Solar cell outdoor longevity?

[Styno]

Background: For a project I need to power some sensors without access to mains power at least two years without maintenance. The electronics (control, sensor readout, RF and sensor connectors) are housed in a tube and installed outdoors in various unspecified locations. Some of the sensors use quite a bit of power (e.g. inductive conductivity) so I don't want to rely on one or more primary battery cells. So I'm researching a solution whereby solar energy harvesting is used to charge a 18650 li-ion cell with a primary cell for backup during long heavily overcast periods in autumn/winter. The solar cell will be glued to the top of the tube.

I found several ic's that are suitable for the energy harvesting, protection and power OR-ing (e.g. adp5090, bq25505, spv1050, s6ae102a, ltc3105) and some small solar cells/panels on Aliexpress. My question is about those solar panels, e.g. this one.



It is no problem when the primary battery cell needs to be replaced every few years but I would like the solar cell to last (much) longer, preferably for the lifetime of the sensor (10+ years). Does anyone have experience with this kind of solar panel in outdoor usage? How long would these last realistically? I'm specifically worried about moisture creeping between the laminate.

[Kleinstein]

The crystalline silicon cells itself usually last very long - more like many decades and it is more the glue to put it on a glass and seals / cables that are an issue. The round disks from the link don't look that good when it comes to the seals.  If it fits, I would prefer a ready made panel/laminate with protective glass. They are available even for marine use, though the small size might be an issue.

The other possible issue might be algae growing on the surface. So it would help to have a really smooth surface to start with. It may like cleaning every few years too.

[splin]

There are at least 3 main enemies here - UV degradation of the transparent cover, sealing against water ingress and damage due to thermal expansion/contraction cycles including seal failure, delamination and mechanical breakage of cells or interconnections. The panels you linked use PET which are better than the cheapest epoxy panels but I doubt that you'd get any manufacturer to guarantee them for much more than two years (they may well last much longer in practice for many users of course). ETFE is better, but more expensive than PET; glass is the best.

It's not easy to find independant studies on the lifetimes of the various materials but module manufacturers and distributers put out marketing statements which may provide some guidance. The numbers vary wildly though with some claiming 25 year lifetimes for ETFE and others say 5 years. Similarly for PET (2 to 5 years).

The actual operating conditions are going to have a big impact - a panel facing north in a location with lots of cloud cover (eg. northern europe) is going to be exposed to very much lower UV than one facing the sun in sunny climes. The temperature and water exposure conditions are going to be very different as well and thus it is very difficult for a manufacturer to specify how long a panel will last.

I think you will need to look for panels with ETFE or glass topcovers but that isn't sufficient to ensure a long lifetime. It's a difficult problem which will probably boil down to finding a manufacturer you trust - which is unlikely to be a cheap, random sellor on Aliexpress. Good luck and please let us know what you find.

Here is one manufacturer's opinion: https://www.voltaicsystems.com/custom-solar-panels#faq_7

What are the coating options?

All solar cells are protected with some sort of coating material. For commercial solar panels, it is often glass. For smaller panels there are a number of different options.

Urethane: Highly UV resistant, but heavier than ETFE. Our standard panels have a urethane coating and last up to 10 years in the sun with minimal power degradation. The urethane coating is poured or "potted" on the solar cells and leads to a highly waterproof panel. Each individual cell is completely encapsulated by the urethane.

ETFE: Slightly less UV resistance than urethane, but lighter weight. ETFE and EVA are attached to the cells via a lamination process. With high quality materials (there is a difference in the production quality of ETFE and EVA), these panels have an expected life of 5 years outside.

Epoxy or PET: Panels made with these coatings will be inexpensive, but we don't recommend this coating as it tends to have a relatively short lifetime. They are the least UV resistant of any of the options.

Glass: Most UV resistant, but also the heaviest and can be challenging to mount in a streamlined manner.

More marketing 'information':

https://qookka.com/en/blog/etfe-and-pet-two-key-materials-in-the-field-of-semi-flexible-panels-n3

Here is another report about a 3M topcover material:

https://www.osti.gov/servlets/purl/1166732

Although not particularly relevant to your problem, it would be interesting to know peoples experience with garden solar lights which are usually either epoxy encapsulated crystalline cells or amorphous glass panels. The former tend to crack or go cloudy with time and the latter suffer water damage - but I bet some people will have panels that worked fine for 5+ years  whilst others have had numerous failures after only a year or so.

[CatalinaWOW]

I have quite a few of those garden lights when I lived in southern Arizona.  The results are very mixed.  You would expect cells that came in the same box to be of the same technology and probably even the same manufacturing lot.  But I had one group that had one cell yellow and crack badly in well under a year.  The rest of those soldiered on until they were retired after their second battery replacement (about four and a half years).  All had similar exposure.  These had some yellowing and very minor cracking but still provided ample power for the application.   Other groups didn't show any real issues with the solar cell.  In some cases for up to ten years.

Sorry, but no real quantitative information.

[boB]

I have seen several modules last 50 years or more...  What usually happens or happened was that they turn a yellow or brownish color.

Of course, their output degrades somewhat too.  Old ARCO modules are some that we still see being used but discolored.

One of the problems with some of today's modules by newer companies that haven't been making PV for a long time is that they can delaminate.

Bypass diodes can also fail.

Don't spray them with water, especially on their backs when they are hot.

Good PV modules should last a very long time.

[splin]

I have seen several modules last 50 years or more...  What usually happens or happened was that they turn a yellow or brownish color.
...
Good PV modules should last a very long time.

Just to make it clear, you're talking about large framed glass laminated panels which are expected to last at least 25 years and warranted for 15 years or more. The OP is asking about small, plastic covered modules which are very different - the vast majority of these are sold for low cost applications such as toys, novelties and solar lights which aren't expected to last long.

Flexible panels use plastic topsheets and often are marketed with warranties and long life claims, but actual lifetimes can be very variable. Yacht owners have plenty of experiance of these, so search on relevant forums. Those panels are relatively expensive, so cheap small modules using similar materials, won't necessarily last as long.

There are Chinese manufacturers making small glass laminated modules in various shapes which ought to have long lifetimes but unfortunately I can't find the links now - you'll have to search. I doubt you'll get any worthwhile warranties unless you are paying accordingly and buying in large volumes.

Amorphous glass panels are common and cheap but the efficiency is low, 4 to 6%. These should last a long time if you can find a reliable way to seal the edges and back to prevent water corroding the film. It might be as simple as using silicon sealant but I don't know how durable that would be.

[boB]

It's hard to say how long these smaller panels will last by new manufacturers.

The silicon might be the most durable part and the packaging of the cells the iffy part.

Also, the 25 year warranties are usually only related to output degradation in output power  and not if they are "broken" some other way.  Which is pretty weird.
You could have a PV panel guarantied for 25 years but if it actually breaks in 22 years, it might not be covered under that warranty.
Seems weird if you consider that its output went to zero percent of its original output power.

[RandallMcRee]

I think marine panels are your best bet--at least the major manufacturers will have done some testing, etc.

https://www.westmarine.com/buy/nature-power--semi-flexible-15w-monocrystalline-solar-panel--14996300?recordNum=1

That is a "cheap" panel and has a 2 year warranty. Seems like a fairly high price. So, perhaps, this tells us (meaning you) that what you want may not be cheap.

I can't imagine anything on aliexpress having a similar guarantee, and if they do, it may be worthless, e.g. no testing to back it up.

[splin]

There are a few glass laminated panels on Aliexpress including these 2W 9V ones (note that I am not recommending this sellor):

https://www.aliexpress.com/item/Wholesale-2Watt-9V-Solar-Cell-Glass-Laminated-Polycrystalline-Solar-Panel-Solar-Module-DIY-Solar-Charger-135/32867801384.html?spm=2114.search0104.3.181.4e04111aqJJXRW&ws_ab_test=searchweb0_0,searchweb201602_2_10065_10068_10130_10547_319_317_10548_10696_10192_10190_453_10084_454_10083_10618_10307_10820_10301_10821_10303_537_536_10059_10884_10887_100031_321_322_10103,searchweb201603_2,ppcSwitch_0&algo_expid=d1e9ff82-d8b7-45e5-a7f9-85739bf5dd09-29&algo_pvid=d1e9ff82-d8b7-45e5-a7f9-85739bf5dd09

The description includes the claim:

Service life: use 20 to 25 years if normally used

Make of that what you will. There is no reason why panels like these couldn't last 20+ years providing they are made properly with good quality materials, especially the encapsulant which is usually EVA. This should prevent any water getting at the cells and interconnects but it is relatively expensive. If they are saving money by buying inferior EVA or material that is cheap because it has passed its shelf life then who knows.

Beware: I bought a couple of very similar panels from Aliexpress for a good price, except that they are faulty, probably factory rejects, because two of the columns are not working - ie. you can completely shade half the panel and it makes no difference to the o/p. Unfortunately I didn't test them properly until it was too late to claim.

Problem with Aliexpress and Ebay Chinese products is that even if you pay a 'realistic' price rather than the 'too good to be true, rock bottom' prices you may still get fakes or rejects whilst giving the vendor a much better profit margin.

Also beware - there are many modules with glass or 'frosted glass' in the title which actually use PET. This one has both glass and PET in the title:

https://www.aliexpress.com/item/Solar-Panel-5-5V-0-88W-Frosted-Glass-Plate-Outdoor-Solar-Charger-Pane-PET-Fast-Charger/32898057987.html?spm=2114.search0104.3.33.7dd9b527gHYy1l&ws_ab_test=searchweb0_0,searchweb201602_2_10065_10068_10130_10547_319_317_10548_10696_10192_10190_453_10084_454_10083_10618_10307_10820_10301_10821_10303_537_536_10059_10884_10887_100031_321_322_10103,searchweb201603_2,ppcSwitch_0&algo_expid=bd10ea18-09cc-4957-b57b-4d095bfd7951-5&algo_pvid=bd10ea18-09cc-4957-b57b-4d095bfd7951