Quick one: unused solar panel on the roof, better short or open circuit?  - Page 1
Forum section: electronics / general

author: Zucca
author_rank: Unknown
timestamp: August 28, 2023, 03:26:22 am
content: 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!

author: nctnico
author_rank: Super Contributor
timestamp: August 28, 2023, 08:01:56 am
content: Open (ofcourse!)

author: Jeroen3
author_rank: Super Contributor
timestamp: August 28, 2023, 08:08:25 am
content: 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.

author: rteodor
author_rank: Frequent Contributor
timestamp: August 28, 2023, 11:09:28 am
content: 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.

author: Zucca
author_rank: Unknown
timestamp: August 28, 2023, 01:54:45 pm
content: 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.Quote from: nctnico on August 28, 2023, 08:01:56 amMay I ask why if it is so obvious? Just curious...

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 28, 2023, 02:26:33 pm
content: Solar panels slowly degrade, so leave it open and put a cover to preserve it.Quote from: Zucca on August 28, 2023, 01:54:45 pmIsn't it obvious? A short circuit would dissipate all the generated power in the panel itself, accelerating aging even more.

author: zilp
author_rank: Frequent Contributor
timestamp: August 28, 2023, 02:42:05 pm
content: Quote from: DavidAlfa on August 28, 2023, 02:26:33 pmAnd when you leave it open, the sun stops shining? Like ... what do you think where the energy goes with the circuit open?

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 28, 2023, 03:28:49 pm
content: 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

author: fourtytwo42
author_rank: Super Contributor
timestamp: August 29, 2023, 08:09:16 am
content: Quote from: DavidAlfa on August 28, 2023, 03:28:49 pmThey can reach RATED VOC and that's it, no higher. No damage occurs to panels operating at VOC.

author: f4eru
author_rank: Super Contributor
timestamp: August 29, 2023, 03:57:44 pm
content: 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.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 29, 2023, 04:47:07 pm
content: Quote from: f4eru on August 29, 2023, 03:57:44 pmAnd 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.

author: Zucca
author_rank: Unknown
timestamp: August 29, 2023, 05:46:00 pm
content: Quote from: f4eru on August 29, 2023, 03:57:44 pmI 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 immensehttps://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

author: nctnico
author_rank: Super Contributor
timestamp: August 29, 2023, 06:05:15 pm
content: Quote from: Zucca on August 29, 2023, 05:46:00 pmWanting 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.

author: gf
author_rank: Super Contributor
timestamp: August 29, 2023, 09:42:05 pm
content: Quote from: nctnico on August 29, 2023, 06:05:15 pmWhy 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 upeven 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.

author: thm_w
author_rank: Super Contributor
timestamp: August 29, 2023, 10:40:26 pm
content: Instead of arguing just read the link DavidAlfa posted, it has actual data instead of assumptions.Quote from: DavidAlfa on August 28, 2023, 03:28:49 pmIf OP can cover them, its going to be the best end result, but may not be an option, or too much effort, etc.

author: fourfathom
author_rank: Super Contributor
timestamp: August 29, 2023, 11:02:31 pm
content: Quote from: nctnico on August 29, 2023, 06:05:15 pmThe current still flows in an open-circuit (unloaded) solar cell, it just flows through the same intrinsic solar cell diode that establishes the VOC.

author: nctnico
author_rank: Super Contributor
timestamp: August 29, 2023, 11:07:53 pm
content: Quote from: gf on August 29, 2023, 09:42:05 pmYou 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.

author: Zucca
author_rank: Unknown
timestamp: August 30, 2023, 01:25:27 am
content: Quote from: nctnico on August 29, 2023, 11:07:53 pmthis is the IV curve of a solar panelhttps://www.pveducation.org/pvcdrom/solar-cell-operation/iv-curvetell me at what output voltage the panel will work when the output are shorted....

author: Zucca
author_rank: Unknown
timestamp: August 30, 2023, 01:32:40 am
content: Quote from: thm_w on August 29, 2023, 10:40:26 pmOf 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.

author: Zucca
author_rank: Unknown
timestamp: August 30, 2023, 02:11:53 am
content: 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.

author: nctnico
author_rank: Super Contributor
timestamp: August 30, 2023, 09:08:26 am
content: Quote from: Zucca on August 30, 2023, 01:25:27 amAgain: 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.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 30, 2023, 06:22:05 pm
content: 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.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: August 30, 2023, 06:58:18 pm
content: Quote from: fourtytwo42 on August 29, 2023, 08:09:16 amThis 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.

author: MF-jockey
author_rank: Regular Contributor
timestamp: August 30, 2023, 09:53:38 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: August 30, 2023, 10:45:04 pm
content: Quote from: CatalinaWOW on August 30, 2023, 06:58:18 pmWhy 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.

author: thm_w
author_rank: Super Contributor
timestamp: August 30, 2023, 10:48:41 pm
content: Quote from: CatalinaWOW on August 30, 2023, 06:58:18 pmSo 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

author: nctnico
author_rank: Super Contributor
timestamp: August 30, 2023, 11:15:39 pm
content: 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.

author: bdunham7
author_rank: Super Contributor
timestamp: August 30, 2023, 11:45:57 pm
content: Quote from: nctnico on August 30, 2023, 11:15:39 pmWhile 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?

author: thm_w
author_rank: Super Contributor
timestamp: August 30, 2023, 11:59:15 pm
content: 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.pdfhttps://static.trinasolar.com/sites/default/files/UM_M_0002_D_TrinaSolarVertexSeriesUserManual_EN_DEG21C.20_2021A_EN_20210512.pdf

author: bdunham7
author_rank: Super Contributor
timestamp: August 31, 2023, 12:13:51 am
content: Quote from: thm_w on August 30, 2023, 10:48:41 pmI 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.

author: nctnico
author_rank: Super Contributor
timestamp: August 31, 2023, 12:41:49 am
content: Quote from: thm_w on August 30, 2023, 11:59:15 pmCommon 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.Quote from: bdunham7 on August 31, 2023, 12:13:51 amWhere 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.

author: Someone
author_rank: Super Contributor
timestamp: August 31, 2023, 12:43:19 am
content: Quote from: Zucca on August 30, 2023, 02:11:53 amIf 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"

author: thm_w
author_rank: Super Contributor
timestamp: August 31, 2023, 12:46:45 am
content: Quote from: nctnico on August 31, 2023, 12:41:49 amWhy would they have instructions for when someone stores the panels in sunlight, which I've already pointed out they say to not do?

author: Someone
author_rank: Super Contributor
timestamp: August 31, 2023, 12:48:40 am
content: Quote from: MF-jockey on August 30, 2023, 09:53:38 pmDIY/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.

author: thm_w
author_rank: Super Contributor
timestamp: August 31, 2023, 01:01:00 am
content: Quote from: Someone on August 31, 2023, 12:48:40 amThis 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

author: Someone
author_rank: Super Contributor
timestamp: August 31, 2023, 01:45:52 am
content: Quote from: thm_w on August 31, 2023, 01:01:00 amI 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.

author: Zucca
author_rank: Unknown
timestamp: August 31, 2023, 02:25:23 am
content: Quote from: bdunham7 on August 30, 2023, 11:45:57 pmThis is so true, this is why I still believe that a short circuit condition is less harming than an open circuit one.Quote from: nctnico on August 31, 2023, 12:41:49 amMy 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....

author: floobydust
author_rank: Super Contributor
timestamp: August 31, 2023, 03:14:21 am
content: 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  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."

author: Zucca
author_rank: Unknown
timestamp: August 31, 2023, 03:28:39 am
content: Quote from: Zucca on August 30, 2023, 02:11:53 amMy bad they are 10AWG from the solar panels strings.Quick disconnect areQO2L30S, 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.

author: bdunham7
author_rank: Super Contributor
timestamp: August 31, 2023, 03:34:18 am
content: Quote from: nctnico on August 31, 2023, 12:41:49 amThe amount of power that comes from near infrared is non-negligible, but that's irrelevant in any case. The total incidence of 1kW/m2includes 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.

author: bdunham7
author_rank: Super Contributor
timestamp: August 31, 2023, 03:36:36 am
content: Quote from: floobydust on August 31, 2023, 03:14:21 amInto 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?"

author: Zucca
author_rank: Unknown
timestamp: August 31, 2023, 03:37:25 am
content: I could not resist...https://diysolarforum.com/threads/quick-one-unused-solar-panel-on-the-roof-better-short-or-open-circuit.68310/

author: Zucca
author_rank: Unknown
timestamp: August 31, 2023, 03:49:52 am
content: Quote from: bdunham7 on August 31, 2023, 03:36:36 amEnergy generated = Energy usedAt 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.

author: bdunham7
author_rank: Super Contributor
timestamp: August 31, 2023, 03:59:35 am
content: Quote from: Zucca on August 31, 2023, 03:49:52 amYes, 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.

author: Zucca
author_rank: Unknown
timestamp: August 31, 2023, 04:21:30 am
content: Quote from: bdunham7 on August 31, 2023, 03:59:35 amThis 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.

author: fourfathom
author_rank: Super Contributor
timestamp: August 31, 2023, 05:58:24 am
content: Quote from: Zucca on August 31, 2023, 03:49:52 amLet's put a fat near-zero-Ohm jumper across the panel connections. (In full sun) the ISCshort-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 VOSand ISC, and the V/I curve, you can figure out pretty closely what the diode characteristics are. The published simulation models also show these effects.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 31, 2023, 08:48:14 am
content: Quote from: floobydust on August 31, 2023, 03:14:21 amThe 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.

author: bdunham7
author_rank: Super Contributor
timestamp: August 31, 2023, 01:43:57 pm
content: Quote from: Zucca on August 31, 2023, 04:21:30 amNo, 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.

author: thm_w
author_rank: Super Contributor
timestamp: August 31, 2023, 09:52:40 pm
content: Quote from: Someone on August 31, 2023, 01:45:52 amIt 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.QuoteI'm completely sure it happens but I've never seen it. Why spend money on a panel that's doing nothing.

author: gnuarm
author_rank: Super Contributor
timestamp: August 31, 2023, 10:22:32 pm
content: Quote from: Zucca on August 28, 2023, 01:54:45 pmIt'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.

author: gnuarm
author_rank: Super Contributor
timestamp: August 31, 2023, 10:24:07 pm
content: Quote from: DavidAlfa on August 28, 2023, 02:26:33 pmSolar cells don't work that way.

author: gnuarm
author_rank: Super Contributor
timestamp: August 31, 2023, 10:31:09 pm
content: Quote from: f4eru on August 29, 2023, 03:57:44 pmIn the open circuit case, there is no external current, so no power. There can be leakage current within the solar cell, but that will be rather slight, otherwise it would significantly impact the performance of the solar cell.In a closed circuit case, there is not external voltage, so no external power. There can be internal resistance, which will dissipate internal heat.None of this will be significantly different from what is observed in normal operation.

author: gnuarm
author_rank: Super Contributor
timestamp: August 31, 2023, 10:31:58 pm
content: Quote from: Siwastaja on August 29, 2023, 04:47:07 pmIt's also surprising how people will offer an opinion, without any supporting information.

author: gnuarm
author_rank: Super Contributor
timestamp: August 31, 2023, 10:35:07 pm
content: Quote from: gf on August 29, 2023, 09:42:05 pmWith a shorted output, you will have maximum current, which produces heat in the solar cell from the internal resistance. With an open output, you have no current, but maximum voltage, which will push current through the internal leakage resistance, producing heat in the solar cell.

author: gnuarm
author_rank: Super Contributor
timestamp: August 31, 2023, 10:38:06 pm
content: Quote from: fourfathom on August 29, 2023, 11:02:31 pmEasy enough to find out. Put a solar cell in the sun, with the output open. Let the temperature settle and measure. Connect the outputs and let the temperature and measure. The heat dissipated with impact the temperature. If you can't measure the difference, then it won't matter.

author: gnuarm
author_rank: Super Contributor
timestamp: August 31, 2023, 10:46:46 pm
content: Quote from: nctnico on August 30, 2023, 11:15:39 pmLet me understand. You find no information telling the user to short the panels, so that means the panels should be left open? I guess you can get very interesting results if your searches are not very good.

author: Someone
author_rank: Super Contributor
timestamp: August 31, 2023, 11:16:51 pm
content: Quote from: thm_w on August 31, 2023, 09:52:40 pmIts not that different at all. If there was a life extension to panels under no load conditions by shorting them, and (as you agree) those conditions exist some of the time in the real world, it would make sense that products would be marketed to match that. Adding a shorting mosfet to an inverter is not a significant cost.We're lacking the evidence that shorting panels will increase lifespan. Not lacking is the occurrence of this, or the low cost solutions.

author: Zucca
author_rank: Unknown
timestamp: September 01, 2023, 04:02:08 am
content: Quote from: gnuarm on August 31, 2023, 10:35:07 pmOh yes, I agree 100% with the above. It would be interesting to know which resistance produces more heat... the internal leakage resistance or the internal resistance.Also I would assume that internal leakage resistance > internal (series) resistance, so if the current is the same the internal leakage resistance is burning more heat than the other one...It looks like you knows how solar cell works... can you please confirming me than it is a device that generate a current and not a power when exposed to light?In other words, the power generated in a solar cell is an effect of the current "created" by the light.Many thanks!

author: Zucca
author_rank: Unknown
timestamp: September 01, 2023, 04:06:49 am
content: Quote from: Someone on August 31, 2023, 11:16:51 pmsurprised as well about the lack of studies in this situation, or.... maybe for the experts it is obvious that in short circuit it will be better and they no bother to study it?Only God knows...

author: f4eru
author_rank: Super Contributor
timestamp: September 01, 2023, 06:40:19 am
content: Quote from: Zucca on September 01, 2023, 04:06:49 amHmm. It is scientifically proven that God does not know thermodynamics of PV panels.

author: gf
author_rank: Super Contributor
timestamp: September 01, 2023, 07:27:56 am
content: Quote from: Zucca on September 01, 2023, 04:02:08 amIn the end, does it matter whether the chicken or the egg came first? For a given light incident, you can observe a particular I vs. V characteristic from the outside, and if you connect a load, then the cell delivers a corresponding amount of power to the load.Furthermore, conservation of energy cannot be circumvented. The solar power absorbed by the cell must be equal to the electrical power deliverd to the load + the power disposed by cell to the ambient (via IR radiation and via thermal transmission to the air). Eventually, the cell reaches a temperature at which an equilibrium between incoming and outgoing power is established (the hotter the cell, the more power it can dispose to the ambient).And does it make a difference at the end, whether (say) 15% of the absorbed solar irradiance power heat up the cell directly, or whether the same 15% are converted electricity first and then this electricity is dissipated inside the cell and heats it up? At least from the power balance POV, it does not make a difference.

author: gnuarm
author_rank: Super Contributor
timestamp: September 01, 2023, 07:53:29 am
content: Quote from: Zucca on September 01, 2023, 04:02:08 amIt's not that I "understand" solar cells more than anyone else here. I look at the data (mostly the I-V curve) and accept what it tells me as the truth.I don't remember data ever lying.I don't think the solar cell produces a fixed current or a fixed power from a fixed illumination. Look at the I-V curve, and tell me what is going on.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 01, 2023, 09:05:23 am
content: "macroheating" of the cell, i.e. heating while ignoring spatially small effects like hotspotting, is the easy part, as the constant nature of emissivity regardless of load is easily observed: both open-circuit and short-circuit conditions give roughly the same heating and hence directly heating-related aging. Only connecting a load (in absence of MPPT tracking, even justsomethingequaling roughly the I_mpp at V_mpp, is an improvement) can reduce the temperature of the cell. This is all obvious from basic conservation of energy, so pretty much common sense to everyone except nctnico.Now the macroscopic temperature probably isn't the only aging mechanism of the cell, andwhat elsehappens requires serious understanding of the detailed cell physics apparently no one here has any idea about, me included.It's not a surprise this is not discussed in cell datasheets. It makes little sense to invest money in this expensive stuff and yet more money to install it, only to not use it. And short periods of non-usage would be irrelevant anyway, clearly the cell does not age while open-circuit for weeks or months, so clearly aging does not increase by orders of magnitude, compared to maximum power point load situation. If aging rate increases by 30%, 50% or even 100%, this does not matter to the manufacturer as the panels are not intended to sit unused for years. 1000% increase would be something that needs to be discussed in datasheets/instructions, and would be a problem when inverters in some situations have to cut production e.g. due to voltage or frequency rise, or utility demand control signals.

author: gf
author_rank: Super Contributor
timestamp: September 01, 2023, 09:21:15 am
content: Quote from: gnuarm on September 01, 2023, 07:53:29 amThe commonly used equivalent circuit consists of an ideal (solar irradiance dependent) current source and a diode (and some resistors). It is supposed to model/approximate the observed I vs. V curve. But this is still just a behavioral (i.e. "as if") model. In practice, of course you cannot dismount the current source and the diode from the cell, as separate components.Quote from: Siwastaja on September 01, 2023, 09:05:23 amThat's true, of course. I guess there exists no trivial model for aging.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 01, 2023, 12:02:20 pm
content: Usually the aging of the crystalline silicone solar cells is not very fast. Looking at hot spots and not the overall temperature is a good point: it is enough to have hot spots fail. There are different types of hot spots:1) points with high current flow (e.g. near contacts), that would get hot especially under short circuit.2) points with locally higher leakage current (lower "diode" drop" that would get hot under open circuit conditions as there is more voltage and thus the chance that areas with defects can get hot.Good modern panels should be checked for both types of weak points, e.g. by IR monitoring the cell under both conditions. So I would not worry too much about the hopefully relatively short time with the PV panels stting unused.

author: mikerj
author_rank: Super Contributor
timestamp: September 01, 2023, 12:41:33 pm
content: This paper covers the accelerated degradation of an open circuit panel in a desert, so not necessarily applicable to less harsh environments.https://www.sciencedirect.com/science/article/pii/S2211379716301280

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 01, 2023, 02:11:09 pm
content: This thread demonstrates one of the features of human behavior. There are a variety of approaches to the answer, with the conclusion similar in a substantial majority of the answers. But the OP doesn't like the result and searches the outliers and creates new explanations of why his preferred answer is best.There is much to learn here, both from the varied ways to skin the cat, and from the demonstrated social behavior.

author: thm_w
author_rank: Super Contributor
timestamp: September 01, 2023, 11:54:37 pm
content: Quote from: Someone on August 31, 2023, 11:16:51 pmIt is different: ~5% of the time unloaded vs 100% of the time.It is a significant cost: FET that can handle full shorted current of all panels + additional heatsinking and thermal dissipation required. And again if you want to do it optimally, you need a huge load dump bank.Its like asking why car manufacturers don't all include an AC charger to keep the internal battery maintained when the car sits unused.Quote from: CatalinaWOW on September 01, 2023, 02:11:09 pmWell OP hasn't even told us how long they expect the panels to sit. If its some days or months or years. They were somewhat clear about not wanting to spend effort or money on it, so all of those solutions are out.Its still an interesting discussion to have.

author: gnuarm
author_rank: Super Contributor
timestamp: September 02, 2023, 12:58:13 am
content: Quote from: thm_w on September 01, 2023, 11:54:37 pmPoor example. All EVs include the AC charger in the car. Perhaps you are thinking of the EVSE that provides the connection from the AC outlet to the car's port. Not all cars include that when you buy the car.

author: bdunham7
author_rank: Super Contributor
timestamp: September 02, 2023, 01:27:26 am
content: Quote from: Someone on August 31, 2023, 11:16:51 pmA shorting FET would be at least some cost, and even more relevant an additional potential failure point. If I were the product manager that would be a hard no unless the benefits were well established, well known and in demand by customers. I think if you raised this issue with 99% of installers and 99.9% of solar customers, you'd get a blank stare, so the last two are out.

author: wraper
author_rank: Unknown
timestamp: September 02, 2023, 02:00:12 am
content: Quote from: gf on September 01, 2023, 07:27:56 amI'm not convinced that when no current is drawn 100% of the light that would otherwise provide electric power actually gets absorbed by the panel and converted into heat rather than reflected. Also I don't think something like "15% are converted electricity first and then this electricity is dissipated inside the cell and heats it up?" will happen with no actual current flow. It either should be converted into electric energy and resistive losses if current flows or directly into heat if no current is drawn with possible change in amount of light reflected. It needs some actual data rather than guesses.IMHO the worst that can happen with open circuit is no change on how much non loaded panel heats up compared with short circuited. I'm surprised about the idea that solar panel somehow can internally convert electric energy into heat with no path for current flow. There will be electric potential but no path for current flow other than small leakage current. Short circuit does not take away the energy from the panel, all dissipation still happens within the panel. The only way to take the energy away from the panel is providing normal load that will dump the heat somewhere else.

author: Someone
author_rank: Super Contributor
timestamp: September 02, 2023, 02:16:31 am
content: Quote from: bdunham7 on September 02, 2023, 01:27:26 amWhich is exactly what I said, there is no well known benefit for this, customers aren't demanding it, neither are manufacturers pushing it.IFshorting during no load was beneficial we'd expect to see it implemented. Therefore it is pretty safe to conclude that there is likely no benefit, even without some solid evidence to prove that.Cost of implementation is trivial compared to possible gains.

author: bdunham7
author_rank: Super Contributor
timestamp: September 02, 2023, 02:50:38 am
content: Quote from: wraper on September 02, 2023, 02:00:12 amA photon of sufficient energy bops an electron up and across the junction. If the photon has energy above and beyond what is required (which it will for any wavelength of photon shorter than the absolute maximum), the electron loses energy (mostl as heat although there could be re-emission, I suppose) until it settles down to whatever the energy level (voltage) is on the other side of the junction. The remainder is your available electrical energy. Once enough of those electrons get bopped across the junction to raise the voltage on that side to the junction voltage, they flow back down through the junction. Since the junction is a diode, the voltage is about half a volt or so. So there's your continuous current flow. The diode nature of the junction is what limits the open circuit voltage of the solar cell. If you short the cell, then the voltage on the other side of the junction is very low and the electrons that get bopped across that junction lose even more energy--almost all of it--as they settle down to that lower voltage level. Then the very low voltage is just enough to propel them around the shorted external loop, but not back down the junction. Either way (open or short) the photons that interact with (bop) electrons end up seeing almost all of that energy dissipated in the cell.I doubt there would be much reason for the emissivity of the panel to change much at the wavelengths sufficient to bop the electrons enough to matter. The panel isn't going to run out of them since if even 1% of available electrons were bopped across the junction at one time you'd probably have thousands or millions of volts, so the supply of available electrons for the photons to interact with isn't likely to go down enough to matter. The current flowing back through the junction at ~0.5V might (I think it does, anyway--I've read about this effect somewhere) cause the panel to emit IR radiation but at a much longer wavelength (0.5eV ~ 2400nM). This would yield a slight cooling effect for the open circuit case.

author: gf
author_rank: Super Contributor
timestamp: September 02, 2023, 07:50:09 am
content: Quote from: bdunham7 on September 02, 2023, 02:50:38 amDou you mean IR radiation like a LED (not just the blackbody radiation due to surface temperature)?

author: nctnico
author_rank: Super Contributor
timestamp: September 02, 2023, 08:57:08 am
content: Quote from: Someone on August 31, 2023, 11:16:51 pmYep. We don't know what solar panel manufacturers do to optimise their products for the actual use case down to the semiconductor physics & chemistry level. Graphs and simplified models won't tell you that. Just like you can't get SOA information from a simplified transistor model. The question from the OP is about how a solar panel ages / deteriorates. The simplified model doesn't answer that so it is not relevant to the question.And you are right about solar panels needing to be able to support / designed for being partially loaded or even unloaded for prolonged periods of time. Inverters and/or the grid may not support solar panels working at full capacity. In the NL there are areas where the grid can't handle the amount of solar energy on a sunny day so the inverters switch off due to the mains voltage becoming too high. That leaves the panels unloaded in what you would call normal operating conditions. And it is not unreasonable to assume panels are oversized for the inverter in a significant number of places. When I look out of my window I see several >4kWp installations and I doubt all of them have an inverter capable of handling the full capacity of the panels. Likely most of these installations are on a 16A (ballpark 3600W) circuit as this is what solar panel installers offer by default.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: September 02, 2023, 09:51:37 am
content: To me this question is rather like asking, should I leave an unused alkaline battery in open or short circuit? The durability of a solar panel is not just a measure of the intrinsic loss of output over time but also the build standard of the panel. No point having high quality mono crystalline silicon when the panel edges and seal fails after five years. Cheap Chinese panels have a cheap Chinese lifespan. Buy German - it lasts.Unless you are building a solar farm where ever joule counts towards the bottom line, arguing over a few amp-hours is a waste of time. Especially if you are designing for capacity redundancy. Let us not forget the inverters too which can burn the spreadsheet numbers when they fail. What is the MTBF of the inverters?Tip: Google "solar farm used solar panels" - the solar farms often off load aging and under performing panels. Also, thanks to green energy grants, panels are financially depreciated to zero over 4 to 5 years. So to make the most of capital allowances, farms upgade to new panels just for the tax-free money. A five year old panel can be a real bargain - ball park, a panel still outputting 200W+ is us$100 re-fitted. So it may pay to use and replace 'preloved' panels for a short period, rather than try squeeze the claimed 25 year lifespan out of full price new ones.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 02, 2023, 06:32:33 pm
content: Quote from: AndyBeez on September 02, 2023, 09:51:37 amBut a battery is a "voltage source", whereas solar panel is a "current source". Especially for laymen, but also for semi-hardened professionals, voltage sources are pretty intuitive but current sources are confusing as heck. Therefore, people understand more easily how capacitors store (and deliver) energy than inductors; they understand how capacitor keeps stored energy by being open, but are confused as hell when they hear that inductors store the energy while being shorted. Because for laymen, "short circuit" means some kind of error condition, dangerous even, which involves heat, sparks, and whatnot. This is true when you short a voltage source. But for a current source, the opposite is true - opening it is dangerous as it generates an infinite voltage, while shorting it means simple that no work is being done.Solar panel of course is not a simple current source. It's a clamped current source so that neither open nor short circuit condition is catastrophic at all.

author: KE5FX
author_rank: Super Contributor
timestamp: September 02, 2023, 06:55:23 pm
content: If a metaphor is called for, an LED might make a good one. An LED under external illumination could be crudely thought of as a current source with compliance limited primarily by the material's band gap, just like a PV panel. Nobody worries about whether LEDs should be shorted when not in use, and it's not clear that any such concerns would necessarily become more pressing as the junction area grows.

author: Zucca
author_rank: Unknown
timestamp: September 03, 2023, 03:31:13 am
content: Thanks guys, I was not expecting 4 pages of discussion.I am also very happy with all the technical/physics explanation. Remarkable.Thanks again to everybody. And yes you changed my mind, I will leave them open, at least there is no heat going into the cables and risk of fire in my laughable wood made home in USA (K I S S).I need a day with 48 hours to finish my solar system.It was not planned to be in a situation like this, I am sure you all don't have time or wanna read for my boring private life details...I pray the eng who designed my solar panels did everything right and they will not degrade too much.Attached the datasheet in case you are interested, I have the 400W variant.

author: bdunham7
author_rank: Super Contributor
timestamp: September 03, 2023, 03:36:42 am
content: Quote from: gf on September 02, 2023, 07:50:09 amYes, sort of like that although less organized. I can't remember where I saw that discussed. If it pops into my head I'll post it.

author: zilp
author_rank: Frequent Contributor
timestamp: September 03, 2023, 11:18:18 am
content: Quote from: wraper on September 02, 2023, 02:00:12 amHave you seen solar panels get brighter when you reduce the electric load?I haven't.

author: gnuarm
author_rank: Super Contributor
timestamp: September 03, 2023, 03:19:30 pm
content: Quote from: zilp on September 03, 2023, 11:18:18 amI don't see IR.

author: zilp
author_rank: Frequent Contributor
timestamp: September 03, 2023, 04:13:35 pm
content: Quote from: gnuarm on September 03, 2023, 03:19:30 pmWhy do you think that solar cells generate electricity primarily from IR?

author: gnuarm
author_rank: Super Contributor
timestamp: September 03, 2023, 04:20:51 pm
content: Quote from: zilp on September 03, 2023, 04:13:35 pmWe must be talking about two different things. I was talking about current creating light. Someone said they couldn't see it and I was explaining why.

author: zilp
author_rank: Frequent Contributor
timestamp: September 03, 2023, 05:14:50 pm
content: Quote from: gnuarm on September 03, 2023, 04:20:51 pmNo, waper suggested that light that under load would be converted to electricity gets reflected under reduced load, and you suggested that the reason why one wouldn't see the increased brightness from the reflection would be because that light would be in the infrared, i.e., that solar cells convert primarily IR to electricity.

author: gnuarm
author_rank: Super Contributor
timestamp: September 03, 2023, 07:20:08 pm
content: Quote from: zilp on September 03, 2023, 05:14:50 pmNo, I never said that.

author: zilp
author_rank: Frequent Contributor
timestamp: September 03, 2023, 07:52:20 pm
content: Quote from: gnuarm on September 03, 2023, 07:20:08 pmSo ... what did you say, then? And no, I don't mean a quote of what you said.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 03, 2023, 11:31:53 pm
content: Quote from: zilp on September 03, 2023, 07:52:20 pmI'll tell you how a person with an engineering or physics background interprets what he said. All objects radiate energy, with a spectrum similar to a black body, but modified slightly by variations in spectral emissivity. So a solar panel radiates energy with a spectrum roughly similar to a 300 to 325 degree Kelvin black body. Such a black body has its strongest emissions in the LWIR band, and very little in the visible band. If you take the derivative with respect to temperature the changes are also strongest in the LWIR band, so in addition to having very little signal in the visible band, the relative change in that signal for the few degree temperature changes is smaller. So if you can't see in the infrared, you can't see how the waste heat in a solar panel is being radiated. Now if you heat your panel up to a cherry red heat or hotter the visible band becomes relatively good for determining temperature. But in that case shorting or opening the leads will make no difference.There is no reason to get into a feud about something that is just the basic way the world works.

author: gnuarm
author_rank: Super Contributor
timestamp: September 04, 2023, 03:05:15 am
content: Quote from: zilp on September 03, 2023, 07:52:20 pmI've written a lot here. How about you reply to the message you are looking for a clarification to.

author: zilp
author_rank: Frequent Contributor
timestamp: September 04, 2023, 05:37:35 am
content: Quote from: CatalinaWOW on September 03, 2023, 11:31:53 pmThat's funny, because in my world, no one with a physics background would ever interpret "reflected" to mean "thermally absorbed and re-emitted as black body radiation", because reflection, by definition, maintains wavelength.Also, it obviously makes exactly zero sense that wraper supposedly intended to say that they weren't convinced that "light that would otherwise provide electric power actually gets absorbed by the panel and converted into heat rather than [absorbed by the panel and converted into heat]", rather than the obvious interpretation of "light that would otherwise provide electric power actually gets absorbed by the panel and converted into heat rather than [redirected to a different propagation angle at the same wavelength]".Quote from: CatalinaWOW on September 03, 2023, 11:31:53 pmExactly my thought.

author: Dacian
author_rank: Contributor
timestamp: September 04, 2023, 06:54:21 pm
content: As bdunham7 already mentioned is a matter of energy conservation.Solar PV cells are basically a very large diode.Voltage drop across that diode (PV cell) is around 0.7V for a cold panel. Divide the open circuit voltage to number of PV cells.Having the panel open or short circuit will make no difference so just leave it open but protect the connectors from water if they are exposed.I actually use PV panels as heating panels to heat my house.Here is a thermal image of a 60 cell PV panel used a heaterThe 60 panels are split in to 3 groups of 20 cells and I shorted the 20 cell group in the middle of the panel to effectively have a 40 cell PV panelThe 40 cells * 0.7V around 28V depending on current it may get close to 0.75V so 30V and that is exactly the max power point of this this 60cell panels.I have two 60 cell panels outside connected in parallel and then those supply the 40 cell PV panel used as heater witch is inside the house.Using a PV panel as heater is a better option than a resistive heating element as it will always work at max power point.In the thermal photos about 17A was flowing trough the heater panel with ambient temperature of +25C and the hottest part of the panel was around +50C so about 25C above ambient.

author: nukie
author_rank: Frequent Contributor
timestamp: August 20, 2024, 04:50:24 am
content: Just put a tarp over it to stop it from generating watts, solves your open or short issues.No tarp? Then get some halogen bulbs and wire them so it bleed off the power.