounted but not grid/load connected isn't wildly unheard of.

I'm completely sure it happens but I've never seen it. Why spend money on a panel that's doing nothing.

[gnuarm]

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...

It's not "obvious".  It makes no difference.  If you leave the solar cell disconnected no current flows and the voltage is the open circuit voltage.  If you short the solar cell output, you have no voltage and the current is the shorted output current.  Neither one does any damage to the solar cells. 

I would leave the leads disconnected and as someone mentioned, protect the connectors.